淬火过程中温度、组织及应力/应变的有限元模拟

为了找到淬火过程中零件产生不规则扭曲变形的原因,选择了H型截面零件为研究对象,建立了相应的有限元分析模型,利用自开发的淬火过程有限元模拟软件对温度、组织和应力/应变进行了耦合分析.通过有限元模拟,得到了温度变化、相变及相变塑性对残余应力及淬火变形的影响,给出了零件内部弹性区及塑性区的演变过程.分析结果表明:零件在淬火过程中经历"弹性-塑性-弹性-塑性-弹性"的演变过程;在淬火过程中,相变塑性导致零件发生不均匀、无规律的扭曲变形.     

基于物理模拟的相变塑性应变分离方法

相变塑性是材料在相变过程中由于外部应力引起的材料的不可逆变形。在大型金属零件的焊接和热处理等复杂热力循环过程中,相变塑性对残余应力和变形的影响不可忽视,因此,对材料的相变塑性开展研究十分必要。文章以马氏体相变塑性为例,采用施加不同单轴载荷水平的热力循环实验确定材料相变塑性;通过对该过程热力循环变形和组织演化的分析,在考虑弹性应变、热应变、组织应变基础上,得到了材料相变塑性应变随温度和应力变化的定量关系式。研究结果为分析和研究钢铁材料焊接和热处理过程中复杂热力行为提供了方法。     

淬火-碳分配-回火钢的低温组织和性能

Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb(质量分数,%)钢通过淬火-碳分配-回火(Q-P-T)工艺抗拉强度可达1250 MPa以上兼具良好塑性(大于17%),显微组织为位错型板条马氏体、微合金碳化物和薄片状残留奥氏体。通过低温拉伸试验分析了Q-P-T钢在-85～25℃下的力学性能并采用透射电镜观察了试样在25℃和-85℃时拉伸前后的显微组织。结果表明,Q-P-T钢在-70～25℃时显示了良好的低温力学性能,仅当拉伸温度低于-70℃时试样塑性开始出现大幅下降;残留奥氏体在未变形前具有良好的低温稳定性,但在变形过程中会发生马氏体相变,产生相变诱发塑性(TRIP)效应,这是Q-P-T钢具有高强度和良好塑性的主要原因。     

TC4合金相变超塑性扩散连接接头组织与性能研究

在非真空条件下对TC4合金进行了恒温超塑性扩散连接及相变超塑性扩散连接。采用莱卡尔金相显微镜、扫描电镜(SEM)及电子万能试验机分别对接头的显微组织、性能及断裂机理进行了研究分析。结果表明,在非真空条件下,TC4/TC4相变超塑性扩散连接接头焊合区界面处Ti和O形成的氧化膜在高温高压下可被破坏,O元素向基体中扩散,其分布均匀,说明TC4合金在非真空条件下进行相变超塑性扩散连接是可行的。与950℃恒温超塑性扩散连接相比,TC4合金在850℃~950℃相变超塑性扩散连接所获得的接头质量更优,界面处晶粒尺寸较小,剪切强度也较高,达到612 MPa,接头断裂方式为韧性断裂。这说明相变可提高原子扩散速率,改善连接质量,并实现低温扩散。     

S355钢焊接温度场和应力场有限元分析

S355钢作为低合金钢,在焊接过程中会伴随着固态相变现象.在考虑S355钢的固态相变效应基础上,建立了焊接过程的热弹塑性有限元模型.通过引入相变转变模型、相变塑性和相变体积模型,计算获得焊后组织分布云图,并分析了焊缝和热影响区典型节点的组织演变规律.结果表明,与不考虑固态相变效应相比,紧邻焊缝两侧的热影响区的Mises...     

