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ASTM 193 B7

ASTM 193 B7

ASTM 193 B7

(OP)
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?

RE: ASTM 193 B7

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.

RE: ASTM 193 B7

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

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