Changes in Tempering and its Effect on Precipitation Behaviour in a Hot‐work Tool Steel

The microstructural characteristics of the hot‐worked and subsequently tempered tool steel grade X38CrMoV5‐1 was studied as a function of the cooling rate using transmission electron microscopy and three‐dimensional atom probe. According to the continuous cooling transformation diagram different cooling rates were chosen to adjust a fully martensitic or mixed microstructure consisting of martensite and bainite. The sample with the highest cooling rate exhibited a martensitic structure with nanometre sized secondary hardening carbides of the type M₃C, M₂C, M₇C₃, and MC. M₃C and M₂C were not stable and transformed to M₇C₃ as the cooling rate decreased. Furthermore, with decreasing cooling rates an increasing number of M₇C₃ precipitates are particularly present at former austenite grain boundaries as well as martensite and bainite lath boundaries, which strongly affects the mechanical properties.     

Study of Property Degradation of T23 Heat‐resistant Steel based on Microstructural Evolution during Creep

In order to study the microstructural evolution and the effect on property degradation of T23 heat‐resistance steel (2.25Cr‐1.6W‐V‐Nb‐B‐N) during creep, creep rupture specimens were investigated at 823K, 873K and 923K. The microstuctural evolution was examined by optical, scanning and transmission electron microscopy. It has been noted that the creep property degradation of T23 is related to the decrease of dislocation density due to the recovery and recrystallization of the bainitic ferrite matrix and the martensite in the carbon‐rich islands, the coarsening of M₂₃C₆ carbides, and even the transformation from M₂₃C₆ to M₆C. Coarsening of M₂₃C₆ is the dominating effect during short‐term creep whereas recovery and recrystallization is the key factor for long‐term creep. Property degradation is advanced at higher temperature due to the quicker recovery and recrystallization.     

Mechanism of Secondary Hardening in Rapid Tempering of Dual-Phase Steel

Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe₃C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo₂C (aspect ratio ranging from 10 to 25) and plate-shaped M₄C₃ carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe₃C, and interlath-retained austenite decomposed into ferrite and elongated carbide.     

Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel

Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain fracture toughness (K _IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows: in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural factor controllingK _IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls theK _IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase bainite have a significant effect on theK _IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite, dramatically improves theK _IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the parent martensite is associated with tempered martensite, it significantly lowers the K_IC.     

Effects of Alloying Elements on Microstructure,Hardness, Wear Resistance,and Surface Roughness of Centrifugally Cast High-Speed Steel Rolls

A study was made of the effects of carbon, tungsten, molybdenum, and vanadium on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by the centrifugal casting method. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The HSS rolls contained a large amount (up to 25 vol pct) of carbides, such as MC, M₂C, and M₇C₃ carbides formed in the tempered martensite matrix. The matrix consisted mainly of tempered lath martensite when the carbon content in the matrix was small, and contained a considerable amount of tempered plate martensite when the carbon content increased. The high-temperature wear test results indicated that the wear resistance and surface roughness of the rolls were enhanced when the amount of hard MC carbides formed inside solidification cells increased and their distribution was homogeneous. The best wear resistance and surface roughness were obtained from a roll in which a large amount of MC carbides were homogeneously distributed in the tempered lath martensite matrix. The appropriate contents of the carbon equivalent, tungsten equivalent, and vanadium were 2.0 to 2.3, 9 to 10, and 5 to 6 pct, respectively.     

Influence of long-term aging at 520 °C and 560 °C and the superimposed creep stress on the microstructure of 1.25Cr-0.5Mo steel

In order to understand the influence of high-temperature aging effects and those of the superimposed creep stress on the microstructural variations in a 1.25Cr-0.5Mo steel, the shoulder as well as gage portions of specimens subjected to stress-rupture tests at 520 °C and 560 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the microstructure of the steel consists of 90 pct ferrite and 10 pct bainite, and M₃C is the only carbide present in bainite and at a few ferrite grain boundaries. On aging at 520 °C for 5442 hours, Cr₂N precipitates in a fibrous form at ferrite-bainite interfaces, and the creep stress has enhanced this mode of precipitation. On holding for 13,928 hours at 520 °C, fibrous carbide is still present but its composition has changed to Mo₂C, while the superimposed creep stress has promoted the precipitation of Mo₂C needles with fine globular precipitates of M₂₃C₆. Aging at 560 °C for 1854 or 10,338 hours has resulted in the precipitation of longer Mo₂C needles and ellipsoidal M₂₃C₆ carbide precipitation; the superimposed creep stress has resulted in a more dense precipitation of shorter needles in both cases. There is some recovery of bainitic ferrite at 560 °C, though the cementite coarsening is negligible.     

