Austenitic stainless steels are currently used for the manufacture of reusable medical instruments, thanks to their combination of elevated strength, corrosion resistance, biocompatibility and low cost. However, during their service life, they are subjected to repeated sterilization cycles, which lead to the loosening of the surface oxide stability and, thus, decrease their durability. In the present study, the influence of different machining cooling conditions on the in-service corrosion behaviour of the AISI 316L stainless steel is investigated with the aim of increasing its resistance to the sterilization cycles. In particular, the effects of low-temperature coolants, namely Liquid Nitrogen (LN2) and gaseous Nitrogen (N2) cooled by LN2 to -100 degrees C, were investigated and compared to those of dry and wet cutting. The machined surface integrity was characterized in terms of microstructure, hardness, residual stresses and surface roughness. Electrochemical studies, making use of potentiodynamic and cyclic polarization tests, were carried out to evaluate the corrosion resistance of the machined surfaces. It was found that the AISI 316L microstructure near the machined surface was significantly affected by the machining cooling strategies The effectiveness of using low-temperature coolants in machining of stainless steels for the manufacture of reusable medical devices was therefore demonstrated.

High strength stainless steel 0Cr16Ni6 treated by quenching+low temperature tempering, and maraging stainless steel 00Cr11Ni11MoTi treated by over aging were studied by means of mechanical properties tests, SEM and XRD, and the effects of retained/reversed austenite in the tested steels on -196℃ cryogenic notched tensile strength and impact property were analyzed. The results show that, with the nearly same strength and toughness of the tested steels at room temperature, the notch tensile strength and impact property of the 0Cr16Ni6 steel are significantly higher than that of the 00Cr11Ni11MoTi steel at -196℃. According to the residual/reversed austenite contents quantitatively calculated based on the relative intensities of the martensite and austenite diffraction peaks far from the fracture and near the fracture, it is found that nearly 90% retained austenite in the 0Cr16Ni6 steel transforms into strain-induced martensite during the formation and propagation of cracks, which improves cryogenic impact property remarkably. But the reversed austenite formed by over-aging of the 00Cr11Ni11MoTi steel has higher stability, which is not suitable for strain-induced martensite transformation to occur, so the improvement of -196℃ cryogenic toughness is limited.

采用力学性能测试、SEM和XRD等手段研究了淬火+低温回火处理的0Cr16Ni6高强度不锈钢和过时效处理的00Cr11Ni11MoTi马氏体时效不锈钢，并分析了残留/逆转变奥氏体对试验钢超低温缺口抗拉强度和冲击性能的影响。结果表明，在两种试验钢室温强韧性相近的情况下，0Cr16Ni6钢在超低温下（-196℃）的缺口抗拉强度和冲击性能显著优于00Cr11Ni11MoTi钢。根据冲击试样远离断口和断口附近马氏体/奥氏体衍射峰的相对强度分别定量计算的残留/逆转变奥氏体含量，发现在裂纹形成和扩展过程中0Cr16Ni6钢有接近90%的残留奥氏体通过应变诱发相变生成马氏体，显著改善了超低温韧性；而过时效00Cr11Ni11MoTi钢形成的逆转变奥氏体具有较高的稳定性，难以发生应变诱发马氏体相变，改善超低温韧性作用程度有限。

利用冲击实验、拉伸实验、XRD和TEM对2种不同N含量的无Ni高N奥氏体不锈钢低温变形行为进行了研究. 结果表明, 高N奥氏体不锈钢在低温下发生明显的韧脆转变和加工硬化现象. 在实验材料的Mn含量水平内, 提高Mn含量能够改善高N奥氏体不锈钢的低温塑性和韧性, 降低其韧脆转变温度. 18Cr-12Mn-0.55N钢在低温拉伸变形时会发生形变诱导马氏体相变, 但马氏体转变量很少,降低温度对马氏体转变量无明显影响. 形变诱导马氏体能提高高N奥氏体不锈钢的加工硬化能力, 但降低了钢的低温塑性和韧性. 加工硬化能力和层错能随温度的降低而降低是Fe-Cr-Mn高N奥氏体不锈钢在低温下发生脆断的主要原因.

The uniaxial tensile property of 316LN austenitic stainless steel (ASS) plate at -40 ℃was examined by strain rates of 5×10-4 s-1 and 1×10-2 s-1 respectively, while the microstructure evolution was characterized by means of OM, TEM, SEM, XRD and 3D profile profiler. The results showed that the deformation induced martensite transformation occurred in 316LN austenitic stainless steel at cryogenic temperature, and the martensite transformation decreased with the increase of strain rate. The yield strength increased with the increase of strain rate, while the tensile strength and elongation decreased with the increase of strain rate. The tensile fractured surface showed typical ductile fracture. The deformed microstructure composed mainly of dislocation tangles and T-M(twin-matrix)lamellar structures. With the increase of strain rate, the dislocation tangles aggravated and the interlamellar spacing of T-M(twin-matrix)lamellar structures reduced.

