The effect of an intercritical heat treatment on temper embrittlement of a Ni-Cr-Mo-V rotor steel

The effect of an intercritical heat treatment on tempor embrittlement has been investigated for a rotor steel containing 0.25 pct C, 3.5 pct Ni, 1.7 pct Cr, 0.5 pct Mo, 0.1 pct V, and deliberate additions of phosphorus, tin, or antimony. Both martensitic and bainitic steels were held at the intercritical temperature of 1380°F (750°C) for times up to 40 h and were then quenched or cooled to obtain martensitic or bainitic transformation. The steels were then tempered, followed by water quenching or step cooling from the tempering temperature. The residual ferrite maintained a fine plate-like shape even after 40 h at the intercritical temperature. Embrittlement induced by step cooling from the final tempering was mark edly reduced by the intercritical treatment as compared to the embrittlement observed after conventional heat treatment; for example, AFATT, the increase in the Charpy V-notch 50 pct shear fracture transition temperature caused by step cooling, was reduced by at least 80°F (45°C) as a result of the intercritical treatment of steels containing 0.02 pct P. Molybdenum effectively reduced AFATT in intercritlcally heat-treated steels as well as in conventionally treated steels. Possible mechanisms for reducing temper embrittlement with the intercritical treatment are discussed.     

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.     

Embrittling characteristics of Ni-Cr-Mo-V steels designed to exhibit low susceptibility to temper embrittlement in large steam turbine rotor applications

Test steels containing 0.25 pct C, 1.0 pct Ni, 3.0 to 4.5 pct Cr, 0.8 to 2.0 pct Mo, 0.12 pct V and two levels of such impurities as phosphorus, tin and antimony, quenched and tempered to a 825 MPa (120 ksi) minimum yield strength level, have been examined for temper-embrittlement susceptibility. The susceptibility is influenced by a combination of chromium and molybdenum contents rather than by contents of individual elements. The susceptibility in steels with 3 pct Cr-0.8 pct Mo and 4.5 pct Cr-0.8 to 1.6 pct Mo was significantly lower than that of a 3.5Ni-1.7Cr-0.5Mo-0.1V steel at the same impurity level.     

Effects of chromium on the temper embrittlement and wear resistance of 0.25% C low-alloy cast steel

The effects of additions of 0.6 to 2.0% Cr on the temper embrittlement behaviour of 0.25 C–1.0 Si–1.3 Mn cast steel under several hardening conditions were studied. The susceptibility to temper embrittlement, transgranular and intergranular fracture were increased as the chromium content increased when the steels were tempered at 350°C and slowly cooled from 550°C. The impact toughness and abrasion resistance of the steels were found to depend to a great extent on the Cr-content and tempering temperature.     

Temper embrittlement of Ni-Cr Steels by phosphorus

Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann. Formerly a Research Fellow at the Department of Materials Science, University of Pennsylvania, Philadelphia, PA 19174.     

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.     

作用应力对2.25Cr-1Mo合金钢回火脆性的影响

在146.7MPa的作用应力下,对加氢反应器材料2.25Cr-1Mo合金钢(%:0.15C、2.32Cr、0.95Mo、0.011S、0.009P、0.0068As、0.0035Sb、0.0079Sn、0.01V)进行468℃125h和400h的等温回火脆化试验。根据加氢反应器母材试块脱脆、脆化和应力作用3种状态冲击功和温度关系曲线,得出各状态回火脆性转变温度VTr54.2(℃)值和回火脆化度ΔVTr54.2(℃)。结果表明,温度和等温时间是导致材料回火脆化的主要因素,作用应力对2.25Cr-1Mo钢回火脆性的影响不显著。     

Phase transformational kinetics and hardenability of low-carbon,boron-treated steels

The phase transformations and hardenability of 0.1 pct C boron-treated and boron-free steels containing Mn, Cr, Ni, or Cr plus Ni, and up to 1 pct Mo were studied. Continuous cooling transformation diagrams, hardenability characteristics, and diagrams of the ferrite start half-cooling time vs alloying were established. An unalloyed 0.1 pct C steel transforms diffusionally in the ferritic-pearlitic range when cooled from an austenitizing temperature, with a negligible contribution of the intermediate (bainitic) transformation occurring at very high rates of cooling. Molybdenum extends the range of the bainitic transformation and markedly delays the decomposition of austenite in the ferritic-pearlitic range. Boron treatment of the unalloyed (molybdenum-free) 0.1 pct C steel permits bainite formation over a wider range of fast cooling programs. At lower rates of cooling, the steel transforms diffusionally into ferrite and pearlite . Alloying additions of Mn, Cr, or Ni result in a slightly higher proportion of the bainitic transformation, which may occur over a wider range of cooling programs. When both nickel and chromium are present, a modest synergistic effect on the delay of the ferritic-pearlitic transformation may be noted. Introduction of molybdenum into all of the boron-treated 0.1 pct C steels strongly delays the decomposition of austenite into ferrite-pearlite structures and vastly expands the range of cooling programs that result in the formation of bainitic structures. In this important action, molybdenum is assisted to a smaller degree by alloying additions of manganese and chromium, and to a greater degree by nickel and chromium plus nickel. In all the steels studied, the alloying elements lower the temperatures of the bainitic transformation, thereby explaining, at least partly, the somewhat higher hardness for any specified cooling program. The observed beneficial effects of boron, molybdenum, and other alloying elements on the phase transformational behavior on continuous cooling are reflected in terms of higher hardenability.     

