Economic pressure to deliver energy at the lowest possible prices forces operators of electric power plants to increase their efficiency by means of increasing the temperature and steam pressure. For current power engineering it is characteristic that the temperature of the steam entering the turbine is 540°C to 565°C.
However, in recent years, there is a tendency to increase the temperature of working medium temperature above 565°C. In some cases, steam temperatures up to 600°C are being used. In the case of using steam in a state of supercritical steam parameters (610°C/30MPa), compared with the current parameters of water vapor (540°C/180MPa), it is possible to achieve an increase of the energy efficiency of equipment up to 8%, while reducing CO2 emissions by up to 20%.
Implementation of these conditions requires the replacement of an obsolete system with a new one, which will be resistant to the conditions of operation, mainly in terms of having higher creep resistance.
One of the structural elements of thermal power plants which is made from steel tubes is membrane walls. For their production steel T12 is normally used, which does not comply with the supercritical parameters of operation. Therefore, in this group of steels research began in the 80s, resulting in the original Japanese steel T/P23 and its European variant under the name T/P24 7CrMoVTi10-10.
These new steels meet the criteria for supercritical operation. Another one is the quality of the P91 (X10CrMoVNb9-1). These are ferritic-martensitic steel (9% chromium, 1% molybdenum) microalloyed with vanadium, niobium and controlled nitrogen content. Tubes made from grade P91 are designed for boiler superheaters, tube heaters for power plants, and tube collectors for high temperature use respectively for furnace tubes in petrochemical aggregates.
Requirements for creep resisting steel complicates the necessity to resist aggressive environments. When operating at high temperatures and pressures in the steels used, material creep phenomenon is applied.
Boiler quality tubes that are used for higher operating temperatures and higher operating pressures need to ensure the creep resistance of heat treatment which ensures their appropriate starting structure. It is a precipitation hardening process, which is caused by the presence of fine precipitates based on carbides and carbonitrides of alloying elements (V4C3, Mo2C respectively. Cr7C3), which forms a barrier for the movement of dislocation by grid.
One of the annealing furnaces in the tube mill of Zeleziarne Podbrezova meets all the requirements for heat treatment of tubes made of creep resistant grades. In the hot part of the furnace in the second to fourth zone are placed ceramic radiant burners, allowing the increase of annealing temperature from normal temperature max. 980°C up to 1,080°C.
Back in the hot part of the annealing furnace is additionally installed electrical heating by electric coils, which ensures the retention of the tube at the required temperature before intensive cooling. At the beginning of the cold part of the furnace is installed a powerful rapid cooler, allowing the tubes to cool down in a protective atmosphere at the speed of 350°C/min., depending on the tube diameter.
A modernised furnace provides the perfect conditions for heat treatment of boiler tubes in such a way so that their properties meet the requirements of the legislation under which the tubes are supplied and meet customer requirements.