装甲钢马氏体相变塑性的实验研究

采用Gleeble热模拟实验机对装甲钢马氏体相变塑性进行了研究。通过对热变形中的热应变、组织应变和弹性应变进行分离,从总应变中分离了相变塑性,并得到了相变塑性的定量关系式,采用Greenwood-Johnson模型得到该种材料的相变塑性系数。分析结果表明,该种材料的相变塑性、组织应变和热应变是引起变形的主要原因,相变塑性与组织应变数值相当,在热处理变形分析中不容忽视;随着温度降低及马氏体体积分数增大,相变塑性迅速增大,马氏体体积分数达到0.8后,相变塑性基本保持不变;随着施加应力的增加,相变塑性逐渐增大,相变塑性与施加应力呈线性关系。文章研究结果不仅为该种材料热处理、焊接过程形状畸变和残余应力研究提供基础,还为该材料复杂热力学行为研究提供参考。     

应力对核反应堆压力容器大型锻件用钢相变动力学和相变塑性影响的研究

以核反应堆压力容器 (RPV)大型锻件常用钢为对象 ,研究了应力对贝氏体相变的相变动力学及相变塑性的影响 ,得到定量描述应力诱导相变及相变塑性的模型 ,为下一步利用数值模拟优化该钢的热处理工艺打下了基础     

γ/ε双相Fe-19Mn合金在拉伸变形过程中的组织演变和加工硬化行为

采用OM、EBSD、TEM、XRD和拉伸实验等方法,研究了γ-奥氏体/ε-马氏体双相Fe-19Mn-0.0017C(质量分数,%)合金在拉伸变形过程中的组织演变和加工硬化行为。结果表明,Fe-19Mn发生了变形诱导马氏体相变,并且随着变形量的增加,相变过程由以γ→ε相变为主转变为以ε→α'相变为主。对比分析加工硬化率的变化与相含量的变化,表明ε→α'相变比γ→ε相变具有更高的加工硬化能力。同时,在变形过程中,ε-马氏体不仅发生了位错滑移,还形成了■孪晶,以满足ε-马氏体的变形协调。在γ→ε和ε→α'双重相变引起的相变诱导塑性(TRIP)效应、γ-奥氏体/ε-马氏体/α'-马氏体中的位错滑移,以及ε-马氏体的孪生变形等机制的共同作用下,Fe-19Mn的抗拉强度和总延伸率分别达到722 MPa和31%,显示出良好的强塑性匹配。     

应力应变,疲劳与脆断,断裂力学

20091057关于焊接塑性应变的计算与讨论/方洪渊…//焊接学报.-2008,29(7):60~63采用数值模拟的方法计算了平板熔焊对接接头的纵向塑性应变的分布和动态演变过程。就目前学者所提出的焊缝存在的拉伸塑性应变的观点与传统的残余压缩塑性应变理论所存在的分歧,对比了考虑熔化现象和不考虑熔化现象两种情况,分析了焊缝中心和热影响区焊接准稳态时纵向塑性应变的变化规律。结果表明,考虑熔化现象和不考虑熔化现象纵向塑性应变结果基本相同,在焊接加热过程中所产生的压缩塑性应变始终大于冷却过程中所产生的拉伸塑性应变,在考虑熔化现象的情况下,其热影响区也始终处于压缩塑性应变状态,只是在移动热源经过后,其压缩塑性应变值在冷却的过程中有所减小。图8参1020091058焊接应力变形原理若干问题的探讨(二)/王者昌//焊接学报.-2008,29(7):69~72提出焊接残余应力形成和消除原理:焊接残余应力不是压缩塑性应变引起的,而是由于焊缝和近缝区金属在“力学熔点“及以下温度冷却收缩受阻产生的;消除焊接残余应力不是产生拉伸塑性应变以减少、抵消和补偿压缩塑性应变,而是将残余弹性应变转变为塑性应变;消除焊接残余应力并不是必须去除固有...     

