Secondary hardening and fracture behavior in alloy steels containing Mo, W, and Cr

In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which contain alloying elements, such as Mo, W and Cr, contributing to the secondary hardening by forming M₂C-type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hardening, while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition, because the cementite of M₃C type was stabilized at higher temperatures and the formation of M₂C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition.     

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M₂C carbides formed during the dissolution of the cementite (M₃C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M₂C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M₂C carbide, while Co increased the nucleation rate of M₂C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M₂C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures.     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

The effect of tungsten on dislocation recovery and precipitation behavior of low-activation martensitic 9Cr steels

The effect of W on dislocation recovery and precipitation behavior was investigated for martensitic 9Cr-(0,l,2,4)W-0.1C (wt pct) steels after quenching, tempering, and subsequent prolonged aging. The steels were low induced-radioactivation martensitic steels for fusion reactor structures, intended as a possible replacement for conventional (7 to 12)Cr-Mo steels. During tempering after quenching, homogeneous precipitation of fine W₂C occurred in martensite, causing secondary hardening between 673 and 823 K. The softening above the secondary hardening temperature shifted to higher temperatures with increasing W concentration, which was correlated with the decrease in self-diffusion rates with increasing W concentration. Carbides M₂₃C₆ and M₇C₃ were precipitated in the 9Cr steel without W after high-temperature tempering at 1023 K. With increasing W concentration, M7C3 was replaced by M₂₃C₆, and M₆C formed in addition to M₂₃C₆. During subsequent aging at temperatures between 823 and 973 K after tempering, the recovery of dislocations, the agglomeration of carbides, and the growth of martensite lath subgrains occurred. Intermetallic Fe₂W Laves also precipitated in the δ-ferrite grains of the 9Cr-4W steel. The effect of W on dislocation recovery and precipitation behavior is discussed in detail.     

Coarsening resistance of M2C carbides in secondary hardening steels: Part II. Alloy design aided by a thermochemical database

In high Co-Ni steels containing the strong carbide-forming elements Mo, Cr, and W, secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides. Thermodynamic stability and coarsening resistance of these carbides depend on the alloy content of these elements. A model for the M₂C coarsening kinetics in multicomponent alloys has been used to identify the optimum alloying addition for maximum coarsening resistance and as a basis for selection of four experimental alloy steels. Necessary information pertaining to the equilibrium in these steels was obtained using the Thermo-Calc software and database developed at the Royal Institute of Technology, Stockholm, Sweden.     

Coarsening resistance of M2C carbides in secondary hardening steels: Part I. Theoretical model for multicomponent coarsening kinetics

The development of very high-strength levels in many alloy steels is achieved by a secondary hardening reaction. In high Co-Ni steels containing the strong carbide-forming elements Mo, Cr, and W, secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides. Coarsening resistance of the M₂C precipitates depends on the alloy content of these elements, and there should be an addition to the alloy of these carbide-forming elements which optimizes the M₂C coarsening resistance. Current Lifshitz-Slyozov-Wagner (LSW) theory^[2,3] cannot properly be used to describe, the coarsening behavior of multicomponent carbides, which involves concentrations and diffusivities of two or more solutes and nonspherical carbide morphologies. A model is introduced for the coarsening resistance of multicomponent carbides. This model treats the coarsening of shape-preserving particle and is applicable to rodlike particles.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised M_s but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raisedM _S but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

Role of Tungsten in the Tempered Martensite Embrittlement of a Modified 9 Pct Cr Steel

The effect of tempering on the mechanical properties and fracture behavior of two 3 pct Co-modified 9 pct Cr steels with 2 and 3 wt pct W was examined. Both steels were ductile in tension tests and tough under impact tests in high-temperature tempered conditions. At T ≤ 923 K (650 °C), the addition of 1 wt pct W led to low toughness and pronounced embrittlement. The 9Cr2W steel was tough after low-temperature tempering up to 723 K (450 °C). At 798 K (525 °C), the decomposition of retained austenite induced the formation of discontinuous and continuous films of M₂₃C₆ carbides along boundaries in the 9Cr2W and the 9Cr3W steels, respectively, which led to tempered martensite embrittlement (TME). In the 9Cr2W steel, the discontinuous boundary films played a role of crack initiation sites, and the absorption energy was 24 J cm^?2. In the 9Cr3W steel, continuous films provided a fracture path along the boundaries of prior austenite grains (PAG) and interlath boundaries in addition that caused the drop of impact energy to 6 J cm^?2. Tempering at 1023 K (750 °C) completely eliminated TME by spheroidization and the growth of M₂₃C₆ carbides, and both steels exhibited high values of adsorbed energy of ≥230 J cm^?2. The addition of 1 wt pct W extended the temperature domain of TME up to 923 K (650 °C) through the formation of W segregations at boundaries that hindered the spheroidization of M₂₃C₆ carbides.     

