In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels

The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γtransformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles de- creased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.

Austenite grain growth of medium-carbon alloy steel with aluminum additions during heating process

In this study, the effects of heating temperature(850–1100°C) and holding time(30–150 min) on the grain growth behavior of austenite in medium-carbon alloy steel were investigated by conducting experiments. The abnormal grain growth and mixed grain structure phenomenon are explained using an equilibrium precipitation phase diagram calculated by Thermo-Calc software package. The Al N particles were observed by field-emission scanning electron microscopy(FESEM), and the amount of AlN precipitations was detected by electron probe microanalysis(EPMA). Based on the research results, it was found that the average grain size of austenite in the test steel increased continuously with the increase of temperature and holding time. Furthermore, the abnormal growth of austenite occurred in the test steel at 950°C, and the heating temperature affected the austenite grain size more significantly. In addition, the decline in the amount of AlN second-phase particle in the test steel, which weakened the "pinning" effect on austenite grain boundaries, resulted in abnormal growth and the development of mixed austenite grain structures. The prediction model for describing the austenite grain growth of medium-carbon alloy steel during heating was established by regression analysis of the experimental data, and the model was verified to be highly accurate.

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.

Austenite grain growth behavior of Q1030 high strength welded steel

The austenite grain growth behavior of Q1030 steel was studied under different heating conditions. The austenite grain size increases with the heating temperature and holding time increasing. Austenite grains grow in an exponential manner with rising heating tem- perature and in a parabolic manner with prolonging holding time. A mathematical model for describing the austenite grain growth behavior of Q 1030 steel was obtained on the basis of experimental results using regression analysis. When the heating temperatures lie between 1000 and 1100℃ at a certain holding time, abnormal grain growth appears, which causes mixed grains in Q1030 steel.

Investigation of the Grain Growth Evolution in the AISI 304H Austenitic Stainless Steel

The austenitic stainless steels usually present an excellent combination of corrosion resistance and mechanical properties such as ductility in the annealed condition and high yield strength after cold deformation. Solution annealing in the AISI 304H is recommended before deformation process in order to improve ductility. However, long annealing during solution annealing can cause GG （grain growth） or AGG （abnormal grain growth） in the AISI 304H. In these cases, ductility is strongly decreased. Therefore, GG or AGG must be avoided during solution annealing. In this article, grain growth during solution annealing of the AISI 304H samples was determined. Samples of the AISI 304H were annealed at 1,100 ℃ for solution-annealing times varying from 15 min to 180 min. The results show that AGG took place for samples annealed at 1,100 ℃ for 30 min. In this condition, grain size reached 70 ± 10 μm. After annealing solution at 1,100 ℃ for 180 min, grain size reached 120 ~ 20 μm. In summary, the results shown that solution annealing at 1,100 ℃ even for relatively short annealing promotes the prompt increase of the grain size.

Carbide dissolution and austenite grain growth behavior of a new ultrahigh-strength stainless steel

The isothermal grain growth behavior for a new ultrahigh-strength stainless steel (UHSSS) is investigated in temperature range from 900 to 1150 ℃ and holding time range from 0 to 20 min. In the temperature range from 1000 to 1050 ℃, a bimodal grain size distribution was induced by different austenite grain growth rates which resulted from the weakened pin-ning effect by the partial dissolution of M6C particles along austenite grain boundaries. Further raising heating temperatures, M6C particles almost dissolved and the bimodal grain size distribution phenomenon became weakened, indicating that the austenite grain coarsening temperature of the new UHSSS was close to 1050 ℃. According to the present experimental results, a pragmatic mathematical model based on the Arrhenius equations was developed to predict the austenite grain growth process, which elaborated the influence of heating temperature, holding time and initial grain size on the austenite grain growth. Predictions for the new UHSSS presented a good agreement with experimental results.

