Grain boundary segregation in a low alloy steel under tensile loading conditions

The influence of tensile stress on the grain boundary segregation behaviour in a NiCrMoV steel is elucidated through the measurement of grain boundary segregation isotherms and the determination of interaction processes amongst trace and alloying elements. The application of tensile stress accelerates and enhances the grain boundary coverage of elements P, N and S presumably through a stress-induced-diffusion process. The qualitative nature of the interaction processes which are operative in the absence of any applied stress remain unaltered on the application of the tensile stress. However, there is an enhancement of the interaction processes, in the presence of the applied stress; site-competitive, P + S, interaction process is particularly activated in the presence of a tensile stress.     

On interactions amongst trace and alloying elements at the grain boundaries of a low alloy steel

Interactions amongst segregants at the grain boundaries of a low alloy steel have been studied using the technique of Auger electron spectroscopy in the temperature range of 750–900 K. Segregation isotherms delineate three kinetically distinct regimes. The first of these is characterized by rapid cosegregation of Cr and N with desegregation of C followed by segregation of V which occurs at relatively higher temperatures. The second stage involves rapid segregation of P which initially attains a maximum coverage and then subsequently decreases with time. The third regime noted at higher temperatures shows slow segregation of S, Sn and Sb. This regime, however, exhibits high sensitivity of site competition to temperature.     

Effects of microstructure on grain boundary segregation processes in low alloy steels

Grain boundary segregation isotherms for trace and alloying elements in a low alloy steel have been investigated in the temperature range of 700–900 K using the technique of Auger electron spectroscopy. Segregation isotherms dilineate a number of cooperative and site-competitive interaction processes which occur in different kinetic regimes and additionally enable grain boundary interaction processes to be depicted in a concise form in terms of interaction maps. An examination of the interaction maps in association with interaction severity (extent of segregation)-temperature plots allows the effect of microstructure on the operating interaction processes to emerge. Interaction severity-temperature plots also unfold the influence of emerging high temperature interaction processes on the processes occurring at low temperatures and give an insight on the relative magnitudes of the operative interaction processes at different temperatures. The results indicate that the degree of interaction (interaction severity) follows the sequence, martensite > bainite > ferrite-pearlite.     

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...     

低合金钢残余元素Cu-As-Sn的晶界偏析对晶间脆性断裂的影响

用Gleeble-1500热模拟机、扫描电镜和俄歇能谱仪研究了0.10Cu-0.07As-0.05Sn对低合金钢(%:0.15C、0.36Si、1.40Mn、0.003S、0.019P)高温延塑性的影响,凝固过程中的偏析和热处理过程的晶界偏析。结果表明,Cu-As-Sn在晶界的偏析明显加剧第Ⅲ脆性凹槽区的深度和宽度,提高该区上限临界温度;连铸坯不存在明显的Cu-As-Sn晶界偏析,850℃拉伸至屈服的试样和热轧板试样存在明显的Cu-As-Sn晶界偏析,说明热变形加剧残余元素的晶界偏析。     

Pb在GH871合金晶界的偏聚

利用PERKIN-ELMERPHI595型俄歇谱仪研究了新型含铌铁基高温合金GH871的晶界上各元素的含量。结果表明Pb含量在5×10     

NiCrMoV钢中铌和锡的非平衡晶界共偏聚

 结合偏聚理论的最新进展,对R.D.K.Misra等认为Ni和Sn的晶界偏聚动力学曲线相互平行,而且2种元素可同时达到一个晶界浓度极大值的实验结果重新进行了分析。分析结果证实,Ni和Sn在恒温过程中发生的是一非平衡晶界共偏聚现象,Ni的非平衡偏聚是由Sn的非平衡偏聚引起的。     

高强IF钢第二相粒子的应变诱导析出行为

在 Gleeble- 3500型热模拟试验机上,利用应力松弛试验研究了含磷高强IF钢第二相粒子的析出行为。结果表明,微合金元素析出钉扎住了位错与晶界,导致应力松弛曲线呈现出3个阶段的特征。试验用钢的PTT曲线呈现典型的“C”曲线形状,最快析出的鼻子点温度约为850 ℃,在此温度下,第二相粒子析出开始时间与结束时间分别为14和246 s,随着弛豫时间的增加,第二相粒子的析出数量逐渐增多,形貌逐渐粗化,析出粒子为Ti4C2S2 和TiC,主要呈圆形颗粒形貌。由于试验钢种采用磷强化,在析出的第二相粒子中存在棒形和长条形FeTiP,同时由于磷的晶界偏聚,所以FeTiP同样存在晶界析出的规律。     

Effects of Thermal History and Microstructure on Segregation of Phosphorus and Alloying Elements in the Heat-Affected Zone of a Low Alloy Steel

The grain boundary segregation of phosphorus and alloying elements in the heat-affected zone (HAZ) of a low alloy steel was studied quantitatively with atom probe tomography. Non-equilibrium segregation mainly occurred during welding and subsequent fast cooling, leading to remarkable segregation of P, C, Mn, and Mo. The segregation of these four types of solutes showed similar microstructure-dependence at this stage, in which the segregation levels are higher in coarse-grained HAZ and intercritically reheated coarse-grained HAZ than in fine-grained HAZ. After simulated aging, P and Mn showed further enrichment at grain boundaries through equilibrium segregation, while desegregation was observed for C and Mo. In addition, it seems that precipitation of Mo at dislocations was greatly promoted during aging, which probably also contributed to the increase of P and Mn at grain boundaries.     

