高温时H13钢中初生碳氮化物的分解研究

利用光学显微镜、扫描电镜和电子探针研究了H13钢中初生碳氮化物高温分解时的形貌、尺寸、成分变化规律.原始初生碳氮化物主要为10~30μm的长条状(Vx,Mo1-x)(Cy,N1-y)及少量方形的(Tix,V1-x)(Cy,N1-y).在1200℃保温2.5 h后碳氮化物边缘变为凹凸不平的锯齿状,然后形成细小的分解颗粒,10 h后碳氮化物平均长度减小为12.9μm,主要为(Tix,V1-x)(Cy,N1-y).当经过1250℃ ×5 h保温后87%的碳氮化物发生分解,(Vx,Mo1-x)(Cy,N1-y)溶解消失,碳氮化物长度在20μm以下,当保温时间延长到10 h后碳氮化物长度均在10μm以下,70%为方形并且93%分解形成细小颗粒,未分解的碳氮化物为(Tix,V1-x)(Cy,N1-y).电子探针分析(Tix,V1-x)(Cy,N1-y)的分解与Fe元素扩散有关,高温时Fe在(Tix,V1-x)(Cy,N1-y)中含量逐渐增加而Ti、V减少,优先在边部曲率半径较小部位或缺陷处分解,形成0.1~1μm的细小分解颗粒,并由外向内以区域溶解方式使原始碳氮化物逐渐消失.双亚点阵模型分析两种碳氮化物的平衡溶解温度和组成有关,试样中大部分(Tix,V1-x)(Cy,N1-y)平衡溶解温度在1200~1246℃之间,与实验吻合较好.     

余热处理含Nb钢筋中Nb的作用

采用热模拟、透射电镜、电解萃取相分析及电子衍射等手段对余热处理含Nb钢筋中Nb的作用进行了考查。发现经余热处理的含Nb钢筋中,微量Nb的2/3以Nb(C,N)的形式存在,其余为固溶态。固溶于奥氏体和由奥氏体析出的Nb(C,N),强烈阻止了形变奥氏体的再结晶,从而细化了形变奥氏体及相变后的组织,由铁素体或从相界析出的极细Nb(C,N)发挥了析出强化作用。细晶强化及析出强化的共同作用减少了焊区强度降。     

基于双亚点阵模型对H13钢中初生碳氮化物的研究

实验证明H13钢中存在三类初生碳氮化物,包括富V和C的(Vx,Mo1x)(Cy,N1-y)、富Ti和N的(Tix,V1x)(Cy,N1-y)及富V和C的(Tix,V1-x)(Cy,N1-y),部分存在形核核心.基于双亚点阵模型和凝固偏析模型分析计算H13钢凝固过程中元素含量变化,考虑初生碳氮化物中元素间的相互作用,根据三类碳氮化物中合金元素含量和C/N比值不同,对各碳氮化物的生成自由能进行分析.理论研究指出富Ti和N的(Tix,V1-x)(Cy,N1-y)在固相率为0.90时即可生成,而富V和C的(Tix,V1-x)(Cx,N1-y)在固相率大于0.96才可生成,两类碳氮化物的临界生成固相率fP均随固溶Ti和N含量的增加而减小.(Vx,Mo1-x)(Cy,N1-y)的临界生成固相率与x值密切相关,x减小时fP降低,粒子尺寸增加,实验与理论分析吻合良好.根据错配度分析氧化物和碳氮化物对H13钢中初生碳氮化物异质形核的作用.     

Coarsening behavior of MX carbonitrides in type 347H heat-resistant austenitic steel during thermal aging

In this work, the growth kinetics of MX (M = metal, X = C/N) nanoprecipitates in type 347H austenitic steel was systematically studied. To investigate the coarsening behavior and the growth mechanism of MX carbonitrides during long-term aging, experiments were performed at 700, 800, 850, and 900℃ for different periods (1, 24, 70, and 100 h). The precipitation behavior of carbonitrides in specimens subjected to various aging conditions was explored using carbon replicas and transmission electron microscopy (TEM) observations. The corresponding sizes of MX carbonitrides were measured. The results demonstrates that MX carbonitrides precipitate in type 347H austenitic steel as Nb(C,N). The coarsening rate constant is time-independent; however, an increase in aging temperature results in an increase in coarsening rate of Nb(C,N). The coarsening process was analyzed according to the calculated diffusion activation energy of Nb(C,N). When the aging temperature was 800–900℃, the mean activation energy was 294 kJ·mol-1, and the coarsening behavior was controlled primarily by the diffusion of Nb atoms.     

Mechanism of generation of large (Ti,Nb,V)(C,N)-type precipitates in H13 + Nb tool steel

The characteristics and generation mechanism of (Ti,Nb,V)(C,N) precipitates larger than 2 μm in Nb-containing H13 bar steel were studied. The results show that two types of (Ti,Nb,V)(C,N) phases exist—a Ti-V-rich one and an Nb-rich one—in the form of single or complex precipitates. The sizes of the single Ti-V-rich (Ti,Nb,V)(C,N) precipitates are mostly within 5 to 10 μm, whereas the sizes of the single Nb-rich precipitates are mostly 2–5 μm. The complex precipitates are larger and contain an inner Ti-V-rich layer and an outer Nb-rich layer. The compositional distribution of (Ti,Nb,V)(C,N) is concentrated. The average composition of the single Ti-V-rich phase is (Ti_0.511V_0.356Nb_0.133)(C_xN_y), whereas that for the single Nb-rich phase is (Ti_0.061V_0.263Nb_0.676)(C_xN_y). The calculation results based on the Scheil–Gulliver model in the Thermo-Calc software combining with the thermal stability experiments show that the large phases precipitate during the solidification process. With the development of solidification, the Ti-V-rich phase precipitates first and becomes homogeneous during the subsequent temperature reduction and heat treatment processes. The Nb-rich phase appears later.     

V-Ti-N微合金非调质无缝油井管钢中碳氮化物的热力学计算

用包括奥氏体γ相和两个碳氮化物相三相溶解度间隙平衡处理的方法计算了V-Ti-N微合金非调质油井管钢中的碳氮化物析出. 计算结果表明,此钢奥氏体中的析出模式为1 473 ℃时TiN即开始析出;其后部分TiN逐渐转变为复合(TixV1-x)(CyN1-y)颗粒,而其他的TiN直到低温仍保持其化学性质;最后富V-C的V(CxN1-x)在846 ℃开始析出. 实验数据验证了这种析出模式. 计算结果支持了中碳含钒微合金钢中800 ℃以下奥氏体中的析出对其后的奥氏体分解相变具有明显的调控作用的观点.     

铌对高钢级管线钢中第二相析出与奥氏体晶粒长大行为的影响

基于双亚点阵模型,计算了两种不同铌含量的高钢级管线钢在不同温度下Nb、Ti和Al的析出量,测定了不同加热温度和保温时间下奥氏体晶粒尺寸,建立两种钢奥氏体晶粒长大模型.发现Nb含量增加提高了其全固溶温度,并且温降过程中Nb析出量显著增多,在晶界两边析出的细小碳氮化物对奥氏体晶粒长大有显著的阴碍作用.高铌钢加热温度为1250℃时奥氏体晶粒显著粗化,预测模型也不同于1050～1200℃的模型,但相同保温温度下晶粒尺寸明显小于低铌实验钢.通过数据拟合计算出高铌钢的长大激活能远远高于低铌钢,再次证明高Nb的管线钢在1200℃以下能够有效地细化奥氏体晶粒,预测模型与实验值吻合较好.