Austenite grain boundaries

On cooling to room temperature after annealing, maraging steels transform completely to martensite. The as-annealed structure consists of packets of parallel lath-like martensite platelets arranged within a network of prior-austenite grain boundaries. The platelets have a high dislocation density but are not twinned. 

Figure 9. Microstructure of 2 V4 Cr-1 Mo steel. Key 0, ferrite grain peppered with M C , pearlite (platelets of alternating M,C and ferrite) and K, ghost boundary of prior austenite grain boundary.

Similar comments are applicable to weld metal hydrogen cracks, with the further observations that cracking in weld metals is frequently transverse to the weld and that in weld metals of the acicular ferrite type, cracking is most common in the primary ferrite (if it is present) at the prior austenite grain boundaries, as in Fig. 1.3(a). In relatively hard, alloyed weld metals (e.g. in the Cr-Mo steels) any transverse weld metal cracking is usually perpendicular to the weld surface, but in C-Mn weld metals the chevron type of cracking (Fig. 1.3(c)) at 45° to the surface is common. 

Intergranular hydrogen cracking in a precipitation-hardening martensitic stainless steel has been suppressed by the addition of palladium. Heat treatment of the alloy produces submicrometer-sized PdAl precipitates that act as reversible traps and are thought to reduce the amount of hydrogen at prior austenite grain boundaries [134]. 

---