Hastelloy molybdenum segregation
The local enrichment of molybdenum and tungsten within interdendritic spaces of nickel-based alloys during solidification, which promotes the susceptible formation of undesirable brittle secondary phases and is minimized through optimized cooling rates.
When welding Hastelloy, molybdenum and tungsten enrich between the dendrites during solidification. That's normal and doesn't need to be prevented. It only becomes a problem when brittle secondary phases form due to incorrect cooling or excessive heat input, which can lead to hot cracking. So, watch out for controlled cooling and avoid unnecessary heat buildup. Keep the interpass temperature low and work with not too high heat input. If you lay the weld cleanly and with controlled heat input, toughness is maintained and you won't get cracks.