List of Papers

The kinetics of phase transformations in 40CrMoV5-1 hot work tool steel

The reason for writing this paper was to describe the kinetics of phase transformation of undercooled austenite in the form of the CCT (Continuous Cooling Transformation) diagram as well as the kinetics phase transformation during heating from as-quenched state in the form of the CHT (Continuous Heating Transformation) diagram of the 50CrMoV5-1 hot work tool steel.

In order to make the CCT diagram of the kinetics of phase transformation of undercooled austenite the samples were heated to a temperature of 1020 °C with a rate of 5 °C/s, hold for 20 minutes and then cooled with various rates (50÷0.02 °C/s) to a temperature of 20 °C. Numerically recorded dilatograms were differentiated to provide more precise readings of characteristic temperatures. In order to make the CHT diagram of the kinetics of phase transformations at continuous heating from as-quenched state, the previously quenched samples (T_A = 1020 °C, t_A = 20 min, cooling with the rate of 50 °C/s) were heated with the following rates: 0.05; 0.1; 0.5; 1; 5; 10; 15; 35 °C/s to a temperature of 700 °C, while changes in the samples elongation in dependence of the temperature were recorded. In this case, numerically recorded dilatograms were also differentiated for a more precise reading of characteristic temperatures. In booth about mentioned case an interpretation of heating dilatometric curves was presented.

The CCT diagram is characterised by the range of diffusive changes shifted strongly to the right, it means to longer times (high hardenability). Hardenability of the investigated steel is so high, that only during the cooling with the rate of 0.05 °C/s a small dilatation effect related to austenite to perlite transformations was recorded.

During continuous heating from the as-quenched state the obtained results confirmed the following order of phase transformations: precipitation of ε carbide, precipitation of cementite with a simultaneous transformation of a part of the retained austenite, independent nucleation of MC carbides followed by the nucleation of M₂C carbides. The increase of heating rate (from 0.05 to 35 °C/s) results in the increase of temperatures of the beginnings and the ends of particular transformations, and in decrease of accompanying dilatation effects.

Presented results may be used to design new heat treatment technologies of hot work tool steel.

The formation of a fine ferrite and low temperature bainitic composite microstructure

A novel thermomechanical processing route was developed in the present study to produce a unique microstructure consisting of fine ferrite grains (i.e. ˜4 (C?X)m on average) and low temperature bainite in a relatively low carbon steel with a modest hardenability. The thermomechanical route consisted of warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to the bainitic transformation regime (i.e. 400-200 °C). The low temperature bainite consisted of high dislocation density bainitic laths and very fine retained austenite films. This microstructure offered a high work hardening rate leading to a unique combination of ultimate tensile strength and elongation. This was due to the presence of ductile fine ferrite grains and hard low temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy, electron backscatter diffraction and atom probe tomography techniques.

Anomalies in the measurement of the carbon content of defective ferrite or austenite using X-ray diffraction

Carbon concentrations are often measured using X-ray diffraction and relating the lattice parameter to the concentration using empirical equations. However, there is strong evidence that something is wrong, in that mass balance is not achieved when duplex mixtures of austenite and ferrite are studied. A solution to this problem is proposed in this work.

The investigation of microstructures and properties of high speed steel PMHSS6-5-3 after laser alloying

Laser treatment as part of the new generation techniques applied in metal surface technology is discussed in this paper. Laser treatment has been applied to the remelting of high speed steel PMHSS6-5-3 with carbide and ceramic powders, especially WC, VC, TiC, SiC, Si₃N₄ and Al₂O₃ particles. Laser treatment is especially promising for solving contemporary surface engineering problems thanks to the physical properties of the laser beam, making it possible to focus precisely the delivered energy in the form of heat in the surface layer. The material behavior during the HPDL processing has been found to be different from the other high-power lasers in the following aspects: fewer cracks and more uniform melt/heating zones, smoother surface, better beam absorption for metallic materials, more consistent and repeatable. It is a well-known fact that the same laser processing parameters may not necessarily produce the same results in laser materials processing. It has already been noticed that HPDL can produce more consistent materials-processing results than Nd:YAG lasers. This feature, together with the lower maintenance costs and longer service life would make HPDL suitable for mass production applications such as the soldering of telephone connectors. Other advantages of the HPDL are: lower running cost, higher energy efficiency (up to 35% wall plug efficiency) thus the cooling requirement is low and the size of laser and cooling unit is small, flexible beam shaping by controlling the intensity of individual beams, theoretically unlimited average power, portable and longer service life (typically 4000 - 5000 up to 10000 h). This type of surface treatment is used for improvement of hardness by changing the structure, and for improvement of the abrasion wear resistance, mostly by introduction of carbide or ceramic particles to the material matrix. On the paper selection of laser operating conditions is discussed, as well as beam face quality after remelting, hardness, micro hardness test, wear resistant, EDX, TEM and X-ray microanalysis results. Fine grained, dendritic structures occur in the remelted and alloyed zone with the crystallization direction related to the dynamical heat movement from the laser beam influence zone. The remelted zone structure is characterized by significant martensite dispersion with its laths several times shorter than of those developed during conventional quenching. The fine grained martensite structure is responsible for hardness increase of the alloyed layer. Micro-hardness changes depend up in the effects of the laser beam on the treated surface, and especially in the alloyed layer. Laser technique features the especially promising tool for solving the contemporary surface engineering problems thanks to the physical properties of the laser beam, making it possible to focus precisely the delivered energy in the form of heat in the surface layer.

Structure and mechanical properties of high-manganese austenitic steels after their heat- or thermo-mechanical treatments

The aim of this paper is to determine the high-manganese austenite propensity to twinning induced by the cold working and its effect on structure and mechanical properties, and especially the strain energy per unit volume of ten new-developed high-manganese Fe – Mn – (Al, Si) investigated steels, including selected high-manganese austenitic TWIP steels containing 25 - 27.5% Mn, 1 - 4% Si, 2 - 3% Al and TRIP steels containing 17 - 18% Mn, about 1% Si, about 3% Al, with various structures after their heat- and thermo-mechanical treatments. The new-developed high-manganese Fe – Mn – (Al, Si) steels provide an extensive potential for automotive industries through exhibiting the twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) mechanisms. TWIP steels not only show excellent strength, but also have excellent formability due to twinning, thereby leading to excellent combination of strength, ductility, and formability over conventional dual phase steels or transformation induced plasticity TRIP steels.

Nanostructured bainite-austenite steel for armours construction

Nanostructured bainite-austenite steels are applied in the armours construction due to their excellent combination of strength and ductility which enables to lower the armour weight and to improve the protection efficiency. Mechanical properties of the bainite-austenite steels can be controlled in the wide range by chemical composition and heat treatment. In the present paper the results of investigation comprising measuring of quasi - static mechanical properties, dynamic yield stress and firing tests of bainite-austenite steel NANOS-BA® are presented. Reported results show that investigated bainite-austenite steel can be used for constructing add-on armour and that the armour fulfils requirements of protection level 2 of STANAG 4569. Obtained reduction in weight of the tested NANOS-BA® plate in comparison with the present solutions is about 30%.

Ultra-Fine and Nanoscale Bainitic Ferrite Structures

Advanced high strength steels for automotive applications were designed to achieve a carbide-free bainitic microstructure after conventional thermo-mechanical processing and a continuous annealing treatment. The microstructure obtained consists of ferrite laths interwoven with thin films of untransformed retained austenite. The sufficiently tough matrix and the control of the heterogeneity in the microstructure will allow an optimum combination of strength, ductility, and formability to be achieved. The designed steels reached far higher uniform elongations than that in commercial dual phase steels and martensitic steels with the same range of ultimate tensile strengths. However, the concepts of bainite transformation theory can be exploited even further to design steels that transform to bainite at temperatures as low as 150 °C. Microstructure obtained is so refined that it is possible to achieve a strength in excess of 2.5 GPa in a material which has considerable toughness (30MPam^½). Such properties have never been achieved before with bainite. It is intended to provide a description of the characteristics and significance of this remarkable microstructure in the context of the mechanism of transformation.

