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#7126 - 04/08/04 10:34 AM materials are not rated over 100F
al k Offline
Member

Registered: 03/22/02
Posts: 9
Loc: DP in CA, Great White North
IN the material tables ASME 8 part IID...
Some materials are not rated for allowable stress over 100F. Examples include SA320 and SA193 B8C. (UNS34700) 18Cr-10Ni-Cb.
Something seems wrong that a stainless is good to only 100F.
Does anybody know why this is, what was wrong with the material (the addition of Cb), or the material table (ASME database) that causes this? We looked on ASME website and found no answer.
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#7127 - 04/17/04 09:20 PM Re: materials are not rated over 100F
Darren_Yin Offline
Member

Registered: 12/15/99
Posts: 40
Loc: Houston, TX, USA
Let me try again to posit an answer to this outstanding question. Though the subject refers to B8C bolts but it touches other SS bolts like B8, B8M and B8T as well.

First, SS bolts for PV&P uses are usually treated, in two Classes. Class 1 bolts typically are "carbide solution" treated and Class 2 bolts are additionally "strain hardened."

One major weakness for Cl. 1 bolts is, being weak. It gives 70 ksi of SMTS and a lowly 30 ksi of SMYS. In consequence Cl. 1 bolts can go no more than 300 # at best. The good news is that with a strain hardened treatment--i.e., upgrading it to Class 2--SS bolts can hold up to 100/125 ksi SMTS and 50/100 ksi SMYS. This is a huge increase in strength, meeting the dire need for higher pressure services.

B8C bolts are composed of 18Cr-10Ni-Cb alloy elements, where Cb abbreviates Columbian. Addition of Cb greatly enhances its resistance to Intergranular Corrosion, and makes H8C the bona fide SS as compared to the virgin austinitic B8 bolts. B8C comes in two spec's, SA193 and SA320. For this forum, let's focus on SA193.

Metallurgists responsible for the Code opinion have long proclaimed that "Cold Work" on austenitic SS can induce a premature failure from so-called "Ductility Impairment." Though the opinion isn't disputed, the degree of impairment is, notably spoken out by expansion bellows manufacturers.

Among all SS bolts, B8C bolts are found to be most susceptible to "cold work," thus resulting in brittle threads. And a strain-hardened treatment for all its good is considered a form of "cold work."

One engineering option to remedy the situation is to control the strain hardening to the extent that the hardness of the threads be less than Rockwell C35. It is a fair quality control proposition, and all PV and Piping Codes except Sect. III have agreed on the C35 number.

However, agreement is one thing and implementation another. In one twist of event, PV and Piping Codes part company big time. Now, for the SA193 B8C Class 2(C35) spec bolt, B31.3 Code lets it go all the way up to 1,000 F. In contrast, Sect. VIII-1 and -2 Codes limit its use to 100 F at which the hardness is measured. So does the B31.1 Code. Not surprisingly, Sect. III does NOT permit its use at all.

The position taken by Sect. VIII Codes reflects a struggled compromise. Obviously neither have the Codes the consensus to downrightly dismiss H8C Cl. 2 bolts as Sect. III does, nor do the Codes accept the use for hot services as B31.3 Code.

All this time process piping industry are using the Cl. 2 bolts for hot services without qualms, and they apparently work just fine to this day. Remember, the biggest SS B8C bolts users come from Process Piping industry.

Notwithstanding we need recognize also that B16.5 Piping Flange Code does NOT forbid B8C Cl. 2 bolts to be used at hot temperatures.

That is how the Code applications of H8C Cl. 2 bolts currently stand.

Yes, the situation can be awkward. Let's assume here is a hot SS process vessel. All the nozzle flanges are joined by H8C Cl. 2 bolts because piping provides the tie-in bolts, which accord to B31.3 Code. However, the vessel can't use the same for the manway because of Sect. VIII-1 Code.

These are two bits of what I know. Hopefully it helps.

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