Carburizing is the term for adding only carbon. This layer can consist of a gamma prime Fe4N or an epsilon Fe2-3 N composition depending on the percentage of each gas in the chamber. Figure 11.2. We perform advanced case hardening on a wide variety of steels. Indeed, Argon and H2 can be used before the nitriding process during the heating up of the parts in order to clean the surfaces to be nitrided. In practice, this distance is limited only by the voltage (Brown, 1994; Engel, 1965). Plasma nitriding. During the last few decades, an impressive number of process variations and applications have been developed. Here, the duration of efficient melting, nitrogen absorption and diffusion is too short. In addition to increasing the steel's abrasion-resistance, the nitride layer also improves the fatigue strength and reduces the friction coefficient. 6.12A, where the flame head provides both flank and root hardening. For a comparison of the two processes choose the Plasma vs. Gas option from the menu. The plasma is ignited between the furnace wall and the screen (metal mesh), which acts as a cathode. 8.03.4.1 Plasma Nitriding. Substrate treatment has considerable influence on the tribological properties of ta-C coated AISI 4140 steel. At the end of the process cycle, the power was switched off and the specimens were slowly cooled to room temperature in the nitriding chamber. The major case-hardening processes include nitriding, carburizing and their combinations, e.g., nitrocarburizing. This in turn reduces the stresses in the coating when loaded, leading to improved tribological properties and coating durability. Furthermore, the heat treatment distortion is significantly lower in dual-frequency method. External spur and helical gears, worm gears, bevel and internal gears, racks, and sprockets are typically induction hardened. He observed and modeled an initial parabolic growth of the compound layer, which then saturates after a certain plasma nitriding time. It is frequently used for forging dies or casting molds to raise resistance to wear and thermal fatigue. The dual-frequency process is a recent version of induction hardening, where two different frequencies, high and low, are simultaneously used for heating. Grün and Günther, 1991; De Sousa and Alves, 1997, Marciniak and Karpiński, 1980; Marciniak, 1983, The Science and Technology of Materials in Automotive Engines, Materials Science of Thin Films (Second Edition), Advanced Gear Manufacturing and Finishing. An immersion quench tank or spraying water through jets passing through the inductor coils is used to quench gear. Each of these gases is mixed with air in particular ratios and burnt under pressure to generate the flame that the burner directs onto the workpiece. The process involves low temperature (350-450°C) nitriding and/or carburizing, which super-saturates the surface of the metal and expands the lattice. Selective heating and, therefore, hardening, is accomplished by designing suitable coils or inductor blocks. As technology progressed so did the plasma nitriding process. In this method, instead of heating the whole gear; the heat can be precisely localized to the specific areas where metallurgical changes are desired (e.g., flank, root, and gear tip can be selectively hardened) and therefore the heating effect on adjacent areas is minimal. The parts are placed in a vacuum chamber and the furnace is filled with process gas containing N2 and H2 to a pressure of 100–800 Pa. The processing time is dependent on the composition of the steel being nitrided and the required case depth. The thickness of the hardened layer should be such that it can withstand the maximum contact stress without collapsing into the softened core of the gear-tooth. Dimitrov has developed a general diffusion model for surface plasma treatment that takes into account the erosion of the material surface (60–62) based on the original work of Wagner (63,64). Upon reaching the desired vacuum, the unit is back-filled with a process gas to begin the preheating cycle. Damage can be avoided by proper adjustment of pressure or by covering the critical size holes with a mechanical mask. Its thickness is usually below 13 µm , which can be reduced further by controlling the ratio of nitrogen in the mixture of nitrogen and hydrogen during ion-nitriding. This is very important with respect to nitriding control and the required understanding of involved phenomena because the inducing processes of coating formation have to be related to correct “physics.” An overview on selected publications within this field will be given in this subsection. Plasma nitrided gears have case hardness of between 58 and 63 HRC and possess excellent wear resistance and extended service life. In this context, surface engineering by pulsed lasers can be assisted by PVD or spraying processes