Effect of post-weld heat treatment on mechanical properties

Mr Malcom
I am not an expert, but this is my opinion.
the question is very broad, there are many variable's. but I will give an example from my perspective.
if the details of an assembly are thermally heat treated, for specific properties. metallurgical and mechanical.
welding will affect the listed properties. test will have to be conducted to verify acceptable metallurgical and mechanical properties. the remelt of material will consume elements and change the hardness. a post heat treat will remove residual stresses, and prevent from indication from propagating such as cracking.

PWHT is to relieve the stress produced during manufacturing, the original material conditions are not changed after PWHT.

Regards

Yes as long as the the post weld is 50 degrees below the original temper on heat treated and quench products.
Thermal changes happens from welding.
And if possible products are welded in the annealed condition, then heat teated after weld to final hardness and properties.

1) In general, PWHT tempers the weld metal and HAZ that has formed martensite on high strength low alloy weld metals from fast cooling, while also relieving residual stresses. Both tempering and stress relief can be beneficial, but tempered martensite is not as strong as untempered martensite, so strength may go down, but should remain higher than the base metal, in which case it isn't hurting to have lower weld/HAZ strength.
2) This completely depends on the selection of welding consumables used in the welding of HSLA steels. Some microalloys used in welding consumables can get reduced toughness after PWHT, but not all. Choose one that has the properties you want after PWHT.

Carbon will also migrate towards the dislocations, so you'll see a slightly improved yield point region.
And depending on the alloy / the temp / the time on elevated temp, there can be some grain growth (even without the gamma/alpha transformation).

In steels it always a tradeoff between strength and toughness.
For materials that have been made by Q&T or N&T routes you would want any PWHT to be above the final tempering temperature or you would change the properties of the entire piece of material.
In other alloys the upper limits are usually set by metallurgical changes.

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P.E. Metallurgy, consulting work welcomed

The deposited weld metal in carbon and low alloy steels has strengthening mechanisms different from that of the base metal. That is why a typical E7018 filler containing 0.05% C and 1% Mn can meet the 70KSI minimum tensile properties similar to A-516-70 with typical C of .20% and 1.2% Mn. Because of this strengthening mechanism the deposited weld metal is also less responsive to loss of strength at normal PWHT temps. But because of the increasingly low carbon content of typical welding filler metals, higher PWHT temps or longer times at typical PWHT temps may lower the strength of the deposited weld metal below that of the base metals to be joined.

Your kind assistance on this are very much appreciated.

At a company, I worked st many moons ago, welded test coupons were required. And test coupons were run. Obtaining actual yield and ultimate tensile test. Removing all doubts.
The tensile test I ran, most of the failures were not at the weld but adjacent to the weld. Haz.