Q1: What is "normalizing," and why is it commonly applied to carbon steel seamless pipes like 20#?
A: Normalizing involves heating the steel to above its critical temperature and then air cooling. It refines the grain structure, improves mechanical property uniformity, and relieves internal stresses from hot working, which is beneficial for subsequent machining or service.
Q2: Why is "quenching and tempering" essential for alloy steel pipes like 42CrMo?
A: Quenching (rapid cooling) produces a very hard but brittle martensitic structure. Subsequent tempering (reheating to a lower temperature) reduces brittleness and relieves stresses, resulting in an optimal combination of high strength, toughness, and fatigue resistance required for demanding applications-4-10.
Q3: What is "stress relief annealing," and when is it performed on cold-drawn seamless pipes?
A: It is a low-temperature heat treatment that reduces internal stresses induced by cold working (drawing) without significantly changing the microstructure. This prevents distortion during machining and improves dimensional stability-7.
Q4: What is "bright annealing," and for what type of seamless pipes is it used?
A: Bright annealing is performed in a controlled atmosphere (e.g., nitrogen or hydrogen) to prevent surface oxidation and decarburization. It is used for precision seamless pipes (e.g., GB/T 3639) to maintain a bright, clean surface and tight dimensional tolerances-5-7.
Q5: How does the heat treatment for boiler tube grades like 15CrMoG differ from that for structural grades?
A: Boiler tube heat treatment (normalizing and tempering) is precisely controlled to develop a specific microstructure (like bainite) that offers excellent long-term creep strength and microstructural stability at high temperatures, which is not a primary concern for general structural grades.
