The Welded Joint Strength Reduction Factors of Modified 9Cr–1Mo Steel for the Advanced Loop-Type Sodium Cooled Fast Reactor

2016 ◽  
Vol 138 (6) ◽  
Author(s):  
Takuya Yamashita ◽  
Takashi Wakai ◽  
Takashi Onizawa ◽  
Kenichiro Satoh ◽  
Kenji Yamamoto
Keyword(s):  
Base Metal ◽  
Creep Strength ◽  
Welded Joint ◽  
High Stress ◽  
Strength Degradation ◽  
Creep Rupture ◽  
Analysis Method ◽  
Type Iv ◽  
Region Splitting

Creep strength enhanced ferritic (CSEF) steels including ASME Gr.91 are widely used in fossil power plants. In the advanced loop-type sodium-cooled fast reactor (SFR), modified 9Cr–1Mo steel (ASME Gr.91) is going to be adopted as a structural material. Modified 9Cr–1Mo steel was registered in the Japan Society of Mechanical Engineers (JSME) code as a new structural material for SFRs in the year 2012. The creep-rupture curve of the base metal of this steel was standardized using region splitting analysis method. According to this method, creep-rupture data were divided into two regions, high-stress and low-stress regimes, and those regions were individually evaluated by regression analyses with the Larson–Miller parameter (LMP). The difference in the creep failure mechanisms between the high-stress and low-stress regions was considered in this method. The boundary between these regions was half of the 0.2% proof stress of the base metal at the corresponding temperature. In the modified 9Cr–1Mo steel welded joint, creep strength may markedly degrade, especially in the long-term region. This phenomenon is known as “type-IV” damage due to creep voids and cracks in the fine-grained heat-affected zone (HAZ). There is no precedent for indicating the obvious creep strength degradation of welded joints under SFR temperatures (550 °C or less). Although obvious strength degradation of the welded joints has not yet been observed at 550 °C, it is fair to assume that the strength degradation will occur due to very long-term creep. Therefore, considering strength degradation due to “type-IV” damage is necessary. This paper proposes the creep-rupture curve and the welded joint strength-reduction factor (WJSRF). The creep-rupture curve of the welded joint was proposed by employing a second-order polynomial equation with LMP using region splitting analysis method, which is used for the base metal as well. The WJSRFs were proposed on the basis of design creep-rupture stress strength. The resulting allowable stress was conservative compared with that prescribed in ASME code and the Japan domestic regulation for thermal plants. In addition, the design of the hot-leg pipe in SFR was reviewed considering the WJSRFs.

Creep Strength Evaluation of Welded Joint Made of Modified 9Cr-1Mo Steel for Japanese Sodium Cooled Fast Reactor (JSFR)

2010 ◽  
Author(s):  
Takashi Wakai ◽  
Yuji Nagae ◽  
Takashi Onizawa ◽  
Satoshi Obara ◽  
Yang Xu ◽  
...  
Keyword(s):  
Creep Strength ◽  
Welded Joints ◽  
Welded Joint ◽  
Strength Degradation ◽  
Creep Rupture ◽  
Primary Circuit ◽  
Allowable Stress ◽  
Type Iv ◽  
Term Creep

This paper describes a proposal of provisional allowable stress for the welded joints made of modified 9Cr-1Mo steel (ASME Gr.91) applicable to the structural design of Japanese Sodium cooled Fast Reactor (JSFR). For the early commercialization of the SFRs, economic competitiveness is one of the most essential requirements. One of the most practical means to reduce the construction costs is to diminish the total amount of structural materials. To meet the requirements, modified 9Cr-1Mo steel has attractive characteristics as a main structural material of SFRs, because the steel has both excellent thermal properties and high temperature strength. Employing the steel to the main pipe material, remarkable compact plant design can be achieved. There is only one elbow in the hot leg pipe of the primary circuit. However, in such a compact piping, it is difficult to keep enough distance between welded joint and high stress portion. In the welded joints of creep strength enhanced ferritic steels including ASME Gr.91 (modified 9Cr-1Mo) steel, creep strength may obviously degrade especially in long-term region. This phenomenon is known as “Type-IV” damage. Though obvious strength degradation has not observed at 550°C yet for the welded joint made of modified 9Cr-1Mo steel, it is proper to suppose strength degradation must take place in very long-term creep. Therefore, taking strength degradation due to “Type-IV” damage into account, the allowable stress applicable to JSFR pipe design was proposed based on creep rupture test data acquired in temperature accelerated conditions. Available creep rupture test data of welded joints made of modified 9Cr-1Mo steel provided by Japanese steel vender were collected. The database was analyzed by region partition 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). Boundary condition between short-term and long-term was half of 0.2% proof stress of base metal at corresponding temperature. First order equation of logarithm stress was applied. For conservativeness, allowable stress was proposed provisionally considering design factor for each region. Present design of JSFR hot leg pipe of primary circuit was evaluated using the proposed allowable stress. As a result, it was successfully demonstrated that the compact pipe design was assured. For validation of the provisional allowable stress, a series of long-term creep tests were started. In future, the provisional allowable stress will be properly reexamined when longer creep rupture data are obtained. In addition, some techniques to improve the performance of welded joints were surveyed and introduced.

