Effect Of Nitrogen Content On Hot Deformation Microstructure And Hot Ductility Of316LN Steels For Reactor Main Pipe

316LN austenitic stainless steel (316LN steel) has excellent mechanical properties and resistance to inter-granular stress corrosion, hence it is widely used in the manufacture of main pipe for reactor. However, due to the more kinds and high content of alloy elements, there are usually many problems in forging process, such as surface cracking, coarse grain and mixed grain structures. It is difficult to meet the requirements of nuclear grade of the components. N as an important element in316LN steel, has a great effect on its hot deformation microstructure and hot ductility, but it is not yet clear.In this thesis, the microstructure change behaviors and hot ductility of316LN steel with two different N nitrogen content (0.08%and0.17%) during hot compression and hot tension were investigated. The effects of N on the deformation characteristics of316LN steel were studied. The main results are as follows:The effects of deformation temperature and strain rate on the microstructure of08N and17N steels in the temperature range900-1200℃and strain rate range0.01-10s-1were analysed by hot compression. The relationship between dynamic recrystallization grain size and Zener-Hollomon parameter and A were D=10.02×(Z/A)-0.212(μm) andD=8.647×(Z/A)-0.198(μm), respectively. The dynamic recrystallization occured more easily and the size of dynamic recrystallization grain was smaller in the material containing more N content.The flow stress of17N steel was higher than that of08N steel during hot tension in the temperature range of900-1200℃and strain rate range of0.01-10s-1. Namely the strength of316LN was higher with the increase of N content. With the increase of deformation temperature, the gap of stress between08N and17N were decreased.The relationships between Z value and peak stress (peak strain) of08N steel and17N steel in the temperature range of900-1200℃and strain rate range of0.01-10s-1were established through hot tensile deformation. The hot deformation equations were ε=1.48×1020[sinh(0.0037σ)]5.59exp(-506000/RT) and ε=2.86×1025[sinh(0.0033σ)]7.105exp(-637000/RT), respectively. With the increase of deformation temperature, the reduction in area of08N steelincreased gradually. The fracture mechanism changed from inter-granular and dimplefracture to dimple fracture. Both the size and depth of dimples increased gradually anddimple wall gradually appeared slip bands. The reduction in area of17N steel increasedfirstly and then decreased, reaching to a maximum at1100℃. Fracture morphology of17N steel changed from inter-granular morphology to dimple morphology.Intergranularfracture morphology appeared locally on the fracture surface at1200℃.The hot ductility of08N is better than that of17N under the same deformationcondition. With the increase of deformation temperature, the hot ductility gap betweentwo steels decreased firstly and then increased, reaching to a minimum at1100℃. Theoccurrence of dynamic recrystallization restrain the nucleation and expansion of thecrack, and improve the hot ductility of steels.