metallurgy of weld joint PWHT

[sandeep monody]

Dear experts.,

Recently I came to know that PWHT does not change the micro structure of weld seam or HAZ.
Please reply me regarding metallurgy of PWHT of weld seam and HAZ in C.S and low alloy steel.

Please clarify why PWHT required for C.S and low alloy steel.

What is residual stress?

How do we remove residual stress during PWHT?

Sandeep.M.V
QA/QC Engineer

[pgoswami]

Sandip,

PWHT is a broad based term which applies to all welds. For C.S and LAS the PWHT's involved in common fabrication are :-

1. Normalizing -- the heating temperature is typically above upper critical temperature AC3
2. Annealing - heating temperature is typically close to lower critical temperature AC1
3. Stress relieving- most common for all fabrications in C.S and LAS. Heating temperature is well below the subcritical temperature. Look at UCS-56 for the recommended temperature ranges for various C.A and LAS.

The first two heat treatments causes changes of microstructures and while the Stress Relieving does not.

 

The intent of any is PHD  is  to  improve resistance to brittle fracture. Stress Relieving  in particular  attempts to improve notch toughness and relax residual stress.

 

Residual stresses are generated due continuous heating and cooling cycle involved during welding. It is the locked up stress within the weld/HAZ, which is usually tensile in nature. The sheer volume of the weld metal adds up to this residual stress. Lesser  the volume of weld , lower is the residual stress, example Narrow Gap bevel vs. conventional bevel. Residual stress is primarily caused by the compressive yielding that occurs around the molten zone as the material heats and expands during welding.

 

Lower heat input or control of interpass temperature or balanced welding etc all are factored to reduce residual stress in the weld.

 

 Hope the above answer clarifies your doubt, which is the very basic of any post weld heat treatment.

 

Thanks.

 

 

Pradip Goswami, P.Eng,IWE
Welding & Metallurgical Specialist
Ontario, Canada
pgos...@quickclic.net
pgos...@sympatico.ca

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[sandeep monody]

Dear sir

Thanks for your reply and I understand your explanations.

But now please clarify how does the brittleness changes after pwht?

I think no physical properties will change without change in chemical properties.

Sandeep.m.v

[Ramin Kondori]

Dear Sandeep:

A few things happen with Stress Relief:

1. residual stress makes every thing brittle. Metals are more tough when they are not in tension and weldments always have residual stress at levels close to yield point.
2. SR will temper the microstructure. This tempering like quench & temper steels, have a possitive effect on toughness and reduces brittleness.
3. SR helps residual austenite transform to ferrite. This improves toughness.
4. SR will reduce Hydrogen level in the weldment.

Hope you get the idea...

Regards

Ramin Kondori

Sr. QA/QC Engineer

SINOPEC

r.ko...@petroyada.com
+98-2123592322
+98-9132150320

[sandeep monody]

Dear Ramon Kondori Sir.

Earlier I was believing the same what you wrote in the reply.

Regarding the 3rd point u mentioned needs more clarification.,because no microstructure changes during pwht as pwht temperature is below the lower critical temperature of the metal but brittleness/hardness changes.

Please clarify your 3rd point

Sandeep.M.V

[Ramin Kondori]

Dear Sandeep:

Cooling after welding is quite fast and when temp drops below Martensite Finish (M_f), it retards the transformation of austenite to some extent. SR is very similar to tempering and will help transformation of Retained Austenite (RA) and improves dimensional stability and soft spot issues.

Of course this RA comes out in Alloys with more than 0.3% Carbon content but it may happen in low carbon grades too. Read the following article, you will get the idea.

Regards

Ramin Kondori

Sr. QA/QC Engineer

SINOPEC

r.ko...@petroyada.com
+98-2123592322
+98-9132150320

[pgoswami]

Sandeep,

 

The issue is back to very basics of welding metallurgy. Please see the attached documents. The types of  PWHT are many, however stress relieving is by far the most common PWHT adopted in BPV, Piping fabrication or any weld repair .

