Normalized A572 Gr 50? 2

[castle74]

As anyone dealt with normalized A572 50, I was under the
impression that it is only supplied in the as-rolled condition.

 

[consultmet]

True, A572 gr 70 has no reguirement for normalization.This material is used extensively for "fracture critial applications" in the AASHTO specs.On occasion we have had clients that had requirements for toughness (CVN)at temperatures which is questionable for this grade (-50F and below).So investigation of normalizing for improvement of low temperature toughness were done.Unfortunately, results were not consistant, with improvements varrying usely dependant on the manufacture. Chemically the columbium additions play a major roll since normal normalizing temperatures do not reverse columbium back into solution.Basically A572 is an excellent material for welding with tensile requirements of 70 min. Either as rolled or in the normalized condition.Hope this comment is useful.

 

[consultmet]

opps sorry i ment gr 50 , 50,000 psi min

 

[metengr]

You don't need to normalize this plate steel for improved strength or notch toughness, as with other plate steels. The micro-alloy additions in ASME A 572 result in a fine grained microstructure that has improved strength and notch toughness.

 

[consultmet]

true, however, not all producing mills, follow the same rolling and finishing temperatures, which effects for one the plates microstructure, example: banded structure.I agree with your comment about micro-alloy additions intent. My comment was directed to castle74 question if anyone had dealt with normalized A572.

 

[castle74]

Heres the situation, one of our weldments is designed using A572 50 (as rolled) plates welded to a 8630 Mod. forging. There are impacts required on the forging but not on the plates; the weldment is PWHT'ed after fabrication at about 1125 F for two hours. What engineering discovered after PWHT is that the hardness was about 150 BHN on the plates, no telling what the impacts were. Engineering now requires impacts and hardness on the A572 50, hence the normalized state.

Originally, the A572 50 did not require impacts or hardness, so the PQR for this job was qualified using A572 50 (as rolled) to 8630M; the 8630M pasted the CVN test at 31/24 ft-lbs. @ -4 F, and the A572 failed (PWHT on PQR [1125 F for 4.5 Hrs]). I was aware that A572 50 does not do very well when stress relieved for that period of time.

I just never heard of normalized A572, so I am curious of its performance when it comes to impacts and hardness after PWHT.

 

[metengr]

Yes, no surprise as well, the high strength low alloy steels were never intended for PWHT because of loss in toughness. In fact, the ASME Boiler and Pressure Vessel Code, Section VIII, Div 1 has the same problem with PWHT of quenched and tempered, UHT steels. The requirement for PWHT needs to be removed as a current requirement in the Code.

 

[consultmet]

I am wondering about the use of term "normalizing". I am aware that the terms normalizing and stress relieving do not always describe fully the process. 1125F is not above the A3 temperature for A572 therefore it is not normalizing at all, and is more indicative of stress-relieving or sub critical annealing. I would expect a lowering of both tensile properties and toughness when employing this heat treatment. The welding procedure should be qualified with the exactly the same thermal history. Qualification testing is the way to assure desired properties following any PWHT, especially in the HAZ of 8630 as well as the A572. What is the condition of the 8630, quench and tempered?

 

[castle74]

creeper,

The Q & T 8630M forgings temper temperatures varys from 1150 F to 1200 F, they require a 85 ksi yield min. and CVN's as stated above; the A572 50 plates were supplied in the as rolled condition. 1125 F is the stress relieve temperature used on the weldment these two materials make. Engineering believes that having the A572 50 plates in the normalized state will produce better results concering impacts.

 

[consultmet]

Castle74, I certainly think normalizing is worth the try. Seems to me your options are (1) welding procedure with carefully controlled interpass temperatures to avoid high hardness in the 8630 HAZ, (2) a lower stress-relief temperature to obtain the desired stress relief of the weldment but not lowering properties of A572. I did a brief search of Impact data we have on this grade at sub-zero test temperatures which we have piles of. At 0,-4, and -22F this material is quite capable of achieving 25 ft-lb. Always check MTR”S, at room temperature charpy values usually are over 100 ft-lb. Again this very much depends on product and producing mill. I remember a weld joint I once was involved with that was 8630 casting to A633 tubular member; this was on an offshore pedestal crane, where sub-zero temperatures are often encountered. You might want to look into a material change to A633. A633 is a low alloy steel, normalized, with a grade C and D with 50 ksi min, suitable for welding, where low temperature properties are better than comparable strength as- rolled steels. Expect the need for suitable welding procedures. A633 is a very nice material where low temp. Toughness and strength is needed in the area structural low alloy steels. Hope this helps some; I will shut up and listen to any other interested parties.

 

[unclesyd]

Would it help if the PWHT temperature is lowered, with longer times, to amerlioate the effects of the 1150F temperature on the A572 material?

 

[metengr]

Are you sure you even need a PWHT? I would explore this option.

 

[metengr]

To ramble on.... or why not butter the 8630 material, PWHT (only if necessary to reduce hardness), then weld the A 572 to the butter layer with no final PWHT? The properties of the A 572 remain as is after welding, and the 8630 Q&T steel has a softened base metal HAZ from the intial PWHT.

 

[consultmet]

I agree with mentengr, using a temperbead welding technique might eliminate the need for PWHT. Unclesyd, my thought was lowering the PWHT temperature might help by not affecting the A572 hardness and toughness, hopefully still providing enough stress relief of the weldment. All of these ideas of course need to be confirmed by qualification testing

 

[castle74]

Thank you gentlemen, I will bring these ideas up to engineering.

 

[unclesyd]

creeper,
You can drop the temperature 200F with longer times and still approach a stress reduction approaching 100%. On some steels you can even go lower with a corresponding reduction in the percentage stress relief.

castle74,

The proposal by metengr to butter the 8630 then PWHT prior to welding on the welding the A572 is the better way out.