贝氏体相变理论—两个一级相变耦合的模型

建立了一种贝氏体相变的Ｇｉｎｚｂｕｒｇ－Ｌａｎｄａｕ理论模型，该模型设计了贝氏体相变的一种耦合机制，即转变过程包含两个一级相变，一个具有重构特征，另一个为位移相变。由数值计算结果，建议了一种扩散诱导形核机制及扩散伴随伴移生长机制。     

NEW VIEW ABOUT BAINITIC TRANSFORMATION IN STEEL

1.IntroductionThemechanismofbainitictransformationhasbeendebatedbym;1lly..hola.s[1--22].Somedrewtheconclusionthatbainitictransformationiscontrolledbydiffusionalmechanismaccordingtothefactthatcarbideconstituentinbainiteresultsfromthediffusionofcarbonatoms.Otherscollsideredthatbainitictransformationiscontrolledbydisplacivemechanismbecausethephenomenasuchassurfacereliefeffectafldthecertaincoherencebetweenausteniteandferritefrombainitictransformatiollaresimilartothatfrommartensitictransformationls…     

Carbon supersaturation of ferrite in a nanocrystalline bainitic steel

The extremely slow transformation kinetics of a nanocrystalline bainitic steel allows the carbon content of the bainitic ferrite away from any carbon-enriched regions such as dislocations and boundaries to be determined by atom probe tomography as the bainite transformation progresses at 200 °C. A high level of carbon, well above that expected from para-equilibrium with austenite, has been detected in solid solution in bainitic ferrite at the early stage of transformation. Results provide strong evidence that bainite transformation is essentially displacive in nature so that the newly formed bainitic ferrite retains much of the carbon content of the parent austenite.     

钢中贝氏体预相变过程的研究

用扫描俄歇显微探针（ＡＥＳ）分析了６５Ｓｉ２Ｍｎ钢中温转变孕育期内碳原子的成分变化。结果表明，在中温转变孕育期等温时，奥氏体晶界附近及晶内均形成可稳定存在的贫碳区。贝氏体预相变的实质是碳原子向晶界及其他晶体缺陷扩散偏聚而形成贫碳区的过程。中温转变孕育期内形成贫碳区具有热力学及动力学可能性。     

Thermodynamic analysis of two-stage heat treatment in TRIP steels

In this work, we present a detailed thermodynamic analysis of the two-stage heat treatment (intercritical annealing (IA) and banite isothermal transformation (BIT)) necessary to stabilize retained austenite in transformation-induced plasticity (TRIP) assisted steels. Through a set of experiments on alloys with nominal composition Fe–0.32C–1.42Mn–1.56Si (wt.%), we monitored the evolution of the volume fraction of retained austenite at room temperature as a function of the IA and BIT temperatures. We also investigated the thermodynamic limit for the bainitic transformation during BIT under the displacive (partitionless) transformation assumption. The fraction of retained austenite at the end of the two-stage heat treatment was calculated by taking into account the corresponding start of the martensitic transformation (T_Ms). Comparisons with experiments suggest good qualitative agreement in the fraction of retained austenite when considering the effect of the IA temperature. On the other hand, the analysis of the effect of BIT on the amount of retained austenite showed qualitative disagreement with the observations. To further analyze this discrepancy, we utilize a modified thermodynamic analysis with empirical observations as input, and conclude that the assumption of thermodynamic equilibrium at IA is not valid at lower IA temperatures. Moreover, the unexpected high carbon enrichment in retained austenite indicates the importance of the kinetic effects. We conclude that the thermodynamic limit for the bainitic transformation can be used at least to provide a lower bound to the expected fraction of retained austenite under specific IA + BIT treatment schedules.     

Si—Mn—Mo系贝氏体钢的CCT图

测定了五种Si-Mn—Mo系贝氏体钢的CCT曲线,在低Mo(<0.25%)的情形下该钢种的高温转变区大幅度右移,且贝氏体转变线在空冷冷速范围内具有扁平的特点在该钢种中,Si使贝氏体转变区右移,使先共析铁素体区左移Mn推迟高温转变区比推迟贝氏体转变区更为剧烈     

Isothermal Transformation of a Commercial Super-Bainitic Steel: Part I Microstructural Characterization and Hardness

The effects of isothermal treatment on the microstructure and hardness of commercial super-bainitic steel were investigated. A series of isothermal treatments were carried out at temperatures of 210-250 °C for different time periods. The results indicate that the bainitic reaction and hardness were very sensitive to the isothermal transformation temperature. The fine super-bainitic microstructure, containing the carbide-free bainitic ferrite lath and the carbon-enriched retained austenite film, can be produced by heating to 210 °C for 30 h, resulting in a hardness of 662 HV. By increasing the isothermal transformation temperature, the bainitic transformation kinetic is accelerated; however, this is at the expense of coarsening bainitic ferrite laths and decreasing the bainitic ferrite quantity. The relationship between hardness and microstructures obtained under different isothermal treatments, which is correlated with the carbon concentration, dislocation density, bainitic amount and super-bainite size, is discussed in detail.     