 

What is stress relief and its' effects are nicely  described in these documents.

 

Stress relief does cause any metallurgical transformation. Metallurgical transformation during PWHT will add to stresses.

 

If the metallurgical transformations are  occurring during  so called stress relief then PWHT is totally out of control. 

 

Thanks.

 

Pradip Goswami, P.Eng,IWE

Welding & Metallurgical Specialist

Ontario, Canada

pgos...@quickclic.net

pgos...@sympatico.ca

 

 

---

From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of Ramin Kondori
Sent: Sunday, June 30, 2013 8:54 AM
To: material...@googlegroups.com
Subject: Re: [MW:18054] RE: 18041] metallurgy of weld joint PWHT

Dear Sandeep:

Cooling after welding is quite fast and when temp drops below Martensite Finish (M_f), it retards the transformation of austenite to some extent. SR is very similar to tempering and will help transformation of Retained Austenite (RA) and improves dimensional stability and soft spot issues.

Of course this RA comes out in Alloys with more than 0.3% Carbon content but it may happen in low carbon grades too. Read the following article, you will get the idea.

Regards

[Shomenath Bagchi]

Transformation to retained austenite is possible, but is a relatively rare case.

---

From: pgoswami <pgos...@quickclic.net>
To: material...@googlegroups.com
Cc: smv.m...@gmail.com
Sent: Monday, July 1, 2013 9:19 AM
Subject: RE: [MW:18056] RE: 18041] metallurgy of weld joint PWHT

[pgoswami]

Mr Bagchi,

 

Thanks for your feedback. Such transformation would occur only when the  the "Lower Critical Temperature"  is  exceed in course of post weld heat treatment. Usually the rule of  thumb rule(say for C-Mn Steel )is :-

• Lower Critical Temperature -723 Deg C

             

       

 

• Tempering Temperature  --675-700 Deg C

 

 

• Stress Relieving Temperature -600-640 Deg C

 

There should not be major surprises, if these rules are followed.

 

Best Regards.

 

Pradip Goswami, P.Eng,IWE

Welding & Metallurgical Specialist

Ontario, Canada

pgos...@quickclic.net

pgos...@sympatico.ca

 

 

---

From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of Shomenath Bagchi
Sent: Monday, July 01, 2013 7:48 AM
To: material...@googlegroups.com
Subject: Re: [MW:18063] RE: 18041] metallurgy of weld joint PWHT

[sandeep monody]

Dear all.,

First of all I would like to thank Goswami sir for his replies., whenever I posted my queries I have got his replies and study materials.

I have gone through all replies and some sites in web and I think both P.Goswami sir and Ramin kondari sir are right in this regard. Here am trying to conclude the matter as given below. Please correct me if I'm wrong.,

1.due to sudden cooling austenite couldn't change to ferrite and forms unstabled or distorted martensite.

2.atoms in the distorted martensite creates the stress in the haz and weld.,which we call residual stress.

3. During pwht, atoms of unstable martensite regain or rearrange its location and form tempered or stabled martensite and reduces the stress up to some level. But there is no phase change or microstructual change.

Please send your feedback.

Sandeep.M.V

[John Henning]

You might want to try and obtain a copy of "Welding Metallurgy" Volume
2, by George Linnert, published by the American Welding Society. This
book has an excellent basic discussion of stress relief heat treatment.
(starting on page 159) After reading this you will have a basic
understanding of what can occur during sub-critical PWHT and any
questions that remain will be more sharply focused.

One exception I have to the discussions I've seen is the comment that no
microstructural changes occur. This is not correct. A subcritical heat
treatment will NOT, by definition, result in bulk microstructural
transformations; ie. delta ferrite to austenite. However,
microstructural changes do occur. Atom movements may result in some or
all of the following: creep, tempering, recrystallization, and changes
in carbide morphology. For example, in simple carbon steels, you may
observe a coarsening of the pearlite plates or spheroidization of iron
carbides. For alloy steels you may be able to see changes from
martensitic structures to tempered martensite and carbides.

