The most important consideration in deciding whether to rework or scrap and re-make a nonconforming part is functionality. Can the part be made to safely provide the service it was designed for? If there is any doubt, you must toss it in the scrap hopper.
If post-heat treat inspections come up with short case depths, low surface hardness, or too much distortion to straighten, your metallurgist’s opinion is final. Supplementary processes such as deep freezing [cryogenic treatment], re-heating and re-quench, or die quenching may be recommended. Some of these may need customer authorization. All of them have to also be done on test coupons so the lab work can be repeated. When it comes to metallurgy, trust the experts.
Very few people can tell what is going on beneath the surface of a part, but once there is a failure you can be absolutely certain they will find out. Regardless of the cause of the failure, an improperly heat treated part will make you at least complicit in the problem.
Differences in the outside appearance of a part can be completely cosmetic, yet make customers nervous. We once had a skid of parts returned because two parts had been subjected to surface temper etching to comply with purchase order requirements. There was no way to make that dull gray color go away, but etching the rest of the lot alleviated the worry.
The lesson from that exercise is that every rework plan must consider the physical appearance as well as the function. Just because the customer would not see the “step key” on an internal shaft does not mean you have free reign to proceed.
There is an expectation of “interchangeability” even in custom manufactured devices. Most of us were taught a lie about Eli Whitney in high school history; he scammed the U.S. Army into believing that his firearms had completely interchangeable parts. They looked very similar, but key components were “fitted” by a cadre of very proud and independent gunsmiths. Recent research blames that “custom fitting” for the poor performance of the U.S. Army during the War of 1812. Think about that when you next hear our national anthem.
So, in addition to functionality, internal soundness, and physical appearance, rework decision making processes must consider future spare parts needs. It is common for some parts to be replaced as set. Others get replaced as sub-assemblies. Whatever your specific plan is, make certain the records reflect any modifications that will be needed as a result of rework on the initial project.