Showing papers on "Tempering published in 2012"

Effect of Austenitizing Heat Treatment on the Microstructure and Hardness of Martensitic Stainless Steel AISI 420

Abstract: The effect of austenitizing on the microstructure and hardness of two martensitic stainless steels was examined with the aim of supplying heat-treatment guidelines to the user that will ensure a martensitic structure with minimal retained austenite, evenly dispersed carbides and a hardness of between 610 and 740 HV (Vickers hardness) after quenching and tempering. The steels examined during the course of this examination conform in composition to medium-carbon AISI 420 martensitic stainless steel, except for the addition of 0.13% vanadium and 0.62% molybdenum to one of the alloys. Steel samples were austenitized at temperatures between 1000 and 1200 °C, followed by oil quenching. The as-quenched microstructures were found to range from almost fully martensitic structures to martensite with up to 35% retained austenite after quenching, with varying amounts of carbides. Optical and scanning electron microscopy was used to characterize the microstructures, and X-ray diffraction was employed to identify the carbide present in the as-quenched structures and to quantify the retained austenite contents. Hardness tests were performed to determine the effect of heat treatment on mechanical properties. As-quenched hardness values ranged from 700 to 270 HV, depending on the amount of retained austenite. Thermodynamic predictions (using the CALPHAD™ model) were employed to explain these microstructures based on the solubility of the carbide particles at various austenitizing temperatures.

Structural changes of tempered martensitic 9%Cr–2%W–3%Co steel during creep at 650 °C

Abstract: The structural changes in tempered martensitic 9Cr–2W–3Co (wt%) steel during creep tests at 650 °C were studied. The starting material was a solution treated at 1050 °C and subsequently tempered at 750 °C. The tempered martensite substructure consisted of a mixture of crystallites with a lath-type morphology and equiaxed subgrains. The tempering resulted in the precipitation of numerous second phase particles. MX-type precipitates were homogeneously distributed within the ferritic matrix, whereas M 23 C 6 -type carbides were located on boundaries. Under creep conditions, Laves phase particles also precipitated at (sub)grain/lath boundaries. Laths and subgrains tended to coarsen during creep. It was shown that M 23 C 6 -type carbides played a major role in the stabilization of the tempered martensite lath structure by exerting a large pinning pressure. The transition to tertiary creep correlated with a coarsening of the carbides and a detachment of lath and subgrain boundaries from the chains of these carbides that led to a decreased pinning force from M 23 C 6 carbides.

Iron and Steel

Abstract: Steels Steels are iron-base alloys usually containing carbon. Figure 7.1 shows the iron-carbon phase diagram. Below 911°C and between 1410°C and the melting point, iron has a bcc crystal structure called ferrite . Between 1410°C and 911°C it has an fcc crystal structure called austenite . Austenite dissolves much more carbon interstitially than ferrite. On slow cooling below 727°C, the austenite transforms by a eutectoid reaction into ferrite and iron carbide or cementite (which contains 6.7%C). The ferrite and cementite form alternating platelets called the eutectoid temperature. The resulting microstucture is called pearlite (see Figure 7.2). Pearlite Formation When a steel containing less than 0.77%C ( hypo-eutectoid steel ) is slowly cooled, some ferrite forms before any pearlite. A steel containing more than 0.77%C ( hyper-eutectoid steel ) will form some cementite before any pearlite. The formation of pearlite from austenite on cooling requires diffusion of carbon from ahead of the advancing ferrite platelets to the advancing carbide platelets as indicated in Figure 7.3. Because diffusion takes time, pearlite formation is not instantaneous. The rate at which pearlite forms depends on how much the temperature is below 717°C. The rate of diffusion increases with temperature, but the driving force for the transformation increases as the temperature is lowered. The result is that the rate of transformation is fastest between 500 and 600°C, as indicated schematically in Figure 7.4.

