The morphology

nature

influence factors and control principles of type Ⅳ cracks in HAZ of 9%Cr steel joints are reviewed.The results show that the type Ⅳ cracks occurring in the fine grained region and critical region of the HAZ of the joints extend in series with creep voids or microvoids generated by coarse precipitates along grain boundaries

which belongs to typical brittle fracture cracks with creep damage properties. The inhomogeneous martensite+carbide+trace δ-ferrite microstructure is the organizational factor for the formation of type Ⅳ cracks. The width of HAZ has an obvious effect on the creep fracture strength of the joints

adding proper amount of B in the base metal gives the welded joints good creep fracture resistance（containing resistance to type IV crack）

the influence of welding procedure factors can not be ignored. Optimizing the composition and microstructure of the base metal is a essential condition for controlling the trend of the type Ⅳ crack

and the correct procedure measures are sufficient conditions for controlling the trend of the type Ⅳ crack.