Page 63 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

are difficult to drive back into solution. This contributes to
the high temperature strength of precipitation hardened
superalloys.

Stress relief
The primary function of post-weld heat treatment (PWHT)
stress relief is to relieve internal stresses in welded fabrica-
tions by permitting the steel to creep slightly at an elevated
temperature. This elevated temperature lowers the material
yield point and allows any high residual welding stresses to
exceed the new yield stress, causing localised plastic
deformation. When the material is cooled down, the yield
point increases again but the residual stresses have now been
reduced to a lower level.
Stress relief is also used to reduce the hardness of
hardenable steels after welding to help prevent brittle
fracture. The procedure is basically a form of subcritical
annealing and most steels are stress relieved in the 550–
700 8C temperature range depending upon their composition.
The holding temperature, holding time and cooling rate are
critical and must be sufficient to permit the required changes
to take place throughout the whole material thickness. For
this reason most codes that specify PWHT stress relief will
normally specify the minimum holding time in minutes per
millimetre thickness (min/mm) or hours per inch (h/in).

48

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200904QG Welding chap4.3d Page 48 of 48