界面条件剧变的淬火过程三维温度场的计算机模拟

用有限单元法建立界面条件剧变的淬火过程三维瞬态温度场的计算机计算数学模型，并用Ｔ１０钢作为试验材料进行冷却曲线测试以验证数学模型。该模型较全面地考虑了淬火过程中界面换热系数剧烈变化及各种物性参数随温度而变化的非线性问题，包括了相变的计算及相变潜热与温度场变化的耦合计算。冷却曲线的计算结果与实测结果吻合较好，表明本数学模型和计算程序能正确预测复杂形状物体的非线性三维瞬态温度场变化。     

界面条件剧变的淬火过程三维温度场的计算机模拟

用有限单元法建立界面条件剧变的淬火过程中三维瞬态温度场的计算机计算数学模型，并用Ｔ１０钢作为试验材料进行冷却曲线测试以验证数学模型，该模型较全面地考虑了淬火过程中界面换质热数剧烈变化及各种特性参数随温度而变化的非线性问题，包括了相变的计算及相变潜热与温度场变化的耦合计算，冷却曲线的计算结果与实测结果吻合良好，表明本数学模型和计算程序能正确预测复杂形状物体的非线性三维瞬态温度场变化。     

过冷奥氏体等温转变曲线数据库的建立

本文用三次样条函数描述钢的过冷奥氏体等温转变曲线，在微机上建立了ＴＴＴ曲线数据库，实现了ＴＴＴ曲线的数据形式存储与动态调用，操作简单，直观，占用计算机存储空间小，并进一步等温转变动力学公式计算出过冷奥氏体等温转变过程中任意转变量曲线，与手册中的曲线吻合得很好。     

S7钢退火TTT曲线的测定及研究

利用Ｆｏｒｍａｓｔｏｒ－Ｄｉｇｉｔａｌ全自动相变测量仪，测定了Ｓ７钢退火用的ＴＴＴ曲线，研究了合金元素，奥氏体化温度对ＴＴＴ曲线的影响，为Ｓ７钢的软化退火提供了依据。     

H13钢退火用TTT图及球化退化工艺的研究

用全自动相变测定仪测定了Ｈ１３钢退火用ＴＴＴ曲线，分析了它与Ｈ１３钢淬火用ＴＴＴ曲线的差别，研究了Ｈ１３钢的球化退火工艺及共在不同工艺下的组织和硬度，为合理制订１３钢球化退火工艺提供了依据。     

钢的过冷奥氏体等温转变图（TTT）的计算机绘制

ＴＴＴ图是制定钢的热处理工艺的重要依据。本软件对所测相变数据进行分段抛物处理，实现了曲线的拟合，用ＢＡＳＩＣ语言设计出具有在半对数坐标系下绘制ＴＴＴ图的软件，该软件再与ＡＵＴＯＣＡＤ联接，在ＣＡＤ绘图软件支持下，可自动绘出精确、实用、美观的ＴＴＴ图。     

过冷奥氏体转变曲线的计算机模拟

利用已知的ＴＴＴ图，通过编制的程序和计算，绘出ＣＣＴ曲线，并预测不同冷速下的组织、硬度等。所编制的程序是针对多种钢材的，最后结果与实测结果相符，从而证明了此种方法的可行性，为热处理工艺的计算机辅助设计奠定了基础。     

S5钢TTT曲线及软化退火工艺的研究

采用日本Ｆｏｒｍａｓｔｏｒ－Ｄｉｇｉｔａｌ全自动相变分析仪测定了Ｓ５钢的临界点及退火用ＴＴＴ曲线的珠光体转变部分，并进行了软化退火工艺的试验，从而得出了最佳退火工艺。     

TRIP590钢贝氏体区的等温转变规律

采用DIL805L型淬火膨胀仪测定了TRIP590钢贝氏体区等温转变曲线,根据膨胀量-时间的关系曲线,计算了其相变过程中的组织转变率以及对应时间的关系,确定了在不同两相区加热温度下贝氏体相变动力学模型的主要参数。结果表明:两相区加热温度一致时,实验钢贝氏体区等温温度越高贝氏体转变速率越快;贝氏体区等温温度一致时,两相区加热温度越高贝氏体转变率越高;在贝氏体等温转变图中,转变量为90%的线呈C形,即"鼻尖"处转变速度最快;相变动力学模型的预测值在转变后期比实测值偏大,但整体来说预测值与实测值吻合度较好。     

