Thermomechanical Treatment of Stainless Steel Piston Rings

Abstract

We study the influence of thermomechanical processing, namely, severe plastic deformation by high-pressure torsion and cryogenic cooling, on the change of the microstructure and mechanical properties of the piston rings made of AISI-316 stainless steel. The deformation of the annular workpieces is carried out in a tool of a new design, the key feature of which is a double helix system. In the first part of the experiment, an estimation simulation of the high-pressure torsion process is carried out in a new matrix design. To estimate the process possibility, the main technological parameters of the process are varied in order to determine the most optimal conditions. Parameters such as the workpiece heating temperature, the upper striker vertical speed, and the friction coefficient at the contact of the workpiece with the tool are selected. The stress–strain state of the annular workpiece during deformation is also studied. At the second stage of the study, a laboratory experiment on the annular blanks deformation is performed. The microstructure evolution and mechanical properties of deformed workpieces after 8 cycles of deformation by high-pressure torsion at cryogenic temperature are studied. All the metallographic studies are carried out using the modern methods such as transmission electron microscopy and analysis of diffraction patterns of back-reflected electrons. As a deformation result, a nanocrystalline structure with a size of 30–40 microns with a large number of large-angle boundaries and a high complex of mechanical properties is obtained. The tensile strength rises from 595 MPa to 1965 MPa, while the yield strength rises from 320 to 1280 MPa. The plasticity value decreases from 55% to 24% as compared to the initial state, but it remains at a sufficient level to be applied.

Keywords

• severe plastic deformation
• piston rings
• stainless steel
• microstructure
• mechanical properties
• cryogenic cooling