Processing of a nanostructured austempered ductile cast iron (ADI)

Austempered ductile iron (ADI) has emerged as a very important engineering material in recent years because of its excellent combination of mechanical and physical properties. It has several advantages over cast/ forged steels like low melting point, high machinability, high damping capacity and low production costs. These excellent properties of ADI are due to its unique microstructure consisting of high carbon austenite and ferrite. Because of these advantages, ADI is now used extensively in many industrial applications such as automotive, defense, railways and earth moving machineries etc.

While there is significant development of various nanostructured materials in recent years, the application of nanotechnology in bulk structural material like iron and steel has been rather limited. In this investigation, nanostructured austempered ductile cast iron with a unique microstructure consisting of high carbon austenite, proeutectoid ferrite and bainitic ferrite was produced by a novel heat treatment process consisting of intercritical austenitization and plastic deformation followed by austempering. This unique nanostructured ADI will deliver cost reductions to component manufacturers in a number of industries via the energy and time saving in the austempering process as well as enabling a reduction in material costs by lowering component weight without sacrificing the component strength.

An unalloyed nodular cast iron was austenitized in the intercritical region (where ferrite and austenite co-exist) and subsequently plastically deformed and austempered at several temperatures ranging from the lower and upper bainitic temperatures. The effect of processing parameters such as the austenitizing temperatures, plastic deformation (percentage of pre-strain) and austempering temperatures on the microstructural features was examined. Effect of these processing parameters on the volume fraction of the phases and the ferritic cell size were examined. In addition, the influence of these processing parameters on the mechanical properties was also examined. Test results shows that the plastic deformation at intercritical temperatures produced very fine ausferritic structures along with proeutectoid ferrite in the matrix.

ADI - The material revolution and its applications at CMRDI

In the last three decades, the revolutionary material; the austempered ductile iron (ADI) with its unique combination of mechanical properties, has been offering the design engineer alternatives to conventional material/process combinations. The excellent properties of this material have opened new horizons for cast iron to replace steel castings and forgings in many engineering applications with considerable cost benefits. Moreover, the sustained efforts worldwide of the automotive industry to use lightweight materials have eroded the market for the heavier iron castings. Currently, ADI with its super strength can successfully compete with the lightweight alloys, a point which has yet to be fully understood by many design engineers.

This review is an attempt to compile the results of the worldwide explosion of research and development that followed the announcement of the first production of this material, meanwhile, reference is made to the work carried at Central Metallurgical R&D Institute (CMRDI) over the past decade. It is not intended to provide an in-depth investigation of any specific technique, but to present a macro-analysis of the current state of metallurgy, processing and applications of ADI.

Better understanding of the strengthening mechanisms of ADI has led to the development of new techniques that contribute to enhance strength and toughness of the alloy, some examples will be discussed in details such as:

• Ausformed ADI; where mechanical processing component was added to the conventional heat treatment as a driving force to accelerate the rate of stage I austempering.
• Squeeze cast ADI; where superior quality ADI castings were produced through squeeze casting of molten iron in a permanent mold, followed by in-situ heat treatment of the hot knocked-out castings in the austenite range followed by normal austempering in a salt bath.
• Two step austempering to achieve finer ausferrite at higher undercooling during austempering treatment followed by austempering at higher temperature where higher austenitic carbon is promoted.

The excellent abrasion resistance of ADI could still be remarkably increased through the development of:

• Carbidic ADI-ductile iron containing carbides subsequently austempered to form ausferritic matrix with an engineered amounts of carbides.
• Bainitic/martensitic (B/M) ADI containing less expensive alloying elements such as Si and Mn in the range of 2.5 - 3.0%.
• Selective surface treatment, where parts of the casting subjected to excessive wear may be locally hardened, either by induction heating and quenching or by surface laser processing.

The review analyses the key features of important processing techniques of ADI such as:

• Cold rolling, where as thin as 3 mm sheets have been successfully produced at CMRDI with enhance strength and hardness properties.
• Welding, whether used as repair welding for the parent ductile iron castings before austempering or to weld already austempered parts to each other or to other materials such as steel and ductile iron.
• Some of the machining difficulties, that appeared with the first use of ADI as engineering material still persist today, which are mainly related to the work hardening and deformation induced martensitic formation from retained austenite. Recent work at CMRDI clearly indicates that cutting force during machining of ADI is closely related to martensitic transformation, which in turn, in a function of cutting depth and speed of cutting a novel development of ADI is the intercritically austempered ductile iron; an exciting engineering material with a favorable combination of good strength and ductility, excellent fatigue strength and good machinability resulting from a microstructure containing colonies of proeutectoid ferrite together with isolated islands of austenite.

Different applications of ADI cover automotive e.g. gears, crankshafts, connecting rods, camshafts transmission as well as suspension components and steering knuckles as well as earthmoving, defense and agricultural consumables such as plough blades. Case studies will be shown with emphasis on the experience of CMRDI in production gears of different types and agricultural parts in its experimental foundry. ADI and steel properties related to gear performance will be compared such as structural integrity, abrasion resistance, bending fatigue, teeth conformation, noise and vibration reduction, manufacturing cost, weight reduction and gear power loss, the comparison seems to be in favor of ADI. The CMRDI experience with the use of ADI in agricultural applications will be discussed. Production of thin wall ADI components may offer potentials for new applications. Thin wall ADI castings are capable to build complex thin wall parts of high strength.

The microstructure of the ductile iron after various austempering heat treatments - the time effect

Our previous studies revealed, that austempering heat treatment of ductile iron in the temperature slightly above the Ms temperature may lead to the formation of the nanocrystalline ausferrite matrix (nanoausferrite) [1]. The nanoausferite is composed of carbide-free bainitic plates separated by thin layers of residual austenite, with a possible presence of retained austenite blocks. Austempered ductile iron (ADI) with this microstructure has relatively high mechanical parameters: hardness c.a 350 HB, tensile strength of 1300 MPa at the elongation of c.a. 5% and impact strength of 9 J/cm². These mechanical properties are obtained after completion of bainitic transformation, which usually takes up to several dozen of hours, in particular at a low temperature. Such a long time of a heat treatment is a disadvantage for industrial production. Hence, it is important to check how the reduction of isothermal annealing time influences the nanostructuring process of ADI. The dilatometric investigations revealed that approximately 80% of bainitic transformation may be achieved just after several minutes. The present study attempts to determine the influence of austempering time on the microstructure of two types of ductile iron – one with addition of molybdenum and the second one with addition of nickel. It has been shown, that the reduction of austempering time, resulting in c.a. 90% of a bainitic transformation, gives rise to the formation of a highly heterogeneous microstructure. This microstructure displays significant differences in ausferrite grain size, as well as in spatial distribution and size of residual austenite blocks. Moreover, after shortened austempering time, martensitic laths are formed during final cooling of the samples. On the other hand, the increase of the austempering time to complete the bainitic transformation, prompts the precipitation of cementite inside the bainitic ferrite plates as well as in the interphase boundaries. The possible effects of the microstructural changes on mechanical properties are discussed.

Nanostructured ductile iron for waste grinders

The aim of the study was to propose new, cheaper material for waste grinders’ cutters. Currently, for manufacturing of cutters, a high alloy tool steel is used, which greatly affects total cost of the equipment. One of the materials, which is substantially cheaper than high alloy steels is the Austempered Ductile Iron (ADI). Moreover, we have recently revealed that by the use of appropriate austempering treatment, it is possible to obtain in the ADI a nano-ausferitic matrix with potentially improved mechanical properties. We have decided to verify whether it is possible to obtain an ADI with mechanical properties suitable to produce the waste grinders’ cutters. For this investigation, a ductile cast iron, containing 3.44 wt.% of carbon and 2.32 wt.% of silicon was chosen. The material samples were submitted to austempering treatment at two different temperatures. After the heat treatment microstructure investigations were performed using light microscopy and transmission electron microscopy (TEM), followed by mechanical tests. Moreover, a prototype cast iron cutters were prepared and submitted to comparative wear test in the operating conditions. It was shown, that the low temperature austempering, with the designed parameters, leads to formation of nanometric microstructure consisting of bainitic ferrite plates separated by films of retained austenite. This process assures very good mechanical properties of the alloy which can be used in manufacturing of waste grinders’ cutters.

Austemperability of large chunk nanostructured steel designed based on thermodynamic model

Nanostructured bainitic steels are known as valuable engineering materials which can be obtained through a simple austempering heat treatment process as large chunks. The well-known MUCG83™ thermodynamic model has been vastly used in order to design the proper chemical composition for obtaining these unique alloys based on thermodynamic theories alone. This article aims to study first how experimental results match with those predicted by thermodynamic theories determining the times needed for both reconstructive and shear transformations being started. Secondly it aims to investigate the austemperability of designed steel which isothermally is going to be kept in salt bath furnaces at different heat treatment temperatures to obtain nano bainite. For this aim, austemperability was evaluated based on the critical diameter of the material. At first step, ideal diameter was measured according to the chemical composition and using the Jominy end quench test method. Consequently the quench severity factors of the salt bath furnaces (H factors) were determined experimentally and were used for determining the critical diameters at each transformation temperature. It has been shown that practical results were in a good agreement with those predicted by thermodynamic theories for estimating the incubation times needed for starting the bainitic transformation even if it was less accurate for reconstructive transformations. Therefore thermodynamic model has been shown to be a good approach for designing the steel in order to attain nanostructured bainite. This in turn could omit the try and error methods and enhances the efficiency for steels production. Moreover, it has been demonstrated that large chunks of steel with enough thick cross sections could be austempered successfully without any diffusional transformation products taking place within the microstructures. Decreasing the transformation temperature increased the H factor of the identical salt bath so thicker steel chunks could be austempered successfully.

Nanostructuring of commercial steels by heat treatment

The paper describes the methods of nanostructuring of low- and medium alloyed commercial steels by means of precisely designed heat treatments. Technology of nanostructuring of steels using the bainitic transformation, developed since about 10 years, relates mainly to steels with a specially designed chemical composition, which allows obtaining the desired phase composition. Moreover, these chemical compositions of steels, prone to nanostructuring, have been patented. Designing of the phase content in steel is based on the concept of α T₀ line, assuming the conditions of para-equilibrium. It has been experimentally shown that the optimum combination of high strength and toughness provides a nanocomposite microstructure containing about 50-88% of bainitic ferrite in the form of plates with the thickness below 100 nm. The rest is the retained austenite in form of thin layers separating the plates of ferritic bainite, and ocassionaly in form of small blocks. The higher the content of bainitic ferrite, and the lower the content of retained austenite, the higher is the hardness and strength of steel. From a practical point of view, the implementation of the newly developed steel into the market is difficult, because the minimum volume of steel melt produced by steel makers is of the order of several tens of tons. However, in the first stage of implementation of a new steel into the market, the manufacturers of steel products are not willing to order such a large volume of steel. The solution to this problem would be to develop the nanostructuring processes of commercial steels, which are currently produced by the steel makers. This problem is the subject of the present work, and it was one of the goals of a structural project 'NanoStal' implemented at the Warsaw University of Technology. The paper presents the nanostructuring processes, developed within the project, for different commercial steel grades: tool, bearing, spring and structural steels. The study revealed that it is not necessary to dispose of a special steel with strictly precised chemical composition to produce the nanocrystalline microstructure through the bainitic transformation. On the contrary, it was shown, that it is possible to produce a nanobainitic microstructure with defined phases composition in many commercial steel grades, by using the appropriately designed heat treatment. It turned out, that the criteria concerning the chemical composition of steel for nanostructurisation are less restrictive than those which are included in patents' claims. It was also shown, how the phase composition of steel and the thickness of the ferrite plates and austenite layers, vary as a function of parameters of the applied heat treatment. The changes of mechanical properties of steels as a function of the phase composition and the refinement of both phases were described. It was found that the commercial steels after the application of the designed nanostructurisation processes achieve high strength parameters and high resistance to cracking, abrasion and fatigue.

Microstructural changes of the nanostructured bainitic steel induced by quasi-static and dynamic deformation

Changes in the microstructure of nanostructured bainitic steel induced by quasi-static and dynamic deformation have been shown in the article. Plastic deformation of steel can induced changes of dislocation density, morphology of the grains and other structural components or can induced phase transformation as a result of which new phase arises. The method of deformation and strain rate have important impact on the microstructure changes and their localization. Localization of strain as a result of experiment set-up and/or conditions of exploitation is the main factor producing deformation bands.

Microstructure of nanostructured steel Fe-0.6%C-1.9Mn-1.8Si-1.3Cr-0.7Mo consists of nanometer size carbide-free bainite laths and 20-30 % volume fraction of retained austenite. Deformation of the nanostructured steel can induce transformation of the retained austenite into fresh martensite. Quasi-static and dynamic (strain rate up to 2x10² s^-1) compression tests were realized using Gleeble simulator. Dynamic deformation at the strain rate up to 7x10³ s^-1 was realized by the SHPB method (Split Hopkinson Pressure Bar). Moreover high energy firing tests of plates made of the nanostructured bainitic steel were carried out to produce dynamically deformed material for investigation. Adiabatic shear bands were found as a result of localization of deformation in dynamic compression tests. Microstructure of the bands was examined and hardness changes in the vicinity of the bands were determined.

Understanding the tempering process of nanocrystalline bainite

Nanocrystalline bainite is obtained by isothermal transformation at low temperatures in specifically designed alloys. The resulting material not only presents a nanostructured structure, consisting of a mixture of bainitic ferrite (α) and retained austenite (γ), but also high tempering resistance, comparable in some cases to secondary hardening tool steels. This work presents some new results on the reasons lying beneath such behaviour. Microstructural observation, in conjunction with XRD advanced data analysis, helped to reveal the insights of the evolution and processes taking place on the two phases, α and γ, during the whole tempering process.

