Q1: Why is the sulfur (S) and phosphorus (P) content strictly controlled in seamless pipe steels like 20#?
A: S and P are considered impurities. High S content can cause "hot shortness" (cracking during hot working), while high P increases "cold brittleness." Standards typically limit them to ≤0.035% each for general grades-5 to ensure good ductility and toughness.
Q2: What is the role of chromium (Cr) in grades like 40Cr and 20CrMo?
A: Chromium increases hardenability (depth of hardening during heat treatment), improves corrosion/oxidation resistance, and contributes to strength through solid solution strengthening. This makes these grades suitable for more demanding mechanical parts-4-5.
Q3: How does the addition of molybdenum (Mo) in 42CrMo and 15CrMoG enhance pipe performance?
A: Molybdenum improves strength at elevated temperatures, increases hardenability, and enhances resistance to "temper embrittlement." This is critical for pipes used in high-pressure boilers (15CrMoG)-1 or high-stress automotive/mechanical components (42CrMo)-4-10.
Q4: What are the typical chemical ranges for key elements in 35# steel seamless pipes?
A: For grade 35#: Carbon (C): 0.32-0.39%, Silicon (Si): 0.17-0.37%, Manganese (Mn): 0.35-0.65%, with S and P ≤0.035%-5.
Q5: Why are "clean steel" practices important for seamless pipe production, especially for alloy grades?
A: Minimizing non-metallic inclusions (oxides, sulfides) is crucial for achieving good internal soundness, which directly impacts fatigue life, toughness, and formability. This is particularly important for pipes subjected to cyclic loading or severe forming.
