Tensile properties and fatigue crack propagation of solution-annealed and aged superaustenitic stainless steel

The effect of aging in air at 650°C for 100~1000 h on the tensile properties of superaustenitic stainless steel in the range RT-750°C and the fatigue crack growth behaviour at RT and 650°C was studied. Yield strength and ultimate tensile strength were almost the same between the as-received and the aged specimen. The fracture strain, however, decreased significantly from aging, and the fracture surface of the aged specimen at RT test was intergranular. The fatigue crack growth rate at RT is enhanced by aging at the high stress intensity factor range. This is due to the occurrence of an intergranular fracture in the aged specimen. At 650°C the fatigue crack growth behaviour of both the as-received and the aged specimen was almost same with no intergranular fracture.     

Co基合金激光熔覆层的局部组织特征

采用定量金相法,提取激光熔覆Ｃｏ基合金熔覆层不同部位的枝晶组织特征,结果表明：随着离熔覆层与基材结合界面距离的增加,熔覆层枝晶特征尺寸随之减小,即组织不均匀;同一高度下不同部位的特征尺寸变化不明显,即组织均匀。     

激光重熔扫描速度对Co基合金凝固特性及显微组织的影响

提高扫描速度可使Ｃｏ基合金经激光表面重熔处理后显微组织明显细化。通过定量金相法测定枝晶二次臂间距查明,扫描速度提高一个数量级,熔池中部的二次臂间距减小一半。依据传统的凝固理论,结合激光重熔三维流场和温度场模型计算出的冷却速率和ＣｏＣｒＣＷ伪二元相图,计算出不同扫描速度下熔池中部的二次臂间距,计算结果和实验结果符合得很好,成功地从理论上解释了实验结果,建立了激光重熔的工艺和组织特征之间的联系。     

ZA合金枝晶臂间距的研究进展

研究ZA合金枝晶臂间距对于提高合金性能有重要意义.阐述ZA合金的枝晶臂间距,综述了冷却速率和合金元素对ZA合金枝晶臂间距的影响,概述ZA合金枝晶臂间距对合金力学性能和耐蚀性能影响规律,分析了ZA合金枝晶臂间距研究中存在的问题,并对ZA合金枝晶臂间距的研究前景进行了展望.     

Hardness, microstructure and surface characterization of laser gas nitrided commercially pure titanium using high power CO2 laser

Surface nitriding of commercially pure (CP) titanium was carried out using high power CO ₂ laser at pure nitrogen and dilute nitrogen (N ₂ + Ar) environment. The hardness, microstructure, and melt pool configuration of the laser melted titanium in helium and argon atmosphere was compared with laser melting at pure and dilute nitrogen environment. The hardness of the nitrided layer was of the order of 1000 to 1600 HV. The hardness of the laser melted titanium in the argon and helium atmosphere was 500 to 1000 HV. Using x-ray analysis the formation of TiN and Ti ₂ N phase was identified in the laser nitrided titanium. The presence of nitrogen in the nitrided zone was confirmed using secondary ion mass spectroscopy (SIMS) analysis. The microstructures revealed densely populated dendrites in the sample nitrided at 100% N ₂ environment and thinly populated dendrites in dilute environment. The crack intensity was large in the nitrided sample at pure nitrogen, and few cracks were observed in the 50% N ₂ + 50% Ar environment.     

Erosion damage of laser alloyed stainless steel in mercury

The effect of laser surface alloying of type 316 stainless steel on the erosion resistance in mercury has been investigated. The alloying was produced by melting predeposited Al-Si powder and a portion of underlying substrate with a pulsed Nd:YAG laser beam. The microhardness of the modified layer was found to be 2.5 times higher than that of untreated steel. The erosion test of laser alloyed surface and steel in mercury was carried out by using the electromagnetic impact testing machine. The laser alloyed surface was found to be less damaged after 10⁵ cycles of impacts compared to untreated stainless steel. However, after 10⁶ cycles the erosion resistance of the modified layer is much lower than that of untreated steel. Liquid metal embrittlement in contact with mercury and residual stresses were considered as factors impairing the erosion resistance of the laser alloyed surface.     

Laser surface hardening of austenitic stainless steel

For the purpose of studying the possibilities of increasing the wear resistance, keeping a high level of corrosion strength, austenitic stainless steel specimens mainly containing 19.2%Cr and 9.4%Ni were two-step surface alloyed using added materials (AMs) with hard particles of carbides (WC), nitrides (TiN), and borides (TiB₂). The simultaneous melting of AM and surface layer was performed by a CO₂ continuous wave laser on a numerically controlled X-Y table. On these specimens, the microstructural characteristics, microhardness, and depth of the molten zone were determined, which allowed definition of the AM with the best hardening effect. The research continued by two-step laser surface alloying of the same base material with different effective AM quantities. The specimens were processed by continuous wave laser radiation, by multiple-pass with 35% overlap. The alloyed layers were described by light optical microscopy, x-ray diffractometry, flash spectrometry, and hardness measurement. The conditions to obtain compact surface layers with 2.5 to 3 times higher hardness than the base material were determined.     

