Performance of Surface-Engineered SAE 4140 Valve Stem Steel Under High-Temperature Cyclic Oxidation and Hot-Corrosion Environment

Abstract

The valve stem of HHP Railway locomotives operates in a harsh thermo-chemical environment and is subjected to fluctuating temperatures, oxidizing exhaust gases, and corrosive salt deposits that promote scale growth, spallation, and premature component failure. To improve corrosion resistance, four surface-engineered conditions were examined: bare, shot-peened, coated, and a hybrid (shot-peened + coated) configuration. Each specimen was exposed to cyclic oxidation conditions, involving 50 heating–cooling cycles at 500°C in the presence of NaCl and 100 cycles at 700°C in a Na₂SO₄–V₂O₅ salt mixture (60:40 wt.%), in order to reproduce aggressive locomotive exhaust corrosion environments. Gravimetric weight-change analysis was conducted to determine oxidation kinetics and parabolic rate constants (K_p), while SEM and EDS were used to evaluate oxide scale morphology, crack formation, and elemental distribution. The study aims to identify microstructural degradation mechanisms and identification of surface engineering approach that provides the greatest improvement in durability and operational reliability of locomotive valve-stem materials.