Controlling the phase composition of X37CrMoV5-1 steel with a nanobainitic structure by means of custom-designed heat treatments

This paper presents the results of research aiming at optimizing the nanocrystalline structure and mechanical properties of steel by means of custom-designed heat treatments. Our earlier work [1,2,3] proved that it is possible to create a nanocrystalline structure in X37CrMoV5-1 hot work tool steel by using an isothermal annealing process in the temperature range of the bainitic transformation. The steel subjected to nanostructuration shows higher plasticity and resistance to brittle cracking compared with the same steel after quenching and tempering, while retaining similar strength characteristics. The improvement in plasticity parameters results from the presence of large quantity of residual austenite (up to 50 % according to stereological TEM observations) in the form of blocks and thin layers separating plates of bainitic ferrite in the nanocrystalline structure. In the course of the research work, it was attempted to increase the content of bainitic ferrite at the expense of residual austenite, in order to enhance the steel's hardness and strength characteristics. To achieve that, dilatometric tests of phase transformations were carried out, in order to design new, heat treatments allowing to decrease the content of the residual austenite. The analyses of phase composition and microstructure confirmed that the newly designed complex heat treatments allow to increase the content of the bainitic ferrite to a significant extent. These treatments allowed to raise the steel's yield strength R_0,2 and to enhance its strength with no detriment to its plasticity characteristics.

[1] W.A. Świątnicki, K. Pobiedzińska, E. Skołek, A. Gołaszewski, S. Marciniak, Ł. Nadolny, J. Szawłowski: Nanocrystalline structure formation in steels using bainitic transformation. Inżynieria Materiałowa, rok XXXIII, nr 6, 524-529 (2012)
[2] S. Marciniak, E. Skołek, W. Świątnicki: The effect of stepped austempering on phase composition and mechanical properties of nanostructured X37CrMoV5-1 steel. Archives of Metallurgy and Materials 2015/1.
[3] E. Skołek, S. Marciniak, W. Świątnicki: Determination of temperature stability of steel X37CrMoV5-l based on the electron microscopy observations. Archives of Metallurgy and Materials 2015/1.

Multiphase ausformed austempered ductile iron

Three ductile irons with different manganese and aluminum contents were subjected to true strain of 0.2 in the austenite region by deformation in a thermo-mechanical simulator. Subsequently, ferrite was introduced by isothermal holding in the intercritical region after austenitization. Samples were quenched from the austenite region as well as from the intercritical region to austempering temperature, T_A = 375°C and held at this temperature for 60 s before applying a deformation φ_A = 0.1 (ausforming). Dilatometric measurements as well as the microstructure examination showed a fast ausferrite transformation directly after applying φ_A and that the introduction of ferrite to the matrix resulted in a remarkable acceleration of the ausferrite formation. The transformation kinetics, microstructure evolution, hardness and compression properties of the three thermo-mechanically processed ductile irons are studied.

Influence of chemical composition and heat treatment on charpy impact resilience of ADI

Three different ductile iron casts were heat hardened with isothermal transformation with reference to two alternatives. The first processing alternative based on single austenitizing treatment at the temperature t= 830, 860 or 900 °C, cooling to the isothermal transformation temperature 300 or 400 C and resisting in the period from 8 up to 64 minutes. The second processing alternative based on double machining austenitizing treatment. Ductile iron was soaked at the temperature t = 950 °C, cooled to the supercritical temperature t = 900, 860 or 830 °C. The isothermal transformation process was performed under the same conditions as in the first alternative. All ductile iron melts had been two stage ferritized before the isothermal annealing . The ductile iron resilience test was performed at ambient temperature.

Wear properties of low temperature NiCu ausferritic ductile iron

The purpose of this study was to determine experimentally the wear properties of ausferritic ductile iron austempered in 250°C for various times. The microscopic observations of heat treated material revealed that the microstructure obtained for longer time is more homogenous, less fragmented and contains smaller blocks of residual austenite in comparison to the microstructure obtained for shorter time. However the treatment time doesn’t affect significantly on the total amount of residual austenite. The wear tests were carried out on a disc-on-disc test rig. The test samples were examined under conditions of sliding mating, while the leading destructive process was microcutting of the surface with loose corundum grain. The loss of mass of the examined samples was measured as a parameter characterizing the wear. Base on it, other wear coefficients were determined, for example the volume loss, the intensity of wear and the wear rate. The volume loss values determined were presented as a function of the initial hardness. Based on the results obtained, it was found that very long time of heat treatment (>6h) causes a not significant increase in resistance to abrasive wear, especially at a high unit load. This may be due to the different mechanisms of wear related to the different areas of residual austenite in the microstructure and various rates of saturation of austenite with carbon.

Effect of nickel on the crystallization and microstructure of compacted graphite cast iron

The paper presents the effect of nickel on the crystallization process, microstructure and selected properties of compacted graphite cast iron. Compacted graphite was obtained in castings with use of Inmold process. The cast iron containing nickel at a maximum concentration providing obtain an austenitic microstructure were examined. The effect of nickel on the eutectic crystallization temperature was given. The effect of the cooling rate (casting wall thickness) on the microstructure of compacted graphite cast iron with nickel was shown too. Nickel concentration for obtaining martensitic and austenitic compacted graphite cast iron was presented. The representative microstructures of martensitic and austenitic compacted graphite cast iron were shown in Figures 1 and 2, respectively. Hardness of compacted graphite cast iron with nickel and microhardness of individual phases were examined.

Review of hydrogen effects in some martensitic steels

We review our recent research using the Linearly Increasing Stress Test (LIST) to study cathodically charged martensitic steels. The equivalent gaseous hydrogen pressure can be evaluated using the permeation cell applroach. For steels with yield strengths 620-800 MPa, there was no decrease in the yield strength or the ultimate tensile strength, but there were some brittle hydrogen influenced fracture events that competed with the overall ductile fracture. For steels with yield strength 900-1400 MPa, solid solution softening by hydrogen caused a decrease in the yield strength, and there was some decrease in ductility, due to the hydrogen tending to cause macroscopically brittle shear fracture at slow applied stress rates, with no effect at fast rates.

The role of microstructure in high temperature tribology of iron alloys

The work presents the issue of wear of tool materials with an iron matrix exposed to working conditions involving a contact with structural steel heated to a high temperature (austenitic range) and in the case when the friction couple has been exposed to a temperature exceeding 727°C. The investigations aimed at determining the role of the microstructure of the materials used for hot rolling metallurgical rolls in their tribological properties. The observations of the wear mechanisms were referred to the microstructure of the metallurgical rolls. The laboratory tests aimed at evaluating the effect of modification and thermal treatment on the properties of cast steels under conditions of the, so called, high temperature tribology. A significant role of the morphology of ledeburitic cementite and secondary cementite on the tribological properties at a temperature close to Ac1f of hypoeutectic cast steels was exhibited. The investigations assumed the presence of an austenitic matrix with primary and secondary carbide precipitations of a varying morphology in the cast steel microstructure. The investigation results make it possible to point to a direction of carbide morphology change with the purpose of obtaining the assumed properties of hot operation tools.

