Three-dimensional observations of proeutectoid cementite precipitates at short isothermal transformation times

Three-dimensional (3D) analysis techniques were used to examine a model Fe–1.34% C–13.0% Mn alloy to reveal connectivities of proeutectoid cementite precipitates whose growth was arrested at an early stage. The present results indicate that grain boundary cementite precipitates nucleate at austenite grain boundary edges and corners, then grow and spread on the grain boundary faces. No cementite precipitates were found to connect solely to austenite twin boundaries, which appear to act as barriers to precipitate growth rather than as potential nucleation sites. Cementite precipitates were all connected to austenite grain boundaries or cementite grain boundary precipitates, confirming 3D observations made earlier on a specimen transformed at a longer isothermal transformation time. Widmanstätten lath precipitates appear to emanate only from grain boundary cementite precipitates. While edges of several Widmanstätten plate precipitates were observed to intersect with areas of ‘clean’ austenite grain boundary, they may or may not nucleate directly on austenite grain boundaries.     

Crystallography of grain boundary cementite dendrites

The crystallography and morphology of proeutectoid grain boundary cementite precipitates were studied in an isothermally transformed Fe-1.3%C-12%Mn steel. Grain boundary cementite precipitates develop with complex variations of a morphology that can be described as fern-like dendrites growing preferentially within and along austenite grain boundaries. The dendritic morphology was made most readily apparent by deep etching specimens of the alloy whose growth had been arrested after relatively short times during the solid state austenite→cementite transformation. Trace analysis revealed that these precipitates usually have no crystallographically preferred primary or secondary dendrite arm growth directions. Electron Backscattered Diffraction pattern analysis showed that most of the grain boundary cementite precipitates approximated one of the known cementite–austenite crystallographic orientation relationships (OR) with at least one of its adjacent austenite grains. Occasionally, the grain boundary cementite exhibited an OR that achieved ‘best fit’ directions between the cementite and austenite in both adjacent austenite grains.     

钢中奥氏体晶界遗传性再探讨

作者用２０Ｃｒ２Ｎｉ４Ａ钢研究了晶界遗传现象和本质、再次处理前的热变形对晶界遗传的影响以及晶界传对力学性能的影响。研究表明，该钢存在典型的晶界遗传现象，其本质在于加热过程中于原晶界处形核的新相奥氏体分属于两个原奥氏体晶粒，并且不能跨越原晶界合并或长大。高温大变形能够彻底消除晶界遗传现象。遗传下来的粗大原晶界对材料的力学性能无特殊影响。     

20Cr2Ni4A钢中奥氏体晶界遗传现象

对２０Ｃｒ２Ｎｉ４Ａ钢经高温预淬处理获得的粗大奥氏体晶粒的晶界在再次加热后的遗传现象进行了研究。研究认为，奥氏体晶界遗传与奥氏体晶粒遗传在产生机理上是有所不同的；奥氏体晶界遗传是由于成分和夹杂物在晶界偏聚，而这种偏聚在随后的加热保温过程中仍大量保留下来并且阻碍晶粒穿越晶界长大或合并而形成的。研究表明，这种奥氏体晶界遗传受热变形的影响，热变形的程度越大，则晶界遗传现象越不明显。     

Prediction model of austenite growth and the role of MnS inclusions in non-quenched and tempered steel

The austenite growth behavior of non-quenched and tempered steels (casted by continuous casting and molding casting processes) was studied. The austenite grain size of steel B casted by continuous casting process is smaller than that of steel A casted by molding casting process at the same heating parameters. The abnormal austenite growth temperature of the steels A and B are 950 °C and 1000 °C, respectively. Based on the results, the models for the austenite grain growth below and above the abnormal austenite growth temperature of the investigated steels were established. The dispersedly distributed fine particles MnS in steel B is the key factor refining the austenite grain by pinning the migration of austenite grain boundary. The elongated inclusions MnS are ineffective in preventing the austenite grain growth at high heating temperature. For the non-quenched and tempered steel, the continuous casting process should be adopted and the inclusion MnS should be elliptical, smaller in size and distributed uniformly in order to refine the final microstructure and also improve the mechanical properties.     

Three-dimensional Monte Carlo simulation of discontinuous grain growth in HAZ of stainless steel during GTAW process

The Monte Carlo (MC) simulations of grain growth in the heat affected zone (HAZ) of titanium alloys and ultra fine austenite steels are widely used. In single-phase alloys, the pinning effect of the grain boundary precipitates is one of the important factors to influence grain growth. However, it has not been considered in most of the simulations. In this study, both thermal pinning and precipitates pinning effects were taken into account to simulate grain growth in HAZ of austenite stainless steel SUS316 during gas tungsten arc welding (GTAW) process. By using heat transfer and fluid flow model, the thermal process of welding that influences grain structure evolution is obtained as well. The results of the simulation agree with the experiment and demonstrate that the grains in HAZ grow discontinuously when a large amount of carbides lie on the grain boundary.     

