Metallographic preparation of nitrided and nitrocarburised components Application Nitriding is a thermochemical process by Notes which the surface of a ferrous metal is en- riched with nitrogen to improve the wear re- sistance of components. In nitrocarburising not only nitrogen but also small amounts of carbon are involved in the process. The result is a nitrided layer consisting of the compound layer (“white layer”) and the diffusion zone immediately below the com- pound layer. The nitriding and nitrocarburising process using gas or salt bath was developed in the early 20th century in Germany and the United States. The development of ion or plasma nitriding started in the 1930s but Nitrided steel, colour etched with Beraha’s reagent was not commercially used until the 1970s. All three nitriding methods have advantages dimensions and do not need costly finish- and the selection of a particular method ing work such as grinding or straightening depends on the specific application of the after nitriding. nitrided component. Nitriding and nitrocarburising are mainly Nitriding produces a hard, wear resistant used for ferrous components such as layer on carbon and low alloy steels and valves, camshafts and piston rods in the cast iron. In addition, it considerably im- mechanical engineering and automotive proves fatigue strength and by oxidizing industries. Other applications are cutting the nitrided surface, it enhances corrosion tools or large forming dies. Cast iron parts, resistance. such as pump and gear houses, can also be nitrided. The main advantage of nitriding and nitro- Metallography of nitrided and nitrocar- carburising over other means of surface burised components is mainly used for hardening is the low process temperature controlling the nitriding process through (500-600°C), preferably 580°C. Compo- examination of the layer. The white layer, nents can often be nitrided in the fully hard- diffusion zone and porous zone are meas- ened and tempered condition without the ured and evaluated. In addition, failure core properties being adversely affected. An analyses of used parts are carried out to additional advantage of the low temperature see if faulty material, surface deterioration process is the low risk of distortion. Conse- or the nitriding process were contributing quently the parts can be machined to final factors. Difficulties in the preparation of nitrided parts: - Chipping and cracking in the layer - Edge-rounding Solution: - Proper mounting - Plane grinding with SiC paper - Fine grinding with diamond - Diamond polishing on hard cloths Fig.1: Shrinkage gap between specimen 500x Fig. 2: Without edge retention, the layer is not in focus and mounting resin can cause flaking at high magnification of the nitrided layer and trap abrasives Formation and Nitriding processes composition of the and application of nitrided layer nitrided parts At nitriding temperature the nitrogen dif- Fig. 4: Nitrocarburised carbon steel after heating for 45 Before nitriding, the components have min/300°C, etched with 1% Nital showing nitride need- fuses into the steel surface and reacts with les in the diffusion zone. to be thoroughly cleaned and degreased. iron, forming γ’ iron nitride (Fe4N), contain- Any surface contamination from grinding ing up to 6 wt % N. With increasing nitro- particles, oil or metal chips will result in gen the ε-phase (Fe3N) is formed, which an uneven formation of the nitrided layer. can absorb up to 11 wt % N. This can cause cracks in the coating which These two iron nitride phases, ε+γ’, form leads to flaking and corrosion (see Figs. 6 the compound layer, also called “white and 7) After cleaning, the parts are dried layer”, because it stays white when the steel and preheated and then transferred to the is etched with Nital. This compound layer actual nitriding environment. does not contain any metal but consists of The various nitriding processes can be dif- a non-metallic phase formed by iron and ferentiated mainly by their nitrogen source nitrogen that can be called a “nitride ce- and the energy supply. Salt bath-, gas- and ramic”. In the outer areas of the compound plasma nitriding have different advan- layer a porous zone can be found (see Fig. 5: Alloyed steel, nitrocarburised, etched with 1% tages regarding investment cost, process Nital, shows dark diffusion zone, and white compound Fig. 3). layer with dark oxide coat. time, environment, safety and quality. The properties of the resulting nitrided or The percentage of γ’ and ε-nitride depends the steel, such as aluminium, molybdenum, nitrocarburised surface are in many cases on the carbon content of the steel: higher chromium and tungsten. independent of the production process. carbon content promotes the formation of The required case depth is determined by ε, lower carbon content forms more γ’ iron Because of their sub-microscopically fine the application of the nitrided component nitride. distribution the nitrides in the diffusion and can