应力对U75V重轨钢珠光体转变的相变塑性影响

基于Greenwood-Johnson理论,采用Gleeble-1500D热模拟实验机,对U75V重轨钢连续冷却转变过程中珠光体转变的相变塑性进行研究。通过测定不同加载应力下的膨胀曲线,对总变形中的组成应变,即热应变、相变应变、弹性应变、相变塑性应变进行分离,建立重轨钢相变塑性模型方程,通过数值模拟对热处理工艺进行优化。计算结果表明,重轨钢U75V在珠光体相变过程中,热应变在总应变中占80%以上;相变塑性应变在总应变中所占比例的最大值随应力而增加,当加载应力为-45MPa、相变在577.1℃结束时,相变塑性应变占总应变的比例最大可达到12.5%。     

RECRYSTALLIZATION-INDUCED PLASTICITY IN 18Ni MARAGING STEEL UNDER ELASTIC STRESS

An approach to study the recrystallization-induced plasticity(RIP)phenomenon in the 18Nimaraging steel under the elastic stress has been made by the analysis of dynamic behaviour atelevated temperatures,the phenomenological analysis of dynamic strain feature curves,theconfirmation of phase transformation point and TEM observation of RIP processing itself.     

ORIENTATION VARIANT SELECTION AND ITS EFFECTING FACTORS IN TITANIUM SHEET

ＯＲＩＥＮＴＡＴＩＯＮＶＡＲＩＡＮＴＳＥＬＥＣＴＩＯＮＡＮＤＩＴＳＥＦＦＥＣＴＩＮＧＦＡＣＴＯＲＳＩＮＴＩＴＡＮＩＵＭＳＨＥＥＴＺ．Ｓ．Ｚｈｕ；Ｎ．Ｐ．Ｃｈｅｎ；Ｊ．Ｌ．ＧｕａｎｄＭ．Ｇ．Ｙａｎ（１）ＩｎｓｔｉｔｕｔｅｏｆＡｅｒｏｎａｕｔｉｃａｌＭａ...     

Influence of Fe addition on phase transformation behavior of NiTi shape memory alloy

Three different NiTi-based alloys, whose nominal compositions were Ni₅₀Ti₅₀, Ni₄₉Ti₄₉Fe₂, Ni₄₅Ti_51.8Fe_3.2 (mole fraction, %), respectively, were used in the current research to understand the influence of Fe addition on phase transformation behavior in NiTi shape memory alloy (SMA). The microstructure and phase transformation behavior of the alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis. The results show that the matrix of the Ni₅₀Ti₅₀ alloy consists of both B19′ (martensite) phase and B2 (austenite) phase. Moreover, the substructures of twins could be observed in the B19′ phase. However, the ternary alloys of NiTiFe exhibit B2 phase in the microstructures. Such microstructures were also characterized by large presence of Ti₂Ni precipitates dispersed homogenously in the matrix of the two kinds of alloys. The addition of Fe to the NiTi SMA results in the decrease in phase transformation temperatures in the ternary alloys. Based on mechanism analysis, it can be concluded that this phenomenon is primarily attributed to atom relaxation of the distorted lattice induced by Ni-antisite defects and Fe substitutions during phase transformation, which enables stabilization of B2 phase during phase transformation.     

高速列车车轮钢摩擦热致相变的计算机模拟

将有限元热力耦合计算与材料相变特征的实验研究相结合，研究了高速度及大载荷条件下的轮／轨摩擦导致车轮热损伤的机理，探讨了滑行速度、时间和载荷等因素对车轮踏面的温度场变化及对奥氏体相变区尺寸的影响．结果表明，在大载荷滑行情况下，接触区的峰值温度可超过材料的相变临界点，相变区深度可能达到1mm以上；滑行速度的提高可以增大相变层出现的几率，但并未增加相变层的深度；接触区峰值温度和相变区尺寸均随载荷的大小呈线性变化。     

奥-贝球铁焊缝金属的连续冷却转变图及其组织分析

为了研究焊态下奥－贝球铁焊缝在不同冷速下的组织变化及其相变规律,本文采用Ｆｏｍａｓｔｏｎ－Ｆ相变分析仪测定了在不同冷速下焊缝金属的膨胀量曲线,然后通过对曲线特征进行分析,并结合金相组织定量分析及硬度分析等方法,找到了在各冷速下的相变临界点,最终建立了奥－贝球铁焊缝的连续冷却转变图（ＣＣＴ图）,为选择焊接工艺提供了科学的基础。     

