1 1/4Cr-1/2Mo fabrication as per API 934 C 2

[Paulettaa]

Dear All
I was reading API 934 C for a new project in our company and I have some doubts with which you might help me.
API 934 C Ed.2008: 7.3.1 All base metals shall be heated to a minimum of 300 °F (150 °C) during all welding, rolling, forming, pressing operations, thermal cutting, and gouging operations.

Here the RP uses the word "shall" making it mandatory to preheat the shell material to 150 C before rolling. I find it really difficult to heat the whole shell up to 150 C and maintaining that temperature during all rolling. Have you ever faced this difficulty before? Please share your idea regarding the issue.

Apart from feasibility of the process, I want to know about the possible benefits of this preheating before rolling.

Also the RP continues with a sentence that is even stranger to my eyes. I highlighted that in the image below.

Please tell me what is "maintaining temperature" in this context?

Warm regards

 

[weldstan]

The minimum preheat temperature is not reduced until the PWHT or dehydrogenation treatment is begun. All of the recommendations shown state "should" not "shall".

 

[Paulettaa]

I uploaded the 2008 edition of the RP. However in the 2019 edition the word "should" has changed to "shall". This shows that they have considered it to be more important than they thought before.

 

[Paulettaa]

The minimum preheat temperature is not reduced until the PWHT or dehydrogenation treatment is begun.

weldstan, is it possible that I give the plates a dehydrogenation heat treatment and omit all later preheat before rolling or forming?

 

[weldstan]

No. The purpose of the dehydrogenation treatment is to lower or eliminate hydrogen induced by welding. After the dehy treatment, you can PWHT at anytime thereafter.

 

[DekDee]

Purely out of interest - how does a "Recommended Practice" become mandatory ?
It states in the foreword of the RP it is to be used in the design, fabrication and certification of ASME VIII vessels but I cannot see it mentioned in that code ?

 

[EdStainless]

I always thought that your option was either to form warm, or de-hyd. That if you did the de-hyd first then you could form parts cold.
But of course after you welded (with pre-heat) then you would de-hyd again.
We used to go through this a lot, which sequence of processes is the easiest.
Sometimes going back to heat is easy, and some times warm working is nearly impossible.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[weldstan]

When the Client requires the use of a Recommended Practice as a condition of the Contract, it becomes mandatory.

 

Purely out of interest - how does a "Recommended Practice" become mandatory ?

When a lawyer finds out you did not follow Recommended Practice.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

There is a problem with terminology here that is being propagated by non-experts.

Yes, dehydrogenation soaking involves heat and yes, it is a treatment, but it is NOT heat treatment, which has a specific Code connotations.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[DekDee]

Thanks Weldstan,
I understand contract requirements can make an RP mandatory.
Just trying to understand this:

This recommended practice (RP) covers materials and fabrication requirements for new 11/4Cr-1/2Mo steel heavy wall pressure vessels and heat exchangers for high-temperature, high-pressure hydrogen service. It applies to vessels that are designed, fabricated, certified, and documented in accordance with ASME Section VIII, Division 1 or Division 2. This document may also be used as a resource for equipment fabricated of 1Cr-1/2Mo Steel.

Why is there no mention of this RP in ASME VIII Div 1 or 2 if API deems it critical ?

 

[XL83NL]

The ASME Codes are not considered nor intended to be cookbook type of codes. ASME VIII-1, by means of using ASME Sec II materials, has some leads to this issue: refer to ASME IID Mandatory appendix A, specifically section A-203.2.

 

[weldstan]

Thanks DekDee. API has been developing Recommended Practices that reflect a lot of the major oil companies' standard practices and the RP for heavy wall (2.5" and over in the past) 1 1/4 Cr - 1/2 Mo in hydrogen service reflect many of those practices. Many of the heavy wall vessel shells were hot rolled by a number of manufacturers, with which I worked in the past. Thinner wall shells were cold rolled.

 

[r6155]

An reputable fabricator consider an recommendation like a requirement.

Regards

 

[Paulettaa]

Thank you all for your replies.
DekDee, the owner of the equipment has asked conformance with API 934 as a mandatory requirement. Besides even if they had not asked for it, in order to give an acceptable fabrication procedure or inspection and test plan or other issues it would be necessary to follow all good practices known to the industry. Actually there is a reference to API 934 A in VIII-2.

ironic metallurgist, I'm not sure if I understand what you mean. If it involves heat and it is a treatment why not call it a heat treatment. API itself is calling it a heat treatment.
Again back to my question:
What is meant by the word "maintain" in the following highlighted text?

If it means that the temperature shall be above 150 C throughout the whole fabrication process it really freaks me out since that is definitely impossible (at least in our shop).
If it means "The minimum preheat temperature is not reduced until the PWHT or dehydrogenation treatment is begun" as weldstan mentioned then it can be interpreted that you can give a dehyd treatment and then start rolling or forming the shells at lower temperatures.

I do not want to eliminate preheating for welding, but I definitely want to get rid of it before forming.
The benefits of preheating to 150 C before welding is understandable:
1- It helps the hydrogen diffuse away since the temperature will remain higher for a longer time and increases diffusibility of hydrogen.
2- It slows down the cooling rate of the weld hence prevents (or reduces) formation of hard and brittle microstructure.
3- It reduces temperature gradient hence reduces residual stress.
On the other hand, I cannot understand the benefits of preheating to 150 C before forming. If we want to make use of Hot-forming, then the minimum temperature of forming shall be at least that of PWHT not simply heating to 150 C.

Warm Regards

 

[Drums4Tay]

The word "maintain", in this section of API 934C, means you don't let the item being worked drop below it's preheat temperature until it can be DHT'd or PWHT'd. I've been involved in the fabrication of HEs and PVs that had API 934A or C applied. They are quite challenging.

 

[CWEng]

Paulettaa does have a point. Forming does not introduce any hydrogen like welding does, so why does it group them together and required PWHT or DHT after forming. Seems like it could be just poorly written to me.

 

[Drums4Tay]

I completely agree it's poorly written and the forms of heat-treating it communicates are unnecessary for forming. In the rare even that a customer interprets this as it's written, we will take "exception" to it.