Sensivity Analysis of the Cellular Automata Model for Austenite-Ferrite Phase Transformation in Steels

The main goal of the present research is to realize a sensitivity analysis of the developed complex micro scale austenite (γ) to ferrite (α) phase transformation model. The proposed solution is implemented in the developed Cellular Automata Framework that facilitates implementation of various microstructure evolution models. Investigated model predicts phase transformation progress starting from the fully austenitic or two-phase regions. Theoretical background of the implemented austenite-ferrite phase transformation model is presented in the paper. The defined transition rules for initiation and subsequent growth as well as internal variables for each particular CA cell are also discussed. Examples of results obtained from the developed model, as well as model capabilities are shown. Finally sensitivity analysis using Morris OAT Design is also presented and discussed.

A Mixed-control Mechanism Model of Proeutectoid Ferrite Growth under Non-equilibrium Interface Condition in Fe-C Alloys

By combining the α/γ interface migration and the carbon diffusion at the interface in Fe-C alloys, a mathematical model is constructed to describe the mixed-control mechanism for proeutectoid ferrite formation from austenite. In this model, the α/γ interface is treated as non-equilibrium interface, i.e., the carbon concentration of austenite at γ/α interface is obtained through theoretical calculation, instead of that assumed as the local equilibrium concentration. For isothermal precipitation of ferrite in Fe-C alloys, the calculated results show that the rate of interface migration decreases monotonically during the whole process, while the rate of carbon diffusion from γ/α interface into austenite increases to a peak value and then decreases. The process of ferrite growth may be considered as composed of three stages: the period of rapid growth, slow growth and finishing stage. The results also show that the carbon concentration of austenite at γ/α interface could not reach the thermodynamic equilibrium value even at the last stage of ferrite growth.

Variations in the Microstructure and Properties of Mn-Ti Multiple-phase Steel with High Strength under Different Tempering Temperatures

There are few relevant researches on coils by tempering,and the variations of microstructure and properties of steel coil during the tempering process also remain unclear.By using thermo-mechanical control process（TMCP）technology,Mn-Ti typical HSLA steel coils with yield strength of 920 MPa are produced on the 2250 hot rolling production line.Then,the samples are taken from the coils and tempered at the temperatures of 220℃,350℃,and 620℃respectively.After tempering the strength,ductility and toughness of samples are tested,and meanwhile microstructures are investigated.Precipitates initially emerge inside the ferrite laths and the density of the dislocation drops.Then,the lath-shaped ferrites begin to gather,and the retained austenite films start to decompose.Finally,the retained austenite films are completely decomposed into coarse and short rod-shape precipitates composed of C and Ti compounds.The yield strength increases with increasing tempering temperature due to the pinning effect of the precipitates,and the dislocation density decreases.The yield strength is highest when the steel is tempered at 220℃because of pinning of the precipitates to dislocations.The total elongation increases in all samples because of the development of ferrites during tempering.The tensile strength and impact absorbed energy decline because the effect of impeding crack propagation weakens as the retained austenite films completely decompose and the precipitates coarsen.This paper clarifies the influence of different tempering temperatures on phase transformation characteristics and process of Mn-Ti typical multiphase steels,as well as its resulting performance variation rules.

A new step-cooling process for strength and toughness matching control of vanadium-containing railway wheels:effect of intragranular ferrite

To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite(γ)on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was studied using an optical microscope(OM),a scanning electron microscope(SEM),a transmission electron microscope(TEM),and mechanical property tests.The results show that when the wheel steel is slightly cooled to an appropriate temperature above A c3 point for a short time after it has been austenitized at an elevated temperature,the solid-solved vanadium is pre-precipitated in the form of V(C,N)second phase semicoherent with the matrix in the originalγgrain.This phase hardly participates in matrix strengthening.Due to the small mismatch between V(C,N)and ferrite(α),during the subsequent-cooling phase transformation stage,the pre-precipitated second phase becomes theαnucleation point,causing granular and ellipsoidal intragranular ferrite(IGF,with an average size of 4-6μm)to nucleate in the originalγ.The IGF production and strength loss increases with the increasing undercooling degree.Based on this,Masteel Co.,Ltd.has developed a new heat-treatment step-cooling process that can promote the formation of IGF,considerably improving the level and uniformity of fracture toughness on the premise that the strength and hardness of the wheel are almost unchanged.

