Reasons for the formation of cracks in boride coatings on steel
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Three types of cracks occur in boride coatings on steel — cracks parallel to the surface, cracks perpendicular to the surface down to the FeB-Fe2B interface, and cracks perpendicular to the surface down to the base metal.
Opposing residual stresses occur in borides — tensile stresses in FeB and compressive stresses in Fe2B.
The residual stresses are due to the difference in the coefficients of thermal expansion of FeB, Fe2B, and the steel. At 200–600° the expansion coefficient of FeB is considerably higher than that of pure iron.
The residual stresses resulting from the difference in the expansion coefficients lead to cracks of the first type when the thickness of FeB is large as compared with the total thickness of the coating. Cracks of the second type occur when the thickness of FeB is small. Cracks of the third type occur mainly after quenching of borided parts.
FeB can be prevented from forming by changing the boriding technique. Then cracks of the first and second types do not occur. Due to the high compressive stresses in Fe2B on alloy steels the coating of Fe2B may peel when the thickness of the coating is large.
The residual stresses in Fe2B depend on the composition of the base metal and the boriding temperature. The composition of the saturating mixture has no effect.
KeywordsIron Residual Stress Thermal Expansion Tensile Stress Compressive Stress
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