The extrusion molding process for precision steel pipes is a core manufacturing technology that uses external force to drive the controllable plastic deformation of the billet metal in the mold cavity, ultimately obtaining a steel pipe substrate with high dimensional accuracy, excellent surface quality, dense microstructure, and stable mechanical properties. This process can be divided into three categories according to extrusion temperature: hot extrusion, cold extrusion, and warm extrusion. It is suitable for various materials such as 40CrNiMoA alloy, 304/316L stainless steel, and 6061 aluminum alloy, and can meet the stringent requirements for precision steel pipes in high-end fields such as aerospace, medical equipment, optical instruments, and semiconductor manufacturing. The core technology control of the extrusion molding process directly determines the final quality of the steel pipe. It requires meticulous operation in five core areas: billet control, mold design and maintenance, process parameter optimization, lubrication and cooling control, and defect prevention, to ensure a stable molding process and controllable defects.

First, billet control for precision steel pipes. 
As the core carrier for extrusion molding, the billet's material purity, dimensional accuracy, microstructure, and surface quality directly affect the uniformity of metal plastic flow, which is a prerequisite for avoiding extrusion defects and ensuring consistent steel pipe quality. Full-process control must be implemented from three dimensions: selection, screening, and pretreatment.
(I) Precise matching of material and purity for precision steel pipes.

Based on the final service conditions of the precision steel pipes, suitable billet materials are selected, and material purity and composition stability are strictly controlled. Alloy steel billets must ensure impurity content ≤0.03%, and carbon equivalent fluctuations controlled within ±0.02%; stainless steel billets must have carbon content ≤0.08%, and the content of alloying elements such as chromium and nickel must meet standards and have uniform fluctuations; aluminum alloy billets must strictly control the content of impurities such as iron and silicon to avoid affecting plastic deformation performance. Through spectral analysis, component titration, and other testing methods, the billet composition is verified batch by batch to prevent unqualified billets from being put into production. 

(II) Strict Dimensioning and Appearance Screening of Precision Steel Pipes.

The dimensions of the precision steel pipe blank must be precisely matched with the extrusion die, with the outer diameter tolerance controlled within ±0.5mm and the wall thickness deviation ≤0.3mm. The length should be rationally planned according to the extrusion press stroke and the finished steel pipe length, with a 10-15mm extrusion allowance reserved to avoid insufficient allowance leading to finished product dimensional deviation or excessive allowance causing material waste. In terms of appearance, the surface of the blank must be free of defects such as oxide scale, rust, cracks, dents, and burrs, with a surface roughness Ra ≤3.2μm. Internal quality is assessed through dual ultrasonic and magnetic particle testing to eliminate primary defects such as porosity, inclusions, and microcracks, ensuring a dense internal structure of the blank.

(III) Implementation of Pre-treatment Process Specifications for Precision Steel Pipes.

The core objective of pre-treatment is to improve the plasticity of the billet, eliminate residual internal stress from the manufacturing process, and avoid problems such as uneven deformation and cracking during extrusion. Different pre-treatment processes are required for different extrusion types:

(a) Hot-extruded billets: Homogenization annealing is adopted. For example, alloy steel billets are held at 850-900℃ for 3-4 hours and then cooled to room temperature in the furnace to refine the grains to grade 8-10, improving the metal's plasticity and microstructure uniformity. Stainless steel billets are annealed at 1050-1100℃ for 2-3 hours to eliminate work hardening effects. 
(b) Cold Extrusion/Warm Extrusion Billets: First, pickle with a mixed solution of hydrochloric acid and sulfuric acid for 20-30 minutes to thoroughly remove surface oxide scale and rust. After pickling, rinse with clean water and dry. Then, perform targeted annealing treatment: annealing temperature for cold extrusion billets is 650-700℃, and for warm extrusion billets, it is 450-550℃, to control the billet hardness at HB120-180, suitable for plastic deformation requirements. Finally, perform stress relief treatment to control residual stress below 50MPa.