EFFECT OF CARBIDE SOLVATION ON FERRITE FORMATION IN AUSTENITIC STEELS

The effect of carbide solvation on the γ/γ+δ boundary temperature Tδ and the equilibrium relationship between the alloying elements at the γ/γy +δ boundary in austenitic steels at high temperature have been studied, and the variation of the ferrite volume fraction with temperature in α+ γ dual phase steel has been investigated. The relative expressions are derived from many experimental results, which provides a basis for quantitative calculation, composition design, determination of the working processes,and prediction of the mechanical properties and the structure of the austenitic steels.

STUDY ON AUSTENITE-FERRITE TRANSFORMATION WITHIN CRITICAL TEMPERATURE REGION

his paper deals with the phase trans formation of austenite to ferrite within the critical temperature region(between Ac1 and Ac3).The results show that the volume fraction of trans formation of ferrite formed isothermally from austenite is mainly varied with the austenitizing temperature.The higher the austenitizing temperature,the more volume fraction of the ferrite formed isothermally from austenite.Besides,the phase trans formation of austenite to ferrite within the critical temperature region was checked by austenitizing isothermal time.The volume fraction of ferrite formed isothermally from austenite within the critical temperature region for different isothermal time has heen examined experimen tally.The lglg(l-fv)-1 vs lgt relation does not follow the Avrami equation strictly and consists of two straight lines.

具有高强匹配焊缝金属的LDX2101钢接头显微组织表征

综述了涉及高强匹配焊缝金属的LDX2101钢接头显微组织的特征。结果表明,节约型双相不锈钢LDX2101焊接材料的选用,可以采用“准成分匹配”选用原则,即焊缝中所加元素的种类与母材相同,但各元素的含量可能有较大的变化,以确保熔敷金属中具有适量的相平衡,获得满意的综合性能。相同的热循环条件下,接头的组织主要取决于化学成分,即Creq/Nieq比值;在一定范围内,随Creq/Nieq比值降低,提高了铁素体固溶温度,在较高温度下发生δ→γ转变,获得较多的奥氏体。随热输入增加,在所有焊缝金属中,γ奥氏体的含量增大,而且γ奥氏体的含量高于δ-铁素体的含量;在HAZ,随热输入增大,γ奥氏体的含量增大。在650℃时效试件中,焊接接头的组织变化主要是Cr2N析出;650℃时效15 min时,主要是铁素体晶内析出Cr2N;650℃时效90 min时,在接头3个区的晶界上均析出Cr2N。铁素体含量的变化:焊缝为37%,HAZ为40%,母材为45%。

不锈钢中亚稳奥氏体-马氏体相变行为的原位分析

采用原位EBSD分析方法,系统研究铁素体不锈钢淬火配分(quenching and partitioning,Q&P)工艺后残余奥氏体特征对其相变行为的影响。结果表明:变形过程中残余奥氏体的相变行为与其晶粒尺寸、分布和形貌有关,其影响程度按降序排列。与马氏体间奥氏体和马氏体与铁素体间奥氏体相比,三叉晶界处的奥氏体和孪晶奥氏体在变形早期更容易发生马氏体相变,这与变形过程中不同分布残余奥氏体所受不同的应变或应力密切相关。与大尺寸奥氏体相比,小尺寸奥氏体在变形后期开始转变,有助于延长均匀伸长率。这可能是由于小尺寸奥氏体具有较高的界面能和平均C含量,并且由于细晶强化效应需要较大的宏观应力/应变来诱导马氏体相变。拉长/等轴残余奥氏体在变形早期容易转变,而薄膜状残余奥氏体的转变主要集中在变形后期,有助于进一步提高塑性。不同的转变行为是由于C和N含量的差异以及层错、位错和滑移等缺陷的存在。

