1. Raw material preparation
Steelmaking and billet casting
The main components of stainless steel include iron, chromium (≥10.5%), nickel and other alloy elements (such as 304 stainless steel contains 18% chromium + 8% nickel). It is smelted by electric arc furnace or medium frequency furnace to control the carbon content (such as food grade 316L carbon content ≤0.03%) to improve corrosion resistance.
The molten steel is cast into round billets (diameter 50-300mm) or square billets by continuous casting machine as raw materials for pipe making.
Billage selection
Choose billets according to pipe diameter requirements:
Small diameter pipes (diameter ≤100mm) usually use round billets;
Large diameter pipes (diameter > 100mm) use steel ingots or continuous casting billets.
2. Pipe making process: hot rolling and cold rolling/cold drawing
(I) Hot rolling stainless steel pipe process (applicable to large diameter, thick wall pipes)
Heating and perforation
The round billet is heated to 1100-1250℃ in the heating furnace to soften it;
The round billet is perforated into a hollow rough tube by a perforator (such as a two-roller oblique rolling perforator). The core is to use a head to pierce a hole in the center of the billet to form a rough tube with uneven wall thickness.
Stretching and pipe rolling
Stretching process: The rough tube is rolled by a stretching machine (such as an automatic pipe rolling machine) to reduce the wall thickness and increase the length, thereby improving the pipe diameter accuracy;
Rolling process: Further rolling by a continuous rolling mill (such as a three-roll continuous rolling mill) to make the pipe wall uniform and the diameter close to the target size (accuracy ±0.5mm).
Sizing and reducing
The sizing mill refines the outer diameter of the steel pipe to ensure uniform size;
The reducing mill is used to produce small-diameter pipes. The pipe diameter is reduced by multiple rolling passes, while improving the surface finish.
Cooling and cutting
The steel pipe is cooled by air or water and cut into fixed lengths (such as 6 meters/piece) as required.
(II) Cold rolling/cold drawing stainless steel pipe process (suitable for small diameter, high precision pipes)
Preparation of rough pipe
Hot-rolled rough pipe or finished pipe as blank needs to be surface treated first:
Annealing: eliminate hot rolling stress and soften the material (temperature 700-900℃);
Pickling: remove oxide scale with hydrochloric acid or sulfuric acid to avoid subsequent processing scratching the surface;
Lubrication: apply lime or phosphating liquid to reduce cold working friction.
Cold rolling process (suitable for mass production)
Use multi-roll cold rolling mill (such as Sendzimir rolling mill) to reduce the wall thickness and diameter of steel pipe through roller extrusion, with an accuracy of ±0.05mm and a surface roughness of Ra≤0.8μm. It is often used in medical equipment and food pipelines.
Cold drawing process (suitable for high-precision, special-shaped tubes)
Put the steel tube on the mandrel and pull it through the die through the tube drawing machine to achieve precise control of wall thickness and diameter;
Special-shaped tubes (such as square tubes and oval tubes) can be produced, but the production efficiency is lower than cold rolling, and it is often used for decoration and mechanical parts.
Intermediate annealing and final annealing
Cold processing will harden the steel tube (work hardening), and multiple annealing (such as bright annealing, heating to 750-900℃ in a protective atmosphere) is required to restore plasticity and prevent oxidation.
3. Surface treatment and finishing
Deburring and straightening
The burrs of the cut tube mouth need to be removed to ensure sealing during welding or connection;
Straightening machines (such as pressure straightening machines) eliminate steel tube bending, and the straightness is ≤1mm/m.
Surface treatment process
Pickling passivation: Use nitric acid + hydrofluoric acid solution to remove surface impurities and form a passivation film (Cr2O3) to improve corrosion resistance, suitable for chemical pipelines;
Electrolytic polishing: Make the surface smooth through electrochemical action (Ra≤0.2μm), commonly used in food and pharmaceutical equipment;
Mechanical polishing: Sanding with a sand belt to a mirror effect (such as 8K mirror stainless steel pipe), used for decoration projects;
Coating treatment: Titanium plating (gold), chrome plating (wear resistance), to improve aesthetics or functionality.
Non-destructive testing
Eddy current testing (ET): Detect surface and near-surface defects (such as cracks, holes);
Ultrasonic testing (UT): Check for internal defects, suitable for thick-walled pipes;
Hydraulic pressure test: Fill with water and pressurize (such as 10-20MPa) to detect leaks and ensure pipeline sealing.
4. Special processes and application scenarios
Welded stainless steel pipe (welded pipe) process
Forming: Stainless steel coils are rolled into tubes by a roll forming machine;
Welding: TIG welding (argon arc welding) or high-frequency welding is used to form straight seam welded pipes. After welding, welding slag and annealing are required;
Advantages: low cost and high production efficiency, suitable for fluid pipelines (such as drinking water pipes) and structural pipes (such as guardrails).
Seamless stainless steel pipe process (high-end application)
Directly formed by perforation + rolling, no welds, high strength, suitable for high-pressure environments (such as oil pipelines, aviation components), but the cost is higher than welded pipes.
Special-shaped pipe processing
Through cold drawing or extrusion dies, round pipes are processed into square pipes, rectangular pipes, and patterned pipes for architectural decoration and furniture frames.
V. Comparison of Typical Processes
Process Type Advantages Application Scenarios Representative Products
Hot-rolled seamless pipes High strength, high pressure resistance, suitable for large diameters Oil pipelines, chemical containers 304 seamless pipes (diameter 219mm)
Cold-rolled welded pipes High precision, smooth surface, low cost Food pipelines, decorative guardrails 316L welded pipes (diameter 50mm)
Cold-drawn special-shaped pipes Various cross-sections, precise dimensions Furniture, mechanical parts Rectangular pipes (50×30mm)
VI. Key points of quality control
Alloy composition control: Insufficient chromium and nickel content will reduce corrosion resistance and need to be tested by spectrometer (such as 304 stainless steel Cr≥18%, Ni≥8%);
Intergranular corrosion test: After sensitization treatment, detect whether corrosion is caused by carbide precipitation, which is common in 304 steel welded parts;
Dimensional accuracy: The outer diameter and wall thickness tolerances must meet the standards (such as GB/T 14976-2012 stipulates that the seamless pipe wall thickness tolerance is ±10%).