Seamless steel pipe heat treatment process and its function

The heat treatment process of seamless steel pipe can be divided into overall heat treatment (annealing, normalizing, quenching + tempering) and surface heat treatment (surface quenching, chemical heat treatment, etc.) according to its purpose and process characteristics. The following is a detailed classification, characteristics and function analysis of each process:
 

Overall heat treatment process
1. Annealing
Definition: The process of heating the steel pipe to above the critical temperature, keeping it warm and then slowly cooling it (furnace cooling, pit cooling, etc.).
Features:
The heating temperature varies according to the type of steel (such as hypoeutectoid steel heated to above Ac3, hypereutectoid steel heated to between Ac1 and Accm).
The cooling rate is slow, and the equilibrium state structure (such as pearlite, ferrite) is obtained.
Function:
Eliminate internal stress: Eliminate residual stress caused by cold working (such as rolling, welding) or casting, and prevent deformation and cracking.
Grain refinement: Improve overheating structure, uniform microstructure, and improve the stability of mechanical properties.
Reducing hardness: Improve cutting performance (such as high carbon steel hardness is reduced after annealing, which is easy to cut).
Typical applications: used for seamless steel pipes after cold rolling and cold drawing, or as a preliminary heat treatment to prepare for subsequent processing.
2. Normalizing
Definition: A process of heating to above the critical temperature, keeping warm and then air cooling (or wind cooling).
Features:
The cooling rate is faster than annealing, and fine pearlite or troostite structure is obtained, and the strength and hardness are higher than annealing.
The process cycle is short and the cost is low.
Function:
Improve the uniformity of the structure: eliminate network carbides (such as hypereutectoid steel) and refine the grains.
Adjust the hardness: For low-carbon steel (such as 20# steel), normalizing can increase the hardness to 170~210HB and improve the cutting performance (after annealing, the hardness is too low and the knife is easy to stick).
Stress relief: Although the effect is weaker than annealing, it can be used as the final heat treatment of ordinary structural parts.
Typical applications: used for seamless steel pipes for mechanical structures (such as gear sleeves), or as a preliminary heat treatment before quenching.
3. Quenching + Tempering
(1) Quenching
Definition: Heating to above the critical temperature, keeping warm and then cooling rapidly (water, oil, polymer solution, etc.).
Features:
Quick cooling (cooling rate is greater than the critical quenching rate), forming martensitic structure, and the hardness increases sharply (can reach above 50HRC), but it is brittle and has high internal stress.
It is easy to deform or crack, and needs to be used with tempering.
Function:
Improve hardness and wear resistance: suitable for seamless steel pipes that require wear resistance (such as hydraulic cylinders and bearing sleeves).
Adjust mechanical properties: provide basic structure (martensite) for tempering.
(2) Tempering
It is divided into three categories according to the tempering temperature:
Low-temperature tempering (150~250℃):
Features: retain high hardness (45~60HRC), reduce quenching stress and brittleness.
Function: used for tools, molds, and wear-resistant parts (such as seamless steel pipes for drilling).
Medium temperature tempering (350~500℃):
Features: obtain troostite structure, high elastic limit (σe can reach 700~900MPa).
Function: suitable for steel pipes for springs and elastic elements.
High temperature tempering (500~650℃, i.e. quenching and tempering):
Features: obtain troostite structure, excellent comprehensive mechanical properties (balance of strength, plasticity and toughness).
Function: used for important structural parts (such as automobile axles, high-pressure pipelines), improve fatigue resistance and impact resistance.

Surface heat treatment process
1. Surface hardening
Definition: The surface of the steel pipe is quickly heated and quenched by induction heating, flame heating, etc., and the core maintains toughness.
Features:
The surface hardness is high (50~60HRC), and the core maintains the original toughness (such as HB≤200 of the core of 20# steel after surface quenching).
Fast heating speed, small deformation, and high production efficiency.
Function:
Improve surface wear resistance: suitable for parts with local wear (such as gear sleeves, hydraulic cylinder piston rods).
Retain core toughness: avoid brittleness caused by overall quenching, suitable for steel pipes that bear impact loads.
Typical applications: seamless steel pipes for engineering machinery, gear sleeves, etc.
2. Chemical Heat Treatment
Improve surface properties by infiltrating elements such as carbon, nitrogen, and boron.
(1) Carburizing
Definition: Heat low carbon steel (10#, 20# steel) to 900~950℃, keep it warm in a carburizing medium, and allow carbon atoms to penetrate the surface.
Features:
The surface carbon content is 0.8%~1.2%, the surface hardness is 58~62HRC after quenching + low temperature tempering, and the core maintains the toughness of low carbon steel.
The depth of the carburized layer is 0.5~2mm, which can be adjusted according to the insulation time.
Function:
Surface wear resistance, core impact resistance: suitable for parts that bear alternating loads and wear (such as steel pipes for gears and piston pins).
Typical applications: seamless steel pipes for automobile gearboxes, bearing rings.
(2) Nitriding
Definition: Heating to 500~570℃, allowing nitrogen atoms to penetrate the surface in ammonia to form a nitride layer (such as Fe3N, Fe4N).
Features:
Extremely high surface hardness (up to 1000~1200HV), excellent wear resistance and corrosion resistance, and minimal deformation (low temperature, no quenching required).
The nitrided layer is thin (0.1~0.6mm) and has a high cost.
Function:
High wear resistance, corrosion resistance, and fatigue resistance: suitable for precision parts (such as high-pressure valves, chemical pipelines), or steel pipes that require no coating for corrosion protection.
Typical applications: seamless steel pipes for aerospace, corrosion-resistant high-pressure pipelines.
(3) Nitrocarburizing
Definition: Carbon and nitrogen are infiltrated simultaneously, the process temperature is lower than carburizing (700~850℃), and the performance of the carburized layer is between carburizing and nitriding.
Features:
High surface hardness (60~70HRC), better wear resistance and corrosion resistance than single carburizing, and better toughness of the carburized layer.
Function:
Comprehensive improvement of surface properties: suitable for parts with medium load and wear resistance and corrosion resistance (such as steel pipes for hydraulic supports).

Special heat treatment process
1. Isothermal annealing
Features: After heating, keep warm in the isothermal transformation temperature range to quickly obtain uniform structure (such as spheroidized body) and shorten annealing time.
Function: Used for high alloy steel seamless pipes to improve cutting performance or prepare for quenching.
2. Stress Relieving Annealing
Features: Heating to below Ac1 (e.g. 500~650℃), slowly cooling after heat preservation, without changing the microstructure.
Function: Eliminate residual stress after welding and cold drawing, and prevent stress corrosion cracking (e.g. steel pipes for high-pressure pipelines and heat exchangers).

Summary
The heat treatment process of seamless steel pipes achieves precise optimization of performance by regulating heating temperature, cooling rate and alloy element distribution:
Overall heat treatment mainly improves the overall mechanical properties of the material and is suitable for structural load-bearing parts;
Surface heat treatment focuses on surface wear resistance, corrosion resistance and other characteristics to meet the needs of local performance enhancement.
In practical applications, the process needs to be comprehensively selected based on the material (carbon content, alloy elements), service conditions (load, environment) and processing cost of the steel pipe to achieve a balance between performance and economy.