The Stress Issue in Steel Pipe Production

The main causes of internal stress in steel pipes are uneven temperature changes, uneven plastic deformation, and asynchronous phase transitions in the structure. These stress phenomena occur in the early, middle, and final stages of the production process and can be classified into transient stress (which disappears after the process is completed) and residual stress (which remains in the finished product and is the core factor affecting quality). There are significant differences in the stress generation stages for different production routes (such as seamless steel pipes and welded steel pipes). The specific details are as follows:

During this stage, the stress is mainly thermal stress, mostly transient stress. Some initial residual stress will be carried over to the subsequent processing steps along with the raw materials.

• Initial residual stress of pipe billets: During the cooling and straightening process of continuous casting pipe billets and rolled pipe billets, internal stress has already been retained. This belongs to the pre-processing stress brought by the raw materials and will be superimposed onto the subsequent processing stress.
• Thermal stress during heating process: When the pipe billets are heated, the surface layer expands first due to heat, while the core layer has a lower temperature and a smaller expansion amount. The surface layer is compressed and the core layer is stretched. If the heating speed is too fast or the temperature in the furnace is uneven, the thermal stress will increase sharply, and in extreme cases, it may cause cracking on the surface of the pipe billets. After uniform heating, the instantaneous thermal stress will be significantly relieved.

This is the most critical stage for stress generation. Large plastic deformation and local thermal cycles generate a large amount of deformation stress and thermal stress, which are the main sources of residual stress in the finished product.

• Core processes of seamless steel pipes

1. Piercing process: During inclined rolling piercing, the tube billet undergoes intense three-dimensional non-uniform plastic deformation under the action of the rolls and the punch head. There is a significant difference in the deformation amount of the wall thickness and the inner and outer surfaces, resulting in significant deformation stress; the uneven cooling after piercing will add thermal stress, jointly forming the initial residual stress of the tube.
2. Rolling tube / Fixed reduction diameter process: During continuous rolling and automatic tube rolling, the wall thickness is reduced and the axial extension occurs. The deformation is unevenly distributed along the wall thickness; in the fixed diameter and reduction diameter processes, the circumferential compression deformation occurs, and the difference in deformation between the inner and outer surfaces is further amplified, which is the main source of axial and circumferential residual stress in hot-rolled seamless pipes.

• Core process of welded steel pipes

1. Forming process: The strip steel is bent multiple times by rollers to form the pipe blank. The bending deformation causes the outer layer of the steel plate to be under tension and the inner layer to be under compression. After cold plastic deformation, residual bending residual stress remains. This belongs to the forming processing stress.
2. Welding process: The weld seam and the heat affected zone are rapidly heated and cooled locally. The thermal expansion of the heating area is constrained by the surrounding cold metal, resulting in compressive plastic deformation; during cooling, the contraction is restricted by the surrounding base metal, and finally a very high residual tensile stress is formed in the weld seam area. The base metal on both sides is under compressive stress to maintain balance, which is the most core stress source of welded pipes. The greater the welding line energy and the faster the cooling speed, the higher the residual stress level.

This stage may introduce new residual stresses, or these stresses can be eliminated through specific processes in the previous stage.

• Cooling process: When hot-rolled and welded steel pipes cool down, the surface cools faster while the core cools slower. The surface first contracts and is under tension, while the core later contracts and is under compression, resulting in residual thermal stress; the faster the cooling rate and the more uneven the cooling, the greater the residual thermal stress.
• Straightening process: Cold straightening corrects the pipe's curvature through repeated reverse bending and plastic deformation, but uneven deformation will introduce new bending residual stress. Usually, the surface layer of the pipe retains compressive stress, and the sub-surface layer retains tensile stress; hot straightening (using the residual heat from rolling to straighten) introduces stresses that are much smaller than those from cold straightening.
• Cold processing stages (cold drawing/cold rolling tubes): Under large plastic deformation at room temperature, extremely strong processing hardening and residual stress will be generated, which is the absolute main source of residual stress for cold-drawn/cold-rolled steel pipes, and the stress level is much higher than that of hot-rolled steel pipes.
• Cutting and pipe end processing: Sawing, threading, and groove processing will cause local plastic deformation, resulting in local stress concentration at the pipe end.
• Heat treatment process: When the process is carried out properly, stress can be eliminated; if the heating is uneven or the cooling is too fast (such as quenching), new thermal stress and microstructural stress will be generated, and in severe cases, the steel pipe will crack.

The instantaneous stress during the process only affects the stability of the process. Residual stress is the core factor determining the quality and service performance of the finished product, and it has two effects: negative impacts and controllable positive effects.