Nonequilibrium grain-boundary cosegregation of nitrogen and chromium in NiCrMoV steel

It is concluded in this article that nonequilibrium grain-boundary cosegregation (NCGS) of nitrogen and chromium occurs in NiCrMoV steel. That conclusion is reached from experimental observations of the parallel segregation isotherms and the maximum coverage of Cr and N at grain boundaries during the isotherms. This means that the nonequilibrium segregation of Cr induces that of N, in NiCrMoV steel.     

低作用应力引起的溶质在晶界非平衡偏聚或贫化

提出了关于应力时效（高温及作用应力）引起溶质原子的晶界非平衡偏聚或贫化机理的一个新模型。此模型基于如下假设：晶界相对于完整晶体而言在强度上是弱化区。当多晶体受到一个在弹性范围内的作用应力时,晶界优先变形。若压应力作用时,晶界作为激发射空位;若拉应力作用时,晶界作为阱吸收空位,应力引起的过饱和空位与溶质原子形成复合体,此复合体在基体中的扩散速率远高于溶质原子。作用应力对晶界溶质偏聚或贫化的影响,是由复合体的扩散和溶质原子的反向扩散之间的平衡所决定。根据此模型,应力时效过程中存在一个临界时间,在此时间,应力引起的晶界偏聚或贫化程度将达到一个极大值。此模型已由ShinodaT等所作的磷在钢中的应力时效实验和MisraR D K所作的硫在钢中的应力时效实验结果所证实。     

不锈钢热塑性降低的非平衡偏聚机理

Mintz于1997年报道了一个有趣的试验现象:随着拉伸应变速率的增加,奥氏体钢韧性降低,铁素体钢韧性反而提高。其机制未得到解释。通过计算试验钢中P原子的非平衡晶界偏聚临界时间,结果发现奥氏体钢拉伸前热过程的等效时间短于其临界时间,而铁素体钢的等效时间长于其临界时间。由于奥氏体钢和铁素体钢分别在850和800℃等效时间最接近临界时间,韧性最低,即试验钢的热塑性降低都是由于非平衡晶界偏聚的临界时间造成的。应变速率降低,弹性应力作用时间增加。晶界偏聚量改变,热塑性降低的程度也随之改变。即热塑性降低的程度随应变速率的改变是由应力引起的非平衡晶界偏聚决定。     

2.25Cr-1Mo合金钢回火脆化过时效现象的研究

研究了2.25Cr-1Mo合金钢在回火温度下时效处理时,发生回火脆化和回火脆化的过时效反偏聚现象。应用非平衡偏聚的动力学理论,确定了成分(%)为0.15C、2.32Cr、0.95Mo、0.009P的2.25Cr-1Mo合金钢在650℃140 h回火时钢中磷的非平衡晶界偏聚规律。试验得出,2.25Cr-1Mo合金钢650℃磷非平衡偏聚的临界时间t_c为20 h,晶界磷的原子浓度,由初始状态的0.016%提高至最大值2.79%。根据试验数据进行的动力学计算,得出的计算曲线与试验结果吻合,验证了非平衡晶界偏聚空位-复合体模型的扩散机制。     

Effect of vanadium on grain boundary segregation of phosphorus in low alloy steels

The phosphorus grain boundary segregation at 853 K was investigated in three low alloy steels with different vanadium content. Kinetic dependence of the phosphorus grain boundary concentration was determined experimentally by means of AES and described theoretically, as well. To assess the influence of the individual alloying elements on the phosphorus segregation, the metal composition of carbide phases at 853 K was predicted by means of thermodynamic calculations and confirmed by experimental measurements (TEM + EDX). The vanadium was found to enhance the phosphorus grain boundary segregation by reducing the amount of dissolved and segregated carbon. Thereby, the equilibrium of mutual displacement C (segregated) ? P (segregated) was shifted to more phosphorus segregation. The results achieved indicate that vanadium indirectly increases sensitivity of low alloy steels to intergranular embrittlement.     

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.     

