When the Appendix G methodology, fracture toughness criteria for protection against failure, was first adopted by ASME Section III in 1972, it included a lower-bound Kir curve for ferritic steels with specified minimum room-temperature yield strength up to 50 ksi. In 1977, Section III Appendix G added a requirement to obtain fracture-toughness data for at least three heats (base metal, weld metal, and heat-affected zone) if the KIR curve is used for ferritic steels with specified minimum room-temperature yield strength between 50 and 90 ksi. The three-heat data requirement has not changed when the lower bound curve was adopted by Section XI, or when the lower-bound crack initiation toughness curve was changed from the dynamic Kir curve to the static KIc curve during the 2000s. Based on the accumulation of fracture-mechanics data of ferritic steels with specified minimum yield strength between 50 ksi and 90 ksi and their use for Class 1 pressure vessel production, Section XI recently expanded the applicability of the KIc curve to SA-508 Grade 2 Class 2, SA-508 Grade 3 Class 2, SA-533 Type A Class 2, and SA-533 Type B Class 2 whose specified minimum room-temperature yield strength is 65 ksi or 70 ksi. This paper describes the technical basis including the fracture-mechanics data to support the expansion of the applicability of the KIc curve by ASME Section XI.