Development of Ultrahigh Strength Low Alloy Steel through Electroslag Refining Process

In this investigation, attempts are made to develop special steel by electroslag refining (ESR) process. The base alloy is prepared with a chemical composition of 0.28% C, 1.0% Mn, 1.0% Mo, 0.35% V, 4.2% Cr with trace amount of S and P This alloy displays yield strength of about 1450 MPa in quenched-and-tempered condition. It is further attempted to increase the strength and toughness of the base alloy by addition of 1 to 3% of nickel. As a result there is a substantial improvement of mechanical properties. 3% nickel steel displays tensile strength of 1758 MPa and yield strength of 1 542 MPa. The austenite grains are marginally reduced in nickel alloys. With the chemical composition of nickel alloys when about 0.07% titanium is inoculated during ESR process, there is a deterioration of the tensile properties. In 3% nickel alloy the effect is more prominent, which displays yield strength of 1455 MPa along with the lowest value of elongation. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA) studies indicate that the microstructures of un-inoculated nickel steel predominantly consist of tempered lath martensites. In titanium inoculated alloys the microstructures are mixture of retain austenite, bainite and martensites. Strengths are deteriorated possibly due to the inhomogenous distribution of carbon because of the formation of different phases and depletion of nickel due to the precipitation of the corresponding intermetallics of (Ni, Ti).