Effects of boron on the creep properties,phosphorus segregation behaviour,and stress relief cracking susceptibility of 1.5% Cr–0.5% Mo steels

The effects of 25 ppm boron and of 0.05% phosphorus on the creep behaviour at 550 °C and on the fracture temperature in the constant load fracture test were tested for a 1.5% Cr–0.5% Mo steel. B and P decrease the creep strength, the rupture elongation is increased by B. B and P lower stress relief cracking susceptibility, however, it is increased at low stress. In the B doped material the rate of P grain boundary segregation is accelerated and the level of equilibrium segregation is somewhat higher, the equilibrium segregation is somewhat lower in the bainitic than in the martensitic structure.     

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.     

Auger electron spectroscopy study of grain boundary segregation in alloy K-500: Part I. Behavior in as-processed state

Increased interest has been paid to grain boundary segregation in alloy K-500 due to severe intergranular cracking recently observed in forged bars. However, little systematic study of this segregation has been performed so far. A detailed auger electron spectroscopy (AES) study of grain boundary segregation in alloy K-500 has been carried out as a function of alloy chemistry. To determine C segregation, the C and O contamination rates in a vacuum chamber were measured and the necessary condition for C grain boundary segregation determination was established. It has been found that severe C, Al, and Cu segregation to grain boundaries occurred and depended on alloy chemistry. High bulk Ni and low bulk Al promoted C and Al grain boundary segregation, and low bulk Ni and high bulk Al significantly enhanced Cu segregation to grain boundaries. The depth profiles of intergranularly segregated elements also showed different features for high and low Ni content alloys. In high Ni alloys, C and Al levels dropped continuously as a function of distance from the grain boundaries but the Cu level dropped only slightly. In low Ni alloys, the Al and C levels rose from relatively low grain boundary levels to a peak at a certain distance from the grain boundary where the high grain boundary Cu level dramatically dropped. Transmission electron microscope (TEM) observation revealed a grain boundaryγ′-depleted zone followed by a region with coarser and denserγ′ particles in low Ni and high Al alloys but quite uniformly distributedγ′ particles with no depleted zone in high Ni and low Al alloys. These can be explained by the observed segregation behavior. The occurrence of Cu segregation is explained according to available theories about surface segregation in binary Ni-Cu alloys, and the segregation of C and Al to grain boundaries is suggested to be probably due to their interaction with Ni and Cu.     

Carbide Evolution in Temper Embrittled NiCrMoV Bainitic Steel

Phosphorus segregation to prior austenite grain boundaries in low alloy steel from exposure to temperatures of 300 to 600°C results in a susceptibility for intergranular fracture referred to as “temper embrittlement”. It has been observed that alloying steel with Mo greatly reduces the phosphorus segregation kinetics. Therefore changes in the ferrite matrix composition from carbide precipitation and evolution involving Mo can influence the segregation phenomenon and fracture properties. This study uses analytical electron microscopy of extraction replicas to characterize the changes in carbide chemistry of a NiCrMoV bainitic steel with 0.25 wt% C that accompany the phosphorus segregation during aging at 480°C for up to 3400 hr. The steel was doped with 0.02 wt% P and tempered at 650°C to two different hardness levels, i.e., two different initial carbide distributions. The amount of grain boundary phosphorus segregation produced by aging at 480°C correlates with the level of molybdenum that remains in solution in the ferritic matrix whereas changes in vanadium and chromium appear to have less influence on the temper embrittlement.     

Effects of impurities in steels on mechanical properties and corrosion behaviour

In steels produced and utilized in the Fed. Rep. of Germany the elements P and Sn may occur as impurities. Both these elements tend to enrich (segregate) at grain boundaries. The equilibria of grain boundary segregation in iron and the effects of alloying elements have been studied for P and Sn by Auger-electron-spectroscopy and were thermodynamically described. For a 3.5% NiCrMoV-turbine steel the grain boundary segregation of P and its effect on ductility have been studied in detail, with the results that the long-term embrittlement of this steel during application at temperatures around 400°C can be predicted and the maximum bulk concentration of P can be given. The effect of Sn on the creep of a 1% CrMoNiV steel at 550°C has been investigated, Sn favours cavity nucleation and growth, therefore tertiary creep starts earlier and premature failure occurs with increasing Sn content. Therefore, the Sn content should be kept as low as possible in heat resistant steels. Since carbon also segregates to grain boundaries and can displace P and Sn if there is enough free C in a steel, plain carbon steels are not subjected to embrittlement by P and Sn. The susceptibility to intergranular stress corrosion cracking in nitrates and other electrolytes is somewhat enhanced by P, however, only in a restricted range of potentials. In the range of maximum susceptibility the impurities have no effect, all carbon steels are susceptible to IGSCC, independently of their purity. So stress corrosion cracking cannot be suppressed by diminishing the content of phosphorus – only by avoiding the critical corrosion conditions concerning electrolyte and potential.