Effects of H2 combustion on scale growth and steel surface quality in reheating furnaces

The use of hydrogen as fuel in the reheating furnaces, to replace (partially or totally) natural gas, causes a modification in the composition of the furnace atmosphere, with low or null concentration of CO2 and an increased concentration of H2O. The new atmosphere may change the scale growth kinetic and may affect the surface properties, in terms of scale removal and formation of decarburized layer. These effects can be studied at laboratory scale simulating the atmosphere generated by H2 combustion considering the working conditions of the burner and the excess oxygen. Hence, the comprehension and quantification of this effect is a fundamental pre-requisite for the use of hydrogen as fuel in reheating and treatment furnaces. For this reason, TenarisDalmine and Rina CSM carried out an experimental activity to simulate reheating and heat treatment process selecting two steels grades: one for line pipe production and one for casing production, two steels in which the more remarkable differences for scale growth and adherence are in the C and Ni content [1,2]. The experimental scenario was defined to verify the effects of the combustion atmosphere simulating the heating of a 20 mm thick pipe at 920 °C and the reheating process at 1230 °C for 180 min. The trials included two possible combustion atmospheres having different H2O concentration and corresponding to 100% natural gas and 100% H2. The steel oxidation and scale formation were studied by TGA (Thermo Gravimetric Analysis) tests. Results showed some increase in scale growth up to 10% during heating at 920 °C and up to 16% in case of re-heating at 1230 °C. The differences between line pipe and casing steel grades were minimal with a slight tendency to higher oxidation for line pipe steel. No decarburization was observed in steel sample heated at 920 °C oxidized in both atmospheres (with 100% natural gas or 100% H2 combustion). Metallographic analysis on TGA samples confirms, for the test with atmosphere form 100%H2, at 1230 °C, the increased thickness of the scale; in these tests, the interface between scale and steel appears more complex and entangled.