Heat Resistant Martensitic Steel P91

The heat resistant martensitic Steel P91, which contains 9% Chromium, 1% Molybdenum with additions of Niobium and Vanadium, is being utilized in conventional power plants for piping systems with operating temperatures of 560 - 600ºC (1040 - 1112ºF) and high pressures in the range of 270 bar (4000 psi). These high operating temperatures and pressures require high quality weldments to ensure safety of equipment and personnel. The design of welded structures requires resistance to the high temperatures and pressures for up to 30 years. Therefore, welding (especially circumferential welds) has to be performed with the utmost care. Also, the selection of the correct welding consumables is of great importance to prevent premature equipment failure. Special attention should be paid to the following: 1. The welding consumable must be optimized for the intended welding process to control the loss of alloying elements across the arc. It is important that critical elements are maintained in a tightly controlled range in the weld deposit, applicable to all welding processes, as follows: Carbon 0.08 – 0.13% Niobium 0.04 – 0.08% Vanadium 0.15 – 0.30% Nitrogen 0.03 – 0.07% Adherence to these critical limits ensures that the creep rupture strength will be sufficient to withstand the high temperatures and pressures over many years. In order for the elements V, Nb and N to be guaranteed within these very tight limits, Bohler Thyssen Welding developed solid wire consumables instead of flux cored wires. Solid wires allow for a more uniform chemical analysis with strict control of the alloy elements, as well as the so-called tramp elements, as opposed to flux cored wires. Adherence to the tightly controlled range of analysis applies to all product forms. Furthermore, the sum Mn + Ni should not exceed 1.5% in total to avoid undesired structure changes during post weld heat treatment (PWHT). 2. The weld metal should exhibit sufficiently high toughness properties after PWHT in order to avoid weldment failures during pressure testing and start up of the power plants. The welding consumables of Bohler Thyssen Welding (Thermanit MTS 3 and Thyssen Brand Chromo 9V mod.) have been optimized in order to obtain toughness values greater than 41J (30 ft-lbs.) after PWHT at 750 - 760ºC (1382 - 1400ºF) for a minimum of 2 hours. In the case of submerged arc welding, the time should be increased to a minimum of 4 hours to obtain these values. If PWHT is applied at lower temperatures, the heat treatment time must be extended accordingly. 3. Strength of the weldment is a critical consideration in determining the long term integrity of the structure. While room temperature strength is important, the strength of the weldment at elevated temperatures is crucial, especially after long term exposure at operating temperatures. To determine the integrity of a particular consumable to this long term exposure at elevated temperatures, the creep rupture strength of the weld metal must be tested to ensure matching properties to the base material. Bohler Thyssen Welding has tested the matching filler metal Thermanit MTS 3 in welded joints for up to 30,000 hours of actual test time, equating to almost 3.5 years in the test fixture. This allows extrapolation of the results to 100,000 or 200,000 hours, which is equivalent to approximately 23 years of service. Only through this extensive testing regime and rigid quality assurance program, we can be assured of metallurgically sound weldments. Detailed test reports, technical papers, and welding procedure assistance are all available upon request. With availability of quality products in all proven supply forms, Bohler Thyssen Welding is prepared to be your partner for the supply of filler metals to meet the most stringent requirements. http://www.btwusa.com/html/ppf2.html