15.22 Material choice for extractor systems

The optimum material from which to build an extractor system has to address chemical compatibility, strength of materials, and compliance to food service and pharmaceutical cleaning standards. 

Polished 300 Series Austenitic 304SS has seen the greatest use, with 316SS next.  The difference between the two being primarily their Chromium and Nickle content, with 304SS containing about 18% Chromium and 8% Nickle.  18-8 kitchen flatware, pot, and pans are a good example of a food service application.

304SS has a ultimate yield strength of about 42,100 psi, and an ultimate tensile strength of around 90,100 psi.  It is extremely stretchy and elongates 55% during the transition from yield to ultimate tensile failure.  Rockwell hardness is about R-82 in a solution annealed state.

It is also a pleasure to weld once its needs for cleanliness and an inert atmosphere are provided, but it is here that metallurgical issues resulting in red rust freckles rear their ugly heads. 

The term solution annealed heat treat state means that the 304SS was heated to at least 1900F and held for one hour per inch of cross sectional density at that temperature, before being rapidly quenched in cold water.

When in solution annealed state, the Chromium and Nickle provide the corrosion resistance, despite the fact that they 304SS still contains about 65 to 74% Iron (Fe).

The fly in the ointment, is that between the temperatures of around 425 and 860C/797 and 1580F, the Chromium and the Carbon fall in love and form Chromium Carbides, which takes the Chromium out of solution and the iron in the alloy loses its corrosion resistance.

That is why the 300 Series stainless is cold water quenched after heat treat, so as to pass through the reactive zone so fast that it doesn’t have time to form Chromium Carbides. 

In the aircraft industry, we typically solution annealed the 300 Series parts after welding, so as to put any carbides back into solution.

To compensate for that problem with our reheat treating, one solution is to go to 304L, which has lower carbon content and therefore is less prone to react with the Chromium.

Another is to go to a higher Nickle content such as they have done in 316SS, where the Chromium fall in the 16 to 18% range, but the Nickle increases to 10 to 14%.

You can weld 304SS with 316L or 317L to make it less prone to post weld corrosion.

Rust on post welded part will be in the heat effected zone, but another source of rust freckles on 300 Series stainless is weld spatter, some microscopic.  The little micro-bursts when welding sends molten iron molecules and particles in all directions and when they stick to the surface of a casting, the don’t receive protection from what is underneath.  An issue even with TIG, but a serious issue with the Metalic Inert Gas (MIG) process.

For that reason it is a good idea to protect the adjacent surfaces with an anti-splatter coating or cover, but an alternative is electro polishing, which will remove the spatters and the exposed iron from carbide formation, returning the surfaces corrosion resistance.

Wall thicknesses are another issue that comes into play when designing equipment, both from a strength standpoint as well as weight.  304SS weighs about .295 lbs/In3, as compared to .283 lb/In3 for A-36 mild steel.

When using sanitary tubing, most of our designs fall between .065” and .109” wall thickness because of ASME standards, but one of the issues that I regularly ran across was that I would design a tubular stainless cart with a wall thickness of .065 and the welder would bid it at .120” wall thickness to make it easier for him to weld.

While that may look like a simple labor versus material decision, but consider that besides the increase in material cost, your cart just doubled in weight.

Welding thin wall tubing is infinitely easier with the right equipment, so I suggest looking at Inverter welders with low start and drop out amps for smooth starts and stops without burn back. 

We like a WP-20 water cooled TIG torch, for its handy size and capacity to run from a .020 to a .125 electrode and a wide range of cup sizes.

When welding 304SS that isn’t going to be subsequently annealed, no pre or post heat is required and it is a good idea to keep beads small and inter pass temperatures low to reduce carbide formation.  It is also a good idea to cool the heat affected zone as rapidly as possible using a stream of argon from either the cup or a secondary source.


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