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

The traditional method to generate creep data requires several long term tests; in some cases upwards of 100,000 hours. These tests are often time and cost prohibitive to perform. Creep data in literature is limited to commonly available materials in set processing conditions. This speaks very little to the properties for new materials, alternate processing of existing materials, as well as 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, namely, the Stress Relaxation Test (SRT), is discussed as the means to evaluate two different materials and their weldments used in the manufacturing of steam turbine casings. Cast 9Cr-1Mo-V material from two different foundries as well as their weldments and Heat Affected Zones (HAZ) were tested at temperatures between 550oC and 700oC. The SRT generated data closely matches that given in literature. In a nearly identical test program, cast 1Cr1Mo¼V steel showed anomalies in the weld and HAZ behavior. Further investigation showed that the incorrect weld metal was used and a second attempt, poor welding practice lead to failure due to Type IV cracking. 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 casting suppliers and welding procedures for high temperature applications.

Numerical Simulation of Residual Stress in P91 Repair Welding Incorporating Martensitic Transformation

The type IV cracking is one of the main reasons for service failure. The repair welding is implemented to solve this problem. The residual stress in P91 repair welding incorporating martensitic transformation (MT) is acquired through finite element simulation. In the simulation, the ABAQUS and the user subroutines FILM, DFLUX, HETVAL, USDFLD, UEXPAN and UHARD are adopted. The effect of MT latent heat on temperature and the effects of the volume expansion, the yield strength change and the transformation plasticity on stress are considered. The results show that there is full MT in the repair welding area and the old welding area. The MT latent heat makes the temperature of the repair welding area increase. The residual stress of the repair welding area decreases because MT relieves the thermal stress. Before repair welding, the residual stress distribution is M-shape, which is consistent with the experimental results. After repair welding, the residual stress of repair welding area decreases and the residual stress of old welding area increases.

Creep Rupture Life Prediction of Grade 91 Circumferential Welded Tube Under Combined Internal Pressure With Axial Load

Grade 91 steels are widely used for high temperature pipes in ultra-super-critical thermal power plants. It was recently reported that the creep damage was detected in the fine grain region within the heat affected zone (HAZ) in the welded pipes, so called “Type IV” damage. So far, studies on creep damage and life assessment methods for welded joints of the Grade 91 steel were concentrated on longitudinal welded pipes. Circumferential welded joints are also susceptible to Type IV damage due to the increase of axial thermal stress superimposed with pipe weight. In this study, the effect of additional axial stress to the axial stress produced by the internal pressure on damage and rupture property is discussed based on internal pressure creep tests adding different levels of the axial loads using the Grade 91 circumferential welded tubes. Rupture time of the circumferential welded tube decreases with increasing additional axial load level. Longitudinal cracking in the weld metal was observed in the specimens tested under lower additional load level, and Type IV cracking was observed in the specimens under higher additional load level. The stress analysis results indicate that the longitudinal stress in the HAZ increases with the axial load. Type IV cracking occurs at a certain value of a ratio of the total longitudinal stress to the circumferential stress. Under the test condition where Type IV cracking occurs, the rupture time is significantly shorter than the predicted rupture time based on the creep rupture data of the longitudinal welded tubes. The limited creep strain concept is introduced to predict the rupture time of the circumferential welded tubes by considering the effect of stress multiaxiality. Eventually, the rupture times caused by Type IV cracking of both the longitudinal and the circumferential welded tubes were accurately predicted by the limited creep strain concept.