Analysis Of Cold Treatment And Stress Relief Process For Seamless Steel Pipes

Seamless pipe cold treatment generally refers to deep cold treatment (low-temperature treatment, usually in a liquid nitrogen environment at -70℃ to -196℃), and is often used in conjunction with quenching, tempering, cold rolling/cold drawing processes. The core function is:

1. Stabilize structure and dimensions：

Eliminate the remaining austenite and promote its transformation into stable martensite to prevent dimensional deformation and accuracy drift during the subsequent use and processing of the pipe, especially for precision hydraulic pipes, instrument pipes, and bearing pipes.

2. Enhance hardness, wear resistance and strength：

The organization becomes denser, the surface hardness and wear resistance are enhanced, and the service life of the pipe is prolonged.

3. Release micro internal stress：

Reduce the micro-stress caused by lattice distortion during cold drawing and quenching, and lower the risks of cracking and warping in the later stage.

4. Improve toughness and fatigue performance：

Refine the grain structure, reduce stress concentration points, and enhance the fatigue and impact resistance of the pipe material.

Distinguishing point:

Cold treatment ≠ normal temperature cold drawing/cold rolling. Cold treatment is a low-temperature deep cooling process and is an auxiliary procedure following heat treatment.

(1) Pre-treatment Preparation

1. Pre-process Control
The pipe material should undergo quenching/tempering/cold drawing first, then be cooled to around room temperature (20-40℃) before being placed in the furnace. It is strictly prohibited to directly introduce high-temperature into low-temperature equipment to prevent excessive temperature difference and cracking.

2. Sorting and visual inspection

Remove pipes with cracks, folds, uneven wall thickness, surface scratches and bumps. Sort by material, specification and batch to avoid mixing materials.

3. Cleaning and Drying

Thoroughly remove surface oil stains, moisture and cutting fluid. Freezing water can crack the pipe wall and corrode the inner wall. Both ends of the long pipe are sealed to prevent low-temperature media from entering the pipe.

4. Furnace Loading Standards
Place the pipe material layer by layer, leaving gaps, without stacking and squeezing. Support thin and long pipes to prevent self-weight bending and deformation under low temperature.

5. Process Parameter Pre-setting
Set the target temperature and holding time according to the material： carbon steel/low alloy seamless pipes are commonly -70 to -120℃, high alloy/bearing pipes -150 to -196℃.

(II) Process Control of Cold Treatment

1. Slow Cooling Rate
Avoid rapid cooling. Control the cooling rate at 1 to 3℃/min. Large temperature differences can cause new temperature stresses, leading to microcracks.

2. Adequate Insulation
After the entire pipe reaches the set low temperature, maintain insulation for 2 to 4 hours (for pipes with large wall thickness and large diameter, extend to 4 to 6 hours). Ensure that the internal and external structures fully transform to avoid uneven performance inside and outside.

3. Sealed Environment and Uniform Medium
The liquid nitrogen/cold air circulation is uniform, eliminating local overcooling; continuously monitor the temperature to avoid extremely low temperatures.

4. Prohibit External Impact and Movement
At low temperatures, the toughness of steel temporarily decreases, and it is prone to latent cracks under stress.

(III) Post-cooling Operations

1. Slow Cooling (Key)
After unloading, allow the pipe to naturally warm up to room temperature. Do not expose it to direct sunlight, blow hot air on it, or immerse it in water for rapid heating. This is to prevent secondary stress caused by alternating cold and hot temperatures. The duration of cooling depends on the wall thickness: for thin-walled pipes, 2 to 4 hours; for thick-walled pipes, more than 6 hours.

2. Timely Tempering/Ageing
After deep cooling, it is necessary to perform low-temperature tempering (at 120 to 200°C) to eliminate the small amount of low-temperature stress generated during the cold treatment. This also helps restore the toughness of the pipe material and prevent brittleness.

3. Re-inspection and Sorting
Inspect the appearance, straightness, and roundness of each pipe individually; take samples for hardness, metallographic analysis, and flaw detection to check for micro-cracks and deformation.

4. Protection and Storage
After drying and cooling, store the pipes in a warehouse to avoid exposure to the elements, rain, and bumps.
 

The main sources of stress in seamless pipes: hot rolling, cold drawing/cold rolling, quenching, welding, straightening, mechanical processing. The stress is divided into macroscopic residual stress and microscopic lattice stress.

1. Preventing Future Failures
The release of residual stress can cause the pipe to bend, twist, and have size deviations; the superposition of stress is likely to cause pipe wall cracking and burst, which poses extremely high risks for high-pressure pipelines and fluid transportation pipes.

2. Stabilizing Processing Precision
During subsequent machining processes such as turning, threading, cutting, and bending, the stress release will lead to deformation of the processed product. Removing stress can ensure the long-term stability of the finished product's dimensions.

