American Welding Society Forum
We have some very thick reinforcement pads being welded around the inside of access hatches on a vertical spire pipe. There is a requirement for PWHT to stress relief the welds and surrounding material. The PWHT procedure is not a problem, my question relates to how to measure the amount of stress relief achieved. The PWHT Contractor has suggested simply measuring hardness before and after with an Equotip instrument. I suspect this will not give a quantitive indication of how much of the stress is relieved, only that we have softened the material by a few Brinnell Hardness points. The Engineer has suggested using strain guages, but this requires a destructive test (hole drilling or saw cut).
I have searched the Web, but can only find procedures involving neutron bombardment, diffraction and the like - far too "exotic" for our application. Can anyone suggest an alternative? What is done when heat treating complex nodes during construction of offshore structures? Is measurement of the amount of stress relief even necessary? PWHT is a fairly standard procedure and the effects of soak temperature and time at temperature have been well established. Is it sufficient to know that if the procedure has been properly followed then it can be assumed that the stresses have been relieved?
Any comments gratefully received.
I suggest You abandon the idea of measuring the ammount of stress relieved, and concentrate on being able to acuratly follow the proper procedure, and verify that You followed it. I don't think there is any merrit to monitoring hardness, You are not annealing it. That is My opinion as a Tool & Die maker, I am not a welding engineer.
As Dave suggests, I don't think there is any merit is attempting to measure the stress.
I also belive your focus should be in researching what the PWHT recommendations are for the material and thickness you are dealing with. Make sure the welding contractor has a properly qualified WPS with PWHT that is consistent with the actual weld conditions of the field weld (ie - thickness, position, filler metal, joint configuration, etc.). You will have to spend some time in the appropriate welding code to determine which variables are "Essential". Once you figure that out, review of the WPS and PQR should be pretty straight forward.
Hardness measurements are imposed by a number of construction codes as a means of determining that the welding and PWHT were performed adequately. That said, if the code you are working under is silent on what the material hardness should be (post weld); you may have to give this a little more thought before the PQR is put into place (ie - check material hardness before welding, check it using a Vickers procedure with a decent spread on a sample with and without PWHT) to determine what to expect for results on your field weld.
I have some pretty decent guidelines on qualifying procedures (with the inclusion of Vickers Hardness Testing). My guidelines are for industrial and pipeline applications, but they could be useful in your situation. If you want them, shoot me and email and I will send you a copy.
Agreed with the above. Stress relief is a function of exposure temperature, more so than time at temperature. I would make sure you have a PWHT procedure and it is followed. The PWHT time and temperature requirements have been in the construction codes for many years and were established based on testing and past safe expierence. There is no need to validate these requirments for each and every job.
Thanks very much, that would be greatly appreciated. You can send them to me at <email@example.com>.
Also thanks to other respondents. As I suspected, the consensus of opinion is that there is no practical way to do this on production welds, which is what I have told the Consulting Engineers. PWHT procedures are in place and will be followed meticulously. I like the idea of doing parallel test pieces with measured Vickers before and after just to keep track (the Contractor suggested Equotip, which of course would be useless for this appication).
Another way of looking at it is this. Just as a thought experiment. Gedanken. Though I understand I am probably not using the word exactly right (Stephen might help me here) or spelling it exactly right. I just like the word.
Anyway, what is tensile strength? Well, atoms have certain magnetic properties and under strain these magentic properties blah, blah, blah, blah, blah. Just put it in a machine and pull it and when it starts to stretch we'll say this represents yield strength. Where it breaks, tensile strength.
What is stress? Well, the atomic matrix has become distorted due to the expansion and contraction of atoms though the input of heat energy blah, blah, blah, blah, blah. Just smack a little ball (or a pyramid-or your finger) with a hammer, measure the indent (or the swelling), and say it represents stress relief. Thats what codes suggest.
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