The influences of impurity content,tensile strength,and grain size on in-service temper embrittlement of CrMoV steels

The influences of impurity levels, grain size, and tensile strength on in-service temper embrittlement of CrMoV steels have been investigated. The samples for this study were taken from several steam turbine CrMoV rotors which had operated for 15 to 26 years. The effects of grain size and tensile strength on embrittlement susceptibility were separated by evaluating the embrittlement behavior of two rotor forgings, which were made from the same ingot, after giving an extended step-cooling treatment. The results reveal that among the residual elements in the steels, only P produces a significant embrittlement. The variation of P and tensile strength of the steels in the ranges investigated has no effect on in-service temper embrittlement susceptibility, as measured by the shift in fracture appearance transition temperature (FATT). However, the prior austenite grain size plays a major role on in-service embrittlement. The fine grain steels with a grain size of ASTM No. 9 or higher are virtually immune to in-service embrittlement. In steels having duplex grain sizes, the embrittlement susceptibility is controlled by the size of coarser grains. For a given steel chemistry, the coarse grain steel is more susceptible to in-service embrittlement, and a decrease in ASTM grain size number from 4 to 0/1 increases the shift in FATT by 61°C (110°F). It is demonstrated that long-term service embrittlement can be simulated, except in very coarse grain steels, by using the extended step-cooling, treatment. The results of step-cooling studies also show that the coarse grain rotor steels take longer time during service to reach a fully embrittled state than the fine grain rotor steels. This difference in the kinetics of embrittlement is believed to be related to the variations in Mo content in the matrix and the grain size of the steels.     

The postweld heat-treatment response of simulated coarse-grained heat-affected zones in a new ferritic steel

The tempering behavior of simulated coarse-grained (CG) heat-affected zones (HAZs) in two ferritic alloy steels, 2.25Cr-1Mo and HCM2S, was investigated. The hardness of HCM2S was found to be stable at longer times and higher temperatures than the 2.25Cr-1Mo steel, even though the “as-welded” hardnesses were approximately equal. Both materials reached a peak secondary hardness after tempering for 5 hours at 575 °C. The increase in hardness of the 2.25Cr-1Mo steel was due to precipitation of Fe-rich M₃C carbides within the prior-austenite grains, whereas the secondary hardening in HCM2S was due to a fine dispersion of intragranular, W-rich carbides. The HCM2S steel retained its hardness at longer times and higher temperatures than 2.25Cr-1Mo steel, because of the precipitation of intragranular, W-rich carbides and V-rich MC carbides that stabilized the lath structure. This study shows that HCM2S should not be heat treated in the same way as 2.25Cr-1Mo steel and also provides a basis for defining the postweld heat treatment (PWHT) of HCM2S.     

The Effects of Tempering Reactions on Temper Embrittlement of Alloy Steels

The effects of tempering reactions which produce molybdenum-rich carbides on the temper embrittlement of NiCrMo, NiCrMoV, CrMo, and CrMoV steels, particularly embrittlement due to phosphorus segregation, are reviewed. Molybdenum can act as an effective scavenger for phosphorus and other embrittling impurities, but the scavenging is lost when the molybdenum is precipitated in carbides as a result of continued tempering during service at elevated temperatures. This leads to very slow embrittlement, controlled by the rates of alloy carbide formation, rather than by the diffusion of phosphorus, for example. The presence of vanadium apparently retards the embrittlement process even more by interfering with the formation of the molybdenum-rich carbides. Observations of the temper embrittlement behavior, and of the effects of service exposure, in three CrMoV steam turbine rotors are also reported and are shown to be consistent with the previous results. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

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.     

Effects of Mn,Si, and purity on the design of 3.5NiCrMoV, 1CrMoV,and 2.25Cr-1Mo bainitic alloy steels

Three high-temperature bainitic alloy steels were evaluated in the laboratory to determine the effects of Mn, Si, and impurities (i.e., S, P, Sn, As, and Sb) on microstructure and mechanical properties. The alloy steels were 3.5NiCrMoV and CrMoV, which are used for turbine rotors, and 2.25Cr-1Mo, which is used in pressure vessel applications. The important effects of Mn, Si, and impurities, which should control the design of these high-temperature bainitic steels, are presented. Key results are used to illustrate the influence of these variables on cleanliness, overheating, austenitizing, hardenability, tempering, ductility, toughness, temper embrittlement, creep rupture, and low-cycle fatigue. Low levels of Mn, Si, and impurities not only result in improved temper embrittlement resistance in these steels but also lead to an improvement in creep rupture properties (i.e., improved strength and ductility). These results have produced some general guidelines for the design of high-temperature bainitic steels. Examples illustrating the implementation of the results and the effectiveness of the design guidelines are provided. Largely based on the benefits shown by this work, a high-purity 3.5NiCrMoV steel, which is essentially free of Mn, Si, and impurities, has been developed and is already being used commercially. T. OHHASHI was formerly Research Scientist, Japan Steel Works, Ltd., Muroran Research Laboratory, 4 Chatsumachi, Muroran, 051 Japan.     

