There are several process-stability items to be aware of when performing induction heat treatment.
Equipment preventive maintenance
Ensure that the operation and maintenance manual(s) are being implemented and acted upon to give regular preventive maintenance before critical maintenance is required.
Material analysis including prior metallurgy before induction treatment
Ensure that the received manufacturing material is the material that the test certificate says that it is. This can be accomplished by a hardness-test comparison of material to test certificate, Jominy end-quench test and decarb depth measurement.
Incoming material grain size
If the material has been forged prior to induction heat treatment (hardening), check the grain size. Coarse grain is easy to machine but more likely to be a potential crack risk. Fine grain is harder to machine but considerably less of a crack risk because of the grain-boundary fineness.
Incoming material grain flow
This can be checked by a macro-etch. The grain flow should follow the geometric design of the forged product.
Material cross-sectional thickness
This item is so often overlooked. Some engineers miss it simply because the material is all the same analysis. But what is overlooked are the cross-sectional heating rates and cooling-rate variation in relation to the thicker area of the part. This will generally lead to serious crack risk (even with PAG).
Required depth of the formed martensitic case
This requires very careful selection of the induction unit’s induction power setting to the induction coil. Too great a power-setting frequency selection will obviously produce a deeper formed martensitic case. A clear understanding will be necessary of the depth of formed case in relation to the component wall thickness, the depth of case required and the engineering drawing.
Residence time within the inductor coil
Remember that the longer the residence time in the induction coil, the hotter the part will get (risk of cracking and overheating) and the deeper the formed case will be. There is also the danger of retained austenite as well as excessive scale buildup on the steel surface.
Blind holes can lead to thinner wall sections and the risk of overheating in that area. There will also be a higher hardness variance and a greater risk of cracking.
If the system is to use “city water,” it will be necessary to have a good incoming water filtration system because of the risk of possible particulate contamination. It would be more suitable to use deionized water as the cooling and quench water.
Polyalkaline glycol analysis
If you are treating alloy steel, then the polyalkylene glycol system will be a critical item in the PM of the system. The PAG system solution strength should be analyzed each day. The reason for this is threefold:
- Solution dilution due to drag out
- Particulate matter (iron oxide from the quenching of the steel surface at austenitic temperatures). The iron-oxide particulate can also affect the electrical characteristics of the induction system.
- Remove a sample of the quench medium from the storage tank (500 ml), filter through a fine filter paper overnight and examine the filter mapper for particulate residue.
We will conclude our discussion on induction in part 3.