I work in a foundry making primarily Ductile Iron castings, but also stainless and other steels. We mske pipes with grooved fittings and flanged fittings. Ductile iron and another iron called Mondi, are generally superior in piping applications. Water, water treatment, sewer, oil and gas, etc, all are generally better with ductile iron. Strength and flexibility without added brittleness are very desirable in these instances, and ductile iron is better and cheaper then stainless. It can also be welded.

Mondi is more for the fertilizer industry, and used in acids. It can handle transporting acids at high temperature and under pressure. Mondi cannot be welded.

Stainless steel is used in many industrial applications. It is more expensive and harder to work with, but if done right it can last a long time.

Anyways, interesting post.