A Study for Proposal of Welded Joint Strength Reduction Factors of Modified 9Cr-1Mo Steel for Japan Sodium Cooled Fast Reactor (JSFR)

Abstract

This paper proposes provisional welded joint strength reduction factors (WJSRF) of modified 9Cr-1Mo steel (ASME Gr.91) applicable to the structural designing of “Japan sodium cooled fast reactor (JSFR)”. In the welded joints of creep strength enhanced ferritic steels including modified 9Cr-1Mo steel, creep strength may obviously degrade especially in long-term region. This phenomenon is known as “Type-IV” damage. The authors had proposed provisional allowable stress for the welded joints made of the steel in PVP 2010 conference, taking creep strength degradation due to “Type-IV” damage into account. Available creep rupture data of the welded joints made of the steel provided by Japanese steel venders were collected. The temperature range was from 500 to 650°C. The database was analyzed by stress range partitioning method. The creep rupture data were divided into two regions of short-term and long-term and those were individually evaluated by regression analyses with Larson Miller Parameter (LMP). The difference in the creep failure mechanisms between short-term and long-term regions is taken into account in this method. Boundary between these regions was half of 0.2% proof stress of the base metal at corresponding temperature. First order polynomial equation of logarithm stress was applied. For conservativeness, allowable stress was proposed provisionally considering design factor for each region. JSME (Japan Society of Mechanical Engineers) published a revised version of the elevated temperature design code in last year. Modified 9Cr-1Mo steel was officially registered in the code as a new structural material for sodium cooled fast reactors. The creep rupture curve for the base metal of the steel was standardized by employing stress range partitioning method, same as for the welded joint. However, second order polynomial equation of logarithm stress was applied in the analysis for the base metal. In addition, the creep rupture data obtained at 700°C were included in the database and data ruptured in very short term, i.e. smaller than 100 hours, were excluded from the analysis. Thus, there are some differences between the procedures to determine the creep rupture curves for base metal and welded joint made of modified 9Cr-1Mo steel. This paper discusses the most feasible procedure to determine the creep rupture curve of the welded joint of the steel by performing some case studies to focus on physical adequacy and harmonization with the determination procedure of the creep rupture curve for the base metal. Then, the WJSRF are provisionally proposed based on the design creep rupture stress intensities. In addition, the design of JSFR pipes was reviewed taking WJSRF into account.

Structural Materials and Code Development for Japanese Sodium-Cooled Fast Reactors

“…Based on the database, the authors have conducted a detailed study and have demonstrated for both 316FR and Mod.9Cr-1Mo steels that adequate conservatism is maintained in long-term regions by using the time-fraction approach based creep-fatigue damage evaluation method as shown in Figs. 3 and 4 [9,10].…”

Creep–fatigue evaluation method for weld joints of Mod.9Cr–1Mo steel. Part I: Proposal of the evaluation method based on finite element analysis and uniaxial testing

“…To adopt Mod.9Cr-1Mo steel, the individual failure mechanism (Type IV cracking) at the weld joints [6][7][8] should be considered in piping and component design. Since the evaluation procedure of primary stress limit for the weld joint of Mod.9Cr-1Mo steel is one of the important subjects for the JSFR design, the creep rupture curve of weld joints made of Mod.9Cr-1Mo steel was proposed based on the available creep rupture data and the stress range partitioning method for the evaluation of creep rupture strength [9][10]. In addition to this creep rupture curve for Mod.9Cr-1Mo steel weld joints, a provisional weld joint strength reduction factor (WJSRF) for Mod.9Cr-1Mo steel has also been proposed to develop the rules that limit primary stress in the JSME FRs code.…”