dissimilar steel welding

Welding P91 (martensitic heat-resistant steel) and 316L (austenitic stainless steel) is a classic and extremely challenging case in dissimilar steel welding. The biggest challenge in selecting welding materials can be summarized as a core contradiction:dissimilar steel welding

How to simultaneously address the significant differences in chemical composition, physical properties, and metallurgical behavior of the two base materials in a single weld joint, while ensuring the long-term safe operation of the joint under high-temperature and high-stress conditions.

一：Specifically, this core contradiction gives rise to the following three key challenges, which are also problems that must be solved when selecting welding materials:

Challenge 1: Chemical dilution and carbon migration

This is the most critical metallurgical challenge.

❶ Composition dilution and harmful phase formation:

⑴ Problem: During welding, the metals of P91 and 316L will simultaneously melt into the weld. P91 is a high-Cr (9%), high-C martensitic steel, while 316L is an ultra-low-carbon austenitic steel. If austenitic stainless steel welding materials (such as E309L) are used, the weld will be “diluted” by P91, and its chemical composition may fall into the martensite + austenite two-phase region. This will lead to the formation of a hard and brittle martensite structure in the weld, drastically reducing its toughness and increasing the risk of cold cracking.

⑵ Carbon migration: During subsequent high-temperature service or post-weld heat treatment, carbon atoms will diffuse from the P91 side, which has higher carbon activity (but relatively lower Cr content), to the weld/316L side, which has lower carbon activity (but very high Cr content). This results in:

● P91 side heat-affected zone: the formation of a decarburized and softened “ferrite band,” significantly reducing strength and creep strength.

● Weld/316L side boundary: the formation of a carbide-hardening “carbide precipitation zone,” increasing brittleness and making it prone to becoming a crack initiation point.

Challenge 2: Significant Differences in Physical Properties

❶ Mismatch in Thermal Expansion Coefficients:

● Problem: The thermal expansion coefficient of austenitic steel (316L) is approximately 30%–40% higher than that of martensitic steel (P91).

● Consequences: During welding heating and cooling, and in subsequent high-temperature start-stop cycles, the different expansion and contraction of the base materials on both sides generates significant thermal stress in and around the weld. This alternating thermal stress is the root cause of thermal fatigue cracks, seriously threatening the long-term lifespan of the joint.

❷ Differences in Thermal Conductivity:

● Problem: P91 has lower thermal conductivity than 316L.

● Consequences: This leads to uneven heat distribution during welding, exacerbating the stress state of the joint and making the control of welding parameters more complex.

Challenge 3: The Dilemma of Post-Weld Heat Treatment

❶ P91 Requirements vs. 316L Risks:

● P91 Requirements: P91 steel must undergo high-temperature tempering (~760°C) after welding. The purpose is to transform the coarse martensite in its heat-affected zone into tempered martensite, restoring toughness and reducing hardness to prevent cold cracking.

● 316L Risks: If austenitic stainless steel is held at this temperature range for an extended period, it will promote the precipitation of chromium carbide along grain boundaries, leading to sensitization and making it highly susceptible to intergranular corrosion in corrosive environments. Simultaneously, brittle σ phases may precipitate, reducing toughness and corrosion resistance.

二: dissimilar steel welding：Strategies and Solutions for Welding Consumable Selection

Faced with the above challenges, the principle for selecting welding consumables is “avoiding forbidden zones and acting as a buffer.” The current standard solution in the industry is:

Preferred Solution: Use nickel-based alloy welding consumables (such as ERNiCr-3/625 alloy).

This is the most common and reliable solution for welding dissimilar joints of P91 and 316L. The reasons are as follows:

❶ Solving metallurgical problems:

● High nickel content: Nickel is miscible with iron but does not form carbides with carbon. Using nickel-based welding materials can effectively block carbon migration, preventing decarburization and softening on the P91 side and carburization hardening at the weld boundary.

● Stable microstructure: The weld metal is a fully austenitic structure, but due to its high nickel content, it possesses excellent toughness and crack resistance, avoiding the risk of forming hard and brittle martensite when using stainless steel welding materials.

❷ Alleviating physical property mismatch:

● The coefficient of thermal expansion of nickel-based alloys is between that of P91 and 316L, providing a good transition and buffering effect, significantly reducing thermal stress caused by differences in thermal expansion, thereby improving the joint’s resistance to thermal fatigue.

❸ Resolving heat treatment contradictions:

● Nickel-based alloys themselves do not require the post-weld heat treatment required for P91. In practice, only the P91 side can be locally heat-treated, with strict control over the heat treatment temperature and holding time, minimizing the impact on the 316L side and the nickel-based weld. This ensures the toughness of the P91 side while minimizing the risk of 316L sensitization.

三: Secondary Option (Under Specific Conditions): Using high-nickel austenitic stainless steel welding consumables (such as E309L)

This can also be used in some less demanding operating conditions, but its limitations must be recognized:

● Risks: As mentioned earlier, there is a risk of carbon migration and formation of brittle phases. The long-term performance (especially creep performance) and reliability of the joint at high temperatures are not as good as those of nickel-based welding consumables.

● Applications: Typically used in non-critical parts or in applications where service conditions (such as temperature and pressure) are not so harsh.

四. dissimilar steel welding：Conclusion

The biggest challenge in welding dissimilar steels, P91 and 316L, lies in designing or selecting a weld metal with chemical composition and physical properties that can act as a “buffer zone and isolation zone,” simultaneously addressing four major challenges: carbon migration, harmful phase formation, immense thermal stress, and the conflicting requirements of post-weld heat treatment.

Currently, using nickel-based alloy welding materials is the best solution to this systemic challenge because it comprehensively addresses these issues from both metallurgical and physical perspectives, ensuring the integrity and lifespan of dissimilar steel joints under harsh operating conditions.