These two differ in strength requirements and testing. A193 Grade B7M bolts are have lower minimum mechanical requirements than Grade B7 and require 100% hardness testing of each bolt
What is the difference between A193 Grade B7 and Grade B7M?
What is the difference between A193 Grade B8 and Grade B8M?
These bolts have similar strength requirements but differ in their corrosion resistance. Grade B8 bolts are made from type 304 stainless steel, while Grade B8M bolts are made from a more corrosion-resistant type 316 stainless steel.
What is the standard thread series for A193 bolts?
Unlike most other ASTM specifications covering construction fasteners, the A193 specification requires 8 threads per inch for bolts larger than 1” diameter. The 8 Unified National thread series (8UN) provides increased tensile and shear strength due to a larger tensile stress area and minor diameter and 8 pitch threads reduce the probability of nuts backing off due to vibration.
Can A193 Grade B7 bolts be galvanized?
Yes they can. A common misconception is that B7 bolts cannot be galvanized. That misconception is wrong. A193 Grade B7 bolts are considered high strength and have developed their strength through a heat treating process. There are two concerns when galvanizing high strength bolts. One issue is the potential for hydrogen embrittlement and the other issue is altering the mechanical properties of the fastener during the galvanizing process.
Hydrogen embrittlement is a phenomenon in which hydrogen can become trapped in the surface of some fasteners during the galvanizing process leading to premature failure. However, this is only of concern for bolts exceeding 150 ksi in tensile strength. Since A193 Grade B7 has a maximum hardness requirement of Rockwell C35, it is highly unlikely that the tensile strength for a given lot would exceed 150 ksi.
The other concern is the effect that the heat of the molten zinc might have on the strength of the A193 Grade B7 bolts. When quenched and tempered bolts are heated to within 100 degrees of the tempering temperature, you run the risk of altering the strength of the bolts. Since hot-dip galvanizing is performed at 840 degrees Fahrenheit and the minimum tempering temperature of A193 Grade B7 bolts is 1,100 degrees Fahrenheit, there is no risk of altering the mechanical properties of the bolt during the hot-dip galvanizing process.
Can A193 Grade B7 bolts be welded?
No they cannot. A193 Grade B7 bolts are manufactured from a medium carbon alloy steel that undergoes a heat treating process to develop its strength. When you apply heat during a welding operation, you run the risk of altering the mechanical properties (strength) of the bolt.