유압 펌프 피스톤 소재 SACM645의 마모특성에 관한 연구

Abstract

본 연구에서는 피스톤 재료로 사용되고 있는 SACM645소재를 다양한 방법으로 열처리를 실시한 후 건조, 윤활 및 고온 윤활 분위기에서 마모 특성을 조사하여 아래와 같은 결론을 얻었다. (1) 인장강도 측정 결과 QT 시험편, QT+shot peening 시험편 및 QT+질화 시험편의 최대응력은 800MPa 부근으로 비슷한 값을 나타내고 있으나, 변형률은 많은 차이를 나타내었다. QT+질화 시험편은 질화처리 후 경도가 Hv 800으로 매우 높으며, 연성이 매우 적게 나타났고, QT 고주파 시험편은 내부까지 전경화되어 1,700MPa로 매우 높았게 나타났다. (2) 경도 측정결과 대부분의 시험편은 모재와 비슷한 경도 값(210~230Hv)을 나타내었지만, 고주파 처리한 시험편은 전경화되어 550∼600Hv 정도로 나타났으며, 질화처리한 시험편에서는 표면 경도가 800Hv, 내부는 모재와 비슷한 경도를 나타내었다. (3) 상온 건조 상태에서 마모시험 결과 질화 시험편, QT+질화 및 고주파 시험편에서 마모 특성이 우수하게 나타났으며, QT, QT+shot peening 시험편 및 모재 시험편은 작은 기울기를 가지는 비슷한 마모 특성을 나타내었다. (4) 상온 윤활상태에서 마모 시험결과, 전체적으로 마모량이 많았으며, 가장 내마모성이 가장 우수한 시험편은 상온 건조상태와 마찬가지로 질화처리 및 고주파 처리를 실시한 시험편이라고 판단된다. (5) 고온 윤활상태에서는 질화처리와 고주파 처리를 한 시험 핀의 마모량이 많은 편이었으며, 모재와 QT처리를 한 시험 핀이 가장 낮은 마모량을 나타내었다. (6) 건조조건의 경우 오일이 없는 상태에서 질화처리나 고주파처리의 경우는 거의 마모량이 발생하지 않을 정도로 좋은 내마모성을 보여주었으며, 저온 윤활조건의 경우에서도 마찬가지로 일반 모재와 QT처리를 한 시험 핀에 비하여 다소 좋은 내마모 특성을 나타내었다. 하지만, 고온 윤활조건에서는 내마모성이 표면경도에만 관계를 가지는 것을 넘어 연성과도 밀접한 관계를 가지고 있다는 것을 알 수 있었다. (7) 상온 건조 조건에서 마모시험 후 마모면 관찰에서는 모재 시험편, QT 시험편, QT+shot peening 시험편의 마모면은 응착마모와 연삭마모에 의한 마모흔적을 볼 수 있었고, 질화 시험편, 고주파 시험편, QT+질화 시험편, QT+고주파 시험편은 핀 마모량이 적은 것에서 미끄럼마모와 산화마모가 일어난 것으로 확인할 수 있었다. 하지마, 저온 및 고온 윤활 시험에서는 실리콘카바이드연마지를 상대재로 사용함으로 인하여 연삭마모의 흔적만 관찰되었다.

The oil hydraulic piston pump and motor generally have been using at an oil pressure system because these instruments can generate high oil pressure and torque with simple structure and small size. The wear occurred at a narrow gap between piston and cylinder has a great influence on the performance and durability of the piston pump and motor. Characteristics of wear at a gap directly affect wear properties at a gap of the swash plate and the slipper pad and a gap of the cylinder block and the valve plate. According to high pressurizing trends of oil hydraulic machines, the importance of surface treatment technology has been increasing as well as a gap of relative motion part and a surface roughness. In case of the oil hydraulic piston that makes highest pressure, outstanding surface hardness of main parts by using the surface treatment technology is essential to improve wear resistance, fatigue resistance and corrosion resistance. Otherwise, the damaged surface by wear results in the fracture of pump. Therefore, it is necessary to study more about the surface treatment technology. In this study, the characteristic of wear had been investigated on dry, lubrication and high temperature lubrication atmosphere after the various heat treatment of SACM645 steel for piston material. The results are given as follows. (1) The maximum tensile strength of QT treated specimen, QT treated and shot peened specimen and QT treated and nitrided specimen have the similar value of all about 800 MPa, but the strains indicated big difference. (2) The hardness of QT treated and nitrided specimen was very high with 800 Hv but the ductility was very low. (3) The tensile strength of QT treated and high frequency induction hardened specimen showed very high as 1,700 MPa because this specimen was hardened completely on all section to the inside. (4) The hardness of most of the specimen was similar to the hardness of base metal (210~230Hv). However, the high frequency induction hardened specimen showed about 550~600 Hv of hardness because of its complete hardening. And the surface hardness of the nitrided specimen was 800 Hv, while the hardness of inside has shown similar to hardness of base metal. (5) As a result of wear test at dried condition in room temperature, the wear characteristic of the nitrided specimen, the QT treated and nitrided specimen and the high frequency induction hardened specimen was very excellent. the QT treated specimen, the QT treated and shot peened specimen and the bulk specimen showed similar wear characteristic. (6) The wear loss in the wear test at the lubricated condition in room temperature was generally large. From this result, the wear resistance was high at the nitrided specimen and the high frequency induction hardened specimen same as the dried condition at room temperature. (7) At the lubricated condition in high temperature, the wear loss of the nitrided pin and the high frequency induction hardened pin was larger than that of base metal and QT treated pin. (8) In case of the dried condition without oil, the nitrided specimen and the high frequency induction hardened specimen have shown good wear resistance that did not generate any wear. Also, it indicated the similar tendency in the result of low temperature. However, the wear resistance in the high temperature lubricated condition is thought to be related to ductility as well as surface hardness. (9) The wear surface of the base metal, the QT treated specimen and the QT treated and shot peened specimen after wear test at the dried condition in room temperature indicated cohesion wear and grinding wear, respectively. However, the nitrided specimen, the high frequency induction hardened specimen, the QT treated and nitrided specimen and the QT treated and high frequency induction hardened specimen showed slide wear and oxidation wear from small wear loss of pin. However, in low and high temperature lubrication test, only grinding wear was observed because of using a silicon-carbide grinding paper as the relative material.