Improvements on properties of vermicular microstructure cast alloys for new generation automobile by gas nitriding

While automobile companies produce a new engine, they have to consider customer expectations and legal regulations. Customer expectations rise as high performance and low fuel consumption. Legal regulations are agreement of exhaust emissions to EURO standarts. To agree these expectations and regulations, automobile companies increase compression ratio in engine and feeding air quantity for combustion. However, this solution intensify the damage mechanisms in engine by causing a rise in internal pressure and the combustion temperature. Therefore, the geometric and material designs are made to overcome this problem. Since cylinder heads have combination of very thin and thick sections, it is the part having the most complex geometry in engine. The nonuniform thickness causes the different cooling rates during casting and also heterogeneous microstructure. As a consequence of that, it brings together lots of difficulties when modelling on thermal and structural analysis programs. Because of these reasons, the geometric and material designs for the cylinder head is the most important factor in determining the competition between automotive companies. In other words; cylinder head is more important than the other engine parts. Nowadays, vermicular cast iron (CGI) is used as a cylinder head material in order to meet customer expectations and legal obligations. Companies make geometric designs on the cylinder head in order to provide the near future EURO standards. However, increasing customer expectations and become more stringent legal obligations with each passing day will be unable to resolve by only the geometric design. This situation that companies are in reveals the necessity of improvement of CGI or finding a new material more resistant to damage mechanisms on the cylinder head. Therefore, automotive companies have already begun to work on the cylinder head material. Damage mechanisms that result from the combustion on the lower surface of the cylinder head are thermo-mechanical fatigue, high cycle fatigue, wear and corrosion. As a result of start-stop of engine or increase-decrease in engine rpm (revolutions per minute), engine gets warm and cool. Therefore, the temperatures and stresses by acting cyclically cause thermo-mechanical fatigue failure on the cylinder head. Sehitoglu model is mostly used for thermo-mechanical fatigue. In this model, the thermo-mechanical fatigue is described as the sum of high temperature oxidation fatigue and creep damage mechanisms. High cycle fatigue occurs on the cylinder head by cyclic stresses result from piston movements and explosion. Also, the motion of the pistons and valves causes wear on the cylinder head. Another important damage mechanism on the cylinder head is corrosion which arises from environmental effects during shipping and usage of the engine. The cylinder head is exposed to humudity of the air during transportation and usage, and so the cylinder head corrodes. When it is considered tahat engines transport by ships, the corrosion resistance is concluded how important. Because, due to chlorine ions in seawater contacts directly or indirectly with the cylinder head, corrosion mechanism starts to act as more severe. Developing engine technology requires to advance material properties such as thermo-mechanical fatigue, high hardness and high corrosion resistance. These properties are directly related with the surface features, because the surface of cylinder head is exposed to higher stresses and abrasive forces than the interior parts. Also corrosion starts from surface of the cylinder head. Thus, improvement of CGI which is current cylinder head material instead of working on a new cylinder head material offers a more economic and quick solution. Even if CGI becomes more resistant to at least one of these damage mechanisms, it provides greater benefits to the automotive industry. For these reasons, gas nitriding is used as a surface modification technique to improve the surface properties of CGI. Nitriding techniques are classified as plasma (ion) nitriding, salt bath nitriding and gas nitriding. Since plasma nitriding has high installation and operation costs and does not allow to nitride multiple parts at the same time, and also because the salt bath nitriding contains cyanide which is harmful to the health and the environment, gas nitriding technique was used in this study. The basic principle of gas nitriding method is based on the diffusion of the nitrogen atoms. In the gas nitriding process, nitrogen atoms are supplied by ammonia gas. When ammonia gas sends over workpiece, it dissociates cataytically and nitrogen atoms diffuse into octahedral intersititial sites of iron lattices. After gas nitriding, iron nitrates ε-Fe3N and γ′-Fe4N are formed on the surface of CGI due to high nitrogen concentration. This zone is named as white layer (or compound layer). Nitrogen concentration decreases from surface to inner region. So, when the nitrogen concentration falls below the solubility limit of nitrogen in iron, solid solution are formed. In this region named as diffusion zone, nitrogen atoms lead to the lattice distortion. Hence, the fatigue resistance of CGI increases by occuring compressive residual stresses. In all experimental studies, CGI GJV-450 CGI samples are gas nitrided by keeping the temperature constant at 525 °C. In order to determine the effect of the nitriding potential, experiments were performed at 0.4, 1, and 2 atm-1/2 the nitriding potential value for 3 hours. Also to examine the effect of the nitriding time, the nitriding potential value was set as 2 atm-1/2 and the samples were gas nitrided for 1, 3 and 6 hours. After gas nitriding, XRD analysis, GDOES analysis and optical profilometer test, SEM analysis, EDS analysis, X-ray mapping, surface and cross sectional microhardness test and accelerated salt spray corrosion test were conducted on the nitrided samples. The results shows that the surface hardness and the corrosion resistance of CGI increases by increasing nitriding potential and/or nitriding time. In SEM images, it was concluded that increment on nitriding potential and/or nitriding time causes the pearlite in the microstructure to coarsen and the pearlite structure to be deformed. İt is also observed from SEM results that, after the white layer is formed on the surface, it proceeds between the grain boundaries and also graphite boundaries. The mechanical and corrosion resistance of GJV-450 CGI have been improved by using the gas nitriding process. Therefore, gas nitriding provides new generation automobile parts such as especially cylinder head and also manifold, blok etc. to be more resistant to the damage mechanisms. Hence, it makes engine parts possible more durable and longer useful life. By this study, it has been concluded that CGI can gas nitrede and also gas nitriding provides an increase in the hardness and corrosion resistance of CGI. Since nitriding increases the fatigue strength of the materials, it is thought that the fatigue resistance of CGI improves by gas nitriding. However, in order to improve this work and to prove that gas nitriding increases the fatigue resistance of CGI, nitrided CGI samples requires to perform fatigue tests.