Abstract
Rolls are the main consumptive parts which can deform the metals and determine the efficiency of the rolling mills and the quality of the mill bars. How to improve the wear resistance and elongate the service life of rolls matters a lot in reducing the consumption of rolls. In recent years, because of its better red-hardness and wear property, high speed steel was used to manufacture the new type composite rolls.
As a new surface treatment technology, laser cladding can fabricate dense coatings with a metallurgical bonding with substrates, thus improving the wear resistance of the substrates. The aim of the present work is to use laser cladding technique to fabricate dense,wear resistant coatings which have a metallurgical bonding with the substrates. It can offer theoretical instructions on repair rolls and manufacture composite rolls by laser cladding, and technical support for their further application.
In this paper, the substrates for laser cladding were nodular cast iron rolls, YAG solid pulsed laser and CO2 laser were used to explore the feasibility of preparation high speed steel coatings. Coatings with good metallurgical bonding with substrates were fabricated with optimal processing parameters. Optical microscopy (OM),scanning electron microscopy (SEM) attached with energy dispersive x-ray spectrometry (EDX), X-ray diffractometer (XRD) microhardness tester and wear tester were used to analyze the microstructure,composition, phase, hardness, wear-resistant and high-temperature oxidation properties of the coatings.
The Nd:YAG pulsed laser was applied to nodular cast iron rolls coated with high speed steel. The optimized laser processing parameters for pre-pasted single-path laser cladding layers were as follow: the current is 240A, scanning velocity is 3mm/s, the defocus amount is 13mm,the thickness of coating is 0.5 mm, frequency is 15Hz and impulse width is 3.0ms.
The multi-channel laser cladding results that the coated layers combined metallurgically with the substrate with a lot of crack. The main strengthening phases are WCi-xand V4C3. The average micro-hardness up to 600HV is about 2 times as high as that of the substrate. The number of the crack reduce when Ni60 add to the coating.
The CO2 laser was also applied to nodular cast iron rolls coated with T15 high speed steel. The optimized laser processing parameters for single-path laser cladding layers were as follow: the power is 1800W, scanning velocity is 6mm/s, the defocus amount is 340mm,the thickness of coating is 1mm. The multi-channel laser cladding results that: the way of combination is metallurgy and there is a bright belt between the substance and coating. The coating was dense,pore and crack free. The main strengthening phases are V4C3,WCi-x,CoCx。The average microhardness up to 750HV is about 2.5 times as high as that of the substrate. The high-temperature wear rate is 40% of nodular cast iron's in 30 min. The coating is excellent in the high-temperature oxidation capacity. The high-temperature wear-resistance capacity of laser cladding YT layer is reduced and the high-temperature oxidation capacity is increase.
Key words: Laser cladding; high-speed steel roll; coating; technique optimization;microstructure property
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