or, for example, by applying mechanical stress fields during processing in order to minimize thermal-induced cracking, especially on the millisecond timescale. Pictures courtesy of Advanced Heat Treat Corp. Monroe, Michigan. While every surface treatment has its advantages and disadvantages, gas and ion (plasma) nitriding are often compared when engineers decide what is best for their application. Some of these methods are discussed in the following section. The glow discharge at the surface of the steel part produces atomic nitrogen by ionisation of the ammonia gas. The surface of the heated piece is additionally heated by the plasma. The well-known fatigue issue during prolonged heat treatments on titanium will be minimized, too. At temperatures below 600 °C, on the other hand, the deposition of oxygen predominates, and there is no extensive nitriding. In addition, the fatigue strength of a gear-tooth may also be significantly increased. After loading the parts in the working chamber the technological program begins. X-ray diffraction patterns of duplex treated (PVD titanium and plasma nitriding) aluminium alloy; (a) plasma nitriding at 500 °C, (b) plasma nitriding at 550 °C. In the tooth-to-tooth method the gear is heated and quenched by the machine itself, which limits the amount of heat going into the gear. Schematic of plasma nitriding of a gear. Additional important properties are also achieved including higher corrosion and abrasion resistance plus improved fatigue strength. The glow discharge in electropositive gases is maintained when the cathode emits electrons and light quanta from the gas under positive-ion bombardment. Ion plating and subsequent plasma nitriding were applied to aluminium alloy in an investigation to improve the tribological properties of such alloys (Ashrafizadeh, 1992). 6.10). After World War II the plasma nitriding process received widespread acceptance in Germany, Russia, China and Japan. At Metal Plasma Technology (MPT) in Valencia California, we are experts in plasma ion nitriding. Consequently, non-uniform nitriding can be expected. As can be seen in Figure 11.1, pressure has an effect on the thickness of the cathode dark space (CDS) and negative glow (NG). During plasma nitriding, three competing processes can simultaneously affect this oxide film; sputtering of the oxide, deposition of oxygen atoms/ions, and solution of some oxygen followed by diffusion of oxygen into the metal. Furthermore, the combination of two techniques, such as hardening and coating, or superfinishing and coating, or finishing and hardening, achieves a significantly enhanced effect than a single treatment. The combination of the heat and energy of the plasma cause the gasses to react with nitride forming elements in the steel. 6.11). No considerable nitriding was observed at, or below, 450 °C. The active screen leads to a better uniformity of temperature in cold-wall furnaces, with the plasma on the screen acting like an internal heating system. Special process engineering features … Instead, the atoms of nitrogen and carbon that deposit on metal surfaces modify them by diffusing into the underlying matrix. In this technique the glow discharge phenomenon is used to introduce nascent nitrogen to the surface of a gear and its subsequent diffusion into subsurface layers . A special technical feature of this process is the possibility of using mechanical masking to provide accurate partial nitriding. (2005) have investigated the DC pulsed plasma nitrided 4140 steels and found that the process improved wear resistance. The main advantages of plasma nitriding over conventional nitriding processes are: reduced cycle time, controlled growth of the surface layer, elimination of white layer, reduced distortion, no need of finishing, pore-free surfaces and mechanical masks instead of copper plating. Plasma Nitriding, also referred to as ion nitriding, is a diffusion process that improves the wear resistance and fatigue properties of the product being nitrided. Accurate analysis of the diffraction patterns of the nitrided surface in Fig. Coatings of titanium, deposited onto the aluminium substrate, were subjected to a series of plasma nitriding cycles in order to form a graded interface and hard metal compounds on the surface. Nitriding of thinner films does not seem to be very beneficial since the films were mostly removed by the sputtering action of the process. 