Microstructure and properties of 13Cr5Ni1Mo0.025Nb0.09V0.06N super martensitic stainless steel

Abstract: The morphological microstructure, the density and dispersion of high angle boundaries, morphology and micro chemical composition of precipitates and the volume fraction of retained austenite of a commercial super martensitic stainless steel (SMSS) normalized and tempered at various temperatures were characterized by optical microscope, scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) and X-ray diffraction (XRD) in the light of equilibrium phase diagram of the alloy calculated using Thermo-Calc software. The mechanical properties and pitting corrosion resistance were determined to correlate with microstructures. Two kinds of morphology of precipitate were observed in tempered commercial super martensitic stainless. Besides the globular Nb and V rich carbo-nitride precipitates, rod-like Cr rich nitrides were formed due to excess N content. While high density of high angle boundaries and precipitates contribute to strength properties, the dislocation softening of the matrix and retained austenite from tempering restore the ductility and impact toughness properties. The poor resistance to pitting corrosion is attributed to the occurrence of Cr rich precipitates. It is demonstrated that by lowering the nitrogen content and adding niobium, the Cr rich precipitates can be suppressed and the mechanical properties and resistance to pitting corrosion can be significantly improved.

Effect of hardness of martensite and ferrite on void formation in dual phase steel

Abstract: The influence of the hardness of martensite and ferrite phases in dual phase steel on void formation has been investigated by in situ tensile loading in a scanning electron microscope. Microstructural observations have shown that most voids form in martensite by evolving four steps: plastic deformation of martensite, crack initiation at the martensite/ferrite interface, crack propagation leading to fracture of martensite particles and void formation by separation of particle fragments. It has been identified that the hardness effect is associated with the following aspects: strain partitioning between martensite and ferrite, strain localisation and critical strain required for void formation. Reducing the hardness difference between martensite and ferrite phases by tempering has been shown to be an effective approach to retard the void formation in martensite and thereby is expected to improve the formability.

Dual phase structure formed by partial reversion of cold-deformed martensite

Abstract: Effect of prior deformation and heating rate on the dual phase (DP) structure formed by partial reversion of cold-rolled martensite was investigated in a low carbon steel (0.15%C–1.0%Mn). The steel plate was quenched after austenitization to obtain full martensitic structure and then cold-rolled with varying reductions. The cold-rolled specimens were continuously heated at a slow (0.083 K/s) or fast (100 K/s) heating rate up to a temperature above A1 point to partially form reversed austenite. Increasing rolling reduction rate or lowering heating rate enhanced recrystallization on heating before the onset of reversion, while the undeformed martensite never caused recrystallization irrespective of heating rate. The matrix of DP structure was changed from tempered martensite to equiaxed ferrite through the recrystallization, which resulted in a large difference in the distribution of fresh martensite (reversed austenite). Tensile testing revealed that the excellent strength-elongation balance was obtained in the DP steel produced from undeformed martensite, while higher strength was realized in the steel with prior deformation. With increasing the rolling reduction and the heating rates, the grain size of recrystallized ferrite becomes finer and the tensile strength is more increased. It was also suggested that the competition between recrystallization and reversion during continuous heating could be predicted by the modified tempering parameter.

Microstructure and fatigue performance of single and multiple linear fiber laser welded DP980 dual-phase steel

Abstract: The aim of this study was to identify the effect of fiber laser welding (FLW) on the microstructure, hardness, tensile properties and fatigue performance of high-strength (UTS ≥ 980 MPa) DP980 dual-phase steel with single linear and multiple linear joint geometry. While FLWed joints showed tempered martensite at the outer heat-affected zone (HAZ) which caused softening in the welds, a lower extent of martensite tempering and higher hardness value were observed in the narrower HAZ of the FLWed joint than in the wider HAZ of the diode laser welded (DLWed) joint due to a higher power density, faster welding speed, lower heat input, and faster cooling rate. A characteristic “suspension bridge”-like hardness profile with the fusion zone (FZ) hardness as a “pylon” appeared in the FLW due to the formation of almost fully martensitic structure in the FZ. Despite the lower ductility after FLW, a joint efficiency of about 96–97% was achieved with the yield strength essentially unchanged. At higher stress amplitudes, fatigue life of the FLWed joints was equivalent to that of BM, but at lower stress amplitudes no direct improvement in the fatigue resistance was observed due to the presence of soft zone and weld concavity. Multiple linear welds appeared to increase the probability of premature dynamic fatigue failure at lower stress amplitudes as a result of increasing number of weld concavities and soft zones. Fatigue crack initiation occurred from the specimen surface where the weld concavity met with a soft zone, and crack propagation was mainly characterized by the typical fatigue striations along with secondary cracks.

