Abstract
This study examines the formation and consequences of delta ferrite (Fe₃P) in heat-treated bolts made from SAE10B30 microalloyed steel. The research demonstrates that when phosphated steel undergoes quenching and tempering in neutral or reducing atmospheres without proper cleaning, residual phosphate coating leads to the formation of delta ferrite—a brittle tetragonal phase with high hardness (approximately 450 HV) but poor toughness. Through comparative analysis of phosphate-treated and phosphate-free specimens heat-treated to 43-44 HRC, the authors identify that delta ferrite promotes micro-crack nucleation at grain boundaries, significantly reducing impact resistance, toughness, and fatigue life. The study confirms that proper alkaline cleaning to remove phosphate coatings before heat treatment is essential for preventing delta ferrite formation and maintaining the structural integrity of high-strength bolts (strength class 12.9), particularly those operating under fatigue conditions.
