Abstract
Bevel gears manufactured from martensitic steels are widely used in marine propulsion systems, where high cyclic contact loads and fluctuating torque promote subsurface damage mechanism like rolling contact fatigue (RCF) resulting in tooth flank fracture. This study aims to optimize material selection for bevel gears based on RCF performance using an in-house testing facility at SINTEF. Three case-hardened martensitic steel variants (18CrNiMo7-6) were investigated: (i) 623, subjected to single quench-cycle and temper with 2D forging (ii) 627, subjected to double quench and temper with 2D forging (iii) ESR, produced via electro-slag refining followed by single quench-cycle and temper with 3D forging. RCF tests run upto 50 million cycles show appearance of sub-surface cracks in both 623 and 627, although 627 performs slightly better with delayed crack initiation. ESR performs the best with no appearance of sub-surface cracks. Detailed microstructural and fractographic investigation revealed that sub-surface crack-initiation and propagation is driven primarily by the presence of brittle non-metallic inclusions along with banding of elemental segregation. A higher number-density of inclusions at sub-surface with low sphericity (morphology) -which are potential stress concentrators- was identified in 623 compared to 627, while such inclusions were nearly absent in ESR, explaining its enhanced RCF resistance. Although segregation banding contributes to crack initiation and/or propagation, inclusions remain the dominant factor, as improved performance in ESR is achieved despite presence of pronounced segregation banding. These findings provide microstructure-sensitive insights for material selection for bevel gears, supporting improved reliability and durability of marine propulsion systems.