A weld that attached the head to the shell of a preheater containing steam at 1.4 MPa (200 psi) and was used in the manufacture of paper cracked in service. The original joint contained a 6.4 by 50 mm backing ring that had been tack welded to the inside surface of the shell in a position to project beyond the fully beveled top edge of the shell. The projecting edge of the ring fitted against a deep undercut on the inner corner of the rim of the head. The internal 90-deg angle in this undercut was sharp, with almost...