Evaluation of the Creep Strength of 9Cr-1Mo-V and 1Cr-1Mo-1/4V Castings and Weldments Using Accelerated Creep Testing

Abstract

Knowledge of creep properties is vital in determining the allowable stresses for rotating equipment design at high temperatures. Unfortunately, the traditional method to generate creep data requires several long term tests; in some cases, upwards of 100,000 hours are needed. These tests are often time and cost prohibitive to perform. Some data is available from sources such as the ASME Boiler and Pressure Vessel Code, but these are limited to commonly available materials in set processing conditions. They speak very little to the properties for new materials, alternate processing of existing materials, and properties for weldments and heat affected zones (HAZ) that occur in the fabrication of large equipment. Due to this, several methods have been developed for accelerated creep testing. One such method is the Stress Relaxation Test (SRT) developed by Woodford. This high precision stress relaxation test can generate five decades of creep data in a single, one-day test. This paper discusses the use of the SRT method to evaluate two different materials and their weldments used in the manufacturing of steam turbine casings. The first material is cast 9Cr-1Mo-V (SA-217, Grade C12A). In this first case, material from two different foundries was tested at temperatures between 550°C and 700°C. Specimens consisting entirely of matching weld metal and those that that include the HAZ centered between weld metal and the base casting were also tested as a means to verify fabrication and casting upgrade procedures. In this case, the data generated for all three sample types very closely match those given in literature. In a nearly identical test program, testing was performed on cast 1Cr-1Mo-¼V steel (ASTM A356, Grade 9). In this second case, the base casting closely matched literature data, while the weldments did not. In one instance, through a significant reduction in properties of the weld metal specimen, the SRT method was able to detect that an under matching filler metal was used. In another instance, the HAZ specimen, from a weldment using matching filler metal, failed during the test. It was found that the welding procedure resulted in overheating the sample. These two case studies illustrate the ability of the SRT method to accurately predict creep properties and its sensitivity to detect variations in properties, which can make it useful for rapid verification of welding procedures for high temperature applications.