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

2013 ◽  
Author(s):  
Takashi Wakai ◽  
Takashi Onizawa ◽  
Takehiko Kato ◽  
Shingo Date ◽  
Koichi Kikuchi ◽  
...  
Keyword(s):  
Base Metal ◽  
Creep Strength ◽  
Welded Joints ◽  
Welded Joint ◽  
Polynomial Equation ◽  
Creep Rupture ◽  
Short Term ◽  
Type Iv ◽  
Rupture Data

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.

Evaluation of Creep Strength Reduction Factors for Welded Joints of Modified 9Cr-1Mo Steel (P91)

2006 ◽  
Author(s):  
Masaaki Tabuchi ◽  
Yukio Takahashi
Keyword(s):  
Base Metal ◽  
Creep Strength ◽  
Welded Joints ◽  
Welded Joint ◽  
Rupture Strength ◽  
Reduction Factor ◽  
Creep Rupture ◽  
Type Iv ◽  
Rupture Data ◽  
Term Creep

In order to review the allowable creep strength of high Cr ferritic steels, creep rupture data of base metal and welded joints have been collected and long-term creep strength have been analyzed in the SHC committee in Japan since 2004. In the present paper, the creep rupture data of 370 points for welded joint specimens of modified 9Cr-1Mo steel (ASME Grade 91) offered from seven Japanese companies and institutes were analyzed. These data clearly indicated that the creep strength of welded joints was lower than that of base metal due to Type IV fracture in HAZ at or above 600°C. From the activities of this committee, the master curve for life evaluation of welded joints of Gr.91 steel could be represented as follows: LMP==34154+3494(logσ)−2574(logσ)2,C=31.4 The reduction factor of 100,000 hours creep rupture strength of welded joint to base metal was concluded to be 0.75 at 600°C and 0.70 at 650°C for the Gr.91 steel.

Evaluation of Stress Rupture Factors for Grade 91 Weldments

2018 ◽  
Author(s):  
Kazuhiro Kimura
Keyword(s):  
Base Metal ◽  
Rupture Strength ◽  
Stress Rupture ◽  
Creep Rupture ◽  
Weld Strength ◽  
Creep Rupture Strength ◽  
Premature Failure ◽  
Type Iv ◽  
Grade 91

Stress rupture factors and weld strength reduction factors for Grade 91 weldments in the codes and literatures have been reviewed. Stress rupture factors for weld metals proposed for Code Case N-47 in the mid 1980’s was defined as the average rupture strength of the deposited filler metal to the average rupture strength of the base metal. Remarkable drop in creep rupture strength of weldments is significant issue of Grade 91, especially in the low-stress and long-term regime. A premature failure of Grade 91 weldments in the long-term, however, is caused by Type IV failure which takes place in the fine grained heat affected zone (FG-HAZ), rather than fracture in the deposited weld metal. The stress rupture factor of the Grade 91 steel, therefore, was based on the creep rupture strength of cross weld test specimens. Time and temperature dependent stress rupture factors for Grade 91 have been estimated based on the average creep rupture strength of cross weld test specimen to the average creep rupture strength of base metal.

Long-Term Creep Behaviour of E911 Heat Resistant 9% Cr Steel Weldments Fabricated With Filler Metals of Different Creep Strength

2007 ◽  
Author(s):  
Peter Mayr ◽  
Horst Cerjak ◽  
Claus Jochum ◽  
Jerzy Pasternak
Keyword(s):  
Weld Metal ◽  
Creep Strength ◽  
Creep Behaviour ◽  
High Stress ◽  
Base Material ◽  
Characteristic Type ◽  
Type Iv ◽  
Stress Levels ◽  
Filler Metals

