Will the high-temperature resistance of 304 stainless steel pipes be affected by other factors

The high-temperature resistance of 304 stainless steel tubes (which usually refers to their mechanical stability, oxidation resistance, and structural stability under high temperatures) is not constant. It is influenced by various factors such as composition control, heat treatment status, processing techniques, usage environment, and stress conditions. These factors directly affect the performance under high temperatures by altering their microstructure or surface state. The specific impacts are as follows: 

1. Component Control: The content of core elements determines the high-temperature resistance foundation. 

The high-temperature resistance of 304 stainless steel is mainly supported by "chromium (Cr ≥ 18%) and nickel (Ni ≥ 8%)". Any fluctuations in the composition will directly affect its high-temperature stability. 

- Insufficient chromium content: Chromium is a key element for forming the high-temperature oxide film (Cr₂O₃). If the chromium content is lower than 18% (for example, the chromium content of inferior 304 is only 16%-17%), the surface oxide film at high temperatures is prone to rupture, unable to prevent further oxidation of the substrate, resulting in rapid "oxide flaking" of the pipe material above 600°C, and wall thickness reduction;

- Low nickel content: Nickel can stabilize the austenite structure and prevent the transformation of austenite to ferrite or σ phase at high temperatures. If the nickel content is lower than 8% (such as non-standard 304 with manganese substitution for nickel), during long-term use at 700-800°C, the austenite is prone to decompose into brittle σ phase, causing a sudden drop in mechanical properties of the pipe material and cracking even under slight external force;

- Excessive carbon content: When the carbon content exceeds 0.08% (the upper limit of standard 304), at 500-850°C high temperatures, carbon easily combines with chromium to form Cr₂₃C₆ carbides, which not only increases the risk of intergranular corrosion but also consumes the chromium content at the grain boundaries, weakening the high-temperature oxidation resistance, resulting in a double decline in corrosion resistance and strength of the pipe material in the medium- and high-temperature range (500-700°C). 

II. Heat treatment state: Affects the stability of the structure and the elimination of stress 

The heat treatment (especially the solution treatment) of 304 stainless steel tubes directly determines their microstructure at high temperatures: 

- Failure to perform solidification or incomplete solidification: Before leaving the factory, the material was not subjected to 1050-1100℃ solidification followed by water cooling treatment, or the heating temperature was insufficient and the cooling speed was slow. This would result in residual undissolved carbides (such as Cr₂₃C₆) in the matrix and coarse grains. When used at high temperatures, the residual carbides would become "stress concentration points", accelerating the expansion of intergranular cracks; the coarse grains would reduce the high-temperature plasticity, and the pipe would easily undergo "creep deformation" (such as the bending of steam pipelines after long-term use at high temperatures);

- Failure to perform stress relief treatment after welding: If no 300-400℃ low-temperature stress relief treatment was performed after welding, the weld heat-affected zone would still retain welding stress. Under high temperatures, the stress and oxidation would combine, causing "stress corrosion cracking", especially above 600℃, the cracks would rapidly expand along the weld grain boundaries, leading to sudden pipe rupture. 

III. Processing Technology: Cold processing or shaping defects reduce the high-temperature performance. 

The processing methods and forming quality during the pipe manufacturing process can lead to "hidden damages", which in turn affect the high-temperature stability: 

- Excessive cold processing: Cold processing methods such as cold drawing and cold rolling can cause "work hardening" in 304 stainless steel. If the cold processing deformation exceeds 30% (such as thin-walled pipe cold contraction forming) and intermediate annealing is not performed, the processing stress at high temperatures will be released, resulting in "high-temperature recovery deformation" (such as pipe diameter contraction and uneven wall thickness) of the pipe material; at the same time, excessive cold processing can refine the grains, but it may also introduce dislocation accumulation, which is prone to intergranular slip at high temperatures and reduces strength;

- Forming defects: When bending pipes using "high-temperature hot bending" (heating temperature > 800℃) and with uneven cooling, the bending section will exhibit "uneven grain size"; when welding, if the backside does not have argon gas protection, the inner wall of the weld seam will form an "oxide layer" (a mixture of FeO and Cr₂O₃). These defects will become the "starting point of corrosion and oxidation" at high temperatures, leading to local performance degradation, such as the bending section oxidizing and perforating first at temperatures above 700℃. 

IV. Operating Environment: Degradation of Acceleration Performance Due to Medium and Atmosphere Factors 

The operating environment under high temperatures (especially in corrosive atmospheres) will significantly accelerate the performance degradation of 304 stainless steel pipes: 

- Chlorine/Sulfur Atmosphere: If used in an environment containing chloride ions (such as high-temperature salt water vapor) and sulfur ions (such as industrial exhaust gas), high temperatures will accelerate the destruction of the oxide film by chloride ions, leading to "high-temperature pitting corrosion", and the corrosion products (such as FeCl₃) will catalyze further oxidation, causing local corrosion perforation of the pipe material even below 500°C;

- Reductive Atmosphere: In hydrogen, carbon monoxide, etc., reductive atmospheres, at high temperatures, the surface Cr₂O₃ oxide film will be reduced to volatile CrO₃, losing its protective effect, and the substrate will be directly exposed to the atmosphere, resulting in "high-temperature hydrogenation corrosion", causing the pipe material to become brittle and lose strength;

- Cyclic Cold-Hot Shock: If frequently experiencing "high temperature (such as 800°C) - normal temperature" cycles (such as intermittent heating equipment pipelines), the surface oxide film will repeatedly crack and regenerate due to thermal expansion and contraction, accelerating the peeling of the oxide film, and at the same time, "thermal stress fatigue" will occur inside the substrate. After long-term use, net-like cracks will appear on the surface of the pipe material, and the high-temperature performance will sharply decline. 

V. Stress Conditions: Failure Caused by the Combined Effects of High Temperature and Stress 

The bearing capacity state of 304 stainless steel tubes under high temperatures will directly affect their performance limits: 

- Excessive static load: If a load exceeding the allowable stress is sustained for a long time under high temperature (such as 600°C) (e.g., when a high-pressure steam pipeline operates under overpressure), "high-temperature creep" will occur - the slow sliding of atoms leads to continuous deformation of the pipe material. Over time, the wall thickness will thin and the pipe diameter will expand, eventually exceeding the safety limit.

- Dynamic vibration stress: If proper vibration reduction measures are not taken during installation (such as placing high-temperature pipelines close to fans or pump bodies), the elastic modulus of the pipe material decreases under high temperature, reducing its vibration resistance. The vibration stress will accelerate the expansion of grain boundaries cracks, especially at stress concentration areas such as welds and bent pipes. Fatigue fractures are prone to occur, and the fracture temperature will decrease with the increase in vibration frequency. 

Core Summary 

The high-temperature resistance of 304 stainless steel tubes is the result of the combined effects of composition, process, environment and stress: standard composition + thorough solution treatment + defect-free processing are the foundation for its stable use within 800°C for a short period and 600°C for a long period; if there are non-compliant compositions, process defects or harsh high-temperature corrosive environments, the upper limit of its high-temperature resistance will significantly decrease, and even oxidation, corrosion or deformation failure may occur at 400-500°C. Therefore, in practical applications, specific working conditions (temperature, medium, load) need to be considered to control these influencing factors specifically, in order to avoid degradation of high-temperature performance.