Machine B7 Without Losing Temper

ASTM alloy A193 grade B7 is a quenched and tempered chromium-molybdenum high-carbon steel used to make fasteners for high-pressure applications. It has a minimum tensile strength of 100,000 psi. This steel can develop a great deal of heat in a machining process. If too much heat is developed in the cut, the material can be hardened by machining.

Not good, since ductility will be lost and the fastener could become unsuitable for its high-pressure application. In this article I will go into the material’s properties and explain machining processes that I have used to work this metal with success. Irecently had a job to deliver 40 B7 full-thread studs 121⁄2" long, 15⁄8"-8 thread, chamfered on both ends with wrench flats cut on one end. Figure 1 shows the part and a chamfer tool I made from a highspeed steel (HSS) tool bit.

(I am fussy about my workmanship, and since these parts were going to the U.S. Navy, well.) The ±0.060" length tolerance could have been hit using the band saw, which has flood coolant, but I don’t like the saw cut surface finish.

Instead, I sawed the parts a little long and faced and chamfered them on a lathe. The quality of a part will be remembered long after the price has been forgotten. If too much heat is developed when machining this material, the chips can be burned black or even get red hot when the chip gets into the metal’s plastic zone. This can return some of the workpiece to the quenched condition, which is harder and less ductile than the B7 condition.

Then, if the quality conremembered long after the price has been forgotten. trol folks at my customer’s shop want to get fussy, they can reject the part. I prefer not to give them an opportunity to get fussy. I want the chips to have the silvery steel color.

Ido three things to prevent overheating the part. 1. I keep the metal removal rate low. 2.

I keep the cutting speed slow. 3. And I use coolant. Iuse Rustlick WS500A, a water-based emulsion, because I have never had problems with skin irritations, like dermatitis, or with rust on the workpiece or machinery.