10.5 PREHEATING

Hardening of the weld and base metal HAZ are important because of
hydrogen assisted cracking that occurs in carbon and low-alloy steels. As the
hardness of the base metal HAZ increases so does the susceptibility to
hydrogen assisted cracking. The hardness limits currently recommended for
steels in refinery process service are listed in Table 11. Hardness values
obtained in excess of these usually indicate that post-weld heat treatment is
necessary, regardless of whether specified on the welding procedure
specification. In those instances where PWHT is needed, an alternate
welding procedure qualified with PWHT is necessary.
Hardness in excess of those listed can result in stress corrosion cracking in
service due to the presence of sulfides in the process. The 200 BHN limit for
carbon steel is equally as important in sulfur containing oils as is the limit for
Cr-Mo steels.