Microstructural Evolution and Change in Hardness of S30432 Heat‐resistant Steel during Creep at 650 °C

The microstructural evolution of S30432 heat‐resistant steel during creep at 650 °C and its effect on the change in hardness was investigated. The change of hardness during creep of S30432 at 650 °C can be divided into three stages. These are related to the precipitation and coarsening of ε‐Cu and M₂₃C₆ carbides, decrease in the number of twins and increase in grain size. The precipitation of ε‐Cu dominantly contributes to the significant hardening at stage I, and the coarsening of ε‐Cu is the key factor to decrease the hardness at stage II. At stage III, the hardness hardly changes since the microstructure of S30432 tends to be stable in the long‐term creep range.     

Microstructure Evolution and Pinning of Boundaries by Precipitates in a 9 pct Cr Heat Resistant Steel During Creep

Structural changes in a 9 pct Cr martensitic steel during a creep test at 923 K (720 °C) under the applied stress of 118 MPa were examined. The tempered martensite lath structure (TMLS) was characterized by M₂₃C₆-type carbide particles with an average size of about 110 nm and MX-type carbonitrides with a size of 40 nm. The M₂₃C₆ particles were located on the packet/block/lath boundaries, whereas the MX precipitates were distributed homogeneously throughout TMLS. TMLS in the grip portion of the crept specimen changed scarcely during the tests. In contrast, the structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. The formation of a well-defined subgrain structure in the gauge section was accompanied by the coarsening of M₂₃C₆ carbides and precipitations of Laves phase during creep. The most pronounced structural changes occurred just at the beginning of the tertiary creep regime, which was interpreted as a result of the change in the mechanism of grain boundary pinning by precipitates.     

Structure and mechanical properties of Fe−Cr−C−Co steels

As part of a continuing program concerning the microstructures and mechanical properties of steels in which particular attention is given to transformation substructures, the present work is concerned with martensite and bainite in Fe−Cr−C steels with and without cobalt. Although cobalt raises theM _s temperature it does not affect the extent of twinning for the same carbon level and so M _s temperature alone does not control transformation substructure. Thus cobalt is not effective in retaining dislocated martensite as carbon is increased and in this regard cobalt is not beneficial to toughness. TheM _s temperatures of the steels were relatively high and hence isothermal transformation yielded mixtures of bainites and tempered martensite depending on the temperature of transformation. The mechanical properties of the isothermally transformed steels were inferior to those of the tempered steels due to the interference of upper bainite or (tempered) martensite during the isothermal transformation. Thus, in the steels having highM _s temperatures the twinning tempered martensitic structure had relatively better mechanical properties compared to the isothermally transformed steels. Attempts to produce desirable autotempered structures by air cooling (single heat treatments) were not successful and did not improve the mechanical properties since the structure consisted of a mixture of bainite and martensite. This paper is based upon a thesis submitted by M. RAGHAVAN in partial fulfillment of the requirements of the degree of Master of Science at the University of California.     

Morphology and properties of low-carbon bainite

Morphology of low-carbon bainite in commercial-grade high-tensile-strength steels in both isothermal transformation and continuous cooling transformation is lathlike ferrite elongated in the 〈11l〉_b direction. Based on carbide distribution, three types of bainites are classified: Type I, is carbide-free, Type II has fine carbide platelets lying between laths, and Type III has carbides parallel to a specific ferrite plane. At the initial stage of transformation, upper bainitic ferrite forms a subunit elongated in the [−101]f which is nearly parallel to the [lll]_b direction with the cross section a parallelogram shape. Coalescence of the subunit yields the lathlike bainite with the [−101]_f growth direction and the habit plane between (232)_f and (lll)_f. Cementite particles precipitate on the sidewise growth tips of the Type II bainitic ferrite subunit. This results in the cementite platelet aligning parallel to a specific ferrite plane in the laths after coalescence. These morphologies of bainites are the same in various kinds of low-carbon high-strength steels. The lowest brittle-ductile transition temperature and the highest strength were obtained either by Type III bainite or bainite/martensite duplex structure because of the crack path limited by fine unit microstructure. It should also be noted that the tempered duplex structure has higher strength than the tempered martensite in the tempering temperature range between 200 °C and 500 °C. In the case of controlled rolling, the accelerated cooling afterward produces a complex structure comprised of ferrite, cementite, and martensite as well as BI-type bainite. Type I bainite in this structure is refined by controlled rolling and plays a very important role in improving the strength and toughness of low-carbon steels. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Influence of Manganese and Nickel on the α´ Martensite Transformation Temperatures of High Alloyed Cr‐Mn‐Ni Steels