In this work an X11CrMoWVNb9-1-1 (E911) pipe with an outside diameter of 355 mm and a wall thickness of 43 mm was welded with three different filler metals by GTAW and SMAW process. The used filler metals differed in creep strength level, compared to E911 grade pipe base material creep strength. The long term objective of this work was to study the influence of weld metal creep strength on the overall creep behavior of the welded joints. Uni-axial creep tests at 600°C (873 K) and stresses ranging from 70 to 130 MPa were performed using cross-weld samples of all three welds. Fractured samples were investigated by optical microscopy, electron microscopy and hardness testing. The results showed that the use of undermatching weld metal of P91-type led to premature fracture in the weld metal at higher stress levels. At lower stresses the fracture location was shifted into the fine-grained heat affected zone (HAZ) and samples failed by characteristic Type IV failure mode. The use of matching (E911 type) and overmatching (P92 type) filler material increased the time to rupture only at high stress levels. The fracture mode for all samples at lower stress levels was identified as characteristic Type IV failure.

Evaluation of Creep Strength Reduction Factors for Welded Joints of Grade 122 Steel

2011 ◽  
Vol 133 (2) ◽  
Author(s):  
Yukio Takahashi ◽  
Masaaki Tabuchi
Keyword(s):  
Base Metal ◽  
Creep Strength ◽  
Heat Affected Zone ◽  
Welded Joints ◽  
Thermal Power ◽  
Strength Reduction ◽  
Creep Rupture ◽  
Piping System ◽  
Type Iv ◽  
Rupture Data

HCM12A (ASME Grade 122) is used for boiler components in thermal power plants because of its high creep strength. However, a type IV creep damage formed in the heat-affected zone can cause a considerable decrease in the creep strength of the weldment and a failure of large diameter piping due to this damage took place recently in a thermal power plant. In order to update the design method and develop life estimation method for this kind of piping system with axial weld, the creep rupture data of base metal and welded joints have been collected and analyzed by the Strength of High-Chromium Steel Committee in Japan. In the present paper, the creep rupture data of over 400 points for welded joint specimens of HCM12A offered from six Japanese organizations are analyzed. These data clearly indicate that the long-term creep strength of the welded joints becomes weaker than that of the base metal at above 600°C due to the type IV fracture in the fine grain heat-affected zone. After the discussions on the effects of product form, welding procedure, specimen sampling procedure, etc., on the creep strength, the master creep life equation for the welded joints is developed. The so-called region decomposition technique was adopted to fit the data in both high and low stress regimes with a reasonable accuracy. The creep strength reduction factor obtained from 100,000 h creep strength of the welded joints and the base metal is given as a function of temperature.

Evaluation of Creep Strength Reduction Factors for Welded Joints of Modified 9Cr-1Mo Steel

2012 ◽  
Vol 134 (3) ◽  
Author(s):  
Masaaki Tabuchi ◽  
Yukio Takahashi
Keyword(s):  
Creep Strength ◽  
Welded Joints ◽  
Welded Joint ◽  
Residual Life ◽  
Strength Reduction ◽  
Life Assessment ◽  
Type Iv ◽  
Rupture Data ◽  
Residual Life Assessment

Creep strength of welded joint for high Cr ferritic heat resisting steels decreases due to Type-IV failure in heat-affected zone (HAZ) during long-term use at high temperatures. In order to review the allowable creep strength of these steels, creep rupture data of base metals and welded joints have been collected, and long-term creep strength has been evaluated in the SHC (strength of high-chromium steel) committee in Japan. In the present paper, the creep rupture data of 370 points for welded joint specimens of modified 9Cr-1Mo steel (ASME Grade 91 steel) offered from seven Japanese companies and institutes were analyzed. These data clearly indicated that the creep strength of welded joints was lower than that of base metal due to Type-IV failure in HAZ at high temperatures. From the activities of this committee, it was concluded that the weld strength reduction factor (WSRF) should be taken into consideration for the design and residual life assessment of boiler components in fossil power plants. The committee recommended the WSRF for 100,000 h creep of Gr.91 steel as 0.85 at 575 °C, 0.75 at 600 °C, 0.74 at 625 °C, and 0.70 at 650 °C. The master curve for residual life assessment of Gr.91 steel welds using Larson-Miller parameter was also proposed.