Effect of Ti on Austenite Grain Growth Behavior in High Carbon Steels

Austenite grain growth behavior of two high carbon steels was observed using Confocal l.aser Scanning Mi croscope （CLSM）. Apparent austenite grain sizes for different holding time under a series of temperatures were measured by employing linear intercept method. Experimental results showed that Ti bearing steel exhibited a much sluggish growth rate compared with Ti free counterpart, which was attributed to the pinning effect of Ti（C,N） nan oparticles with the size of 20 to 40 nm on austenite grain boundaries. Based on the research conducted by using Transmission Electron Microscope （TEM） observation and Thermo Calc calculation, Ti（C, N） was confirmed to be the dominant phase at elevated temperature. Some models were introduced to predict the grain sizes of both steels. By comparison, the results predicted by the modified Gladman equation are found to be closest to the experimental resuits, which could be employed to predict accurately the austenite grain growth of high carbon steels.

Isothermal Growth Kinetics of Ultra-fine Austenite Grains in a Nb-V-Ti Microalloyed Steel

Ultra-fine austenite grains with size of i-3 μm were prepared in a Nb-V-Ti steel through repetitive treatment of rapid heating and quenching. A model for the growth kinetics of these ultra-fine austenite grains was successfully created through successive 2 processes, and the activation energy Q for growth was estimated to be about 693.2 kJ/mol, which directly shows the inhibition effect of microalloy elements on the growth of ultra-fine austenite grains.

Growth behavior of ultrafine austenite grains in microalloyed steel

Ultrafine austenite gains （UFAGs） with size of 1-5 μm were prepared through repetitive treatment, four times, of rapid heating and quenching, and the growth behaviors of these UFACs during both the reheating and cooling stages were investigated. The results indicated that UFAGs without pinning particles appeared with significant coarsening when the reheating temperature reached 1000 ℃. Although coarsening still occurred in the cooling stage, the growth was obscured during the isothermal holding process at temperatures between 900 ℃ and At3.

Microstructure and Mechanical Properties of Gear Steels After High Temperature Carburization

High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are not suitable for high temperature carburization due to abnormal grain coarsening. The gear steel 20CrMnTiNb, which is microalloyed with 0. 048% Nb and 0. 038% Ti, has been compared with the gear steel 20CrMn in terms of microstructure in the case of hardened layer and in the core after carburizing at 1000 ℃ for 4 h and mechanical prop- erties after carburizing and pseudo-carburizing. The results indicate that the fine austenite grains exist in the carbu- rized case of 20CrMnTiNb steel, while there is abnormal coarsening and duplex grain structure in the case and core of steel 20CrMn. The average prior austenite grain sizes are 19.5 and 34.2 μm for the steels 20CrMnTiNb and 20CrMn, respectively. In addition, the mechanical properties of 20CrMnTiNb steel are superior to those of 20CrMn steel. In particular, the HV hardness of the former is higher than that of the latter by about 40--70 in the range of less than 0. 7 mm in depth. Therefore, the steel 20CrMnTiNb is suitable for high temperature carburization.

Effects of Quenching Process on Microstructure and Mechanical Properties of Low Carbon Nb-Ti Microalloyed Steel

The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high plasticity for building facilities. The effects of quenching process on microstructure and mechanical properties of tested steel were investigated. The results showed that prior austenite grain size, phase type and precipitation behavior of （ Nb, Ti） （ C, N） play important roles in mechanical properties of the steel. Through modified appropriately, the model of austenite grain growth during heating and holding is d^5.7778 = 5. 6478^5.7778 ＋ 7.04 × 10^22t^1.6136 exp（- 427. 15 ×10^3 /（RT））. The grain growth activation energy is Qg = 427. 15 kJ. During quenching, the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations. The content of phases, fine and coarsening （ Nb, Ti ） （ C, N ） precipitated changes during different quenching temperatures and holding time. Finally compared with the hardness value, the best quenching process can be obtained that heating temperature and holding time are 900 ℃ and 50 mins, respectively.

