My understanding is that the intent of ASME is that all B31 codes will reference B31J for SIFs and Ks. While many of the B31 2018 editions reference B31J as an option, with the 2020 editions B31J B31J is now referenced in place of Appendix D. B31.3 2020 has deleted Appendix D.

Most piping codes (not just B31 codes) have stated for years that "if more applicable data is available, it should be used instead of the SIF and K-factor equations". In almost all instances, the SIF and K-factor equations presented in the codes are all based on the work of Markl, and Markl stated in his papers that his equations should be used only if there was no better alternative.

B31J is that better alternative. Check this earlier post from 2016 B31J post . There are many other posts on this topic you can find using the "Advanced" search tool of this forum.