If insufficient tempering, leaf spring fracture easily in preloading process , stress blasting and Loading using. Furnace charging amount and tempering equipment must be considered.
The purpose of tempering is to reduce or eliminate the high frequency quenching stress, reduce steel brittle. Considering the two aspects, we can determine whether tempering fully through the test or instrument test. We usually use mesh belt furnace to temper the spring.
Personal understanding: low tempering temperature, or insufficient tempering time result in insufficient tempering.
The simple way is to test the hardness: if hardness is not down in the extended time then tempering should be sufficient.
In addition, different workpiece hardness requirements are different, corresponding to the tempering temperature also has a certain range, tempering temperature and time should be determined in accordance with the actual production.
We usually do tempering stability experiment by hardness method. Impact experiment also needs to contras with hardness method.