Die section repair

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I have a piece of hardened A-2 tool steel which required repair.  I preheated the section which is about 9X13 by 1.5 inches thick @ 600 degrees for over an hour.  The middle of the section contains the trim area in a complex shape but no sharp angles which occupies approximately 60 % of the overall section.  The area to be welded is a U-shaped notch which I scarfed out with a carbide burr.  There is no visible fracture after scarfing the parent metal.I am using tig welding with a tool steel filler and multiple passes peening to stress relieve and elongate between passes.  There is approx. one half inch of land for die life which welds fine but their is a one percent draft angle in the rest of the section thickness which cracks as I weld it every time.  I try to weld it but after post heat it comes out cracked.  All this regardless of which direction  I travel or how much amperage I use.Any suggestions this one has me stumped?  Thanks!
Reply:is it possible to fully anneal the part ? then repair and re heat treat ?insert thoughtful quote from someone else2000 Thermal Arc 300GTSW 3.5 hours1946 Monarch 20 x 54 Lathe1998 Supermax 10x54 Mill2004 Haco Atlantic 1/2" Capacity Lasernot mine but i get to play with it
Reply:A picture would really help here, as I'm not sure that I can correctly picture the situation you've described.  That said, I'm also not sure what your expertise level is, so bear with me if this is overstating the obvious.  A2 is a air-hardening tool steel.  So it's going to form a Martensitic microstructure, which is very hard and brittle, just as it sits and cools on your worktable.  The material is then toughened by tempering for a prescribed time, at a specific temperature.  According to my references on hand, the preheat and interpass temperature for welding A2 is 300-500°F.  Here are some questions and thoughts that came to mind.  I don't know if any are the root cause of the problem, but perhaps something I say will ring true.  You mention that the cracking is occuring during welding, and that you're finding it after post-welding heat treatment.  Does this mean you've seen both occuring?  My tool steel references also mention that interpass temps should never exceed the drawing temperature for the alloy.  The drawing temperature for A2 is 350-400°F.  Although it's counterintuitive to me, have you considered lowering your preheat temps?How are you preheating?  An oven or furnace, or a torch?Hope these questions help lead you to the fix for your problem.  Wish I had a more definite answer for you.Regards,DaveIs it possible that the region you're having trouble welding is thin enough that it's cooling enough to harden while you're peening the previous pass?  If the peening is occurring on quenched material, could you be inducing cracking by excessive peening, or by peening at too low a temperature?How are you monitoring the interpass temperature of the part?  Tempil sticks, a thermocouple, or an infra-red thermometer?
Reply:I have not tried to anneal the section followed by welding, that may be an option.  As to Dave's questions, the cracking is visible immediately as I remove the torch after the post gas flow has ceased.  We have tried preheating at 400 degrees with the same effect.  Pre and post heat is done in an electric industrial oven.  To be honest I didn't check the interpass temperature during the process.  A tempil stick is the only tool I currently have available for such.Both of your inquiries/suggestions have been informative and are appreciated.  I will post further info as I work on a solution.  Have a great weekend.
Reply:i would fully anneal before welding.  then preheat to @ 400 degreeskeep it at 400 degrees while welding, harder on large parts.heat to 600 degrees after finished welding then bury in quick lime or hot sand to cool.then anneal and rehardenA2 being an air hardening steel is very hard to keep from crackinginsert thoughtful quote from someone else2000 Thermal Arc 300GTSW 3.5 hours1946 Monarch 20 x 54 Lathe1998 Supermax 10x54 Mill2004 Haco Atlantic 1/2" Capacity Lasernot mine but i get to play with it