Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel

https://doi.org/10.1016/j.ijpvp.2009.04.008Get rights and content

Abstract

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.

Introduction

Mod.9Cr–1Mo steel has been widely used for high temperature structures such as boiler components in ultra-supercritical (USC) thermal power plants operating at about 600 °C. It is also a candidate material for a steam generator, intermediate heat exchanger and secondary piping of a liquid metal reactor, and a pressure vessel and some internal structures of gas cooled reactors due to its high creep strength. However, it is found that the creep properties of the welded joints are inferior to those of the base metal and weld metal due to the Type IV damages forming in the fine-grained HAZ paralleling the fusion line at high temperature [1], [2]. Scientists and researchers have been investigating the mechanisms of Type IV fracture both from the microstructural approaches and mechanical approaches [3], [4], [5]. However, mechanisms of Type IV fracture have not been fully understood. The complicated stress state distributed in the welded joint affects the formation and growth of creep voids prior to the crack growth in fine-grained HAZ, which occupies a large fraction of creep life.

During the past years, several creep damage models were proposed in order to describe the nonlinear creep behavior of initially isotropic solids at high temperatures. After Kachanov first devised the continuum damage mechanics to evaluate the creep life of a structure in 1958 [6], the damage mechanics have been applied for predicting creep damage and lives of components under complicated stress conditions [7]. Hyde et al. predicted the creep damage distributions and crack growth life of circumferential pipe welds by damage mechanics using creep data of parent, weld and HAZ of new and serviced materials [5]. Eggeler et al. investigated the effect of hoop, axial and radial stress distributions of a P91 pressure vessel with welds using the creep data of base metal, weld metal and simulated inter-critical HAZ [4]. Bauer et al. analyzed the stress–strain situations and multiaxial stress state in various pipe models including welds for predicting creep life, and clarified the effect of creep strength of weld metal [8].

The experimental creep damage distributions and evolutions of high Cr steel welds, however, were scarcely investigated, and the comparison between experimental creep damage evolutions and numerical analysis was scarcely conducted. In the present paper, we have investigated the formation and growth processes of Type IV creep damage (voids and cracks) quantitatively using the large scale thick welded joints with double U grooves of Mod.9Cr–1Mo steel. The failure mechanism is discussed, comparing the experimental results with the stress–strain distributions and stress triaxial condition in the welded joint calculated by finite element method (FEM).

Section snippets

Creep tests

The material investigated in the present study is a Mod.9Cr–1Mo steel plate of 25 mm in thickness. The chemical compositions of base metal and the welding filler are given in Table 1. The plates were welded by using gas tungsten arc welding (GTAW) method with double U grooves. The post weld heat treatment (PWHT) adopted was to keep temperature at 745 °C for 1 h. The simulated fine-grained HAZ specimens were produced by rapid heating (60 °C/s) to the peak temperature 900 °C and followed by the gas

Creep properties of welded joint, base metal and simulated HAZ

The creep test results for the welded joints, simulated fine-grained HAZ (FG-HAZ) and base metal (BM) of the Mod.9Cr–1Mo steel at 550, 600 and 650 °C are shown in Fig. 6. The failure location of welded joint is shown with subscript in this figure. The creep rupture times of the simulated fine-grained HAZ were smaller more than one order than those of base metal at all temperatures. At 550 °C, the fracture location of welded joint was base metal for the applied stress of 240 and 220 MPa, and it

Discussion

From Fig. 13, comparing the results of numerical analysis with the creep voids measurement, the stress triaxial factor is reasonable to explain why the fine-grained HAZ is the most damaged zone in the whole welded joint, because it accelerates the creep void formation and growth. However, the distributions of maximum principal stress and stress triaxial factor in the fine-grained HAZ shown in Fig. 14, Fig. 15 are not thoroughly consistent with the experimental void distributions of Fig. 8,

Conclusions

In the present paper, damage processes in the fine-grained HAZ of thick welded joint of Mod.9Cr–1Mo steel during creep were investigated. Stress–strain distributions in the welded joint were computed by FEM analysis with a three-dimensional, three-material model (base metal, simulated fine-grained HAZ and weld metal). The results can be summarized as follows.

  • (1)

    It is found that the creep voids of Mod. 9Cr–1Mo steel weld form at the early stage of creep rupture life (0.2 of life), the number of

Acknowledgements

The present study includes the result of “Development of damage prevention technology for welded structures in the next-generation high temperature nuclear plant” entrusted to Central Research Institute of Electric Power Industry by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

References (12)

There are more references available in the full text version of this article.

Cited by (58)

  • Experimental investigation of creep crack growth behavior in the heat affected zone of boron added modified 9Cr–1Mo steel weld

    2021, Materials Science and Engineering: A
    Citation Excerpt :

    A complex microstructural gradient is developed across HAZ due to repeated thermal cycling during multi-pass welding. The resulting fine grain (FGHAZ)/inter-critical (ICHAZ) region, which are creep weaker regions, makes the HAZ preferential region for creep crack initiation in weldments [6,7]. Notably, two main factors - coarsening of martensitic substructure with the reduction in dislocation density and coarsening of M23C6 precipitates, during inter-critical heating, are attributed to the loss of creep strength, creep strain localization, and cavitation in the FGHAZ/ICHAZ region.

View all citing articles on Scopus
View full text