Thick-section welding of Modified 9Cr-1Mo (P-91) steel

The weldability of thick sections of Modified 9Cr-lMo steel (Grade 91) has been investigated. The alloy has excellent fabricability. It is not susceptible to hot cracking on stress relief cracking. It is, as are most highly hardenable steels, susceptible to hydrogen assisted cracking but it can be avoided by good shop practice. A soft zone can occur in the heat-affected-zone which can result in Type IV cracking in service. This can be avoided through design or fabrication sequence. Strength reduction factors may also be applied. Thick sections have been successfully welded with the submerged arc, shielded metal arc and gas tunsten arc processes. The alloy because of its higher allowable stress values (2 1/2 times higher than 2 1/4CR-lMo steel at 1050{degrees}F) (566{degrees}C) permits the design of considerable thinner header sections. Further, the alloy is a viable replacement for austenitic materials at temperatures up to 1100{degrees}F (593{degrees}C) thereby eliminating or minimizing austenitic-to-ferritic dissimilar metal joints.