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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 | F E B R U A R Y / M A R C H 2 0 2 0 2 3 zone is low (230 to 245 HV). The highest hardness was in the OHZ (290 HV). This is attributed to the formation of Wid- manstätten structure, which reduced ductility and toughness and increased hardness. Due to lower heat input and high cooling rate in the surface region, hardness of the OHZ in this region was higher than that at the bottom and mid- dle, and lower hardness was obtained the farther away from the weld zone in the HAZ. In general, the hardness met the requirement of ISO EN15614 stan- dard, “Specification and qualification of welding procedures for metallic ma- terials—Welding procedure test—Part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys.” Average tensile test results (Ta- ble 2) of 578.5 MPa transverse tensile strength, 476 MPa yield strength, and 32.5% elongation meet the require- ments of ISO EN15614. Figure 6 shows the fracture position; necking and frac- ture all occurred at the junction of the HAZ and the base metal; no fractures occurred in the weld. This shows that the strength and ductility of the weld seam were higher than those in the HAZ and base metal. In addition, a large number of dimples distributed on the fracture surface indicate a ductile frac- ture mode. Charpy-V notch impact testing was performed at temperatures of −40 ° C and −20 ° C. The notch was located at the center of the weld. Table 3 lists the values of absorbed energy. The aver- age absorbed energy decreased 76 J at the lower test temperature, indicat- ing that toughness decreased, but the impact toughness was far higher than 27 J, which is required in the relevant standard. Figures 7a and 7b show the macrostructure of the impact specimen fracture surfaces at −20 ° C and −40 ° C, re- spectively. Examination of the fracture surfaces via SEM reveal a large number of dimples on the surface of the −20 ° C test sample (Fig. 7c) indicating good toughness and massive dimples and a spot of cleavage on the surface of the −40 ° C test sample (Fig. 7d). The main cause of the latter is that the quantity of slip systems of ferrite decreased at the lower temperature and twinning defor- mation was strengthened. In addition, the opposite growing structure also weaken the toughness of the joint. Acknowledgment This work was supported by the National Key Research and Develop- ment Program of China (NO.2018Y FC0310400); Natural Science Foun- dation of the Higher Educational In- stitutions of Jiangsu Province, China (NO.18KJB460007), and the Post- graduate Research & Innovation Pro- gram of Jiangsu Province, China (NO. SJCX19_0585). ~AM&P For more information: Yong Zhao, Pro- vincial Key Lab of Advanced Welding Technology, Jiangsu University of Sci- ence and Technology, Zhenjiang Jiang- su 212003 China, yongzhao418@just. edu.cn . References 1. Z. Shu, et al., “Study on Micro- structures and Mechanical Properties of Laser-Arc Hybrid Welded S355 (a) (b) Fig. 6 — (a) fracture position in weld joints in tensile tests and (b) SEM image of fracture surface. Fig. 7 — (a) Macrostructures of impact test specimen fracture surfaces tested at −20°C; (b) macrostructures of impact test specimen fracture surfaces at −40°C; (c) SEM image of impact test specimen fracture surface at −20°C shows ductile fracture mode; and (d) SEM image of impact test specimen fracture surface at −40°C shows mixed fracture mode. (a) (b) (c) (d)