不锈钢表面激光熔覆技术研究现状与展望

对不锈钢表面激光熔覆技术的研究现状进行了综述,详细介绍了现阶段不锈钢表面激光熔覆材料的研究进展以及影响熔覆层质量的各项因素,并展望了不锈钢表面激光熔覆技术的发展方向。     

Modifying the steel surface by laser heating

Methods of increasing the service properties of steel components by producing hardened modified layers on the surface using laser heating are described. It is proposed to use a combined technology based on laser alloying the steel surface with nitride-forming elements and nitriding. It is shown that laser alloying in the continuous mode results in the formation of a layer with a uniform fine-grained structure with a thickness of 600 μm in the surface layer. Subsequent nitriding eliminates the unfavourable residual stresses and increases the microhardness of low-carbon steels to 20,000 MPa, cracking resistance 1.5–1.8 times and wear resistance 1.5–3 times.     

不锈钢表面激光合金化Mn-Al2O3强化层的组织及性能

基于高速混合机桨叶所用奥氏体不锈钢的失效情况,利用激光合金化技术在奥氏体不锈钢上制备合金化强化耐磨层。利用光学显微镜、扫描电镜、X射线衍射仪、显微硬度计和光学分析仪研究合金化层的显微组织、成分、物相、横截面的硬度分布,同时利用正交试验得到激光合金化技术的优化参数。结果表明:当硬质相Al2O3的含量为70%,激光功率为2 kW,扫描速度为15 mm/s时,合金化层的显微硬度最大,达到440.8 HV0.1;所制备的合金化层无裂纹和气孔等缺陷,与基体形成冶金结合;合金化层由枝晶和枝晶间共晶组成,其相组成为奥氏体、Al2O3、Cr7C3、Mn7C3和Mn2O3;合金化层的耐磨损性能较基体的显著提高。     

不锈钢活性激光焊工艺研究

以不锈钢SUS304作为试验对象,分别研究了焊接速度、散焦离焦量、激光功率和保护气体等工艺参数以及活性剂和活性元素硫对不锈钢YAG激光焊焊缝成形的影响.试验结果表明,各焊缝表面成形良好,焊接速度和激光功率能明显影响焊缝成形,散焦离焦量和保护气体种类对YAG激光焊焊缝成形的影响较小.与硫含量较低的情况相比,活性剂和活性元素硫含量较高的情况下均能使得焊缝熔宽发生明显收缩,随着热输入量增加,活性剂增加熔深的效果也越明显,表明活性剂对熔深的影响效果与焊接过程的热输入量有密切联系.     

27SiMn钢表面激光熔覆304不锈钢的组织和性能

利用激光熔覆技术在液压支架立柱母材27SiMn钢表面进行了不同激光功率的单道激光熔覆304不锈钢试验,选择出熔覆层质量最佳时的激光功率,并在该功率下进行多层累加激光熔覆304不锈钢试验。分析了熔覆层材料的显微组织,对比分析了27SiMn钢基体和304不锈钢熔覆层的拉伸性能和断口形貌。结果表明,熔覆层和基体之间实现了良好的冶金结合,熔覆层材料中呈现出了具有典型定向凝固特征的柱状晶;熔覆层材料的抗拉强度与基体相当,伸长率明显高于基体;熔覆层和基体材料的拉伸断口处均出现了具有典型塑性断裂特征的韧窝,且熔覆层材料的韧窝尺寸及深度明显大于基体材料。     

模具钢激光表面改性技术的研究进展

介绍了激光表面淬火、激光熔覆及激光表面合金化在模具钢表面改性技术方面的应用特点和研究现状,提出了实际应用中存在的问题及今后需深入研究的方向,为激光表面处理技术应用于模具钢的表面处理提供参考。     

Microstructural characterization of Inconel 718 alloy after pulsed laser surface treatment at different powers

An annealed Inconel 718 alloy was surface-treated by pulsed laser at three different powers (100, 50 and 25 W). Microstructural changes induced by the laser treatments were characterized by use of electron backscatter diffraction and electron channeling contrast imaging techniques. Results show that both annealing twins and strengthening precipitates profusely existing in the as-received specimen are dissolved at elevated temperatures during the laser irradiation. Meanwhile, in the melting zone (MZ), densities of low angle boundaries (LABs) are greatly increased with a large number of Laves phases preferentially distributed along such LABs. For different specimens, widths and depths of their MZs are found to be gradually reduced with decreasing the laser powers. Orientation analyses reveal that the columnar grains in the MZ of the 100 W specimen could inherit orientations existing in the matrix while lower laser powers promote the formation of more nuclei with scattered orientations to grow to be granular grains in the MZ. Hardness tests reveal that the MZs of all laser-treated specimens are softer than the matrix probably due to both precipitate dissolution and grain coarsening.     

