Development and performance evaluation of an agitated quenching tank/bath

Thesis Overview

<p> 1.0 INTRODUCTION<br>1.1 Background to the Study Agitation of a quenching bath is one of the techniques used to create turbulence in a liquid (water or oil) during quench hardening heat treatment of steel/cast iron components which is usually applied to manufactured machine components before they are finally put into service or the market. The hardening process is usually carried out by using heat treatment equipment comprising a heating furnace and a quenching tank (bath) containing a liquid medium (usually water, brine, oil, caustic soda solution, or polymer solution) at room temperature (Higgins, 1984). Although, there are some types of hardening methods that do not required quenching in a liquid medium. Allen (1979) defined hardening as a number of heating and controlled cooling operations that will provide desired properties in a metal or alloy. According to Rajan et al, (1988), heat treatment is a process of heating and cooling operation(s) applied to metals and alloys in the solid state so as to obtain the desired microstrcutures and properties. Among engineering metals and alloys, steels and cast irons are more versatile and this versatility is based on the fact that their properties can be controlled and changed at will by heat treatment.<br>There are various types of heat treatments, and they include: – annealing, normalizing, quench hardening, spheroidizing, case or surface hardening, tempering, martempering and austempering (Rajan et al., 1988). In this research work the quench hardening heat treatment method is under investigation. It involves heating a steel or cast iron component to the austenite range (800 – 950ï‚°C), allowing the component to<br>2<br>stay at that temperature for some time (for uniform temperature throughout the component), then removing it and dropping it into a liquid medium (quenchchant) in a container called a quenching bath at room temperature. This process is referred to as quenching, and it makes the quenched component acquire high hardness due to the formation of martensite which is known to be hard and brittle. This resultant high hardness makes possible the use of steel for metal cutting tools which can maintain sharp cutting edges under severe operating conditions, and for dies which can resist abrasion and wear Quench hardening is also used for hardening machining tools to make them resistant to wear during machining operations and to increase their life span. There are basically two types of quenching baths: –<br>a. Static or unagitated quenching baths<br>b. Agitated quenching baths<br>Considering the importance of heat treatment as one of the industrial finishing operations which help to increase the service life of components in service, there is need to contribute through research, the development of the metal industry. Therefore, the objective of this work is to design, fabricate and evaluate the performance of an agitated quenching bath. The agitation rate will be varied using a variable speed electric motor drive, while carbon steel and ductile cast iron alloys will be used to test or evaluate the performance of the fabricated agitated quenching bath. 1.2 Statement of the Problem<br>Quench hardening of machine components for the purpose of improving their mechanical properties is done either in a static quenching bath or in an agitated quenching bath. The result of survey of most small and medium scale metal casting<br>3<br>and fabrication industries and metal workshops of mechanical engineering departments of most tertiary institutions in the country show that a majority of these use the static quenching baths for their quench hardening operations. The reason for this may be unavailability of agitated quenching bath due to high cost of importation of these baths. But in terms of achieving better mechanical properties, the use of an agitated quenching tank is preferred, but it costs more because it is imported. Hence there is need to design and construct locally an agitated quenching bath that would be affordable to indigenous metal casting and fabrication industries and mechanical workshops of tertiary institutions. 1.3 Justification of the Study Among all engineering metals/alloys, steels and cast irons are more versatile. This versatility is based on the fact that their properties can be controlled and changed at will by heat treatment. The mechanical properties of metals generally are related and dependent on their composition, microstructural make-up and metallurgical properties (Rajan et al., 1988). Quenching is a critically important process in the development of desired mechanical properties of many steels and cast irons. Proper agitation of the quench bath is often considered as a single parameter that dictates the success of the quenching process itself, hence there is need to pay attention to the study and improvement of quench tank agitator design in order to assist and contribute to the development of industrial finishing operations. 1.4 Significant of Study<br>The research work is important because, if successful, it will help indigenous metal producing industries to improve the quality of their products. The agitated quenching tanks will be cheaper than imported ones and will reduce, hence save the<br>4<br>country some foreign exchange. The quench tank will also be useful in tertiary institutions for research and teaching. The fabricated equipment could also be exported to earn our country foreign exchange. The equipment will also assist in the further development of industrial finishing units of metal producing industries. 1.5 Aim and Objectives This research work was aimed at the successful design; construction and testing of an agitated quenching bath that is suitable for effective quench hardening of steel and cast iron components not more than 300 mm long and 100 mm width and upto 5 kg in weight. The specific objectives are as follows: –<br>(i) to design a portable agitated quenching bath with the ability of varying the<br>speed of agitation.<br>(ii) to construct the designed agitated quenching bath.<br>(iii) to use carbon steel and ductile cast iron alloys to evaluate the performance of the quenching bath (iv) to determine the effect of speed of agitation on the hardness and find the optimum speed. (v) to compare the results of agitated and unagitated or static baths. 1.6 Scope of the Study Based on available resources and associated cost, this research work shall be limited to the following:<br>(i) The designed and constructed tank will be limited to a volume of 20 litres and the shape will be of rectangular section (for faster heat removal during quenching as explained in section 2.7).<br>5<br>(ii) Variable speed electric motor with four speed drives will be used to vary the agitation<br>(iii) Plain carbon steel and ductile cast iron samples will be used to test the performance of the agitated quenching bath. (iv) The tests to be carried out on the quenched samples (both in static and agitated baths) are hardness, tensile strength, impact strength and microstructural analysis. (v) Water at room temperature will be used as quenching medium in both static and agitated conditions. The choice of water is due to some of its good properties, for instance, water is the most popular quenching medium; it meets the requirements of low cost, easy availability, ease of handling and safety (non- harzadous). Water has high cooling power (high cooling rate). According to Vigendra (2009), the cooling rate of water falls between that of brine and oil. High specific heat and high latent heat of vapourization of water are responsible for its high cooling rate. <br></p>