Stainless, but…

Stainless, but…

5 May 2021 0 By Facto Edizioni

/ by Aldo Avancini /

Are you sure that stainless steel is truly stainless? /

Ing. Aldo Avancini / Proposta Srl

In Italy and around the world, water parks represent a significant portion of the recreational offering.  What they propose to end users is the result of careful sourcing of the most competitive materials and technologies (on an industrial as well as economic level), which are tested in the field in real operating conditions.

In relation to construction steel, for obvious reasons water parks mostly use stainless steel. Let’s look in particular at austenitic stainless steel: in practice, grades 316 or 316L. Their use, even if recommended by standards and supported by thousands of applications all over the world, can still in some situations (just a few, fortunately) cause not insignificant problems, which are worth examining.

First of all, welded joints with the aforementioned materials (austenitic stainless steel) that are not stabilised with titanium and niobium, can trigger corrosion that worsens significantly over time.

But that’s not all. Two other phenomena can occur, the first being pitting. This refers to the phenomenon by which a part of the base metal partially segregates the chromium in the metal, which obviously reduces resistance to corrosion in the point where this occurs.  

I can recall a very effective metaphor told by my professor of metallurgy. He said that oxidation is like knocking down a brick wall by rubbing it with sandpaper, while pitting is like knocking down the wall with the same sandpaper, but only rubbing the mortar between the bricks, making the bricks fall intact; all involving much less energy.

The second is often identified as oxidation by infiltration and occurs when bolted joints are subjected to alternating thermal cycles. This creates tensions on the joints, causing instability, which leads to the infiltration of water vapour into the joint, inducing localised oxidation.

The solution can be to use 2-phase matrix stainless steels known as ‘austenitic-ferritic’. Also called ‘duplex’, these are 2-phase alloys with both the austenitic and ferritic matrices in the microstructure, obtained through careful dosage of nickel and chromium. 

A similar situation occurs with bolted joints. A2 and A4 austenitic stainless steel bolts are normally used in bolted joints. The problem is that while austenitic stainless steel has excellent corrosion resistance characteristics and is a material with mechanical advantages up to temperatures of 400°C, it is however the hardness that is needed to give the bolt its required mechanical characteristics.

At times seats are made for A2 or A4 self-tapping screws by first using a normal steel screw, which is then removed and replaced with the stainless steel screw. This solution, in limited cases, might be fine, but it will never be the ideal solution.

So what is the difference between A2 and A4 stainless steel and other construction steels? A2 and A4 stainless steel (widely known for ’marine’ bolts), due to the non-corrosive nature of stainless steel, is used in many fixing applications, ranging from automotive and marine, to architecture and DIY and home improvements; however, these are 18/8 (A2) steels which, with the addition of 3% molybdenum, become A4, improving resistance to corrosion especially in the presence of chlorides.

Taken from Games&Parks Industry April 2021, page 76

Ing. Aldo Avancini /  Proposta Srl / proposta_design@yahoo.it

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