Hydrogen embrittlement in high-strength low-alloy steels

Hydrogen embrittlement has been the cause of failures in high-strength constructional steels used in many industry branches, e.g. shipbuilding and offshore, aircraft and car industry.

Corrosion of marine constructions is prevented by coatings and cathodic protection. The later may be harmful for steels resulting in their hydrogen embrittlement. Steel could be charged with hydrogen in the vicinity of polarisation anode (over-protection), and by presence of some sulfate-reducting bacteria (SRB) in sea-water. Additionally, welded joints contain some amount of hydrogen introduced into a material during welding. This could increase a risk of hydrogen degradation during service.

The aim of the paper is to evaluate susceptibility to hydrogen embrittlement in high-strength low-alloy steel and its welded joints.

A quenched and tempered plate 12 mm in thickness made of S690Q steel grade with submerged arc welded (SAW) and shielded metal arc welded (SMAW) joints were used for investigations. Electrochemical permeation test was performed to evaluate hydrogen uptake and diffusion.

Susceptibility to hydrogen embrittlement of steel and welded joints has been evaluated using monotonically increasing load. Slow strain rate test (SSRT) was carried out on round smooth specimens in air, and seawater under cathodic polarization. Elongation and reduction in area were chosen as measures of susceptibility to hydrogen embrittlement. Fractographic examinations with the use of scanning electron microscope (SEM) were performed to establish mechanisms of hydrogen-enhanced cracking.

High-strength low-alloy steels and its welded joints are susceptible to hydrogen embrittlement when evaluated with the use of SSRT under cathodic polarization. The loss of plasticity is higher for welded joints then for the base metal. However, tested S690Q steel and its welded joints could be safely utilized in marine constructions under cathodic protection provided that overprotection and severe hydrogen charging does not take place.

Optimization and characterization of a quenching and partitioning heat treatment on a low carbon steel

A low carbon Quenching and Partitioning (Q&P) steel grade was subjected to optimized heat treatment cycles. The heat treatment cycles were optimized with respect to the quenching temperature, partitioning temperature and partitioning time and tested by carefully designed dilatometric experiments. A wide range of heat treatment cycles were applied to the cold rolled sheet material by Gleeble thermo-mechanical simulator resulting in a variety of mechanical properties and microstructures. The mechanical properties were evaluated by tensile testing and linked with the retained austenite fraction (cfr. Fig. 1) and its carbon content, which was determined with X-ray diffraction experiments.

Detailed characterisation of the retained austenite and the carbides was carried out by Electron Backscattered Diffraction, Scanning Electron Microscopy and Transmission Electron Microscopy (cfr. Fig. 2).

The Koistinen-Marburger relationship was determined experimentally and evaluated in terms of the kinetics of partitioning. The retained austenite fractions and carbon contents of samples heat treated with different partitioning times are compared with theories of grain boundary mobility. Conclusions are drawn about the influence of the annealing parameters on the microstructure in terms of optimal austenite fraction in the martensitic matrix and ensuing mechanical properties. These results contribute to the optimal design of Q&P heat treatment, producing the microstructure with desired balance of strength and ductility.

Effect of shortening of the austempering time on the nanocrystalline structure and properties of the 67SiMnCr6-6-4 bearing steel

Increasing requirements concerning economy environment and security of industrial products impose incessant technological changes on the production of structural steels. One of the latest trends is formation of nanocrystalline structures within steels using precisely designed heat treatment processes. Efforts to develop steels with nanomartensitic, nanoaustenitic or nanobainitic structure have been described in literature. Formation of nanobainitic steel structures seems to be the most promising due to the expected results.

However, preparation of a nanobainitic structure has a serious limitation - a long time of isothermal annealing in the range of bainitic transformation. The completion time of bainitic transformation ranges from several days to several weeks, thus being a significant issue limiting the industrial implementation of these steels.

Interruption of heat treatment before completion of bainitic transformation may lead to formation of an unstable structure resulting in significant deterioration of steel properties.

The present study attempts to shorten the isothermal hardening process leading to the formation of nanostructures within the 67SiMnCr6 -6 - 4 bearing steel while maintaining the overall properties resulting from complete bainitic transformation.

Characterization of nanobainitic structure obtained in 100CrMnSi6-4 steel after industrial heat treatment

Novel high silicon, high carbon steel have been developed by Bhadeshia et al. with the purpose of obtaining nanobainitic structure by isothermal quenching. The nanobainitic structure consists of extremely fine laths of bainitic ferrite and thin films of carbon enriched retained austenite. This type of structure guarantees high mechanical properties while maintaining high hardness and toughness compared to the steels of similar chemical composition and microscale structure.

The study attempts to characterize the nanobainitic structure produced in the commercial bearing steel – 100CrMnSi6-4.

In order to produce this structure austempering was performed, which parameters have been selected based on dilatometric measurements. It is worth noting that the present structure was formed during the process carried out in industrial furnaces. The obtained structure was characterized using transmission electron microscopy. In order to investigate the effect of the microstructure parameters on the material’s mechanical properties, the hardness, impact strength and static tensile tests have been conducted.

The influence of tempering temperature on the morphology of the iron carbide precipitations and its influence on the mechanical properties

This work contains a detailed description of the influence of the temperature on the morphology and quantity of the iron carbides occurred after tempering in as-quenched medium carbon Cr-Mn-Mo alloy steel. The microstructure of the investigated steel was investigated by a light microscope Axiovert 200 MAT and the HITACHI SU-70 scanning electron microscope. Hardness was measured using Vickers diamond pyramid hardness method. The impact strength was obtained from Charpy impact test.

This steel was water quenched from 870 °C (which means 50 °C above Ac₃ = 820 °C). In as-quenched state the microstructure of investigated steel was consisted of martensite and retained austenite. After tempering above 200 °C transition carbide ε transforms into cementite and retained austenite into fresh martensite. Also during tempering of this steel above 200 °C the unfavourable irreversible temper brittleness effect occurred which is very interesting problem. This effect is often explained by the transformation of the epsilon carbides as well formation of cementite.

It can be seen in the SEM images various precipitations of which the length are in the range from 300 nm to 3.5 µm. These are most likely ε and cementite carbides. The width and length of the observed precipitates were measured at three different tempering temperatures 200, 250 and 300 °C. The obtained data has been compared to toughness and hardness measures.

The modification of microstructure to increase toughness of high – strength nanostructured bainite – austenite steel

In the recent decade the class of steel with the microstructure named "nanocrystalline bainite" or "nanostructured bainite" has been intensively investigated and several steel grades with carbideless bainitic structure, containing also significant amount of retained austenite, have been introduced into production. Various grades of the nano – bainitic steels exhibit the tensile strength typically in the range of 1.5-2.5 GPa and good plasticity of 15-25% as measured by total elongation. However, required toughness value or fracture resistance of products made of the nano-bainitic steels could be difficult to meet for some applications. In this paper results of experimental work aimed at increasing toughness of nano – bainitic steel through microstructure modification are reported. As experimental material a nanostructured bainite – austenite steel named NANOS-BA® was used characterised with following properties: ultimate tensile strength over 1.9 GPa, yield strength in the range of 1.2-1.5 GPa and total elongation 14-20%. To improve toughness of the investigated steel a number of multi-stage heat treatments were tested intended to refine steel microstructure, and specifically to reduce the length of the packets of the nano-bainite laths. To produce specimens for investigation of the correlation between parameters of the applied heat treatments and obtained microstructure experiments in a dilatometer were used. Microstructure of the dilatometric specimens was examined by optical microscopy and scanning electron microscopy whereas amount of retained austenite was determined by X-ray diffraction. The parameters characterising mechanical properties of bulk specimens of NANOS-BA® steel subjected to heat treatment procedures designed in this study were: ultimate tensile strength, yield strength, total percentage elongation, hardness and – as the measure of toughness – Charpy notched impact strength. It has been found that the designed heat treatment could raise Charpy impact toughness by 20-30% over the testing temperature range of (-40 °C) ÷ (+20 °C).

