Yes, there is. A193 and A320 grades B8 and B8M are assumed to be Class 1 unless otherwise specified. Class 1 fasteners are carbide solution treated, while class 2 fasteners are carbide solution treated and strain hardened. The work hardening that occurs in the manufacturing of Class 2 fasteners increases their strength. Class 2 fasteners are differentiated by the letters “SH” after the grade marking on the fastener and an underline for both A193 and A320 fasteners. The marking typically appears on the heads of bolts or the ends of threaded studs. For both A193 and A320, the marking requirement historically was only the underline, the “SH” lettering was added in approximately 2006.
| ASTM Specification | Stainless Grade |
Application | Marking Class 1 |
Marking Class 2 |
|---|---|---|---|---|
| A193 Grade B8 | Type 304 | High temperature / High pressure | B8 | B8SH |
| A320 Grade B8 | Type 304 | Low temperature | B8 | B8 |
| A193 Grade B8M | Type 316 | High temperature / High pressure | B8M | B8MSH |
| A320 Grade B8M | Type 316 | Low temperature | B8M | B8M |
In what cases will a A-194 Class 2 hex nut be used with the A-193 hex bolt? Is it even necessary to use class 2 hex nuts?
@Dan- it is up to the buyer to request which nut they need, and ideally before that the designing engineer will specify. The A193/A320 bolting specifications do not mandate any specific nut other than one of the nuts within A194. Side note – the term class 2 is not used with A194 gr.8/8M nuts. Instead the strain hardened grades are designated by calling out supplement S1.
Can we use B8MSH StudBolts in place of B8MA?
@BK- Those grades are not mechanically identical, so any substitution should be approved by the project engineer.
Can Grade A 4-70 be used where ever B8M Cl 1 bolts are required.
@Anil- those material call outs are not exactly identical, so any substitutions should be approved by the project engineer.