The effects of tempering reactions on temper embrittlement of alloy steels

The effects of tempering reactions which produce molybdenum-rich carbides on the temper embrittlement of NiCrMo, NiCrMoV, CrMo, and CrMoV steels, particularly embrittlement due to phosphorus segregation, are reviewed. Molybdenum can act as an effective scavenger for phosphorus and other embrittling impurities, but the scavenging is lost when the molybdenum is precipitated in carbides as a result of continued tempering during service at elevated temperatures. This leads to very slow embrittlement, controlled by the rates of alloy carbide formation, rather than by the diffusion of phosphorus, for example. The presence of vanadium apparently retards the embrittlement process even more by interfering with the formation of the molybdenum-rich carbides. Observations of the temper embrittlement behavior, and of the effects of service exposure, in three CrMoV steam turbine rotors are also reported and are shown to be consistent with the previous results.     

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     

The effects of composition and carbide precipitation on temper embrittlement of 2.25 Cr-1 Mo steel: Part II. Effects of Mn and Si

The effects of additions of 0.7 pct Mn and/or 0.6 pct Si on the temper embrittlement behavior of 2.25 Cr-1 Mo steel at several hardness levels were determined. As in Part I, the tendency for the P segregation was dependent upon the Mo concentration in the ferrite, which is controlled by the types of carbides formed during heat treatment or aging. Additions of either Mn or Si increase the fraction of grain boundaries which adsorb P (although they do not increase the P concentration on the embrittled boundaries significantly) thereby raising the amount of temper embrittlement as measured by the transition temperature shift. Mn and Si appear to act independently and their effects appear to be additive; this is rationalized in terms of their expected influences on the segregation free energy for P in Fe.     

The effects of double austenitization on the mechanical properties of a 0.34C containing low-alloy Ni-Cr-Mo-V steel

This article considers five different microstructures of a tempered martensitic 0.34C, 3Ni-1.3Cr-0.4Mo-0.1V steel through various heat treatments, including double austenitization (DA) treatments, and how the impact toughnesses are influenced by microstructure. Of the four mechanisms considered to explain the beneficial effect of DA treatment, the roles of retained austenite, grain-boundary embrittlement by impurity segregation, and matrix flow stress are discounted. The 50 pct fracture appearance transition temperature (FATT) of this steel is found to be dependent on both the grain size and the carbide dissolution. The conventionally treated steel contains mainly platelike M₃C carbides. The DA treatment helps to dissolve VC carbides and coarsen and spheroidize M₃C carbides in favor of the precipitation of short rodlike M₇C₃ carbides with a lower aspect ratio. The improvement of impact toughness (upper shelf energy, ductile-to-brittle transition temperature (DBTT), and lower shelf energy) by DA treatment, explained in detail, is attributed to a change of this material’s tensile and work-hardening behavior affected by a variation of carbide morphology.     

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.     

Tempering of 2.25 Pct Cr-1 Pct Mo Low Carbon Steels

The effect of carbon level on the tempering behavior at 700°C of 2.25 pct Cr-1 pct Mo steels having typical weld metal compositions has been investigated using analytical electron microscopy and X-ray diffraction techniques. The morphology, crystallography and chemistry, of each of the various types of carbides observed, has been established. It has been shown that each carbide type can be readily identified in terms of the relative heights of the EPMA spectra peaks for iron, chromium, molybdenum, and silicon. A decrease in the carbon level of the steel increases the rate at which the carbide precipitation reactions proceed, and also influences the final product. Of the carbides detected, M₂₃C₆ and M₇C₃ were found to be chromium-based, and their compositions were independent of both the carbon level of the steel and the tempering time. The molybdenum-based carbides, M₂C and M₆C, however, showed an increase in their molybdenum contents as the tempering time was increased. The rate of this increase became greater as the carbon content of the steel was lowered.     