以两种应变速率(5×10^-4 s^-1和1×10^-2 s^-1,分别代表慢速拉伸和快速拉伸)对316LN奥氏体不锈钢板状试样进行低温(-40℃)单轴拉伸实验,借助金相显微镜(OM)、透射电镜(TEM)、扫描电镜(SEM)、X射线衍射仪(XRD)及三维形貌轮廓仪分析了拉伸变形过程中的微观组织演变和力学性能变化规律。结果表明,316LN奥氏体不锈钢在低温拉伸条件下发生形变诱导马氏体相变,且马氏体转变量随着应变速率的增加而减少;屈服强度随着应变速率的增加而升高,抗拉强度和延伸率则随着应变速率的增加而降低;拉伸断口形貌均呈现出典型的韧性断裂特征。变形组织均以位错缠结和T-M(Twin-matrix)层片状组织为主,随着应变速率的增加,位错缠结程度加剧,T-M层片状组织的层片间距减小。

The microstructure and texture evolution in a cold-rolled AZ31 magnesium alloy during electropulsing treatment (EPT) are investigated and correlated with the mechanical properties. The microstructure is effectively refined, and a tilted basal texture develops gradually during EPT. The yield stress in the treated samples is lower than that in the cold-rolled sample, indicating that texture softening is dominant over strengthening because of grain refinement. The phenomenon is primarily the result of the tilted basal texture. EPT improves the tensile ductility of the EPT samples significantly, albeit slightly compromising the tensile strength. The mechanism of the microstructure evolution during electropulsing is discussed from the viewpoint of grain-boundary motion. Moreover, the ductility enhancement is discussed in terms of the deformation mechanism and texture of the Mg alloy.

Advanced material processing techniques have been successfully used to produce metals or alloys with submicro- or nano-sized grain structures with some possibly required harsh working environment that limits their industrial application. Cryogenic deformation might promote extensively severe deformation or distortion of metals or alloys (such as Al or aluminium alloys, Cu or copper alloys, Ti, Zr, etc.) so as to accumulate higher deformation energy (e.g., higher defect density) for the depression of the (dynamic) recovery, which will contribute to the microstructure refinement. Presently, the macro-/micro-structural evolution, the martensitic transformation as well as its effect on the mechanical property during the cryogenic and room temperature rolling of 304 metastable austenitic stainless steel were studied. It shows that the cryogenic rolling can effectively accelerate the martensitic transformation, e.g., after 20% cryogenic rolling the volume fraction of the transformed martensitic is equal to that after 50% room temperature rolling, and finally the cryogenic rolling can promote the complete martensitic transformation. Also the through-thickness uniformity of the martensitic transformation after cryogenic rolling is significantly better than that of the room temperature rolled one, which can help to improve the through-thickness performance uniformity. It is found that the deformation mechanisms are different for cryogenic and room temperature rolling metastable austenitic stainless steel: the martensitic transformation and its deformation occur in the former while austenitic deformation in the latter. The cryogenic rolling can quickly induce higher hardness than that of the room temperature rolled one, and the hardness tends to be equal finally because of the minimized dislocation density difference between these two rolled steels. TEM results indicate that the orientation relationship between the transformed martensite and the old austenite in the cryogenic and room temperature rolled sheets can still keep the K-S (Kurduumov-Sachs) relationship.

研究了304亚稳态奥氏体不锈钢在超低温和室温轧制变形过程中的宏、微观组织演变, 变形引起的马氏体转变及其对合金性能的影响. 结果表明, 超低温轧制比室温轧制能更有效地加速马氏体转变, 其中20%超低温轧制变形便可实现50%室温轧制变形下的马氏体转变量, 且超低温轧制变形最终可实现完全的马氏体转变. 同时, 超低温轧制引起的马氏体转变在板厚方向上较均匀, 显著优于室温轧制板材的板厚方向均匀性, 有助于提高亚稳态奥氏体不锈钢板厚方向性能的均匀性. 分析认为, 亚稳态奥氏体不锈钢在超低温和室温轧制过程中具有不同的变形机理, 前者主要以马氏体转变及其变形为主, 后者以奥氏体变形为主. 超低温轧制所获板材的硬度比室温轧制板材增长迅速, 但随变形量增大位错密度差距缩小, 最终导致两者硬度趋于一致. TEM表征结果表明, 超低温和室温轧制过程中引起的马氏体与母相基体间的取向关系遵循K-S (Kurduumov-Sachs)关系.