Affect of the tempering temperature on the microstructure and mechanical properties of dual phase steels

Abstract: In this study, affect of tempering temperature on the microstructure and mechanical properties of dual phase steels was studied. The C–Mn steel specimens were put under intercritical annealing treatment (ICT) at 760 °C and quenched in water to obtain 31% martensite and then tempered within the range of 100–600 °C. The tempering changes the mechanical properties of the dual phase steels through influencing both martensite and dislocation density of ferrite. Low tempering temperature results in formation of fine carbides in the martensite phase. Density of carbides increases and then carbon concentration of martensite decreases as tempering temperature increases. The continuous yield behavior of dual phase steel remains until tempering up to 300 °C. At tempering temperatures above 300 °C the discontinuous yielding and upper–lower yield points return which means degradation in mechanical properties. Yield strength increases by tempering up to 200 °C slightly, but it decreases at higher tempering temperatures. Ultimate tensile strength does not change at tempering temperature up to 200 °C considerably, but it decreases at higher tempering temperatures. Uniform elongation and total elongation increased slightly with increasing tempering time. Hardness of martensite decreases after tempering but this is not the case with ferrite resulting in a decreased hardness difference between martensite and ferrite. According to obtained results it seems that the tempering temperatures lower than 300 °C (1 h) is suitable for attaining continuous yielding and optimum strength and ductility.

Diffusionless transformations, high strength steels, modelling and advanced analytical techniques

Abstract: Part 1 Diffusionless transformations: Crystallography of martensite transformations in steels Morphology and substructure of martensite in steels Kinetics of martensite transformations in steels Shape memory in ferrous alloys Tempering of martensite in carbon steels. Part 2 Phase transformations in high strength steels: Phase transformations in high strength low alloy (HSLA) steels Phase transformations in transformation induced plasticity (TRIP)-assisted multiphase steels Phase transformations in quenched and partitioned steels Phase transformations in advanced bainitic steels Phase transformations in high manganese twinning induced plasticity (TWIP) steels Phase transformations in maraging steels. Part 3 Modelling phase transformations: First principles in modelling phase transformations in steels Phase field modelling of phase transformations in steels Molecular dynamics modelling of martensitic transformations in steels Neural networks modelling of phase transformations in steels. Part 4 Advanced analytical techniques for studying phase transformations in steels: Application of modern transmission electron microscopy (TEM) techniques to the study of phase transformations in steel Atom probe tomography for studying phase transformations in steels Electron backscatter diffraction (EBSD) techniques for studying phase transformations in steels Application of synchrotron and neutron scattering techniques for tracking phase transformations in steels.

Tensile Properties and Work Hardening Behavior of Laser-Welded Dual-Phase Steel Joints

Abstract: The aim of this investigation was to evaluate the microstructural change after laser welding and its effect on the tensile properties and strain hardening behavior of DP600 and DP980 dual-phase steels. Laser welding led to the formation of martensite and significant hardness rise in the fusion zone because of the fast cooling, but the presence of a soft zone in the heat-affected zone was caused by partial vanishing and tempering of the pre-existing martensite. The extent of softening was much larger in the DP980-welded joints than in the DP600-welded joints. Despite the reduction in ductility, the ultimate tensile strength (UTS) remained almost unchanged, and the yield strength (YS) indeed increased stemming from the appearance of yield point phenomena after welding in the DP600 steel. The DP980-welded joints showed lower YS and UTS than the base metal owing to the appearance of severe soft zone. The YS, UTS, and strain hardening exponent increased slightly with increasing strain rate. While the base metals had multi-stage strain hardening, the welded joints showed only stage III hardening. All the welded joints failed in the soft zone, and the fracture surfaces exhibited characteristic dimple fracture.

