Damaged Section of a Worm Drive

A corrosion resistant chromium nickel steel (X 2 Cr-Ni-Mo 18 10) worm drive used in a chemical plant at 80 deg C and 100 to 200 atm pressure to transport media containing chloride failed during normal operation. Visual inspections showed that the entire surface of the gear was covered with fine branching cracks and was flaking off. Microscopic examination showed that the unetched polished material had disintegrated to an average depth of 1 mm below the surface. A micrograph of the etched surface revealed numerous deformation lines and transgranular cracking. The failure was thus due to stress-corrosion cracking and additional corrosion due to ventilation elements. Because austenitic chromium nickel steels are prone to stress-corrosion cracking, particularly in the presence of chlorine compounds at high temperatures, and because austenitic rust- and acid-resistant steels are prone to smearing and work hardening during machining, it was recommended that these types of steels be machined only with sharp, short tools mounted in rigid structures. In addition, residual stresses should be eliminated by post-process annealing in a protective atmosphere.