Quite often in-house process specs will reference industry standards for common procedures like temper etch inspection of finish ground gear teeth. If you simply want to read up on temper etch inspection of finish ground carburized surfaces, MIL-STD-867C is a publicly accessible standard. Another current aerospace standard for general temper etch inspection of steel parts is AMS 2649, but it must be purchased. There is also ANSI/AGMA 2007-C00 (ISO 14104:1995) which is a standard focused more on temper etch inspection of finish ground gear/spline teeth.
The standards listed above don't provide specific accept/reject criteria, they only provide guidance on classifying various conditions indicated by the inspection process. An in-house process spec (such as your W.I. P1026) or drawing general note will usually provide specific accept/reject criteria. It is usually best to specify detailed accept/reject criteria in the engineering documentation for each part number. Providing detailed accept/reject criteria specific to each part number, based on factors such as type/severity of indication, location of indication, size/distribution of indications on a surface, etc., will minimize the rejection rate for this inspection procedure.
