Failure mechanisms in surface treated tool steels

Abstract

An investigation of the failure mechanisms associated with cyclic and static stressing of hot work tool steels has been undertaken. The materials used were BH13 and Ni-Cr-Mo (No.5) die steels, both were evaluated in the hardened and tempered conditions and after the application of various surface treatments. Five test procedures were employed, namely - wear, thermal fatigue, mechanical fatigue, impact,and slow bend. The surface treatments employed were plasma nitriding, Tufftriding, brush plating of a Co-Mo alloy, electroless deposition of a Ni-P alloy and electrodeposition of hard and Micro-cracked chromium. Simple upset forging tests, aided by microscopic examination of die surface and subsurfaces were performed:- (1) to establish the wear mechanisms of both steels during dry hot forging. (2) to assess the effect of surface quality on wear. Four main mechanisms (oxidation, erosion, abrasion, delamination) were observed, while it was shown that die wear is a sensitive function of surface quality. Thermal fatigue test results indicated that plasma nitriding and brush plated Co-Mo alloy may delay craze-cracking of dies during hot forging and die-casting. Semi-range S-N curves for three-point bend fatigue conditions were established for No.5 die steel in the hardened and tempered condition,and after coating with electrodeposited hard and Micro-cracked Cr, brush plated Co-Mo alloy and electroless Ni-Palloy. The mechanisms observed on the fracture surfaces included, brittle and ductile failure, void coalescence and cleavage facets. Standard Izod impact test results indicated reductions in Notch-toughness of between 1.7% to 12.3% for similar coatings to those applied to fatigue testpieces above. The failure mechanisms were also similar to those observed on the fatigue testpieces. Finally slow bend test results showed that, all except hard Cr improved ductility of the composites.