Nitration effect on high temperature wear resistance of high speed tool steels

High speed steels (HSS) is a specialized group of highly alloyed tool steels and they used for applications requiring long life at relatively high operating temperatures such as for heavy cuts or high-speed machining. The characteristic properties of high speed steel are high working hardness, high wear resistance, excellent toughness and shock endurance at room temperature and elevated temperature due to their high alloy content. They have ability to retain their high hardness at elevated temperature, which allows to reach high cutting speeds. In cutting tool industry, in comparison with other cutting tools, high speed steels are more economical. Besides, they used for hammer drilling and cutting processes that require high toughness. Although wear resistance of high speed steels are lower than cement carbides, their surface hardness and wear resistance can be improved by developing surface treatment technology. In this dissertation work, DIN 1.3343, DIN 1.3243 and DIN 1.3247 quality high speed steels which are used for cutting tool, are hardened and tempered before nitriding. After nitriding process their wear resistance at room temperature and high temperature were investigated. Prepared specimens were hardened in salt bath furnace for 210 seconds at 850-1050-1180/1200-580°C respectively. Then they were cooled in air. 4 stages tempering process at 550-550-570-550°C for 90 seconds is performed for preventing retained austenite in hardened high speed steels and then cooled in air again. Part of hardened and tempered steels is nitrided via controlled gas nitriding process. In this process ammonia flowed into furnace and total gas amount in the furnace is calculated by control system, based on the furnace volume and incoming gas flow. The furnace was preheated for 3 hours and equalized to the pre-set temperature (500°C). During each process stage of nitriding, the temperature and gas flow were controlled to achieve the set-point Kn values, which depends upon ammonia dissociation percentage. When the system temperature rises above 400⁰C, Kn value decreases with dissociation of ammonia. Nitriding consists of two stages, stage 1 begins when the temperature reaches 500°C. During stage 1, Kn value continues to decrease when desired Kn value achieved, stage 2 begins. Operating parameters were as follows: stage 1, set-point Kn=14 (atm)−1/2; stage 2 set-point Kn=0,8 (atm)−1/2. Kn value was high selected during first stage to ensure fast diffusion of nitrogen into surface, after that it was reduced and fixed for second stage not to reach oversaturation (causes brittleness) of system. After preheating of system, the temperature is constant until cooling stage. When the second stage is finished, cooling stage was automatically activated until the pre-set cooled temperature (100°C) level is achieved, which takes about 2,5 hours. The heat and process time for stage 1 is about 1 hour and second stage is about 8 hours. However, the whole process takes about 14,5 hours, which includes actual nitriding time (9 hours) of two stages. Prepared hardened and nitrided or only hardened specimens were grinded step by step using SiC abrasive papers starting from 600 grit to 2500 grit followed by fine polishing with diamond paste. After metallographic preparation of specimens, microstructure examination was carried out by using optical microscopy between 5 and 50X magnifications. As a result of this examination, matrix consists tempered martensite along with carbides; also diffusion layer near surface of nitrided steels was observed visually. To create nitrogen profile of diffusion layer observed with optical microscopy, Energy-dispersive X-ray spectroscopy (EDX) analysis was conducted. The measured thickness of the diffusion layers of steels as follows: DIN 1.3343 40μm, DIN 1.3243 37μm and DIN 1.3247 25μm that was observed by drawn nitrogen profile. Nitrided specimens which have different composition also have different thickness of the diffusion layers depending on the alloy amount they have. By the increasing alloy ratio of specimens, the thickness of the diffusion layer is decreased, due to the difficulty of the diffusion of nitrogen to the structure. According to results of EDX analysis from spots, MC and M6C type carbides were determined which had seen brightly coloured in the results of optical microscopy examination. To see the distribution of nitrogen and iron in the structure, elemental mapping is performed by using EDAX detector by selecting iron and nitrogen elements. Nitrogen was found in the region close to the surface more intense by elemental mapping. X-ray diffraction (XRD) analysis of the nitrided steels was carried out to detect which phases composed after nitriding. During this investigation, Cu-Kα radiation was used to obtain diffraction patterns of the specimens. The measured 2θ range between 10 and 90° was scanned with the velocity of 1°/min. Based on the study results, hard and brittle phases; γ ́-Fe4N and ε-Fe3N between the formed phases are absent which also known white layer. Instead of γ ́-Fe4N and ε-Fe3N phases, α-Fe, MC and M6C phases were found. The applied nitriding process has proven not to have white layer by the optical examination and X-ray diffraction analysis. After having demonstrated the success of nitriding process, effect of nitriding on the material was examined. Surface hardness studies of material were conducted by applying load ranging from 0,3 to 2 kg to the material surface, and these studies indicated that nitriding process increases the surface hardness of the materials. The different compositions owned by the selected samples; the hardness was determined to be dependent on the amount of alloy that increases with increased amount of alloy. Hardness increases with increasing alloy ratio, due to the use of high amount of elements, which form primary carbide that is underlying mechanism of hardness. However, DIN 1.3343 and DIN 1.3243 nitrided steels have not a significant difference between their hardness, while their non-nitrided samples have different hardness. The hardness of nitrided steels from their cross section was examined that applying the maximum load of 25 g(250 mN) with Vickers indenter for 2 second. There is a decrease in hardness from the surface to the centre of material, and the points, which have significantly decreased hardness, were concluded that they were out of the diffusion zone. After all of these investigations, ball on wear tests were performed to examine the effect of nitriding to the wear resistance at room and elevated temperature of high speed steel. Ball on wear tests parameters were as follows: applied load 3 N, distance 300 meters, velocity 16 cm/sec and alumina ball used. Wear tests were carried out at three different temperatures including room temperature, 400°C and 600°C. Charts of friction coefficient which were drawn from wear analysis specify that there is no significant change on friction coefficient of specimens in the same temperature, but it was observed that the friction coefficient reduces with increasing temperature. When the wear areas of specimens compared, nitrided specimens have less wear area than non-nitrided ones for all temperatures. While minimum wear area was observed at room temperature, maximum wear was observed at 400°C. Wear areas of specimens are maximum at 400°C compared with other temperatures, due to the brittle and vulnerable formed oxides on specimens' surface. The formed oxide layer at 600°C is more than at 400°C. Thus, the formed oxide layer at 600°C provides protecting layer on the wear surface of specimen which also known glazing effect. Specimens increasing hardness affects the wear resistance that was observed on specimens, which have high hardness also have higher wear resistance.