NANOBAIN - Industrialization of a nanostructured steel

Nanostructured microstructures consisting of a mixture of very thin plates of bainitic ferrite separated by C enriched austenite are the main characteristics of the so called NANOBAIN steel family. This work deals with the theoretical approach followed in the design of a new, industrially viable, generation of NANOBAIN steels, a process in which industrial demands as, simpler chemical compositions, faster transformation kinetics and suffice hardenability have been also considered. The microstructural and mechanical characterization of the bainitic microstructures obtained by isothermal transformation at different temperatures, come to confirm and to validate the followed theoretical approach.

Optimisation of the Complex Phase Steels microstructure and its properties through a new heat treatment process

It was shown that it is possible to form a ferritic - bainitic microstructure with a retained austenite in the 35CrSiMn5-5-4 structural steel. This microstructure was produced during intercritical annealing which allowed determined volume fractions of austenite and ferrite to be obtaned. The subsequent austempering performed in the temperature range of bainitic transformation led to obtaining a microstructure composed of carbide-free bainite with retained austenite and of the ferrite grains with carbides. After the aforementioned treatment, this steel may be classified as one of the multiphase steels (CP - complex phase) which belong to the broader group of Advanced High Strength Steels (AHSS). However, the obtained microstructure exhibited strong heterogeneity and significant density of coarse carbides in the ferrite grains, which did not dissolve during the incomplete austenitization. These carbides may contribute to the processes of heterogeneous deformation of steel and, consequently, to premature cracking.

It was shown, that the use of additional heat treatment prior to intercritical annealing resulted in a radical change of the microstructure formed after the final austempering process. The light and transmission electron microscopy observations revealed that the obtained microstructure is more homogenous and much more fine grained as compared with the previous one. Due to this preliminary step of heat treatment, 35CrSiMn5-5-4 steel gains higher strength and plasticity parameters in comparison to the steel after the treatment without the preliminary heat treatment step.

Weldability of thermomechanically treated steel with high yield point

The article concerns the issue of weldability of S700MC steel, treated thermo-mechanically, with high yield of plasticity and strengthened by precipitation. The treatment process leads to a defected structure and its supersaturation with the reinforcing components. Suitable mechanical and plastic properties of S700MC steel were ensured by thermomechanical treatment by segregation strengthening, solution-based and hardening, as well as the processes associated with grain grinding. Elements responsible for the strengthening of the S700MC steel are Ti, Nb, N and C. The strengthening is dependent on the separation precipitation of dispersion type (Ti,Nb)(C,N) with a size range from a few to several nanometers. The segregation of the dispersion in the size range of a few nanometers, anchoring dislocations movement are released during cooling of steel in the ferrite, causing significantly strengthening, while particles larger than several nanometers are limiting the grain growth of recrystallized austenite, favouring formation of fine-grained structure. The welding process is significantly different from the parameters of thermomechanical processing, leading to reduction of plastic properties in both the HAZ area and the weld. In the case of steel with high yield of plasticity, treated thermomechanically, in which there is segregation strengthening by reinforcing phases of titanium and niobium, coal equivalent and phase transition γ-α, cannot be a basis for assessing the weldability. Decisive influence on the properties of welded joints of this steel group has a stability of strengthening phases, changing their dispersion and any aging processes. The conducted research on the effect of heat treatment on the properties of S700MC steel, the study of simulated HAZ areas, heated to different maximum temperatures have shown that the thermal cycle strongly influences the mechanical and plastic properties of welded joints. The worst properties were characterized by a high temperature, coarse HAZ area, in which due to the nucleation effect of the dissolved phases, strengthening the matrix and their subsequent uncontrolled separation precipitation in the form of finely dispersed, resulting in a rapid decrease in toughness. Arc welding tests performed here have shown that in order to ensure high quality of welded joints, it is necessary to limit the welding linear heat input. During the welding process of S700MC steel, it is not recommended to use pre heating before the welding process and heat treatment after welding, and the number of repairs should be kept to a minimum, because it leads to a reduction of strength and plastic properties in the HAZ area, as a result of aging processes, dissolution of strengthening phases in the matrix and their subsequent uncontrolled precipitation during cooling.

Microstructure-property relationships in TRIP-aided medium-C bainitic steel with lamellar retained austenite

The work concerns the design of multi-step cooling of medium-C steel containing increased contents of Si and Al in order to obtain multiphase structures consisting of bainitic ferrite laths and metastable retained austenite, which enables the utilization of the TRIP (TRansformation Induced Plasticity) effect during technological cold forming of sheets. The CCT (continuous cooling transformation) and DCCT (deformation - continuous cooling transformation) diagrams were developed on the basis of dilatometric tests. The sheets were produced by thermomechanical rolling and controlled cooling with their isothermal holding at 450 °C for 600s. The detailed analysis of the microstructure was performed by means of light, scanning and transmission electron microscopy.

The image analysis is not a useful method for evaluating the fraction of austenitic phase because it does not allow to detect fine granules and interlath regions of this phase distributed in a bainitic matrix. It was done using X-ray and EBSD (Electron Back-Scattered Diffraction). The volume fraction of austenitic phase mainly of interlath morphology is about 16% and its C content is 1.6 wt.%. Mechanical properties were determined using uniaxial tensile tests.

Study on non-metallic inclusions in laser-welded TRIP-aided Nb-microalloyed steel

Mechanical properties and technological formability of Advanced High Strength Steels (AHSS) for the automotive industry are dependent on relative volume fractions and mechanical properties of individual microstructural constituents. Ferrite forms a matrix of AHSS whereas strengthening phases are martensite and/or bainite. A very attractive combination of high strength and ductility can be obtained in multiphase steels with a ferritic matrix containing bainitic-austenitic islands. Their final mechanical properties are formed during cold working under conditions of strain-induced martensitic transformation of metastable retained austenite. The further increase of strength properties of multiphase steels can be achieved in Nb/Ti/V-microalloyed TRIP-aided steels.

The work is focused on issues related to laser welding of Si-Al type TRIP steel containing Nb and Ti microadditions. The tests of laser welding of thermomechanically processed sheet steels were carried out using keyhole welding and a solid-state laser. The tests carried out for various values of heat input were followed by detailed microscopic investigations using light microscopy (LM) and scanning electron microscopy (SEM). A microstructural analysis was carried out in the penetration area and in various areas of the heat affected zone (HAZ). Special attention was paid to the influence of cooling conditions on the stabilisation of retained austenite as well as to a type of non-metallic inclusions formed during air atmosphere welding. The non-metallic inclusions occurring in base metal, HAZ and fusion zone were identified using EDS mapping.

Microstructural features of strain-induced martensitic transformation in medium-Mn steels with metastable lamellar retained austenite

Advanced high-strength steels (AHSS) dedicated to the automotive industry have received much interest in recent years due to their outstanding combination of strength, ductility, technological formability and superior capability of energy absorption under dynamic deformation conditions. The newest achievements include TRIP-aided medium-Mn steels containing a large fraction of metastable retained austenite. Their strength-ductility balance is dependent on strain-induced martensitic transformation of retained austenite upon straining.

The work deals with the microstructural analysis of the thermomechanically processed and subsequently cold deformed 3Mn-1.5Al steel showing a TRIP effect. The thermomechanically processed sheet samples were subjected to 5%, 10% and 15% elongation in uniaxial tension. The evolution of the bainitic-austenitic microstructures and especially the identification of strain-induced martensite was carried out using light microscopy, electron transmission microscopy and electron scanning microscopy equipped with EBSD (Electron Backscatter Diffraction). Morphological details influencing the mechanical stability of retained austenite were indicated.