基于正交实验法的BS960E高强钢的淬火工艺优化

基于正交实验法研究了淬火过程中加热速率、保温温度、保温时间和冷却速率等参数对BS960E贝氏体高强钢显微组织和力学性能的影响。结果表明,淬火后BS960E钢的组织均为板条马氏体;通过正交实验法设计的淬火工艺获得最小平均晶粒尺寸的最优参数组合为:加热速率50℃/s、保温温度920℃及保温时间2 min;获得最大维氏硬度的最优参数组合为:加热速率50℃/s、保温温度1010℃、保温时间2 min及冷却速度100℃/s。通过增设对照实验组验证了正交实验法的正确性,正交实验设计的试样最小平均晶粒尺寸为6.36μm,马氏体板条群、马氏体板条块和马氏体板条分别为5.2μm、1.24μm和336.3 nm。正交实验设计的试样最大硬度为424.3 HV,马氏体板条群、马氏体板条块和马氏体板条分别为8.5μm、1.65μm和333.5 nm。相比于前者,后者在冷却速率相同(100℃/s)的情况下,由更高的温度(1010℃)降低至室温,产生了更大的过冷度,马氏体相变驱动力增加导致位错密度增加,大角度晶界占比由70.5%提高至83.3%,因此硬度更高。

20CrMnTi齿轮钢动态再结晶模型的研究

齿轮是机械传动系统的重要组成部分,20CrMnTi作为用途广泛的齿轮钢种,对棒材组织均匀性的要求越来越高。但是随着棒材尺寸增大,组织均匀性的控制难度进一步增加。为提高棒材组织均匀性,通过Gleeble热模拟实验,对齿轮钢20CrMnTi的奥氏体高温变形行为进行了研究,得到了变形温度为950~1150℃,应变速率为0.01~10 s^(-1),压缩应变0.7的真应力-真应变曲线。通过真应力-真应变曲线建立了该钢种的动态再结晶模型,模型包括动态再结晶临界应变模型、动态再结晶体积分数模型和动态再结晶晶粒尺寸模型。为进一步验证模型的准确性,采用阶梯试样进行热轧实验,得到轧后奥氏体晶粒尺寸。将轧制实验得到的奥氏体晶粒尺寸与20CrMnTi钢热-力耦合有限元模型中的再结晶模型计算得到晶粒尺寸进行对比,发现计算值与实验值良好吻合。证明该模型预测精度较高,可为20CrMnTi棒材轧制工艺优化提供相应的理论依据。

42CrMo钢加热时奥氏体晶粒长大演化规律

对42CrMo钢在不同加热温度(850~1150℃)和保温时间(0~1200 s)下的奥氏体晶粒长大规律进行了研究。采用金相定量法对加热后材料的奥氏体晶粒度进行测量,建立42CrMo钢加热时奥氏体晶粒长大演化模型。结果表明:奥氏体晶粒尺寸随加热温度升高呈指数关系长大,随保温时间的延长呈近似抛物线形式长大;利用基于唯象理论的Sellars模型,通过非线性回归方法建立42CrMo钢加热时奥氏体晶粒长大演化模型;将该模型导入有限元软件中预报42Cr Mo钢加热时奥氏体晶粒变化,预报结果与实验结果吻合,验证了该模型的正确性。

GCr15钢奥氏体晶粒长大规律研究

利用Gleeble-3800型热模拟试验机研究不同加热温度和保温时间下GCr15钢的奥氏体晶粒长大规律。结果表明,奥氏体晶粒随加热温度的升高呈指数关系长大,随保温时间的延长近似呈抛物线关系长大,同时晶粒平均直径与保温时间的关系符合Beck方程,温度越高,晶粒生长指数越大。在已有模型的基础上,通过对试验数据进行非线性回归得到了描述GCr15钢奥氏体晶粒长大规律的数学模型。

