Understanding Ferritic vs Austenitic vs Martensitic Stainless Steel

Industries have trusted stainless steel to ensure strength and corrosion resistance in their metal structures. However, the term “stainless steel” does not communicate the range of properties offered by this diverse metal. To better communicate its variation, stainless steel can be classified as either ferritic, austenitic, and martensitic stainless steel.

Stainless steel generally consists of elements like carbon, nickel, molybdenum, aluminum, copper, titanium, and at least 10.5 percent chromium. The different variations of stainless steel are developed by altering its chemical composition and crystal structure. These stainless steel types offer versatility in attributes that facilitate their use in a wide range of industries. However, when deciding which type of stainless steel to use, the comparison of ferritic vs. austenitic vs. martensitic stainless steel is important.

Ferritic vs. Austenitic vs. Martensitic Stainless Steel

Choosing the correct type of stainless steel can be difficult, and it will require a thorough investigation into your own manufacturing needs, as well as the benefits and metallurgical challenges inherent in each type of stainless steel. Here, we will discuss ferritic vs. austenitic vs. martensitic stainless steel in detail.

Ferritic Stainless Steel

As with all stainless steel types, ferritic stainless steel is named for the microscopic crystal structure associated with the metal: in this case, ferrite. It has a body-centered cubic (BCC) grain structure, which gives ferritic stainless steel its magnetic property. Because ferritic steel has high-chromium, low-carbon content, it offers excellent ductility and formability. Ferritic stainless steel also provides good thermal and corrosion resistance. For this reason, ferritic stainless steel is suitable to avoid stress corrosion cracking in applications where high chloride exposure poses a significant problem.

Austenitic Stainless Steel

Austenitic stainless steel is the most commonly used stainless steel type due to its excellent mechanical properties and aesthetic appearance. The steel has high corrosion resistance thanks to its high chromium content. This makes austenitic stainless steel suitable for harsh operating conditions. The presence of austenite with a face-centered cubic (FCC) grain structure gives austenitic steel a non-magnetic nature. However, it is possible to induce a certain level of magnetism and increase the strength of the austenitic steel through cold-rolling. Cold-rolling also fosters aesthetic appeal by providing an excellent surface finish, which is why aerospace, pharmaceutical, food and beverage industries, and many others use austenitic stainless steel for their application.

Martensitic Stainless Steel

Martensitic stainless steel is known for its strength, durability, and corrosion resistance. At high temperatures it has an FCC structure of austenite that when quenched and tempered can form martensite with a BCC structure. This martensite is a crystalline structure of very hard iron with varying properties depending on the carbon content. Thus, heat treatment can enhance the strength of this stainless steel. However, this also makes the martensitic steel difficult to weld and fabricate. The strength of the steel makes it suitable for components in valves or turbines where hardness is the key requirement.

The table below compares ferritic vs. austenitic vs. martensitic stainless steel properties.

Challenges in Welding Ferritic vs. Austenitic vs. Martensitic Stainless Steel

Due to different mechanical properties and heat treatment reactions, welding each stainless steel type requires special consideration.

Welding ferritic stainless steel presents several challenges, including the possible loss of ductility and toughness, as well as grain growth. To meet these challenges, welders should:

• Weld thin sections, preferably less than 6mm, to decrease the overall significance of the loss of strength in the component.
• Apply an interpass temperature between 100 to 120℃ to restrict the grain growth.
• When welding sections thicker than 6mm, preheat at low temperatures to reduce stress and avoid the formation of cracks in the joint.
• Use low-carbon austenitic steel, as well as Ni-based consumables to enhance weld strength.

Austenitic stainless steel, on the other hand, is easier to weld than other stainless steel. However, the possibility of embrittlement and cracking cannot be overlooked. To ensure weld quality, follow these welding tips.

• Ensure the maximum interpass temperature is 150℃ and heat input is less than 50 KJ/inch to avoid issues like hot cracking.
• Use filler material with low carbon content to improve the strength of the weld.
• Depending on the level of ferrite required, consumables with 3-5 percent ferrite content can help prevent hot cracking.

Martensitic stainless steel has a greater hardenable property, which also makes it less ductile. Sudden heat from welding and sudden quenching can result in the formation of cracks. To counter this:

• Use a lower hydrogen process like Metal Inert Gas (MIG) or Tungsten Inert Gas (TIG) welding to minimize the problem of hydrogen cracking.
• To avoid hardening in a heat-affected zone, preheat the metal to reduce the temperature difference and enhance controlled and slow cooling.
• Ensure that the filler metal matches the base metal.

Understanding Differences to Ensure Quality Weld Results

The unique chemical composition and metallurgical properties of ferritic, austenitic, and martensitic stainless steel demand different treatments when welding. Before commencing any operation, manufacturers should understand the complexity of the metal and the demands it may be exposed to in its operating environment. Only after understanding the different requirements of ferritic vs. austenitic vs. martensitic stainless steel can industries optimize their welding processes to foster productive and cost-effective operation.

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