Atom probe analysis of 2.25 Cr1Mo steel

Atom probe mass analysis was conducted on 2.25 Cr1Mo and 2.25 Cr steels containing a small amount of P and problems associated with this analysis were described. A reliable compositional analysis by the atom probe of these low alloying steels was demonstrated by comparing the compositions determined by the atom probe and by a chemical analysis. In order to apply the atom probe technique to the study of temper embrittlement of 2.25 Cr1 Mo steel, compositional variations with aging time at 773 K of alloy elements and P in ferrite phase were examined. It was found that Cr and P concentrations in ferrite increased within 10 h of the early stage of aging and then gradually decreased with further aging, while Mo concentration simply decreased with aging. It was suggested that P atoms in as tempered steel were concentrated in ferrite-cementite boundaries and they were dissolved into ferrite during a process in which M₃C was replaced by M₂C in the early stage of aging. These dissolved P atoms were then segregated to grain boundaries causing the temper embrittlement. It was found that the presence of Mo retarded the decrease of P concentration in ferrite with aging thus suppressing the grain boundary segregation of P.     

The effects of composition and carbide precipitation on temper embrittlement of 2.25 Cr-1 Mo steel: Part I. Effects of P and Sn

Temper embrittlement of 2.25 Cr-1 Mo steel doped with P and Sn was studied systematically. Carbide extraction by electrolysis, X-ray diffraction, transmission (replica) electron microscopy, chemical analysis of the matrix, and scanning Auger microprobe analysis were conducted to determine the effect of carbide precipitation and subsequent variation of the Mo concentration in solution on the segregation of P. These analyses were correlated with the ductile-to-brittle transition temperature (measured by use of a slow-bend test), as well as hardness measurements and fractographic information obtained by scanning electron microscopy. The results indicate that the principal role of Mo is to suppress embrittlement by scavenging of P, presumably by a Mo-P compound formation, thereby diminishing P segregation. However, due to the stronger interaction between Mo and C, Mo is precipitated in an M₂C carbide during tempering or aging, and the matrix is depleted of Mo. The P thereby released segregates at a rate consistent with the rate of M₂C precipitation. At a Mo concentration >0.7 pct the beneficial effect of Mo is decreased due to enhanced M₂C precipitation, the content of Mo in solution remaining essentially constant. The M₂C is formed at the expense of Cr-rich M₇C₃; this results in more Cr in solution, thereby permitting more Cr-P cosegregation, and embrittlement increases. Tin was found not to produce temper embrittlement in this steel when present at concentrations up to 0.04 pct.     

The microstructure of chromium-tungsten steels

Chromium-tungsten steels are being developed to replace the Cr-Mo steels for fusion-reactor applications. Eight experimental steels were produced and examined by optical and electron microscopy. Chromium concentrations of 2.25, 5, 9 and 12 pct were used. Steels with these chromium compositions and with 2 pct W and 0.25 pct V were produced. To determine the effect of tungsten and vanadium, three other 2.25Cr steels were produced as follows: an alloy with 2 pct W and 0 pct V and alloys with 0 and 1 pct W and 0.25 pct V. A 9Cr steel containing 2 pct W, 0.25 pct V, and 0.07 pct Ta also was studied. For all alloys, carbon was maintained at 0.1 pct. Two pct tungsten was required in the 2.25Cr steels to produce 100 pct bainite (no polygonal ferrite). The 5Cr and 9Cr steels were 100 pct martensite, but the 12Cr steel contained about 25 pct delta-ferrite. Precipitate morphology and precipitate types varied, depending on the chromium content. For the 2.25Cr steels, M₃C and M₇C₃ were the primary precipitates; for the 9Cr and 12Cr steels, M₂₃C₆ was the primary precipitate. The 5Cr steel contained M₇C₃ and M₂₃C₆. All of the steels with vanadium also contained MC.     

