API 934 C Preheat requirement

[mechmania]

Hello all,

We have a vessel with Shell material of SA-387-12 CL2 and shell thickness of 105mm.This vessel needs to follow the API 934-C requirements.
API 934-C para 7.3.1 (Preheat) mandates that "All base metal should be preheat to a minimum of 300 def F during rolling.

Out vessel supplier is asking waiver to this requirement.

My question is how difficult is it to meet this requirement?

Regards,
Mechmania

 

[metengr]

Preheat is required for this material chemical composition and thickness to control the level of hardening which can occur in the weld region. Preheat can be easily applied using electric resistance heating elements or other means (neutral flame torches). The only reason I can think of where you could not achieve local preheat is a confined space within a pressure vessel where welding is being performed. Even then, controls can be put into place to apply local preheat from an exterior surface and keep the welders comfortable on the inside of a vessel.

 

[weldstan]

mechmania,
Are you defining prior to rolling of the shell course or prior to welding? If prior to welding, see metengr.

If rolling the shell course or prior to welding, understand what "should" means. Should is a recommendation; shall is a mandate.

 

[weldtek]

To answer your question directly, it can be difficult to preheat a plate for rolling. The degree of difficulty depends on the size of the plate, and the equipment available to the fabricator. Some shops are set up to preheat plate for rolling in that they have a furnace designed for this operation in close proximity to the rolls. Along with this they need the proper handling equipment and some expertise.
As to whether or not it's necessary is another matter, but I can say we're rolling 387-22 in 2.375" thickness to a ~ a 50" radius at ambient temperature. So the necessity for preheat is a function of material type/ grade / condition, thickness and final radius. There are of course other factors that play into the situation such as the type of rolling equipment available, pre-bend etc.

 

[metengr]

I find it odd that there is a suggested preheat temperature requirement for only 300 deg F for plate rolling because normally rolling can be done hot, warm or at ambient temperature. Forming is a process detail not typically spelled out in Codes and Standards because it is dependent on the power requirements for your roll forming equipment.

However, I must confess when I reviewed the OP, I focused on preheat for welding and not forming. If it is for forming, it makes no sense.

 

[mechmania]

Yes, when I referred to API 934-C pre heat requirement, i was referring in regards to preheating before rolling.
And yes the Standard says “SHOULD” so it’s more of a recommendation than mandate.

The supplier is suggesting following in lieu of Pre heat before rolling.
1) UT (Grid wise 9" x 9") and MT (100%) examination after rolling (from inside).
2) Hardness check with grid length of 9 inches from inside.

Does these alternate adequate provides the properties ensured by pre heating?

Supplier wants waiver siting the difficulty in accommodating this pre heat (before rolling)requirement in actual fabrication procedure.

What if we do not get adequate properties with pre heat waiver? What is possible fix than?

Should we enforce this suggested requirements or alternate suggested by supplier would be okay?

Thanks
Mechamania

 

[metengr]

Mechamania;
I see no reason to perform the UT and hardness prior to rolling simply because the preheat recommendation is being waived. In this case, 300 deg F is too low a temperature and will provide little benefit in terms of roll forming. I would waive the 300 deg F preheat for rolling if it cannot be done using a furnace to heat the entire plate thickness.

 

[mechmania]

metengr,

Thanks for quick response.

Grid UT, MT(100%) and Hardness check will be performed after rolling to make sure there are no defects.

So as i understand from your suggestion that 300 deg F would not do much and its okay to allow them to cold roll the shell followed by above mentioned check(Grid UT,MT and hardness Check)?

Thanks.

 

[metengr]

Yes, based on your reply it seemed like this was going to be done before rolling. I would agree with the 100% MT and thickness. What are going to compare the hardness against? After rolling, will a thermal treatment be performed to remove forming strains?

 

[weldstan]

Agree with metengr, a 300F preheating of the Grade 12 plate prior to rolling is pointless. The need for heating of the base plate for rolling shells is based on thickness of the plate and the capacity of the plate rolls.

 

[Fernandiux]

Dear all,
These posts have been very helpfull to us since we are dealing with something very similar but in grades 11 and 22.
It seems that our client is in agreement to void preheating before rolling. However, it is not the same for thermal cutting, we believe that preheating has been achieved during cutting operations and we are having very smooth profiles and PT/MT are being applied to all edges.
Your comments are highly appreciated.

Regards,

 

[metengr]

Fernandiux;
Preheat for thermal gouging or cutting process is entirely different, and I agree with your client for not voiding the preheat for thermal cutting processes for air hardenable materials. Yes, surface NDT of base material after heat affected material has been removed is prudent for detection of cracks.

 

[Fernandiux]

Metengr,
Thank you for your prompt reply; I am refering to cutting prior to rolling with pieces flat on the cutting machine, we cut and bevel in a single operation with CNC Machine, after preheating, welding, DHT complete vessel will be PWHT.
Is it still required a preheat before cutting? plates are 26 and 50 mm thk.

Thanks,

 

[metengr]

Yes, preheat should be used for cutting.