PREVENTING HYDROGEN-INDUCED CRACKING AFTER WELDING OF PRESSURE VESSEL STEELS BY USE OF LOW TEMPERATURE POSTWELD HEAT TREATMENTS (To Purchase Call 1-800-854-7179 USA/Canada or 303-397-7956 Worldwide)
Hydrogen-induced cracking occurs either in the heat-affected zone microstructure or in weld metal when four factors react simultaneously. These factors have been defined as (1) presence of hydrogen@ (2) welding stresses@ (3) a susceptible microstructure@ and (4) a low temperature. Hydrogen can become available during welding from base and welding materials and extraneous contaminating matter. With the current processes of steel making where hydrogen is reduced to a low level (in particular for certain applications within the nuclear industry where the hydrogen content in weld material is closely controlled)@ the presence of hydrogen in the starting metallic materials has been diminished@ but not eliminated. However@ the possibility of introducing hydrogen still exists to some degree@ for example@ from welding fluxes that have not been properly dried. Welding stresses are inevitably present and can only be minimized prior to welding by consideration of joint geometry@ fit-up@ external restraint@ and yield strength of the weld metal. The heat-affected zone microstructure is dependent upon the composition and hardenability of the steel and the cooling rate after welding@ which@ in turn@ is governed by the weld heat input@ preheat@ and thickness. The one factor that is most subject to fabrication control is the preheat temperature and holding time after welding. Thus@ dependence is placed on the control of preheat as an effective means of reducing hydrogen concentrations (if present) during welding and heat treatment. Data are presented to show the effects of preheat and postweld heat treatments. These data are principally concerned with the type of steels used for nuclear pressure vessels which@ for classification purposes@ have carbon equivalent (CE) values less than 0.70. Many tests have indicated that with relatively high preheat temperatures@ hydrogen induced cracking in steels over 13/8 in. thick will not occur even in the presence of high moisture concentrations.