论文部分内容阅读
奥氏体不锈钢等离子渗氮时会形成膨胀奥氏体（γN），其强度、韧性和耐蚀性均高于传统的氮化物层。然而，膨胀奥氏体在热力学上是亚稳的，其性能会因其发生分解而降低。本文对AISI 316L 不锈钢进行了等离子渗氮，工艺为450℃×5 h，压力500 Pa。对渗氮后试样采用XRD、OM和TEM等进行了显微组织表征。试验结果证明了面心立方膨胀奥氏体的存在，其晶格常数比未经渗氮奥氏体增加多达9．5％。薄片试样的TE M分析表明，N层中有细小的氮化物形成，并且发现一些区域呈奇特的层片状形貌，与碳钢中的珠光体团很相似。选区电子衍射（SAED）分析表明，这些区域由膨胀奥氏体局部分解产生的体心立方铁素体和立方晶系氮化铬组成。在所研究试样中的某些区域，发现有非晶态膨胀奥氏体。N的分解与膨胀奥氏体层局部区域中铁素体稳定元素（Cr，Mo）的显微偏析和奥氏体稳定元素（Ni）的贫化有关。“,”Expanded austenite (γN ),which can be produced during plasma nitriding of austenitic stainless steels, provides high levels of strength,toughness and corrosion resistance by comparison with traditional nitride layers. However,expanded austenite properties can be lost due to decomposition caused its thermodynamic metastability. In the present work,austenitic stainless AISI 316L steel was plasma nitrided at 450 ℃ for 5 h at 500 Pa and microstructurally characterised by X-ray diffraction (XRD ),and optical and transmission electron microscopy (TEM)which confirmed the presence of fcc expanded austenite with a lattice parameter up to 9.5% larger than untreated austenite.TEM analyses of thin foils showed that fine nitrides were formed in the N layer and some areas were observed with a singular lamellar morphology very similar to the pearlite colonies found in carbon steels. Selected area electron diffraction (SAED)analysis suggests that these areas are composed of bcc ferrite and cubic chromium nitrides produced after a localised decomposition of the expanded austenite layer.Amorphous expanded austenite was observed in some areas of the investigated samples.The occurrence of N decomposition was associated with microsegregation of ferrite stabilisers (Cr,Mo)and depletion of an austenite stabiliser (Ni)in localised regions of the expanded austenite layer.