贝氏体相变简介

钢、有色合金和一些陶瓷材料中都存在贝氏体相变。贝氏体钢正成为有益的工程材料。总结评述切变学派和扩散学派作者们以形貌、动力学或晶体学对贝氏体相变机制所持的论点。钢中贝氏体相变以过饱和铁素体开始形成之说迄未得到支持。一些实验已发现替代（置换）型合金元素在相界面上的偏聚,并以此所呈现的拖曳效应说明相变的不完全现象,切变学者以切变机制来解释这个现象,但此现象不是钢中贝氏体相变的普遍情况。贝氏体形成时呈现帐篷形浮突,不具不变平面应变特征;有时马氏体相变晶体学表象理论能近似地应用于贝氏体相变晶体学,但不能以此来判定其相变机制为扩散或切变。溶质拖曳效应以及高分辨电镜对相界面结构实验、热力学研究、磁场和应力场对贝氏体相变影响以及一些预相变现象都确证贝氏体相变籍扩散机制进行。本文作者定义贝氏体为：在Ms温度以上,经扩散相变的产物,多呈片状,形成时会在自由表面上呈现帐篷形浮突。提出贝氏体相变机制进一步研究和应用的展望。     

CHARACTER OF CARBON-DEPLETED REGIONS IN UNDERCOOLED AUSTENITE AND THERMODYANMICS OF BAINITIC TRANSFORMATION

The chemical driving forces for the phase transformation from austenite in three different car-bon-depleted regions into ferrite with the identical composition have been derived respectively.The starting temperature of bainitic transformation,B_s,has also been calculated accordingto the shearing mechanism of bainitic transformation in carbon-depleted regions of austenite.The maximum driving forces for banitic transformation at B_s in both 3% Cr and the CrMosteels were obtained.It was shown that the proposed shearing mechanism of the bainitic trans-formation is thermodynamically possible.     

Effect of Ni Addition on Bainite Transformation and Properties in a 2000 MPa Grade Ultrahigh Strength Bainitic Steel

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.     

贝氏体相变的激发－台阶机制

简要总结了贝氏体相变切变及扩散控制台阶长大理论面临的主要问题．并首次基于台阶长大理论，提出贝氏体相变的激发－台阶机制及其相变模型．     

MECHANICAL STABILIZATION OF DEFORMED AUSTENITE DURING CONTINUOUS COOLING TRANSFORMATION IN A C-Mn-Cr-Ni-Mo PLASTIC DIE STEEL

1.IntroductionTOmeettherequirementsofmachinability,polishedquality,alldoillerpropertiesofcompletedieproducts,quellchedafldtemperedprehardenillgtreatmentfortheblallkshasbeenwidelyusedintheplasticdiemanufacturingprocess.Recently,atllerlllomechallicalprehardeningprocesshasbeendevelopedforaInediumcarbonMll-Cr--Ni--Moplasticdiesteel(718steel)usedtosubstitutetheaboveheattreatmentprocesseswiththeaimofcuttingdownmanufacturingcosts.Becausethesteelhadsufficienthardenability,polygonalferritetransformat…     

奥氏体贫碳区性质与贝氏体相变热力学SCIEI

奥氏体内贫碳区可分为三类,即平衡贫碳区、非平衡贫碳区及随机贫碳区,分析计算了各类贫碳区内奥氏体转变为同成分铁素体的相变化学驱动力,根据奥氏体贫碳区内贝氏体切变转变机制计算了贝氏体相变开始温度B_s,3％Cr钢和CrMo钢在其B_S温度可获得的最大相变驱动力。在整个贝氏体相变温区,贝氏体铁素体以部分过饱和碳量切变形成的构想在热力学上是可能的。     

IN SITU OBSERVATION OF LOWER BAINITIC CARBIDES PRECIPITATION

The lower bainitic carbides precipitation in 40CrMnSiMoV steel previously austempered at310℃ was observed in situ by employing a high temperature stage of an ultra high voltageTEM(JEM-1000).The typical lower bamitic carbides were found to have precipitated fromwithin the carbide-free bainitic ferrite after tempering for certain period at a temperaturehigher than that of the isothermal transformation.This revealed that the lower bainitic ferriteis supersaturated with carbon to some extent.The carbide may also precipitate from austenite,but they have no typical morphological features of lower bainitic carbide.     