3. Enhancing Mechanical Properties
Eliminating stress concentration improves the pipe's resistance to fatigue, corrosion, and brittle fracture, significantly enhancing safety in high-pressure and low-temperature, and cyclic loading conditions.

4. Improving Subsequent Surface Treatment
After stress removal, the pipe's structure becomes uniform. Processes such as galvanizing, phosphating, and coating are less likely to have peeling or pinholes.
 

1. Low-temperature stress relief annealing (most commonly used, mainstream process)

Applicable to: Cold-drawn pipes, quenched and tempered pipes, precision seamless pipes, medium and small-diameter pipes

• Process: Heat to 180~300℃, hold for a period of time, then slowly air-cool or furnace-cool
• Principle: Atoms diffuse slowly, releasing lattice distortion stress, without changing the hardness and strength of the matrix, only eliminating residual stress.
• Key points: The temperature of the entire furnace should be uniform, the holding time is 30~60 minutes per 10mm of wall thickness; the cooling rate should be ≤ 2℃/min.

2. High-temperature stress relief annealing (complete stress relief, large deformation in pipes)

Applicable to: Large-diameter thick-walled pipes, hot-rolled pipes, elbows, processed pipe fittings, welded pipes

• Process: Heat to 550~650℃ (below the transformation temperature), hold for a while, then slowly cool in the furnace (take out of the furnace when the temperature drops below 200℃).
• Features: The stress relief effect is the most thorough, almost eliminating all macroscopic and microscopic stresses; disadvantage is that it slightly reduces hardness and strength, suitable for structural pipes and transportation pipes with low hardness requirements.

3. Vibration Aging (Physical Stress Relief, No Heating Required)

Applicable to: Ultra-long pipes, large-diameter pipes, finished pipes that do not allow heating to change color or soften.

• Principle: By resonance, the internal stress of the pipe material is relaxed and redistributed.
• Advantages: No thermal deformation, no oxidation decarburization, low energy consumption; Disadvantages: Limited for eliminating deep stress, mostly used as an auxiliary stress relief method.

4. Natural Aging (Low Cost, Only for Low-Requirement Pipes)

Applicable to: Ordinary low-pressure structural pipes, non-precision pipes

• Principle: Placed at room temperature for a long time, stress gradually releases naturally.
• Disadvantages: The period is extremely long (several weeks to several months), the stress relief is not thorough, and it is rarely used in industrial batch production.

5. Deep cold treatment (previously referred to as cold treatment, serving as an auxiliary stress relief method)

It is usually used in conjunction with tempering. First, the structure is stabilized through deep cold treatment, and then tempering is carried out at a lower temperature. The combined effect of stress relief is more effective.
 

Combining cold treatment + stress relief + forming, there are two main pipeline processing methods:

Solution 1: Precision cold-drawn seamless pipes (used for hydraulic, instrumentation, and precision machinery)

1. Hot-rolled pipe billet → Cold drawing/cold rolling forming (resulting in cold work stress)
2. Low-temperature stress relief annealing (200~280℃): Eliminate cold drawing stress
3. Straightening, cutting the head, and finishing
4. Deep cold treatment (-80~-120℃, slow cooling + sufficient holding time): Stabilize the structure and enhance hardness and wear resistance
5. Secondary low-temperature annealing (150~200℃): Eliminate cold treatment stress and restore toughness
6. Non-destructive testing, size inspection, and finished product storage

Effect: Stable dimensions, uniform hardness, excellent fatigue performance, and no deformation over a long period of use.

Solution 2: High-pressure/ thick-walled hot-rolled seamless pipes (oil and gas, boilers, fluid transportation)

1. Hot rolling forming → Rough straightening
2. High-temperature stress relief annealing (580 - 630℃, slow cooling in furnace): Completely eliminate the stresses from hot rolling and straightening
3. Machining, beveling, bending pipe, etc. for further processing
4. Product flaw detection, hydraulic test
 

Effect: Resistant to cracking, withstands high pressure, stable stress, suitable for heavy-duty conditions.
 

1. Core of Cold Treatment: Maintain dimensions, enhance hardness, promote tissue transformation. The entire process must involve slow cooling, slow re-heating, and subsequent re-tempering; otherwise, it is prone to cracking.

2. Core of Stress Relief: Prevent deformation, cracking, and dimensional drift. For stress relief during heat treatment, prioritize "high-temperature annealing (complete)" and "low-temperature tempering (to maintain hardness)." 

3. Logic for Performance Enhancement: Perform stress relief during forming → Implement cold hardening → Conduct secondary stress relief. The combination of these three processes takes into account strength, hardness, toughness, and dimensional stability.