Heat treatment effect on resistance to wear and rolling contact fatigue of 100CrMnSi6-4 steel

The paper describes a comparative study of resistance to wear and rolling contact fatigue of 100CrMnSi6-4 steel after two kinds of treatments: quenching & tempering or austempering. Standard quenching and tempering, typically used for this kind of steel, led to obtaining low-tempered martensite. Replacing a standard heat treatment with the austempering process allowed a nanocrystalline bainite microstructure to be obtained instead of martensite.

The wear resistance of studied steel was investigated with the use of block-on ring wear tester. Afterwards, the width of a track for each sample was measured by optical profilometer in order to specify the volume loss of the tested block. The rolling contact fatigue tests were carried out with the use of a modified four-ball machine in which the upper ball was replaced with a cone. On the basis of the obtained test results, the Weibull distribution plots and rolling contact fatigue L10 lives were determined.

The results obtained for the austempered steel samples were compared with the results obtained for steel samples subjected to the standard quenching and tempering heat treatment. It was concluded that steel with nanocrystalline structure obtained through the austempering treatment provided a higher wear resistance and a longer rolling contact fatigue life.

Phase transformations in modern steels: interaction of alloying elements with transformation interface

The development of modern steels depends on the increased understanding of the function of alloying elements in steels and in particular of their role in the phase transformations. The process of phase transformation depends on complex interactions between elements and transformation interfaces. Their analysis is often difficult for the reason that they occur at the nanoscale and in the vicinity of an interface of few atomic plans thick. However, the experimental and theoretical techniques had become much more powerful over the last decade, that is seems fruitful to return to this topic of interest with fresh eyes. In the present work, by coupling 3D Atom Probe Tomography (ATP), TEM and modelling at the scale of phases, we propose to deal with two main topics:

• the interfacial contact conditions at moving ferrite/austenite interface in Dual-Phase steels,
• the mechanisms of carbon enrichment in austenite during quenching and partitioning (Q&P)

the interfacial contact conditions at moving ferrite/austenite interface in Dual-Phase steels, The mechanisms of carbon enrichment in austenite during quenching and partitioning (Q&P)

The principle of a sufficient hardenability of the light CCT diagrams

The risk of applying traditional hardenability measures [1] based on the so-called critical diameter D_k, ideal critical diameter D_ik, basic diameter D_o together with the hardenability coefficient f, which are worldwide popularised, among others by papers [2,3], is emphasised in this paper. The problem with traditional hardenability measures is that none of them indicates places, on the cross-section of the hardened rod, where the - so-called - upper bainite, the component of a very low crack resistance, is formed. Whereas a majority of steels for quenching and tempering has a high Bs temperature and - in practice - each cooling with a rate lower than the critical rate causes formation of the upper bainite zone on the hardened rod cross-section. The formal and practical ways of avoiding the upper bainite zone formation in the - so-called - reliable cross-section of the hardened rod [4], are presented in this paper.

Thermodynamic analysis of phase transitions in advanced multiphase steel showing the transformation-induced plasticity effect

The study addresses monitoring phase transitions from the heavily deformed austenite of the 0.24C-1.5Mn-0.9Si-0.4Al type steel. The pseudo-binary Fe-C diagram of the steel was calculated using Thermo-Calc. The next step of the study included the thermodynamic calculations of the volume fraction change of individual phases as a function of temperature. The calculations were performed with the use of JMatPro programme under conditions of thermodynamic equilibrium. The research included the determination of the time-temperature-transformation (TTT) diagram and continuous-cooling-transformation (CCT) diagram of the investigated steel. The calculated equilibrium diagrams were compared with the experimental diagram determined using dilatometric tests. The dilatometric analyses were conducted with the use of DIL805 dilatometer equipped with the LVDT type measuring head. The physical simulation tests were performed using a thermomechanical simulator under conditions of multi-step deformation to reflect cooling strategies designed on the basis of thermodynamic data. Microstructural features of phases were revealed using light microscopy and scanning electron microscopy techniques.

Transformation diagrams of selected steel grades with consideration of deformation effect

The aim of this article was to assess the effect of previous plastic deformation on the transformation kinetics of selected steels with a wide range of chemical composition. Transformation (CCT and DCCT) diagrams were constructed on the basis of dilatometric tests on the plastometer Gleeble 3800 and metallographic analyses supplemented by measurements of hardness HV. Effect of previous deformation on transformation was evaluated of the critical rate of formation of the individual structural components (ferrite, pearlite and bainite) in the case of formation of martensite respect to temperature Ms. Previous plastic deformation accelerated especially diffuse transformations (ferrite and pearlite), temperature of Ms was lower after previous plastic deformation and bainitic transformation was highly dependent on the chemical composition of steel.

Evolution of microstructure and its influence on tensile properties in thermo-mechanically controlled processed (TMCP) quench and partition (Q&P) steel

The advantage of Q&P process is being recognized in recent times in producing AHSS for automotive applications. Partitioning of carbon and stabilizing of retained austenite has lead to wide scope of possibilities to develop a combination of strength and ductility properties. Combining a thermo-mechanical controlled process with Q&P treatment is promising process route to enhance the strength properties further and is gaining much attention of late. In the present work, controlled hot rolling was performed on a low carbon Q&P steel at 1100°C for two different strain levels viz. of about 40% reduction (ε = 0.51) and 70% reduction (ε = 1.1), and was subsequently followed by a direct Q&P (DQP) treatment and a separate Q&P treatment (SQP). Microstructure consisting of fine martensite laths and inter lath austenite were observed in all the cases. Comparatively higher martensite volume fractions and fine lath packets were noticed in the DQP method and tendency of higher retained austenite volume fraction was observed in the SQP method. High grain refinement was noticed for the higher applied strain in both DQP and SQP process. XRD studies suggest that the application of a prior strain has influenced the kinetics of partitioning and consequently the carbon enrichment. EBSD analysis has shown a major distribution of high angle grain boundaries (HAGB) in the martensite laths which indicate a fully recrystallized prior austenite grains. TEM studies revealed a standard K-S orientation relationship in addition to many other orientation relations between the martensite and the retained austenite crystallographic planes. Tremendous improvement in strength was evidenced with respect to that obtained by the basic Q&P treatment alone. Maximum strength of about 1398 MPa with a true elongation of 14% was achieved in the DQP steel. The study suggests that performing a TMCP prior to Q&P promotes grain refinement as well as favourable grain boundary characteristics like HAGBs that are beneficial in enhancing the final microstructure and resultant properties.

