Coated Tungsten Carbide Inserts: Effects on Machinability in Turning of AISI 1020 Steel

How the different types of cutting inserts coatings influence on surface roughness and tool wear, taking these two factors as machinability criteria, in turning carbon steel bars AISI 1020 was investigated. Specimens of 36 mm in diameter and 200 mm in length tested were made. Uncoated inserts and coated WC tungsten carbide inserts with: TiN; TiN / Al2O3 and TiC / Al2O3 / TiN, were use. The tests were carried out on automatic parallel lathe, using the cutting parameters: 1500 rpm; a= 0,25 mm/rev; p= 0,4 mm; therefore, the cutting speed was Vc= 170 m/min. Roughness was measured in the roughness meter: Mitutoyo, Surftest-211, and a high resolution optical microscope was used to measure the flank wear, complementing with SEM electron microscopy. It was found that the lowest roughness Ra is obtained using Triple inserts showing a variation Ra = 2μm. The maximum flank wear VB = 300μm was taken as machinability criterion, to later determine the tool life for each insert. The uncoated insert life was 8 mim, and TiN coated insert life was 14.5 min, while the bilayer and trilayer coating inserts did not exhaust their life for 12,000 mm length machining. Uncoated and TiN-coated inserts showed strong wear and workpiece material build-up (BUE). The double and triple layer inserts did not show this phenomenon due to the Al203 compound which is chemically inert at high temperatures. The most prevalent mechanisms in the inserts wear was adhesion and abrasion. No direct or inverse relationship was found between wear and roughness.