A comparative study on surface topography and micro hardness of laser polished hardened AISI D2 tool steel

In this work, a laser polishing-hardening (LPH) method with integration and high efficiency for the treatment of AISI D2 tool steel was proposed, and the effects of laser hardening (LH), laser polishing (LP) and LPH treatments on the surface topography and microhardness were examined. The results show that LH method had a negligible effect on the surface roughness of the treated sample, while the surface roughness Ra of LP and LPH specimens was reduced by 74.6% and 80.9% respectively, indicating that the milled surface topography had been significantly improved, especially LPH was more effective in reducing the roughness. Besides, the polishing efficiency of LPH was 10 times that of LP approach. In terms of hardness improvement, the near-surface microhardness of LH and LPH samples increased by 1.5 times and 1.3 times respectively, and the effective hardened zone (EHZ) depth was 0.42 mm and 0.24 mm respectively, demonstrating that these two laser processing methods had a beneficial effect on the cross-section microhardness of D2 tool steel, while the increase of LP on the microhardness was insignificant. The comprehensive analysis of the surface morphology and microhardness of LPH specimen indicates that LPH was a feasible laser surface treatment method for D2 tool steel. On the premise of ensuring a high surface finish, the polishing efficiency can be remarkably improved, the subsurface microhardness and EHZ depth of processed specimen can be also significantly enhanced, which provided a feasible idea for the application of laser surface treatment technology in industrial mold production.

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si3N4) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H2-25 pct N2, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si3N4-β, Si3N4-γ, Fe4N, and Fe3N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

Significant Contribution of Temperature on AISI D2 Tool Steel Ground Surface Characteristics and Integrities

The Controlled grinding is governed by the maximum surface temperature in the wheel-workpiece interaction. In this study, we demonstrate that temperature is the significant controller on the surface characteristic grinding, the surface integrity, the productivity and the fatigue life. Moreover, high temperature generated in abrasive processes is the main factor responsible on ground surface damage and its impact on the induced consequences in grinding of AISI D2 tool steel. The combined effects of abrasive type, cooling mode according to the cutting depth, on the temperature and residual stress distribution were highlighted by exploiting FEM. Obtained numerical results were validated with the experimental ones.