Different routes for forming high-aluminium steel

Today’s trends in engineering design bring a demand for ever more lightweight materials with high mechanical properties. One of available ways to reduce the weight of steel involves adding aluminium as an alloying element. This, however, negatively affects the possibilities of subsequent treatment. In the present experimental programme, a steel with 6.5 % Al and 0.2 % C alloyed with Mn, Si and Cr was designed and manufactured. With the exception of the high aluminium content, similar chemistries can be found in steel sheet intended for TRIP or Q-P processing where strengths above 1500 MPa are achieved. The present experimental ingot was first inspected in several locations to confirm its uniform chemical composition and microstructure. It was then sectioned into small blocks for further processing. One part of the ingot was hot open-die forged into bars. Another block was used for the unconventional process of thixoforming. The resulting microstructures were compared using optical and electron microscopy. Tension tests were carried out to determine mechanical properties.

Development of structural steel containing 3÷5 wt%Al with microlaminated microstructure

Alloying of structural steels with aluminium (in amounts exceeding contents needed for grain refinement) is not widely used and till now only automotive TRIP grades have been commercialised. Because the addition of aluminium reduces steel density and produces significant solid solution strengthening, many research activities are underway aiming at development of structural lightweight steels based on Fe-Al, Fe-Al-C and Fe-Mn-Al-C systems with aluminium content in the range of 4÷12wt%. In this work the potential to develop lean-alloy structural steel based on Fe-(0.1÷0.35%C)-(0.5÷2.0%Mn)-(3÷5%Al) composition was investigated (Fig. 1). To determine the basic parameters of heat treatment and hot working operations an experimentally established in this work metastable pseudo-binary phase diagram (Fe+0.1%C+1.4%Mn)-Al (Fig.2) was used. Hot deformation behaviour of the investigated alloys was characterised using the results of heating/deformation/cooling tests carried out in a Gleeble3800 simulator and in a hot rolling line. Microstructure of specimens subjected to various sequences of heat treatment and/or hot working operations in the dilatometer, the Gleeble simulator and the rolling line was investigated using light microscopy, SEM, TEM and XRD techniques. Specimens subjected to specific processing variants were tested for mechanical properties. In the temperature range typical of hot working, the investigated steels characterised with a two-phase microstructure comprising ferrite and austenite. This allows to produce a multi-phase microstructure after the final treatment (martensite, bainite and retained austenite in ferritic matrix). The investigated alloys are suitable for producing a fine lamellar (microlaminated) microstructure rather than a regular equiaxed microstructure. As a result of the work, the parameters of thermomechanical treatment were established allowing to obtain the microlaminated microstructure in the developed steel with very attractive mechanical properties.

Structure refinement of spring steel 51CrV4 after accelerated spheroidisation

Material research of the spring steels tries to meet requirements of the industry, which are mainly higher yield and tensile strength. Steel 51CrV4 is widely used for spring production. Optimization of its properties lies in tensile and yield strength enhancement without decrease in ductility in quenched and tempered (QT) state. This can be accomplished by structural refinement. One possible way to refine final QT structure is refinement of the soft annealed structure before quenching. The article is devoted to accelerated carbide spheroidisation and refinement (ASR) and subsequent hardening (quenching and tempering) of the 51CrV4 spring steel. Samples with different carbide size were prepared by conventional soft annealing in atmosphere furnace (fig. 1) and ASR process by induction heating (fig. 2). Influence of the structural refinement on the properties of QT state was studied.

The microstructure evolution in the heat treatment process of UFG-TRIP steel

The main subject of this study was to analyse the ultra-fine grained (UFG) multiphase microstructure of 35CrSiMn5-5-4 structural steel after two unconventional heat treatments. In order to fully characterise the microstructure of steel, light microscopy and electron microscopy were used. The microscopic observations allowed the description of the microstructure of steel after the two processes of heat treatment. Two different types of the microstructure were obtained as a result of the applied heat treatments. Both microstructures consisted of carbide-free bainite with retained austenite in the ferritic matrix. However, the second microstructure was much more homogeneous and it exhibited strongly refined grain sizes in comparison with the first one. The phase composition after the second heat treatment was similar to that of Transformation Induced Plasticity Steels; therefore, steel with such microstructure has been called the UFG-TRIP steel.

The analysis of the microstructure indicated higher grain refinement after applying the second heat treatment (Fig. 2) in comparison to the first heat treatment (Fig. 1). In both cases, significant differences in the morphology of the microstructure were observed. The electron microscopic measurements revealed that both bainitic and ferritic grains were elongated (Fig. 2), whereas the TEM analysis indicated the presence of austenite.

Applied heat treatments resulted in the development of the structures which consisted of carbide-free bainite with retained austenite in the ferritic matrix.
The second heat treatment led to the greater refinement of the structure, in which the width of ferritic areas was in the range of 200-700nm.
The thickness of bainite plates was about 100-200 nm in both cases, while the average thickness of the austenite layers was about 100 nm.

Effect of strain on phase transformations of recrystallized and non-recrystallized austenite in TRIP-aided steels

The work discusses the effect of strain on phase transformations occurring upon cooling the 0.17C-3Mn-1.5Al type medium-Mn steel from the recrystallized and non-recrystallized region. The pseudo-binary Fe-C diagram was calculated using Thermo-Calc. Ferritic, bainitic and martensitic phase transformations are investigated in detail in respect of their temperature range forming and microstructures produced under various conditions of both continuous and isothermal cooling. The equilibrium temperatures of A_e1 and A_e3 and phase composition of the investigated steel were initially calculated whereas critical temperatures of A_c1 and A_c3 were determined upon heating. The major tests included the determination of the continuous-cooling-transformation (CCT) diagram and the DCCT diagram of the steel deformed prior to cooling. The dilatometric analyses were conducted with the use of DIL805 dilatometer. Deformation of samples at the temperatures of 900 and 1050°C was to determine the effect of austenite state (non-recrystallized and recrystallized) on temperatures of phase transitions during cooling, as well as its impact on steel microstructure. The microstructure of the steel was examined with the use of light microscopy (LM) and scanning electron microscopy (SEM). X-ray examination was carried out with filtered Co radiation and an X-pert PRO diffractometer.

High versatility of niobium alloyed AHSS

The effect of the number of deformation steps on the microstructure and properties was investigated. For this reason, 6 processing schedules were proposed with the same soaking, cooling and bainitic hold conditions. The only difference was in the number of deformation steps and the overall deformation applied during the processing. Among other strategies tested were also heat treatment without deformation, conventional quenching and TRIP steel processing with continuous cooling. Various microstructures were achieved, consisting of the mixture of ferrite, bainite, retained austenite and M-A constituent. They possessed ultimate tensile strength in the range of 780-859MPa with high ductility just above 30%. Volume fraction of austenite was for all the samples around 13%. The only exception was reference quenched sample with high strength 1186MPa, ductility 20% and only 4% of retained austenite.

Study of relationships between parameters of holding in bainitic range and microstructural components in medium carbon TRIP steel

The comprehensive studies of the influence of parameters of isothermal holding in the bainitic range on the Ms temperature location as well as volume fractions of retained austenite and bainitic ferrite in the microstructure TRIP steel were presented in the paper. A simple TRIP steel (0.4%C, 1.5%Mn and 1.2%Si) was used for research. Such steel was austenitised in the intercritical range (bounded by Ac1s Ac1f) by 600 sec. before isothermal holding.

