Improvement in Yield Strength of Deformed Steel Bar by Quenching Using Taguchi Method

Abstract

This paper is on improvement in Yield strength of deformed steel bar by quenching using Design of experiment (DOE) for robust process design. The production of high quality deformed steel bar for concrete reinforcement will reduce the construction cost. It is found that the bar produced having variation in yield strength, this major quality problem is considered for improvement. Yield strength of bar is deviated from standard value specified in IS: 1786-1985. Here the study is focused on the robust parameter design for cooling system, to control the process of quenching to get the desired Yield strength of deformed steel bar. The controlling parameters of cooling system were controlled to improve the Yield strength of bar produced. Taguchi method of Orthogonal Array was used to find the effect of parameter on output quality parameter such as yield strength. DOE by Taguchi approach was used for the improvement in Yield strength. We got satisfactory result of yield strength of deformed steel bar having minimum variation from its mean value. Thus the Yield strength of bar has been improved by controlling the Quenching process parameters at optimum level which was found from factor effect plot using DOE by Taguchi Method. In this paper, the study of hot bar quenching process has been investigated. In order to predict the effect of noise factors and signal on response factor of water quenching process. The controlling parameters such as water pressure, speed of rolling, Cooling rate and Finish rolling temperature of bar were the four independent factors and each of them have three levels were considered for DOE and the response was yield strength. An orthogonal array- L9 had been used for the experiment. Here we used DOE by Taguchi method of orthogonal array to find the effect of parameter on Yield strength. Before conducting the experiment, the quenching setup was made ready with all respect to conduct experiment. The experiments were conducted and experimental data was recorded and analyzed by excel statistical method. The graph of main effect plot of factors was drawn, which was used to find the main effect on response. The parameters were controlled properly to minimize variation in yield strength and obtained the desired quality of bar. Here it is found that water pressure had the largest effect on the yield strength. The water pressure was controlled at different level for many experiments as per L-9 array and yield strength for each experiment was noted. The experimental data analyses were done by Minitab Statistical software. We found the optimum level of parameters setting to get optimum level of Yield strength. For this action plan was designed to control water pressure to a specific level as per the conclusion of this experimentation. Then the action plan had been implemented. The deformed bar were produced and checked for its quality; we found satisfactory quality steel bar having optimum Yield strength. The conclusion has been found that the cooling rate affects on yield strength, but cooling rate is controlled by water pressure which is operated using control valve. Hence water pressure has been controlled in quenching process to get the desired quality with minimum variation from standard value. The bar of 8 mm was produced by controlled rolling and quenching process. Speed of rolling was kept constant and PLC control water pressure valve was used instead of manual control valve for controlling the water pressure. II. Cooling system For the given operation conditions, the temperature and its distribution within the bar entering into the quenching play the crucial role in controlling the mechanical properties of the steel bar such as yield strength. Hence understanding of the cooling system is essential. There are three types of most frequently used cooling system in hot rolling; water jet cooling system, water spray cooling system, and mist cooling systems.(3)It is found that, the yield strength depends on cooling rate. Hence to control cooling rate we made the modification in the present system. The water pressure need to be controlled according the speed of rolling of bar and temperature of bar before entering the quenching system. Fig.1 shows the cooling system having water injector, a cooling tube, and a water stripper. As per size of bar to be produced, the length of cooling pipe is selected and the speed of rolling is controlled accurately.Fig.2, shows water pressure control valve in cooling system, the primary and secondary valves are used to control the water supply, another valve is used to control the air