Thermomechanically Controlled Erosion in Army Cannons: A Review

Abstract

Metallographic characterization is presented of thermal damage of Cr-coated steel in a fired cannon; Cr and Ta coated steel in a vented-erosion-simulator; and bulk Si3N4 in laser heating. Common features of rapid crack-induced erosion are noted. (i) Cracks form normal to the surface, often permanently open, indicating tensile stress was present at some point during thermal damage. (ii) Softening of Cr and Ta coatings and Si3N4 occurs near the heated surface, verified by metallography and hot hardness. The transformation of steel beneath the coatings is used as an in-situ verification of temperatures that cause thermal damage. (iii) Crack-induced under-cutting of thermal-damage islands is observed for coatings and bulk Si3N4. A thermomechanical analysis of rapid crack-induced erosion observed in severe cannon firing and firing simulation suggests the following key failure mechanisms common to metals and Si3N4. (i) High near-bore transient temperatures increase thermal expansion compression and concurrently decrease the elevated temperature strength. For metals, the thermal compression stress greatly exceeds strength, to depths of about 0.5mm. (ii) Thermal stress exceeding strength produces compressive yielding, which, upon cooling, causes tensile residual stress and cracking. The near-bore residual tension is high enough to cause one-cycle cracking of both Cr and Ta coatings; hydrogen from combustion enters via the cracks and causes cracking in the steel below the coating. For Si3N4, cracks are encouraged by the low fracture toughness of Si3N4. (iii) Repeated thermal cycles deepen and widen cracks to form islands that can be undercut, leading to island removal and rapid erosion failure of the cannon. For Cr and Ta coatings, undercutting is by hydrogen cracking in the steel and degradation of the coating interface by combustion products that enter via the cracks. For Si3N4, undercutting is by direct thermomechanical cracking.