Why is it necessary to increase ferrite testing after cold forming of austenitic stainless steel heads?

Properties of austenitic stainless steels and the effects of cold working: Austenitic stainless steels typically have a fully austenitic structure (face-centered cubic) at room temperature, which endows them with excellent corrosion resistance and toughness. However, the metastability of this type of steel means that when plastic deformation (cold working) is applied, part of the austenite structure transforms into strain-induced martensite. Martensite is a metastable phase with a body-centered cubic or body-centered tetragonal structure; it has high hardness and strength, but poor toughness. Deformation characteristics in the cold forming process: The cold forming of end caps (disk-shaped, oval, spherical, etc.) usually involves complex and non-uniform plastic deformation. The amount of deformation varies greatly across different regions: in areas with large deformation, such as the top of the head (the spherical cap region), the deformation is greatest, resulting in significant strain-induced martensitic transformation. Moderate deformation zone: Such as transition zones (corners). Small deformation zone/no deformation zone: Such as the edges of straight sections.

Potential risks of strain-induced martensite: reduced corrosion resistance 🛡: The martensite phase itself has lower corrosion resistance than austenite. More importantly, an excessively high content of local martensite leads to uneven microstructural distribution and stress in certain areas, significantly increasing the risk of pitting, crevice corrosion, and especially stress corrosion cracking in materials exposed to environments containing chlorides or acids.      Decreased toughness (brittleness): The high hardness of martensite reduces the toughness of the material in the deformed areas. Although the effects may not be apparent at room temperature, the risk of brittle fracture increases in low-temperature applications or under impact loads.       Magnetic property generation: Austenitic stainless steels are usually non-magnetic or weakly magnetic (paramagnetic), whereas martensitic ones are ferromagnetic. The head section after cold forming (especially the highly deformed area) will thus acquire magnetism.       Change in weldability: If welding is required in the vicinity of the cold-formed area later on, the high-hardness martensitic structure increases susceptibility to weld cold cracks (hydrogen-induced cracks).

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