Lu Shanping

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.

FINITE ELEMENT SIMULATION OF WELDING RESIDUAL STRESS FOR BUFFER BEAM OF CRH2A HIGH SPEED TRAIN

A finite element model of the buffer beam is established and the distribution of the welding residual stress is investigated by the finite element method.The results show that the calculated stress agrees well with the measured stress by the indentation strain-gage method.There are large and nonuniform residual stresses in the edge of the bottom flange and the processing hole.The welding of the workpieces hanging has an important effect on the residual stresses of the bottom flange.The replacement of A7N01 aluminum alloy with A6N01 aluminum alloy as the base metal can effectively reduce the residual stresses of the buffer beam.When the reinforcement plate is integrally formed with the buffer beam,the residual tensile stresses near the original weld are reduced remarkably.Two welders operating simultaneously on the opposite welds can significantly reduce the residual tensile stresses of the bottom flange.

HIGH EFFICIENCY WELDING PROCESS FOR STAINLESS STEEL MATERIALS

The high efficiency tungsten inert gas (TIG) welding process has been developed, including active flux welding process, mixed shielded welding process and double shielded welding pro- cess, to increase the weld depth/width ratio (D/W ) of conventional TIG welding method. Compared to the active flux method, mixed shielding method can make penetration deeper and the industrializa- tion can be realized easily due to the simplification in operation. Double shielded method can avoid the oxidation of tungsten electrode. The results of experiment and simulation show that the change of the Marangoni convection direction which arises from the adjustment of the oxygen content in the weld pool is one of the main factors contributing to the increase in TIG weld penetration, and the large D/W ratio can be obtained by adjusting the active element content in the liquid pool. High efficiency TIG welding process is not sensitive to welding parameters (welding speed, welding current and electrode gap) and therefore is suitable to be applied in industry easily.