Research On The Stress Corrosion Cracking Susceptibility Of The Stainless Steel With Surface Nanocrystallization By Small Punch Test

The technology of surface nanocrystallization (SNC) is becoming a hot area since the1990s, more and more scientists focus on this domain. However, the present SNC methods could not realize industrialization and batch processing easily, a new method is needed urgently. After SNC, the property, mechanical behavior and chemical behavior of materials will change greatly, mastering these knowledge could enable people to maximize the advantages and avoid the deficiencies of these SNC materials in engineering application. At the same time, the scientists want to find out whether the materials with SNC could retard the stress corrosion cracking (SCC). More and more researching reports on SCC of the SNC materials are brought out. However, there are also many problems and confusions in this area due to the deficiency of experimental technique. As we all know, the SCC is the common failure for austenitic stainless steel (ASS) application, so it is necessary to make clear the SCC mode and mechanism of ASS with SNC. In this work, a newly developed SNC method-surface mechanical rolling treatment (SMRT) was studied which can be applied in different environment and easily industrialized. Then, the comparison betweent the SMRT and the traditional SNC method-surface mechanical attrition treatment (SMAT) was drawn in detail. The mechanical properties of the SS304L with SNC were firstly analysed by small punch test (SPT) in different areas (base metal, heat affected zone and welded seam). At the same time, a new SCC evaluating technique was developed on the basis of SPT, which is a useful complement to the traditional SCC testing methods. The SCC susceptibility of SS304L with SNC was firstly studied in different areas (base metal, heat affected zone and welded seam) by SPT-SCC technique.The main study contents and conclusions are listed as follows:(1) The SNC technique-SMRT was used to generate nano-grain layer on the surface of SS304L in both liquid nitrogen and normal temperature environment successfully. The evolutions of structural and mechanical properties were characterized and analyzed. A detailed comparison between SMRT and the surface mechanical attrition treatment (SMAT) was drawn. It is proven that the SMRT is a useful method to realize material’s SNC which has the industrialized potential. The SNC experiments using SMRT could be carried out in different environments with very high energy output.(2) The SNC of SS304L in different areas (base metal, heat affected zone and welded seam) was conducted by SMAT. The SNC mechanism and mechanical properties were studied in different areas (base metal, heat affected zone and welded seam) by kinds of testing techniques. SNC mechanism of metal with low stacking fault energy was supplemented, a refinement mode of mechanical twins generated during the SNC process was identified.(3) A new SCC testing technique was developed on the basis of SPT. It is proved that the SPT-SCC is a useful SCC evaluation method with lots of advantages, such as short experimental period, reproducible results, rich mechanical and fracture information and non-destructive characteristics. The SPT-SCC technique could also be used for testing the materials with different micro-structures or single side with special treatment. The SPT-SCC technique is a useful supplement to the traditional SCC evaluation methods.(4) The SCC susceptibility of surface nanocrystallized SS304L were evaluated in different areas (base metal, heat affected zone and welded seam) by SPT-SCC. The SCC mechanisms of surface nanocrystallized SS304L (base metal, heat affected zone and welded seam) were analyzed and explained in detail. The relationship between SNC and SCC susceptibility was obtained.