# The Investigation of Rotary Bending Fatigue Properties of 4Cr14Ni14W2Mo Engine Valve Steel Processed by Surface Mechanical Rolling Treatment

## TL;DR

This study improves the fatigue resistance of engine valve steel using a surface treatment, but excessive treatment or high temperatures can reduce its effectiveness.

## Contribution

The study introduces surface mechanical rolling treatment to enhance the fatigue properties of 4Cr14Ni14W2Mo engine valve steel under different temperatures.

## Key findings

- Surface mechanical rolling treatment increases fatigue limits by modifying the surface layer with refined grains and compressive stress.
- At 25°C, the fatigue limit increased by 40.3% with 10 rolling passes, but plasticity decreased.
- High temperatures (650°C) reduce the benefits of the treatment and accelerate crack propagation.

## Abstract

In order to address potential fatigue fractures at the valve stem-neck junction during engine operations, surface mechanical rolling treatment (SMRT) was introduced to enhance the rotary bending fatigue (RBF) performance of 4Cr14Ni14W2Mo engine valve steel in this study. The results indicate that the increasing number of rolling passes induces a modified surface layer characterized by refined grains and dislocations, increased hardness, and compressive residual stress (RS). SMRT specimens exhibited improved tensile strength but plasticity performance was decreased. At room temperature (RT) about 25 °C, the fatigue limit at 1 × 10 7 cycles of specimens treated with 10 rolling pass was increased from 437 MPa to 613 MPa (40.3%). At 400 °C, the fatigue limit of specimens treated with 10 passes was increased from 376 MPa to 425 MPa (13.0%) at 400 °C, but decreased at 650 °C. The enhanced fatigue performance is attributed to a modified surface layer, leading to the shift of the crack initiation to the subsurface. However, excessive rolling passes and high temperature (650 °C) significantly reduce the material plasticity, accelerating crack initiation and propagation, thus compromising performance.