Application of high-power microwaves for heat-treatment of large P91 steel samples

P91 steel is being used in boiler pipes, headers and tubes in welded condition for many years. Generally, these steels are being heat-treated by using conventional heat-treatment furnaces for getting the desired properties. The P91 steel samples were heat-treated in the temperature range of 1073 to 1573 K as they are normally experienced in the heat-affected zone (HAZ) during welding. The aim of this study is to prove that similar or even better uniform properties, in addition to considerable savings in terms of time and money, are being achieved by heat-treating large steel samples in the high-power hybrid microwave furnace. Till now, generally microwave furnaces are industrially used for sintering purposes i.e. heating of ceramics, metal powder, powder metal composites etc. only. As per the studies, the metal reflect microwaves i.e. they are very poor absorber of microwaves, so they cannot be heat-treated directly in the microwave field. Microwave technology heats the metal directly, avoiding the electrical arcing and magnetron destruction that are expected in heating a metal in a microwave oven. The microwave furnace system encloses the metal component within a ceramic insulating cavity containing microwave-absorbing plasma. The plasma heats the metal and this technology works quickly. It can reduce cycle time by two-thirds compared to the conventional heating process and also requires less energy to heat the object compared to a conventional furnace as only the object is heated by microwave radiation and there is no loss of energy in heating the total furnace volume. This results in huge savings in terms of time and money.