Hardness in the HAZ is usually caused by too quick of a cooling cycle, and is extremely common in MIG/GMAW and on thicker material. As I mentioned before, a pre/post heat, and slow as possible post-weld cool down will reduce the possibility of that happening. Read up on cold cracking and hydrogen di...

Nope, no pre heat, just ambient temperature which was around 28°c in the shop Andy At the school I go to, 12mm (1/2 inch for us barbarians), is about the beginning of the range where we start to pre-heat. Not sure if your spec allows for it or not. Is there a interpass temp you have to maintain? We...

Is there a pre-heat on those plates?