Thermomechanically rolled medium-Mn steels containing retained austenite

Multiphase steels with retained austenite belong to advanced high-strength steels (AHSS), which are used in the automotive industry. They include different ferrite-based grades covering a wide range of mechanical and technological properties. Their characteristic feature is the good combination of strength, ductility and technological formability. At present, there is the tendency to further improve a strength-ductility range of steel sheets to satisfy the needs of the automotive market. One of the main idea is to replace a ferritic matrix by carbide-free bainite containing a large fraction of interlath austenite. It is a relatively low-cost concept covering medium-Mn steels with strength above 1000 MPa and elongation from 15 to 30%.

The aim of the work is to assess a level of hot flow stresses and microstructure of thermomechanically rolled and controlled cooled medium-Mn steels with increased Al addition. Four steels of various Mn (3 and 5%) and Nb concentrations were thermomechanically rolled with a finishing deformation temperature of 950 and 750 °C. Hence, the supercooled austenite transformations were monitored upon cooling from the recrystallized and non-recrystallized austenite. Temperature-time and force-energetic parameters of hot rolling were continuously registered and assessed. It was found that the steels investigated have high total pressure forces due to the high total content of alloying elements. For the purpose of the detailed analysis of all the microstructural constituents combined methods of light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used. Additionally, the orientation imaging microscopy (OIM) using SEM was applied. It was proved that it is possible to stabilize up to 18% metastable retained austenite located between bainitic ferrite laths.

Effect of Heating Rate on Accelerated Carbide Spheroidisation (ASR) in 100CrMnSi6-4 Bearing Steel

The processing of bearing steels often includes an annealing stage, at which the initial lamellar pearlite is to be transformed to ferrite matrix with globular carbides. The purpose of this treatment is to improve the machinability of the material prior to hardening. In general, carbide spheroidisation occurs through diffusion close to temperature A1, which is a time-consuming and thus energy-intensive process. The annealing times, sometimes up to tens of hours, make soft annealing one of the most expensive heat treatment processes. The process that was newly designed at the company COMTES FHT reduces the carbide spheroidisation times to a fraction and delivers considerable time and cost savings. The heat treating experiments were performed using induction heating, which provides high heating rates and rapid transformations of microstructure. The accelerated carbide spheroidisation process relies on rapid thermal cycling in the vicinity of the transformation temperature A1. The purpose is to obtain globular carbides uniformly dispersed in the matrix and to achieve overall softening in the material.

Unlike experimental facilities, industrial plant conditions do not always allow the required process parameters to be met. Changes in the heating rate are accompanied by changes in the transformation temperatures. As a result, the accelerated spheroidisation process takes place within a certain temperature interval. The present paper gives a description of the process window (temperature interval) and its dependence on the heating rate applied in the accelerated spheroidisation and refinement process (ASR). The total heat treating time is on the order of minutes.

Microstructural analysis of twinning mechanism in high manganese TWIP steel after static and dynamic deformation

TWIP (Twinning Induced Plasticity) steels offers with high ductility and high plasticity high strength and excellent energy absorption ability has an attracted materials for automotive, railway and military industry. When the Mn is up to 25 wt.%, Al is more than 3 wt.%, and Si is between 2 and 3 wt.%, calculated SFE is between 25 and 40 mJ/m² the austenitic steel exhibits TWIP effect.

The energy-absorption capacity is higher than conventional deep punching steel. Most studies about the Mn-Al steels are focused on static properties, such as the tensile properties fatigue behaviours, welding properties, alloying effects, and deformation mechanism. Dynamic mechanical properties are requested and play an important role in describing the mechanical properties and structure evolution in TWIP steel for understanding the great properties of these group of materials. In the present study, the dynamic mechanical properties of TWIP steel were investigated using a flywheel machine with high strain rates at room temperature. The structure was analysed by using a SEM, STEM and TEM with characterization of the dislocation and twin substructure. It was found the presence of mechanical twins and of dislocation cells after the dynamic deformation.

Disk laser welding of armor steel

The paper describes the application of an Yb:YAG disk laser with a maximum output of 3.3 kW for the welding of armor steel ARMOX 500T. Both bead-on-plate and butt joint welding were conducted on ARMOX 500T plates 3.6 mm thick without an additional material using keyhole welding mode. The influence of laser welding parameters such as laser power beam, welding speed, focal point position on weld quality and mechanical properties of joints was studied. The joints were examined by optical metallography and scanning electron microscopy (SEM). The mechanical properties of the joints were determined by means of tensile, bending and Charpy impact tests, and also micro hardness tests.

The results indicated that the laser power level directly determines the range of welding speed for fully penetrated weld, thus simultaneously determines a minimum heat input level required to achieve a full penetration butt welds. The welding speed, at given laser power level, has a major effect on the weld aspect ratio (the depth to width ratio). A proper selection of disk laser welding parameters provides non-porous, fully-penetrated welds with the aspect ratio up to 6.0. The minimum heat input required to achieve full penetration butt welded joints with no defect was found to be 50 J/mm. The hardness values at the fusion zone (FZ) and the base metal (BM) were similar. There was approx. 40 % reduction in the hardness of heat affected zone (HAZ) in comparison to hardness of the BM and FZ. The joints exhibited about 10 % lower ultimate tensile strength when compared with that of the BM. Charpy absorbed energy of the joints was approx. 50% of that of the BM.

Nanobainitic structure recognition and characterization using transmission electron microscopy

Various transmission electron microscopy techniques were used for recognition of different kinds of bainitic structures in 100CrMnSi6-4 bearing steel after isothermal quenching. Conventional bright and dark field images with corresponding selected area diffraction patterns are very useful in nanostructure characterization. Upper and lower bainite are morphologically different, so it is possible to distinguish between them without problem. A lot of work was done on classical bainitic structure. For new bainitic structure, consisting of mutual ferritic and austenitic lamellae, there is still controversy. It is really carbide free structure? What is the difference between dense lamellar pearlite and nanobainite?

Crystallographic relationship between ferrite and austenite it is the key for successful phase identification in this case. Proper orientation of TEM sample can help significantly in this analysis. Some specific orientation relationships between ferrite and austenite may be expected; Nishiyama-Wassermann or Kurdjumov-Sachs. Superimposed selected area diffraction patterns for these orientation relationships may be used to determine common directions and planes in both phases. The needles of austenite and ferrite appear as packets of smaller sub-units. Depending of the chemical composition of the steel the packets are interpenetrated or isolated. In studied bearing steel the ferrite surrounding the austenite lamellae has a high density of dislocations.

Q-P processing of steel with 0.2% C

Carbon content of Q-P (quenching and partitioning) processed steels is one of the key parameters, because carbon plays an important role in retained austenite stabilization and also in the increase of strength. The highest strengths are commonly obtained for martensitic steels. Therefore, there have been attempts to apply new methods of heat treatment to low carbon steels to enable them to obtain martensitic microstructure with retained austenite and subsequently to reach high strength and ductility.

Q-P processing of two low alloyed steels with carbon content of 0.2% have been optimized in experimental program. The first steel, CMnSi, was alloyed by 1.5% of manganese and 1.8% of silicon. The second steel, CCrMnSi, was further alloyed by chromium, which has positive effect on suppression of bainite transformation and increase of strength. Various austenization temperatures were tested and also the effect of cooling rate from heating temperature on quenching temperature and different quenching and partitioning temperatures were investigated.

The results showed that besides suitable austenitzation temperature, cooling rate had significant effect on microstructure development. In the case of high cooling rates, even low alloyed steels with low carbon content were able to reach predominantly martensitic microstructure with low ferrite fraction. Chromium alloyed steel possessed ultimate strength around 900MPa with 30% ductility.

Influence of austenitizing temperature on the kinetics of phase transformations in the TRIP steel with medium carbon content

Comprehensive studies of the kinetics transformations of undercooled austenite in the TRIP (0.4%C; 1.5%Mn; 1.2%Si) steel during cooling from temperature range: 750 ÷ 1030 °C were carried out. The selection of temperatures was made on the bases of the critical temperatures (i.e. Ac_1s, Ac_1f and Ac₃) of the investigated steel, which were estimated during its dilatometric analysis, performed previously. Based on this research four Continuous Cooling Transformation (CCT) diagrams were made and presented in the paper.