I don't generally make recommendations, but for any of you who are weak
on the basics of welding metallurgy, I would strongly recommend getting
the two volume set of books by Linnert. Although it may seem somewhat
dated the basics are well detailed without requiring extensive
background knowledge.

John A. Henning
Welding & Materials

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From: material...@googlegroups.com
[mailto:material...@googlegroups.com] On Behalf Of sandeep monody

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[John Henning]

Sandeep,

What you have stated below is fine for alloy steels and carbon steels
with a higher CE (such as HSLA's or high carbon steels) with respect to
the formation of brittle microstructures. But typical carbon steels (CE
< 0.45) will most likely have little or no martensite formation - the
cooling rates, even during welding, are not fast enough. For carbon
steels with CE on the high side or for thicker sections (more mass -
more quench effect), one of the reasons for preheating is to suppress
the formation of any martensite by decreasing the cooling rate of the
material. This is usually done to result in a material less susceptible
to hydrogen induced cracking but also to control hardness - remember
that 200BHN max the petrochems like.

There are a number of factors that affect what will happen when a steel
or alloy steel cools from the austenitizing temperature. Chemistry
(carbon content, alloy additions), grain size, cooling rates, all affect
the resulting microstructural components present after cooling.
Transformation diagrams (IT, TTT, CCT) are discussed in almost all basic
metallurgy and heat treating texts. There are a lot of discussions etc.
available on the internet. You will find that most of these discussion
are on how to achieve greater hardenability and low carbon steels are
only briefly discussed because they are, well, boring from a heat
treating standpoint.

You have to remember that the largest component of residual stress is
due to shrinkage of the weld nugget and HAZ when the weld is restrained.
The more complex the structure or the more highly restrained the weld
the higher the residual stress will be - nominally up to the yield
strength of the material.

John A. Henning
Welding & Materials

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[Ramin Kondori]

Dear Sandeep:

The first thing you should consider for stress relief is the stress itself that is independent of microstructure or type of alloy. Residual stress is always present in weldments and SR is primarily applied to reduce the level of this residual stress. That you should consider as the main reason for SR.

Other items like tempering the martensite or help retained austenite transform are bonuses of SR and of course sometimes these bonuses are as valuable as the wage...!!!

But about your conclusions:

1.due to sudden cooling austenite couldn't  change to ferrite and forms unstabled or distorted martensite.

Few Austenite grains remain Austenite. That's why we call them retained austenite. They transform later in an uncontrolled manner and cause trouble (dimensional unstability, soft spots etc.).
It doesn't always happen. This "may" happen in carbon steels with C>0.3% and sometimes in lower ranges if the cooling rate is high enough. By SR, you will accelerate this transformation because we don't like to be surprised later.

2.atoms in the distorted martensite creates the stress in the haz and weld.,which we call residual stress.

Residual stress has nothing to do with martensite. It will be created in any weldment with any microstructure. Moreover, Martensite is a stressed (and somehow distorted) microstructure itself and the only thing that happens to martensite (during SR) is that it will be tempered. Also the Hydrogen level of the weldment will be reduced after SR.  

3. During pwht, atoms of unstable martensite regain or rearrange its location and form tempered or stabled martensite and reduces the stress up to some level. But there is no phase change or microstructual change.

You do not need martensite for residual stress. Even in ferrite microstructures you still have high level of residual stress. It is independent of microstructure.

For existing Martensite, yes. there will be no phase change, only tempering of Martensite. But do not forget that still the best result of SR (even in martensite) is that it releases residual stress and tempering is the bonus of SR. Again, the main function of SR is that it will release residual stress.

Regards

Ramin Kondori

Sr. QA/QC Engineer

SINOPEC

r.ko...@petroyada.com
+98-2123592322
+98-9132150320

Sandeep.M.V