Effects of Ni on austenite stability and fracture toughness in high Co-Ni secondary hardening steel

Three kinds of high Co-Ni secondary hardening steels with different Ni contents were studied.The nanoscale austenite layers formed at the interface of matensite laths were observed.Both observation and diffusion kinetic simulation results showed that both Ni and Co did not obtain enough time to get the equilibrium content in this system.The Ni content in austenite layers decreased significantly,and Co content increased slightly with the decrease of Ni content in overall composition.The austenite stability was estimated by Olson-Cohen model,in which both chemical and mechanical driving force could be calculated by equilibrium thermodynamic and Mohr′s circle methods,respectively.Simulation and mechanical test results showed that the decrease of Ni content in austenite layers would cause the change of austenite stability and decrease the fracture toughness of the steels.When the Ni content in the overall composition was lower than 7wt.%,the Ni content inγphase would be lower than 20 wt.%.And the simulation value of Mσs(stress-induced critical martensite transformation temperature)would be up to 80°C,which was about 60°C higher than room temperature.Based on the analysis,the Ni content in the overall composition of high Co-Ni secondary hardening steels should be higher than 8wt.% in order to obtain a good fracture toughness.     

Variation in electric resistivity in metastable alloys during thermomechanical loading: Effects of temperature,elasticity, plasticity and phase transformation

301L metastable stainless steel is a ductile material in which there is strong coupling between plasticity and phase transformation, leading to strongly non-linear thermomechanical behaviour. To determine phase transformation kinetics, isothermal tensile tests at different temperatures were carried out and in situ voltage (electric resistance) was measured. We then chose an appropriate post-processing method based on observed voltage using strain and temperature. It was thus possible to determine the effect of temperature, elasticity, plasticity and phase transformation on the electric resistivity of the studied material. After identifying the effect of each strain mechanism, a volume phase fraction determination method based on electric resistivity variation was developed and used to determine the kinetics of phase transformation. Finally, we compared our results with those of two classic methods: the neutron diffraction method and the magnetic method.     

奥氏体不锈钢件的等离子弧焊接工艺研究

等离子弧焊接技术是近年来发展起来的机械零部件焊接新技术。本文研究了等离子弧焊接技术在奥氏体不锈钢零部件损伤修复中的应用,提出了等离子弧焊接修复奥氏体不锈钢件的工艺及条件。     

Theoretical investigation of the interaction of oxygen with pure and K-doped NiTi shape memory surface alloys

Density functional based theories and experiments agree well on the value of the heat of formation of TiO₂ on NiTi surfaces. However, experimental studies of polycrystalline NiTi surfaces tend to indicate that Ti atoms are always abundantly available at the surface. This theoretical study indicates that whether Ti atoms are dominantly available at the surface depends on the surface index. On NiTi(001) in the B2 phase, the surface can be Ti or Ni terminated, with equal probability, while on NiTi(110) in the B2 phase and NiTi(010) in the B19? phase, Ti atoms are favored to be present at the surface.     

铁—锰—铝—铬奥氏体钢难切削性的对策

本文报导了铁锰铝铬奥氏体钢切削加工的变形系数、摩擦系数、切削温度、切削力的测定和分析,切屑层片结构形貌和切屑根部显微组织观察和研究。结果表明,这类钢的切削机理及特点仍符合金属切削的一般规律,但其最大摩擦系数和最小切屑变形系数都出现在较低的切削速度范围内。同时,根据最佳切削温度原则,采用径向磨损率最小的方法,进行切削用量实验优化,建立了可供现场应用的进给量和最佳切削速度的关系方程。     

等通道热挤压00Cr19Ni10奥氏体不锈钢的晶粒细化机制

Ф8mm00Cr19Ni10不锈钢（％：0．025C、18．75Cr、10．96Ni）经700℃、挤压速度9mm／s、8道次等通道挤压后获得150～350am超细晶组织。通过光学显微镜，扫描和透射电镜，X-射线衍射，分析了在等通道挤压变形过程钢的微观组织演变，提出晶粒细化的位错、孪晶和动态再结晶机制，研究发现在1～4道次以孪晶细化机制为主，5～8道次以动态再结晶细化机制为主。     

奥氏体中碳活度的计算

根据奥氏体中碳原子与铁原子间的关系,推导出奥氏体中碳活度与浓度的关系式为: α_C=(x_(CS)~(-1)-m)/(x_C~(-1)-m)。按此式计算出的结果与实验值相符程度远优于捷姆金公式。另外还提出一个用来计算在一定碳活度值下,不同温度的碳浓度值的经验式: ％C=d b(t-800)~n。     

管线钢奥氏体晶界的浸蚀及测定

介绍一种显示管线钢奥氏体晶界的浸蚀剂及奥氏体晶粒平均尺寸的测定方法。结果表明，此浸蚀剂是管线钢一种理想的奥氏体晶界浸蚀剂，所采用的测定方法的精确度较高。     

硼向奥氏晶界的扩散机制

通过解变温扩散方程 ,导出了硼在非平衡晶界偏聚的理论公式 ,建立了晶界贫硼区宽度与淬火加热温度、冷却速度以及非平衡晶界偏聚扩散激活能与扩散常数之间的关系 ,理论预言与实验结果能较好地吻合 .     

Role of transformed martensite in the cracking of stainless steel plumbing lines

Austenitic stainless steel AISI 321 is used to fabricate plumbing lines to transfer cryogens from storage to output collector of propulsion system. Cracks were found in some of such tubes during one of the qualification tests of pipelines. The crack occurred at the bent region of the fuel feed pipeline along its neutral axis. Detailed metallographic investigations revealed presence of numerous longitudinal fine cracks on the inner surface of the tube that resulted from improper extrusion of the tube. Further, deformation induced martensitic transformation occurred, as the tube was bent at liquid nitrogen temperature. The transformed martensite, in presence of fine cracks on inner surface of the tube, due to improper extrusion, resulted in crack opening. This paper highlights the details of investigations carried out.