Tempering Treatment Effect on Mechanical Properties of F82H Steel Doped with Boron and Nitrogen

Effects of tempering treatment on mechanical properties and microstructures have been studied for martensitic steel F82H doped with 60 ppm B and 200 ppm N (F82H þ B þ N). The tempering treatments were performed at 700–780 � C after the normalizing treatment at 1000 � C. Yield stress of the F82H þ B þ N steel tempered at 700, 750 and 780 � C was 740, 580 and 500 MPa, respectively, and ductile-brittle transition temperature (DBTT) of the specimens was � 55, � 85 and � 85 � C, respectively. The areal density of dislocations decreased from 1:1 � 10 14 to 2:5 � 10 13 m � 2 with increasing tempering temperature from 700 to 780 � C. The number density of precipitates decreased with increasing tempering temperature from 700 to 750 � C, while the number density was almost equivalent as increasing tempering temperature from 750 to 780 � C. The results indicate that the change of DBTT, depending on tempering temperature, is related with the change of yield strength, size and number density of carbides. Hardening behavior of the F82H þ B þ N steel irradiated by 10.5 MeV Fe 3þ to 10 dpa at 360 � C has been also studied by using a micro-indentator. The micro-hardness of the F82H þ B þ N steel tempered at 780 � C was changed from 3.6 to 4.8 GPa by the irradiation. Because hardening behavior of the F82H þ B þ N steel was found to be similar with that of F82H non-doped, doping effects of B on irradiation hardening were suppressed by co-doping of B and N.