The martensite start temperature (M_s), the martensite austenite re‐transformation start temperature (A_s) and the re‐transformation finish temperature (A_f) of six high alloyed Cr‐Mn‐Ni steels with varying Ni and Mn contents in the wrought and as‐cast state were studied. The aim of this investigation is the development of the relationships between the M_s, A_s, A_f, T₀ temperatures and the chemical composition of a new type of Cr‐Mn‐Ni steels. The investigations show that the M_s, A_s and A_f temperatures decrease with increasing nickel and manganese contents. The A_f temperature depends on the amount of martensite. Regression equations for the transformation temperatures are given. The experimental results are based on dilatometer tests and microstructure investigations.     

Influence of long-term aging and superimposed creep stress on the microstructure of 2.25cr-1Mo steel

To understand the influence of high-temperature aging and superimposed creep stress on the microstructural variations in a 2.25Cr-1Mo steel, the shoulder and gage portions of the specimens subjected to stress-rupture tests at 540 °C and 580 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the steel exhibited a tempered bainitic structure and the carbides were present as M₃C globules, M₂C platelets, and M₂₃C₆ rectangular parallelepipeds. Aging the steel at 540 °C for 7022 hours or 17,946 hours resulted in considerable coarsening of M₂C and caused precipitation of M₆C carbides. The superimposed creep stress enhanced the M₂C precipitation. The ferrite matrix exhibited some recovery in the specimens exposed for 17,946 hours. While M₂C platelets were observed in a few areas after 14,836 hours of aging at 580 °C, this carbide was virtually nonexistent when a stress of 78 MPa was superimposed. Amounts of M₂₃C₆ persisted throughout the tests at both 540 °C and 580 °C. The M₆C carbide became more predominant after long exposure at 580 °C. The ferrite matrix recovered considerably in specimens subjected to creep stress at 580 °C for 14,836 hours.     

Competing Martensitic,Bainitic, and Pearlitic Transformations in a Hypoeutectoid Ti-5Cu Alloy

This article discusses the competing mechanisms of martensite formation vs eutectoid decomposition via pearlitic or bainitic mechanisms during continuous cooling of a Ti-5 wt pct Cu hypoeutectoid alloy, which falls under the category of active eutectoid systems. Faster cooling rates result in a mixed microstructure of nanoscale bainite consisting of a far-from-equilibrium Ti₂Cu phase and martensitic alpha plates, as determined from three-dimensional atom probe (3DAP) coupled with energy-filtered transmission electron microscopy (EFTEM). Slower cooling resulted in near-equilibrium eutectoid-based microstructures.     

The role of alloy composition in the precipitation behaviour of high speed steels

The role of alloy composition in determining the microstructure and microchemistry of a series of related high speed steels has been investigated by a combination of analytical electron microscopy and atom-probe field ion microscopy. The four steels which were investigated (M2, ASP 23, ASP 30 and ASP 60) cover a large range of C, V and Co contents. Excepting the Co content, the composition of primary MC and M₆C carbides and as-hardened martensite was similar in all four alloys and the major effect of increasing the content of C and V was to increase the volume fraction of MC primary carbides. Precipitation of proeutectoid carbides (mainly MC and M₂C) occurred during hardening of all four steels and the extent of this was greatest in the highly alloyed ASP 60. Tempering at 560°C resulted in the precipitation of extremely fine dispersions of MC and M₂C secondary carbides with very mixed compositions in all four steels. It was found that, as well as hindering the formation of autotempered M₃C in the as-hardened martensite, additions of Co refined the secondary carbide dispersion and delayed overaging reactions. Overaging at 600°C resulted in the precipitation of M₃C, M₆C and M₂₃C₆ at the expense of the fine MC and M₂C secondary carbide dispersion.     

