PWHT Question

Date 04-01-2014 20:22

Daniel,
First of all A106B is not a low grade of carbon steel. Its a medium strength carbon steel intended for high temp service.
Second, any detrimental effects of PWHT would occur based upon time at temperature not number of times cooked.
Heat treats of those materials of 8, 10 or more hours are not rare.
So, how much time at temp are you talking about?
Also, what do you mean by detrimental?
If you are considering damage pertaining to charpy's, which I doubt since its A106, then you might have a concern.

By daniel.earp Date 04-01-2014 21:32

Soak time is 1 hour at 1150.

It is my understanding (which I must admit is limited) that PWHT has an increased effect on hardness on weaker materials (GrB vs X65.)

Do you think the limited wall thickness would factor in?

Most importantly, I am being told by an inspector that this is referenced in code. I've reviewed every code I could think of and have not seen any mention of this.

By nantong (**)

Date 04-02-2014 05:08

No restrictions on this. Time at temperature is only regulated as a supplementary essential variable and I take it you are not impact testing A106B. Refer ASME IX QW 407.1 and 407.2.

By js55

Date 04-02-2014 12:00

Daniel,
Actually, for the most part heat treat will reduce hardness as stress is relieved and secondarily, carbon evolves out of the iron solution to become carbides. Those alloys that increase in hardness will be those alloys that have a precipitation mechanism, such as precipitation hardening stainless steels.
Also, at temperature things happen very fast as far as stress relief is concerned. Actually within seconds on a micro level. The soak times are based upon the idea of homogeneity. In other words making sure the entire volume of material is effected. Metallurgical changes such as solid solution mechanisms like carbide formation take more time. This is why, as nantong accurately reports heat treat time at temp is a supplementary essential variable because these metallurgical mechanisms effect toughness to a great extent.

By js55

Date 04-02-2014 12:02

Daniel,
One other piece of advice. When an inspector tells you something is referenced in the Code, respectfully ask him to show you. Tell him you want to know where it is so that you can reference it yourself in the future.

By daniel.earp Date 04-02-2014 18:09

This has been very helpful. Thank you. As a new member to the forum, I can see myself utilizing everyone's wealth of knowledge here to my advantage in hopes that someday I can return the favor.

As for many other inspectors (and contractors) I encounter, generally when I hear someone reference code, the next words that come out of their mouth are either false or completely misread... I've tried not to form the same bad habits.

By 46.00

Date 04-03-2014 00:26

If it aint in the book.............!

By Joey

Date 04-03-2014 01:19

You should have the approved PWHT procedure prior to work start.
Comments of a trouble maker Inspector's are always welcome but must be in writing.

~Joey~

By nantong (**)

Date 04-03-2014 12:29

Daniel, it is clear that B31.3 does not require PWHT on 3" STD wall thickness. On your original post you state "engineering requirements specify PWHT of a particular line. To me this indicates your Client has, within his specifications, considered particular service requirements for this line. Please bear in mind codes such as ASME cannot cover all conditions and it is up to the Client to ensure that all service conditions are considered during the FEED stage.
To me this indicates a Client's concern with Amine/Sour Service and additional requirements such as Nace to avoid the likes of stress corrosion cracking which can be reduced by limiting hardness through PWHT. Have a look at your Client's specification. API 5L allows quite a high carbon content for this material which may not meet the required hardness limits (I cannot remember what the Nace requirement is but under the BS 4515 pipeline code it was 248 HV10 which can be hard to achieve.

By js55

Date 04-03-2014 12:50

The old standard MR0175 limit has always been 22HRC. I don't know what the conversion is offhand, and I don't know (haven't used the standard in awhile) if the standard is still using Rockwell or has changed to Vickers.

By nantong (**)

Date 04-03-2014 13:17

JS55 22HRC equates to HV 248 according to ASTM E140. Have always used Vickers for carbon steels and Rockwell for duplex.

By daniel.earp Date 04-04-2014 15:29

FYI, it is an amine service. I've escalated this question to the design engineer and am awaiting the response. There was no procedure or specification provided by our engineering other than the piping spec that requires the pipe to be heat treated per B31.3 as well as the accompanying WPS we performed the PQR on. Of course, 31.3 doesn't require material of this size and type to be relieved, which also makes me question the soak time at which it has been treated. Is it possible the soak time should have been less than the standard minimum required of 1hr/in?

