You pinpointed a question which has been asked thousands and thousands of times and answered quite very often in different manners. From a strictly technical point of view, it can't be said that a particular case is identical with another. Too many variables influence the HAZ to establish a general rule applicable to all cases. Always from a strictly technical point af view, only a metallographic test of YOUR HAZ will dictate whether and how many times the weld can be repaired.
In too many cases, of course, this is unfeasible. HAZ metallographic analysis is a destructive test which, as the name itself says, requires the weld to be destroyed.
For this reason, there exist a few "rules of thumb" which are followed on jobsites, structural steel and plateworking shops, maintenance departments and other places where welds are done. These "rules" are not based on theoretical principles nor metallurgical consideration but on the experience gathered after years and years of experience and, of course, are somewhat conservative rather than not. It's better to limit the repairs and be in the safe side instead of being "generous" and risk a weld failure when it is in service.
Back in my days of erector engineer, the current "rule of thumb" spreaded through jobsites was that, for carbon steel, three repairs were the maximum, whereas for alloy steel it was only one.
Does anyone have a different experience?
Giovanni S. Crisi
Sao Paulo - Brazil

I have to agree with Prof. Crisi that this is not an easy question to answer.

Let us considder some of the variables:

1) When repairing, are you removing the old weld and HAZ before re-welding?
2) How thick is the component?
3) What pre-heats were applied?
4) What post weld heat treatments have, and will still be applied?
5) How did you remove the "defective" material to perform the repair? Did this cause the material to become very hot?

To place some perspective on the problem, let's considder the following example: Let us suppose that you have a 30mm thick component that you are wleding with the SMAW process with a single V weld prep. It can take you anything up to 30 weld runs to perform this weld. Each weld run has an effect (either positive or negative) on the material around it. How is this substantially different from welding an 8mm thick plate and after the final run, grinding out the weld and re-welding it say ten times?

If suitable welding parameters are used, subsequent weld runs actually tend to improve the properties of earlier runs. (You can see this from the fact that multi-run weld test pieces generally have better properties that single run weld test pieces.) If your repair peocedures are suitable, there should not be any substantial reduction in weldment properties when you perform the repairs. What does however often happen is that excessive weld residual stresses and distortion build up in the components. This can often render a component unsuitable for its duty.

Prof. Crisi's rule of thumb of three repairs for plain C steels seems good for new fabrications. (After all, if you are a customer, you want a "new" component when you are paying for it.) If one however works in an old plant as I do, which is 50 years old, one comes accross items that have been repaired ten times or more in their lives. This includes vessels that have been post weld heat treated each time. An analysis of the material (as far as one can do it without doing too much dammage) shows that there has been very little degradation of the properties.

If you are in doubt, a metallurgical consultancy can take some replicas of the surface of the material, and tell you if you have a problem. Hardness tests are also very usefull for this purpose.

Hope this helps

Regards
Niekie Jooste
South Africa

One of the other things to consider is the service conditions of the material being repaired. The number of repair cycles on a statically loaded item may not affect the fitness for service however a dynamically loaded item that is subject to corrosion and/or heating-cooling cycles may not be as forgiving.

The two previous gentleman may have better details about what is actually effected by multiple repairs. I think AISI HAZ tensile strength stays about the same after multiple repairs (This is only a guess and I welcome correction). Other properties such as impact, resistance to creep, yield and corrosion resistance may be much more affected.

On any critical item in which failure could cause death, injury or serious property damage I would always recommend testing to determine the true effects.

Have a nice day

Gerald Austin
Weldinginspectionsvcs.com