Effect of Cryogenic Treatment on AISI M2 High Speed Steel: Metallurgical and Mechanical Characterization

Abstract: This study aims to present the metallurgical and mechanical characterization of cryogenically treated AISI M2 high speed steel (HSS) in terms of carbide precipitation and wear behavior. The samples of commercially available conventionally quenched and tempered AISI M2 HSS were procured and subjected to cryogenic treatment at two levels −110 °C (shallow treatment) and −196 °C (deep treatment) of temperature. The microstructures obtained after cryogenic treatments have been characterized with a prominence to comprehend the influence of cryogenic treatment vis-a-vis conventional quenching and tempering on the nature, size, and distribution of carbides. The mechanical properties such as hardness and wear rate of the specimens have also been compared by performing Rockwell C hardness test and pin-on-disc wear test, respectively. Microstructures, hardness, wear rate and analysis of worn surface reveal the underlying metallurgical mechanism responsible for the improving mechanical properties of the AISI M2 HSS.

Microstructural characteristics and impact fracture behavior of a high-strength low-alloy steel treated by intercritical heat treatment

Abstract: Effects of the intercritical heat treatment (IHT) on microstructural evolution and Charpy impact fracture behavior of a high-strength low-alloy (HSLA) steel were investigated. The toughening mechanism was clarified by analyzing microstructural characteristics and crack propagation paths. Results showed that a composite microstructure of ferrite phase separated by globular, rod and irregular shape martensite was obtained by adding the intercritical quenching to the conventional heat treatment of quenching and tempering. And 3.6% retained austenite was detectable in the microstructure. The percentage content of high-angle (15° or more) boundaries reached 78.5%. It was also found that the steel had a high ratio of propagation energy (average: 152 J) to the total absorbed energy (average: 212 J) during impacting at −40 °C. Two crack propagation path models were observed: along the long axis direction of banded ferrite, and across the grains and corresponding interfaces. The improvement of impact toughness was attributed mainly to the retained austenite, the interlocking arrangement of banded ferrite and the ferrite–martensite interfaces with high-angle misorientation, which exhibited effective resistance to the crack propagation.

Effects of tempering on the mechanical properties of high strength dual-phase steels

Abstract: Dual-phase (DP) steels are generally characterized by the combination of high strength with good ductility, but some formability characteristics of these steels are low compared to other steel classes. In particular, stretched edge ductility and bendability are poor by comparison with other less ductile strip steel classes such as complex-phase (CP) steels. This paper describes the effect of short time tempering of the order of few seconds to few minutes on the mechanical properties of two cold rolled and hot-dip galvanised dual-phase (DP) steels. The uni-axial tensile characteristics, hole-expansivity, Erichsen cupping height and bendability were assessed after tempering at temperatures between 200 and 450 °C for relatively short time scales up to 10 min. The short tempering treatments significantly changed the character of the DP steel leading to a decrease in tensile elongation and an increase in hole-expansitivity and bendability. The properties observed in tempered DP steels where found to be closely comparable to those expected for CP steels. Microstructural analyses using the SEM showed carbide precipitation within the martensite and ferrite that increased with tempering.

Effect of Heat Treatment Processes on the Mechanical Properties of Medium Carbon Steel

Abstract: The importance of various form of heat treatment operations on medium carbon steel in order to forester the problem that may arise in making a wrong choice of these steel materials or faulty heat treatment operations which may give rise to serious disruption in terms of human safety, higher cost and untimely failure of the machine components is of great concern. The mechanical properties such as ductility, toughness, strength, hardness and tensile strength can easily be modified by heat treating the medium carbon steel to suit a particular design purpose. Tensile specimens were produced from medium carbon steel and were subjected to various forms of heat treatment processes like annealing, normalizing, hardening and tempering. The stiffness, ductility, ultimate tensile strength, yield strength and hardness of the heat treated samples were observed from their stress-strain curve. The value of the yield strength (σy) was observed to be higher for the tempered specimen possibly as a result of the grain re-arrangement, followed by the hardened, normalized and annealed specimens. The value of the ultimate tensile strength (σu) were also observed to be in the order; hardened> tempered>normalized>annealed.

