Improvement in Mechanical Properties of Ultra High Strength Steel through Induction Melting and Electroslag Refining
A. K. Rajak1, S. K. Maity2, Nagendra Prasad3
1A. K. Rajak, Department of Metallurgical Engineering, Birsa Institute of Technology, Sindri, Dhanbad, India.
2S. K. Maity, CSIR- National Metallurgical Laboratory, Jamshedpur (Jharkhand), India.
3Nagendra Prasad, Department of Metallurgical Engineering, Birla Institute of Technology Sindri, Dhanbad (Jharkhand), India.
Manuscript received on 15 February 2016 | Revised Manuscript received on 25 February 2016 | Manuscript Published on 28 February 2016 | PP: 52-57 | Volume-5 Issue-3, February 2016 | Retrieval Number: C4412025316/16©BEIESP
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Abstract: The objective of the present study is to develop the ultrahigh strength steel by induction melting and electroslag refining, which is followed by thermomechanical treatment with yield strength in excess of 1600 MPa and elongation of 9-10% . Ultrahigh strength steels are used in fabrication of rocket motor casings, aircraft undercarriages, turbine motors, pressure vessels and offshore platforms. Some of the currently employed imported steels, like maraging steel is highly alloyed and is expensive . In the first part, the alloys were prepared by induction melting with addition of calculated amount of scrap and ferroalloys. The molten metal was tapped at 16000C and poured in preheated cast iron mould of 48x52x250 mm in dimension. The other alloy is prepared by addition of 0.024% Ti to the base composition. This alloy exhibits better mechanical properties than previous one. In the second part of investigation, Attempts were made to develop steel containing low sulphur and low phosphorous through electroslag refining (ESR) process followed by thermomechanical treatment (TMT). The other alloy was prepared by inoculation of about 0.058% titanium during ESR process. Alloys developed by ESR process resulted in sound ingot with low inclusions. The ESR ingots were further undergone for thermomechanical treatment (TMT) to convert it into plates. The process consist of pre-rolling of the ESR ingot to a bar at 1200C, followed by hot rolling in two passes starting from 950C and finishing at 850C with equal deformation of 25% in each pass to convert the bar into plates and were immediately cooled in oil. The mechanical properties and some microstructural features were characterized with the specimens prepared from plates. The base alloy displayed UTS of 1792 MPa, yield strength of about 1580 MPa and elongation of 7.6%. Titanium inoculated alloy displayed UTS of 1885 MPa, yield strength of 1700 MPa and elongation of 8.3 %. It can be construed that the mechanical properties of the titanium inoculated alloy were substantially improved compared to base alloy. Optical and SEM microstructures of the TMT specimen’s reveals predominantly lath martensites. However, the microstructure of titanium inoculated alloy consisted of small packets of finer lath martensite. Titanium addition reduces the grain size and refines the martensite laths that lead to improvement in mechanical properties
Keywords: Ultrahigh Strength Steel, Electroslag Refining, Themomechanical Treatment, Microstructure, Mechanical Properties
Scope of the Article: Mechanical Design