Effect of Rare Earth Element Ce and Heat Treatment Process on Impact Toughness and Microstructural Developments of 27Co-Fe Alloy

A systematic study of impact energy, microstructure, fracture section and residual stress of 27Co-Fe was carried out using charpy impact tester, metalloscope, scanning electron microscope and x-ray residual stress detector. The forged C27, C27R1 and C27R2 were held at 650 °C, 700 °C, 750 °C, 775 °C and 800 °C for 2 h, respectively, quenched with water and tempered at 300 °C for 2 h to test the impact energy. The impact energy of forged C27R1 increased to more than 50 J compared with 1.2 J of C27 and the impact fracture changed from brittle fracture to ductile fracture with dimples. Microcracks and microvoids were reduced. Ce increased the nucleation rate of 27Co-Fe, refined and homogenized the grain. It’s obvious when Ce content was 0.03 wt.%, the grain size decreased from 20.2 to 12.9 μm. The grain refinement trend decreased when the Ce content was 0.06 wt.%. After quenching and tempering, C27, C27R1 and C27R2 were basically increased to more than 50 J when quenching temperature was 700–750 °C. C27R2 was held at 800 °C for 2 h, quenched with water and tempered at 300 °C for 2h, 4h and 8h, respectively, to test impact energy and residual stress. The residual stress of C27R2 decreased from 547 to 281 MPa when the tempering time was 8 h, and the impact energy increased from 2.5 to 78 J.