Second, Optimization of Process Parameters for Precision Steel Pipes.
Extrusion process parameters are the core element in controlling the plastic flow of metal. They need to be precisely matched according to the billet material, steel pipe specifications, and extrusion type. The core objective is to achieve a balance between uniform metal flow and forming efficiency, avoiding defects such as over-deformation, cracks, and dimensional deviations. 
(I) Precise Temperature Parameter Control for Precision Steel Pipes. Temperature parameters directly affect the plasticity and microstructure of the metal. They must be precisely set according to the extrusion type and material characteristics, with temperature fluctuations controlled within ±20℃. Infrared thermometers are used to monitor the billet and die temperatures in real time and make dynamic adjustments:
(a) Hot Extrusion: Alloy steel pipe extrusion temperature 850-950℃, stainless steel pipe 1100-1200℃, aluminum alloy pipe 450-550℃. Excessive temperature can lead to coarse grains and severe surface oxidation, while insufficient temperature will result in insufficient metal plasticity and cause extrusion cracking.
(b) Cold Extrusion: Temperature controlled at room temperature -150℃, suitable for materials with good plasticity, improving the dimensional accuracy and surface quality of the steel pipe.
(c) Warm Extrusion: Temperature controlled at 200-400℃, suitable for medium to high hardness materials, balancing metal plasticity and processing efficiency, and reducing die wear. 
(II) Segmented Speed Control for Precision Steel Pipe Extrusion
A segmented speed control strategy is adopted to avoid defects such as wall thickness deviation and surface ripples caused by uneven metal flow. The speed is set differently according to the extrusion stage:
Initial Extrusion: Speed is controlled at 5-10 mm/s, with slow feeding to ensure a tight fit between the billet and the cavity, preventing billet eccentricity and localized stress concentration.
Middle Extrusion: Speed is increased to 15-30 mm/s to achieve efficient forming and ensure production efficiency.
Late Extrusion: Speed is reduced to 8-12 mm/s to reduce end deformation and avoid "neck-in" defects.
For thin-walled precision steel pipes, the overall extrusion speed needs to be reduced by 30% to further improve the uniformity of metal flow and prevent uneven wall thickness and surface scratches.
(III) Closed-Loop Control of Extrusion Pressure for Precision Steel Pipes.
The extrusion pressure needs to be precisely calculated based on the hardness of the billet material and the deformation of the steel pipe. A hydraulic extruder is used to achieve closed-loop pressure control, with pressure fluctuations ≤ ±50MPa: alloy steel pipe extrusion pressure controlled at 1500-2000MPa, stainless steel pipe 1200-1800MPa, and aluminum alloy pipe 800-1200MPa. The pressure curve is monitored in real time during extrusion. If a sudden pressure increase occurs, the machine must be stopped immediately for investigation, which may indicate problems such as die blockage, billet defects, or lubrication failure, to avoid equipment damage and mass scrapping of steel pipes.
IV. Lubrication and Cooling Control for Precision Steel Pipes.
Lubrication and cooling are key auxiliary processes in extrusion molding. Lubrication reduces the frictional resistance between the billet and the die, preventing surface scratches and metal adhesion in the precision steel pipe; cooling controls the molding temperature, suppressing defects such as hot cracking and coarse grains, ensuring the surface quality and dimensional stability of the steel pipe, and extending the service life of the die. 
(I) Selection of Lubrication Process for Precision Steel Pipes
Select a suitable lubricant based on the extrusion temperature to ensure a uniform, dense, and durable lubricating film that is easy to clean after extrusion and does not affect subsequent processing:
(a) Hot Extrusion: Employ a composite lubrication scheme of "glass lubricant + graphite coating." The melting point of the glass lubricant matches the extrusion temperature, forming a dense, high-temperature lubricating film. The graphite coating enhances lubrication durability. The coating thickness is controlled at 0.2-0.5 mm to ensure coverage of the blank surface and mold cavity.
(b) Cold/Warm Extrusion: Use extreme pressure grease with added extreme pressure additives such as MoS₂. Apply a coating thickness of 0.1-0.2 mm to improve lubrication and wear resistance. After extrusion, ultrasonically clean with anhydrous ethanol for 15 minutes to thoroughly remove surface lubricant residue and avoid affecting subsequent surface treatment and processing accuracy. 
(II) Efficient Operation of the Precision Steel Pipe Cooling System
A targeted cooling system is designed based on the extrusion type to achieve precise temperature control of the die and billet, avoiding defects caused by excessive temperature:
(a) Hot Extrusion: Equipped with a dual cooling system of "die water cooling + billet air cooling." The die cooling water inlet temperature is 20-30℃, and the outlet temperature is ≤60℃. The die working zone is uniformly cooled through dedicated water channels to prevent die overheating, deformation, and wear. The billet is immediately air-cooled after extrusion, with a cooling rate controlled at 5-10℃/s to avoid slow cooling leading to coarse grains and surface oxidation.
(b) Cold Extrusion: Cooling oil is sprayed onto the contact area between the die and billet through cooling nozzles, with the forming temperature controlled in real time to ≤150℃ to prevent lubricant failure and surface burning, while also reducing die wear.