be regulated through the nitriding zone of low carbon steels generally can not With an optical microscope the differen- temperature and time. be seen after etching of the metallographic tiation of ε and γ’ iron nitrides in the com- sample. However, after heating the sample pound layer is only possible by using very In the following paragraphs the different to tempering temperature (200-400° C for special and difficult etching methods. nitriding processes are briefly described 15-30 min), the nitrogen in solid solution A correct analysis of the composition can and the application of the nitrided parts precipitates in the form of γ’ nitride needles. only be made by quantitative structural mentioned. These nitride needles can be etched so that x-ray analysis using deeply penetrating the diffusion zone becomes visible and its radiation. thickness can be measured (Fig. 4). The compound layer is relatively hard and On alloy steels the diffusion zone will be the hardness increases with increasing con- etched dark with Nital, but the nitrides can tent of nitride forming alloying elements, not be resolved with an optical microscope at the same time the case depth decreases. (Fig. 5). Nitrided carbon steels have a surface hard- The thickness of the white layer and the dif- ness of 300-400 HV and alloyed steels from fusion zone depends on various parameters 700 HV to more than 1000 HV. of which the most important ones are time, Below the compound zone is the diffusion temperature and steel composition. The zone containing nitrogen in solid solution. white layer can be between 0-20 µm and In addition, it has stable metal nitrides the diffusion zone up to 0.8 mm, depending formed by the various alloying elements of on the requirements of the application. =18 µm { Porous zone Compound layer ε-Nitride { γ ‘-Nitride 1000:1 Fig. 3: Details of composition of Enlarged • = Fe nitrided layer ° = Possible N-Positions Fig. 6 and 7: Impurities in the steel and on the surface can lead to faulty areas in the nitride layer and cause cracking or corrosion. Nitrocarburised cases are particularly resistant against abrasive wear, scuffing, sliding friction and corrosion. The porous surface can retain lubricants which adds to the running properties of, for instance, camshafts. (Fig. 8) Salt bath nitrocarburising is a fast, flexible and economical process. Typical applica- tions are parts for the automotive industry such as piston rods, crank and camshafts, Fig. 8: Salt bath nitrocarburised alloyed steel (16MnCr5), valves and gears. In addition, nitrocarbu- Fig.10: Gas nitrocarburised carbon steel, 580°C/1.5 hr. etched with 1% Nital, diffusion zone is etched dark, rised components are used in the aircraft compound layer with porous zone shows white. and off-shore industry and in mechanical engineering. bration and torsion. Components with bore Salt bath nitrocarburising holes, undercuts and cavities, for instance Salt bath nitrocarburizing is carried out in Gas nitriding and nitrocarburising gener- sintered steels, are suited for gas nitriding gas or electrically heated crucible furnaces. ally takes place in a sealed, bell-type nitrid- because the gas can easily enter every- The preferred material for the crucible is ing furnace which provides good gas cir- where as it circulates in the retort. Copper titanium. After preheating to 350°C, the culation. The process is mainly controlled plating or special resistant materials can be components are submerged into the salt- by the degree of dissociation of ammonia. used for masking areas that should not be bath, either hanging or lying in charging The ammonia gas reacts at 500-520°C with nitrided. racks, or as bulk material in stainless steel the steel surface and decomposes, thereby Gas nitrided parts are typically machine or Inconel baskets. The salt bath consists releasing nascent nitrogen which diffuses spindles, ductile iron pump housings, door of alkaline cyanate and alkaline carbonate. into the steel surface. As gas nitriding uses lock mechanisms, water pump compo- Through oxidation and thermal reaction a lower temperature, process times are 40- nents and pistons for gas compressors. with the immersed component surface, at 80 hrs. By adding gases containing carbon, nitriding temperature the alkaline cyanate gas nitrocarburising is also possible (Figs. Plasma nitriding and nitrocarburising is releases nitrogen and carbon which diffuse 9 and 10). As a consequence process carried out in a nitrogen - hydrogen at- into the surface of the component. Pure times are reduced. mosphere at 400-600°C and a pressure of nitriding is not possible with the salt bath approx. 50-500 Pa. For nitrocarburising, as small amounts of carbon will always The formation and properties of the com- gases containing carbon, such as meth- diffuse into the surface. The usual process pound layer and diffusion zone are similar ane, are added. The plasma is produced parameters are 90 min at 580°C. to those produced by salt bath nitriding.