A193 B7 REPLACING A193 B7M

[chanin]

CAN WE USE A193 B7 REPLACE A193 B7M?

 

[Metalguy]

You should try to find out why B7M was specified first. B7 is a little stronger-same material, slightly different HT (tempering temp.). Also, I think B7M requires 100% hardness testing.

 

[Carburize]

B7M has controlled maximum hardness of 235 HB for use in applications such as sour service areas. If strength is the only issue B7 can be substituted but if hardness control is required than they cannot.

 

[PAN]

You should also select nut A194-2HM for combination with A193-B7M.

 

[stanweld]

Replacement of B7M with B7 could lead to catostrophic failure depending on the service environment. There was an instance involving the Engineering company for which I worked over 25 years ago when B7 was mistakenly used in place of the specified B7M and all the internal bolting in the vessel failed within a few weeks of startup. Never, unilaterally make such a change without approval of the Engineer or Owner!

 

[Metalguy]

Stanweld,
Do you know why the bolts failed? Service conditions? Correctly HT'd B7 bolts are resistant to SCC in all but the most severe conditions.

 

[stanweld]

Metalguy,
Bolts were in H2S/amine service.

 

[Carburize]

Failure seems to echo my comment concerning the major difference between B7M and B7 ie the hardness control.

 

[stanweld]

Carburize,
The grade "B7M" was created to provide resistance to SCC and has been specified for use in "Sour" service for at least 3 decades.

 

[Carburize]

That's what I said in my post of December 17

 

[PAN]

stanweld and Carburize,
I believe that you have the same comment.

Please clarify more about the service environment. Normally piping stud bolts will not directly contact to H2S/amine in the pipe. What is the reason to select bolt B7M instead of B7. If the adjacent pipes contain other non-corrosive fluids, should we select B7M because of in the sour service area?

In our plant, we also use B7M for H2S/amine service piping. However, sometimes the maintenance people replace them with B7. Should we worry for external bolting?

 

[Metalguy]

It's the H2S that causes the SCC concern, not the amines, unless they are very dry. *IF* there is a possibility of exposure to H2S from adjacent piping, then the slightly softer B7M bolts are called for (that would be considered to be severe service). If not, B7's should be fine-but you have to check any committments your company may have made to authorities wrt B7M's.

If you are able to perform Rc tests on all B7 replacement bolts and select only the softer ones, that might work too.

 

[ThomasEun]

The replacement depends on the client’s requirement and/or the applicable codes and standards. (mainly in low temperature service as well as SSC and SCC environment)

Please refer the following code and standard.
1. NACE MR 0175, Section 6 Bolting.
2. ASME Section VIII Div.1 Fig.UCS 66, General Note (e)

ThomasEun