1. Thermal Expansion: Structural Displacement and Stress Concentration Caused by Temperature Changes
Stainless steel is a metal material with high coefficient of thermal expansion, which means that it will obviously elongate at high temperature, but it will shrink rapidly when it is rapidly cooled. If the structural design cannot effectively absorb these displacements, it will bring the following problems:
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Loose or deformed joint
In large-scale cold storage equipment, refrigeration pipelines or offshore platforms, repeated temperature difference cycles will make flanges, welds and bolted connections bear additional thermal stress.
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Internal stress accumulation of equipment
Thermal expansion and cold contraction will gradually accumulate stress in the material, and long-term action will lead to microcrack initiation, which will eventually lead to structural fatigue or rupture.
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The change of assembly clearance affects performance
Special equipment, such as low-temperature cabin and high-pressure vessel, needs very accurate sealing structure. The gap change caused by temperature difference may affect the safety or sealing performance.
Therefore, expansion joints, flexible pipe sections or reasonable expansion space should be considered in the design stage to avoid stress concentration.
2. Stress corrosion risk: temperature difference and corrosive environment work together
Although stainless steel has good corrosion resistance in marine engineering or humid and high salt environment, extreme temperature difference may increase the risk of stress corrosion. The reasons include:
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The surface passivation film is damaged
Temperature change may lead to local instability of the passivation film, which makes chloride ions easier to penetrate, thus accelerating pitting corrosion or crevice corrosion.
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Alternating cold and hot deepens the infiltration of corrosive medium.
When the material encounters hot and cold cycles, micro-cracks are repeatedly opened and closed, providing access channels for corrosive media.
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Superposition of residual stress and environmental stress
Under the action of wave impact, mechanical vibration or internal pressure, stress corrosion cracking (SCC) is more likely to occur due to the superposition of external force and thermal stress.
In marine structures, seawater heat exchangers, condensers and coastal equipment, duplex stainless steel or nickel-based alloys are often used to enhance chloride corrosion resistance.
3. Cold brittleness: the toughness of materials decreases at low temperature.
When the temperature drops to a certain range, the toughness of some stainless steels will decrease, that is, the materials are more prone to brittle fracture under impact or sudden stress. The main performance is as follows:
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The impact toughness is obviously reduced
For example, austenitic stainless steel usually performs well, but the toughness of ferritic or martensitic stainless steel decreases obviously at low temperature, so it should be cautious when used in low temperature equipment.
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The welding area is more prone to brittle fracture
Weld metal and heat affected zone (HAZ) may show different embrittlement trends at low temperature, so welding materials and processes need to be strictly controlled.
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Material selection should be suitable for actual working conditions
For example, the refrigeration system of cold storage, liquefied gas equipment and outdoor structures in extremely cold areas 316L prefer to use 304,316l or special low-temperature steel with excellent low-temperature toughness.
4.Suggestions in engineering application
Based on the above performance changes, stainless steel should pay attention to the following engineering strategies in extreme temperature difference scenarios:
- Choose an alloy suitable for low temperature and high temperature stability, such as austenitic stainless steel or duplex stainless steel.
- Reserve thermal expansion space in the design stage to avoid stress concentration.
- Use materials with stronger chloride corrosion resistance or adopt surface strengthening treatment.
- Strengthen welding process management in key parts to improve the overall structural reliability.
- Conduct periodic maintenance inspection, especially for welds, flanges and long-term stress areas.