用有限元法计算35CrMo钢大锻件淬火过程的瞬态温度场

根据大锻件的特点及其淬火过程的复杂性，本文提出了有限元法计算锻件淬火过程中温度场的传热学数学模型。应用ＴＴＴ曲线的回归方程及孕育期叠加原理，对大锻件淬火过程非等速冷却过程中奥氏体组织转变进行判断。计算过程中所涉及的热物性值，如比热，导热系，换热系数等，均采用分段线性回归处理，相变潜热则采用温升法处理，对３５ＣｒＭｏ钢Ф３５０ｍｍ的长圆柱体锻件进行了计算，并测定了温度场及组织分布。     

无钛热冲压成形钢的连续冷却与等温转变规律

利用Thermo-Calc热力学软件(TCFE 9数据库)、DIL805A/D变形热膨胀相变仪和场发射扫描电镜(FE-SEM)研究了连续冷却转变及等温转变过程中无钛热冲压成形钢的微观组织演化规律。结果表明:试验钢的Ac₁=749 ℃,Ac₃=863 ℃。绘制了CCT曲线和TTT曲线;无钛热冲压成形钢的马氏体相变开始温度Ms=385 ℃,马氏体相变结束温度Mf=130 ℃。过冷奥氏体冷却过程中,发生马氏体相变的临界冷却速度为5 ℃/s;当等温温度高于750 ℃时,热冲压成形后可获得全马氏体组织。     

23MnNiCrMo煤机链条钢TTT曲线的研究

采用Formastor Digital全自动相变测量仪测定了钢的临界点和TTT曲线 ,进行了组织分析 ,研究了该钢的相变特征 ,为热处理工艺的制定提供了依据     

20CrMnTi钢的等温相变行为分析及动力学建模

利用Gleeble-3500热模拟试验机对20CrMnTi钢在不同温度和保温时间进行了等温膨胀试验,得到其相变热膨胀曲线。结合光学显微镜分析了20CrMnTi钢的等温相变行为,绘制了该钢的等温相变曲线(TTT曲线)。引入Johnson-Mehl-Avrami(JMA)方程和Koistinen-Marburger(KM)方程分别建立了该钢的扩散型相变动力学模型和非扩散型相变动力学模型。结果表明:20CrMnTi钢的TTT曲线呈“双C型”,鼻温分别为630和530 ℃。在730~580 ℃等温时,奥氏体转变为珠光体+铁素体,随着温度的降低,等温相变速度先加快后减慢;580~430 ℃等温时,奥氏体转变为贝氏体,随着温度的降低,等温相变速度也是先加快后减慢;低于430 ℃等温时,奥氏体转变为马氏体,随着温度的降低,马氏体的体积分数先较快增大后减缓。所推导的20CrMnTi钢的动力学模型计算结果与试验结果一致性较好。     

Computation of isothermal transformation diagrams of 42CrMo4 steel from dilatometer measurements with continuous cooling

Abstract

To generally predict the mechanical properties of steels following heat treatment, the isothermal time–temperature–transformation (TTT) diagrams must be known. The isothermal kinetics of the phase transformation is influenced by the austenitisation conditions and the deformation processes. Owing to the requirements of the process chain 'integrated heat treatment following hot-forging', the steels are austenitised at a comparatively high temperature and deformed before quenching. The TTT diagrams found in the literature only treat limited austenitisation temperatures and do not generally consider different deformation levels. However, the measurements of the corresponding TTT diagrams are both costly and time consuming. In addition to this, the isothermal transformations of low alloyed steels, which transform very quickly, cannot be measured using those dilatometers which are readily available.¹ In this paper, the TTT diagrams of the 42CrMo4 steel austenitised at 1200^°C and deformed at different levels are computed from the dilatometer measurements with continuous cooling using the methods developed by Buza et al.² and Rios.³     

Determination of nucleation and growth modes from evaluation of transformed fraction in solid-state transformation

On the basis of an analytical transformation model, the transformed fraction has been evaluated for isothermal and isochronal solid-state transformations. From the evolution of Avrami exponent n with annealing temperature T and the evolution of effective activation energy Q with transformed fraction f, the prevailing modes of nucleation and growth in the isothermal transformation can be determined. As for the isochronal transformation, however, a conversion of the experimentally obtained continuous-heating-transformation (CHT) diagram to the temperature–time-transformation (TTT) diagram should also be performed. Because the modes of nucleation and growth remain unchanged upon CHT–TTT conversion, the prevailing modes of nucleation and growth in the isochronal transformation can be determined. Using numerical calculations, the above evaluation has been tested. Finally, application in the crystallization kinetics of amorphous Pd–Ni–P–Cu and Mg–Cu–Y as measured by differential scanning calorimetry was described.     

