HOME and how to join › Forum › Open Area › General Scott topics › Brum head separation answer › Re: stainless steel for head studs
With the preceding build up the following is unfortunately is going to seem like showing off. But I can add some information to the discussion on the matter of stainless steels. So here it goes-
Stainless steel alloy 316 has the best corrosion resistance properties of all the stainless steels. 316, or specifically the low carbon 316L, is used for medical implants. But even that will corrode invitro, particularly where you have micro-motion or fretting in bolted constructs that continually break the protective surface oxide layer. The small wear particles turn the surrounding tissue a disgusting, but apparently harmless, black color. Besides playing havoc with the MRI image, about six percent of the population is allergic to the nickel content in stainless, and break out post surgery with hives!
316 can not be heat treated, but it can be strengthened by cold working to about 860-1100MPa. There are some cold drawn and extra-hard states that can push that to 1350-1600MPa but it becomes a bugger to machine. It is not the strongest stainless steel available. 440C is a high-carbon martinsitic stainless that can be heat treated to a hardness of RHc60 and 1965MPa. It is used for rust-proof cutting instruments. While it has a high tensile strength, that is solely due to the hardness and it is far too brittle to use for head studs. The 0.2% yield strength is 1896MPa, dangerously close to the ultimate tensile strength of 1965MPa. This means very shortly after it starts to give, it breaks! 420B is also a martensitic like the 440 series and typically used for making injection molds. It can be heat treated to about RHc52, 1724MPa, 0.2% yield at 1482 MPA. A lot of medical instrument forgings (hemostats, forceps) in Europe are made of 420 alloy. It would probably make a decent head stud, but none of the 400 series has exceptional corrosion resistance.
For modest strength applications, like Scotts and side valve Dougles, I will use 17Cr-4Ni, a martinsitic precipitating hardening stainless. Strength and ductility at 1365MPA at RHc44, 1262MPa 0.2% yield. Its best feature is its a dottle to heat treat as it only entails elevating to 900C and allowing to air quench (air cool.) At such low temperatures (relatively speaking) no special atmosphere controlled furnace is required to combat oxidation. The parts do turn a pretty golden brown, but that is about it. 400 series and particularly 440 are more reactive at elevated temperatures and will pit, requiring refractory coatings or an inert purge gas in the furnace. At work we have all 440 processed by vacuum heat treat, or anything else for that matter where you want the parts to stay clean and bright.
Long ago I did try making head bolts for another project in 440C, well tempered so they would not be quite so hard and hopefully more ducal. They snapped under the head radius anyway. Since they actually did not need to be very strong for the intended application, I also tried 304 the next time some was loaded in the bar-feeder on the CNC lathe I was running. They torqued up a bit ‘queer’, and on removal and examination, the 5/16-18 bolt elongated to 5/16-16 threads where the thread emerged from the cylinder! Apparently even though it was not a high strength application, 304 was clearly not good enough! Since then I have paid more attention to mechanical property tables.
Chemical passivation of stainless steels will increase the corrosion resistance. This basically entails a short soak in dilute nitric acid to etch the iron out of the surface, and accelerate to formation of the protective chromium oxide film. 400 series will rust and stain in a steam autoclave if you do not passivate the material. Un-dilute nitric acid will dissolve broken steel taps from aluminum parts, and create evil looking and organically unfriendly brown fumes. Electropolishing also creates a passive layer. Electropolish also takes about 0.02mm off the surface (depending on how long you leave it in the tank and the amount of current passing through the part!) and can affect fine part tolerances.
You could also coat the stainless with one of the Physical Vapor Deposition (PVD) processes, as mentioned by Mike. You still passivate before PVD to remove surface contamination. These coatings are often used on medical instruments to increase wear resistance, and biocompatibility on certain implants. It is also a fairly high temperature process (500C ), so can effect the heat treatment of the material if that is above its tempering range. Coating thickness is typically around 0.005mm. I have used it at lower temperatures of 220C, but was not impressed with the adhesion. There are different methods, Evaporative, Sputtering, and Cathodic-Arc. Primarily the coatings are intended to increase wear resistance- TiN, TiCN, TiAlN, AlTiN, ZrN, etc.; the ones that you see in the cutting tool catalogs. The tempering temperature of tool steels and carbide are safely above the PVD temperature. PVD will also help alleviate galling, which stainless steel threads are prone to, more so than steel. This is commonly encountered with the anti-seize lube provided with the stainless steel spoke and nipple kits.
