Piston ring excellent in resistance to scuffing, cracking and fatigue and method for producing the same, and combination of piston ring and cylinder block

Abstract

A piston ring having improved scuffing resistance, cracking resistance and fatigue resistance, consists of a high-chromium martensitic stainless steel and a sliding nitriding layer formed on the surface of said steel. The stainless steel consists of C: 0.3 to 1.0%; Cr: 14.0 to 21.0%, N: 0.05 to 0.50%, at least one of Mo, V, W and Nb: 0.03 to 3.0% in total, Si: 0.1 to 1.0%, Mn 0.1 to 1.0%, P: 0.05% or less, S: 0.05% or less, the balance being Fe and unavoidable impurities. The sliding nitriding layer comprises on its surface hard particles mainly consisting of nitrides in a range of from 0.2 to 2.0 mum of average particle size, 7 mum or less of the longest diameter, and from 5 to 30% in area ratio.

Description

BACKGROUND OF INVENTION[0001] 1. Technical Field[0002] The present invention relates to a piston ring used in an internal combustion engine, particularly, a piston ring consisting of high chromium martensitic stainless steel with nitriding, having improved scuffing resistance (seizure resistance), cracking resistance (failure resistance) and fatigue resistance. The present invention is also related to a production method of the piston ring.[0003] 2. Background Technique[0004] Along with recent demands for low fuel consumption, weight reduction and high performance of internal combustion engines, the piston rings are thinned to reduce weight and to follow the high rotation of the engine. Material properties of the piston rings, such as wear resistance, scuffing resistance and fatigue resistance, and the like must be improved to enable thinning of a piston ring. The conventional cast-iron piston rings have, therefore, been replaced with steel piston rings particularly from a view poin...

AI Technical Summary

Benefits of technology

[0016] In the high chromium martensitic stainless steel with nitriding, the microstructure of the nitriding layer generally shows mainly hard nitrides dispersed in the tempered martensite matrix. The mechanism of scuffing is strongly dependent upon the microscopic unevenness on the sliding surface. In the nitriding layer, hard particles disperse in the relatively soft matrix. The microscopic unevenness is, therefore, defined by the size and dispersion state of the hard particles. When the cross section of the surface layer having such structure is observed, the following is apparent. The convex hard particles are brought into contact with an opposite sliding surface, while the relatively soft matrix is relatively concave. The lubricating oil retained in the concave portions is subjected to pressure during sliding. Frequency of the entire direct contact of the steel with nitriding and the opposite member is low, since the steel with nitriding has the microstructure as described above. As a result, the contact pressure between both sliding members is decreased. In addition, the oil is fed to the convex portions mentioned above. The scuffing can, therefore, be prevented.

[0041] In the present invention, high hardness of from Hv 900 to 1400 is obtained up to a satisfactory depth from the surface by the nitriding treatment for a relatively short period of time. This feature is attributable to the relatively fine .gamma. crystal grains formed at low quenching temperature and thus to the increase of the grain boundaries which are the main diffusion passages of N during the nitriding treatment.

Problems solved by technology

This phenomenon results in scuffing of the piston ring.

Specifically, in the case of the cast iron monolithic cylinder block having a non-uniform structure as described above, the surface of this block is liable to be roughened by the grinding.

Images