What are the performance differences between 316L stainless steel welded pipes and 304 stainless

The performance differences between 316L stainless steel welded pipes and 304 stainless steel welded pipes under high temperatures mainly lie in aspects such as oxidation resistance, mechanical property stability, and corrosion resistance. The specific comparison is as follows: 

1. Differences in High Temperature Oxidation Resistance

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Typical Data:

After being exposed to air at 800℃ for 1000 hours, the oxidation weight gain of 316L was 0.12%, and that of 304 was 0.35%.

In high-temperature flue gas containing Cl⁻ (such as in power plant desulfurization systems), the retention time of the oxide layer integrity of 316L was 2-3 times longer than that of 304. 

II. Changes in Mechanical Properties at High Temperatures

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Application Impact:

316L can withstand higher internal pressure in high-temperature and high-pressure reactors (such as hydrogenation units) without undergoing creep deformation.

304 is prone to σ phase precipitation when used for a long time at temperatures above 600℃, resulting in a decrease in toughness. 

III. Comparison of High Temperature Corrosion Resistance

1. Resistance to Cl⁻ Corrosion (such as seawater desalination, chemical pipeline)

316L:

In 600℃ flue gas condensate containing Cl⁻ (Cl⁻ 500 ppm), the annual corrosion rate is only 0.3-0.8mm.

The critical crevice corrosion temperature (CCT) reaches 60-80℃, which is 20-30℃ higher than that of 304.

304:

Under the same conditions, the corrosion rate is 1.5-3mm/year, and the CCT is only 40-50℃.

2. Resistance to Sulfur Corrosion (such as petroleum cracking furnace)

316L:

It has excellent resistance to H₂S/CO₂ corrosion, with an annual corrosion rate of less than 0.1mm at 800℃.

It can inhibit sulfide stress corrosion cracking (SSCC), suitable for the transportation of sulfur-containing oil and gas.

304:

In environments containing H₂S, sulfide corrosion is prone to occur, and the temperature needs to be limited to <500℃.

3. Oxidation Resistance and Carburization (such as combustion chamber components)

316L:

In the combustion environment of sulfur-containing fuels (such as power plant boilers), a stable Cr₂(MoO₄)₃ layer forms on the surface, reducing oxidation weight loss.

Its anti-carburization ability is superior to 304, suitable for high-temperature rich fuel combustion scenarios.

304:

At high temperatures, it is prone to combine with carbon to form Cr₂3C6, resulting in surface decarburization and brittleness. 

IV. Intergranular Corrosion Sensitivity

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Case:

The 316L welded pipe did not experience intergranular corrosion during the high-temperature sterilization (121℃ steam circulation) in the pharmaceutical factory within 5 years; while the 304 welded pipe developed cracks within 2 years under the same conditions. 

V. High Temperature Processing and Weldability

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VI. Selection Suggestions

1. For high-temperature scenarios, prioritize 316L:

Temperature > 600℃ and containing Cl⁻/S (such as incinerators, flue gas desulfurization systems)

Requires long-term oxidation resistance and anti-rust properties (nuclear power steam pipelines, aircraft engine components)

2. Consider 304 for high-temperature scenarios:

Temperature ≤ 650℃ and without Cl⁻ (such as ordinary boiler feedwater pipelines)

Dry gas transportation (such as high-temperature steam pipelines) 

VII. Cost vs. Lifespan Trade-off

Cost: The price of 316L is 30% - 50% higher than that of 304, but its lifespan at high temperatures can reach 2 - 3 times that of 304.

Overall Benefits: In high-temperature corrosive environments, the maintenance cost of 316L is lower (reducing the frequency of shutdown and maintenance). 

Summary

316L, through the incorporation of molybdenum and a low-carbon design, forms a more stable composite passivation film at high temperatures, significantly outperforming 304 in terms of oxidation resistance, creep resistance, and resistance to Cl⁻/S corrosion. It is suitable for high-temperature and highly corrosive core scenarios. On the other hand, 304, due to its economic nature, is applicable to conventional high-temperature environments with lower temperatures or weaker corrosiveness.