Steel pipe hot working is a core technology in steel pipe production. It involves using high temperatures to induce plastic deformation in the billet, achieving shaping and performance optimization. Widely used in petrochemical, mechanical manufacturing, and other fields, its quality directly determines the mechanical properties, dimensional accuracy, and service life of the steel pipe.
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Basic Principles of Steel Pipe Hot Working Technology
It utilizes the characteristics of increased metal plasticity and decreased strength and hardness at high temperatures. External forces are applied to cause plastic deformation of the billet, while internal stresses are partially eliminated through atomic diffusion, preventing fracture.
Core Processes of Steel Pipe Hot Working Technology
Billet Preparation
Billet preparation includes three steps: selection, cutting, and surface treatment. Billets matching the finished product requirements must be selected, cut to the appropriate size, and surface scale, oil stains, and other impurities removed to avoid affecting subsequent processing quality.
Heating Process
Heating is a critical step and must follow the principle of "uniform heating and precise temperature control." Commonly used equipment includes walking beam furnaces, rotary hearth furnaces, and induction heating furnaces, to prevent overheating, burning, or uneven temperature distribution of the billet.
Walking beam furnaces are suitable for large-volume, multi-specification billets, rotary hearth furnaces are suitable for continuous heating of seamless steel pipe billets, and induction heating furnaces offer fast and environmentally friendly heating. The heating temperature is determined by the material, such as 1100-1250°C for ordinary carbon steel and 1050-1200°C for stainless steel.
Forming Process
Seamless steel pipe forming includes three steps: hot rolling piercing, continuous rolling, and sizing: piercing creates a hollow tube, continuous rolling adjusts the specifications, and sizing calibrates the outer diameter accuracy.
Welded steel pipe forming includes plate rolling and welding: the plate is rolled into a tube shape, and then welded using methods such as submerged arc welding and high-frequency induction welding.
Plate rolling requires ensuring uniform gap between the edges and acceptable roundness. Submerged arc welding is suitable for large-diameter, thick-walled pipes, while high-frequency induction welding is suitable for mass production of small-diameter, thin-walled pipes.
Cooling Treatment
Cooling treatment should be selected according to the material and performance requirements, using natural cooling, air cooling, or water cooling: natural cooling is suitable for ordinary carbon steel, air cooling is suitable for alloy steel, and water cooling is suitable for steel pipes requiring high strength. The cooling rate needs to be controlled to prevent cracking.
Subsequent Processing
Subsequent processing includes straightening, cutting the ends, non-destructive testing, surface treatment, and marking and packaging. Straightening ensures straightness, cutting the ends ensures length accuracy, non-destructive testing eliminates defects, surface treatment provides rust and corrosion protection, and marking and packaging facilitate storage and transportation.
Thermal Processing Requirements for Different Steel Pipe Materials
Carbon Steel Pipes
Carbon steel pipes are classified into low, medium, and high carbon steel based on carbon content: low-carbon steel has a hot working temperature of 1150-1250℃, with good plasticity and easy processing. Medium-carbon steel is processed at 1100-1200℃, requiring prevention of grain coarsening. High-carbon steel is processed at 1050-1150℃, requiring strict control of heating and holding times, and slow cooling to prevent cracking.
Alloy Steel Pipes
The hot working suitability of alloy steel pipes depends on the alloying elements: chromium alloy pipes are processed at 1100-1200℃, requiring sufficient holding time. Manganese alloy pipes are processed at 1150-1250℃, followed by normalizing after forming. Nickel alloy pipes are processed at 1100-1250℃, followed by natural or air cooling. Molybdenum alloy pipes are processed at 1150-1200℃, followed by tempering after forming.
Stainless Steel Pipes
Stainless steel pipes are divided into austenitic, ferritic, and martensitic types: austenitic stainless steel is processed at 1050-1200℃, avoiding deformation below 900℃, followed by solution treatment after forming. Ferritic stainless steel is processed at 1000-1150℃, with controlled heating and natural cooling; martensitic stainless steel is processed at 1050-1150℃, followed by quenching and tempering.
