深过冷合金中的晶粒细化

深过冷熔体在一个临界过冷度下凝固,在很窄的过冷区间内晶粒尺寸将急剧减小,称之为晶粒细化.从研究过冷Ni基合金的晶粒细化现象入手,对迄今为止关于产生晶粒细化的各种机制做了较为系统的归纳和分析.并在此基础上,提出了存在的问题和今后的发展方向.     

过冷对共析钢动态相变和组织演变的影响

将共析钢在600-700℃的过冷奥氏体状态下进行单轴热压缩,获得了亚微米级别等轴铁素体 纳米级别弥散分布球化渗碳体的复相组织,其组织演变经历动态相变、片层渗碳体球化、铁素体动态再结晶和纳米级别渗碳体颗粒析出等过程.随着形变温度的降低,过冷度增加,相变动力学过程加快.片层渗碳体的球化程度由球化时间和球化速度控制,形变温度升高使渗碳体球化速度加快,但是相变开始时间及动力学延迟使得用于渗碳体球化和熟化的时间相应缩短,导致球化程度降低.铁素体再结晶和等轴化过程则主要受位错迁移、渗碳体颗粒钉扎的影响,形变温度升高导致较高的等轴化发展速度.     

钢铁材料组织超细化处理工艺研究进展

综述了钢铁材料在组织超细化技术方面的研究成果，阐述了各种超细化技术的适用条件，介绍了近期这方面的研究结果。     

小样品在过冷液氮中的淬冷沸腾传热

本文用沉浸法研究小样品沉浸入过冷液氮的沸腾传热特性。实验发现,沉浸样品的直径减少、液氮过冷度增大,均导致沸腾的热流密度急剧向右上方移动。当小直径足够小,过冷度足够大时,稳定膜态沸腾段消失了,“Ｍ”形的沸腾曲线变为“Λ”形。     

SrCO3在AZ31镁合金中的细化效果及机理

研究了SrCO_3对AZ31镁合金凝固组织的影响。结果表明:在AZ31中添加0.6%的SrCO_3,于760℃时保温10min细化效果最佳,α-Mg晶粒的尺寸由基体合金的570±15μm降至110±10μm,降幅约80.7%。通过能谱分析,结合能计算及自由能计算证实细化机理是SrCO_3反应后生成的部分Al_4C_3质点作为异质核心细化晶粒。多余的Al_4C_3质点钉扎晶界阻碍晶粒长大。Al元素随固/液界面前沿被快速推至晶界,生成沿晶界生长的β-Mg_(17)Al_(12)相,起到进一步固定晶界的作用。     

MgCO3在AZ31镁合金中的细化效果及机理

为改善AZ31镁合金铸态组织,用MgCO3对其进行细化,采用扫描电子显微镜、X射线衍射仪和金相显微镜研究了细化工艺参数对AZ31镁合金显微组织及其物相组成的影响.结果表明:在AZ31中添加质量分数为0.6%的MgCO3,于760℃保温10 min细化效果最佳,α-Mg晶粒的尺寸由基体合金的570μm降至100μm,降幅约82.5%.少量多次添加MgCO3的细化效果明显优于单次添加MgCO3的细化效果.研究认为,细化机理是MgCO3反应后生成的部分Al4C3质点作为异质核心细化晶粒,多余的Al4C3质点钉扎晶界阻碍晶粒长大.Al元素随固/液界面前沿被快速推至晶界,生成沿晶界生长的β-Mg17Al12相,起到进一步固定晶界的作用.合金元素的分布均有改变.     

高碳钢中珠光体在温楔横轧过程中的组织演变

采用温楔横轧方法制备出表层具有超微细复相组织的高碳珠光体钢棒件,研究了珠光体组织在温变形过程中的演变.结果表明,珠光体组织中的渗碳体片层主要以弯曲扭折的形式协调塑性变形,表现出较强的塑性变形能力;剧烈塑性变形促进了渗碳体片层的球化,表层球化完全的渗碳体颗粒粒径均小于0.2μm;温楔横轧后铁素体基体发生了动态连续再结晶,等轴铁素体平均晶粒尺寸为0.3～0.4 μm,0.5R处和心部的渗碳体球化不完全,铁素体再结晶也不完全;铁素体晶粒的超细化和渗碳体片层的球化明显改善高碳珠光体钢棒件的塑性,温变形过程中应变、应变速率及温度分布的不均匀是引起组织性能差异的根本原因.     

Homogenisation of austenite during non-equilibrium heating in hypereutectoid steels

The homogenisation (Stage III) of austenite during non-equilibrium heating of hypereutectoid steels has been characterised for the first time through dilatometric strain measurements and electron microscopic techniques. It has been found that the homogenisation temperature (A_ch) is significantly higher than the cementite dissolution temperature (A_cm). Though there is a complete conversion of low-temperature constituents to austenite above the A_cm, a significant segregation of carbon in the austenite is observed below the A_ch. Performing heat treatment with an incomplete austenitisation between the A_cm and the A_ch temperature can affect the subsequent on-cooling phase transformations. It is proposed that the A_ch temperature should be used as the full austenitisation temperature in hypereutectoid steels.     

Effect of austenite deformation on crystallographic texture during transformations in microalloyed bainitic steel

Abstract

The evolution of the texture of ferrite as a function of the coiling temperature has been studied in a hot rolled Nb alloyed CMnMoCrB complex phase steel by means of electron backscatter diffraction. Coiling that steel at 720 ° C led to ferrite and pearlite, and coiling at 550 ° C produced a bainite-martensite microstructure. The presence of residual austenite in the steels coiled at 680 and 550 ° C allowed for texture measurements in γ. Analyses of texture gave fundamental information on the decomposition of γ in both the recrystallised state and the deformed state. It was found that austenite, initially deformed below the non-recrystallisation temperature T_nr, recrystallised statically d partially during the γ α and the γ ^d α _b transformations. In the specimen coiled at 680 ° C, primary ferrite and bainite could be distinguished based on the confidence indexof the diffraction pattern. A clear variant selection was observed for the γ ^d α _b transformation, as arotation of ? ₁ = 30 ° occurred inthe austenite between the ferrite and the bainite formations. The bainite was found to result mainly from the decomposition of the brass {110} 〈 112 〉 and Goss {110} 〈 001 〉 orientations of deformed austenite. The residual austenite was found to be recrystallised γ γ austenite with the cube{001} 〈 100 〉 orientation. Coiling simulations were performed in a dilatometer starting from different austenite grains sizes and deformation states. In the most deformed specimens, the deformation state of the austenite and the combined effects between the different alloying elements presentin the steel were responsible for a solute drag like effect.     

奥氏体变形对22CrSH齿轮钢连续冷却相变的影响

为了研究奥氏体变形对22CrSH齿轮钢连续冷却相变的影响,在Gleeble 1500热模拟机上,将22CrSH钢在950℃变形0.4及未变形处理,然后连续冷却.利用膨胀曲线、光学显微镜、透射显微镜,结合各种腐蚀方法,分析了22CrSH钢相变行为及相变组织.研究表明:奥氏体变形使多边形铁素体加珠光体混合组织的临界冷速增大;当奥氏体变形及降低冷速时,大量的晶界仿晶型铁素体占据了奥氏体晶界,贝氏体相变向针状铁素体相变转变;变形使奥氏体在中温相变区稳定性增加,室温组织中M/A岛的数量增多.     

奥氏体变形对铌微合金钢贝氏体相变的影响

用Gleeb le-1500热模拟机研究了铌微合金钢奥氏体变形后连续冷却条件下贝氏体相变规律.研究表明,随变形温度的升高,先共析铁素体量较少,贝氏体的板条束变宽.在连续冷却条件下,贝氏体的转变量随变形量的增加而减少,随应变速率的增加而减少,但应变速率对贝氏体转变量的影响随冷却速度的增加而减弱.随着变形后保温时间的延长,奥氏体稳定性增加,在较快冷却条件下转变产物中存在残余奥氏体.     

中碳合金钢奥氏体变形应力应变分析及屈服强度的预测

奥氏体变形会影响合金钢中的相变和组织,奥氏体变形时的应力-应变影响因素比较复杂,涉及变形温度、应变速率甚至应变诱导相变的影响。本文以0.38C-1.44Si-0.82Mn-0.9Cr-0.25Mo-0.078V钢为对象,在Gleeble3500试验机上进行热模拟压缩变形实验,研究了不同变形温度下应力-应变的变化规律以及变形压缩样品的显微组织。结果表明,随着变形温度增加,应力逐渐降低,当变形温度为400℃时,因过冷奥氏体变形会诱发相变,产生了束状组织,导致应力随应变急剧增加,相较于其它较高温度的变形,呈现更大的加工硬化现象;此外,通过添加拟合参数对现有Medina模型和Eres-Castellanos模型进行拟合修正,使用修正后的模型计算本实验钢的屈服强度,结果表明其预测屈服强度与实际屈服强度基本吻合,建立了本实验钢奥氏体屈服强度预测模型,可以为其奥氏体变形工艺提供指导。     

双电极奥氏体不锈钢焊条单弧焊工艺研究

本文研究了双电极奥氏体不锈钢焊条单弧焊电弧的静特性、焊接电流、电弧电压、焊芯间隙对双电极焊条单弧焊的工艺性能和焊缝成形的影响;通过热电偶测试技术,对双电极焊条单弧焊焊接过程中不同部位焊芯表面温升进行了测定.研究结果表明焊芯直径为φ4.0mm的双电极A102焊条,其合适焊接工艺参数为:焊接电流140～160A,电弧电压45V左右,焊条两芯间隙1.2～1.5mm,焊接板厚8mm的1Cr18Ni9Ti材料,焊缝成形良好.     

Kinetics of austenite formation during continuous heating in a low carbon steel

The kinetics and microstructural evolution of austenite formation in a low carbon steel, with initial microstructure composed of ferrite and pearlite, were studied during continuous heating, by using dilatometric analysis and measurements of microstructural parameters. The formation of austenite was observed to occur in two stages: (a) pearlite dissolution and (b) ferrite to austenite transformation. The critical temperatures of austenite formation in continuous heating increase with increasing heating rate, with greater influence on the finishing temperature of austenite formation. For both the 1 °C/s and 0.1 °C/s heating rates, the formation rate of austenite reaches a maximum at approximately the finishing temperature of pearlite dissolution, and the formation rate of austenite as a function of the temperature is greater at the higher heating rate.