The influence of the austenitization temperature of the investigated TRIP steel on the formation ranges of new phases and structural components during the continuous cooling (with different rates) was determined on the bases of the obtained results.

It was also shown, that the austenitization of the investigated steel at the lowest temperature, i.e. in the Ac_1s and Ac_1f range, resulted in the strongest lowering of Ms temperature. Simultaneously it was found, that the tested steel was characterized by the lowest hardenability, when the quenching started from Ac_1f-Ac_1s range. However, after austenitizing at higher temperatures, an increased hardenability of the tested steel accompanied by the Ms temperature increase, was noticed.

Based on the analysis of the stated relationships, the modifications of the annealing in the critical temperatures range, adapted to the investigated steel chemical composition, was proposed.

Influence of Thermo-Mechanical Treatment on the Microstructure and Texture Evolution of a Cold-Rolled Austenitic High Manganese Steel

High manganese austenitic steels are widely used in the automotive industry for car safety parts due to their good formability and high energy absorption during deformation. These two features are caused by extensive twin formation – TWIP effect (Twinning Induced Plasticity) [1] or martensitic transformations (γ_fcc→ε_hcp→α´_bcc) referred to as TRIP effect (Transformation Induced Plasticity) [2].

The work presents a study on the influence of thermo-mechanical treatment on the microstructure and texture evolution of a cold rolled high-manganese steel, containing 24.4% Mn, 3.49% Si, 1.64% Al and micradditions of Ti (0.075%) and Nb (0.029%). The received steel sheet was subjected to cold rolling with deformation rate up to 40% in two variants: directly from the as-received state and after a thermo-mechanical treatment process. The microstructure changes were analysed with Optical Microscopy and Transmission Electron Microscopy. The phase analysis and texture determination were conducted by the means of X-Ray diffraction investigations. The X-Ray phase analysis was conducted on the inner and outer layer of the steel sheets.

It was shown, that the fully austenitic structure of the initial state changes with increasing deformation rate due to the γ→ε→α´ martensitic transformation, whereby γ, ε and α´ deformation takes place simultaneously. Moreover the phase composition and the preffered grain orientation (deformation texture) of these phases are different on the steel surface and in the inner layer of the investigated steel. These changes are disappearing during rolling with high thickness reduction of the steel sheet after thermo-mechanically treatment. This finding is important, since the texture influences the directionality of mechanical properties of investigated high manganese TRIP/TWIP type steel sheets applied in automotive industry.
1. D. Barbier, N. Gey, S. Allain, N. Bozzolo, M. Humbert, Materiasl Science and Engineering A, 2009, vol. 500, 196-206.
2. S. Kołodziej, J. Kowalska, W. Ratuszek, W. Ozgowicz, K. Chruściel, Solid State Phenomena, 2013, vol. 203-204, 71-76

The plane strain fracture toughness (KIc) of new steels with nickel content with regarding to the carbides morphology

The reason for writing this paper was to describe the influence of Ni addition on the microstructure and K_Ic parameter of structural steels after tempering. In this investigation, four model alloys of the variable concentration of Ni and constant concentration of carbon and other elements were used. All samples for investigations were used in quenched and tempered conditions. Quenching was performed in oil at room temperature. After quenching samples were tempered at 200 °C for 2 hours. The microstructure of the investigated steels were investigated using transmission electron microscope.

An increase of Ni content in investigated structural steels causes a decrease of ε carbide concentration in their microstructure after tempering at 200 °C for 2 hours. Cementite precipitates in these steels independently on the boundaries of martensite needles and on the twin boundaries in the areas in which the precipitates of ε carbide dissolved. Morphology of ε and M₃C carbides has influence on the plane strain fracture toughness of investigated steels.

Presented results may be used to design new technologies of tempering of structural steels with nickel addition.

The effect of alloying on mechanical properties of advanced high strength steels

Q-P (quenching and partitioning) process is one of the method of heat treatment used for processing of advanced high strength steels. The processing results in martensitic microstructure with metastable austenite in the form of thin foils. To prevent or suppress undesirable reactions, it is necessary not only to utilize proper technological parameters, but also to choose convenient alloying strategy. One of the most important alloying element is carbon, which effects all phase transformations and also the stability of retained austenite. Manganese, silicon and chromium are other elements which influence retained austenite stability and strengthen solid solution. Manganese and silicon further prevent carbide precipitation and pearlite formation.

Six different high strength steels were tested in experimental program. 42SiCr steel with 0.43% C, 0.59% Mn, 2% Si and 1.33% Cr was used as a reference material. The second steel possessed higher silicon content of 2.6% and the third one had higher contents of both, manganese and silicon, to investigate their effect on microstructure development. The influence of carbon was investigated using two steels with carbon contents of 0.19% and 0.56%. Chromium-less steel was also used to analyse the effect of chromium in 42SiCr steel.

Q-P processing was gradually optimized with respect to the final microstructures and properties. The most convenient austenization temperature was established for each steel to ensure homogeneous austenite microstructure. The effect of cooling rate on development of proeutectoid ferrite and pearlite was further investigated. In the next step, attention was paid to undercooling temperature and quenching temperature. Besides processing parameters, chemical composition was another factor which significantly influenced mechanical properties. The most pronounced effect was created by higher carbon content. The increase in carbon content from 0.19% to 0.56% resulted in increase of ultimate tensile strength from 988MPa with A_5mm 32% to 2438 MPa with ductility A_5mm over 10%.

Welding of thermomechanically rolled fine-grain steels by different types of lasers

The autogenous laser welding of thermomechanically rolled fine-grain steel S700MC and S420MC was investigated.

The Yb:YAG Disk laser TruDisk 3302, emitted at 1,03 µm, was used for welding of 5.0 mm thick butt joints of the S700MC steel and high power diode laser HPDL ROFIN SINAR DL 020, emitted at 808 nm, was used for welding of 2.5 mm thick butt joints of the S420MC steel.

The influence of laser welding parameters such as laser output power, welding speed, heat input on the mode of welding (key hole or weld pool mode) and weld shape, structure of weld metal and heat affected zone (HAZ) was investigated. The laser welding test were divided into two phase. The first phase was focused on bead-on-plate welding to simulate the process of autogenous laser welding of butt joints and to determine the range of welding parameters for fully penetrated I-joints. Next the test joints were produced for further investigations by light microscope, scanning electron microscope (SEM) and chemical analysis by energy dispersive X-ray spectroscopy (EDS), hardness and microhardness measurements, tensile, bending and impact tests. As the results of investigations the differences in welding modes of butt joints produced by the applied two different types of lasers were showed.

It was found that the high power diode laser may be applied successfully for one-side welding of the S420MC steel butt joints 2.5 mm thick. Additionally it was found that in the optimal range of laser welding parameters the high performance and high quality, with no internal imperfections and flaws, joints can be produced. While in the case of Disk laser, the butt joints were characterized by significantly higher penetration depth, higher welding speed, and also different depth/width ratio of weld metal compared to the butt joints produced by HPDL laser. However, in the case of Disk laser welding tendency to weld porosity was found, which is related to instability of the key hole in a certain range of laser parameters.

Acceleration of regeneration treatment by rotary impacting trailed welding

Regeneration treatment has been successfully used for welding nanostructured bainitic steel. However, it needs a long time to assure the completion of bainite transformation at low temperatures, which limits the industrial applications of this technique. The method called rotary impacting trailed welding (RITW) is proposed to accelerate the regeneration treatment. Stress and strain behaviors of welding metal under this mechanical process have been analyzed by finite element software MSC. Marc. Detailed changes of microstructures in the welded joints are characterized by optical microscopy and scanning electron microscopy. Rotary and impacting synchronously take effect to generate large plastic deformation in the welded joint. Large deformation of the austenite in the welded joint can accelerate the bainite transformation and reduce the regeneration time. At the same time, the large shear deformation in the austenite generated during welding has a strong effect on the bainite growth, which results in curved bainite plates.

