INVESTIGATION OF THE MICROSTRUCTURE AND IMPACT PROPERTIES OF THE HIGH NITROGEN STAINLESS STEEL WELD

The high nitrogen stainless steel(HNS) is widespread attention in recent years because of the high strength,good toughness and corrosion resistance.Weld ability and welding efficiency are received extensive attention in the HNS welding.Using the double shielded TIG welding method,the active element,oxygen,was transferred into weld pool so as to change the convection of the liquid metal from outward to inward.This proposed welding process could effectively reduce the width of the pool surface,which is beneficial to reducing the overflow of nitrogen content in the weld.The influence of heat input(different welding current) on the weld pool shape and the impact properties of welds were investigated.The weld penetration increased with the increasing heat input and the 10 mm thick work piece was welded thoroughly under the 250 A welding current.The XRD test and calculation results showed that the 5 ferrite content in the welds was increased with the increasing welding current,while the impact properties of welds change little.The poor impact properties of the weld are another focus of this study.The fracture surfaces of the Charpy V-notch specimens were characterized using the scanning electron microscope(SEM).A large number of short-deep cracks and "skeleton-like" surfaces were observed on the fracture surfaces where were identified as containing (?) ferrite under energy spectrum analysis(EDS) using SEM.The interface between the (?) ferrite and austenite with low binding strength is considered to be the location of the cracks initiation.The cracks propagate along the interface between the two phases,and then causing brittle fracture of weld joints.