Caustic Stress Corrosion Cracking of a High-Strength Alloy Steel Bolt in a Turbomachinery Coupling

A high-strength alloy steel bolt fractured after approximately eight years of service in a turbomachinery coupling connecting a steam turbine drive to a centrifugal compressor. The bolt was designed for infinite life, as the coupling incorporated a flexible disc pack intended to absorb all cyclic loading. The unexpected failure prompted a comprehensive failure analysis to determine the failure mechanism, contributing factors, and root cause, with particular relevance to fastener performance in aggressive service environments.

Visual and optical examination revealed a predominantly flat fracture surface with a semi-elliptical region identifying a progressive fracture origin. Optical microscopy under oblique lighting showed a fan-like fracture morphology characteristic of stress corrosion cracking (SCC) in steel. SEM examination of the fracture origin revealed extensive intergranular fracture with significant grain boundary decohesion, while the final fracture region exhibited dimpled rupture associated with overload. EDS analysis detected sodium on the fracture surface, suggesting exposure to a caustic environment, such as sodium hydroxide, known to induce intergranular SCC in high-strength steels.

Metallographic evaluation confirmed a quenched and tempered martensitic microstructure consistent with properly heat-treated 4140 alloy steel. Extensive intergranular cracking was observed near the fracture origin and along the advancing crack front. Microhardness testing verified that the bolt hardness met the specified requirements, and chemical analysis confirmed compliance with SAE/AISI 4140 composition. No evidence of manufacturing defects, improper heat treatment, or material nonconformance was identified.

The failure mechanism was determined to be caustic stress corrosion cracking driven by the combined presence of a susceptible high-strength steel, sustained tensile stress, and exposure to an aggressive chemical environment. This case highlights the vulnerability of high-strength fasteners to environmental degradation mechanisms in turbomachinery and industrial service and underscores the importance of environmental control, material selection, and fastener protection strategies in preventing SCC-related failures.