The effect of step austempering on phase composition and mechanical properties of X37CrMoV5-1 steel

This paper presents the results of studies of X37CrMoV5-1 steel subjected to quenching processes with a one-step and a two-step isothermal annealing. The one-step process consisted on austenitisation and quenching with an isothermal stop at the temperature of the bainitic transformation, just above the Ms temperature, for the time necessary to complete the transformation. As a result, a microstructure of carbide-free bainite with nanometric thickness of ferrite plates separated by thin layers of residual austenite was obtained. Steel subjected to such a treatment showed higher strength, and higher ductility than steel subjected to a conventional quenching and tempering treatment. Apart from enhanced strength parameters in, the one-step annealed samples exhibited increased resistance to brittle cracking, and to abrasion wear, as weel as reduced quenching distortion. The increase of cracking resistance is due to a microstructure consisting of alternating layers of hard bainitic ferrite and ductile austenite. The tests showed that steel subjected to one-step isothermal quenching contains about 50 % of the of the austenite volume fraction. Reduction of the content of austenite, which is the softer phase than bainite, and the resulting increase of the content of bainitic ferrite should further improve the strength parameters. In order to achieve this goal and to reduce the overall time of heat treatment, a quenching with two-step isothermal annealing was applied. The temperature and time of each step were designed on the basis of dilatometric measuremnts. As expected, the increase of the bainitic ferrite content resulted in improvement of steel's strength with no detriment of steel ductility.

Nanoausferritic matrix of ductile iron

Austempered ductile iron (ADI) is known for its excellent mechanical properties, which are due to the special phase composition. The typical ADI microstructure consists of ferrite plates of micrometer size submerged in untransformed austenite matrix. The morphology of the microstructure depends on the heat treatment parameters. It has been recently shown, that by use of an appropriate chemical composition of cast iron and a well-targeted heat treatment parameters it is possible to strongly reduce the ferrite plates width to sub-micron or even nanometric size. This creates the potential to achieve even higher mechanical properties of ADI cast iron. The paper describes the influence of applied heat treatments on the microstructure of selected ADI cast irons. The conditions necessary to reduce the size of phases to the nanometric scale of ADI cast iron by heat treatment are discussed.

Manufacturing of toothed elements with nanoausferritic ductile iron

The technology currently used in the fabrication of the toothed wheel, gear couplings and chain drums assumes teeth induction hardening processes or hardening and tempering after carburizing. These processes are long and result in adverse changes in the dimensions of the device and the surface changes, which must be removed by machining cavities. The paper proposes the implementation of gear elements of ductile iron with nanoausferritic matrix obtained by appropriate new heat treatment process. The new material offers good performance and reduce the possibility of technological processes used in the manufacture of gears.

Influence of dynamic surface treatment on microstructure of austempered ductile iron in correlation to processes of wear

Dynamic finishing the elements of austempered ductile iron causes hardening and deformed martensite transformation in the surface layer. This is important for wear processes that occur during contact with loose abrasives in terms of utility. Research has indicated characteristic changes caused by dynamic process in the microstructure of the material subjected to wear in the presence of hard abrasive quartz. The measurements provide information about the effects of the combined action of shot peening processes and wear on structural transformation of different grades of austempered ductile iron.

Elaboration of thermomechanical treatment conditions of Ti-V and Ti-Nb-V microalloyed forging steels

The work goal is to elaborate forging conditions of the thermomechanical treatment for Ti-V and Ti-Nb-V microalloyed steels. Conditions of hot processing and cooling after its finish, allowing to obtain desired mechanical properties of forgings were selected taking into consideration: precipitation analysis of MX-type interstitial phases in austenite; research of the influence of austenitizing temperature on austenite grain size; research of the kinetics of phase transformations of supercooled austenite; investigation of the continuous compression of specimens; examination of the kinetics of recrystallization of plastically deformed austenite.

The precipitation analysis of MX-type (M - metal, X - metalloid) interstitial phases in austenite was done on the basis of a simplified thermodynamic model for equilibrium conditions proposed by Adrian, assuming that individual MX phases are soluble in austenite.

The solubility of TiN, TiC and NbC as a function of temperature as well as the simplified temperature sequence of precipitation of these phases have been determined. The effect of austenitizing temperature in range from 900 to 1200 °C on grain size of prior austenite was investigated to verify the analysis. The analysis of the kinetics of precipitation of MX-type interstitial phases in austenite allows to select a proper temperature range of forging for studied steels, which should correspond to the temperature range of precipitation of these phases.

The CCT (continuous cooling transformation) and DCCT (deformation-continuous cooling transformation) diagrams were developed on the basis of dilatometric tests. Performed dilatometric research revealed that the steel is characterized with A_c3=843 °C, A_c1=707 °C and a relatively low M_s temperature equal 370 °C. Plastic deformation of steel at the temperature of 885 °C prior to the start of phase transformations results in distinct acceleration of pearlitic transformation and slight translation of bainitic transformation towards shorter times. Elaborated curves of supercooled austenite transformations of studied steels fully predispose them to production of forgings quenched directly from finish forging temperature and successively subjected to high temperature tempering.

The work presents also results of impact of Nb, Ti and V microadditions on flow stress, recrystallization kinetics and microstructure. The study was performed with the use of Gleeble 3800 simulator. Stress-strain curves were determined during continuous compression test in a temperature range from 900 to 1100 °C and at a strain rate of 1, 10 and 50 s^-1. In order to determine recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of specimens were done with a given strain at the rate of 10 s^-1, in a temperature range from 900 to 1100 °C, with isothermal holding of samples between successive stages of deformation for 2 to 100 s. Basing on the analysis of the form and the course of curves obtained in the compression test, it was found that in the studied range of parameters of hot plastic deformation, the decrease of strain hardening of studied steels is caused by the process of continuous dynamic recrystallization. This is also confirmed by calculation results of activation energy of plastic deformation process. Performed two-stage compression tests revealed that microadditions considerably influence the kinetics of static recrystallization.

The studies provide the basis for proper design of manufacturing process of thermo-mechanical treatment of forged machine parts obtained from high-strength microalloyed steels.

Microstructure and mechanical properties of structural steel after dynamic cold working deformation

The results of the selected mechanical properties i.e. ultimate tensile strength (UTS), yield strength (YS), engineering strain (e), reduction of area (RA), hardness (HV) and impact strength (KCV) of the common, S235JR grade steel, are presented in the paper. A strong relationship between the above mentioned properties and cooling rates after hot rolling of rods, made of this steel, was found.

Additionally, the possibility of further enhancing of mechanical properties (UTS and YS) by the controlled, dynamic cold working, was shown. The use of such deformation, through changes in the microstructure (mainly of ferrite), allows for the upper yield stress (YS) increase - app. 10% and ultimate tensile stress UTS - app. 5%.

Simultaneously, very high indicators of plasticity (e, RA) and ductility (KCV) are retained, as they were immediately after the rolling. The possibility of improving the mechanical properties of rods made of this steel grade has a great technological and commercial importance for its manufacturers, as well as for their final users.

Interpretation of cooling dilatograms in range of bainitic and martensitic transformations

The method of interpretation of cooling dilatograms of undercooled austenite during its transformation into upper and lower bainite (UB and LB) and into martensite (M), applied in the Laboratory of Phase Transformations of AGH University of Science and Technology in Krakow, is presented in the hereby paper. The relation of the mechanism of the mentioned above transformations to places on the dilatometric curve (temperature) - where transformations start and where they are continued - was indicated. The appeal for the uniform, correct interpretation of dilatograms of phase transformations, in all laboratories of the world, was formulated. This will allow the formation of equally accurate diagrams of the phase transformation kinetics of undercooled austenite at its continuous cooling (CCT). In consequence this could provide a development of more advanced heat treatment technologies. Such approach will also allow for the correct assessment of the influence of the alloying elements on the stability of austenite and phase transformation temperatures at its cooling.

