표면처리에 따른 유압펌프용 피스톤 재료 SACM645의 기계적특성

Abstract

When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. To become steel, it must be melted and reprocessed to reduce the carbon to the correct amount, at which point other elements can be added. This liquid is then continuously cast into long slabs or cast into ingots. Approximately 96% of steel is continuously cast, while only 4% is produced as cast steel ingots. The ingots are then heated in a soaking pit and hot rolled into slabs, blooms, or billets. Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel, such as I-beams and rails. In modern foundries these processes often occur in one assembly line, with ore coming in and finished steel coming out. Sometimes after a steel's final rolling it is heat treated for strength, however this is relatively rare. This study was investigated the tensile properties and fatigue crack propagation characteristics with heat treatment and surface treatment using the material, SACM645, for hydraulic piston pumps. Fatigue limit was examined by using base metal, QT specimen, QT nitriding specimen and QT inductive hardening specimen. Wear characteristics was investigated in the atmosphere of dry, lubrication and high-temperature lubrication after performing the heat treatment in a variety of ways. The mechanical properties of DLC-coated SACM645 were investigated. The results obtained were as follows: The depth of QT nitriding specimen depth were approximately 300 ㎛ from the surface, some parts were affected by the heat. The inner parts were observed a tempered martensite structure by the QT treatment. In a tensile test of four kinds of test specimens, the maximum stress of QT specimen and QT nitriding specimen is similar to about 820 MPa and the strain showed a large difference. The surface hardness of QT nitriding specimen appears higher than the 800 Hv, while the base metal and QT specimen at a depth of 300 ㎛ from surface were almost the same hardness. The base metal showed the longest fatigue life and fatigue crack growth rate was the most late. Hardness of QT nitriding specimen is highest and the crack initiation at the surface is slow. Thus, it is judged that QT nitriding treatment is an ideal surface treatment methods because inner has a ductile even if cracks occur. Fatigue limit is QT specimen of 492 MPa, base metal of 533 MPa, QT inductive hardening specimen of 615 MPa and QT nitriding specimen of 656 MPa, respectively. Fatigue limit of QT nitriding specimen was about 80% of tensile strength. Fatigue limit of QT specimen was 60% of tensile strength although it has the lowest fatigue limit. In the room temperature dry wear properties, Nitriding specimen, QT nitriding specimen and QT inductive hardening specimen were highly represented and QT specimen, QT shot peening specimen and base metal were similar. Overall, the room temperature lubrication wear was a lot of wear. Such as the room temperature dry wear, nitriding specimen and inductive hardening specimen showed the best wear resistance. High temperature lubrication wear was lots of wear of the nitriding specimen and inductive hardening specimen. Base metal and QT specimen showed the lowest wear rate. QT nitriding DLC coating specimen does not nearly exist graphite from the Raman analysis of DLC coating layer and peak appears at 1560 cm-1. It can be confirmed that mostly made ​​of DLC structure. But, nano-indentation measurements appear to 2,158 Hv, it was satisfied with the values ​​of DLC coating layer. The surface roughness of QT nitriding DLC coating layer was Ra = 0.02 ㎛. The friction coefficient of DLC coating is less than 0.9 up to 1 km, since then, it was approximately 0.9 like the QT nitriding specimen.