The knife steel hardening procedure
Following is a description of how hardening is carried out in practice, what distinguishes the various furnace types, and what should be borne in mind in the various furnace types and processes.
In addition, the importance of quenching the material as quickly as possible from the hardening temperature, and how hardness is affected by different tempering temperatures are described. The required hardness can be achieved by varying the tempering temperature within the specified range.
Furnace types
Knife steel can be hardened in two basically different ways, depending on the equipment:
- Piece hardening in a small furnace or hardening in a belt furnace. These two procedures are equivalent in terms of times and temperatures, and share the same hardening program.
- Batch hardening of larger batches, e.g. in a vacuum furnace.
Regardless of the method used, the purpose is the same: to heat treat the material to increase the hardness and improve the corrosion resistance.
Piece hardening
Piece hardening of individual blades in a small furnace or a belt furnace is normally carried out by heating the steel to 1080°C / 1976°F (for Sandvik 12C27) and the time during which the blade is held (soaked) in the furnace depends on the material thickness.
The table below shows the approximate soaking times in the furnace as a function of the material thickness. When the blade has been soaked in the furnace for the time specified above, it is removed and immediately quenched, preferably in oil intended for quenching.
In belt furnaces, the material is either placed together on a long belt, or the knife blades are placed on a mesh belt that transports them through the furnace. The most critical operation for hardening in a belt furnance is quenching, which should be given extra attention for an optimal result.
| Thickness | Thickness | Time in furnace |
|---|---|---|
| mm | in. | minutes |
| 2.50 | 0.100 | 5 |
| 3.00 | 0.118 | 6 |
| 3.25 | 0.128 | 7 |
| 2.50 | 0.138 | 8 |
| 3.75 | 0.148 | 10 |
| 4.00 | 0.157 | 12 |
When the blade has been soaked in the furnace for the time specified above, it is removed and immediately quenched, preferably in oil intended for quenching.
In belt furnaces, the material is either placed together on a long belt, or the knife blades are placed on a mesh belt that transports them through the furnace. The most critical operation for hardening in belt furnance is queching, which should be given extra attention for optimal result.
Hardening of larger batches
When larger batches are hardened, there is a risk of temperature difference within the batch. The material should therefore be given a chance to achieve uniform temperature by soaking it at 850°C (1562°F) during the heating process. The material should also be given a longer soaking time and slightly lower temperature in relation to piece hardening.
Quenching is equally critical in this type of hardening, and it is very important that the furnace equipment has high cooling capacity in order to meet the requirement for lowering the temperature to 600°C (1112°F) within 2 minutes.
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