中温临氢钢12Cr2Mo1R回火脆性研究

对正火回火状态的中温临氢钢12Cr2Mo1R进行模拟最小程度焊后热处理试验,采用阶梯冷却加速脆化法,进行了回火脆化处理,并对12Cr2Mo1R钢的回火脆化敏感性进行了研究。试验研究结果表明:12Cr2Mo1R具有良好的抗回火脆化性能,该钢在阶梯冷却试验前后的组织形貌无明显变化。     

Aging embrittlement and grain boundary

“Clean” 3.5NiCrMoV steels with limited contents in trace elements (P, Sn, As, Sb) are commonly provided for manufacturing big rotor shafts. The possible increase in temperature in future steam turbines has promoted the development of “superclean” steels characterized by an extra drastic decrease of manganese and silicon contents. Their higher cost in comparison to “clean” steels leads to concern above which temperature they must be considered as mandatory for resisting aging embrittlement in operation. 3.5NiCrMoV “clean” steel samples (Mn = 0.30 pct; Si = 0.10 pct) were aged at 300 °C, 350 °C, and 400 °C for 10,000 hours up to 30,000 hours. No embrittlement results from aging at 300 °C and 350 °C, but holding at 400 °C is highly detrimental. Auger spectroscopy confirms that, when aging at 400 °C, phosphorus is the main embrittling trace element. It is suggested that grain boundary embrittlement is associated with the building of a layer that contains, on the one hand, Ni and P and, on the other hand, Mo and Cr. Head of the Testing and Head of the Testing and Head of the Testing and     

Temper embrittlement of Ni-Cr steel by Sn

Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr steels containing 0.2 and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens. The ductile-brittle transition temperature varied linearly with Sn concentration on grain boundaries at a rate which increased with hardness of the steel. The rate of approach to the steady state level of embrittlement was faster in the higher C steel. The grain boundary concentrations of Ni and Sn bear a unique relationship in a manner analogous to the behavior found previously in Sb-doped steels. The potency of Sn as an embrittling element is somewhat less than that of Sb, but considerably greater than that of P. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia,PA Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with the Research Staff, LRSM, University of Pennsylvania     

氮合金化对马氏体不锈钢00Cr13Ni4Mo回火性能的影响

测定了控氮00Cr13Ni4Mo(S13-4N)与低氮00Cr13Ni6Mo(S13-6)的性能,对比分析了氮元素对00Cr13Ni4Mo的强度、韧性、耐蚀性的影响。力学性能测试结果表明,回火温度≤550℃时,控氮的S13-4N比低氮的S13-6的强度更高,韧性更低;回火温度≥550℃时,规律逐渐变得相反;S13-4N在450℃左右出现回火脆性现象。电化学测试结果表明,S13-6的耐点蚀性能优于S13-4N。XRD和EBSD结果表明,两种钢在550℃以上温度回火时出现逆变奥氏体,600℃附近含量达到最大值,此时的韧性最佳,S13-6中的逆变奥氏体总量多于S13-4N,但形态不同。分析认为:氮比镍稳定奥氏体的能力强,而镍形成逆变奥氏体的能力则更强,逆变奥氏体的含量和稳定性对韧性的影响很大;S13-4N的450℃回火脆性主要是由于碳化物和氮化物的析出引起的。     

