Machining and tapping of normalized 4340

Q: We have trouble tapping nuts of heat-treated 4340 alloy steel. Are we missing something?

Q: I have been manufacturing a slotted hex nut out of 4340 alloy steel heat-treated to Rc 32-38, for a government customer. I buy the material in bar form and as forged nuts, depending on size and quantity. These are specified to be in the normalized and tempered condition, after which we machine them completely and then heat-treat to final properties. While we can drill the holes with some success, my machinist has been having trouble tapping these nuts even in the normalized condition. He has used cobalt-titanium-coated taps and still complains. Are we missing anything?

A: The process of normalizing 4340 steel is almost like hardening since in small parts or bar size it is nearly an air-hardening grade. Tempering will soften it sufficiently for most turning operations, but tapping requires a microstructure that encourages shorter chips due to a tendency for long chips to bunch up in small holes.

I suggest a cycle anneal instead. The first type of cycle anneal consists of annealing at a temperature similar to normalizing, followed by an air cool to about 1,200-1,250°F, followed then by a slow cool to room temperature. This process tends to eliminate the tempering cycle.

The second type of cycle anneal involves a true "cycle" of temperatures above and below the critical followed by a slow cool. This more time-consuming process can result in a hardness of closer to 25Rc or lower.

The first cycle can cost slightly more than normalizing because of the slower cooling rate and the longer furnace time, but it leads to a coarser carbide microstructure that, in turn, promotes chip fracturing (shorter chips).

Another alternative is to resort to a partially resulfurized calcium-treated steel that is much more machinable. The calcium prevents the long sulfur stringers from forming while still improving machinability. Anyway, your mill supplier should be able to help you in this matter. Your government customer may not want to use a resulfurized steel because it tends to reduce the ductility of heat-treated steels. Thus, this might not be an option. I would try the cycle anneal first.

H. James Henning answers forgers' technical and operational questions. For more than 40 years he held key technical positions in the forging industry, most recently as director of technology for the Forging Industry Association. He is president of Henning Education Services, Columbus, OH, specializing in customized education and training in forging technologies.

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ASK JIM
c/o FORGING
1300 E. 9th St.
Cleveland, OH
44114-1503

Or e-mail: [email protected]

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