Evaluation of Creep Strength Reduction Factors for Welded Joints of HCM12A (P122)

2006 ◽  
Author(s):  
Yukio Takahashi ◽  
Masaaki Tabuchi
Keyword(s):  
Base Metal ◽  
Creep Strength ◽  
Heat Affected Zone ◽  
Welded Joints ◽  
Thermal Power ◽  
Strength Reduction ◽  
Creep Rupture ◽  
Piping System ◽  
Type Iv ◽  
Rupture Data

HCM12A (ASME Grade 122) is used for boiler components in thermal power plants because of its high creep strength. However, type IV creep damage formed in heat affected zone brings about considerable decrease in creep strength of the weldment and a failure of large diameter piping in a thermal power plant due to this damage took place recently. In order to update the design method and develop life estimation method for this kind of piping system with axial weld, creep rupture data of base metal and welded joints has been collected and analyzed in the SHC (Strength of High-Chromium Steel) committee in Japan since 2004. In the present paper, the creep rupture data of over 400 points for welded joint specimens of HCM12A offered from six Japanese organizations were analyzed. These data clearly indicated that the long-term creep strength of welded joints becomes weaker than that of base metal at above 600C due to Type IV fracture in fine grain heat-affected zone. After discussing the effects of product form, welding procedure and specimen sampling etc. on the creep strength, the master creep life equation for the welded joints was developed. So-called region decomposition technique was adopted to fit the data both in high and low stress regimes with a reasonable accuracy. The creep strength reduction factor obtained from 100,000 hours creep strength of welded joints and base metal was given as a function of temperature.

Region Splitting Analysis on Creep Strength Enhanced Ferritic Steels

2007 ◽  
Author(s):  
Kazuhiro Kimura ◽  
Hideaki Kushima ◽  
Kota Sawada ◽  
Yoshiaki Toda
Keyword(s):  
Creep Strength ◽  
Rupture Strength ◽  
Creep Rupture ◽  
Ferritic Steels ◽  
Creep Rupture Strength ◽  
Delta Ferrite ◽  
Time To Rupture ◽  
Term Creep ◽  
Region Splitting

Overestimation of long-term creep strength of creep strength enhanced ferritic steels is caused by inflection of a relation between stress and time to rupture. Creep rupture strength of those steels has been re-evaluated by a region splitting analysis and allowable tensile stress of some steels regulated in METI (Ministry of Economy, Trade and Industry) Thermal Power Standard Code in Japan has been reduced. A region splitting analysis method evaluates creep rupture strength in the short- and the long-term individually, which is separated by 50% of 0.2% offset yield stress. Inflection of stress vs. time to rupture curve is attributable to longer creep rupture life with a stabilized microstructure of creep strength enhanced ferritic steels, since tensile strength property, which determines short-term creep rupture strength, remains the same level. Accuracy of creep rupture strength evaluation is improved by region splitting analysis. Delta ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite at the normalizing temperature. It increases driving force for diffusion and promotes recovery of tempered martensite adjacent to delta-ferrite. Concentration gap may be produced also in heat affected zone (HAZ), especially in fine grain HAZ similar to that in dual phase steel, and it has possibilities to promote recovery and, therefore, to decrease creep strength.

Strain Rate Dependence of Yield Strength and Long-Term Creep Strength Evaluation of ASME Grade 91 Steel

2009 ◽  
Author(s):  
Kazuhiro Kimura ◽  
Kota Sawada ◽  
Hideaki Kushima
Keyword(s):  
Yield Strength ◽  
Strain Rate ◽  
Yield Stress ◽  
Creep Strength ◽  
Life Prediction ◽  
Total Strain ◽  
Analysis Method ◽  
Term Creep ◽  
Region Splitting

Accuracy of long-term creep life prediction of creep strength enhanced ferritic steels is improved by region splitting analysis method in consideration of 50% of the 0.2% offset yield stress. According to JIS and ASTM standards for tensile test, however, specified strain rates for evaluation of yield strength are slightly different from each other. The conditions specified in JIS G0567 and ASTM E21-03a are 0.3 ± 0.2%/min and 0.5 ± 0.2%/min, respectively. Strain rate influences on yield strength and 0.2% offset yield stress of ASME T91 at 600 and 650°C under strain rate of 0.5%/min was about 10% higher than that under strain rate of 0.3%/min. Influence of difference in strain rate between JIS and ASTM regulations on the long-term creep rupture life prediction by region splitting analysis method was insignificant. With decrease in stress, magnitude of creep strain at the onset of accelerating creep stage decreased from about 2% in the short-term to less than 1% in the long-term. Life fraction of the time to 1% total strain tended to increase with decrease in stress. It was indicated that the initiation of tertiary creep should be a more important parameter for the stress intensity limit, St, than time to a total strain of 1%, since 80% of the minimum stress to cause initiation of tertiary creep was definitely smaller than 100% of the average stress required to obtain a total strain of 1% in the long-term.

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