Multiphase Microstructure Optimization to Enhance the Mechanical Properties of AISI M35 High-Speed Steel

It is crucial to conduct in-depth research on the cryogenic-treatment mechanism to promote the standardization and industrialization of cryogenic treatment in the high-speed steel (HSS) industry. In this study, the microstructure and mechanical properties (microhardness and impact toughness) of AISI M35 HSS after deep-cryogenic treatment (DCT) and conventional heat treatment (CHT) are investigated, and the microstructural characteristics at different stages of CHT and cryogenic treatment are studied. It is indicated in the results that DCT of the steel leads to the formation of fresh martensite from residual austenite, as well as the introduction of more dislocations due to plastic deformation. In addition, the deep-cryogenic-treated specimen that is tempered shows increased numbers of martensite blocks and secondary carbide precipitation. The carbides in the steel are mainly V-rich (MC), W–Mo-rich (M₆C), and Cr-rich (M₂₃C₆). The hardness of the deep-cryogenic-treated samples increases by approximately 50 HV₁ because of the transformation of residual austenite and dislocation strengthening. Furthermore, specimens that are both deep-cryogenic treated and tempered exhibit a 30% increase in impact toughness and a more uniform distribution in hardness, likely due to the more homogeneous precipitation of secondary carbides and refinement of martensite.     

Effect of Co on Creep Behavior of a P911?Steel

The microstructure and creep behavior of a 3 pct Co modified P911 steel and standard P911 steel were examined. It was shown that the nanoscale M₂₃C₆ carbides and MX carbonitrides in the 3 pct Co modified P911 steel are not susceptible to significant coarsening under creep conditions. Also, coarsening simulations of M₂₃C₆ particles were performed for both steels. The rates of lath and particle coarsening in the P911 + 3 pct Co steel are remarkably lower than those in the P911. Increased stability of a tempered martensite lath structure in the 3 pct Co modified P911 steel provides enhanced creep resistance at an exceptionally high temperature of 923 K (650 °C).     

Analysis of the Strain‐Induced Martensitic Transformation of Retained Austenite in Cold Rolled Micro‐Alloyed TRIP Steel

The stability of retained austenite and the kinetics of the strain‐induced martensitic transformation in micro‐alloyed TRIP‐aided steel were obtained from interrupted tensile tests and saturation magnetization measurements. Tensile tests with single specimens and at variable temperature were carried out to determine the influence of the micro‐alloying on the M_s^σ temperature of the retained austenite. Although model calculations show that the addition of the micro‐alloying elements influences a number of stabilizing factors, the results indicate that the stability of retained austenite in the micro‐alloyed TRIP‐aided steels is not significantly influenced by the micro‐alloying. The kinetics of the strain‐induced martensitic transformation was also not significantly influenced by addition of the micro‐alloying elements. The addition of micro‐alloying elements slows down the autocatalytic propagation of the strain‐induced martensite due to the increase of the yield strength of retained austenite. The lower uniform elongation of micro‐alloyed TRIP‐aided steel is very likely due to the presence of numerous precipitates in the microstructure and the pronounced ferrite grain size refinement.     

The effect of tungsten on creep

The effect of tungsten on creep behavior and microstructural evolution was investigated for tempered martensitic 9Cr steels with various W concentrations from 0 to 4 wt pct. The creep rupture testing was carried out at 823, 873, and 923 K for up to 54 Ms (15,000 hours). The creep and creep rupture strength increased linearly with W concentration up to about 3 wt pct, where the steels consisted of the single constituent of the tempered martensite. It increased only slightly above 3 wt pct, where the matrix consisted of the tempered martensite and δ-ferrite. The minimum creep rate was described by a power law. The apparent activation energy for the minimum creep rate showed a tendency similar to the W concentration dependence of the creep-rupture strength and was larger than the activation energy for self-diffusion at high W concentrations above 1 wt pct. The martensite lath microstructure with fine carbides along lath boundaries was responsible for a high resistance to creep deformation. With increasing W con- centration, the martensite lath microstructure became stabilized, which decreased the minimum creep rate and increased the apparent activation energy for the minimum creep rate.     

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.