X12CrMoWVNbN10-1-1铁素体耐热钢奥氏体晶粒长大行为的研究

将超超临界发电机组高中压转子材料X12CrMoWVNbN10-1-1铁素体耐热钢试样在1010-1200℃之间经5—1200 min等温奥氏体化处理,通过测量处理后的奥氏体晶粒尺寸对其奥氏体晶粒长大规律进行了研究.结果表明:当奥氏体化温度低于1050℃时,晶粒长大表现为正常长大过程,即使等温1200 min,晶粒仍能保持均匀细小;1050—1120℃之间等温奥氏体化处理,奥氏体晶粒出现异常长大现象;当奥氏体化温度高于1150℃时,奥氏体晶粒以较大的速率正常长大.拟合实验结果.得出了不同温度下奥氏体晶粒长大的动力学方程,并确定了获得不同尺寸奥氏体晶粒所对应的温度和时间参数.

低碳齿轮钢18CrNiMo7-6奥氏体晶粒度长大规律

利用金相实验法研究了低碳齿轮钢18CrNiMo7-6在不同加热条件下奥氏体晶粒的长大行为,建立了Arrhenius奥氏体晶粒长大模型,并利用时间指数对模型进行了优化。结果表明:在1173~1373 K范围内,奥氏体晶粒平均尺寸随着温度的升高及时间的延长而增大,并且对温度的敏感性高于对时间的敏感性,显著粗化温度为1273 K;建立了Arrhenius奥氏体晶粒长大模型D=2.223×10^(6)exp[-132086/(RT)]tn,其中激活能Q=132.086 kJ/mol;时间指数n随温度的变化近似服从S型函数,并建立了相关的数学方程式n=0.0775+0.2317/(1+10(1326.73-T)×0.0235),用该方程式对考虑合金元素的Arrhenius方程中的时间指数进行了优化,将优化后由该模型得到的奥氏体晶粒平均尺寸的计算值与实测值进行了对比,结果显示其吻合性较未优化前的模型更好。

40CrNi2MoE钢奥氏体晶粒长大的数学模型

利用金相实验方法,基于实验数据,应用Beck、Hillert、Sellars数学模型研究了40CrNi2MoE钢在加热温度850~1200℃和保温时间30~480 min下的奥氏体晶粒长大规律。结果表明,随加热温度升高和保温时间延长,40CrNi2MoE钢奥氏体晶粒逐渐长大,当加热温度超过1050℃或保温时间超过120 min时,试验钢奥氏体晶粒开始粗化。通过对Beck、Hillert和Sellars 3种晶粒长大数学模型对比分析,Sellars模型对40CrNi2MoE钢的奥氏体晶粒尺寸预测具有较高的精度,其奥氏体晶粒长大模型方程为:当温度为850℃≤T≤1050℃时,D5.49Sellars=7.64×1021texp(-390081/(RT));当温度为1050℃≤T≤1200℃时,D8.13Sellars=8.04×1041texp(-771322/(RT))。

Cr8钢奥氏体晶粒长大规律

为了分析Cr8钢再结晶过程中加热温度和保温时间对奥氏体晶粒长大尺寸的影响,给出Cr8钢再结晶过程中奥氏体晶粒的长大规律,对Cr8钢试样在不同加热温度和不同保温时间下进行了水淬处理,并对实验结果数据进行了数据处理和线性拟合。结果表明,Cr8钢奥氏体晶粒长大尺寸随着加热温度的提高和保温时间的延长而不断增大;其晶粒长大过程可分为抑制长大阶段和自由长大阶段;在抑制长大阶段,其奥氏体晶粒尺寸与加热温度近似呈指数关系;在整个长大过程中,奥氏体晶粒尺寸与保温时间近似呈幂函数关系。利用Sellar公式对实验数据进行非线性回归分析,得到了Cr8钢奥氏体晶粒长大的数学模型。