A335-P22 3

[mcsteel]

My concern is whether or not to preheat pipe before thermal cutting. As per API 582:

8 Preheating and Interpass Temperature
8.1 Preheating, where required, applies to all welding, tack
welding, and thermal cutting of ferritic steels except for thermal
cutting of P1 materials. Minimum preheat requirements
shall follow the applicable code such as ASME Section VIII,
Appendix R; ASME B31.3, Table 330.1.1; and AWS D1.1,
Annex XI. Any recommendations or requirements for preheat
listed in the relevant code shall be considered mandatory.

What are the possible dangers (metallurgy) of not preheating to code temperature before cutting?

Note: The cut pipe will eventually be welded and recieve PWHT as per B31.1

Thanks for your help!

Mike

 

[Metalguy]

You'd run the risk of making untempered martensite next to the cut, which is quite brittle and crack-prone. It's possible the weld prep might not remove all such cracking, and possibly cause a rejected weld or worse if postwelding NDE doesn't find it.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[mcsteel]

Metalguy,

Thanks for your prompt response. I have been looking at the CCT diagram for P22 and considering the thermal cutting is done in a shop I am questioning whether the cooling rate is fast enough to produce Martensite. Is Bainite equally susceptable to cracking?

Does anyone have hardness and strength data for the different microstructures (Martensite, Bainite, Fine/Coarse Pearlite) for P22?

Mike

 

[Metalguy]

With the 2 1/4 % Cr and 1% Mo, P22 is a fairly deep hardening steel--far deeper than the same C level of a plain carbon steel. If it's flame/thermal cut cold, the surrounding metal will most probably pull heat away from the HAZ fast enough that martensite will form. You don't need to be concerned with bainite unless the pipe is very thick--more than a few inches of wall thickness.

The mechanical properties of the various microstructures can be easily looked up, but there are a lot of factors which will influence them.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[metengr]

If the flame cut edge is being completely removed for use in welding/fabrication, which it should be, preheat is not required for flame cutting P22. P22 material is low alloy steel and is air hardenable, which is why preheat and PWHT are required for most welding applications. ASTM/ASME standard material specifications limit the maximum carbon content to reduce high hardness because hardness is a direct function of carbon content.

 

[Metaljon]

It is good practice to apply preheat before thermal cutting. You are definitely at risk of forming martensite for P22 if thermal cutting is applied. It will be impossible to establish whether the HAZ region has been all removed by grinding.

 

[mcsteel]

metengr,

Yes, the flame cut edge is cleaned up by a grinder prior to welding. How do you ensure that sufficient material is removed? In other words, how will a welder determine when enough material is removed to eliminate any cracks and remove all of the martensite.

Mike

 

[Metaljon]

Yes exactly. Why leave this to chance when all you need is some preheating to be applied and then there is no problem at all.

 

[metengr]

Normally, preheat is applied to reduce the amount of heat affected metal removal and to reduce the risk of hardened base material from the thermal cutting process by slowing the rate of cooling, period.

If preheat was not applied or used for some reason, I would recommend 1/4" removal of heat affected material from the cut edge followed by a surface NDT to ensure no cracks. Typically, 1/4" removal of material would suffice without preheat, and keep a safe distance from locally hardened material. Depending on thickness, you can also perform portable hardness testing to determine if any hardened material remains, the surface NDT would be a requirement.

 

[mcsteel]

Gentlemen,

Thank you all for your input. I think I will request preheat to code temperature prior to cutting for these reasons:
-Preheat will take no more time than grinding off 1/4" of material
-Eliminate potential for lazy welders not removing enough material
-Preheating to code temperature is easily measured and managed
-Peace of mind

Mike