Welding Inspection & Metallurgy API ICP Self Study Notes

The design of items should be based on elastic limit (yield strength).
Permanent deformation, resulting from plastic flow, occurs when the elastic
limit is exceeded. A material subjected to loads beyond its elastic limit may
become strain hardened, or work hardened. This results in a higher effective
yield strength, however, the overall ductility based on the strain hardened
condition is lower than that of a material which has not been subjected to
loads exceeding the elastic limit.
Some materials also deteriorate in terms of ductility due to thermal cycling in
service. Reduction in ductility in these cases may fall so far that in-service
repair welding without cracking becomes very difficult if not impossible. This
is sometimes experienced during the repair welding of complex alloy
exchanger tubesheets.