DRIVING FORCE OF BAINITIC TRANSFORMATION FOR Fe-Mn-C ALLOYS

本文应用三元合金的规则溶液模型计算了Fe-C-Mn三元合金在贝氏体相变温区内以不同方式等温转变的相变驱动力,从热力学上论述了以不同的微观机制进行等温贝氏体转变的可能性。     

Effect of copper additions on the isothermal bainitic transformation in hypereutectoid copper and copper-nickel steels

During the isothermal bainitic transformation in hypereutectoid steels alloyed with copper or copper and nickel, it was found that at all the transformation temperatures studied, the formation of equally copper supersaturated bainitic ferrite and cementite always occurred. This observation implies the formation of bainitic ferrite and cementite from the parent austenite phase without redistribution of the alloying elements, since the solubility of copper in cementite is negligible and very low in bainitic ferrite. By carefully designing suitable tempering treatments, it is possible to produce copper precipitation not only within tempered bainitic ferrite in both types of steel using low tempering temperatures, but also within the tempered bainitic cementite of the copper steel at higher tempering temperatures. The interpretation of these experimental data strongly supports the theory that bainite formation is promoted through a shear controlled type of mechanism.     

Kinetics model of bainitic transformation with stress

Thermal simulations were conducted on a Gleeble 3800 simulator. The main purpose is to investigate the effects of stress on the kinetics of bainitic transformation in a Fe-C-Mn-Si advanced high strength bainitic steel. Previous studies on modeling the kinetics of stress affected bainitic transformation only considered the stress below the yield strength of prior austenite. In the present study, the stress above the yield strength of prior austenite is taken into account. A new kinetics model of bainitic transformation dependent on the stress (including the stresses below and above the yield strength of prior austenite) and the transformation temperature is proposed. The new model presents a good agreement with experimental results. In addition, it is found that the acceleration degree of stress on bainitic transformation increases with the stress whether its magnitude is below or above the yield strength of austenite, but the increasing rate gradually slows down when the stress is above the yield strength of austenite.     

PREBAINITIC TRANSFORMATION IN A Cu-Zn-Al ALLOY

使用微机自动控制相变内耗仪,以0.7Hz频率测量Cu-25.83wt-％Zn-3.96wt-％Al合金在贝氏体相变温区等温过程中的内耗和频率随时间的变化,实验结果显示,在孕育期内随时间出现内耗峰值Q_(max)~(-1)且Q_(max)~(-1)随等温温度升高(孕育期缩短)而升高,达到Q_(max)~(-1)所需时间却随等温温度升高而缩短.在等温初期频率最低,随时间延长逐渐上升至稳定值.在施加8×10~5Pa拉应力的条件下,重复上述等温测量表明,Q_(max)~(-1)升高,达到Q_(max)~(-1)所需的时间缩短,且初期频率下降更为显著. 本文揭示:Cu-Zn-Al合金贝氏体相变存在类似于热弹性马氏体相变的点阵软化现象。在贝氏体相变孕育期内存在预相变的主要过程为形核。等温内耗的变化规律可根据相变理论和局部软化理论,通过孕育期内形核率J~*的变化作出解释。     

INVESTIGATION ON LATER STAGE OF INTERMEDIATE TRANSFORMATION IN Fe-C-Mo ALLOYS

The transformation process and the products of long term isothermal transformation aroundthe kinetic B_s temperature(the bay temperature in TTT diagram)in Fe-C-Mo alloys havebeen investigated.It was found that,at the final stage of intermediate transformation,a kindof granular structure formed accompanying the growth of bainitic ferrite.In comparison withthe carbide Mo_2C in the bainitic ferrite plates or blocks,the carbide m granular eutectoid hasbeen determined to be cementite containing a tiny bit of molvbdenum,(Fe,Mo)_3C.     

Development of a kinetic model for bainitic isothermal transformation in transformation-induced plasticity steels

In this work, we modify existing models to simulate the kinetics of bainitic transformation during the bainitic isothermal transformation (BIT) stage of a typical two-stage heat treatment – BIT is preceded by an intercritical annealing treatment – for TRIP steels. This effort is motivated by experiments performed in a conventional TRIP steel alloy (Fe–0.32C–1.42Mn–1.56Si) that suggest that thermodynamics alone are not sufficient to predict the amount of retained austenite after BIT. The model implemented in this work considers the non-homogeneous distribution of carbon – resulting from finite carbon diffusion rates – within the retained austenite during bainitic transformation. This non-homogeneous distribution is responsible for average austenite carbon enrichments beyond the so-called T₀ line, the temperature at which the chemical driving force for the bainitic transformation is exhausted. In order to attain good agreement with experiments, the existence of carbon-rich austenite films adjacent to bainitic ferrite plates is posited. The presence of this austenite film is motivated by earlier experimental work published by other groups in the past decade. The model is compared with experimental results and good qualitative agreement is found.