• Dimension and shape instability: Residual stress is an internal self-balancing force. When subsequent processing steps such as cutting, threading, bending, etc. disrupt the stress balance, the stress release will cause the steel pipe to bend, exceed the ellipticity limit, expand or contract in diameter, and affect the dimensional accuracy; slow stress release during long-term storage can also lead to deformation failure.
• Reduced pressure-bearing and fatigue performance: Residual tensile stress will be superimposed with the working stress generated by working pressure and alternating loads, causing the steel pipe to actually bear more force than the design value, thereby reducing its pressure-bearing capacity; under alternating loads such as fluid pressure fluctuations and vibrations, residual tensile stress will accelerate crack initiation and significantly reduce fatigue life, and in severe cases, cause pipe rupture or fracture.
• Induce stress corrosion cracking: In corrosive media containing sulfur, chloride ions, etc. (such as oil and gas transportation, chemical pipelines), residual tensile stress will interact with corrosion, inducing stress corrosion cracking (SCC), which is one of the core failure forms of oil and gas transportation pipes and refining and chemical pipes.
• Degradation of subsequent processing performance: For steel pipes with large residual stress, during secondary processing steps such as bending, expanding, and contracting, they are prone to cracking and excessive rebound, resulting in a significant decrease in processing qualification rate.

Not all residual stresses are detrimental: The surface residual compressive stress can counteract some external tensile loads, enhancing the fatigue strength and stress corrosion resistance of the steel pipe. In industry, controlled surface compressive stress is actively introduced through processes such as shot peening and hydrostatic testing to optimize the service performance of the steel pipe.

Regarding stress, it is divided into two types of approaches: "Eliminating/controlling the residual stress that has already been generated" and "Reducing the generation of stress at the source".

• Heat treatment method (most core, highest elimination rate)

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The most commonly used method is stress-relieving annealing: heat the steel pipe below the recrystallization temperature (for carbon steel, generally 550-650°C, for alloy steel, adjust according to the material), hold for a while, and then cool slowly. Through micro plastic flow and atomic diffusion, internal stress is released, with an elimination rate of 70% to 90%, while not significantly reducing the material strength. It is a standard process for cold-drawn pipes, welded pipes, and high-pressure boiler pipes.

The normalizing + tempering process eliminates residual stress simultaneously while adjusting the structure and optimizing mechanical properties, mostly used for the final heat treatment of seamless steel pipes.

• Mechanical regulation method

1. Hydrostatic test: apply overpressure water pressure to cause micro plastic deformation of the pipe wall, release residual stress and redistribute the stress, forming beneficial compressive stress on the surface layer, having the dual functions of quality inspection and stress regulation, and is a necessary process for pressure-bearing steel pipes.
2. Shot blasting/peening treatment: high-speed projectiles impact the surface of the steel pipe, causing plastic deformation of the surface layer, introducing controllable surface residual compressive stress, counteracting harmful tensile stress, and simultaneously enhancing fatigue strength and corrosion resistance. It is commonly used for pre-treatment before anti-corrosion.
3. Vibration aging: use resonance to make microscopic defects in the steel pipe slip, release residual stress, suitable for large-diameter and large-size steel pipes, with low energy consumption and short cycle, but the stress elimination rate is lower than that of heat treatment.

• Natural aging: store outdoors to allow stress to release slowly and naturally, with a long cycle and limited effect, only used as an auxiliary method.

• Optimize heat processing techniques: Control the heating speed of the tube billet to ensure uniform furnace temperature; adopt controlled cooling after hot rolling to avoid rapid cooling and uneven cooling, thereby reducing thermal stress.
• Optimize plastic deformation techniques: Improve the design of piercing, tube rolling, and constant reduction die configurations to ensure uniform deformation along the wall thickness and length direction; reasonably distribute the deformation amount in each pass to avoid excessive deformation in a single pass.
• Welding process optimization (welded pipes): Control the welding heat input, adopt preheating and postheating processes to reduce the cooling rate, and minimize the temperature gradient at the weld seam; use symmetrical welding and multi-pass welding to counteract some deformation and stress.
• Cold processing technique optimization: Distribute the deformation amount in cold drawing/cold rolling reasonably, schedule intermediate annealing to eliminate work hardening and stress during processing, and then proceed with subsequent cold processing.
• Straightening process optimization: Prioritize using hot straightening instead of cold straightening, optimize the straightening reduction amount and roll pattern, reduce the stress introduced by straightening; use precision sawing instead of shearing to reduce stress concentration at the pipe ends.