Ferrite content

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01 Overview: Austenitic stainless steels possess good corrosion resistance, heat resistance, and low-temperature resistance, as well as excellent formability and weldability. They represent an important category among stainless steel materials, accounting for about 70% of the total output of stainless steels. The main material used in stainless steel valves is almost entirely austenitic stainless steel, yet the understanding of austenitic stainless steel in the valve industry currently only covers its chemical composition and mechanical properties. With technological progress, regulations have been established regarding the ferrite content in the base metal and welds of austenitic stainless steel for nuclear power plants, nuclear reactor engineering applications requiring nuclear-grade valves, special valves for defense and military industries, as well as important \"SHA-grade\" pipeline valves in large-scale chemical plants. Therefore, it is necessary to master the methods for measuring and calculating the ferrite content in austenitic stainless steels. 02 The role of ferrite in austenitic stainless steels. Analyzing the role of ferrite in austenitic stainless steels is a crucial technical foundation; only through in-depth research and a thorough understanding of both the positive and negative effects of ferrite can it be utilized or controlled properly. The role of ferrite in austenitic stainless steel: For valves, the most important aspect is its impact on weldability; followed by its effect on the material’s corrosion resistance, mechanical properties, and workability. 2.1 Composition: The pressure-bearing components of stainless steel valves (valve body, valve cover, and valve disc) are primarily made from CF-grade stainless steel castings as specified in ASTM A351, as well as F304 and F316-grade stainless steel forgings as specified in ASTM A182. These are austenitic stainless steels of the 18-8 and 18-12 types (where the numbers indicate the approximate content of Cr and Ni). Stainless steel is classified into austenite, ferrite, and martensite according to its crystal structure. Austenite has a face-centered cubic crystal structure and is non-magnetic. Ferrite has a body-centered cubic crystal structure and is magnetic. It should be noted that the term \"austenitic stainless steel\" for metallurgical products does not mean that its microstructure must be 100% austenitic. When inspecting stainless steel valves and components, it is common to use magnets to attract the objects being tested; if a weak magnetic response is detected, it is assumed that there is a quality issue with the product. In fact, this is a misunderstanding of austenitic stainless steel, and such an approach often leads to incorrect judgments. Austenitic stainless steels usually contain a certain amount of ferrite. According to Volume 3 of the Metal Handbook, \"Properties and Selection: Stainless Steels,\" the section on \"Properties of Cast Stainless Steels\" states that CF-type cast stainless steels typically contain 5% to 25% ferrite. To this end, the American Society for Testing and Materials (ASTM) has defined the standard name for austenitic stainless steel castings used in valves as ASTM A351: \"Austenitic-austenitic-ferritic (duplex) cast steels for pressure-containing components.\" 2.2 Welding properties The main issues associated with welding austenitic stainless steels are hot cracks in the weld and heat-affected zone, as well as corrosion resistance; these issues also serve as indicators of the weldability and serviceability of austenitic steels. 2.2.1 Preventing thermal cracking in welds Ferrite plays an extremely important role in the welds of austenitic stainless steels. A certain amount of δ-phase ferrite (4% to 12%) needs to be formed in the welds of austenitic stainless steels to prevent solidification cracks (thermal cracks) in the welds. δ-ferrite is the ferrite that is formed in austenitic stainless steels (including weld metal) during the primary crystallization process (solidification process) and remains at room temperature. Due to its very low carbon content, ferrite has properties similar to those of pure iron: it possesses good plasticity and toughness, but low strength and hardness. The advantage of ferrite is its high solubility for elements such as S, P, Si, and Nb, which prevents the segregation of these elements and the formation of low-melting eutectics, thereby avoiding the occurrence of solidification cracks. The welding process is essentially a metallurgical and heat treatment process that takes place locally on the welded structure, between the base metal and the welding material. The ferrite in the weld can effectively prevent the formation of eutectics with low melting points, reduce the degree of segregation, and inhibit the displacement of secondary grain boundaries; thus, it helps to prevent cracks in the heat-affected zone as well as cracks resulting from high temperatures and low plasticity. In summary, δ-ferrite in welding certainly plays a role in preventing and reducing hot cracks and microcracks in the austenitic weld metal; it significantly improves weldability and enhances the safety of welded structures. δ-ferrite has a certain negative effect in welds. For weldments that require heat treatment at temperatures above 600°C after welding, or those that operate for extended periods at temperatures between 600 and 850°C, δ-phase ferrite precipitates into β-phase ferrite at such high temperatures. The β-phase has a tetragonal crystal structure and is rich in Cr, which leads to chromium depletion in the surrounding area and thus causes brittleness in the weld metal. At this point, the ferrite content in the weld should be kept between 3% and 8%, or a re-solution treatment can be employed to dissolve the β-phase ferrite back into the matrix. 2.2.2 Improving the corrosion resistance of welded joints A welded joint refers to the entire welding area, including the weld seam, the fusion zone, and the heat-affected zone. Welded structures made of austenitic steel often get damaged or even rendered unusable due to corrosion, with intergranular corrosion and stress corrosion being the most common types. Since ferrite exists in the form of dispersed, evenly distributed small pits among the austenite grains, it weakens the directionality of the austenite columnar and dendritic crystals, interrupts the continuous network of chromium carbide precipitates at the austenite grain boundaries, thereby preventing intergranular corrosion; hence, ferrite is beneficial for improving resistance to intergranular corrosion. Tests have shown that since ferrite is insensitive to stress corrosion cracking, the stress corrosion resistance of austenitic steel welds containing ferrite is superior to that of austenitic steel welds with the same composition but containing little ferrite. 2.3 Corrosion resistance The δ-phase ferrite in welding materials (base metal and filler metal) can significantly improve the mechanism of resistance to intergranular corrosion and stress corrosion in the weld and heat-affected zone. Based on the same mechanism, it can be concluded that a small amount of ferrite (5%~12%) in the base material of austenitic stainless steel castings and forgings generally contributes to improving the material’s resistance to intergranular corrosion and stress corrosion. On the other hand, in certain specific corrosion environments, such as in media containing urea and acetic acid, ferrite undergoes selective corrosion; therefore, it is necessary to limit the ferrite content.