Critical Considerations of Composition and PropertiesFor P 91 Welding Consumables

Grade 91 was originally developed as an alloy with improved strength and creep resistance as compared to conventional materials for application in power plants for header systems and reheat piping, with the goal of improvement in plant efficiency. Since introduction, additional applications have surfaced in other industries as well. Due to the commercial demand for the alloy and the desire for standardization, the various forms of the consumables have been included in classification documents around the world. In the AWS filler metal specifications, classifications for coated electrodes, submerged arc wire, gas metal arc wire, and gas tungsten arc rods are in the current documents and are indicated by the “B9” designator. A request for classification of flux cored wire has also been made, with anticipated acceptance in the near future. According to the alloy design, most if not all applications of the material call for operation of the fabricated equipment at temperatures within the creep range. Therefore, it is essential that all materials used, including welding consumables, be formulated to optimize creep resistance. Additionally, it is often times desirable to have a certain notch toughness of the alloy to avoid catastrophic failure during hydrotesting and start up of the equipment. Unfortunately, notch toughness and creep resistance are contradictory terms. The alloy additions that are generally used to improve the creep properties are precisely the alloys that are detrimental to good toughness. The culprit in the case of Grade 91 is Niobium (Columbium). A certain amount of Niobium is required in the weld metal in order to be within the scatter band of the base metal in creep resistance. However, this minimum amount makes it extremely difficult to meet the minimum toughness requirements, which typically are 30 ft-lbs at 68 F (41 J at 20 C). Therefore, very careful control of the chemical composition, especially for the key alloying elements, is required. Often times, meeting the chemical analysis requirements and passing the tests required for classification of an alloy are not indicative of the acceptance of the material for the intended application. This is certainly the case for the consumables for the welding of Grade 91. According to the AWS requirements for classification, the only tests that are required for the “B9” classifications are chemical analysis, tension test, and radiographic test. Understandably, specification of toughness and creep properties is left up to the end user. Since the range of chemical analysis is fairly broad and the requirements for mechanical properties are minimal for classification, care must be taken to insure that the delivered materials will in fact meet the demanding requirements of the critical applications for Grade 91. Through extensive research and material testing, as well as more than 10 years of field experience, the consumables manufactured by Bohler Thyssen Welding for the welding of Grade 91 are proven to meet even the most demanding requirements. Numerous published papers and test reports detailing our experiences are available upon request, including actual creep rupture data for 30,000 hours of testing. Please contact us for further information and detailed discussion of your specifications.