How does grain boundary sensitization in stainless steel springs lead to failure?

Sensitization occurs when stainless steel is exposed to temperatures between $800^{\circ}F$ and $1500^{\circ}F$, causing chromium carbides to precipitate at the grain boundaries. This depletes the adjacent areas of chromium, destroying the corrosion-resistant passive layer. In a wave spring, this leads to intergranular corrosion, where the spring becomes brittle and fails under minimal load. This is often seen in springs that were improperly heat-treated or exposed to exhaust heat. The failure is identified by a 'sugar-like' appearance of the fracture surface under magnification. Using low-carbon grades or stabilized grades like 321 can prevent this.