Effect of alloying additions on secondary hardening behavior of Mo-containing steels

The effect of alloying additions on secondary hardening behavior in Fe-Mo-C steels has been investigated by means of the successive alloying additions of Cr, Co, and Ni. The Cr additions promote M₃C cementite formation. The Ni additions destabilize the cementite formation, while the Co additions retard dislocation recovery and present the necessary sites for M₂C formation which provides the secondary hardening. Professor Kwon is jointly appointed at the Center for Advanced Aerospace Materials.     

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.     

Carbide precipitation,grain boundary segregation,and temper embrittlement in NiCrMoV rotor steels

This paper presents a study of carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor steels. One of the steels was high purity, one was doped with phosphorus, one was doped with tin, and one was commercial purity. In addition, two NiCrV steels, one high purity and one doped with phosphorus, were examined. Carbide precipitation was studied with analytical electron microscopy. It was found that after one hour of tempering at 600 ‡C only M₃C carbides were precipitated in the NiCrMoV steels. These were very rich in iron. As the tempering time increased, the chromium content of the M₃C carbides increased significantly, but their size did not change. Chromium rich M₇C₃ precipitates began to form after 20 hours of tempering, and after 50 hours of tempering Mo-rich M₂C carbides were precipitated. Also, after 100 hours of tempering, the matrix formed bands rich in M₃C or M₇C₃ and M₂C particles. Tempering occurred more rapidly in the NiCrV steels. Grain boundary segregation was studied with Auger electron spectroscopy. It was found that the amount of phosphorus and tin segregation that occurred during a step-cooling heat treatment after tempering was less if a short time tempering treatment had been used. It will be proposed that this result occurs because the low temperature tempering treatments leave more carbon in the matrix. Carbon then compctes with phosphorus and tin for sites at grain boundaries. This compctition appears to affect phosphorus segregation more than tin segregation. In addition to these two impurity elements, molybdenum and nickel segregated during low temperature aging. The presence of molybdenum in the steel did not appear to affect phosphorus segregation. Finally, it will be shown that all of the steels that contain phosphorus and/or tin exhibit some degree of temper embrittlement when they are aged at 520 ‡C or are given a step-cooling heat treatment. Of the NiCrMoV steels, the phosphorus-doped steel showed the least embrittlement and the commercial purity steel the most. The phosphorus-doped NiCrV steel was also more susceptible to temper embrittlement than the phosphorus-doped NiCrMoV steel. This latter difference was attributed to molybdenum improving grain boundary cohesion. It was also found that as the segregation of phosphorus or tin to the grain boundaries increased, the measured embrittlement and the amount of intergranular fracture increased. However, there was a large amount of scatter in all of these data and the trends were only qualitative. All parts of this study are compared in detail to others in the literature, and general trends that can be discerned from all of these results are presented. Formerly with the University of Pennsylvania, Department of Materials Science, Philadelphia, PA     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel—IV. Precipitation behaviour and distribution of elements at grain boundaries

The distribution of elements and the precipitation behaviour at grain boundaries have been studied in boron containing AISI 316L and “Mo-free AISI 316L” type austenitic stainless steels. A combination of microanalytical techniques was used to study the boundary regions after cooling at 0.29–530°C/s from 800, 1075 or 1250°C. Tetragonal M₂B, M₅B₃ and M₃B₂, all rich in Fe, Cr and Mo, precipitated in the “high B” (40 ppm) AISI 316L steel whereas orthorhombic M₂B, rich in Cr and Fe, was found in the “Mo-free steel” with 23 ppm B. In the “high B steel” a thin (<2 nm), continuous layer, containing B, Cr, Mo and Fe and having a stoichiometry of typically M₉B, formed at boundaries after cooling at intermediate cooling rates. For both types of steels a boundary zone was found, after all heat treatments, with a composition differing significantly from the bulk composition. The differences were most marked after cooling at intermediate cooling rates. In both types of steel boundary depletion of Cr and enrichment of B and C occurred. It was found that non-equilibrium grain boundary segregation of boron can affect the precipitation behaviour by making the boundary composition enter a new phase field. “Non-equilibrium phases” might also form. The synergistic effect of B and Mo on the boundary composition and precipitation behaviour, and the observed indications of C non-equilibrium segregation are discussed.     

