Tensile testing

[Padmasanker S]

Dear all,

Please refer to attached stress -strain graph obtained during tensile testing of SA 106 Gr B pipe.

While performing tensile testing of SA 106 pipes for Div 1 vessels ,we were not getting yield point even though required tensile strength is obtained.

The specimen were taken in both  transverse and longitudinal direction from half thickness.

Kindly share your experience  in finding the root cause..

Please inform if any further details is required for clarity.

Best regards,

Padmasanker,

Quality coordinator,
Bilfal heavy industries ltd.

[george....@gr.bureauveritas.com]

That is because we define as Yield Point the stress at 0,2% permanent elongation

best regards

Dr. Georgios Dilintas

Authorized Nuclear Inspector
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BUREAU VERITAS PIRAEUS - GREECE

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[Harish Kannepalli]

Did u use extensometer or not?

You have to construct 0.2% offset on the strain axis and get the 0.2% yield strength. Probably ur software would have that option!

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Regards,

Mr. Harish.

[Mohanananthanarayanan KR]

Dear all,

In order to get the 0.2% yield or proof stress, in the load vs extension curve take a point on x-axis at 0.2% of gauge length ( if 50 is GL then this point is  0.1mm on the x-axis ie along the extension). From this point draw a parallel line to the curve obtained by tensile test. This line would meet the curve at a point. Load corresponding to this is the 0.2% proof load and the stress based on this will give 0.2% Proof stress. Since some alloy steel unlike mild steel does not have a specific point of yield, this method is suggested for estimating the yield strength allowing a permanent yield of 0.2%. Software will have this but you have to verify that 0.2% of GL marked is taken for getting the value.

With warm regards

Mohan Ananthanarayanan
DGM QC VSSC


On Fri, 30 Oct 2015 20:28:50 +0400, Harish Kannepalli wrote

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[Alan Denney]

Dear all,

 

Just to endorse the statements below and to confirm that there is nothing unusual in finding that many grades of steel (and most other metals) do not have a defined yield point and we take the 0.2% proof stress as being ‘yield’ for design purposes. One exception is pipeline steels (many of which do not have clear yield point) but where we follow US practice and take as ‘yield’ the intercept with the stress strain curve corresponding to the 0.5% total strain. However if in doubt take the 0.2% proof stress as defined in Mohan Ananthanarayanan’s email below.

 

Alan

 

Alan Denney

AKD Materials Consulting Ltd

 

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[Padmasanker S]

Dear all,

Thank you all for giving information regarding offset method for calculating yield point.

kindly clarify my below query too,

While going through articles related to yield strength ..its learned that because of "Cottrell atmosphere", an yield point is obtained in CS materials.

We have carried out another heat of same above said material(SA 106 gr B) and in that testing , yield point was observed, but in this heat. we were not getting any yield point.

We verified the composition of elements in MTC  and observed that all elements are within the acceptable range.

Is the material acceptable and whether  is there any metallurgical reasons ?

Padmasanker

Quality Coordinator,

BILFAL Heavy Industries Ltd.

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[Alan Denney]

I have to comment, since I do not agree that 'the yield point is observed in all carbon steel material' [but it may depend on what you consider to be a carbon steel]. That statement is probably true for as-rolled and normalised carbon steels but, notably, the TMCP [thermo-mechanical controlled processed] steels which are carbon-manganese micro-alloyed steels and widely used for structures and pipelines do not generally show a distinctive yield point.

Alan Denney
AKD Materials Consulting Ltd

-----Original Message-----
From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of Pravin Nimbalkar
Sent: 16 November 2015 07:14
To: Materials & Welding <material...@googlegroups.com>
Subject: Re: [MW:23883] Tensile testing

Definitely

construct a straight line parallel to the initial linear portion of the stress-strain curve, but offset from it by e = 0.002 (0.2%.) The yield strength is taken as the stress level at which this straight line intersects the stress–strain curve.

Generally the yield point is observed in all carbon steel material.
The reason behind yield point is Cottrell atmosphere. Its related to how dislocation pinned in mild steel metals by carbon and nitrogen interstitials. These atoms distort the lattice shortly and there will be associated residual stress field surrounding the interstitials. This stress field relaxed by interestitail atoms diffusing towords dislocation. Once the dislocation has become pinned a small extra force is required to unpinn and the dislocation prior the yielding and produce upper yield point in stress-strain strain diagram. After unpinning dislocation are free to move in Crystal which results means lower yield point and material will deform in more plastic manner.

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[Pravin Nimbalkar]

Definitely

construct a straight line parallel to the initial linear portion of the stress-strain curve, but offset from it by e = 0.002 (0.2%.) The yield strength is taken as the stress level at which this straight line intersects the stress–strain curve.

Generally the yield point is observed in all carbon steel material.

The reason behind yield point is Cottrell atmosphere. Its related to how dislocation pinned in mild steel metals by carbon and nitrogen interstitials. These atoms distort the lattice shortly and there will be associated residual stress field surrounding the interstitials. This stress field relaxed by interestitail atoms diffusing towords dislocation. Once the dislocation has become pinned a small extra force is required to unpinn and the dislocation prior the yielding and produce upper yield point in stress-strain diagram. After unpinning dislocation are free to move in Crystal which results means lower yield point and material will deform in more plastic manner.

Pravin Nimbalkar
Welding engineer
Bilfal heavy industries ltd

[Pravin Nimbalkar]

Dear Mr. Alan

 Thermo-mechanical controlled processed micro allying steel resist the dislocation motion for certain period of time but shows the Luders band of yeild point in very short area. 

Recently, new methods have been started to be used for investigation of plastic flow of metals such as thermography please see the attachement of ;

1) Influence of Niobium on the Beginning of the Plastic Flow of Material during Cold Deformation.

 

2) For reference 3% Ni steel -Normalised+ Acc. Cooled+ Tempered.( Its shows the Yeild point)

Please clarify your suggestion