Types of annealing treatment of seamless steel tubes
According to different processing requirements (such as reducing hardness, relieving stress, refining grains), annealing can be divided into several types. Each process differs in heating temperature, holding time and cooling rate, and should be selected based on tube material and subsequent processing.
♢ Full Annealing
For: hypoeutectoid steels (e.g. 45#)
Structure: fine pearlite + ferrite
Why: reduce hardness, eliminate stress, refine grains
Typical use: mechanical supports, drive shafts
⏲️ 820°C (45#) • 1‑5h hold • ≤50°C/h to 500°C
♢ Spheroidizing
For: high‑carbon steels (T8, alloy tool)
Structure: spheroidized pearlite
Why: lower HB (280→180), improve machinability
Typical use: taps, drills, high‑strength bolts
⏲️ 2‑4h slow cool or 3‑6h isothermal
♢ Stress Relieving
For: any steel (thin‑wall, welded, precision)
Why: remove >90% residual stress, no phase change
Applications: rolled tubes, welded flanges, hydraulic valves
⏲️ 1‑2h + extra 0.5h/10mm • cool ≤100°C/h
♢ Incomplete Annealing
For: medium‑carbon steels (45#)
Result: partial recrystallization, σₛ 300‑320 MPa
Why: balance machinability & service strength
Use: gear shafts requiring local quenching
⏲️ 1‑2h hold, furnace cool to 500°C
♢ Diffusion Annealing
For: high‑alloy steels, cast/forged tubes
Why: eliminate segregation (Cr, Mo), homogenize composition
Applications: corrosion‑resistant valves, furnace tubes
⏲️ 4‑8h hold • ≤20°C/h to 600°C
Selection guidance: Each annealing modifies the microstructure in a unique way — from coarse grain refinement to complete spheroidization. The table below summarises the core parameters and applications.
| Annealing type | Temperature range | Hold time / Cooling | Microstructure / Hardness | Typical tubes / parts |
|---|---|---|---|---|
| Full annealing | Ac₃+30~50°C (e.g. 820°C for 45#) | 1‑5h; ≤50°C/h to 500°C | Pearlite+ferrite; HB↓, grains refined | Low/medium‑carbon structural tubes |
| Spheroidizing | Ac₁+20~30°C / isothermal ~700°C | 2‑4h slow or 3‑6h isothermal | Spheroidized cementite; HB 180‑220 | High‑carbon / tool steel tubes (taps, drills) |
| Stress relieving | 550‑650°C (below Ac₁) | 1‑2h (+0.5h/10mm); ≤100°C/h to 300°C | No phase change; stress reduced >90% | Thin‑wall SS, welded flanges, hydraulic valves |
| Incomplete | Ac₁–Ac₃ (750‑800°C for 45#) | 1‑2h; furnace cool to 500°C | Partial recrystallization; σₛ~300 MPa | Gear shafts, parts with local quenching |
| Diffusion | 1100‑1200°C | 4‑8h; ≤20°C/h to 600°C | Homogenised composition; dendritic seg. eliminated | High‑alloy cast/forged tubes (valves, furnace tubes) |
♻️ Process notes
• Full annealing for 45# steel: wall thickness >20 mm → extend to 5h hold.
• Spheroidizing can be done isothermally: hold 3‑6h at ~680°C.
• Stress relieving cools at ≤100°C/h to avoid new stresses.
• Incomplete annealing retains σₛ 300‑320 MPa, good for partially quenched parts.
• Diffusion annealing requires extremely slow cooling (≤20°C/h).
⚙️ Applications in seamless tube production
✔ Thin-walled stainless tubes (≤5mm) after rolling → stress relieving to avoid bending cracks.
✔ Heat exchanger tube sheets welded assemblies → stress relieving to prevent leakage.
✔ High-strength bolt tubes → spheroidizing for cold extrusion.
✔ Drive shafts / supports → full annealing for machinability.
✔ Corrosion-resistant valves from alloy cast tubes → diffusion annealing + subsequent quenching.
The right annealing transforms tube performance — choose wisely.