Cadmium plating will help, but I think a passive layer would work just as well and be cheaper and safer, since cadmium is now considered so dangerous. (Cadmium oxide, particularly as a dust, is fatally toxic.) I use to use stainless safety wire to hang parts in the cadmium cyanide solution, and the plating on the support wires would flake and break off quite easily. No, I do not intentionally try to live dangerously! Perhaps it was because the layer got too thick with internal surface stress, I have seen the similar results trying to heavy hard chrome plate worn transmission shafts, coincidently also a high-nickel content alloy. But it raised doubts as to the cadmium’s ability to really stick to the stainless. On the other hand I read just now in my Handbook of Practical Electroplating that cadmium plating is indeed used on stainless steel aircraft components to prevent galvanic couples with aluminum and magnesium. Personally I have only ever seen cadmium plating on steel aircraft hardware, but then as I said above stainless is not a primary choice for a high-strength fastener. It is probably used on a special-need basis, and an example has not turned up in my surplus hardware; though I have come across the occasional monel fastener.
Any head studs, or any fasteners for that matter, that need strength would be better off in alloy steel. 4140 in B7 (half-hard) condition makes an excellent high-strength stud material and is readily available in small quantities (except the supplier I use skips over 5/16” diameter!) Purchased pre-heat treated to HRc 26-32 it is not so hard that it can not be machined with conventional metal working cutters, and can yield north of 1300MPa with excellent toughness. Heat treated to a full hard state can push that over 2000MPa and still have excellent shock resistance. You can then surface treat it to you liking (phosphate, PVD, Cadmium) for corrosion resistance.
I made new head studs for my Brum in 17Cr-4Ni stainless steel. It has not been Moss tuned but remains in factory specification, so I do not think the studs need to be particularly strong. Consider- the whole block is only held to the crankcase with four slightly larger bolts! Besides I had two studs twist off flush with the head before I tried electrocuting them, and had to make some anew. I do not think the original studs were heat treated (they did not twist off like they were) but they were definitely annealed by the time I was done! There was very little arcing at the top when contact was made. I still have them about somewhere if someone wants to do a gas emissions spectrograph on them to see exactly what their chemical make-up is. I have done this on critical parts like connecting rods and crankshafts on the Dougies, but it did not seem to warrant the expense for a low powered Brumie, the lab test now running about $65. Indeed the rusty originals, annealed and all, would have probably sufficed in this application, but I wanted to make a better job of it and was in the grip of a ‘replace everything with stainless’ fad at the time.
But all this is only needed if your head gasket leaks, as galvanic corrosion only occurs when an electrolyte is present. Bare steel against bare aluminum would be fine if the parts remained dry. O.k. perhaps one can not guarantee sans leaks, you only notice the leaks when the commute to the outside or into the combustion chamber, or you just want an extra margin of protection in case it does leak. So what about the corrosion inhibitors in anti-freeze solutions? These work perfectly well in millions of modern cars with iron and aluminum combinations. I believe there was (still is?) a bias against anti-freeze in Scotts, but that is perhaps best left for a separate topic. Wandering into stainless steels itself has sort of hijacked this post on how to remove stuck cylinder heads!
Disclaimer- I am not a metallurgist. I do design engineering on orthopedic implants, so have come to know stainless steel and titanium alloys in that context. There is not much scope in spinal surgery for cadmium, cyanide, concentrated nitric acid, and high voltages, so I play with those at home…
There are obviously many more stainless steel alloys than mentioned above to suit other requirements. Any good metals handbook can provide the specs and properties, as well as being numerous websites like MatWeb or Carpenter Specialty Steels.