TA15钛合金焊接热影响区组织演变的数值模拟

基于相变热力学和动力学经典理论,构建了TA15焊接热影响区β-α相变组织演变元胞自动机模型,并将其应用于不同焊接热影响区位置不同冷却速度的TA15组织演变模拟.该模型考虑了冷却速度对相变过冷度和固溶体溶解度、相变形核率以及相界面迁移率的影响,充分体现了焊接快速冷却过程的相变组织演变规律,计算结果表明,在临界冷却速度范围内,冷却速度越快,扩散型相变组织产物越细小,新相长大速度越快,但最终相变分数降低.综合考虑工艺、组织与力学性能的关系,通过模拟获得最佳焊接工艺应为中等热输入.     

热处理温度对NiTiNb5相变温度和组织的影响

采用差式扫描量热仪(DSC)、扫描电镜(SEM)和金相显微镜,研究了新型材料NiTiNb5合金经200～900℃热处理后的相变温度、相变滞后(As-Ms)、相变焓以及微观组织的变化。结果表明,当热处理温度低于700℃时,不论是锻态还是轧态,合金都没有出现马氏体及其逆相变,表明铸态NiTiNb5形状记忆合金经锻造或轧制后不再具有形状记忆效应,转变成结构材料;当热处理温度提高到800℃后,NiTiNb5合金锻态或轧态又出现较明显的马氏体及其逆相变,表明NiTiNb5合金经过合适的热处理,恢复了形状记忆效应。组织分析表明,800℃后的再结晶是NiTiNb5合金恢复"功能材料"特性的主要原因。     

Determination of the continuous cooling transformation diagram of a high strength low alloyed steel

The continuous cooling transformation diagram of a high strength low alloyed steel was determined by a dilatometer and microscopic analysis (OM, SEM) as well as thermodynamic analysis. As expected, Widmanstätten ferrite, bainite and martensite coexisted for most cooling rates, which made it difficult to determine the transformation kinetics of individual phases. However, peaks were clearly observed in the dilatometric

$$\frac{{d\left( {LVDT} \right)}}{{dT}}$$

curves. By overlapping the

$$\frac{{d\left( {LVDT} \right)}}{{dT}}$$

curves, which were determined using various cooling rates, peaks were separated and the peak rate temperatures, as well as the temperature at the start of transformation (5%) and the end of transformation (95%) of an individual phase, were determined. A SEM analysis was also conducted to identify which phase existed and to quantify the volume fraction of each phase. It was confirmed that the additional

$$\frac{{d\left( {LVDT} \right)}}{{dT}}$$

curve analysis described the transformation behavior more precisely than the conventional continuous cooling transformation diagram, as determined by the volume measured from the microstructure analysis.

     

Characterization of Oxide Scales Formed on Plain Carbon Steels in Dry and Wet Atmospheres and Their Eutectoid Transformation from FeO in Inert Atmosphere

Thermal gravimetric analysis (TGA) was used to investigate plain carbon steel exposed to dry air and wet air containing 4 vol% water vapor at 750 °C to simulate the formation of tertiary scale during hot-strip rolling. After the exposure to oxidation condition, the specimens were isothermally held at temperature in the range of 300–500 °C in inert argon (Ar) atmosphere for 10–240 min, and the isothermal phase transformation behavior of FeO was investigated. The morphology, element distribution, and phase composition of oxide scales after isothermal transformation were characterized by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction, respectively. The experimental results indicate that wet air accelerated the growth rate of the oxide scale. Moreover, the time–temperature-transformation (TTT) diagrams of oxide scales grown in dry and wet air conditions were constructed to elucidate the isothermal phase transformation behavior of the FeO layer. The rate of isothermal phase transformation in the scale formed in wet air was significantly delayed compared to that in dry air, which caused the C-type TTT diagram to shift to the right. Two mechanisms elucidating the effects of initial oxidation atmosphere on the subsequent isothermal phase transformation behavior of FeO during inert atmosphere annealing were proposed.