PRODUCTION OF CASE HARDENING COMPOUND FOR SURFACE TREATMENT OF MILD STEEL FOR IMPROVE MECHANICAL PROPERTY
CHAPTER ONE
INTRODUCTION
1.1 Background Information
Most engineering materials have unique properties that render them useful in many applications. Before any engineering material is subjected to used, it must satisfy certain range of properties suitable for that purpose, the usage is determined by its availability, cost, corrosion, wear resistance, durability and other functional requirement (Linus and Ihom, 2013). Mild steel is one of classes of steel that contain low quantity of carbon about 0.1-0.25% and it has a density of 7850kg/m3. Mild steel has wide range of application in the production of engineering element like gears, cam, shaft, pinions, keys etc, in developing countries like Nigeria, mild steel is commonly used for machinery construction because of its availability, dominance, workability, ductility, cost-effectiveness, wear resistance etc among the class of steel (Ihom and Aniekan, 2014).
There are numerous limitations encountered in the usage of the mild steel particularly as regarding wear resistance, hardness and strength, most machines constructed in the developing world like Nigeria using this engineering material, do not last long especially when moving components of the machine like gear, shaft, valves, disc, cam etc which requires a tough core and a hard surface are constructed of mild steel without casehardening heat treatment (Ihom, Nyior Alabi, Segun Ogbodo, 2012).
According to Ihom and Aniekan, (2014), casehardening of a material can be accomplished by subjecting the component to high temperature in the presence of a carbonaceous material which may be solid, liquid or gaseous, energizer are often used to speed up the process. It is a process used to produce a high surface hardness for wear-resistance supported by tough, shock-resistance core. It is also the process of saturating the surface layer of steel with carbon to about 0.19% or some other process by which case is hardened and core remain soft. A survey of Nigeria industrial stock market reveal a dearth of commercial pack carburizing compound that fabricators, machinist and blacksmiths who produce or recondition these vital engineering element could conveniently use to carburize or case harden them. The resultant effect of this is the non-treatment of such produced parts with the consequent results of fast wear, tear and failure of the parts made up of mild steel (Dempsey, 2002).
Production of case hardening compound for surface treatment of mild steel using different local carbonaceous material cannot be overemphasized. This is because of machine component fabricated from mild steel, when subjected to friction, it easily get wear and tears, for effective usage of the machine part, the mild steel can be subjected to casehardening compound formulated from local carbonaceous element like coal, bone, charcoal, snails shell, periwinkle shell, above all, the hardness of mild steel pack carburized with each of the local carbonaceous element above will be determined, also the case depth will be determine.
Case hardening is the process that is used to harden the outer layer of case hardening steel while maintaining a soft inner metal core. The case hardening process uses case hardening compound for the carbon addition. Case hardening is a technique in which the metal surface is reinforced by the addition of a fine layer at the top of another metal alloy that is generally more durable. Case hardening steel is normally used to increase the object life. This is particularly significant for the manufacture of machine parts, carbon steel forgings and carbon steel pinion. Case hardening is also utilized for other application. Case hardening has been in use for many centuries and was frequently used for producing horseshoes and different kinds of cooking utensils that were subjected to substantial wear and tear. Case hardening is performed normally on top surface and for a limited depth. Greater hardness is usually related with cover wear and fatigue resistance (Ogo and Ette, 1994).
Nwoke (2014) report that carburizing is one of the most commonly performed steel heat treatment. Over the years it was performed by packing the low carbon iron parts in charcoal, then raising the temperature on the back to red heat for several hours. The resulting interstitial solid solution is harder than the base material.
Components that are subjected to severe impacts and high pressures are generally case hardened. The surface that needs special hardness may be selectively hardened, without performing case hardening of the remaining object. Firearms are usual item that is case hardened, as they need accuracy in machining and higher hardness for performing the desired function. Another general application of the case hardening is on camshaft and special purpose screws, mainly the self drilling screws. Case hardening is less complex for fasteners and screw since it is performed simply by heating and quenching. Case hardening of smaller items is performed by repetitive heat application. Case hardening is useful for objects that need to be hardened externally to endure wear and tear but soft internally to withstand shock.
