| A hot tool steel, SKD61, nitrided in DC plasma and gas environments was investigated to see the properties of the nitrided layers and the effect of reheating on them. It was observed that as nitriding time increases, the thickness of the nitrided layers (compound and diffusion layers) increases, while surface hardness decreases. X-ray diffraction analyses showed that the compound layers obtainded from the both methods are composed of r`-Fe₄N and ε-Fe_(2-3)N phases which are completely decomposed by reheating at and above 550℃ As reheating temperature went up, surface hardness became low and the hardness difference between surface and bulk matrix became small. At and above 660℃ which was higher than the tempering temperature of the steel, the surface and bulk matrix hardnesses decreased very sharply. It was suggested that to improve the life time of dies, their service temperatures be kept below the tempering temperatures. |
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