Failure Analysis of Basic Oxygen Furnace Lance Head Assembly

Abstract

In a steel plant, basic oxygen furnace (BOF) is a method to convert liquid hot metal and steel scrap into steel with oxidizing action of oxygen blowing into the melt. A water-cooled lance is used for blowing of pure gaseous oxygen (> 99.5%) at high pressure and flow rate of 11–13 kg/cm2 and ~ 1000 Nm3/min, respectively. A failure of BOF lance head assembly occurred in one of the integrated steel plants. A small oxygen leak ignited near the flange and a significant portion of the oxygen pipe elbow was consumed. The failure analysis was carried out, and likely reasons for the failure were identified. During the BOF process, oxygen is blown at high velocity into molten, carbon-saturated hot metal as received from blast furnaces. Oxygen reacts with carbon to form carbon monoxide (CO) and carbon dioxide (CO2) gases, among other reactions. The occurrence of flames, metal spitting and post-combustion of CO gas generates high-temperature conditions that can damage the oxygen lance assembly. They can also cause changes in microstructure of the parent material (low-carbon-grade SS310S) of the elbow and weaken the material over a period of time. It was found that the presence of a minute puncture in the oxygen pipe line may, under thermodynamically favorable conditions, ignite the leaking oxygen. Such a minute leakage may have occurred due to failure of material, weld joint or detachable joint.