The influence of age-hardening on turning and milling performance of Ti-Al-N coated inserts

Ti-Al-N coatings are well known for their excellent properties and age-hardening abilities. Here we show that the life-time of coated inserts during turning of stainless steel can be increased to 200% by post-deposition vacuum annealing at 900 degrees C combined with a similar to 1 K/min vacuum furnace cooling. During milling of 42CrMo steel an increase in tool life-time to 140% is only obtained if the cooling condition after annealing at 900 degrees C contains a fast segment with 50 K/min from 800 to 700 degrees C. Thereby, the Co-binder in cemented carbide exhibits a retarded phase transformation from cubic to hexagonal. Consequently, the fracture toughness of the cemented carbide is reduced only from similar to 10.8 to 10.4 MPaV root m while the coating still has an adhesive strength of similar to 65 N. Our results indicate that best machining performances of coated inserts are obtained after annealing at 900 degrees C where the supersaturated Ti0.34Al0.66N coating undergoes spinodal decomposition to form nm-sized cubic TiN and AlN domains resulting in a hardness increase from 34.5 to 38.7 GPa. Additionally, we demonstrate that careful attention needs to be paid on the influence of annealing conditions on adhesive strength and fracture toughness of coated inserts. (C) 2008 Elsevier B.V. All rights reserved.