Case hardness of carburized steel is primarily a function of carbon content. When the carbon content of the steel exceeds, about 0.50% may not dissolved, which would thus require temperature, high enough to ensure carbon-austenite solid solution. Case hardening depth of carburized steel is a function of carburizing time and the available carbon potential at the surface. When prolonged carburizing times are used for deep case depth, a high carbon potential produce a high surface-carbon content, which may thus result in excessive retained austenite or free carbides. These two microstructural elements both have adverse effects on the distribution of residual stress in the case-hardened part. Consequently, a high carbon potential may be suitable for short carburizing times but not for prolonged carburizing.
Carburizing steel for case hardening usually have base-carbon content of about 0.20%, with the carbon content of the carburized layer generally being controlled at between 0.8 and 1% carbon. However, surface carbon is often limited to 0.9% because too high carbon content can result in retained austenite and brittle martensite, (liwellyn,1992).
Most steel that are carburized are killed steels, (deoxidized by the addition of Aluminum), which maintain fine grain sizes to temperatures of about 1040oC, steel made to coarse grain particles can be carburized if a double quench provides grain refinement. Double quenching usually consists of a direct quench and then a requench from a lower temperature.
Many alloy steels for case hardening are now specified on the basis of core hardenability. Although the same consideration can be apply to the selection of uncarburized grades, there are some peculiarities in carburizing application.
In case hardened steel, the hardenability of both case and core must be considered, because of the difference in carbon content, case and core have quite different hardenability and this difference is much greater for some steels than for other. Moreover, the two regions have different in service functions to perform. Until the introduction of lean alloy steels such as the 86xx series, with and without boron, there was little need to be concerned about case hardenability because the alloy content combined with the high carbon content always provides adequate hardenability. This is still generally true when the steels are direct quenched from carburizing so that the carbon and alloying elements are in solution in the case of austenite. The parts that are reheated for hardening and in heavy-sectioned parts, however, both case and core hardenability requirements should be carefully evaluated (Dara, 2007)
The relationship between the thermal gradient and the carbon gradient during quenching of a carburized part can make a measurable difference in the case depth as measured by hardness. That is, an increase in base hardenability can produce a higher proportion of martensite for a given carbon level, yielding an increase measured case depth. Therefore, a shallower carbon profile and shorter carburizing time could be used to attain the desired result in properly chosen steel. Gears are always oil quenched because distortion must be held to the lowest possible level. Therefore, alloy steels are usually selected, with much debate about which particular alloy is most useful and durable. The lower-alloy steels such as 4023, 5120, 4118 etc with a carbon range between 0.15 and 0.25% are widely used and generally satisfaction, (Chongxue,2009).
1.8 Statement of the Problem
Due to numerous short fall encountered in mild steel usage particularly were resistance to wear, tear most machines constructed in the developing world like Nigeria using this engineering material do not last long, especially when moving components of the machines like gear, shaft, valve, disc etc are involved.
Analysis of Nigeria industry stock market reveal a death of commercial pack carburizing compound that fabricators, machinist and blacksmiths who produce or recondition these vital engineering element could conveniently use to carburized or case harden them, the resultant effect of this is non-treatment of such produced part with the consequent result of fast wear, tear and failure of the component. The failure of engineering material is undesirable for several reasons, which include loss of human lives, injuries, economic losses and interference with availability of products and services. The used causes are improper selection and processing of materials and inadequate design and misuse of component.
1.9 Objective of the Study
Main objective: The production of case hardening compound for surface treatment of mild steel for improved mechanical property.
The specific objectives:
- To produce case hardening compound for surface treatment of mild steel for improved mechanical properties
- To test the developed compounds
- To evaluate the efficacy of the developed case hardening compounds through hardness test.
1.10 Justification of the Study
This work is justified by the fact that it will give rise to the development of the carburizing compound that will be used for carburization of mild steel. This will improve the mechanical properties of component and spare parts made of mild steel.
1.11 Scope of the Study
The scope of this work does not extend beyond the production of case hardening compound for surface treatment of mild steel for improved mechanical properties, and testing of the developed compound and evaluation of the developed compounds for efficacy using hardness test and microscopy of treated mild steel.
