We Super Steel Manufacturing Co.,Ltd(SMC) are amongst the top manufacturers and suppliers for the seamless steel pipe, welded steel pipe, OCTG products and fittings from China. We have been manufacturing and supplying these products to the industry since 1993, and received repeat orders and appreciation from our valued customers.
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Always up to date. In this column you can find the steel pipe related articles, including industry trends, product standards and materials, and some technical guidelines.
After hot rolling and cold drawing, seamless steel pipes (SMLS pipes) often contain high residual stress and coarse grain structures. If used without proper heat treatment, these internal defects may lead to deformation, cracking, or premature failure under service loads.
Heat treatment modifies the steel’s internal microstructure through controlled heating, holding, and cooling processes. It is a core metallurgical technology for optimizing hardness, toughness, wear resistance, and dimensional stability, enabling seamless pipes to meet demanding conditions such as high pressure, cyclic loading, precision tolerance, and harsh environments.
Six Core Heat Treatment Processes for Seamless Pipes
1. Annealing
Process principle:
Heating to 30–50 °C above the critical temperature (Ac₃ for hypoeutectoid steel, Accm for hypereutectoid steel), followed by slow furnace cooling (≤50 °C/h).
Main types:
Full annealing
Spheroidizing annealing
Stress-relief annealing
Purpose of annealing:
Eliminates residual stress from welding or cold drawing, preventing deformation in subsequent machining
Reduces hardness by over 30%, improving cold bending, flaring, and forming performance
Homogenizes microstructure, preparing the material for quenching or precision machining
Typical applications:
Precision hydraulic pipes, instrument tubing, and structural components requiring extensive machining.
2. Normalizing
Process principle:
Heating to a temperature similar to annealing, followed by air cooling, resulting in a fine pearlite structure.
Key benefits:
Grain size refined by 20–30% compared with annealed condition
Strength increased by 10–20%
Moderate hardness (HB 170–230) with good machinability
Eliminates Widmanstätten structures caused by hot rolling
Typical applications:
General fluid transport pipes, pressure vessel shells, turbine and mechanical structural components.
3. Quenching and Tempering (Q&T)
Representative application: API 5L X70 pipeline steel
Typical process:
Quenching: 920–950 °C heating + water quenching
Tempering: 600–630 °C high-temperature tempering
Achieved properties:
Tensile strength ≥ 570 MPa, yield strength ≥ 485 MPa
Impact energy ≥ 40 J at −20 °C, meeting low-temperature crack resistance requirements
Uniform tempered sorbite structure, suitable for alternating and complex loads
Typical applications:
High-strength line pipes for long-distance oil and gas transmission.
4. Surface Quenching (Surface Hardening)
Process principle:
Only the surface layer is heated to the quenching temperature and rapidly cooled, while the core microstructure remains unchanged.
Main methods:
Induction hardening: Electromagnetic heating, controllable hardened layer (0.5–10 mm), minimal deformation
Flame hardening: Oxyacetylene heating, suitable for localized hardening of large-diameter pipes
Performance advantages:
Surface hardness up to HRC 55–60
Core toughness retained
Fatigue strength increased by 2–3 times
Typical applications:
Hydraulic cylinder barrels, drive shaft sleeves, wear-resistant pipes in mining and heavy machinery.
5. Chemical Heat Treatment
Process principle:
Altering surface composition through atomic diffusion processes such as carburizing or nitriding.
Key advantage:
Nitriding produces minimal deformation and can achieve dimensional accuracy within 0.01 mm, often eliminating the need for post-machining.
Typical applications:
High-precision mechanical pipes requiring superior surface wear and fatigue resistance.
6. Special Heat Treatment Processes for Extreme Conditions
Isothermal quenching:
After quenching, holding at 200–400 °C to form lower bainite, reducing cracking risk and improving toughness in thick-walled, high-pressure pipes.
Aging treatment:
Holding at room temperature or 100–200 °C to stabilize dimensions of precision pipes, commonly used in aerospace and high-precision applications.
Frequently Asked Questions
1. Why is tempering required after quenching seamless pipes?
Quenched steel pipes contain residual stresses exceeding 300 MPa, making them extremely brittle. Tempering relieves over 90% of internal stress while restoring toughness and service reliability.
2. How should annealing and normalizing be selected?
Annealing is preferred for cold working and machining due to lower hardness, while normalizing is suitable for direct service where higher strength is required and costs are approximately 40% lower.
3. Do stainless steel pipes require heat treatment?
Yes. 304 stainless steel pipes typically undergo solution treatment at 1050–1100 °C followed by water quenching to dissolve carbides and improve corrosion resistance, similar in purpose to annealing.
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