Active wear and failure mechanisms of TiN-Coated high speed steel and tin-coated cemented carbide tools when machining p
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INTRODUCTION
AUSTENITIC and duplex stainless steels are normally recognized as materials difficult to machine, because of their following traits: (1) high tensile strength and high work hardening rate and low thermal conductivity, leading to high cutting temperature and accelerated tool wear; (2) high fracture toughness, resulting in poor chip breakability and poor surface finish; and (3) strong bonding to the tool, especially to cemented carbide tools,m causing some pieces of material to be tom from the cutting tool and carried away by the chips. Until now, the main mechanisms for tool wear when machining austenitic and duplex stainless steels are unclear, although there are some studies on them. It has been reported that the high work hardening rate, combined with low thermal conductivity, results in serrated chips when machining stainless steels, t2j The serrations of the chips cause vibration of the cutting forces and attrition wear of the cutting tool, especially the cemented carbide tool.t3] Besides vibration of the cutting forces, it has been recognized that a strong bonding between the tool and the workpiece material is also a necessary condition for attrition wear to occur. Little data concerning work hardening of stainless steels during machining have been presented until now. Also, the bonding mechanisms between stainless steel and the cutting tool, especially the cemented carbide tool, have not been investigated, although it has been generally ac-
LAIZHU JIANG, formerly with the Laboratory of Engineering Materials, Helsinki University of Technolo..gy, is with AB Sandvik Steel, 81811 Sandviken, Sweden. HANNU HANNINEN, Laboratory of Engineering Materials, and JUKKA PARO and VEIJO KAUPPINEN, Laboratory of Workshop Technology, are with the Helsinki University of Technology, 02150 Espoo, Finland. Manuscript submitted September 7, 1994. 2796--VOLUME 27A, SEPTEMBER1996
cepted that the resulting high cutting temperature when machining both austenitic and duplex stainless steels is a very important factor causing bonding. The machining cost of workpiece materials normally occupies at least 30 pet of the total cost of the final product. For the materials difficult to machine, such as stainless steels, it may be up to 50 pct. Powder metallurgy employing hot isostatic pressing (HIP) technology has recently been used to produce stainless steel products; this process, on one hand, reduces the machining costs because the size and shape of the products can be very close to the final product, but on the other hand, possibly causes the mathinability of HIP steels to be poorer than that of the conventional steels due to considerable amounts of hard oxide particles. As the applications of HIP austenitic and duplex stainless steels are increasing due to their excellent mechanical properties and high corrosion resistance, it becomes more and more important to also know their machinability. The machinability of materials depends not only on their properties but also on the cutting tools. Although there are some new cut
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