OPTIMIZATION OF NITRIDE SURFACE TREATMENTS OF AISI H-13 TOOL STEEL USE FOR EXTRUSION DIES

In the aluminum extrusion process, friction and wear plays an important role in the process efficiency, products quality and dies life. These effects directly on the production cost and thus, it is customary to harden the dies by a nitriding process.

This work presents a comparative study of three different nitriding methods, gas, plasma and molten salt bath nitriding. The goal of this work is to understand the influence of the nitriding process on the nitrided layer structure, to evaluate the method capability to penetrate to cavities and to evaluate the nitride layer performance in wear and extrusion conditions.

It was found that the plasma nitriding process produced the thickest layer, 385±5 µm, compared to 69±5 µm and 107±5 µm in gas and liquid nitriding, respectively. However, the gas and liquid nitriding had the capability to penetrate and form a nitride layer in cavities, as small as 1 mm (diameter), while plasma nitriding had a penetration limit of 3 mm. In the plasma and gas nitriding, the nitrided layer was composed from outer- compound layer composed of Fe₃N and Fe₄N followed by inner-diffusion layer. Fe₃O₄ was found as an outer layer in the liquid nitriding process followed by a diffusion layer. No nitride phases where found in liquid nitriding XRD analysis. The plasma nitriding process had the highest surface hardness of

1376±116HV_0.1 compared to 1250±191HV_0.1 and 927±11HV_0.1 in gas and liquid nitriding, respectively. The plasma nitrding layer had a room temperature friction coefficient of 0.24 ±0.04 against an aluminum pole, compared to 0.62 ± 0.05 and 0.57 ± 0.04 in the case of liquid and gas nitriding, respectively. No difference in the ring compression test was found between the three processes (all of them were around friction coefficient 0.9).