November/December AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 8 2 5 *Xiao-Ting Deng is a Ph.D. candidate at the University of Chinese Academy of Sciences, Beijing. EFFECT OF MARTENSITE TRANSFORMATION ON RESIDUAL STRESS IN STAMPED AUSTENITIC STAINLESS STEEL SHEET Deformation-induced martensite phase transformation during room temperature forming of austenitic stainless steel leads to higher residual stress. Xiao-Ting Deng,* Ming Cheng, Shi-Hong Zhang, and Hong-Wu Song Institute of Metal Research, Chinese Academy of Sciences, Shenyang Jin-Song Liu Shenyang Ligong University R esidual stress exists within a com- ponent when external stress is removed.Thisinternalstresscanre- sult in changes to a component’s size and shape [1] . Different metalworking process- es, such as casting, forging, welding, cut- ting, heat treating, and assembly, create various degrees of residual stress [2] . Two methods used to measure residual stress are mechanical (which causes some part damage) and nondestructive examina- tion, or NDE (without part damage). Me- chanical approaches include blind hole, milling ring groove, full released, and layered peeling methods. NDE methods include magnetic, ultrasonic, inherent strain, and x-ray diffraction, which is the most widely used technique. Under certain conditions, defor- mation-induced martensite phase transformation can occur in austenitic stainless steel during processing [3,4] . This article discusses the results of a study to determine the relationship between deformation-induced mar- tensite phase transformation and re- sidual stress in AISI Type 304 austenitic stainless steel (nominal 18% Cr, 8% Ni) after stamping. EXPERIMENTAL METHODS Test coupons for residual stress testing were obtained from dome- shaped specimens (Fig. 1) formed from 124-mm diameter by 2-mm thick stamping blanks using a 100-ton stamp- ing press. Stamping was carried out at room temperature and 150 ° C, the tem- perature at which martensite transfor- mation disappeared completely. X-ray diffraction was used to measure residu- al stress and phase content of the spec- imen outer wall. Measurement of residual stress by x-ray diffraction is based on elastic mechanics and x-ray crystallography, measuring the change of interplanar spacing (i.e., different grains change with crystal orientation and stress). The testing principle is shown in Fig. 2. The x-ray diffraction method is divided into tilting and roll; tilting is the direc- tion plane co-located with the scanning plane, and roll is the direction plane perpendicular to the scanning plane [5-7] . Residual stress of the head out- er wall was measured using an Xstress 3003 x-ray stress meter (Stresstech Oy, Finland). Figure 3 shows the arrange- ment of measurement points. Six points are selected equidistant from the bot- tom of the domed specimen to the straight segment, and the direction of tangent is the 0 ° measuring direction. Residual stresses ( σ r ) on the position at 0 ° measuring direction are σ r1 and σ r2 at the bottom of specimen, σ r5 at the tran- sition period of specimen, and σ r6 at the straight section. EXPERIMENTAL RESULTS Figure 4 shows residual stresses of the specimen outer wall in the 0 ° Fig. 1 — A 100-ton stamping press was used to formdome-shaped test specimens.