Effect of sigma phase precipitation on microstructure and properties of cast ZG0Cr26Ni5Mo3Cu3 duplex stainless steel under different heat treatments

Abstract

ZG0Cr26Ni5Mo3Cu3 Duplex Stainless Steel (DSS) was solution treated at 1,060 ìC for 3 h,followed by water cooling. Tempering treatments were conducted at 720, 750 and 780 ìC, respectively, for 16 h, followed by air cooling. The microstructures of ZG0Cr26Ni5Mo3Cu3 duplex stainless steel samples treated at different tempering temperatures were observed by scanning electron microscope (SEM) and energy dispersal spectroscopy (EDS), and the phase consitutions were analyzed using X-ray diffraction (XRD). The effects of the precipitation of sigma (σ) phase on the duplex phase percentage, hardness, impact toughness and corrosion resistance of the DSS were studied. Results showed that microstructures of ZG0Cr26Ni5Mo3Cu3 after solution treatment consists of ferrite (α) phase and austenite (γ) phase; after being tempered at different temperatures, σ phase appeared due to a eutectoid-type reaction of α—σ+γ2 during tempering treatment. It was observed that σ phase distributed along the grain boundary. The volume fraction of σ and γ phases increased with increasing tempering temperature in the range of 720 to 780 °C, whereas the volume fraction of α phase showed the opposite trend. When the percentage of σ phase increased, the hardness of steel also increased. In the solution treated steel, hardness was measured to be only 244.0 HB, because σ phase did not appear. However, it increased to 391.8 HB when the DSS was tempered at 780 ìC because a great of deal of σ phase appeared. The impact toughness and corrosion resistance of DSS decreased when the percentage of σ phase increased.

Keywords

• Duplex stainless steel
• tempering; sigma phase
• hardness
• impact toughness
• corrosion