Type IV cracking is creep cracking which occurs at the parent material/heat affected zone (HAZ) interface in ferritic steel weldments. Thus, it occurs at the surface of the HAZ in the over-tempered, intercritical or refined region of the HAZ. It was first detected in the HAZ of girth welds in 0.5%CrMoV steel steam pipework operating at temperatures around 550 to 570°C. It has since been detected in most low alloy ferritic steels which rely on dispersion strengthening for creep strength, e.g. 1-1.25%CrMo, 2.25%CrMo, 9%CrMoVNb. This form of cracking is found in elevated temperature pipeline girth welds and seam welds, pressure vessel end-cap welds, pressure vessel nozzle intersections, pipe branch connections, etc.
Sometimes known as 'mid-life cracking', under typical operating conditions Type IV cracking can occur after as little as 40,000 hours in service and up to 200,000 hours or more. The phenomenon occurs as a result of an inherently weak zone at the edge of the visible HAZ. This low creep strength region is attributed to partial transformation of ferrite to austenite during the welding thermal cycle and/or the over-tempering or softening of the material in this region.
The cracking initiates from localised formation and growth of creep voids in the 'Type IV zone'. A significant feature of the subsequent cracking is that it can be relatively rapid. The crack can be regarded as an 'unzipping' of an already creep-damaged zone. For pipe girth welds, the phenomenon usually leads to a leak-before-break situation. However, for pipe seam welds or end-cap welds, failures have often been catastrophic ruptures.
Type IV cracking is detectable by means of ultrasonic NDT. Automated ultrasonic methods can be deployed for large lengths of pipework. Surface metallographic replication can be used to find cavities which reveal the pre-cracking stage. However, cracks can form sub-surface, so that care is required in interpreting findings. Magnetic particle inspection has a similar deficiency. Owing to the relatively rapid growth of such cracks, a repair or frequent inspection is usually advocated once advanced creep damage or cracking is detected.
See further information about Materials and Corrosion Management, or please contact us.