Subject index

Abstract

The commercial attractiveness of rolls with greater resistance to wear and fatigue damage is substantial, providing reduced rolling mill down times, increased roll life, reduced product surface defects and improved stock gauge tolerances. Relatively little is known about the wear mechanisms of these materials which limit the useful life of cold rolling mill work rolls. Two steels of composition 0.8%C3%Cr and 0.8%C5%Cr, which are representative of industry standard materials, have been tested in a rolling-sliding configuration (slippage, 8%) against a high speed steel (M2) counterface. A comparison has been made between samples manufactured by conventional processing (casting and forging) and by spray forming. Spray forming provided a microstructure similar to that of the heavily forged material but with a finer average carbide size (134 nm in the spray formed and 317 nm in the forged, both for the 3%Cr steels) and the spray formed material did not contain the occasional coarse carbides (∼2 μm) found in the forged material. The increase in Cr content from 3% to 5% changed the carbide from (Fe,Cr)3C (3%Cr steel) to (Fe,Cr)7C3 (5%Cr steel) in both spray formed and forged materials. However, both the carbide size and carbide type had little effect on wear rate when tested in the quenched and tempered condition. Tempering of the quenched steel was undertaken in the temperature range 200–500 °C, which yielded hardness in the range 540–890 Hv for the 3% Cr steel and 470–880 Hv for the 5% Cr steel. The wear coefficient decreased significantly for an increase in hardness from to 470 Hv (κ=1.72 × 10−5 mm3 N−1 m−1) to 750 Hv (κ=1.35 × 10−6 mm3 N−1 m−1) but only fell slightly for hardness values above 750 Hv. The friction coefficient was 0.49–0.63 irrespective of load or material temper. Characterisation of worn surfaces indicated that surface strain was limited to a depth of ∼5–15 μm. In the hardest conditions, the wear debris was predominantly oxide, while the soft tempers produced metallic delamination sheets as well as oxide. The wear behaviour of the four materials is discussed in relation to the microstructure.