Improper interpretation of dilatometric data for cooling transformation in steels

Dilatometry is the most commonly method of producing CCT diagrams and analyzing phase transformations during cooling (as well as transformations during heating) and it permits the real-time monitoring of the extent of reaction in terms of dimensional changes due to transformation. All modern dilatometers are fitted with computerised systems which collect dimensional change signals versus temperature to plot a dilatometric curve and also to calculate and plot the derivative of the relative dimensional change with respect to temperature. Unfortunately, elaborated by dilatometers manufacturers software, in many cases could lead to wrong interpretation of phase transformations during cooling.

Study of transformation products obtained by the isothermal decomposition of austenite in AHSS below MS

The transformation products in Advanced High Strength Steels (AHSS) have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Isothermal holding treatments after rapid cooling to various temperatures below martensite start (MS) were carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite (IM) and untransformed austenite. The purpose of the above heat treatments was to create conditions in which the phase transformations during isothermal treatment below the MS temperature can take place. The transformation kinetics was monitored by means of dilatometry and microstructural characterization by Scanning Electron Microscope (SEM), Electron Backscatter Diffraction (EBSD) and X-ray diffraction (XRD). Hardness measurements of the resulting microstructures were analysed. The results revealed that different transformation products such as lower bainite and tempered martensite were obtained depending on the isothermal holding temperature and time.

An attempt to develop the methodology of examining the boron content in construction materials with the use of EDS method

Borided layers are mainly produced during the process of heat-chemical treatment as a result of reactive diffusion of boron inside the material. The borided layers are particularly useful in difficult exploitation conditions. Generally, the borided layers are characterized by the increased hardness, heat resistance and good corrosion resistance. It is commonly known that the major disadvantage of X-ray microanalysis with EDS method is the difficulty in light-elements analysis such as boron. However, due to the appropriate formation of source region and taking measurements during a definite period of time , the microanalysis of boron is possible. The obtained results are characterized by relative accuracy and precision. SEM Vega 5135 Tescan, detector of Si(Li) in Prism 2000 PGT spectrometer and the PGT software Spirit 1.06 were used. The model specimen was made from regular boron nitride (RBN). The estimated parameters of the analysis were verified on 42CrMo4 steel and borided Armco Iron, with satisfactory results having been obtained.

Nanostructured pearlitic steel

Application of high density short duration electric current pulse to cold worked pearlitic steel causes the microstructure transformation toward nanostructured materials. The cementite plates are fragmented and spherodized to grains whose average grain size is around 30 nanometers. Both the mechanical property and electrical properties are modified consequently. This paper reported the experimental and the theoretical investigation of the mechanism behind this microstructure transformation.

Fig. 1 Nanostructured pearlitic steel with average grain size in 30 nanometers

Relation between the cooling rate of X210CrW12 steel and its microstructure

Microstructure changes in X210CrW12 steel cooled at different rates from liquid and semi-solid state were studied. Very high cooling rate of 10⁶ °C/s can be obtained by electrical discharge machining (EDMed). Arc discharges on the surface of X210CrW12 steel cause local melting followed by rapid solidification, thereby forming a white layer. TEM surface studies of the steel confirmed the presence of nanocrystals, martensite and defects in the microstructure which had the average hardness of 1300 HV_0.02. Rapid solidification of X210CrW12 steel from 1450 °C (liquid phase superheated by 70 °C) was also conducted by the melt spinning method (5 x 105 °C/s). It led to the obtaining of ribbons about 5 mm wide and 15 µm thick. The microstructure of the ribbons consisted of fine austenitic dendrites (nearly 3 µm thick and several micrometers long) with fine M₇C₃ carbides among them. The average hardness measured at the cross-section was 1250 HV_0.02.

X210CrW12 steel was also cast into a sand mold (cooling rate: 8 x 10 °C/s) which led to the obtaining of a microstructure which consisted of coarse ferritic dendrites with average size 200 µm surrounded by eutectic mixture. Another applied method was Semi-Solid Metal Processing in which steel was firstly heated up to 1250 °C, which corresponds to 30% of the liquid phase, and then cooled in a metal die (1,5 x 102 °C/s). The microstructure of steel after cooling from solidus-liquidus range consisted of austenite globular grains (average size of globules – 44 µm) surrounded by very fine eutectic mixture (M₇C₃ carbides and ferrite).

The phase transformations in the hypoeutectoid steels Mn-Cr-Ni

Present work corresponds to the research on the kinetic of phase transformation of undercooled austenite and analysis of the microstructure of hypoeutectoid steels Mn-Cr-Ni. The kinetic of phase transformation of undercooled austenite of investigated alloy were presented on CCT diagrams (continuous cooling transformation). The paper presents metallographic research, measurements of the average diameter of austenite grain as well as hardness measurements.

The test material was hypoeutectoid steels. The austenitizing temperature was assumed, in a standard way, which means higher by 30 °C ÷ 50 °C than Ac₃ temperature for hypoeutectoid steels. For steels the technology of full annealing was proposed.

The microstructure of the investigated material was examined by the light microscope Axiovert 200 MAT. The hardness measurements were performed with the Vickers HPO250 type, which imposes a force equal to 10 kG and 30 kG. The dilatometric measurements were performed with the L78R.I.T.A. dilatometer. The CCT diagrams were made on the grounds of dilatograms recorded for samples cooled with various rates.

Nanocrystalline steels' resistance to hydrogen embrittlement

The problem of hydrogen embrittlement affects many structural materials working in environments containing or producing hydrogen, including steels. The greater the strength of a steel, the greater its susceptibility to hydrogen penetration, and hence to hydrogen-assisted cracking. The above is related to high stresses occuring in high-strength steels and also to their phase composition, primarily to the presence of cementite precipitations, which form during tempering and which are not coherent with the base. The aim of this study is to determine the resistance to introduction of hydrogen into the material and the susceptibility to hydrogen-assisted cracking in steels of a structure of submicron and nanocrystalline carbide-free bainite. It has been shown that the formation of carbide-free bainite plates separated by layers of retained austenite results in a significant reduction of the size (length and width) of the cracks forming during hydrogen loading in comparision with a steel subjected to a heat treatment. The run of the cracks also changes - in a nanobainitic steel they run very close to the surface, while in a heat treated one they propagate deeper inside the material.

Description of the structure of welded joints in heat exchangers

In this work, the evaluation of corrosion resistance of welded joints of various carbon steel grades intended for constructing heat exchangers operating in the energy industry was performed. Qualitative and quantitative electrochemical corrosion resistance of welds in 3.5 NaCl environment was evaluated and a structural examination of the materials was conducted after the corrosion tests. In the environment examined (3.5% NaCl), P355NH steel grade shows nearly twice as high corrosion resistance as P235JR steel grade. SMAW welded joints are nearly three times more corrosion-resistant than MAG welded joints, regardless of the steel grade of welded plates. The application of the SMAW technology to welding heat exchangers exposed to corrosion in the seawater environment ensures a longer lifespan of constructions due to higher corrosion resistance of welded joints, among other things. Structural examination of welded joint surfaces after corrosion (LM, SEM, EPMA) have confirmed the correct microstructure of welded joints.

