1. Introduction
Plasma nitriding (PN) is a common low temperature thermo-chemical surface treatment that is applied to the machine parts and tools made of steels, aluminum alloys and titanium alloys to increase their reliability and performance[1,2]. In this process, a plasma is developed around the workpiece within a chamber by application of a high electric potential between anode (earth-connected chamber wall) and the cathode (the workpiece) in a low pressure N2-H2 atmosphere. N-containing ions and/or radicals are produced within the plasma. Ions accelerate to the cathode where they discharge or dissociate and resulting nitrogen atoms are introduced and diffuse into the surface. Sputtering caused by surface bombardment with ions within …show more content…
Element C Si Mn Cr Mo Ni Co Cu V W wt % 0.80 0.26 0.27 3.80 0.68 0.20 4.70 0.13 1.60 18.9

Table 2: Plasma nitriding conditions.
U E D C B A Sample Code
Un-nitrided sample 425 425 575 500 425 Temperature (°C) 25:75 50:50 75:25 75:25 75:25 Gas composition (N2:H2)
3. Results and discussion
In this section, nitrided layer microstructure and composition, surface hardness and hardness profile, surface topography, and wear test results of the samples treated at different temperatures and gas mixtures are presented and compared with those of the un-nitrided one.
Fig. 1 presents the typical microstructure of the heat treated tungsten tool steel. The microstructure consists of a matrix of martensite and spheroidized carbide precipitates. High speed steels contain a high content of elements such as molybdenum, chromium, tungsten, vanadium, and cobalt along with enough carbon to provide a large amount of stable carbides in the final structure. Secondary hardening after tempering is due to precipitation of mixed carbides. Image analysis by ImageJ software [29] revealed the percentage of alloy carbides to be about 15%.The overall hardness of the steel as reported in table 3 was 910