الفهرس 
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Abstract
Nowadays, modern surface treatment commonly uses laser process. Laser surface process means thermal hardening of metals surface which is based on local heating of a surface under the influence of radiation and subsequent fast cooling of this surface. The basic advantage of laser surface treatment process is fast heating of a thin surface layer (i.e. there is no necessity of spending energy on heating of the whole volume of material as happens in conventional heat treatment). Thermal influence zone is reduced to a minimum and consequently the distortions of surface are also minimal in comparison with other methods (i.e. laser allows hardening of small details and partial hardening of separate sites of a surface). nickel-chromium white cast iron (NHWCI) alloys are promising engineering materials, which are extensively used in applications where good resistance to abrasion wear is required. Examples are grinding balls, mining, mineral processing, cement, copper and iron manufacturing equipment where appropriate impact resistance and excellent wear resistance are required. After traditional hardening process such as flame hardening, induction hardening, carburizing, nitriding, and different hard facing techniques, nickel-chromium white cast iron exhibits high wear resistance. However, the core of the alloy suffers from low toughness. Therefore, it would be beneficial to harden the surface of nickel-chromium white cast iron via laser surface technology and keeping the core without hardening to be tough and resist high impact shocks This experimental investigation of laser surface treatment was carried out on different type of nickel-chromium white cast iron (S1 2.77 % C, 9.4 % Cr, 2 % Si, 5.5 % Ni), (S2 3.1 % C, 2.44 % Cr, 0.7 % Si, 4.0 % Ni). The laser Nd:YAG a 2.2-kw continuous wave was used. The aim was to compare the chemical and physical properties achieved by this type of selective laser surface treatment and those achieved by the conventional heat treatment. The latter treatment comprises heating to 800 C° and holding for 8 hours, then slow cooling at the rate 30 °C / hour to room temperature. This is followed by tempering for 4 hours at 300 °C. Laser power of different values (600, 800 and 1000 Watt) with corresponding three different laser scanning speeds (3, 4 and 5 m/min) were adopted to reach the optimum conditions for impact toughness and wear resistance for laser heat treated (LHT) samples. The treated samples were characterized by microstructural investigation, Energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), hardness and impact toughness. Impact energy of LHT samples indicated higher values compared to the conventionally heat treated (CHT) samples. The hardness levels and wear resistance are improved at high laser power density and high heat input apparent the grain refinement and homogenized structure, microhardness was traced to the assess the depth under the treated surface. The optimum conditions for impact toughness and wear resistance for laser heat treated (LHT) samples were recorded at a power of 1000 Watt at scanning speed 3, 4 m/min with heat input 2, 1.6 J/mm2.