Tempering Effects on the Stability of Retained Austenite and Mechanical Properties in a Medium Manganese Steel

Abstract: Tempering effects on the austenite stability and mechanical properties of 0.2C–5Mn steel were investigated in the temperature range from 100°C to 600°C with 1 hour. It was found that tempering doesn’t result in a significant change of the austenite plus ferrite duplex structure, which was developed in the previous annealing through austenite reverted transformation, whereas significant decreasing of the austenite fraction and carbon concentration was found in the specimens tempered at 200°C and 500°C due to the precipitation of carbides. Correspondingly tempering slightly deteriorates the ductility when the specimens were tempered at 200°C and 500°C without effects on mechanical properties around 400°C. Based on the analysis of relationship between mechanical properties and retained austenite, it was found that the product of tensile strength to total elongation (Rm*AT) was strongly dependent on the product of the volume fraction and carbon concentration of retained austenite (fA*Cγ ). Furthermore, the optimal mechanical properties with tensile strength 1 000 MPa and total elongation 40% could be obtained after tempering at 400°C with 1 hour, which means that galvanization is feasible in the 0.2C–5Mn steel with ferrite and austenite duplex structure.

Vacuum heat treatment, deep cryogenic treatment and simultaneous pulse plasma nitriding and tempering of P/M S390MC steel

Abstract: Vacuum heat treatment, deep cryogenic treatment and pulse plasma nitriding are efficient techniques to improve the properties of tool and high speed steels. Sometimes the influence of subzero treatment could be directly ascribed to a specific metallurgical transformation. It is the case of the transformation of retained austenite into martensite, causing a general increase in hardness and higher wear resistance. In other cases, however, the increase in wear resistance is not supported by higher hardness and several theories were proposed to explain the observed results. However, poor experimental evidence was reported in the literature for this phenomenon. Specific attention is paid to the influence of subzero treatment just after quenching and solubilization in the vacuum heat treatment or simultaneous pulse plasma nitriding and tempering (PPNT) cycle of the P/M S390MC high speed steel, respectively. Special emphasis was put on resistance to galling and abrasive wear resistance under dry sliding conditions. From obtained results it can be concluded, that the application of deep-cryogenic treatment results in a significantly higher wear resistance of high speed steels, but no significant improvements in fracture toughness have been noticed.

Effect of Uniform Distribution of Fine Cementite on Hydrogen Embrittlement of Low Carbon Martensitic Steel Plates

Abstract: The effect of uniform distribution of fine cementite on resistance of ultra-high strength steels to hydrogen embrittlement was studied. The materials used were directly-quenched and tempered 1000–1300 MPa class low carbon steel plates for welded structures with lath martensite structure. Cementite morphology was different at different heating rates to tempering temperatures. Finer cementite was distributed in rapidly-heated steels (20°C/s) than in slowly-heated steels (0.3°C/s). The rapidly-heated steels showed higher resistance to hydrogen embrittlement than the slowly-heated steels for a slow strain rate test (SSRT), whereas they showed almost the same resistance to hydrogen embrittlement for a constant load test (CLT). The specimens fractured in a plastic region for the SSRT, on the other hand, the CLT was conducted in an elastic region. The difference in hydrogen embrittlement resistance between plastic and elastic loading methods was concluded to result from a change in the hydrogen trap state at cementite in association with plasticity. Hydrogen is more strongly trapped at and/or around the strained interfaces between the matrix and cementite after plastic deformation. A close observation of fracture surfaces, hydrogen thermal desorption analysis and hydrogen microprint technique revealed that the high resistance of the rapidly-heated and tempered steels to hydrogen embrittlement for the SSRT is due to a shift of the fracture mode from quasi-cleavage fracture to ductile fracture. This shift was caused by the suppression of the quasi-cleavage fracture due to less hydrogen at lath boundaries accompanied by the uniform distribution of fine cementite.