Al对SWRH62A钢奥氏体晶粒长大的影响

采用高温光学显微镜研究了Al对SWRH62A钢在加热过程中奥氏体晶粒长大的影响。结果表明,添加0.015%~0.045%的Al可将SWRH62A钢在低于1050 ℃保温时的晶界激活能由474.14 kJ/mol提高至1175.73 kJ/mol,奥氏体晶粒长大速度由0.18 μm/℃降低至0.12 μm/℃,具有显著细化晶粒的作用。     

Influence of Heat Treatment on Cr18Mn18 High Nitrogen Steel Precipitating Behavior and Microstructure

The grain size and precipitate amount which are affected by heat treatment have significant impact on the properties of high nitrogen austenitic stainless steel. In this study, Cr18Mn18 high nitrogen steel sheet is employed to investigate the effects of precipitate on austenitic grain size. It can be seen that the lamella precipitates which are rich in nitrogen and chromium nucleate in the austenite grain boundary and grow inward into grain when aged at 800 ℃ through electron probe micro-analyzer. The transmission electron microscopy results demonstrate that the precipitate is Cr2N and its morphology are detected as ellipsoid-like with major axis of 100-300 nm and minor axis of 50-100 nm roughly. The experiment show that coarsen of the austenite grain is quite critical at 1000-1100 ℃. However, the samples which pre-precipitated at 800 ℃ for 240 min to obtain the most nitride precipitate exhibits much smaller grain size than the as-rolled samples after solid solution treated at 1000, 1050 and 1100 ℃ for 240 min. The results show that the nitride precipitates in the grain boundary can effectively pin the austenite grain boundary and inhibit the grain growth.     

Influence of tempering temperature on both the microstructural evolution and elemental distribution in AISI 4340 steels

In the present study, the influence of tempering temperature on the microstructural evolution and prior austenite grain boundary segregation of AISI 4340 steels was investigated by transmission electron microscope and atom probe. The transmission electron microscopy results showed a variation in the microstructure and the morphology of carbides with a change in tempering temperature. Additionally, the chemical compositions of the prior austenite grain boundaries and carbides were quantified by atom probe tomography. An increase in the tempering temperature led to a decrease in the amount of carbon segregated at the prior austenite grain boundary from 7.9 to 1.3 at.%. It was found that a higher tempering temperature can accelerate the diffusion of carbon from the prior austenite grain boundary into carbide. However, phosphorus atoms were segregated mainly at the prior austenite grain boundary in steel tempered at 400°C (up to 0.18 at.%). It was found that formation of film-like carbide and phosphorus segregation along the prior austenite grain boundary is the main cause of embrittlement in steel tempered at 400°C.     

15SiMn钢奥氏体等温相变的元胞自动机模拟

建立了15SiMn钢奥氏体一铁素体相变的二维元胞自动机介观模型.模拟结果显示,对于某一给定温度,铁索体的长大速度在相变过程中逐渐降低.当生长速度接近0时,奥氏体相中相界面处平衡碳的质量分数为0.523 95%.此外,模拟结果还显示,由于奥氏体晶界上的碳原子扩散和奥氏体-铁素体相界面移动都较奥氏体晶内更快,所以奥氏体晶界上铁素体晶粒形貌为椭圆形.     

低碳钢中晶界铁素体/原奥氏体界面对贝氏体转变的影响

采用电子背散射衍射 (EBSD) 研究了低碳Fe--C--Mn--Si钢中晶界铁素体/原奥氏体界面对贝氏体形核的影响. 通过两阶段等温热 处理, 获得了晶界铁素体和贝氏体的混合组织. 结合金相观察和取向测量, 发现晶界铁素体与贝氏铁素体之间的界面分为两种, 一种界面不清晰, 一种界面清晰. 分析表明, 在晶界铁素体/贝氏体界面不清晰一侧, 晶界铁素体与原奥氏体保持取向关系, 贝氏体在这类界面形 核生长, 且取向与晶界铁素体保持一致; 在晶界铁素体/贝氏体界面清晰一侧, 晶界铁素体与原奥氏体无取向关系, 且贝氏体与晶界铁素体之间取向差较大.     

等温淬火球铁(ADI)的微观组织与力学性能

ADI的显微组织由奥氏体加上针状铁素体的混合组织组成。其每一束针状铁素体由许多位相相同,厚度大约200 nm的薄铁素体片组成。其奥氏体有两种形态:一种是存在于针状铁素体之间的近似于等轴形的块状奥氏体;一种是存在于针状铁素体内的薄条形奥氏体。从晶粒尺寸数量级来说,针状铁素体的厚度约为200 nm,而铁素体内奥氏体的厚度仅为几到10 nm数量级。金属强化的几种主要方式,细晶强化、位错强化、晶界与亚结构强化、第二相强化、固溶强化等都在ADI得到了体现。正是由于ADI这种特有的微观组织使其具有了优越的力学性能。     

In situ observation of bainite to austenite transformation of microalloyed high strength steel during simulated welding cycle

A confocal scanning laser microscope, equipped with a high temperature stage, was used for in situ observation of the bainite to austenite transformation in a microalloyed high strength steel. Austenite grain growth was observed and measured directly from 1040°C to the peak temperature of 1340°C during the heating process, as well as the subsequent cooling down to 1040°C. The grain growth rate versus temperature was analysed. It was demonstrated that the austenite grains grew up simultaneously by means of grain boundary migration and ‘grain swallowing’ phenomena. The variation in grain growth rate was attributed to the presence of impeding precipitates, e.g. carbonitrides Nb,Ti(C,N). The work also showed that coarsening and/or dissolution of carbonitride precipitates, above a certain temperature, led to a fast grain growth. The in situ observation by confocal scanning laser microscope can provide valuable information on the austenite reformation and final microstructure of weldments.     

