A History of Cold-Forming Stainless Machine Screws
The term “18-8” covers a multitude of alloys, specifically 302 (UNS S30200) or 304 (UNS-S30400). These are basically the same alloy, 18% Chromium, 8% Nickel. 302 has higher carbon content: .15 max, whereas 304 is .08 max. These grades are supplied from the mill in strip, sheet and bar form. Strip and sheet is used for stainless ware (spoons, forks, etc), kitchen sinks and the front of your refrigerator. Bar is used for Hot-forging (it is the material that Pilgrim uses to make Hex Bolts in its Arizona facility). Neither of these alloys can be cold headed. In order to supply a more workable material, the mills created 304L (UNS S30403) by reducing the carbon content to .03 max. This reduced the work hardening rate sufficiently so that it could be cold headed. It is, however, still too hard to use for manufacturing parts with a recess (cruciform, socket, 6-lobe) because of the resulting stress cracking. Pilgrim uses this alloy for hex cap and slotted machine screws.
Back in the 1950’s, if you wanted a stainless machine screw with a Phillips recess, it was usually 430 stainless (iron with 12% Chromium). 430 is very ductile, easy to cold head, as strong as low carbon steel (55ksi), but with only minimal corrosion resistance. 300 series screws with a recess were supplied by the laborious operation of drilling and broaching the recess in a very costly secondary operation. To overcome these problems, Carpenter Steel created an alloy they named “Carpenter #10”. Also referred to as 16-18 (16% chromium -18% Nickel), designated now as Type 384 stainless (UNS S38400). The work hardening rate, because of the high Nickel content, is very low and produces beautiful parts with no stress cracking and tensile strengths of 80ksi minimum.
BUT THEN CAME THE BIG NICKEL STRIKE IN CANADA IN THE EARLY 70’S AND NICKEL COSTS SPIKED SEVERELY, CAUSING THE COST OF 16-18 TO SOAR! THERE HAD TO BE ANOTHER ALTERNATIVE…. AND THERE WAS:
Back in the early 50’s, ARMCO (now AK STEEL), recognizing that copper in solution with chromium produces the same desired characteristics as nickel (both have similar atomic structure), they developed an alloy which they named “18-9LW”. This was accomplished by adding 3% copper to 9% nickel in a basic 304 stainless alloy to achieve low work hardening rates (which is what the “LW” stands for). Armco did not aggressively pursue the cold heading market with this new alloy (although it did find its way into the socket head cap screw manufacturing business as XM-7).
Carpenter Steel (now CARTECH) began producing this alloy under their designation 302HQ (HQ meaning “heading quality”). Initially there remained some issues, particularly stress cracking around the recess due to hardness issues and occasional seams in the wire. To find a cure, over a period of time in the late 70’s, Carpenter formed a partnership with Pilgrim Screw to run samples of various heats of material in our cold heading plant in Providence. The result was the development of a material much less expensive than Carpenter #10, with excellent corrosion resistance and tensile strengths greater than the minimum 80ksi when cold worked. This alloy is used universally for all cold headed fasteners, with or without recess, and accepted on AN-MS-NAS prints (although under different names). It is recognized in various material specs (AMS-QQ-S-763, ASTM F593, AIR4127, and others) also under different names.
The correct and most recognizable designation is UNS S30430. The “3” in the fourth position stands for 3% Copper. On some AN/MS/NAS prints it is referred to as “18-8 Chromium-Nickel alloy developed for cold heading “. This is 302HQ, 18-9LW, XM-7, 304CU (whatever you want to call it). Despite the length of time this alloy has been standard in the industry, it still creates controversy because of the addition of copper. Many users do not have a full understanding of the purpose of alloying copper along with nickel in 300 series austenitic stainless steel and believe it to be inferior to pure 304. Nothing can be further from the truth.
In the latest Revision of NASM51959 (Rev. 2 dated 4/30/2015), here are the materials designated to produce a Cruciform (Phillips) 82 degree Flat Head. Errors like this can drive screw manufacturers nuts! (pun is intended). Asterisks added.
AUSTENITIC CORROSION-RESISTANT STEEL SCREWS SHALL BE MANUFACTURED FROM TYPE 302**(UNS S30200), TYPE 304**(UNS S30400), TYPE 304L*(UNS S30403),TYPE 305*(UNS 30500),TYPE 316**(UNS 31600), TYPE 316L(UNS S31603),TYPE 384(UNS S38400), OR TYPE XM-7 (UNS S30430) IN ACCORDANCE WITH CHEMICAL COMPOSTION IN AMS-QQ-S-763.
**CANNOT BE COLD HEADED WITH CRUCIFORM RECESS!
*Can be cold headed, but is subject to stress cracking and tool life is considerably reduced resulting in much higher cost
It should be noted that Type 384 (UNS S38400) is no longer readily available in small quantities – it must be purchased in mill quantities (usually 7-1/2 tons) with long lead times.
All of the above information refers to cold heading of screws. Wire is fed into the machine and plastically deformed into the final shape of the head, including the recess, with no loss of material. Some drawings also include in their list of materials 303 and/or 303SE. These are alloys produced for machining and cannot be cold headed. 303 stainless is 304 with the sulfur content increased to .15% minimum. This creates a free machining material in which the cuttings come off in chips, rather than the “streaming” you get with 304. But sulfur has some limitations, mechanically and chemically, so 303SE was developed (selenium instead of sulfur). However, selenium is toxic when heated and can give off fumes. It is bad for the environment and dangerous to the workers.
300-series austenitic stainless steel is a fabulous material for fasteners. It provides excellent corrosion resistance and strength. Machine screws with recessed drives need a material that is malleable enough to withstand the severe plastic deformation produced in cold heading. 302HQ, 18-9LW, XM-7, 304CU, 30430, or whatever it is called – IS that material. Pilgrim Screw was in the vanguard of its development throughout the 1960’s and 1970’s and is the recognized expert today in stainless machine screws.
Pilgrim Screw Corporation
Gary E. Grove, Chairman