Tapping forged nuts

Q: We manufacture a slotted hex nut out of 4340 alloy steel heat treated to Rc 32-38. We buy the material in the normalized and tempered condition, then machine it completely followed by a quench and temper operation. Our machinist has been having trouble


A: The process of normalizing 4340 steel is almost like hardening, because in smaller bars it is nearly an air-hardening grade. Tempering does soften sufficiently for most turning operations, but tapping requires a microstructure that encourages shorter chips to get good clearances during tapping.

I suggest a cycle anneal instead. This can cost slightly more than normalizing because of the slower cooling rate, but it leads to a coarser carbide structure, which in turn promotes chip fracturing (shorter chips). This provides a more pearlitic grain structure that is usually easier to machine than the very fine structure typical of normalized parts.

The other alternative is to resort to a partially resulfurized calcium-treated steel, which is much more machinable. The calcium prevents long sulfur stringers from forming while still improving machinability. Your mill supplier should be able to help you in this matter.

Be forewarned that your customer may not want to consider this option for a heat-treated nut, because the sulfur can decrease some properties.

For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.

Guidelines and recommendations offered in this column are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.

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