They are tough to drill because they are very tough, i.e. difficult to fracture. Their wear resistance is poor to the extent they gall readily. This is the cold welding of the fine points of contact on the surface to a mating surface. This occurs because the passive film is easily broken mechanically, permitting the two bare metals to weld. This absorbs a lot of energy when the weld breaks and, thus, friction.
The primary explanation is that austenitic (300 series)stainless steels work-harden significantly and rapidly as the saw or cutting/machining tool tries to shear a layer of material from the base metal.
Normal wear resistance depends on hardness. These materials are not hard unless they are work-hardened and most abrasive or other wear is not as intense as machining or saw cutting, so no work hardening results and wear rates are high.
rustbuster1 is right. The more stable the austenite, the gummier the grade. 201 and 301 are the most machinable. 304 is intermediate. 305, 316 and the rest are worse. 303 with big chunks of sulfur is machinable because of the sulfide chip breakers.
If you dont mind me asking how do the austenitic stainless steels work harden. I know that they cannot be hardened by heat treating due to low carbon content, they can only harnded by cold work, is work hardend By sawing and drilling similar.
Work hardening is more appropriately called strain hardening. Stainless steels strain harden like any other strain-hardening material: induced plastic strain generates dislocations. These dislocations prevent the motion of new dislocations, so any subsequent plastic strain requires extra input energy. This translates into an increasing stress-strain response. Strain hardening by sawing and drilling are similar on a microstructure level, though the affected volumes may be different.
The other thing that happens to austenitic stainless is that it transforms to much harder martensite when it is deformed, as in machining. The leaner the alloy the more of this occurs. I'll post the formula in a FAQ in this section.
MCGUIRE is right. The squeal you often hear in machining an austenitic stainless steel is due to the strain-induced transformation of the austenite to the very much harder nartensite.
I Need to know if anyone has any background information on how to reduce galling (and friction) on Stainlees Steel adjustment screws. The screw and blocks are M20 x 1 with a thread class equivelent to a 3A/B. I am using a grease on the threads in an attempt from keeping the oxide layer from breaking down. The screw part is 440c HRC 52 and the block is 304. The stainless is required because of a clean room environment. The screw adjuster are loaded in compression with about 90kg of mass. I need a way of reducing the friction of the threads. Maybe using plated screws, but will the plating remain in tact with the loading.
anyones insight to my conundrum would be appreciated.
jimchest: Suggest you try a burnished-in layer of molybdenum disulfide. See book "Molybdenum Disulphide Lubrication" by Lansdown, published by Elsevier.
Joe Greenslade, a noted fastening expert, recently recommended a product called Fastorq A/G for reducing or eliminating galling of stainless steel. You can obtain more information using the following link:
