How to make internal threads on case hardening steel parts - [Leave Soft]

Often when taping case hardening steel parts many manufactures make a mistake. They apply case hardening to the thread. This could cause strength problems especially if the thread is under dynamic loading.

It is good practice not to heat treat threads. Instead of making all the threads and sending the work for carburizing and final case hardening, one must follows the below procedure.

• Drill for the thread an initial diameter smaller than the thread minor diameter (depends on carburizing depth).
• Carburize the work.
• Drill the initial thread diameter up to thread minor diameter in order to remove the carburizing layer from the thread surfaces.
• Make a chamfer or spot face.
• Case harden the work.
• Tap the thread as usual.

This procedure is indicated as [LEAVE SOFT] on the blueprint.