ASTM 193 B7

[Jmund]

Why is it so common to see ASTM 193 Grade B7 bolt with ASTM 194 Grade 2H nut? Why not use a ASTM 194 Grade 7 heavy hex nut with the same chromoly composition? The same spec. commonly calls for zinc plating both items. Why zinc plate a "stainless" bolt? Especially for pipe flanges that may encounter saltwater/other minerals or bolt together components that are not all on the same galvanic series, such as ductile valve w/ galv. pipe. Wouldn't the zinc wear away rather quickly?

 

[CoryPad]

It is common to use a 2H nut because it meets the strength requirements with the least cost. The extra Cr and Mo in Grade 7 is not necessary and just add extra cost. Grade 7 is not even close to being stainless - it will corrode easily without a protective coating. The zinc plating may be consumed quickly in certain environments - it is sometimes used because it is common, not because it is suitable.

 

[MikeMet]

The other reason to 2H nuts to that if there is a galling or fretting problem, it will typically effect the nut which is easier to replace and cheaper to buy