碳含量对Mn-B钢氢致延迟断裂行为的影响

采用电化学阴极充氢、氢热分析(TDS)和慢应变速率拉伸等试验方法,研究了4种不同碳含量Mn-B钢经不同热处理制度处理后的氢致延迟断裂行为。结果表明,在低于400℃回火时,随着碳含量的增加,试验钢的氢脆敏感性升高,当碳的质量分数高于0.3%后,试验钢的氢脆敏感性几乎不再增加;碳含量一定时,试验钢的氢脆敏感性随回火温度的升高而降低,且以20MnB试验钢的降低趋势最为明显;当回火温度达到600℃时,各试验钢对氢几乎不再敏感;TDS分析表明,试验钢充氢后的氢含量明显增加,其中以可扩散性氢量的增加为主;随碳含量的增加,试验钢充入的氢量增加;当碳含量一定时,随回火温度的升高,试验钢充入的氢量减少;SEM断口观察表明,试验钢充氢后的脆性断裂倾向性增加;随着碳含量的升高,试验钢的断裂方式由韧性断裂向脆性断裂转变;碳含量一定时,随回火温度的升高,试验钢由淬火态的脆性断裂向高温回火态的韧性断裂转变。     

热处理对1Cr12Ni3Mo2VN马氏体耐热钢组织和性能的影响

试验1Cr12Ni3Mo2VN钢（/%:0.13C,0.16Si,0.70Mn,11.42Cr,2.78Ni,1.67Mo,0.30V,0.0360N）的冶金流程为30t EAF-LF-VD-3t ESR-锻造成Φ350mm材。研究了950~1100℃淬火和200~700℃回火对1Cr12Ni3Mo2VN钢组织与性能的影响以及500℃,500~10000h时效的拉伸性能。结果表明,淬火温度950~1100℃对1Cr12Ni3Mo2VN钢力学性能的影响不明显;该钢的回火脆性区在600℃左右,但对钢的塑性的影响较小。经1040℃淬火、540℃回火的1Cr12Ni3Mo2VN钢,在500℃时效500h后,其抗拉强度和屈服强度分别下降了7.7%和5.8%,时效10000h后,其抗拉强度和屈服强度分别下降了13.4%和14.6%,断面收缩率下降了40%,主要原因是杂质元素在晶界处偏聚以及碳化物在晶界处析出。     

0Cr17Ni钢回火脆性机理分析

 对淬火后不同温度回火的0Cr17Ni不锈钢进行了冲击试验,发现该材料存在较严重的回火脆性。通过透射电镜（TEM）、扫描电镜（SEM）、X射线衍射仪（XRD）和俄歇电子谱仪（AES）的观察分析,研究了0Cr17Ni不锈钢产生回火脆性的原因。结果表明：该材料发生高温回火脆性的断裂属于解理与准解理复合型断裂,而并非是典型的沿晶断裂,导致回火脆性断裂的主要原因是回火过程中出现的针状碳化物。     

钒微合金化低碳贝氏体钢的连续冷却相变特性

 采用热膨胀法测定6种不同成分低碳贝氏体钢的连续冷却转变(CCT)曲线。CCT曲线表明,加入微量硼能使含钒低碳贝氏体钢在大于03℃/s的冷速下获得贝氏体组织,而V-N微合金化的低碳贝氏体获得全贝氏体的临界冷速要高于V-B钢,且贝氏体转变的开始温度也要较V-B钢高20℃左右。在含钒、氮低碳贝氏体钢中加入钼、铬将会促进钢的贝氏体相变,但钼的作用要优于铬;钼、铬的加入可使含钒、氮低碳贝氏体钢的贝氏体转变温度降低至少30℃,且贝氏体组织得到了细化,钢的维氏硬度也提高了HV10~30。     

合金元素对Cr-Mo钢第二类回火脆性影响研究综述

对于一些长期在中温条件下服役的Cr-Mo钢(2.25Cr1Mo),由于钢中P、S等有害杂质元素在服役过程中不断向晶界偏聚,降低了晶界结合力,致使该类钢出现第二类回火脆性这一关键问题.为了探寻降低这类Cr-Mo钢第二类回火脆性倾向的有效方法,提高材料安全服役可靠性,从溶质元素偏聚机制出发,总结分析了合金元素晶界偏聚及改善...     

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.     

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     

1Cr12Ni3Mo2VN耐热钢的回火工艺

利用金相显微镜、扫描电镜和透射电镜等研究了1Cr12Ni3Mo2VN耐热钢的回火工艺,结果指出试验钢产生第一类回火脆性的主要原因是马氏体板条界存在聚集长大的Fe_3C及M_3C脆性相,导致冲击韧性显著下降。Mo_2C与基体处于共格状态,使基体周围晶格产生很大的静畸变是次要原因;产生第二类回火脆性的原因,一是由于碳化物M_(23)C_6沿原奥氏体晶界和马氏体板条界迅速聚集并粗化,二是板条间残余奥氏体膜因碳贫化而发生热失稳分解。结合技术协议要求,为了有利于组织的稳定性,本试验钢的最佳回火工艺为580℃×2h空冷。