[논문]Microstructure and Hardness of Surface Melting Hardened Zone of Mold Steel, SM45C using Yb:YAG Disk Laser

Lee, Kwang-Hyeon; Choi, Seong-Won; Yoon, Tae-Jin; Kang, Chung-Yun

doi:10.5781/jwj.2016.34.1.75

Microstructure and Hardness of Surface Melting Hardened Zone of Mold Steel, SM45C using Yb:YAG Disk Laser 원문보기

Journal of welding and joining = 대한용접·접합학회지, v.34 no.1, 2016년, pp.75 - 81

Lee, Kwang-Hyeon (Korea Institute of Machinery & Materials, Busan Laser Application Support Center) , Choi, Seong-Won (Dept. of Material Science and Engineering, Pusan National University) , Yoon, Tae-Jin (National core research center, Pusan National University) , Kang, Chung-Yun (Dept. of Material Science and Engineering, Pusan National University)

Abstract ▼ AI-Helper

This study applied laser surface melting process using CW(Continuous wave) Yb:YAG laser and cold-work die steel SM45C and investigated microstructure and hardness. Laser beam speed, power and beam interval are fixed at 70 mm/sec, 2.8 kW and $800{\mu}m$ respectively. Depth of Hardening layer(Melting zone) was a minimum of 0.8 mm and a maximum of 1.0 mm that exceeds the limit of minimum depth 0.5 mm applying trimming die. In all weld zone, macrostructure was dendrite structure. At the dendrite boundary, Mn, Al, S and O was segregated and MnS and Al oxide existed. However, this inclusion didn't observe in the heat-affected zone (HAZ). As a result of interpreting phase transformation of binary diagram, MnS crystallizes from liquid. Also, it estimated that Al oxide forms by reacting with oxygen in the atmosphere. The hardness of the melting zone was from 650 Hv to 660 Hv regardless of the location that higher 60 Hv than the hardness of the HAZ that had maximum 600 Hv. In comparison with the size of microstructure using electron backscatter diffraction(EBSD), the size of microstructure in the melting zone was smaller than HAZ. Because it estimated that cooling rate of laser surface melting process is faster than water quenching.

주제어

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가설 설정

1) The depth of surface melting hardening layer was minimum 0.8mm, maximum 1.0mm.

제안 방법

Although the SM45C of die steel is used in many cases, the study on the mechanism for laser surface melting hardening is nothing at all. Therefore, in this research, systematically study the correlation between the change in microstructure of surface melted part and heat-affected zone and hardness on the SM45C, medium carbon steel, on laser surface melting hardening process with minimum over 600 ㎛ hardness depth using Yb:YAG high power disc laser.

성능/효과

2) The hardness of surface melted zone inside was 650-660 Hv without reference to the location, and it is about 60 Hv higher than HAZ maximum hardness (590-600 Hv). Based on the comparison of micro structure between melted zone and HAZ by EBSD, the hardness of melted zone is higher because the microstructure is minute.
3) The cooling speed of laser surface melting hardening treatment is 605 ℃/s (calculated with t85 formula) and it is faster than water cooling, so the melted zone transformed as complete martensite, and the microstructure is very minute.
4) The macrostructure inside surface melted zone has dendrite form; also there are Mn, Si element segregate at the dendrite boundary, MnS at dendrite boundary, and Al based oxide in transgranular. However, there are no such inclusions in HAZ.

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