Two methods frequently used are Visual inspection and Hardness Traverse.
Visual
If the metal is steel, etching with Ammonium persulfate (10 mL (NH[sub]4[/sub])[sub]2[/sub]S[sub]2[/sub]O[sub]8[/sub] with 90 mL water) will give excellent contrast of the various microstructures-- base metal, HAZ, weld metal. The surface should be rubbed with absorbetn cotton during etching. Time depends on the desired contrast.
Hardness
Perform a hardness traverse of the area in question, going from base metal, through the HAZ, into the weld, and then back through the opposite side HAZ, and then into the opposite side base metal. The hardness will drop as you go from base metal into the HAZ. There are a number of standards that cover this sort of testing. One example is the following:
DIN EN ISO 14271 Vickers Hardness Testing of Resistance Spot, Projection and Seam Welds (Low Load and Mirohardness)
Sorry, I guess I wasn't specific enough. Is there a "rule of thumb" to determine the approximate HAZ when designing spot welds in a tightly spaced configuration?
Why do you want to measure the HAZ ? Are you concerned about HAZ's are overlapping ? Shouldn't you be more concern about producing an acceptable spot weld of required strength. How about a detructive test ? There are a few different types of destructive tests you can perform.
Yes, I am concerned with HAZ's overlapping. I know that I can create a satisfactory and repeatable spot weld under certain conditions, but if I want to create a group of them, I am concerned that the effect of the HAZ will degrade the quality of the joint.
If I create two spot welds very far apart then I expect the strength of the joint to be twice as strong as with a single spot weld. But as I move the two welds closer and closer together, at some point, the "second" spot weld (and/or surrounding region) won't be as strong as the first and so my joint strength is no longer double what it was with one weld.
ASM HANDBOOK Volume 6 Welding, Brazing, and Soldering recommends a minimum spacing of ~ 15 to 25 times the sheet thickness. Handbook of Product Design for Manufacturing edited by J. Bralla is the reference cited. Other automotive process standards have similar recommendations.
