A6 Tool Steel to A520 sch40 pipe.....

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Hi everyone,I am a welder who has experienced quite a bit over the years, however my boss just threw this new project at me.  We will be fabricating a heavy object using 1" thick A6 tool steel welding it to a 22" diameter sch 40 pipe (A520) using 1/4" fillet welds.  I've never welded tool steel before.  Can this be done using flux core?  Standard MIG wire?  Or only with some special SMAW rods?  Preheat needed?  Any help would be appreciated!!!Ryan from Nevada.
Reply:A6 tools steel chemistry:http://www.matweb.com/search/datashe...82eefa5&ckck=1It's a medium-high carbon steel 0.7%, with some chromium, manganese, and hefty dose of Molybdenum.A520 is a withdrawn standard from ASTM:http://www.astm.org/DATABASE.CART/WITHDRAWN/A520.htmI don't have the chemistry for that at my fingertips.  But, that pipe for high temp boiler tube applications, which means that it's also probably got a healthy dose of chromium and Moly in the pipe as well.  This is good, as the chemistry mismatch should be less.You or your boss should consult with a company that makes filler metal to select the right filler for this application.  At a guess you'll need a B3 class electrode, but it might be a B6 or B9 to get the best compatibility with the tool steel.A company like Hobart, Lincoln, ESAB, or maybe Weldmold(specialist in tool steel filler metals) can give you  the best filler metal recommendation.You'll also need an appropriate pre-heat, interpass, and cool down temp procedure to avoid cracking issues.  I'd guess you'll also need to do some post weld heat treatment to get the best properties from the base metal and weld metal for this joint.This one's going to need careful attention to detail to get it perfect...There's a high risk of cracking as this steel will readily harden upon coolingBenson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector