Materials of Butt Weld Pipe Fittings

Why do manufacturers offer such a large variety of metals and materials ?

Pipes and fittings are used in many applications and many industries that have widely different operating conditions.
To reach best performance (resistance to various types of corrosive fluids, operating temperature, …) at acceptable costs, many metals have found unique niche applications in fittings.

Why choose a specific material, or grade, or schedule for a fitting ?

Choice of material or grade or schedule is not made by distributors or by fitting manufacturers, but by engineers who design pipelines or systems based on performance requirements and acceptable costs.
Engineers take into account key operating conditions and environment: pressure, temperature, abrasion, corrosion, vibrations, …
Fitting materials also have to be chemically compatible with pipe materials to which they are connected to avoid inter-metal corrosion, and for weldability.
Finally, cost of various materials varies greatly.

What does “high yield” (“HY”) mean for pipe fittings ?

Yield strength refers to ability of materials to maintain a load or pressure without permanent deformation. (In this case, yield does not refer to manufacturing productivity of materials or equipment.)
Fitting design is based on yield strength.
Steel products do not all have similar potential. Yield strength can be changed by steel thickness, its chemical composition and heat treatments.