On the basis of dilatometric, XRD and metallographic tests it was stated that a transformation austenite into ferrite with increased carbon content (banitic ferrite) occurs in the microstructure of the tested steel during holding. It was found that volume fraction of this new phase decreased with increasing of temperature and duration of holding in bainitic range.

Simultaneously, by dilatometric analysis there was no state a negative dilatation effects which should be caused by carbides precipitation during bainitic transformation. Such a scheme of transformations may have contributed to the redistribution of carbon atoms from bainitic ferrite to the untransformed yet austenite what would favor the stabilization of this phase in the microstructure of tested steel.

Such predictions has been validated by XRD tests, through which the volume fractions of retained austenite remaining in the microstructure of the tested after each variant of holding were estimated. Additional verification was done by dilatometric analysis of curves which was recorded during cooling from bainitic range to the room temperature. This has enabled to determine the Ms temperature of the austenite which remains in the microstructure before cooling started.

Based on this results, it was found that an additional increasing of retained austenite volume fraction in the microstructure of tested steel is possible after unconventional austenitising temperature range Ac1s-Ac1f. To achieve this it is desirable to conduct the treatment in the bainitic range bainitic so that it takes place at the highest temperature (450°C) and for a sufficiently long time (1200 sec.).

The effects of finish rolling temperature and niobium microalloying on the microstructure and properties of a direct quenched high-strength steel

This paper comprehends the effects of finish rolling temperature (FRT) and Nb-microalloying on the microstructure evolution and resultant properties of a low carbon direct quenched steel in the yield strength category of 900 – 1180 MPa. To elucidate the influence of niobium, two low-alloy steels of nearly similar compositions (base: 0.1C-0.2Si-1.1Mn-0.15Mo-0.03Ti-0.002B) with or without 0.04% Nb were subjected to hot rolling at pilot scale followed by direct quenching to room temperature. The rolling schedule comprised stages of recrystallization controlled rolling, followed by controlled rolling in the Tnr regime according to specific thermomechanical schedules with different FRTs. It was found that a decrease in FRT close to Ar3 temperature significantly influenced the microstructure following phase transformation, especially at the subsurface of the rolled strip. FESEM microstructural characterization confirmed that the higher FRT promoted formation of lower transformation products i.e. auto-tempered lath martensite and upper bainite throughout the thickness of the strip. However, on decreasing the FRT, the subsurface microstructure revealed a fine mixture of ferrite and granular bainite obviously as a result of strain-induced transformation, whereas the structure at the mid-thickness remained essentially martensitic. Such formation of ferrite-granular bainite mixture in the subsurface region has been found to favorably promote excellent bendability in this class of high-strength direct quenched steel. On the other hand, niobium microalloying promoted the formation of ferrite and granular bainite even at higher FRTs, presumably due to high deformation strain owing to an increase in the recrystallization stop temperature (RST) thus widening the Tnr regime. FESEM-EBSD results showed that the misorientation distribution of the grain boundaries varied as a function of the transformation microstructures, thus revealing the obvious difference because of Nb-microalloying. Additionally, CCT-diagrams were constructed for both unstrained and strained matrices deformed at 850°C, thus simulating the thermomechanical processing in the Tnr regime, using a Gleeble 3800 simulator to corroborate the transformed microstructures of the hot rolled and direct quenched strips. Recent results in respect of influence of FRT and niobium microalloying on the microstructures, hardenability and properties of pilot scale processed and direct quenched steel strips will be presented in this paper.

Effect of austempering process on microstructure and mechanical properties of 27MnCrB5-2 steel

The effect of austempering process on microstructure and mechanical properties of 27MnCrB5-2 steel (numerical designation 1.7182 EN 10083-3: 2006) has been investigated.

The heat treatment cycle has been studied obtaining the time temperature transformation diagrams (TTT) and martensite start temperature (Ms) by means of dilatometry.

Samples, austenitized at 850°C and 900°C for 20 and 40 min, were directly quenched in salt bath between 240 and 400°C for different holding times. Microstructures were examined by X ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS). Tensile tests at room temperature and Charpy V-notch (CVN) impact tests at room temperature, -20°C and -40°C were carried out and mechanisms of failure were studied by SEM analyses of the fracture surfaces.

The microstructual analyses showed that at higher austempering temperature upper bainite developed, while at lower temperatures a mixed bainitic-martensitic microstructure formed, with different amount of bainite and martensite and different size of bainite sheaf depending on temperature.

Tensile tests highlighted superior yield and tensile strengths (≈30%) for the mixed microstructure, with respect both to fully bainitic or quenched and tempered microstructures, with only a low reduction (≈10%) of elongation to failure

Impact tests confirmed that mixed microstructures have higher impact properties both at room and low temperature, with respect to completely bainitic or tempered ones; the impact properties are influenced by the relative amount of lower bainite and martensite.

Fractographic analyses of both tensile and impact specimens showed a mixed mode of fracture (ductile and cleavage); however, while tensile fracture surfaces were mainly characterized by a ductile morphology, in the CVN impact specimens an increasing amounts of brittle fracture was present with decreasing testing temperature. Moreover, while the cracks propagate easily along the low angle boundaries of the bainite sheaves in samples with lower martensite content, crack propagation seems hindered by higher fraction of martensite.

The influence of a cooling rate on evolution of microstructure and hardness of 27MnCrB5 steel

The aim of performed experiments was to determine the influence a cooling rate on evolution microstructure and hardness of steel 27MnCrB5. Firstly was quantified influence of heating temperature (in range 830 – 930 °C) on austenite grain size with a linear dependence, with the exception of some abnormal grain coarsening during heating to a temperature of 930 °C. By using dilatometric tests performed on plastometer Gleeble 3800 and by using mathematical modeling in software QTSteel was constructed continuous cooling transformation diagram for heating temperature 850 °C. Conformity diagrams constructed for both methods is relatively good, except for the position and shape of the ferrite nose. Unfortunately transformation diagram assembly by calculation in QTSteel, unlike of dilatometric measurement, does not reflect the temperature drop of the beginning of martensitic transformation with decreasing cooling rates. Were determined values of hardness, temperatures of phase transformations and proportions of structural phases upon cooling from a temperature of 850 °C at a rate of 0.16 °C·s^-1 to 37.2 °C·s^-1. Mathematically predicted proportion of martensite with real data relatively solid conformity, but the hardness values evaluated by mathematical modeling was always higher.