By js55

Date 04-04-2014 16:25

Extra soak time won't do any damage but it seems a waste of time. The mins in 31.3 are based upon greater wall thickness and adequate time for conduction. Thinner walls would not require such time. If you are cooking what does not require cooking then your are not required to comply with the mins either.

By nantong (**)

Date 04-04-2014 17:50

Daniel, if it contains amines then your designers have got it right. They have given you a piping specification (data sheet?) for this line which shows that it must be stress-relieved. As I said before ASME B31.3 cannot cover everything but your designers are probably looking at something like API RP945- Avoiding Environmental Cracking in Amine Units. This standard clearly states a minimum of one hour with no limit on minimum thickness. Speak to your designers in a friendly way and I am sure they will be more than glad to explain the technical reasons for this requirement.

By js55

Date 04-04-2014 18:23

That's an interesting point. I have to get a copy of the standard (should have one in the library here anyway). I wonder if the requirement is based upon empirical evidence or if there is a considerable fudge factor built in. Given a wall thickness of what is slightly less than .216 whatever is going to happen with A106B is going to take place in minutes at temperature. Essentially if the entire volume is at temperature nothing much else is going to happen, the mechanical phenomena like stress is done. In the long term the chemical stuff, the diffusion stuff, takes over.

By nantong (**)

Date 04-05-2014 19:23

Heuristic with a bit of a safety factor thrown in maybe. Best asking the powers that be how they arrive at these requirements.

By js55

Date 04-07-2014 11:53

Tea leaves and the entrails of owls.

By js55

Date 04-08-2014 12:03

Many people would be shocked at how little empirical justification there is for many Code and Standard requirements. The general history of Codes and Standards in general has been to err on the side of extreme caution and then use empirical evidence to liberalize.

By nantong (**)

Date 04-08-2014 18:10

JS55, and I thought it was all down to scientific experiments and metallurgy and developments in steel making and the like.

I have always believed in this Scots' theory on how to make good quality steel products:

Double, double toil and trouble;
Fire burn and cauldron bubble.
Fillet of a fenny snake,
In the cauldron boil and bake;
Eye of newt, and toe of frog,
Wool of bat, and tongue of dog,
Adder’s fork, and blind-worm’s sting,
Lizard’s leg, and howlet’s wing,
For a charm of powerful trouble,
Like a hell-broth boil and bubble.

Mind you it is a few years since I was at school in Dunsinane.

Tea leaf reading as you say maybe a more accurate option however I have my doubts. I was born in St. Andrews in Scotland and I remember when I was a kid my parents took me to Arbroath for the day. There was a fair on and as we wandered round we saw a tent with the sign "Madam Zsa Zsa tells the future. As we watched Madam Zsa Zsa came out the tent with a friend. I shouted to my mum "hey Madam Zsa Zsa is Mrs Cunningham who lives round the block from us in our home town. Madam Zsa Zsa, teller of the future, holds her hand out and says to her friend "I wonder if it will rain today!".

Maybe better to play safe and just throw the bones in the air (or have a look at API 579).

By js55

Date 04-08-2014 18:54

nantong,
That's good stuff.
We of lesser wisdom must make use of the stone tablets handed down to us from on high.
And lest there be pandemonium in the land of mortals, we shall continue on bended knee to show great deference.
Ah yes, tea leaves, the entrails of owls, and scratching thy regal azzes, the stuff of erudition.

By SCOTTN

Date 04-08-2014 20:08

I visited a fortune teller recently. She studied my palm for only a couple of seconds, then immediately said, "I can tell you're lonely, single and you have not had a partner for some time." Amazed by her answer, I asked if she could tell all that just by the life lines in my hands. "No," she said,"by the callouses and blisters.

By flawfinder (*)

Date 04-10-2014 06:08

By nantong (**)

Date 04-09-2014 12:56

JS 55, where does that come from? Brought a tear from my good eye!

By js55

Date 04-09-2014 13:37

nantong,
LOL!!!!
Just a little sarcasm fun.
We often treat the Codes as being stone tablets. And this is not without a modicum of good cause.
However, the extremely knowledgeable and concerned people who participate in writing these things are acutely aware of their own limitations.
Most of them.
The Codes we must treat as black and white. Though what goes into them can be disturbingly not.