Stress Corrosion Cracking in a Mild Steel Autoclave Used in the Manufacture of Dyestuffs' Intermediates

A steam jacketed autoclave of orthodox design was fabricated from mild steel for a working pressure of 320 psi. The only unusual feature in its construction was a protective layer of weld metal, which was deposited on the internal surface of the upper half of the 1 in. thick shell. The first indication of latent trouble was provided by the bolts which attached the stirring paddles to the shaft and the stationary scraper blades to the shell, either failing in service or breaking off when an attempt was made to remove them. It was the practice to renew them all annually. Microscopic examination of a failed bolt showed the path of the fracture and the secondary cracking associated with it were intergranular, suggesting that failure resulted from stress corrosion. A steel of the rimming type had been used to make the bar from which the bolt was forged. Cracks which originate at the root of threads generally result from fatigue but, in this instance, their intergranular mode of progression indicated that they were due to stress-corrosion. Examination of shell material showed that the cracks in the vessel were wholly intergranular. It was apparent from this evidence that this cracking was also due to stress-corrosion.