奥氏体化温度对改良型T23铁素体耐热钢微观组织与力学性能的影响

对改良型T23铁素体耐热钢试样进行不同奥氏体化温度的热处理试验,通过金相显微镜、扫描电镜、透射电镜对经不同奥氏体化温度处理试样的微观结构进行表征,且对其进行常温力学性能进行测试分析;结合采用Thermo–Calc热力学软件计算的T23铁素体耐热钢平衡相图,探讨C,N,V等元素在钢中基体和第二相间的分配关系,揭示奥氏体化温度对改良型T23铁素体耐热钢显微组织和力学性能的影响。结果表明:在1080~1140℃奥氏体化温度范围,随温度的升高,钢中的晶粒尺寸逐渐增大,贝氏体板条不断粗化,碳化物尺寸逐渐减小。当奥氏体化温度为1080℃时,细化的贝氏体板条可为碳化物聚集长大提供条件,钢中碳化物呈现团簇聚集倾向;当奥氏体化温度升至1120℃时,晶粒内部的贝氏体板条明显粗化,合金元素在基体中的固溶度提高,碳化物的形核位点减少且弥散化程度大大提升。同时,随奥氏体化温度的升高,改良型T23铁素体耐的屈服强度和抗拉强度呈现先降低后增加的趋势,延伸率呈现先增加后降低的趋势,这主要是由于晶粒尺寸、碳化物的尺寸与分布、合金的固溶强化随奥氏体化温度变化而产生的耦合作用所致。

超级奥氏体不锈钢热轧工艺研究

利用热力学模拟软件计算了904L、254SMO、654SMO超级奥氏体不锈钢和316L常规奥氏体不锈钢相图以及析出相的析出行为,并通过数学回归方法研究了其高温强度。研究表明:超级奥氏体不锈钢凝固模式为全奥氏体凝固,固相线温度普遍偏低,较316L低80~110℃,σ相析出温度较316L高,高温强度较316L高,热加工难度大。

形变参数对中碳钢组织演变的影响

通过热模拟实验研究了不同形变温度及在700℃下不同形变量时中碳钢35K的组织演变过程。结果表明,中碳钢通过形变可获得形变诱导铁素体(DIF);形变提高奥氏体向铁素体转变温度,随着形变温度的降低,DIF含量呈反“S”形增加,即先缓慢增加,随后快速增加,当DIF量超过平衡态铁素体量时,其增加趋势趋缓。随700℃形变量的增加,DIF含量呈“S”形增加,在形变量为0.7时可获得良好的球化组织。在中碳钢形变后的控冷过程中,根据形变量和形变温度所影响的未转变的奥氏体尺寸、形变储能、富碳程度和能量与成分起伏等,未转变的奥氏体将发生不同于传统未变形奥氏体的转变。因此,控制轧制和控制冷却后可获得离异珠光体、球粒状或短棒状渗碳体的微观组织。

热处理工艺对双相等温淬火球墨铸铁组织的影响

利用扫描电镜分析了热处理工艺对双相ADI(等温淬火球墨铸铁)组织的影响。结果表明:当奥氏体化温度在820～880℃时,随着奥氏体化温度的升高,铁素体的含量逐渐减少,奥氏体的含量逐渐增加,当奥氏体化温度达到880℃时,基本全部奥氏体化。当等温淬火温度为250～370℃时,随着等温淬火温度的升高,组织由细针状铁素体、残余奥氏体及破碎状铁素体转变成大量的羽毛状贝氏体型铁素体、破碎状铁素体和较多的残余奥氏体。当等温淬火时间为30～90 min时,等温淬火时间较短时,组织为少量马氏体、破碎状铁素体、针状铁素体和残余奥氏体,当等温淬火时间超过90 min时,奥氏体容易发生分解,生成铁素体和碳化物。

