What need butter???

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I am new to the welding community, and i had a few questions regarding the dissimilar metal weld used in the nuclear industry.  I have a low-alloy metal being connected to a stainless steel metal.  I have to use a butter (alloy 82) welded to the low-alloy steel and then have it post weld heat treated.  Then I have to place a filler metal (alloy 182) to connect the stainless steel metal.Here are my questions:1. Why is it necessary to have a butter weld2.  Why is the buttering heat-treated, do all buttering have to be heat-treated before applying the filler weld metal (e.g. inconel, alloy 182)3.  Why is the filler metal weld not heat-treated afterwardThanks.
Reply:Buttering, or coating the sides of the joint with an intermediate filler metal is done to help match the chemistry of the base metal to that of the filler metal.  A welding engineer will specify this if there is a significant risk of cracking in the weld bead or HAZ.  A weld joint is a mixture of the base metal chemistry and the filler metal chemistry.  In your situation, the chemistry for one side of the joint, which is a low alloy steel, and the other side, which is a stainless steel, are most likely incompatible.  If you weld them straight together, there is a good chance the weld will crack during cooling; or that the weld will be too brittle and the weld will crack in service.Less likely, but possible, is the chance that the weld chemistry that results from a mixture of the two base metals will not have the desired resistance to corrosion or stress-corrosion cracking.  A third possibility is that the weld metal won't have the necessary toughness because of the chemistry that is created when they are directly welded together.The buttering, and use of an inconel filler metal are all meant to prevent these sorts of problems from occurring.The post weld heat treat after the buttering is meant to repair the steel microstructure and restore the desired mechanical properties to the weld base metal.  Without the details it's difficult to say exactly what the end result of the PWHT will be.Without more details about the welding procedure and the exact alloys involved it's impossible to say for certain why this specific procedure is necessary.  But the short answer is that it's meant to prevent problems with the weld...Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Welder182,Lots of times buttering is necessary so as to avoid PWHT after completion of the welding.So I could suppose that at your case it is preferable to butter and perform PWHT before completion of the welding.With regard to your queries:1. Buttering is done to avoid PWHT after completion of the welding or to provide suitable weld deposit transition for the subsequent completion of the welding, 2. No, the PWHT depends on the specific project requiremnts (code, materials, etc)3. No reason for doing PWHT afterward provided that you have done it after completion of buttering and based on the required PWHT for this specific materialRegardspw35Last edited by 7A749; 09-30-2013 at 08:16 PM.