钢中残余元素及其对钢性能的影响

简要地论 钢中残余元素的行为及其对钢性能的影响,主要内容有钢中残余元素的来源,残余元素在铸锭过程中的凝固偏析和偏处理时的晶界偏析,残余元素在热加工过程中表面热脆现象中的作用,残余元素在钢的第二类回火脆性现象中的作用,残余元素对钢材耐蚀性能,应变时效行为的影响,以及抑制晶粒长大的作用等,最后提出了纯净钢 我国钢铁工业面临的重大项目。     

分步冷却过程中磷的非平衡晶界偏聚

研究了分步冷却过程中12Cr1MoV钢磷的非平衡晶界偏聚。从理论上计算了磷的晶界偏聚等效时间和晶界偏聚量，并用实验验证了根据动力学计算得出的磷的晶界偏聚浓度。试验结果与理论计算值相符．可为预测钢在实际服役状况下的晶界脆性提供依据。     

淬火钢高温回火时稀土、磷、锰等元素的晶界偏聚

文章初步研究了稀土和砱在淬火钢回火时在原奥氐体晶界的偏聚。许多研究者都发现在Ni—Cr—P钢中,伴随着高温回火脆性的发展,发生砱在原奥氐体晶界的(?)完全,同时还发生镍随着砱的突集而在原奥氐体晶界突集。在Mn—Sb钢和Ni—Cr—Sb钢中也都发现合金元素锰和杂质元素锑的偏聚,并分析了合金元素和杂质元素的相互作用。本文在原来的研究基础上进一步测定了合金元素稀土.锰和杂质元素砱在DZ—40钢和重轨钢中淬火后高温回火时在原奥氐体晶界的偏聚。     

加钛的镍铬钢回火脆性的锑非平衡晶界偏聚机理

回火脆性的溶质非平衡晶界偏聚机理的本质是溶质非平衡晶界偏聚的临界时间引起回火脆性的临界时间.在此基础上,研究了加钛的镍铬钢回火脆性的临界时间、回火脆性的极大值和充分回火后2.7J脆-韧转变温度随回火温度的变化,并结合锑的非平衡晶界偏聚量的测量结果,论述了加钛的镍铬钢中回火脆性的锑的非平衡晶界偏聚机理.     

钢中元素偏聚的研究现状及其发展趋势

 固溶状态下，钢中合金元素会在晶界、缺陷等处优先出现偏聚现象，对钢材的组织和性能产生显著影响。由于元素在钢中的偏聚量小，不易观察，而且影响偏聚的因素很多，导致钢中元素偏聚的理论研究进展缓慢。为了揭示不同元素在钢中的偏聚机理，充分发挥钢中元素偏聚的积极作用，从偏聚类型出发，总结分析了元素偏聚的影响因素和控制手段，重点介绍了几种常见元素的偏聚现象，深入分析其偏聚量及位置对钢材性能的影响，同时还介绍了几种观察元素偏聚的手段。期望能为揭示钢中偏聚行为、发挥其作用提供参考，也为后续偏聚行为的研究提供一些参考。     

Gleeble模拟热轧工艺对磷非平衡晶界偏聚的影响

 采用Gleeble 3500热模拟试验机研究了轧制和卷取温度下磷元素晶界偏聚与热轧板高温热塑性的关联机制。研究结果表明,含磷钢中磷元素晶界偏聚符合空位-磷原子复合体非平衡共偏聚理论,800 ℃以上轧制温度对磷元素晶界偏聚引起的钢板脆性影响较小,而在500～650 ℃温度范围存在磷元素偏聚导致的热塑性低谷区,且在该温度区施加1%拉伸预应变将加剧磷元素晶界偏聚和钢板脆性。因此,本试验成分含磷钢热轧生产中卷取温度设定应避免550 ℃塑性低谷。     

X120管线钢回火过程中断口分离问题研究

针对一种高强度X120管线钢高温回火条件下出现的夏比冲击试样断口分离现象。利用扫描电镜、透射电镜及冲击试验机等,从磷、硼、锰等元素的偏析,晶界处第二相如碳化物、合金析出物等的析出及长大角度进行讨论分析。研究结果表明：随回火温度升高,断口分离加剧,分离平均长度及数量增加。夏比冲击功降幅达60％;断口分离先于主断口形成,导致试验钢表面能增加,刚度、主断口裂纹形成功及裂纹扩展功下降;磷、硼、锰等元素在晶界偏聚引起的回火脆性导致分离裂纹产生,夹杂物对分离起促进作用,尺寸较大的第二相弱化了晶界强度。     

磷在钢回火脆性发展时的非平衡偏聚及钼的影响

用俄歇能谱仪和扫描电镜研究了中碳钢和含钼中碳钢在回火脆性发展过程中Ｐ在晶界上的偏聚。发现在４８０ ℃回火时,两种钢均在晶界出现Ｐ的浓度峰值。含钼中碳钢中的Ｍｏ 减小了晶界断裂比例,但并不改变Ｐ的浓度峰值。同时还观察了Ｓ、Ｃ、Ｍｏ 等组元在晶界的偏聚。Ｐ在晶界的浓度峰值可以用磷－空位复合体的非平衡偏聚机制加以解释。     

磷元素在钢中的晶界偏聚

磷元素在钢中的晶界偏聚可以分为两类,即平衡偏聚和非平衡偏聚.介绍了这两类偏聚现象以及偏聚理论.以磷对晶界结合力、晶界扩散和晶界能的影响机理为基础,分析了磷元素对钢力学性能的影响和磷在钢中发生晶界偏聚的原因.     

NiCrMoV转子钢回火脆性的研究(Ⅲ)——某些合金元素(锰、钼)对回火脆性敏感性的影响

本文就某些合金元素Mo和Mn加入到NiCrMoV转子钢中对回火脆性敏感性的影响进行了研究。当加入0.3％Mn以后,该钢的塑脆转变温度(FATT)在所有的时效时间都比不含Mn的相同钢大约提高了一倍。尽管Mn-P有某种共偏聚效果,但FATT的提高主要与Mn本身对晶界结合强度的固有影响以及P的脆化能力提高有关。 比较NiCr转子钢和NiCrMoV转子钢的杂质导致的脆性行为,根据FATT的测定就可清楚地看到Mo的有益贡献。试验结果显示了Mo的清扫剂作用,在体内Mo-P的交互作用而抑制了脆性,Mo与P在晶界的共偏聚从而降低了磷的脆化能力。本文还报告了在长时时效时碳化物中富集磷的试验结果。     

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     

Relationship Between Grain Boundary Segregation of Antimony and Temper Embrittlement in Titanium-Doped Nickel-Chromium Steel

The antimony segregation at grain boundary was observed and the temper embrittlement in titanium-doped nickel-chromium steel was analyzed. It is concluded that the antimony segregation at grain boundary is nonequilibium and the kinetics of temper embrittlement agrees well with those of nonequilibrium antimony segregation at grain boundary. Besides, the mechanism of nonequilibrium antimony segregation at grain boundary proved to be the most satisfactory one among the existing mechanisms to interpret the antimony-induced embrittlement kinetics in the nickel-chrominm steel. Based on these, the activation energy and frequency factor of diffusion of antimony-vacancy complexes were obtained according to the concept of critical time in nonequilibrium grain boundary segregation theory.     

Critical Time and Temper Embrittlement Isotherms

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Mechanism of Embrittlement and De-Embrittlement for 2.25Cr-1Mo Steel

 The constant embrittlement curve for constant segregation concentration on grain boundary of impurity element P and relationship between equilibrium grain boundary segregation concentration and operation time for 225Cr-1Mo steel were derived based on the theory of equilibrium grain boundary segregation. The mechanism of step-cooling test and mechanism of de-embrittlement for 225Cr-1Mo steel were explained. The segregation rate will increase but equilibrium grain boundary segregation concentration of impurity element P will decrease as temperature increases in the range of temper embrittlement temperature. There is one critical temperature of embrittlement corresponding to each embrittlement degree. When the further heat treating temperature is higher than critical temperature, the heat treating will become a de-embrittlement process; otherwise, it will be an embrittlement process. The critical temperature of embrittlement will shift to the direction of low temperature as further embrittlement. As a result, some stages of step-cooling test would change into a de-embrittlement process. The grain boundary desegregation function of impurity element P was deduced based on the theory of element diffusion, and the theoretical calculation and experimental results show that the further embrittlement or de-embrittlement mechanism can be interpreted qualitatively and quantitatively by combining the theory of equilibrium grain boundary segregation with constant embrittlement curve.     

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.     

Temper embrittlement in a 3-pct Cr-Mo turbine disc steel

The kinetics of isothermal temper embrittlement have been studied in a 3 pet Cr-Mo turbine disc steel at tempering temperatures from 450 to 580‡C. In the cast examined embrittlement results principally from the segregation of phosphorus to prior austenite grain boundaries and it is shown that the present results are consistent with an equilibrium segregation process. These data have been used to predict the degree of embrittlement resulting from continuous cooling from the tempering temperature and the results have been compared with laboratory tests for a wide range of cooling rates. The probable effect of post-tempering cooling rate on toughness and the feasibility of using retempering treatments as a means of rehabilitating temper embrittled components are also considered.      

Temper embrittlement in a 3-pct Cr-Mo turbine disc steel

The kinetics of isothermal temper embrittlement have been studied in a 3 pet Cr-Mo turbine disc steel at tempering temperatures from 450 to 580‡C. In the cast examined embrittlement results principally from the segregation of phosphorus to prior austenite grain boundaries and it is shown that the present results are consistent with an equilibrium segregation process. These data have been used to predict the degree of embrittlement resulting from continuous cooling from the tempering temperature and the results have been compared with laboratory tests for a wide range of cooling rates. The probable effect of post-tempering cooling rate on toughness and the feasibility of using retempering treatments as a means of rehabilitating temper embrittled components are also considered.     

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.     

Effect of prior austenitic grain boundary composition on temper brittleness in a Ni-Cr-Sb steel

Grain boundary segregation during temper embrittlement of an Sb-containing, Ni-Cr steel has been examined both by Auger electron analysis and by chemical analysis by neutron activation of residues of surface layers dissolved by etching intercrystalline fracture surfaces. No grain boundary segregation of either alloying additions or impurities was detected during austenitization or tempering. Redistribution of Cr, Ni, and Sb between carbide and ferrite was observed during tempering, but no grain boundary segregation was noted. Both Ni and Sb were observed to segregate to the boundaries during embrittling. The segregated Sb was shown to be uniformly distributed along the prior austenitic grain boundaries and to control the ductile brittle transition temperature of the alloy studied. Ni segregating to the prior austenitic boundaries during embrittling was shown to be localized in a phase other than the ferritic portions of the boundaries. A possible location was shown to be the ferritecarbide interfaces in the grain boundaries. Weakening of these normally tenacious carbide and ferrite interfaces could account for the change in mode of brittle failure from transcrystalline cleavage to intercrystalline along the prior austenitic grain boundaries that is observed in temper brittle steels.