Effects of carbon content on high-temperature mechanical and thermal fatigue properties of high-boron austenitic steels

High-temperature mechanical properties of high-boron austenitic steels (HBASs) were studied at 850 degrees C using a dynamic thermal-mechanical simulation testing machine. In addition, the thermal fatigue properties of the alloys were investigated using the self-restraint Uddeholm thermal fatigue test, during which the alloy specimens were cycled between room temperature and 800 degrees C. Stereomicroscopy and scanning electron microscopy were used to study the surface cracks and cross-sectional microstructure of the alloy specimens after the thermal fatigue tests. The effects of carbon content on the mechanical properties at room temperature and high-temperature as well as thermal fatigue properties of the HBASs were also studied. The experimental results show that increasing carbon content induces changes in the microstructure and mechanical properties of the HBASs. The boride phase within the HBAS matrix exhibits a round and smooth morphology, and they are distributed in a discrete manner. The hardness of the alloys increases from 239 (0.19wt.% C) to 302 (0.29wt.% C) and 312 HV (0.37wt.% C); the tensile yield strength at 850 C increases from 165.1 to 190.3 and 197.1 MPa; and the compressive yield strength increases from 166.1 to 167.9 and 184.4 MPa. The results of the thermal fatigue tests (performed for 300 cycles from room temperature to 800 degrees C) indicate that the degree of thermal fatigue of the HBAS with 0.29wt.% C (rating of 2-3) is superior to those of the alloys with 0.19wt.% (rating of 4-5) and 0.37wt.% (rating of 3-4) carbon. The main cause of this difference is the ready precipitation of M (C,B),type borocarbides in the alloys with high carbon content during thermal fatigue testing. The precipitation and aggregation of borocarbide particles at the grain boundaries result in the deterioration of the thermal fatigue properties of the alloys.