冷却速度对铸件二次枝晶臂间距影响的模拟研究

设计了45#钢阶梯凝固试验并进行了有限元分析,研究了不同冷却速度对二次枝晶臂间距尺寸的影响。根据模拟结果拟合了冷却速度与二次枝晶臂间距经验公式,为判断铸件冷却温度、建立高温扩散退火预判机制、降低铸件偏析和避免裂纹的产生提供理论依据。     

低碳钢表面激光熔覆不锈钢的组织和性能

采用JHM-1GY-400型脉冲Nb∶YAG固体激光器和316L不锈钢粉末在20低碳钢表面制备了激光熔覆层。利用OM、XRD、SEM等表征方法分析了不锈钢熔覆层的物相组成和显微组织,并分别利用旋转摩擦试验机和电化学工作站对熔覆层和基材的耐磨损和耐腐蚀性进行了研究。试验结果表明,不锈钢熔覆层厚度约为50 μm,由γ相(奥氏体)和α相(铁素体)组成,其显微组织主要包括细小的树枝晶、粗大的胞状晶以及平面晶;不锈钢熔覆层表面硬度约为基材的2倍,摩擦因数比基材低0.0418,磨损量更低,不锈钢熔覆层比基材具有更高的耐磨性。与基材相比,不锈钢熔覆层具有更低的自腐蚀电流和更高的自腐蚀电位,其耐腐蚀性能更优异。     

激光表面强化技术在钢铁企业中的应用

激光表面强化技术目前已成为表面工程一个十分活跃的新兴领域,其独有的特点满足了钢铁冶金机械设备中一些关键零部件的维护修复需求。详细介绍激光表面强化技术在宝钢冶金机械设备备件修复中的应用实践,如采用激光熔覆技术对飞剪转鼓扁头、万向节轴承孔、轧机主传动轴和轧机牌坊进行了表面修复及强化,大大延长了部件的使用寿命;同样通过对轧辊表面激光合金化和对CDQ旋转密封环、烧结台车轨道密封板进行激光表面淬火处理,取得了良好的效果。探讨该技术在生产实际应用过程中存在的问题,为该技术在钢铁行业中的应用提供借鉴和指导。     

Microsegregation-related pitting corrosion characteristics of AL-6XN superaustenitic stainless steel laser welds

Pitting corrosion resistance of laser welds of AL-6XN superaustenitic stainless steel (SASS) was investigated in acidic chloride ion medium. It was found that the critical pitting temperature (CPT) of the laser welds increased with increasing welding speed or decreasing laser power. Pitting attack preferentially occurred at selective dendrite cores of the laser welds. Analytical electron microscope (AEM) microanalysis revealed that depletion of Mo at dendrite cores due to microsegregation is the basic cause for the pitting corrosion susceptibility. The higher partition coefficient k_Mo and Mo concentration at dendrite cores of laser welds were attributed to the lower heat input welding parameters.     

Influence of processing conditions on structural characteristics of hybrid plasma surface alloyed austenitic stainless steel

A hybrid plasma surface alloying process has recently been developed for austenitic stainless steels to improve their surface hardness, wear resistance and corrosion resistance. The process is carried out in nitrogen and methane gas mixtures at temperatures below 450°C and facilitates the simultaneous incorporation of nitrogen and carbon into the surfaces of austenitic stainless steels, forming a dual layer structure with an extremely hard nitrogen-enriched layer on top of a hard carbon-enriched layer. The present paper discusses the influence of three most important processing parameters, i.e., gas composition, temperature and time, on the structural characteristics of the alloyed zones produced on AISI 321 stainless steel, in terms of layer morphology, growth kinetics and chromium compound precipitation. It was found that the development of the alloyed layers is diffusion-controlled, and under proper processing conditions, a precipitation-free dual-layer structure can be produced, with nitrogen and carbon dissolved in the relevant layer forming N-expanded austenite and C-expanded austenite respectively. Based on the experimental results, a threshold temperature-time curve has been established for the investigated austenitic stainless steel.