Abstract: According to an aspect of the present invention, a piston ring for use in internal combustion engines is provided that is made of matensite-based stainless steel material containing C of not less than 0.4 and not more than 0.8 weight percent, Si of not less than 0.16 and not more than 0.4 weight percent, Mn of not less than 0.1 and not more than 0.4 weight percent, and Cr of not less than 2.0 and not more than 11.0 weight percent, where the steel material of the piston ring has mainly chromium carbide precipitated in the base texture thereof. The steel material of the first compression ring then has a nitriding layer formed thereon. According to another aspect of the present invention, a combination of compression rings, including a first compression ring 1 and a second compression ring 2, for use in internal combustion engines, is provided. The first compression ring 1 is made of martensite-based stainless steel material containing C of not less than 0.16 and not more than 1.

Abstract: A method of producing a piston ring to be incorporated in an internal combustion engine includes first nitriding surfaces of a steel base material in a high temperature range and then continuously nitriding in a low temperature range. Finally a porous layer of at least a sliding surface is removed to expose a diffusion layer under the porous layer.

Abstract: A compression ring as a piston ring adapted to by received in an annular groove formed in a piston disposed in a cylinder assembly of an internal combustion engine includes an annular ring base member having a cutout portion, the ring base member having an outer sliding surface, upper and lower surfaces continuous to the sliding surface through corner portions and an inner peripheral surface to be fitted to the piston groove, the sliding surface contacting an inner wall of the cylinder when the piston reciprocates therein. A nitriding layer is formed on all the surfaces of the ring base member by a nitriding treatment. A white portion of the nitriding layer formed only on the sliding surface of the ring base member is substantially completely removed and white portions of the nitriding layer formed on the first and second surfaces and corner portions between the sliding surface and the upper and lower surfaces are removed so as to reduce thicknesses thereof to not more than 5 .mu.m.

Abstract: The object of the present invention is to provide a piston ring which is free from occurrence of cracks on the nitrided layer and having enhanced abrasion resistance and fatigue strength, and to provide a process for manufacturing the piston ring. In the invention, a surface of the steel body is subjected to a two-stage nitriding treatment consisting of low-temperature nitriding and high-temperature nitriding. A white layer on the slidable surface along the inner periphery surface of a cylinder is removed, and a white layer on each of the upper and lower surfaces and the corner portions each being adjacent to the slidable surface is removed to have a thickness of not more than 5 .mu.m.