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When working on construction projects that involve stainless steel, one common question arises: can you mix different stainless alloys within the same construction? What guidelines should you follow to ensure success? Let’s explore some scenarios to understand this better.
Combining Type 316L and Type 304L in Piping Systems
Imagine you work for an equipment fabricator and you've quoted a job requiring specific stainless alloys. However, not all parts (plates, pipes, fittings) are available in the required alloy, or the delivery times are too long. Alternatively, you might be the customer facing this issue. Here’s what you need to consider:
Using Type 316L for components in a Type 304L piping system is generally feasible. Type 316L typically matches or exceeds the corrosion resistance of 304L, though there are exceptions, such as with nitric acid. Both alloys are austenitic and have identical mechanical properties, making welding straightforward with either 304L or 316L filler metal.
However, incorporating 304L components into a 316L system is riskier if 304L exhibits a significantly higher corrosion rate. In such cases, galvanic effects can occur, with a small anode (304L) and a large cathode (316L), necessitating caution.
Mixing Different 6Mo Alloys
There are various 6% Mo alloys (e.g., UNS S31254, N08367, N08926) that offer similar corrosion resistance and mechanical properties and can be welded with the same filler metals. Generally, these alloys can be mixed in construction. However, alloys like S34565, which have higher strength, may pose welding challenges due to the potential for distortion when joining to lower strength alloys.
Combining Lean and Standard Duplex Alloys
Lean duplex alloys are often used in tank construction, but sourcing attachments (pipes, fittings, flanges) can be challenging. Mixing different versions of lean duplex alloys is typically possible if their corrosion resistance and mechanical properties are suitable. Duplex 2205 components are more readily available and can be used with lean duplex tanks. However, careful consideration is needed as 2205 may have lower strength than some lean duplex alloys. A full evaluation of welding properties and thermal expansion differences is necessary before approval.
Mixing Ferritic and Austenitic Stainless Steel
Joining ferritic and austenitic stainless steels is possible, but the significant difference in thermal expansion, especially during welding, must be accounted for. Ferritic alloys are weaker than austenitic ones at high temperatures, leading to potential distortion during cooling. If equipment operates at extreme temperatures, differences in toughness and high-temperature strength are critical. Additionally, from a corrosion perspective, while austenitic alloys may be prone to chloride stress corrosion cracking, ferritic alloys are highly susceptible to hydrogen cracking.
Mixing and matching stainless alloys in construction can be successful if you carefully evaluate the specific properties and compatibility of the materials involved. By considering factors such as corrosion resistance,weld-ability, mechanical properties, and the intended application environment, you can ensure the durability and performance of your construction projects.