Microstructure, wear resistance and mechanical properties of 35CrSiMn5-5-4 steel after quenching and partitioning processes

The paper presents the effect of the Quenching & Partitioning (Q&P) heat treatment parameters on the microstructure, wear resistance and mechanical properties of 35CrSiMn5-5-4 structural steel. The Q&P process consists of martensitic quenching (Q) of a sample to a temperature lying between M_s and M_f temperatures, followed by partitioning (P) during isothermal annealing at increased temperature. During partitioning the atoms of carbon diffuse from martensite laths to untransformed austenite. Steel samples were subjected to two different Q&P processes with distinct partitioning parameters. The Q&P process parameters were chosen on the basis of phase transformation studies with the use of computer simulations and dilatometric tests. The morphology and phase composition of the microstructure were examined by SEM, TEM and XRD analysis. It was shown, that both heat treatments lead to the formation of similar microstructures consisting of carbon-depleted martensitic matrix and carbon-enriched retained austenite. In order to determine mechanical and performance properties of the 35CrSiMn5-5-4 steel, after Q&P processes, the hardness tests, wear resistance and tensile tests were conducted. It was revealed, that the change of the partitioning parameters strongly influences the mechanical properties of the 35CrSiMn5-5-4 steel. The different factors responsible for that effect have been discussed. Moreover, it was found that the wear resistance of the 35CrSiMn5-5-4 after the Q&P processes is better than after the conventional quenching and tempering heat treatment.

Mechanical properties of a bainitic steel producible by hot rolling

A carbide-free bainitic microstructure is suitable for achieving a combination of ultra high strength and high ductility. In this work, a 0.31C-2Mn-1.5Si-1Cr (wt.%) steel was produced via hot rolling route and laboratory heat treatments for austenitization (870, 900 and 950 °C, 10-30 min.) and austempering (300-400 °C, 3 h) were done in salt bath furnaces. The different austenitizing treatments were intended for varying the prior austenite grain size, and the austempering treatments were designed to approximately simulate the coiling step when the bainitic transformation would take place and certain amount of austenite would be stabilized due to suppression of carbide precipitation. Various mechanical properties (tensile, bendability, flangeability, and room and subzero temperature impact toughness) relevant for applications were determined for the bainitic strips. It was found that the mechanical properties were highly dependent on the stability of the retained austenite, presence of initial martensite and the size of the microstructural constituents. The highest amount of retained austenite (~ 27 wt.%) was obtained in the sample austempered at 375 °C but due to low austenite stability and coarse overall microstructure, the sample exhibited lower tensile ductility, bendability, flangeability and impact toughness. The sample austempered at 400 °C also showed poor properties due to presence of initial martensite and coarse microstructure. The best combination of mechanical properties was achieved for the sample austempered at 350 °C with a lower amount of retained austenite. The highest austenite stability was also confirmed for this sample by interrupted tensile tests. The prior austenite grain size did not have appreciable effect on the mechanical properties. The effect of sample preparation (flangeability and impact toughness) on the austenite stability was also investigated, however no influence of this effect was found on the final mechanical properties.

Carrying out the "hot" drawing process of TRIP steel wires at different initial temperatures

During the cold TRIP steel wire drawing process, transformation of retained austenite into martensite occurs due to the deformation, which continues until its complete depletion at large deformation degrees. Therefore, as a result of the cold drawing process, at its final stages, the TRIP effect is cancelled through the depletion of the retained austenite in the structure.

It is assumed that using the hot drawing process and, consequently, increasing the temperature of the material being worked, will block the transformation of retained austenite into martensite, which is expected to reduce the intensity of the material hardening process due to three factors: material deformation taking place at an elevated temperature, the presence of a large quantity of the plastic retained austenite in the structure, and the absence of the structure hardening-causing martensite.

So conducted drawing process should enable final-diameter wire obtained, which, in spite of considerable hardening, will have the appropriately high quantity of retained austenite in its structure, which will allow the TRIP effect to be utilized in the final product.

In the work the results of preliminary research the "hot" drawing process of TRIP steel wires at different initial temperatures has been shown.

The study is expected to find whether the "hot" drawing process, and so the increase in the temperature of the material being drawn, will block the transformation of retained austenite into martensite and, as a consequence, influence the properties of drawn wire.

Experimental verification of carburizing-quenching process of AISI 9310 steel gear shaft

Case carburization followed by quenching is a commonly used heat treatment process for machine components in the automotive industry. Within presented research, the carburized layer was developed using vacuum carburizing and high-pressure gas quenching to study the accuracy of effective case depth on non-linear surface of gear shaft. Changes in the microstructure and hardness profile on convex, linear and concave surface were measured and submitted to comparative analysis. The differences were observed and their impact on the final resistance properties of treated elements after machining operations would be showed.

Induction hardening of tool steel for heavily loaded aircraft engine components

Induction hardening is an innovative process allowing modification of the materials surface with more effective, cheaper and more reproducible way to compare with conventional hardening methods used in the aerospace industry. Unfortunately, high requirements and strict regulation concerning this branch of the industry force deep research allowing to obtain results that would be used for numerical modelling of the process. Only by this way one is able to start the industrial application of the process. The main scope of presented paper are results concerning investigation of microstructure evolution of tool steel after single-frequency induction hardening process. The specimens that aim in representing final industrial products (as heavily loaded gears), were heat-treated with induction method and subjected to metallographic preparation, after which complex microstructure investigation was performed. The results obtained within the research will be a basis for numerical modelling of the process of induction hardening with potential to be introduced for the aviation industrial components.

Low pressure carburizing of Pyrowear® Alloy 53

Pyrowear® Alloy 53 steel is an excellent material for the construction of drive transmission components (gears, shafts, etc.) that must operate in difficult conditions, primarily in the aviation industry. Among the properties of such items is high case hardness and abrasion resistance, while the core remains flexible and is capable of carrying large impact loads. They can work at elevated temperatures with limited lubrication. The thermal treatment of such materials is based on case hardening by carburizing.

This presentation shows the vacuum-carburizing technology of Pyrowear® Alloy 53 steel. It discusses the methods of establishing different carbon concentration profiles in a layer and their influence on the hardness profile. The effects of subzero treatment and tempering on the properties of the hardened case are also considered.

It also presents a SimVaC® simulator of vacuum carburizing, designed especially for Pyrowear® Alloy 53 steel, which provides highly accurate predictions of the outcome of the process (e.g., profile of carbon concentration in the layer or vice versa) by establishing the process parameters that will guarantee the required profile.

Deep cryogenic treatment and tempering at different temperatures of HS6-5-2 high speed steel

The paper presents properties of HS6-5-2 high speed steel subjected to deep cryogenic treatment (DCT) and subsequent tempering at different temperatures. The study includes measurements of hardness, microscopic observations, resistance to wear test and impact strength test. The results are compared with the characteristics of steel subjected to a conventional heat treatment, ie. without DCT.

DCT process of HS6-5-2 steel led to shifting of maximum hardness peak to the lower temperature and the reduction of the obtained maximum hardness by about 1 HRC. Therefore, to achieve the maximum hardness of the steel, it is recommended to carry out tempering at a temperature of about 40°C lower than maximum hardness peak from the tempering curve designated for steel without sub-zero process. These changes in hardness may be due to the shifting of the stage of nucleation and growth of carbide phases to lower temperatures or the changes taking place in the matrix, connected with the additional transformation of the martensite at sub-zero temperatures and more extensively occurring precipitation processes, lowering the content of the carbon in the martensite, determining thereby its lower hardness.

