L7 versus B7 Bolting 2

[ProjEngKLS]

We have a low temperature piping application (-60 °C) we are installing. Our pipe specification calls for either L7 or B8 bolting.

Most of this has been installed with B7 bolting instead. The person that approved the use of B7 based his approval on the fact that the material specifications for B7 and L7 are virtually the same.

I reviewed this and they are very, very close with only slight deviation in allowable composition ranges.

So what's the real difference between L7 and B7? Is it just a matter of pedigree? Is there really some differences - i.e. heat treatment or other? Bottom line: are we facing a potential failure if we leave the B7 bolting in place?

I'd really like to know.

Thanks.

ProjEngKLS

 

[grampi1]

L7 and B7 are both 4130 composition. Both are quenched and both should be tempered. The toughness check an L7 gets is a good indication that it was tempered. We occasionaly see B7 that appear not to be tempered (Rc45) and would expect toughness to be poor. But there is no check on a B7.

Note 4130 should not be used at a section over 2.5" as it has inadequate hardenability to assure bolting strength requirements.

 

[metengr]

The main difference between SA-320 Grade L7 and SA-193 Grade B7 bolting is low temperature service. L7 bolting requires impact testing with 20 ft-lbs at -150 deg F. B7 bolting is for elevated temperature service. Impact testing is not required.