Pearlitic steel wire: High carbon steel based natural nanomaterial by lead patenting process

Abstract

The high strength springs are made from fully pearlitic hypereutectoid steel wires. This fully pearlitic structure in hypereutectoid steel wires are obtained by a special heat treatment process called lead patenting process. This pearlitic structure in steel wires is the best example of natural nanomaterials. It consists of alternate very fine layers of ferrite lamellae and cementite lamellae. The thickness of cementite lamellae ranges from 6 nanometres to 30 nanometres. The thickness of ferrite lamellae ranges from 25 nanometres to 220 nanometres. The finer is thickness of ferrite lamellae and cementite lamellae better is mechanical properties of pearlitic steel wire. The parameters which affect the quality and quantity of pearlitic microstructure in lead patenting process are lead bath temperature, transformation time and wire diameter. A lot of study on quality aspect of lead patented wire is reported but limited study has been carried out on quantative aspect in industrial set up. The mainly focused in this work is to study quantity aspect of pearlitic microstructure formation in a lead patenting process, in an industrial set up. The percentage of pearlitic microstructure developed in lead patenting process has direct influence on ultimate tensile strength, torsion strength and reduction in area of patented wires. The optical images of lead patented wires are obtained by using Olympus microscope. The quantity analysis of pearlitic microstructure in lead patented wires is done by using phase segmentation module of Magnus Pro image analysis software. The optimization of lead patenting process is done using Taguchi technique. The L9 orthogonal array of Taguchi method is selected for optimization of process. The analysis of variance shows the influence of process parameters on mechanical properties of patented wires. The results of optical image analysis shows very good agreement with results obtained using Taguchi technique.