Reversible temper embrittlement of rotor steels

The susceptibility to temper embrittlement of eight different rotor steels has been studied in terms of the effects of composition, of cooling rate from tempering temperature, of isothermal aging, of steel-making practice and of strength level and tempering temperature. The Ni Cr Mo V steels tested showed increasing susceptibility to temper embrittlement with increasing nickel content. The normally marked susceptibility of a high phosphorus 3 pct Cr Mo steel was eliminated by the removal of manganese. Embrittlement in a 3 pct Ni Cr Mo V steel was caused by the equilibrium segregation of solute atoms to the prior austenite grain boundaries. Two Cr Mo V steels tested were not susceptible to temper embrittlement. Electroslag remelting and refining had very little effect on the susceptibility of the steels tested. Strength level and tempering temperature had no effect on the degree of embrittlement of the 3 pct Ni Cr Mo V disc steel. The possibilities of remedial action include an adjustment of the post tempering cooling rate, to optimize the conflicting interests of minimum temper embrittlement and adequate stress relief, and the production of very low manganese rotor steels.     

Identification of M5C2 carbides in ex- service 1Cr-0.5Mo steels

Rod-shaped precipitates up to 6μm} long and 0.25μm wide, observed as a common feature within proeutectoid ferrite grains of ex-service lCr-0.5Mo steels, have been characterized using electron microdiffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The majority of the rods have been identified as M₅C₂ carbides, although some were M₃C. The M₅C₂ carbide, also known as the Hägg orX-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M₅C₂ rods appeared to nucleate heterogeneously on M₂C carbides and persist in ferrite regions from which the needlelike M₂C carbides had disappeared. This suggests that the M₅C₂ carbide is more stable thermodynamically than M₂C in lCr-0.5Mo steels under typical service conditions. The metallic element compositions of the rodlike carbides varied, but the average compositions were in the range 48 to 56 at. pct Fe, 32 to 42 at. pet Cr, 8 to 12 at. pct Mn, and about 1 at. pct Mo. The Mn content of the rods varied systematically with exposure temperature and thus might be applied to the estimation of the effective service temperature of lCr-0.5Mo steel components.     

热处理工艺对新型Cr8Mo2VSi和D2冷作模具钢扁钢组织和力学性能的影响

 研究了热处理工艺对高碳、高铬冷作模具钢Cr8Mo2VSi和D2钢的组织和性能的影响,结果表明：Cr8Mo2VSi比D2钢的碳化物更细小、均匀;1020～1040 ℃淬火,Cr8Mo2VSi的硬度略低于D2,经高温回火后,Cr8Mo2VSi钢表现出更好的二次硬化效应,其硬度高于D2钢1~2（HRC）,并且比D2钢具有更好的强、韧性配合。     

Effect of Silicon on Carbide Precipitation after Tempering of H11 Hot Work Steels

The formation of secondary carbides during tempering of H11 hot work steels at 898 K (625 °C) was studied by transmission electron microscopy (TEM) and related to the previously established effects of Si content on mechanical properties. Lower Si contents (0.05 and 0.3 pct Si) and higher Si contents (1.0 and 2.0 pct Si) were observed to yield different carbide phases and different particle distributions. Cementite particles stabilized by Cr, Mo, and V in the lower Si steels were found to be responsible for similar precipitation hardening effects in comparison to the M₂C alloy carbides in the higher Si steels. The much higher toughness of the lower Si steels was suggested to be due to a finer and more homogeneous distribution of Cr-rich M₇C₃ carbides in the interlath and interpackage regions of the quenched and tempered martensite microstructure. The present effects of Si content on the formation of alloy carbides in H11 hot work steels were found to be the result of the retarding effect of Si on the initial formation of cementite, well known from the early tempering stages in low alloy steels.