직접에너지적층 공정으로 제조된 고망간강 소재의 초음파 나노표면개질에 따른 특성 변화에 관한 연구

Abstract

HMS has high work-hardening and impact toughness as well as excellent wear-resistant. In this study, DED was employed to deposit HMS onto the surface of 316L stainless steel, to improve wear performance. This study focused on three types of HMSs(13Mn, 18.5Mn and 24Mn) according to the manganese content. The deposition characteristics were analyzed according to major process parameters of DED, such as laser power and powder feed rate. From the results, the lowest amount of pores and oxide were observed when the laser power/powder feed rate were 1,050 W/4 g/min for 13Mn, 1,050 W/4 g/min for 18.5Mn and 850 W/4 g/min for 24Mn, respectively. In the as-built HMS, the higher the manganese content, the finer the microstructure and the lower the hardness. Meanwhile, as a post-DED building treatment, UNSM was applied to improve the surface and mechanical properties of the as-built HMS. As a result, the microhardness improvement was the highest under a static load 80 N and an interval 10 ㎛. Wear test was performed on the HMS. As a result, of all HMS, UNSMed 13Mn had the best wear-resistance, which is attributed to the high surface hardness of UNSMed 13Mn. UNSMed 13Mn has a high surface hardness, and wear stops in the middle of the SPD layer. On the other hand, UNSMed 18.5Mn and UNSMed 24Mn, which have relatively low surface hardness, wear occurs beyoud the SPD layer, and then wear occurs rapidly. On the other hand, TRIP and TWIP effects were observed, and the dislocation density of the surface increased in all types of HMS. This suggests that HMS exhibits high wear-resistance, and even if wear occurs beyoud the SPD layer, work-hardening can occur and wear can be reduced. Meanwhile, impact test and tensile test were performed on the 13Mn and UNSMed 13Mn. As a result, the impact absorption energy and tensile strength of UNSMed 13Mn were lower than those of 13Mn. This is because the martensite formed by the UNSM treatment provides a site crack is initiated. As a result of the study, surface roughness, microhardness and wear characteristic could be improved by applying UNSM to as-built HMS. However, impact and tensile properties were decreased due to martensite formed on the surface during UNSM treatment.