Third, Defect Prevention and Control in Precision Steel Pipes 
Common defects in the extrusion molding process of precision steel pipes include surface scratches, cracks, uneven wall thickness, eccentricity, and necking. Targeted prevention and control measures must be implemented based on the characteristics of each process step, establishing a dual mechanism of "process inspection + finished product inspection" to ensure early detection and handling of defects.
(I) Targeted Prevention and Control of Common Precision Steel Pipe Defects
(a) Surface Scratches/Adhesion: Strictly control the surface finish of the die working surface, regularly polish and repair worn areas; ensure the billet surface is free of oxide scale and impurities, optimize the lubrication process, and ensure a uniform and dense lubricating film; avoid collisions and friction between the billet and equipment/tooling during extrusion, and use soft isolation materials during transfer.
(b) Precisely control the extrusion temperature and speed to avoid stress concentration caused by excessively high/low temperatures or excessively high speeds; optimize the die fillet radius and working zone design to reduce billet deformation stress; strengthen billet pretreatment to eliminate internal stress and original defects; avoid rapid cooling of the billet after hot extrusion, and control the deformation amount during cold extrusion to prevent excessive work hardening. 
(c) Uneven Wall Thickness/Eccentricity: Ensure the dimensional accuracy of the billet and the clearance between the die; adjust the concentricity of the extruder to ensure the billet enters the cavity centered; adopt a segmented speed control strategy to control uniform metal flow; monitor the outer diameter and wall thickness of the steel pipe in real time during extrusion and dynamically adjust process parameters.
(d) Narrowing: Optimize the speed in the later stage of extrusion to reduce uneven metal flow at the ends; reasonably reserve extrusion allowance to avoid insufficient material at the ends, leading to necking; strengthen the structural design of the die ends to guide uniform metal filling.
(II) Establishment of a Full-Process Inspection Mechanism for Precision Steel Pipes.
Establish an inspection system of "process sampling inspection + finished product full inspection" to ensure that the quality of each batch of precision steel pipes meets the standards:
(a) Process Sampling Inspection: Randomly select 3-5 billets and semi-finished products from each batch to inspect dimensional accuracy, surface quality, and microstructure. 
(b) Finished Product Full Inspection: Dimensional and positional accuracy are inspected using a laser diameter gauge and roundness meter to ensure outer diameter tolerance within ±0.005mm, wall thickness deviation ≤0.05mm, and roundness ≤0.002mm; surface quality is checked for defects using a laser surface roughness meter, magnetic particle testing, and penetrant testing, with surface roughness Ra ≤0.2μm; mechanical properties are verified through tensile and hardness tests to ensure tensile strength, hardness, and other indicators meet design requirements.


In summary, the core of the precision steel pipe extrusion molding process lies in meticulous control throughout the entire process. Through scientific billet management, optimized mold design, precise parameter control, efficient lubrication and cooling, and strict defect control, precision steel pipe substrates that meet the needs of high-end fields can be produced. In the future, with continuous upgrades in process technology, extrusion molding will play an even more important role in precision steel pipe manufacturing, providing solid support for the development of high-end equipment manufacturing.