Elbow의 인장 성질에 미치는 제조공정의 영향

Abstract

The following conclusions have been reached by evaluating and comparing microstructure, tensile properties of each part of Elbow, which is manufactured through the cold and the hot-expanding forming using 304Lseamless and welded pipe. Hot-expanding formed specimen showed the presence of austenite single-phase, while the cold displayed the presence of some martensite in austenite structure. This result was obtained from 304L Elbow before heat treatment. Heat treated Elbows after the cold and the hot-expanding forming also consist of austenite single-phase. It can be well known that martensite produced by cold forming is all inversely transformed into austenite, and therefore there is no difference in microstructure of specimens regardless of heat treatment and forming methods. Grains of the cold formed specimen are larger than that of hot-expanding, which is mainly attributed to a growing of grain. The growing of grain was caused by heat treatment after the cold and hot-expanding forming. The cold formed specimen displayed higher strength but lower elongation than hot-expanding specimen before heat treatment. This is clearly attributed to transformation of some austenite into martensite, dislocation, and vacancy generated during heat treatment. The Elbows subjected to the heat treatment after the cold and the hot-expanding forming exhibit no significant difference in tensile strength, elongation regardless of forming methods. Microstructures remained unchanged regardless of forming methods. The Elbows manufactured using the Seamless Pipe and the Welded Pipe display no significant difference in tensile strength, while elongation have a tendency to be lowered in the Welded Pipe than in the Seamless, but the difference can be negligible by heat treatment. No significant difference in strength is not located in both the Seamless Pipe and the Welded before and after heat treatment. However, elongation is slightly lower in the Welded than the Seamless before heat treatment, after which the difference is negligible. In conclusion, it can be found that there is no significant difference in structures and tensile properties between the Elbows produced the cold and the hot-expanding forming.