Effect of external loading on the martensitic transformation – A phase field study

In this work, the effect of external loading on the martensitic transformation is analyzed using an elasto-plastic phase field model. The phase field microelasticity theory, incorporating a non-linear strain tensor and the effect of grain boundaries, is used to study the impact of applied stresses on an Fe–0.3%C polycrystalline alloy, both in two and three dimensions. The evolution of plasticity is computed using a time-dependent equation that solves for the minimization of the shear strain energy. Crystallographic orientation of the grains in the polycrystal is chosen randomly and it is verified that the said assumption does not have a significant effect on the final volume fraction of martensite. Two-dimensional (2-D) and three-dimensional (3-D) simulations are performed at a temperature significantly higher than the martensitic start temperature of the alloy with uniaxial tensile, compressive and shear loading, along with hydrostatic stresses. It is found that the 3-D simulations are necessary to investigate the effect of external loading on the martensitic transformation using the phase field method since the 2-D numerical simulations produce results that are physically incorrect, while the results obtained from the 3-D simulations are in good agreement with the empirical observations found in the literature. Finally, it is concluded that the given model can be used to predict the volume fraction of martensite in a material with any kind of external loading.     

Precipitation Behavior of σ Phase in Ultra-Supercritical Boiler Applied HR3C Heat-Resistant Steel

The precipitation kinetics of σ phase in commercial HR3C heat-resistant steel during aging at 650–800 °C was studied in the paper. Through morphology, composition and structural analyzing on the second phase in the HR3C steel, it was confi rmed that the precipitations after aging were mainly NbCrN, M_23C_6 and FeCr type σ phase. The time-dependent mass change of the three precipitated phases showed that the linearly increased σ phase after aging at 750 °C–2000 h was transformed from NbCrN phase or M_23C_6 phase. According to the calculation on the volume fraction of electrolytically dyed σ phase, the time–temperature transformation(TTT) curve for σ phase at 1 vol% in two kinds of commercial HR3C steel(different in grain size) was obtained and analyzed. The nose of the TTT curve was located at around 750 °C for the two kinds of HR3C steel, and the larger grain size HR3C steel displayed a inhibit effect on the precipitation of σ phase. The impact energy of the HR3C steel after aging at 700 °C decreased obviously with the fracture mechanism changing from ductile fracture to intergranular brittle fracture, which was considered to be related to the density of σ-brittle phase after aging.     

稀土对30CrMnSi铸钢TTT曲线的影响

研究了稀土对 30 Cr Mn Si铸钢 TTT曲线及相变点的影响 ,其结果为开始转变曲线右移 ,终了转变曲线左移 ,相变临界点滞后 ,Ms点升高     

Additivity rule,isothermal and non-isothermal transformations on the basis of an analytical transformation model

In the present work, the “additivity rule” was proved to be compatible with an analytical transformation model assuming mixed nucleation (with site saturation and continuous nucleation as two extremes), for some specified conditions. Combining the analytical model with numerical calculations, it is practicable to allow a conversion of continuous heating or cooling transformation (CHT or CCT) data into isothermal transformation data (TTT), and also a conversion of TTT data into CHT or CCT data, under a restrictive assumption of constant cooling or heating rate. From a comparative study between TTT and CHT data, the effective activation energy was found to increase and decrease, respectively, as compared with that before the CHT–TTT and TTT–CHT conversions; the growth exponent was proved to hold almost invariably, independent of the conversions. Real applications in the crystallization of amorphous Mg–Ni alloy and in the gamma–alpha massive transformation in Fe–Mn alloy show that the above conclusions are reasonable.     

TiAl合金冷却相变研究进展及细晶组织调控

TiAl合金是目前航空航天轻质高温结构的热点材料,其最终显微组织很大程度上决定于其冷却过程。由于TiAl合金相图和相变的复杂性,通过冷却过程控制,获得细晶组织结构成为关注的重要问题。综述了TiAl合金冷却相变的研究成果,并给出了关于Ti-48Al-4Nb-2Cr合金相关的进展:首先介绍了TiAl合金的凝固路径与相变行为,然后分别分析了连续冷却转变(CCT)和等温冷却转变(TTT)行为。进一步建立了Ti-48Al-4Nb-2Cr合金CCT图,并指导获得了一种细晶近片层组织。