|Annealing, Hardening, Tempering - Course: Working Techniques of Heat Treatment of Steel. Trainees' Handbook of Lessons|
As to small workpieces, hardening is effected in the entire structure; as to greater workpieces, the degree of hardness decreases internally.
Unfavourable shapes of workpieces or working imperfections can cause crack formation after hardening.
The following problematic points can cause defects:
- irregular breakthroughs
- sharp edges
- boreholes directly made into outer surfaces
- abrupt transitions between thick and thin parts
Sharp edges have always to be rounded; in case of abrupt transitions, a separated individual-part-hardening with subsequent joining of the individual parts is more favourable.
Quenching cracks can be determined when a dilute colour paint is coated over the hardened surfaces - existing cracks ink and become visible!
Particularities for hardening in melting baths:
- When suspending into melting baths it has to considered that the workpieces are correctly emerged according to their shape.
- The workpieces must evenly be wetted; air cushions must not be produced! Air cushions slow down the quenching process and cause soft spots in the hardness layer!
- Before being suspended into the melting bath, the workpieces must be predried. Minor amounts of moisture cause, at those great temperature differences, explosive evaporation of water. The vapour throws off the heated bath fluid from the melting tank.
- Vapours produced in the melting baths must be sucked off the hardening room by means of effective suction devices.