Tempering temperature of Cr/Mo steels 1

[VeryPicky]

Hi all.

One of my client specifies that the PWHT temperature for 1 1/4 Cr 1/2 Mo shall be 50°F lower than the tempering temperature.
That spec was not available initially to the fabricator and they worked out their material and fabrication heat treatments: PWHT was set on 1275°F and the tempering temperature on 1200°F.

Now, with revised requirements as per spec the mill refuses to guarantee the properties of the material.
Equipment is pressure vessel in hydrogen service with impact requirements.

I wonder why the requirement for that 50°F and what would happen if the material was delivered as initially planned?
I appreciate any thought posted on this forum.

 

[davefitz]

If the PWHT is above the first critical temp then it is possible that the crystal structure may have austenized, and the fsubsequent rate of cooling would then determine if the final crystal structure had the correct ratio of ferriteeralite:bainite:martensite.

You can guess what crystal structure resulted by knowing the cooling rate and component thickness or determine it by hardness test plus photmicrographs.

The 1st critical temperature of the parent material might not be the same as the 1st critical of the weldments, so it is wise to also stay below the weldment 1st critical temp.

I guess in hydrogen service you do not want any martensite or bainite, so a slow cool would be requred.

 

[Carburize]

The 50 degree F rule is to try to ensure that the hardness/strength of the material does not drop significantly below the values reported on the material test report.

 

[metengr]

The lower critical for 1.25% Cr-1/2% Mo is approximately 1430 deg F. Based on your information, all may not be lost. In addition to what was stated above, this material may still be quite capable for service. At this point, if your quality program permits it I would label the PWHT plate as a nonconformance item, and attempt to disposition it versus complete rejection.

For disposition, I would obtain a sample of the plate material and duplicate the PWHT. Submit the plate for metallurgical tests to determine if the mechanical properties and Charpy impact values have been altered. This can be easily done and is not that expensive.

You might be surprised that the time at temperature for PWHT at 1275 deg F may not have significantly altered the specified mechanical properties, especially if the original tempering temperature was 1200 deg F (this may be a minimum tempering temperature - have you verified this?). The response of 1.25% Cr- 1/2% Mo plate material to changes in hardness and mechanical properties for PWHT at 1275 deg F may not affect the minimum specified propoerties for tempering at 1200 deg F (as long as you did NOT exceed the lower critical transformation temperature). As a side note, the minimum PWHT temperature for P4 (1.25% Cr-1/2% Mo) material is normally 1300 deg F. It is worth the evaluation.

 

[VeryPicky]

Thank you guys for input.

Let me stir it a bit more.

We have requested a full PWHT simulation test to prove the mechanical properties after multiple cycles (100% not 80% as per code).
With heat treatment temperatures around 1200-1300F I don't see how we would exceed the lower critical temp, so the speed of cooling should not influence the final structure the degree of residual stress, rather.

The PWHT temperature is set to meet the licensor requirements for specific service conditions. Following what Carburize says I would expect the minimizing of the tempering temperature to be beneficial for mechanical properties. We are looking at trade off - hardness v/s UTS. On one hand we want to minimise local hardness values for service purpose ane the other we need strength to withstand the pressure.

I guess the follow up question is:
is there any relationship between tempering temperature and PWHT temperature in relation to material condition (mechanical and metallurgical) other than a possibility of ovetempering by extensive PWHT, provided the both temperatures don't exceed the lower transfomation range?

In other words: if we need to PWHT at 1275°F should we raise the tempering temperature to ensure the 50°F gap between the both temperatures?

I think that action does not find any technical justification because the final product is overtempered material with inferior strength (UTS). Correct me if I am wrong, please.

 

[VeryPicky]

Sorry Metengr, I was not clear enough. The plate I am talking about is about to be manufactured by the mill and we are trying to be proactive. So the drawback is: I have no material to test yet.
The positive side is: we don't deal with an NCR yet.

We are actually setting the original requirements and streching ourselves between the licensor, who enforces the 50F rule and the mill who cries out loud over guarantees they suppose to give regarding material strength and impact properties.
Please read my previous posting, which I was busy with while your kind message arrived.

 

[metengr]

Jackscan;
What is the original plate specification because most ASME SA plate specifications for 1.25% Cr-1/2%Mo call for a minimum tempering temperature of 1150 deg F, not a maximum.

As far as tempering and PWHT, these are two different heat treatment methods. PWHT is performed to assure adequate stress relief and to reduce the hardness of the weld deposit and base metal heat affected zone. Normalizing and tempering is a heat treatment operation to achieve desired mechanical properties. I am amazed that the tempering temperature of the plate was held at 1200 deg F, especially when the typical PWHT temperature for this material is specified at 1300 deg F, by Code. As you indicated, increasing the tempering temperature to 1300 deg F to fall within the PWHT range could alter the customer's specified mechanical properties. But again, I have to ask what was the original plate material specification?

My experience with 1.25% Cr-1/2%Mo plate material is that you should have adequate mechanical properties with tempering at 1300 - 1350 deg F.

 

[VeryPicky]

Metengr at al,

the plate is SA-387 Gr.11 Class 2 - 4" thick.

It is supposed to be certified with 4 PWHT cycles. I guess this is the portion that worries the mill. If they overtemper the material in 4 cycles following the tempering at relatively high tempering temperature they might not meet the properties.

I don't mind following either procedure but the bottom line is:
1. I want to avoid unnecessary heating the material and, ultimately damaging it and
2. need to satisfy the client and licensor and
3. need the mill to guarante the properties to satisfy the fabricator.

It is a handful of disgruntled people so far in this issue.

 

[metengr]

jaceksan;
To provide you some technical background on SA 387 plate steels, I have attached a web site from ISG (International Steel Group) that contains excellent information;

http://www.intlsteel.com/content/products/prod_plate_lit.aspx

I would suggest you contact these folks and discuss your specific issues regarding mechanical properties and PWHT.