Can you please provide additional information about the application? Temperature, environment, etc.? Why is L7M currently specified? What is the exact specification being quoted for HT 12.9, meaning is it ISO 898 property class 12.9? Is the steel chemical composition and heat treatment provided in the quote? In general, no, "HT 12.9" is not an acceptable replacement for something that is currently specified as L7M according to ASTM A 320.
OK. I understood. The application is for valve(low temperature environment). For the big stud I still use L7M.
Only the small screw I wonder to use HT12.9. Because I found that the tensile strength much more higher than L7M but the elongation much smaller than L7M. Also the hardness much more higher than L7M.
I found difficulty to source hex. socket cap screw with material L7M/B7. Most market only produce stainless steel and high tensile. In this case can I use stainlesssteel to replace L7M?
It may be possible to use property class 12.9 SHCS (ISO 898-1) or stainless SHCS (ISO 3506) depending on the environment, preload requirements (stress in fastener), etc. If the engineer/designer who specified SHCS using ASTM A 320 L7M is someone other than you, then you need to contact them about this issue. I would be concerned using PC 12.9 SHCS in a low temperature environment unless the 12.9 SHCS were made from an alloy steel like SAE 4140 or 4340 and tempered at a relatively high temperature (> 425 C).
