PWHT Stress Relief Time for Q&T

[tc7]

I will be working on several PQR's soon on Q&T (120-125ksi) versions of 4130 (welded in the Q&T condition). Will be using several different welding processes (GTAW, SMAW & GMAW). All will use some variant of 120xx rod or wire. Some will be stress relieved and some will remain as-welded.

My question is in regard to stress relief. On all normalized 4130 weld procedures we use the time honored "hour per inch" rule at about 1100-1150 deg F. Now I know the tempering temperature for my new PQR test plates will be about 1000 and 1050 degrees so stress relief temp will have to be not greater than 950 degrees F. Do I still follow the "hour per inch rule" ?
Thanks

 

[metengr]

tc7;
Good morning. Yes, the one hour per inch rule is to assure complete temperature soak with minimal thermal gradients. This rule applies to either base material thickness or as nominal thickness of the weld deposit.

 

[tc7]

Hello Met-
You probably will have anticipated my next question so here goes: Since stress relief is intended to unlock stresses (duuh!) as well as some tempering of the hardened HAZ, and presumably welding stresses are thought to approach yield point in intensity, why then would we not require longer stress relief period for the higher strength Q&T material? especially since we would be at a lower temperature than the lower strength notmalized material!

 

[metengr]

tc7;
Time and temperature are synergistic for tempering steels, as you may recall from other posts on this web site discussing the effects of either higher temperature or shorter time or lower temperature and longer time to achieve the same tempering results; temperature, in lieu of time is the predominant parameter for only stress relief after welding (not tempering).

 

[tc7]

I guess I'm having trouble accepting that 1150 deg F for one hour per inch on normalized steel is a compatible synergy for 950 deg F on Q&T steel also for one hour per inch.
How can these two combinations have similar effect? I know it has been said several times in this Forum that 90% of all stresses will be relieved in the first 10-15 minutes so I guess that sort of answers my question. But this claim has never been substantiated with references. I have searched high and low for confirming evidence or authoritative documentation and have not seen any data for that 90%idea. Any thoughts/references on that?

 

[metengr]

tc7;
You have misunderstood my post above. Tempering achieves two purposes; stress reduction and softening of the base metal heat affected zone and weld metal deposit. If one intends to target a specific hardness value it requires reaching a certain tempering temperature and holding at that temperature for a specified length of time to allow precipitate coarsening to ocurr. Normally, for most tempering data, material is held for 1 hour per inch of thickness. Beyond this 1 hour, the kinetics of precipitation coarsening during tempering slows down considerably.

If you intend to lower the tempering temperature to achieve a similar hardness, the time has to be increased. If you need more background information, Google tempering parameter for steels.

 

[metengr]

tc7;
You might want to re-visit your last two posts;

thread330-199888

thread330-197462

 

[tc7]

This question was only about the difference between stress relieving normalized and stress relieving Q&T material.

 

[metengr]

tc7;
Understood. Short answer - stress relieving after welding can be performed at 50 deg F below the original tempering temperature for Q&T materials, and for materials subjected to only a normalizing heat treatment and welded, the temperature for stress relieving can be selected up to 50 deg F below the lower critical transformation temperature.

 

[pwtomlin]

Just my two cents,

The hour per inch rule is a pretty standard rule of thumb, but it's application is to ensure that the part is stress relieved throughout. Larger sections will reach the furnace temperature in the center more slowly than smaller section. The rate at which the center heats up is largely governed by the thermal conductivity of the steel, which would vary somewhat based on composition. The furnace temperature also has an effects, but the impact of all these variables is small (in most cases) relative to the hour per inch rule.

The short answer is there is plenty of variation between parts, alloys, and HT but the hour per inch rule gives you enough leeway to account for most of the variables without testing each setup.