Stainless Steel

Thursday, 1 June 2023
We as a company are getting increased calls for Stainless Steel Fasteners, but it is noticeable that there is often some confusion as to the grade appropriate for the intended use.The two most common grades are:•    A2 (Metric), G304 (Imperial)
    A4 (Metric), G316 (Imperial)
You will note that Metric Fasteners are designated A2 & A4, while Imperial are G304 & G316.   Almost all off the shelf fasteners are manufactured from one of these two austenitic (non magnetic) grades.A2, often referred to as “Food Grade”, has outstanding corrosion resistance in wet surroundings, oxidizing and organic acids, and in many alkaline solutions.
A4, often referred to as “Marine Grade” is the more corrosion resistant. Performing better in sea climates (chlorides), and industrial atmospheres (sulphur dioxide) and wherever pitting may occur.  The slightly higher Nickel content, and the touch of Molybdenum significantly improves resistance to surface pitting and increases tensile and creep strength properties at elevated temperatures.
Both grades need protection from uncured timber preservatives such as CCA.  
Care must be exercised when using Stainless Steel fasteners.  The greater ductility of austenitic Stainless Steels, not only causes higher friction on the threads, and under the head or nut whichever is being turned to tighten, but also a greater “scatter” of preloads for any given torque.  Further, compared to steel a much lower preload is achieved for the same torque.
A good lubricant   can diminish the friction resulting in the same torque creating more preload in the bolt, but it will do little to reduce the scatter.  Please do not use anti-seizes containing graphite.  The graphite destroys the passive oxide coating on the Stainless, causing severe pitting.
The most common problem experienced with Stainless Steel Fasteners is the phenomena of Galling.   Galling is not as common with Metric Course threads due to their positive clearance, as with imperial threads.   It is unlikely to be a problem unless there are burrs on the threads, foreign matter such as sand or metal particles present or once sided tightening which may occur through misalignment.   Don’t use impact wrenches, and tighten as smoothly as possible.  If there is any doubt use Lanotec anhydrous lanolin.  Fasteners can merely be dipped, and allowed to dry, obviating the expensive anti-galling coatings sometimes specified.  Lanotec’s “anhydrous lanolin” (in grease or liquid form) has the added advantages of being clean, almost invisible, endorsed as food-safe aids to manufacturing, and meet the requirements of a HACCP Based Food Safety Programme.