Pulsed Laser Nitriding of Stainless Steel at Atmospheric Pressure

Abstract

A novel nitriding method using a pulsed laser was developed to resolve several shortcomings of conventional nitriding: difficulty of local processing and batch processing, and the need for high-temperature treatment. Specifically, this report describes pulsed laser nitriding of stainless steel at atmospheric pressure using only nitrogen gas. The samples were irradiated at 20 kHz by a focused beam（spot diameter 0.7 μm）of 355 nm laser pulses（<15 ns duration）. The experimental parameters were scanning speed, pulse energy, and nitrogen concentration in a shielding gas. Results show that surface modification occurs when the distance between laser shots is less than the spot diameter, and that removal processing occurs when the distance is greater than the spot diameter. The laser nitriding layer consists of nitrogen solid solution matrix and some fine crystal grains of 1 μm or less. The laser nitriding layer exhibited no nitrogen concentration gradient from the surface to the inside, in contrast to the diffusion layer formed in conventional nitriding. Crystal grain refinement and the nitrogen solid solution produce a laser nitriding layer that is four times harder than the original stainless steel matrix.