ASME STP/NU-019-1

Description / Abstract: ABSTRACT

Part I Base Metal - The database for the creep-rupture of 9Cr-1Mo-V (Grade 91) steel was collected and reviewed to determine if it met the needs for recommending time-dependent strength values, St, for coverage in ASME Section III Subsection NH (ASME III-NH) to 650°C (1200°F) and 600,000 hours. The accumulated database included over 300 tests for 1% total strain, nearly 400 tests for tertiary creep and nearly 1700 tests to rupture. Procedures for analyzing creep and rupture data for ASME III-NH were reviewed and compared to the procedures used to develop the current allowable stress values for Gr 91 for ASME II-D. The criteria in ASME III-NH for estimating St included the average strength for 1% total strain for times to 600,000 hours, 80% of the minimum strength for tertiary creep for times to 600,000 hours and 67% of the minimum rupture strength values for times to 600,000 hours. Time-temperature-stress parametric formulations were selected to correlate the data and make predictions of the long-time strength. It was found that the stress corresponding to 1% total strain and the initiation of tertiary creep were not the controlling criteria over the temperature-time range of concern. It was found that small adjustments to the current values in III-NH could be introduced but that the existing values were conservative and could be retained. The existing database was found to be adequate to extend the coverage to 600,000 hours for temperatures below 650°C (1200°F).

Part II Weldments - A creep-rupture database that was used to develop stress rupture factors (SRFs) in ASME Section III Subsection NH (ASME III-NH) for weldments of 9Cr-1Mo-V (Gr 91) steel was reassembled. The intent was to review the original work, supplement the database with newer data and validate the applicability of the SRFs to longer time service to meet the needs for the Generation IV nuclear reactor materials program. After a review of the augmented database, approximately 85 of 200 data on weld metal and weldments were selected for the re-evaluation of SRFs. Data were processed using a lot-centered Larson Miller parametric analysis similar to the model used to correlate stress-rupture data for base metal. It was found that the weldments did not follow the same stress dependency in stress-rupture as base metal. As a result, the SRF values depended on both time and temperature. Some SRF values were estimated, but the long-time, low-stress SRF values were found to be lower than those values which formed a basis for the SRFs in 2007 ASME III-NH. Moreover, the lack of long-time data above 540°C (1000°F) made the database unsuitable for the estimation of SRFs for application to all the St values covered in ASME III-NH. The coverage needed for the Generation IV nuclear pressure vessels, however, was expected to be for temperatures below 540°C (1000°F). A review of European and Asian work on Gr 91 weldments provided helpful information in this respect. Although significant differences in behavior were reported from one research effort to another, special notice was taken of recent work in Japan to develop weld strength reduction factors (WSRFs) for use in the fossil and petrochemical industries. Here, the WSRFs were based on stress-rupture models applicable to welded components for long-time service to at least 600°C (1110°F). Further testing of Gr 91 weldments for long times and low stresses was recommended.