6.12B where only the flank is hardened, leaving the root area untreated . The process was developed in the 1920's as an alternative to the gas nitriding process. Tooth-at-a-time flame hardening. The composition of the white layer provides natural lubricity and corrosion resistance, provided its depth does not exceed 10–12 µm [41,42]. In plasma nitriding processes nitrogen gas (N2) is usually the nitrogen carrying gas. In common, these processes all rely on the undersized atoms of N, C, and B. Plasma nitriding process techniques can also be known also as Glow discharge nitriding, or Plasma nitriding Continuous DC nitriding, Pulsed Plasma nitriding. This method is generally not applicable for a tooth size finer than 16 DP. Improved controls and, in later years, the microprocessor have allowed engineers to consistently control the metallurgical properties of the nitride layer. The properties obtained in this duplex treatment were much better than those obtained by either nitriding or ion plating. Nitriding is usually done by heating steel objects in gaseous ammonia (NH 3) at temperatures between 500 and 550 °C (950 and 1,050 °F) for periods of 5 to 100 hours, depending upon the desired depth of diffusion of the nitrogen. The limited space for the glow in the tube results in disturbance caused by compression of CDS and NG. As the process gasses react with the elements in the steel, a wear resistant layer is formed. For partial carburizing, the plasma effect may be prevented by covering with metallic conducting masks or sheet metal where it is not required. If the temperature of the hollow cathode is controlled, this phenomenon can be used for a very effective nitriding of various parts with a need for hardening small holes, slots, etc. Laser nitriding is a very efficient technique, allowing accurate spatial control of the surface treatment without any undesired heating of the substrate. Gas nitriding is only applied, as a rule, for alloyed steels. Table 8.3 summarizes the major case-hardening processes. The other method is shown in Fig. To achieve greater depth of heat penetration, low-frequency current is used, whereas heat treatment at shallow depth requires high-frequency current [1,41]. The results indicated that a significant improvement could be achieved when titanium films of adequate thickness are nitrided to produce hard compounds on the surface, above the aluminium–titanium interface. The microstructure developed according the solidification conditions in needle-like TiN phases and dendrites. In carbonitriding, the main element is carbon with a small amount of nitrogen. MPT plasma ion nitriding vessels are computer controlled and utilize the latest software programs to control the entire nitriding process. Due to the nitrogen input, a diffusion layer and a compound/precipitate layer develops. This has been done by developing advanced coatings such as hybrid- and nanocoatings and their deposition methods, which include magnetron sputtering and plasma-enhanced deposition; modern mechanical hardening methods such as ultrasonic, laser and cavitation jet peening; and advanced case-hardening methods like plasma nitriding and induction and flame hardening. With the nitriding furnaces fully automated, we are able to run 24/7. The heating response of the two identical blocks of steel with different emissivities nitrided in the cold-wall DC plasma is shown in Figure 11.3. Higher surface, case, and core hardness than gas nitriding. Case-hardening is regarded as thermomechanical treatment to modify the surface properties of gear-teeth. Induction hardening of gears is done by two methods: spin hardening and tooth-to-tooth or contour hardening . Manisekaran (2005) has reported 2-fold increases in erosion resistances for plasma nitrided 13Cr–4Ni steel compared with as-received 13Cr–4Ni steel but poor performance compared with laser hardened 13Cr–4Ni steel. Despite this, plasma carburizing of steel has not displaced conventional heat-treating to the same extent that its low-temperature plasma-nitriding counterpart has. Table 8.3. In particular, the specimen plasma nitrided at 500 °C had negligible wear under various loads (Table 10.4). This white layer is brittle and relatively inert. There is also perturbation of cathode fall in the tube caused by the sputtered iron atoms contributing to formation of the dusty plasma (Choi et al., 1991). Table 2.2 shows important aspects for different timescales. Except for the plasma assist and low pressures, these processes are very much like traditional nitriding and carburizing, which in essence are high-temperature, atmospheric-pressure CVD treatments. The increasing interest in plasma surface treatment is manifested in the growing number of conferences on this topic (51–55). free of toxic salts, ammonia and any other toxic gases) Phase controllable compound zone (or white layer) Less distortions than gas nitriding because plasma nitriding is performed at a lower temperature and under a vacuum. After World War II the plasma nitriding process received widespread acceptance in Germany, Russia, China and Japan. Due to the ion bombardment via the plasma and potential difference, both the nitrogen diffusion mechanisms and the parabolic law of layer growth are changed (56–59).