Applications of Nanoindentation Hardness Measurements to Assessments of Microstructural Constituent and Bulk Mechanical Properties of Selected Advanced High Strength Steels

Advanced high strength steels (AHSS) derive their novel properties from controlled combinations of microstructural constituents including martensite, austenite, bainite, and ferrite. The mechanical properties and formability response have been shown to depend on the constituent volume fractions, distributions, and mechanical properties. Nanoindentation measurements offer important data to assess individual constituent properties in new AHSS products. In this presentation, the unique experimental procedure required to obtain systematic and reproducible property measurements on individual constituents will be discussed and critically evaluated. To illustrate the applicability of the technique, nanoindentation measurements were obtained on eight commercially-produced DP980 dual-phase steels to quantify the hardness of the individual constituents, ferrite and martensite, in each steel. Each microstructure was also evaluated to determine grain size, martensite volume fraction (MVF), and retained austenite content. Nanoindentation hardnesses and quantitative microstructural measurements were correlated with tensile properties and performance in hole expansion tests to assess the importance of the individual constituent properties. Hole expansion samples were prepared with both sheared edges produced by mechanical punching, and non-deformed edges produced by electric discharge machining (EDM). Average material hardness based on nanoindentation data correlated directly to Vickers hardness measurements, verifying the capability of the nanoindentation technique to produce data consistent with traditional measurements of hardness. Yield strength (YS) correlated directly to ferrite hardness indicating that, for a similar MVF and microstructural morphology, the YS is controlled by the strength of the softer matrix phase (ferrite). Hole expansion ratios (HER) on EDM samples decreased with an increase in both martensite and ferrite hardness, indicating that EDM HER values can be enhanced by softening both constituents. Punched-hole HER values decreased with increasing martensite hardness and martensite-to-ferrite hardness ratio, but were independent of ferrite hardness, indicating that softening the martensite could produce a higher HER. Punched-hole HER values reflect the microstructural properties of the ferrite and martensite within the sheared zone adjacent to the hole, indicating that caution should be used when interpreting punched-hole HER based on the as-received ferrite and martensite hardness values reported in this study. It is suggested that a method to increase HER, while still maintaining a similar UTS, is to simultaneously decrease martensite hardness and increase ferrite hardness, thus maintaining a similar average hardness, while also lowering the martensite-to-ferrite hardness ratio. In addition to the results on the DP steels, applicability of nanoindentaiton measurement techniques to the assessment of properties in other AHSS grades, such as quenched and partitioned (Q&P) steels, are currently being considered as candidates for third generation AHSS products, and will be briefly discussed.

Fe-Cr-Co based magnetic steels: evolution of structure and properties

Historically alloys on iron base were used as magnetic materials: pure iron and low carbon steels demonstrate soft magnetic properties while high carbon alloys with additions of other elements – hard magnetic properties. Fe-Cr-Co steels are an excellent base for permanent magnets: cheap, corrosion resistant and ductile. Results of magnetic hardening during step aging at a presence of external magnetic field strongly depend on parameters of complex heat treatment. Ideal structure consists of ferromagnetic α₁ nanoparticles oriented along <100> direction in paramagnetic α₂ matrix.

Evolution of Fe-Cr-Co alloys structure was investigated and correlation between structure and properties was observed. It was shown that aging at temperatures below 560 °C leads to degradation of hysteresis properties because of secondary decomposition process.

Microstructure and mechanical properties of 35CrSiMn5-5-4 steel after quenching & partitioning process

The carbide-free bainitic microstructures in steel which contains retained austenite have good wear resistance and high tensile strenght while maintaining sufficient ductility [1-3]. The Q&P (Quenching&Partitioning) process allowed obtaining microstructure consisting of of carbon-depleted martensitic matrix and carbon-enriched retained austenite. The stability of retained austenite is obtained by carbon partitioning from martensite to untransformed austenite. These Q&P processes are suitable for steels containing alloying additions of silicon and aluminum, that suppress formation of iron carbides [4].

The paper presents the description of microstructure and mechanical properties of 35CrSiMn5-5-4 steel after Q&P process. The parameters of the Q&P were chosen on the basis of computer simulations and dilatometric measurements.

Properties of low-temperature bainite obtained in surface layer of carburized 38CrAlMo steel

The aim of the study was to produce and characterize a nanobainitic structure obtained in surface layers of carburized 38CrAlMo6-10 structural steel. The total content of 1.31% Al+Si, which elements hinders the cementite precipitation, was considered to be adequate for desired microstructure to be formed. Steel samples were submitted to two different vacuum carburization processes in order to obtain two different contents of carbon in surface layer. To produce a nanobainitic structure a heat treatment consisting on austempering at temperatures close to Ms of the layer was applied after carburization process. The obtained microstructures of carburized layers were examined using light microscope (LM) and on transmission electron microscope (TEM). Results showed that the microstructure of carburized layer depends strongly on carbon content. Nanometric size bainite ferrite plates with carbon-enriched austenite film was formed in one case and the ultra-fine lower bainite in the other. In order to characterize the mechanical properties of surface layers the hardness and wear resistance tests were performed. Acquired results confirmed that austempering resulting in formation of low-temperature bainite leaded to better mechanical properties of carburized surface layers than after conventional treatment consisting on quenching and tempering.

Assessment of the cumulative effect of wear processes and the dynamic interaction on the structure of ausferritic ductile iron

Austempered ductile iron (ADI) is known to have good abrasive wear resistance. These properties under use conditions with a total destructive influence of different processes, such as in the presence of abrasive wear and dynamic shocks, arising during cooperation with the other components of the device, for different types of ADI will not be the same. Tests carried out on a test bench that allows reconstruction of the actual working conditions of chain conveyors provide a basis to analyze the relationship operational factors of material properties. The results indicate that the balance of grain refining by supercooling and the corresponding content of austenite allow optimum properties of ductile iron.

Assess the wear resistance of structural materials used in harsh conditions

Materials used in harsh conditions, especially in the mining industry, must demonstrate a particularly high resistance to abrasion wear resistance is a key feature, the value of which determines the use of selected material groups: steel and alloy cast steel or ductile iron after heat treatment. The article presents the results of wear in dry abrasives based on Si sand of these materials on the basis of which are presented in graphical form of weight loss, depending on the parameters characterizing the material characteristics such as hardness and tensile strength. The obtained results show that the best features of typical species are characterized by a heat-treated ductile iron.

Application of artificial neural networks to study transformation phases using acoustic emission in bearing steel

The research carried out on the austempered steel l00CrMnSi6-4 resulted in forming martensitic-bainitic duplex structure. The kinetics of transformation was controlled by the acoustic emission method. Low-temperature heat treatment of bearing steel induces complex phase transitions caused by the segregation and carbide banding. At the same time, takes a stand martensite tempering with carbide ε release and austenite enrichment in carbon as well as the further lowering of M_S temperature. At the temperature close to M_S, a certain temperature range occurs and an effect of the first product of martensitic transition on the bainitic transformation can be observed. The method proposed here enables the separation of varied components of the structure with different spectral characteristics and energy of acoustic emission signals. Three groups of AE events of different signal energies were taken into account. Software to identify AE events with different sound energy values separated AE events amongst the different phase transition process. A procedure to detect the events operates in segments of the recorded signal of length 7.35 milliseconds, whereas a procedure to classify the signals uses neutral network properties. It was found that the method of acoustic emission completed with the application of neural networks is a sensitive tool to identify the mechanism of bainitic transformation and to show the interaction between martensitic and bainitic transformations.

Effect of intrinsic mechanical properties of ferrite on the overall elongation in ultra-ductile δ-TRIP alloys

Recently developed high aluminium added δ-TRIP alloys have exhibited ultra-ductility of over 40% in total elongation combined with high ultimate tensile strength of approximate 800MPa. The excellent properties have been proved partially due to the TRIP effect and ability of severe deformation of retained austenite. In these alloys, addition of aluminium could effectively inhibit cementite precipitation for microstructure evolution of TRIP-aided steels and fully replaced the silicon. High silicon addition of ˜1.5 wt% in conventional TRIP-aided steels strengthen ferrite matrix effectively and deteriorate the ductility of ferrite. The solid solution strengthening on ferrite by aluminium is relatively weaker than silicon does. The effect of silicon on the intrinsic mechanical properties of ferrite, especially ductility, will be investigated in this research and compared with that of aluminium. The direct investigations on the microstructures-properties relationship by comparison between silicon-added and silicon-free δ-TRIP alloys will be carried on further in order to explain the mechanism of ultra high ductility in the silicon-free δ-TRIP alloys besides the contribution of retained austenite.