Light microscope observations (magnification up to 2500x) of metallographic samples etched with Murakami reagent allowed to notice the differences between carbides distribution for all tempering temperatures. Carbides with unrecognized size with dimensions smaller than 1 µm were disclosed in much higher amount in microstructure of deep cryogenically treated samples, which may be associated with the changes taking place in the matrix of material, affecting the increase in the quantity of nucleation sites. On the other hand, SEM imaging showed a significant difference between the substructure of the martensite plates, whose length and width were several times smaller for the steel subjected to DCT. There was also a significant difference in the number and size of secondary carbides present in the form of rods arranged parallel to one another, which may suggest their nucleation and growth at the same network planes. Carbides in the form of rods in the deep cryogenically treated steel had several times larger dimensions, reaching a length up to approx. 6 µm and a width of approx. 0.5 µm. Larger sizes of carbides may be due to the phenomenon of displacement to the lower temperatures of processes of their nucleation and growth, shown also during DSC (Differential Scanning Calorimetry) studies in previous work of the author of the paper.

The use of transmission electron microscopy TEM allowed observing of differences in the substructure of steel treated with or without DCT. Particularly significant were the differences in the size of the martensite crystals whose dimensions in case of DCT were about twice smaller. The change in size of the crystals may be related to the proceeding of the isothermal martensitic transformation occurring in high carbon steels at temperatures -100 ÷ -196 °C. Freshly formed in this way orthorhombic κ martensite is relatively soft and, due to the differences in specific volumes of martensite and retained austenite, is subjected to considerable deformation leading to an increase in dislocations density. The consequence of the plastic deformation of the martensite is increased density of dislocations, movement of dislocations and capturing by them of the carbon atoms which form clusters that are sites of nucleation of carbide phases precipitations. Processes taking place during the isothermal martensitic transformation and subsequent processes of nucleation and growth of carbides, occurring during the heating up to the tempering temperature and soaking at this temperature, seem to play a key role in explaining the effect of DCT on the properties of the steel.

The effect of DCT and tempering at different temperatures on the impact resistance and resistance to wear of HS6-5-2 steel is unclear due to their inconsistency with the results obtained by other authors.

Friction stir welding of ODS steel: processing, characterization and stability

Oxide-Dispersed Strengthened (ODS) steels are promising structural materials for the cladding of Gen IV fission reactors, and also for first-wall components in magnetically-confined fusion reactors, due to their outstanding combination of high-temperature strength and radiation resistance. Their microstructure comprises a fine dispersion of γ(Al,Ti) oxide nano-particles embedded in a ferritic matrix. Those 'nano-oxides' act as effective barriers for dislocation glide and increase the material strength at high temperatures. The oxide/matrix interface acts as a preferential sink for trapping radiation-induced defects and helium atoms, and therefore confers an outstanding resistance to radiation damage to these steels.

Despite these beneficial properties for nuclear reactor applications, joining ODS steel components has so far remained a technological challenge limiting their potential use, since standard joining techniques would cause the nano-particles to either agglomerate or dissolve in the matrix. Therefore, the weld would present inferior mechanical and radiation resistance as compared to the base material. We have been able to join ODS steel plates, with the chemical composition 21.7Cr-5.77Al-0.38Y₂O₃-0.33Ti-0.11Ni-0.06Mn-0.05Si-0.03C-bal.Fe (in wt.%), using solid-state Friction Stir Welding. The measured temperature close to the welding tool at constant rotation speed was 950 °C. We have combined high-resolution electron microscopy and small-angle neutron scattering experiments to characterise the distribution of the nanoparticles across the weld cross section, together with the grain size and orientation in the ferritic matrix, for different transverse speeds of the welding tool. Our results reveal a clear correlation between the nano-particle size distribution and the ferrite grain size.

We have subsequently assessed the stability of the nano-particles both at high temperatures and in the presence of radiation, in order to simulate the environmental conditions that these steels will be facing in future nuclear reactor environments. We have simulated the damage caused by neutron bombardment using the intense proton beam produced by a 5MV tandem ion accelerator. The dose achieved during proton irradiation of these materials was approximately 1dpa. These results open the door to optimising the friction stir welding conditions for improved resistance of the nano-particles to thermal and radiation fields, and therefore for improved structural integrity of ODS steel welds in next generation reactors.

Modified twin spot laser welding of complex phase steel

The work addresses microstructure-property relationships of hot-rolled complex phase steel sheets subjected to various conditions of twin spot laser welding. The effect of laser beam distribution on the macrostructure, microstructure and hardness is investigated. The results obtained for the distributions: 50%-50%, 60%-40% and 70%-30%. Joints were made using a Yb:YAG disc laser with a maximum power of 12 kW and a welding head by means of which it was possible to focus a laser beam on two spots. It was found that the change of the laser beam distribution affects geometrical features of the joint. The application of the second beam of lower power enables to obtain tempering-like effects, which finally lead to the beneficial hardness reduction both in the fusion zone and heat-affected zone. Light microscopic micrographs and scanning electron images were used for identification of various microstructural constituents in different zones of the joint.

Comparison of microstructural characterization and mechanical properties of electron beam welded joints of high strength steel grade S960Q and Weldox 1300

In the paper the results of metallographic examination and mechanical properties of electron beam welded joint of quenched and tempered steel grade S960Q and Weldox 1300 are presented. The aim of the study was to examine the feasibility of correct electron beam welded joints without filler metal.

Metallographic examination revealed that the concentrated electron beam significant affect the changes of microstructure in region of weld and the heat affected zone (HAZ) for both steel grades. The microstructure of the welds is not homogeneous and the four zones depending on the distance from the weld face can be distinguished. Basically, the microstructure of the weld consists of a mixture of martensite and bainite. However, the microstructure of HAZ depending on the distance from the fusion line is composed of martensite near the fusion line and a mixture of bainite and ferrite in the vicinity of the base material.

Significant differences in the mechanical properties of joints were observed. For a butt joint with steel grade S960QL mechanical properties are at the level of the strength of the base material Rm=1074MPa. The required bending angle of 180 ° was achieved. The impact strength at -40 °C was 71,7J/cm2. In the case of Weldox 1300 steel grade butt joints are characterized by high mechanical properties (Rm = 1470MPa), although the plastic properties are on the lower level.

The influence of manual metal arc multiple repair welding of long operated waterwall on the structure and hardness of the heat affected zone of welded joints

Welded installations failures of power plants, which are often result from a high degree of wear, requires suitable repairs. In the case of cracks formed in the weld bead of waterwall, weld bead is removed and new welded joint is prepared. However, it is associated with consecutive thermal cycles, which affect properties of heat affected zone of welded joint. This study presents the influence of multiple manual metal arc welding associated with repair activities of long operated waterwall of boiler steel on properties of repair welded joints. The work contains the results of macro and microscopic metallographic examination as well as the results of hardness measurements.