Enhanced Wear Resistance of API5CT L80 Type 1 Steel Via QPQ Treatment: A Comprehensive Microstructure and Tribology Study

This study investigates the influence of quench–polish–quench (QPQ) thermochemical treatment on the microstructure, hardness, friction behavior, and wear performance of L80 Type I steel, a material widely used in oil and gas applications. A titanium-grade crucible salt bath was employed to nitrocarburize the steel at 580 ± 10 °C for 1–3 h, followed by surface polishing and an oxidizing treatment at 480 ± 10 °C. Microstructural analyses confirmed the formation of a dense and uniform ε/γ′ nitride compound layer accompanied by a deep diffusion zone. The QPQ-treated samples exhibited a substantial increase in surface hardness, exceeding 850 HV, which is nearly three times higher than the untreated steel. Pin-on-disk tests demonstrated that the QPQ layer significantly reduced the coefficient of friction and decreased both wear rate and specific wear rate by an order of magnitude across all applied loads. SEM examinations revealed a transition from severe adhesive and delamination wear in the untreated steel to mild abrasive wear with no substantial surface damage after QPQ treatment. The combined effects of a hard nitride layer and a lubricious Fe₃O₄ film resulted in superior tribological stability and enhanced load-bearing capacity. These findings highlight the effectiveness of QPQ treatment in dramatically improving the surface durability of L80 steel, making it a promising candidate for demanding tribological conditions in oil and gas service environments.