Feb_March_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 | F E B R U A R Y / M A R C H 2 0 2 0 2 2 uniform distribution of fine ferrite and pearlite. Two recrystallization phases occurred in the RCZ during heating and cooling processes. A phase trans- formation from ferrite and pearlite to austenite occurred during the heating process and the phase transformation from austenite to ferrite and pearlite did not occur during the cooling pro- cess, as austenite did not have enough time to grow. This process is similar to a RESULTS OF MECHANICAL TESTING Microindentation hardness was measured across the weld at the sur- face, middle, and bottom regions, as shown in Fig. 5. Hardness distributions in the three regions were all similar with the hardness of base metal approxi- mately 190 HV. Considering that the metal in the WZ was mainly formed af- ter the welding wire was melted, the hardening tendency was small and grain size was refined in the welding process. Ferrite and pearlite have exe- cellent plasticity and toughness, so the microindentation hardness in the weld Fig. 4 — Microstructure of weld HAZ: (a) different heat-affected zones of the joint, (b) overheated zone (OHZ), (c) recrystal- lization zone (RCZ), and (d) partial recrystallization zone (PRCZ). TABLE 2 — TENSILE PROPERTIES OF LASER WELDS Sample No. Max load, kN Tensile strength, MPa Yield strength, MPa Elong., % Fracture location Avg. tensile strength, MPa 1 230.6 581 466 33.5 Base metal 578.5 2 231.1 576 486 31.5 Base metal TABLE 3 — RESULTS OF CHARPY-V IMPACT TESTS (a) Sample No. Temperature, °C Absorbed energy, J Avg. absorbed energy, J Result 1 −40 74 75 Qualified 2 −40 68 3 −40 82 4 −20 152 151 5 −20 148 6 −20 154 (a) Notch position: weld center Fig. 5 — Hardness distributions across the weld at surface, middle, and bottom regions. normalizing heat treatment. Therefore, the grain size was fine and mechanical properties were excellent. The PRCZ was located between RCZ and base metal. Part of the grains in this zone re- crystallized and became fine, while the other part did not recrystallize and the microstructure was consistent with that of the base metal. Because phase trans- formation occurred for only a part of the grains in this region, it is also called an incomplete normalized zone. The microstructure of this zone was mixed with fine ferrite and pearlite (after re- crystallization) and coarse ferrite and pearlite (without recrystallization), as shown in Fig. 4(d). (a) (b) (c) (d)