Materials for pipe fittings (elbow, tee, union): For pressure-tight pipe fittings that must be threaded and shock-resistant, which material is generally preferred?

Introduction / Context:
Pipe fittings such as elbows, tees, and unions need adequate strength, good machinability for threading, and toughness to survive handling and service shocks. The choice of cast ferrous materials directly affects sealing reliability and resistance to cracking at threads and thin sections.

Given Data / Assumptions:

• Fittings are commonly made by casting, followed by machining threads.
• Service involves internal pressure, tightening loads, and occasional impact.
• Economy and mass production are relevant.

Concept / Approach:
Malleable iron is produced by heat-treating white cast iron to convert carbides into temper carbon nodules, substantially improving ductility and toughness while retaining castability and machinability. This combination is ideal for threaded fittings. Pig iron is a raw ironmaking product, not a finished material for fittings. High carbon steel is overly hard/brittle for standard threaded casting routes, and spheroidal graphite (SG) iron, while tough, is more often used for larger ductile iron pipe and fittings; traditional small threaded fittings have long used malleable iron due to its balance of properties and cost.

Step-by-Step Solution:
Define requirements: castable, machinable, tough at threads.Match material family: malleable iron meets these needs.Eliminate non-ideal choices: pig iron (intermediate product), high carbon steel (poor thread toughness), grey iron (flake graphite → low tensile/impact strength).Conclude malleable iron is preferred.

Verification / Alternative check:
Industry standards and catalogues list “malleable iron fittings” for threaded services in plumbing, gas, and steam applications.

Why Other Options Are Wrong:
Pig iron is not a service material for fittings.

SG iron is tough but less common for small threaded unions versus malleable iron; it favours push-on or flanged systems.

High carbon steel is difficult to cast into complex, thin-threaded shapes economically.

Grey iron has poor impact strength due to flake graphite.

Common Pitfalls:
Assuming any cast iron works for threaded fittings; ignoring the embrittling effect of flake graphite at stress concentrators like threads.

Final Answer:
Malleable iron