Work hardening and why feed matters more than speed
Austenitic stainless hardens when it is deformed. A cutting edge that rubs rather than cuts deforms a thin layer of material, which then becomes harder than the tool is expecting to cut, and the next pass meets a work-hardened skin.
That is why the countermeasure is counter-intuitive: increase the feed per tooth so the edge gets under the hardened layer rather than skating across it. Reducing speed and feed to be gentle produces exactly the hardening that ruins the tool.
| Grade | Behaviour | Tooling direction |
|---|---|---|
| 303 free-machining | Sulphur addition improves chip breaking | General-purpose; corrosion resistance is lower |
| 304 austenitic | Work hardens, galls | Sharp PVD edge, firm feed, M-grade insert |
| 316 and 316L | As 304 plus molybdenum; more gummy | Sharp edge, generous coolant, avoid dwelling |
| 17-4PH | Precipitation hardening, abrasive when treated | Stronger edge, reduced speed after hardening |
| Duplex 2205 | High strength, abrasive, notch sensitive | Robust edge, conservative speed, rigid setup |
Turning stainless: M grades and sharp edges
ISO M covers austenitic and duplex stainless. The conventional colour code is yellow, and the coating direction is PVD rather than CVD because a sharp, thin coating cuts rather than rubs.
The chip breaker matters as much as the grade here. A breaker that suits the feed range will curl and break the chip; one that does not will produce a stringy chip that wraps around the tool and damages the finished surface.
Drilling and tapping stainless
Stainless is where tapping fails most often, for two reasons: the material work-hardens ahead of the tap, and the tap has limited chip room and coolant access.
A sharp, PVD-coated tap run at a firm feed with adequate lubrication is what works. Reducing the thread engagement from 75 percent to 65 percent by opening the tapping drill slightly reduces the torque substantially with no practical loss of strength, and it is the single most effective change when taps keep breaking.
Coolant and heat
Stainless conducts heat poorly, so the cutting heat concentrates at the edge rather than leaving with the chip. Through-tool coolant delivery is the only way to reach the edge in a deep hole or a deep pocket, and flood cooling alone frequently is not enough at depth.
Coolant also lubricates, which matters for the galling tendency of austenitic grades. A high-pressure, high-flow condition is preferable to a fine mist on stainless work.
Frequently asked questions
Why does stainless steel work harden so badly?
Because the austenitic grades harden rapidly when deformed. A cutting edge that rubs rather than cuts deforms a surface layer that then becomes harder than the tool expected, so the next pass is cutting a harder material than the drawing specifies.
Should I reduce feed to be gentle on stainless?
No, the opposite. Increase feed per tooth so the edge cuts under the work-hardened layer instead of skating across it. Low feed and low speed produce the hardening that ruins the tool.
Why do my taps keep breaking in stainless?
Usually an undersized tapping drill combined with work hardening ahead of the tap. Open the tapping drill to reduce thread engagement from 75 to 65 percent, which cuts the torque substantially, and use a sharp PVD-coated tap with adequate lubrication.
What coating is best for stainless steel?
PVD coatings such as TiAlN or AlCrN with a sharp edge preparation. CVD coatings are thicker and blunter, which encourages the rubbing that causes work hardening.
Do you supply tooling for duplex stainless?
Yes, with a robust edge geometry and conservative speeds, because duplex is both stronger and more abrasive than 316 while also being more notch sensitive. Tell us the grade and the operation so the edge preparation can be matched to it.