This review, prepared in 1990, considered the available literature on the phenomenon of Type IV or midlife weldment cracking, see Figure. The metallurgical and service factors that combine to render a weld prone to this failure mechanism were considered. Case studies from service experience and laboratory investigations dealing with Type IV cracking were collated. The methods available for detection, inspection, repair and avoidance were described.
Better plant life extension and improved availability should be possible with the described level of understanding of this failure mechanism.
Background
Type IV or midlife weldment cracking is a creep induced failure mechanism. It occurs in the intercritical heat affected zone (ICZ) of weldments in dispersion strengthened ferritic steels after prolonged elevated temperature service under conditions where there are high stresses transverse to the weld.
To the date of this review there had been no systematic investigations of the many factors that may possibly affect a welded joint's propensity to fail by Type IV cracking. Little information was available on the effects of composition (particularly residual elements), design, welding procedure, postweld heat treatment (PWHT), grain size, ICZ width and transverse stresses.
With the evidence described it was possible to say that the most promising method for avoiding Type IV cracking is by careful control of stresses transverse to the weld.