Crystallography and structural evolution during reverse transformation in an Fe–17Cr–0.5C tempered martensite

The mechanism and the crystallography of austenite and δ-ferrite formation from tempered martensite at temperatures of 900–1200°C have been studied by means of transmission electron microscopy in an Fe–17Cr–0.55C alloy. It was found that austenite nucleates within ferrite at low angle, high angle and twin-related lath boundaries as well as at high angle equiaxed grain boundaries in contact with M₂₃C₆ grain/lath boundary carbides. The austenite grains are in a cube–cube relationship with the M₂₃C₆ carbide particles and bear the Kurdjumov–Sachs orientation relationship with at least one of the adjacent ferrite grains. They are often in the Kurdjumov–Sachs relationship with both ferrite laths separated by a high angle boundary as far as the laths had formed from the same austenite. The {111}_A close packed plane of γ precipitate is parallel to the {110}_F plane most parallel to the grain boundary. The close packed planes of some austenite grains nucleating at the high angle lath boundaries are parallel to the close packed planes of both ferrite laths. These crystallographic features often result in a single variant of austenite orientation at a grain boundary. After nucleation, the austenite grains grow by the migration of both semicoherent and incoherent interfaces. These results demonstrate that a specific orientation relationship is preferred for the austenite nucleation, but is not necessary for the subsequent growth. The kinetics of austenite growth are controlled by chromium diffusion. The δ-ferrite particles precipitate at high temperatures as a non-equilibrium phase. No rational orientation relationship between δ-ferrite and retained austenite was found. The experimental results are discussed qualitatively in terms of the thermodynamic predictions using the software ThermoCalc, assuming local equilibrium at the moving interfaces.     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe C Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

钢中珠光体相变机制的研究进展

简要总结了钢中珠光体相变的机制，特别是珠光体形核的晶体学和长大的动力学。基于形核时的最小形核能和渗碳体与铁素体“合作”生长的要求，珠光体通常在奥氏体晶界形核并只向某一晶粒内生长，因而晶界珠光体与相邻晶粒存在特定取向关系而与生长进入的晶粒无特定取向关系。晶体学研究表明，珠光体形核的领先相与碳浓度及渗碳体与铁素体间的取向关系(分别为Pitsch-Petch、Bagaryatsky或lsaichev)无关。最近的研究发现，具Bl结构的非共格杂质粒子界面是过共析钢中晶内珠光体形核的有效位置，初步的结果表明，晶内珠光体形核的原因在于杂质粒子生长造成的局部区域碳贫化及低能的珠光体，杂质界面取代高能的奥氏体，杂质界面。特定过冷度下的层片间距与正向界面推移速率的关系是珠光体生长动力学的主要问题。基于珠光体与奥氏体间的无序界面假设和稳态扩散方程分别发展了以体扩散控制和以界面扩散控制为主的动力学理论。两种理论均与实验结果符合较好，但后者似更合理。最近的实验在珠光体中发现了生长台阶与结构台阶，表明生长动力学方程需要修正。     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe-C-Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

The theory and inhibition of abnormal grain growth in steels

Abnormal grain growth results from the presence of a few grains of sufficient size advantage to overcome the pinning forces of a particulate array. When grain growth inhibition fails, the growth process is abnormal, and extremely large grains are produced. This article reviews the theoretical background for grain growth and particle pinning, and particular attention is drawn to the important effects of particle distribution (random or grain boundary precipitation) and the grain size distribution prior to coarsening. Attempts to produce complete inhibition of austenite grain growth are described.     

奥氏体形变对先共析铁素体等温形成孕育期的影响SCIEI

将Feder相变孕育期表达式引入形变奥氏体的分解中,建立了奥氏体形变条件下先共析铁素体等温形成孕育期的计算方法,计算了奥氏体形变储能和奥氏体晶界能量变化对相变孕育期的影响,计算结果与实验所得规律一致。     

EFFECT OF AUSTENITE DEFORMATION ON ISOTHERMAL INCUBATION TIME OF PROEUTECTOID FERRITE NUCLEATION

A calculation method for the isothermal nucleation incubation time of pro-eutectoid ferriteformed from deformed austenite has been developed by introducing Feder's incubation timeexpression into the decomposition of deformed austenite.The effects of austenite deformationstored energy and austenite grain boundary energy change on the incubation time have beentheoretically calculated and the rule of calculated results is consistent with that ofexperiments,this confirms that austenite deformation stored energy and the change ofaustenite grain boundary energy are the natural effect of austenite deformation on ferritenucleation.