What steel?

[Ed Weldon]

Neville - I think when someone says he made it out of "stock" what he is saying it is some unidentified material that he had that could only be identified by outward appearance, i.e. steel, brass, aluminum, copper or what ever the size and shape or some other appearance or history suggests.
Usually when you make something you pick a material that offers specific features your ganeral application requires, primarily resistance to the chemical and mechanical environment of the use, and size, shape and cost that make the fabrication job easiest. What would be "stock" in your shop would be scrap, leftovers and offcuts of the material you most commonly use. The stainless steels you mention are pretty common in most parts of the world. They may be the only types stocked by dealers in South Africa where you are well away from most highly industrialized areas of the world.
Here in the USA your customers' type of "sanitary" application requires easy to clean surfaces and a combination of structural durabillity to survive the production environment. Some reasonable light weight is also needed to allow economic handling of portable components or parts that require frequent disassembly for cleaning. 304 (18-8 composition) probably has all the needed corrosion resistance, is easy to weld especially in low carbon grades, and availabiity in sheet form with nice surface finishes. 316 offers better strength to weight ratios, is more expensive and harder than 304 to fabricate. Again, a low carbon version of 316 means more reliable welds. The presence of commercial fishing on your local coasts suggests there may also be some nitrogen strengthened stainless steels of composition similar to 304 but strength properties well above 316. In the USA they are known by the brand name "Nitronic" followed by a number like 40 or 50. A common use is in propellor shafts for fishing boats operating in heavy sea conditions.
Another common stainless steel in the USA is 17-4ph known as a "precipitation hardening" stainless. We have foundries here that make beautiful 17-4ph investment castings with surface finishes that are smooth enough to invite secondary polishing. This technology may well be applicable to dairy equipment. There are also foundries that do 17-4 investment casting in China 17-4 selling product into American specialty automotive equipment market through US retailer/importers. 17-4 ph has a corrosion resistance somewhere between 304 (and CF8M the casting alloy) and the 400 series chromium stainless steels. Note also that 17-4 ph is shipped in its maximum strength state called "Condition A". A simple premaching heat treat to what is called "condition H1150" makes it easier to machine and this retains most of the desired strength properties of the alloy.
There are also "free machining" versions 416 and 303 which can't be properly welded, give away some acid corrosion resistance and may have alloy constiuents like selenium unacceptable for food processing at least where product contamination is a possibility. These alloys may be useful for specialty fasteners and and other small machned parts where the strength and thread galling resistance are needed but product contamination is not an issue. And on the subject of galling resistance between surfaces that move relative to each other and sustain loads the Nitronic alloys have proven highly resistant to this failure. If you have failure prone shafts running in bushings or cylindrical pieces that must be assembled with tight fits the Nitronic steels may be worth the cost of purchasing from afar and international shipment.
I know this is specialized data dump; but I wanted to share some of my own mechanical engineering experience with folks who work out in the far reaches of technologies most of us take for granted.
Ed Weldon, Los Gatos, California