What is double certified stainless steel pipe?
What is double-certified stainless steel pipe?
Dual Grade/Certified means that the same stainless steel pipe meets the chemical composition and mechanical property requirements of two similar grades (such as 304 and 304L, 316 and 316L). Its core value lies in "one material for multiple uses", which not only retains the strength of the standard grade, but also has the corrosion resistance of the low-carbon grade, which is especially suitable for welding-sensitive scenarios.
304/304L Dual grade Stainless Tube
Stainless steel has become a core material in the industrial field due to its corrosion resistance and versatility, among which 304 and 304L are the most commonly used austenitic stainless steel grades. Despite the similar names, there are key differences between the two in composition, performance and application scenarios.
304/304L
Differences in chemical composition
304 stainless steel (UNS S30400):
Carbon content ≤0.08%, and contains 18-20% chromium (Cr), 8-10.5% nickel (Ni). The higher carbon content gives it better strength at room temperature.
304L stainless steel (UNS S30403): "L" stands for low carbon, with a carbon content of ≤0.03%. The chromium and nickel content is similar to 304, but the low carbon design makes it more resistant to intergranular corrosion.
Key point: 304L reduces the risk of chromium carbide precipitation during welding or high temperature environments by reducing the carbon content.
Corrosion resistance comparison
304 stainless steel:
It has excellent corrosion resistance in normal environments (such as fresh water and atmosphere), but when it is welded or exposed to high temperatures of 450-850°C for a long time, carbon and chromium are easily combined to form chromium carbide, resulting in "sensitization" and reducing the ability to resist intergranular corrosion.
304L stainless steel:
The low carbon property significantly reduces the risk of sensitization, and it can maintain corrosion resistance without additional heat treatment after welding, which is especially suitable for corrosive environments such as chemical and marine.
Welding performance
304 stainless steel:
After welding, the heat-affected zone may become brittle due to the precipitation of chromium carbide, and needs to be restored through solution annealing (rapid cooling), which increases the complexity of the process.
304L stainless steel:
The low-carbon design fundamentally avoids sensitization problems and can be used directly after welding. It is the first choice for large welded structures (such as pipelines and storage tanks).
Mechanical properties
Room temperature strength:
304 is more stable at high temperatures (>500°C), while 304L has a slightly weaker high temperature strength due to its low carbon content.
Typical application scenarios
304 stainless steel:
Food processing equipment, building decoration, household appliances, low-temperature pressure vessels and other non-welding or low-corrosion environments.
304L stainless steel:
Chemical pipelines, pharmaceutical equipment, marine engineering, welded structural parts, and environments that may be exposed to chlorides (such as salt water).
Summary:
The choice between 304 and 304L depends on the specific working conditions:
Choose 304: focus on room temperature strength, non-welded parts, low temperature applications.
Choose 304L: welding processing is required, exposed to corrosive environments or high temperature sensitive scenes.
Q1: Can 304 and 304L be used interchangeably?
If the environment is not corrosive or requires welding, 304 can replace 304L; otherwise, be cautious and give priority to 304L.
Q2: Which material costs more?
The prices of the two are similar, but 304L may reduce the overall cost due to reduced post-weld treatment.
Q3: How to distinguish between the two?
The carbon content needs to be confirmed by chemical composition testing (such as spectrometer), and the appearance cannot be directly identified.
Dual certified 316/316L Stainless Tube
316 and 316L stainless steel are two representatives of superior performance in austenitic stainless steel, widely used in chemical, medical, marine engineering and other fields. Although the names are similar, there are significant differences between the two in composition, performance and application scenarios.
316/316L
Differences in chemical composition
316 stainless steel (UNS S31600):
Carbon content ≤0.08%, and contains 16-18% chromium (Cr), 10-14% nickel (Ni), and 2-3% molybdenum (Mo). The addition of molybdenum significantly improves its corrosion resistance in chloride environments.
316L stainless steel (UNS S31603):
"L" stands for low carbon, with a carbon content of ≤0.03%, and other elements (Cr, Ni, Mo) are similar to 316. The low-carbon design reduces the risk of chromium carbide precipitation during welding or high temperatures, thereby improving resistance to intergranular corrosion.
Corrosion resistance comparison
316 stainless steel:
Performs well in acidic, alkaline and chloride environments (such as seawater), but when welded or exposed to high temperatures of 450-850°C for a long time, carbon combines with chromium to form chromium carbide, resulting in "sensitization" and may cause intergranular corrosion.
316L stainless steel:
The low carbon characteristics significantly reduce the risk of sensitization, and corrosion resistance can be maintained without heat treatment after welding, which is especially suitable for harsh environments such as seawater equipment and chemical pipelines.
Mechanical properties and processing characteristics
Strength:
Due to its high carbon content, 316 has a better tensile strength (about 620 MPa) than 316L (about 480 MPa), which is suitable for high-strength demand scenarios.
Welding performance:
The low-carbon design of 316L makes its welding performance better, reducing the need for welding cracks and subsequent solution annealing, and is suitable for large welded structures (such as storage tanks and pipelines).
High temperature performance:
316 has more stable strength in high temperature environments above 500°C, while 316L has a slightly lower high temperature strength due to its low carbon content, but has stronger resistance to stress corrosion cracking.
Application scenarios and material selection recommendations
Occasions where 316 is preferred:
High-temperature equipment (such as heat treatment furnaces)
Non-welded parts with high strength requirements (such as mechanical parts)
High-temperature welded parts that need to avoid sensitization risks.
Cost and market status
Price: Due to the low-carbon process and welding convenience, the cost of 316L is slightly higher than that of 316, but the difference is small (about 5%-10%)112.