Some Draw Backs on Stainless Steel Fasteners.

[Craig Weis]

Reading my BOATUS Magazine July-2009 I was enlightened by the following.
Doug Cohen, a marine surveyor and an industrial supply house owner says "All That's Silver Is Not Stainless."

Stainless-steel is probably the most misunderstood metal used for fasteners encountered by the average do-it-yourseler. If you ever tried to cut or drill a stainless-steel bolt or screw, you know it's a great way to fry drill bits, and wear out saw blades. As a result, you may think that stainless steel is strong right? Maybe. Stainless steel is what  metallurgists refer to as "tough," but not always strong. In fact, stainless-steel fasteners are only about as strong as SAE Grade 2, a rating exceeded in strength by some alloys of aluminium and even some species of wood.

Stainless, originally called "rust-less," steel is an alloy of steel  [very low carbon]  with a chromium content of a minimum of 12 percent. Although there are over 100 differing alloys of stainless steel, divided into three main classes~austenitic, martensite, and ferritic~most commercial fasteners are of two alloys: 304  [18-8]  or 316. The designation refers to the alloy's content with 18-8 comprised of 18 percent chromium and 8 percent nickel. Type 316 has 16 to 18 percent chromium and 11 to 14 percent nickel, with the addition of between 2 to 3 percent of molybdenum which increases corrosion resistance and strength.

Side bar: Henry Ford was impressed with an Alfa's performance, that happened to  blowup in front of him on a  European race track, spilling it's guts all over the track. Henry slipped the still hot piece of molybdenum steel con rod bottom in his pocket and brought that back to the States. Once figured out, molybdenum was included in all Ford steel. Henry was a fox!

Most stainless-steel sheet steel metal or machine screws are made of 410 stainless-steel, although other alloys are sometimes utilized.

Originally, stainless-steel fasteners came with no head markings to distinguish them, other than the familiar pair of lines at 90 degrees to each other on the head of a type 304  [18-8]  bolt head or the "316" marking on a type 316 bolt head. Additionally, there's a requirement for manufacturer's logo or trademark, registered with the Industrial Fasteners Institute, making the fasteners traceable and the manufacturer accountable for quality.

Don't make the mistake of replacing a graded fastener  [Grade 5 or 8]  with a stainless-steel one. The result will be fastener failure, as stainless-steel is nowhere near as strong as the graded fasteners. Using stainless-steel fasteners presents some issues not found regularly with graded, or plated fasteners. First, stainless-steel fasteners are not as smooth on the thread flanks, either on the bolt or the nut. This can result in galling and, if used with an impact drill or mechanical driver, causes cold welding of the threads. This prevents the proper tightening of the connection and makes disassembly impossible.

Stainless steel is weaker than graded fasteners, so be mindful of the installation torque used. For instance, a 3/8-16 Grade 5 fastener, clean and dry, is installed with 31 foot-pounds of torque. The comparable bolt in 304 stainless-steel is only installed to 19 foot-pounds. Note that bronze or aluminum fasteners also aren't noted for strength, so be careful of application and installation torque.

Many stainless-steel applications require protection from vibrational loosening, normally by using nylon insert hex nuts. Stainless-steel fasteners cannot be used with anytype of all-metal locknut, as the cold welding process discussed above occurs, resulting in the nut and bolt becoming impossible to disassemble.

Corrosion protection is a basic part of the stainless-steel chemistry; it's not a surface-applied or plated process and offers an interesting advantage over plated steel fasteners. In the presence of oxygen, the material "heals" itself if scratched, forming a film of chromium oxide on the surface. The film is invisible, thin, inert, and is what creates the resistance to corrosion. When deprived of oxygen, however, the stainless-steel is subject to crevice corrosion, eliminating the chromium oxide protective layer. This is why you need to be watchful of "hidden" potential failures, such as a sailboat's chainplates just below deck level or a fastener inserted through deck level hardware. If water gets under the caulking and stagnates, it creates an oxygen-poor environment, in which the stainless-steel fasteners begin to corrode, eventually failing completely.

Side Bar: Liquid thread tighteners like "Permatex's Loc-Tight" work when the oxygen is squeezed out of the threads, so with no oxygen present "Loc-Tight" becomes hard up,   [anirobic]  locking the thread to the bolt or tapped hole. In the presents of oxygen Loc-Tight will stay a liquid. So would you use it on stainless-steel?

Well any way that's what Doug Cohn says in a most round about way.
skip.

[Bob23]

Thanks again, Skipper for keeping us informed. Some of that I knew but most I did not. Maybe Henry Ford's foxiness was passed on to future generations. Maybe that's why present day Ford didn't need any Obama-bucks! If I ever buy an American car, it's gonna be a Ford. I do not, and never will support these government owned car companies and the way they screwed the bondholders. Don't ya love it when the government blattantly breaks the law and everyone looks the other way?
Sorry about the rant. I know I shouldn't get you upset.
Bob23