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Tough nitriding treatment used under harsh conditions of rapid heating and quenching

SKD61 material system limited “D200”

The effective temperature range of nitriding treatment is limited to 800°C.
The cause is decomposition of nitride at high temperature ➡ Decrease in nitride hardness due to evaporation. In addition, repeated rapid heating ➡ rapid cooling cause early cracks, progressive fractures, and large cracks.

Therefore, a white ceramic layer is used as a barrier on the surface of the mold to create a “Flexible & Tough coating” that is resistant to rapid heating ➡ rapid cooling.

For die-casting dies used in the temperature range of around 700°C, it is thought that the white layer on the outermost surface causes initial cracks due to the cycle time of repeating heating ➡ cooling.
As an effective countermeasure against the occurrence of cracks, these initial cracks can be delayed by making the white layer, which is sensitive to rapid heating and cooling, as thin as possible.

In addition, there is the idea of reducing the nitriding depth with a gentle hardness gradient in the range of 50 to 100 μm so that the heat cracks do not progress too deeply. However, at high temperatures of 900°C or higher, the current situation is that production is not possible unless the mold contact temperature is high and the protective function of the glass-based coating agent can be effectively utilized. Therefore, it is required to improve the retention of the glass-based coating agent and develop a coating that protects from heat shock. The completed solution is the “strong white layer.”
This strong and flexible white layer is a nitriding method that makes full use of hard and soft elements of surface treatments, rather than being chemically manufactured.

D200 cross-section metal structure photograph
(SKD61) × 160
D200 cross-section metal structure photograph
(SKD61) × 400
D200 cross-section metal structure photograph
(DH31) × 400
Strong white layer that does not crack in cold tests
Prove the flexibility of the white layer with a hardness tester (compared to EH treatment)
The film quality was verified by driving an indenter directly above the nitrided surface using a Rockwell hardness tester and observing the cracks generated in the indentation.
This is the result of conducting the HRC (150kgf) test.
EH treatment
EH treatment
Surface hardness including nitrided layer
51.0HRC (+0.5)
Nitriding layer
Base material hardness 50.5HRC
D200
D200
Surface hardness including nitrided layer
51.0HRC (+0.5)
D200 nitriding layer
Base material hardness 50.5HRC
D200 has a surface hardness of up to 59.5HRC, indicating increased hardness, hard and deep
Surface section and hardness curve
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