变形、冷却条件下低碳钢铁素体相变的元胞自动机模型

建立了一个二维元胞自动机模型 ,模拟了低碳钢在形变与快冷作用下铁素体最终微观结构 ,讨论了不同初始奥氏体γ晶粒尺寸对于奥氏体γ→铁素体α相变后微结构的影响·分析了模拟结果与实验存在一定偏差的原因·模拟结果表明 ,形变前的奥氏体晶粒尺寸粗大 ,相变后铁素体转变不完全 ,铁素体晶粒粗大且不均匀 ;形变前的奥氏体晶粒细小 ,相变后铁素体转变较为完全 ,铁素体晶粒细小·

γ-α相变中不同晶界特征下铁素体生长形貌的相场模拟

利用多相场模型模拟了奥氏体(γ)-铁素体(α)相变过程中不同晶界特征下铁素体晶粒的形貌与生长动力学.模型中通过能量梯度系数和耦合项系数的协同变化定量表达晶界能与晶界迁移率的各向异性,同时固定相场界面宽度来保证计算精度.模拟结果显示:随着原奥氏体晶界能与铁素体-奥氏体晶界能比值σ_(γ,γ)/σ_(α,γ)的增加,三叉相界面处的平衡角β减小,铁素体晶粒沿原奥氏体晶界与垂直于奥氏体晶界方向的生长速率差变大.铁素体与奥氏体晶粒间的晶粒取向越接近,铁素体生长越缓慢.模拟结果可描述铁素体晶粒生长形貌的多样性,与实验结果符合.

锰元素对形变强化相变完成时临界应变量的影响

利用形变强化相变机制研究了两种锰含量不同的低碳锰钢过冷奥氏体在780℃,以10s-1的速率变形时,锰元素对相变完成时临界应变量εc的影响。结果表明,锰元素对εc的影响很大,主要表现为对组织演变和转变动力学的影响。锰含量增加,主要推迟了相变过程的进行,εc增大。低碳锰钢过冷奥氏体形变强化相变组织演变和转变动力学都分为两个阶段:第一阶段对应着曲线上的斜率接近4,铁素体在原奥氏体晶界形核;第二阶段在曲线上表现n值减小,大约1.0-2.0,对应着铁素体在奥氏体晶内高畸变区大量反复快速形核,直至相变完成。锰含量增加,铁素体晶粒细化显著。

00Cr22Ni5Mo3N双相不锈钢的组织分析

介绍了00Cr22Ni5Mo3N双相不锈钢的性能特点,采用扫描电镜(SEM)和能谱分析法(EDS)研究了该钢固溶油冷后的组织和各析出相的性能,分析了该钢离子渗氮层的组织。试验表明,该不锈钢在固溶处理时必须快速冷却,并在低于600℃时效。提出了降低离子渗氮层脆性的措施。

核电站用国产铸造双相不锈钢的热老化

对国产铸造双相不锈钢Z3CN20-09M进行了450℃长达1200 h的加速热老化试验。研究了它的热老化脆化行为。结果表明:随着老化时间的增加,冲击功显著下降,老化1200 h后冲击功为161 J,断裂方式由韧性转变为脆性;奥氏体相显微硬度基本不变,而铁素体相显微硬度显著增加,老化1200 h后铁素体相显微硬度为470 HV10。

奥贝双相钢力学性能及塑变分析

采用等温处理方法获得了奥氏体含量和铁素体中含碳量不同的奥氏体──贝氏体双相钢，研究了等温处理温度对奥氏体量和铁素体中碳含量的影响，测定了双相钢的力学性能；建构了奥贝双相钢等应变模型，计算了奥贝钢中两相的塑性应变分配，对双相钢的塑性变形及影响进行了初步的理论探讨．

核电站冷却剂主管道奥氏体-铁素体双相不锈钢铸件铁素体含量检测方法研究与实践

核电站冷却剂主管道奥氏体-铁素体双相不锈钢铸件中铁素体含量对主管道的使用性能具有重要影响,因此,铁素体含量的检测具有重要意义。本文以主管道90°弯管铸件为例,介绍了铁素体含量检测的各种试验方法,阐明了各种方法的检测原理、检测时机和具体步骤,简要分析了不同检测方法所检测结果的差异和原因,指出了各种方法的适用范围。