Microstructure and properties of an as-deposited and post treated high strength carbide-free bainite steel fabricated via laser powder deposition

A carbide-free bainite steel of 0.29C–2.50Si-1.50Mn-1.0Mo-0.97Cr (wt.%) with no voids and cracks was fabricated via laser powder deposition (LPD). Different post-treatment plans were conducted and then microstructures and properties of the steel were studied through optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD), transmission electron microscopy (TEM), hardness meter and tensile test machines. Obtained results show that the microstructure of the deposited specimen consisted of granular pearlite and retained austenite (RA). The yield strength, tensile strength and elongation were 662 ± 71 MPa, 1051 ± 65 MPa and 13.03 ± 1.33 %, respectively. After austempering at 578 K, the microstructure of the specimen changed to bainite ferrite (BF) and RA. Moreover, it was found that the yield strength and tensile strength significantly increased to 1156 ± 60 MPa and 1289 ± 110 MPa, respectively. However, it had poor strain hardening capacity and elongation remained unchanged. After hot rolling and austempering, the RA content of the specimen significantly increased and the strain hardening capacity improved. However, when the isothermal temperature was less than the Ms temperature (i.e. 553 K), the corresponding microstructure consisted of martensite, BF and 13.6 vol% of RA. In this case, the yield strength, tensile strength and elongation were 895 ± 74 MPa, 1157 ± 92 MPa and 14.59 ± 1.13 %, respectively, indicating slight improvement. Meanwhile, it was found that when the isothermal temperature exceeded Ms temperature, the mechanical properties improved significantly. The present study demonstrated that specimens with an isothermal temperature of 578 K had the best mechanical properties. In particular, the yield strength, tensile strength and elongation of the optimized alloy were 1177 ± 44 MPa, 1398 ± 57 MPa and 19.80 ± 0.79 %, respectively.