Pipe fittings have applications in plumbing, electrical, pneumatics, HVAC (heating ventilation and air-conditioning), and other industrial uses. When picking out a pipe fitting, first select the fitting type and then select your material.
Major categories of fitting types includes threaded, flanged, and nipples. Within these categories, there are subcategories. For example, thread pipe fitting are further divided into bushings, caps, couplings, cross, elbow, tees, return-bends, unions, laterals, locknut, plugs, and so on. Flanged fittings can come in various varieties that include blind flange, floor flange, reducing flange, slip-on flange, threaded flange, and weld-neck flanges. For some manufacturers, all they do is pipe fitting. That is enough, because the combinations can be endless, especially if they make custom fittings for clients as well.
The material is cast from different alloys. An alloy is a metallic solid that is composed of multiple elements such copper, iron, aluminum, zinc, titanium, chromium, and others. Different alloys will have different mixtures of these elements, giving slightly different properties. Depending on your application, you select your alloy based on your desired properties such as tensile strength, yield strength, elongation, weight, electrical/thermal conductivity, melting range, specific gravity, and so on. As this can easily become quite technical, engineers with degree in material science at the manufacturing shop can work with you on this.