Degradation of mechanical properties of Cr-Mo-V and 2.25Cr-1Mo steel components after long-term service at elevated temperatures

The influence of operating temperature on in-service degradation of mechanical properties of high temperature stream turbine components has been investigated. Material samples for this study were taken from a Cr-Mo-V rotor and several 2.25Cr-1Mo cast steel components which had operated over 200,000 hours. The test results revealed that the degree of in-service degradation of strength, toughness, and the fracture appearance transition temperature of both steels were very sensitive to the service temperature. Both steels softened only when they were exposed at a temperature greater than 454°C (850°F) and the degree of softening increased with further increase in service temperature. In Cr-Mo-V steel, the loss in strength was accompanied by an improvement in ductility and toughness. Despite softening of 2.25Cr-1Mo steel in service, elevated temperature exposure resulted in a marked decrease in ductility and toughness. The loss of toughness in this steel was in part irreversible. In contrast, a severe increase in fracture appearance transition temperature, due to reversible temper embrittlement, occurred in both steels at a service temperature of around 427°C (800°F), but not at the highest service temperature. In fact, the Cr-Mo-V steel did not temper embrittle as a result of service exposure at the highest operating temperature investigated. These results are rationalized in terms of changes in microstructure and grain boundary chemistry that occur in service as a function of operating temperature.     

Effects of alloying elements on fracture toughness in the transition temperature region of base metals and simulated heat-affected zones of Mn-Mo-Ni low-alloy steels

This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment, and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and increased Mo and Ni content, the number of fine M₂C carbides was greatly increased and the number of coarse M₃C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to precipitate fine M₂C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base metals.     

The 885° f (475° c) embrittlement of ferritic stainless steels

The 885odgF (475°C) embrittlement of seven heats of chromium steels was investigated: four vacuum-melted heats with C + N < 0.008 pct and 14 pct Cr, 14 pet Cr-2 pet Mo, 18 pct Cr, or 18 pet Cr-2 pet Mo, and three air-melted heats with C + N > 0.09 pet and 18 pet Cr, 18 pct Cr-2 pet Mo, or 18 pet Cr-2 pet Mo-0.5 pct Ti. The steels were heated at 600° (316°), 700° (371°), 800° (427°), 900° (482°), and 1000°F (538°C) for various times up to 4800 h and the influence of this aging was investigated by hardness measurements, impact tests, and electron metallography. It was demonstrated that the embrittlement due to 885°F (475°C) exposure was caused by precipitation of a chromium-rich α^’ phase on dislocations. The nucleation rate of α^’ was calculated with the aid of Becker’s theory and the results were used to extrapolate experimental data obtained in this study. After an exposure of about 1000 h at 1000°F (538°C), a decrease in room temperature toughness was observed for all steels investigated. The decrease in toughness was not caused by immobilization of dislocations by α^’, but by precipitation of carbonitrides.     

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

In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which cointain 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.     

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.     

Influence of strain-induced martensitic transformations on fatigue crack growth rates in stainless steels

The fatigue crack growth rates (FCGR) of two unstable austenitic stainless steels (Fe-16 Cr-13Ni) and (Fe-18Cr-6.5Ni-0.19C) were determined in theM_s-M_d temperature range where a strain induced μ → α′ martensitic transformation occurs near the crack tip. These FCGR were compared to the rates measured in the stable austenitic phase of a Fe-31.5Ni and a Fe-34 Ni alloy and in the martensitic phase obtained by quenching the Fe-31.5 Ni alloy below M_s. In the Fe-31.5 Ni, the FCGR are an order of magnitude higher in the martensitic than in the austenitic structures for ΔK ≤ 40 ksi in. The FCGR of the stainless steels decrease markedly when the test temperature approachesM _s in theM _s - M_d range. The FCGR for the alloy Fe-18Cr-6.5 Ni-0.19 C in a warm-worked condition are consistently higher than for the same alloy in the annealed condition for ΔK ≤ 40 ksi √in.. The results are discussed in terms of the influence of phase structures, stacking fault energy and work hardening exponent on the FCGR.     

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.     

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.     

Carbide precipitation and high-temperature strength of hot-rolled high-strength,low-alloy steels containing Nb and Mo

The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass hot rolling and coiling at 650°C, 600°C, and 550°C. Microstructural analysis of the carbides has been performed using field-emission gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both Nb and Mo exhibited a higher strength at high temperatures (∼600 °C) in comparison to the steel containing only Nb. The addition of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides had one variant of the B-N relationship ((100)_MC//(100)_ferrite, [011]_MC//[010]_ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus, believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles.