Increasing of the contact fatigue of bearing components made of steel EI-347SH

Abstract

The new generation bearings of aircraft engines are becoming increasingly complex products, which meet high requirements for the contact fatigue, including one at elevated temperature. The surface plastic deformation (SPD) is used to increase the contact fatigue. One promising direction of SPD is micro-ball strengthening. The majority of works does not take into account the influence of temperature onto the contact fatigue of parts after SPD. Purpose. The aim of the study is to increase the contact endurance of steel EI-347SH through the rationalization of modes of micro-ball strengthening. Methodology. Contact fatigue tests were carried out with the setup MKV-T at temperatures of 20°C and 300°C. All the samples were polished before strengthening. For this purpose a special device, which ensures a constant pressure on the sample during polishing was designed. The determination of residual stresses was performed on surveillance samples by the blind hole drilling method. To determine the influence of fluid pressure on the contact endurance a group of tests was carried out on four batches of samples: the samples in the original state, the samples strengthened within 3 minutes at the working fluid pressure of 0.16 MPa and the same but with pressure of 0.2; 0.25 MPa respectively. Findings. Selected strengthening modes lead to an increasing of surface roughness for about one quality class if to compare to after-polishing state. The lowest change in surface roughness was observed with the working fluid pressure of 0.2 MPa. For all modes, the depth of the strengthened layer is in the range of 0.08 ... 0.1 mm. The decrease in microhardness, which is related to instability and unevenness of the strengthening process, was observed at the working fluid pressure of 0.16 MPa. The mean log durability and the guaranteed durability was determined; straight lines approximating the distribution functions of logarithms of numbers of cycles were built. It was found that the highest durability of the samples was obtained by strengthening mode with the working fluid pressure of 0.2 MPa that is caused by a favorable combination of levels of residual stresses, strain hardening, microhardness, in conjunction with a minimal increase in the surface roughness. Originality. Regularities of influence of the microballs strengthening modes on the steel EI-347SH contact endurance were defined. More advanced testing methods were applied. Practical value. The parameters of micro-ball strengthening that provide increased durability under contact loading for steel EI- 347SH samples were determined. This enables one to assign the modes of SPD when designing the bearings for promising type of gas turbine engines more efficiently. The test procedure has been improved, resulting in improved reliability of the experimental results.