Heat treatment can be used to accomplish several different things. Different alloy systems respond to heat treatments in different ways. For instance, if high carbon steel is heated to a high temperature, i.e., austenized, and then quenched in water, it becomes hard and brittle. The same is not true with copper. If pure copper is heated to a high temperature and quenched, it becomes soft and pliable. In either case, the heat treatment affects the mechanical properties of the base metal, but in different ways.
Generally, any heat treatment that affects the mechanical properties is important to the end user and the design engineer. If post weld heat treatment is required, it must be approved by the engineer. The engineer must understand what post weld heat treatment is to be used and how it affects the properties of the base metal.
While I believe the language used by D1.2 could be much clearer on the subject, most people recognize the influence of post weld heat treatment. In the case of aluminum for structural applications it is not usually the fabricator that will perform the final post weld heat treatment. The fabricator rarely has the facilities or the personnel with the expertise required to perform post weld heat treatment on the aluminum alloy. An engineer with experience working with aluminum alloys may separate the welding operation from the post weld heat treatment and separate the qualification of the WPS and PWHT since they are performed by two different parties/contractors. It all depends on how the contract is developed. Make no mistake; PWHT is a variable that must be controlled whether the alloy is ferrous or nonferrous.
Welding overages heat treatable aluminum alloys. Over aging the alloy allows the alloying constituents to precipitate to the grain boundaries thereby weakening the metal. PWHT usually consists of heating the weldment to a high temperature so all the alloying elements go back in to solution. The alloy is quenched so that a super saturated solution is formed. Aging, either naturally at ambient temperatures or artificially at a slightly elevated temperature allows some of the alloying elements to "clump" within the atomic lattice. The clumping action strains the atomic lattice and strengthens it. The mechanism is much like taking a length of GMAW solid electrode in which the atoms are "aligned" by drawing the wire through a die. The wire can be bent to a sharp radius rather easily, but it is much more difficult to straighten the wire. Instead of becoming straight, the wire will bend to either side of the original bend. The mechanism is often called “strain hardening” or “work hardening.” However, in the case of precipitation hardened aluminum alloys, the "strain hardening" is the result of the clumping action that distorts the lattice just as bending the wire did in the previous sentence.
As mentioned, over aging occurs when the alloy constituents precipitate (diffuse) to the grain boundaries (clumping is no long present within the lattice) or when the alloy is remelted by the high heat of welding thereby allowing the alloy constituents to diffuse (precipitate) to the grain boundaries, thereby removing some of the alloying constituents that would otherwise be available to strengthen the alloy. The end result of over aging either by holding the alloy at a high temperature for a period of time or by remelting the alloy constituents by the high temperatures of welding, the alloy is weakened. In the case of the 6061-T6 alloy, the tensile strength is reduced by about 40%. Some of the diminished properties can be recovered by using the appropriate PWHT.
Heat treatment, whether it is associated with welding or as a separate operation can affect hardness, tensile strength, yield strength, ductility, impact properties, electrical resistivity, conductivity, and magnetic permeability. There are other properties that I may over overlooked, but considering the influence of heat treatment, I find it hard to believe anyone would not consider it to be a variable that must be considered when designing a component.
Best regards - Al
By tom cooper 
Date 06-02-2013 13:21
Edited 06-02-2013 13:24
Thanks Al, that was a very scholarly explanation and your post should be bookmarked by everyone who welds, inspects or design with aluminum.
However, the question remains unanswered - what does the code require?
In view of pwht being specifically labeled as a "variable associated with the welding process", the code is not requiring me to include pwht as part of PQR. To satisfy Clause 4.2.4, my only requirement is to demonstrate I can "restore T6 condition", I don't even have to do this on a welded plate - I can do this on any piece of scrap aluminum.
Therefore, if a welding firm already has an existing D1.2 qualified procedure based on Table 3.2 'as-welded' tensile strength then all that is then required is a demo heat treatment on a unwelded plate according to a heat treat recipe copied from the ASM handbook, followed by a single tensile test that meets a 42ksi breaking strength and walah! the requirement has been met.
Agree?
That is what I didn't understand. My limited experience of using PWHT was solely for stress relieving or straightening. It was my assumption that more specific terms, like 'quenching' and 'tempering' would have been used for increasing ksi, especially if it was going to be a separate operation. I had pictured welding on a large vessel (tank, ship) that could not be put into a furnace. Guess this is what happens when not having all the information.
I do understand the importance of heat treatment in the design of a product, but wouldn't the Code list PWHT as an essential variable for re-qualification if it was detrimental to the weldment?
I hadn't realized that. When I was welding copper, the WPS required a preheat of 900 degrees, but no PWHT.
As always, I learn something new from your posts.
Al said it very well, but it can not be over emphasized that heat treating will affect different materials in different ways.
Where most people get confused is in the terminology and application of a particular process. When speaking of heat treating one can mean: annealing, normalizing, quenching, tempering, preheating, postheating, and/or stress relieving. Now, it is narrowed down in this case as it is PWHT so we know we have excluded Pre-Heat. Also, some of the others are more applicable to preparing a material for welding while others are for restoring them to their original state after welding or even changing that state to something more desirable after the welding is completed.
From here I am not even going to begin to add to what Al said as it would be above my head anyway and I would only be quoting other sources that you can check out yourself. AWS has many invaluable reference materials that dicuss these processes in much depth.
Have a Great Day, Brent
