CREEP PROPERTIES OF T92/HR3C WELDED JOINTS 91 among which the shear m dulus is commonly regarded as one-tenth of the elastic modulus The results of the cor onal analy presente in Fig. 13, which shows the Von Mises equivalent stress distribution across the welded joint. It can be obviously observed from Fig. 13a that residual stress mainly ac cumulates at the two HAZ regions. The largest resid ual stress occurs at the HAZ of T92 region, seen in Fig b, which has already reached around 330 MPa. Mean while, the residual stress of the haz of hr3c is also up to around 250 MPa. This could be ascribed to the preferable ductility of austenite HR3C, as it is able to off- set stresses through plastic deformation. The FEM result verified the fact that residual stresses usually accumulate at HAZ of the undermatched part, i. e the ferrite HAZ, in ferrite/austenite dissimilar steels welded joints. A conclu sion can then be put forward that the selection of welding wires with similar strength of ferrites could effectively lieve the residual stresses of the joints after welding; in other words, it can increase the comprehensive strength of the joir Compreh In terms of creep voids, they are commonly generated un- der the interaction among high temperature, load stress and ageing time. Till date there are several classic con troversial theories of the emergence of creep voids. 9, #0 By Greenwood, Argon, #. Raj and Ashby, #4 tion positions of void nucleation were argued; by Hull and Fig9 SEM of the cross-section of creep ruptured sample under Rimmer, +5 Needleman# and Hancock, "the controlling 30 MPa(a)total morphology() shallow voids factors of void growth were debated; and by Stowell, + s Nicolaou and Semiatin 49,50 and Chokshi, 5I the coales cence procedures of voids were discussed. In this paper, stress of the T92/HR3C dissimilar steels welded joint af- based on their works, a four-stage nucleation to crack ter welding was calculated by the FEM software ANSYs mechanism of creep voids was put forward to clearly un- 10.0. derstand the mechanisms of creep voids generation for the width of the welded joint is not sufficiently large, the T92/HR3C dissimilar steels welded joints their effect on residual stress distribution is generally ne- glected. Thus, the three-dimensional (3D)thermal-stress 1 Nucleation coupled field analysis can be simplified as 2D axisymmet- In this stage, the creep voids with irregular shapes are ric problem. The 2D FEM model (after being meshed) generated from the effect of grain boundaries sliding is shown in Fig. 1la by using PLANE 13 2D coupled d/or grain matrix deformation under specific load field solid element. Also, birth-death element was applied In in the weld seam with 21 lavers to simulate the welding the voids commonly nucleate at the grain boundaries of procedure, seen in Fig. llb, of all the layers that were the weakest part or the part with largest residual stress of initially dead, the lowest layer was firstly activated when tested material. As for the t92/HR3C dissimilar steels being welded, then the rest ones would be activated layer welded joints, the rupture positions i. e the voids nucle by layer above the former ones sequentially. Displace ation positions varied under different creep condition ment in horizontal and vertical directions of the left and seen in Table 5. This may be accounted for the different the right borders of the welded joint set zero were the properties of the five regions across the joint. boundary conditions. The physical properties of T92 and 2 Growth HR3C base materials as well as ErNiCr-3 welding wire Creep voids continuously grow under constant temper used in calculation are listed in Fig. 12 and Table 7, 12.3 ature and load stress in this growth stage. However, the @2010 Blackwell Publishing Ltd Fatigue Fract Engng Mater Struct 34, 83-96C R E E P P RO P E RTI E S O F T 9 2/H R 3 C W E LD ED JOINT S 91 Fig. 9 SEM of the cross-section of creep ruptured sample under 130 MPa (a) total morphology (b) shallow voids. stress of the T92/HR3C dissimilar steels welded joint af￾ter welding was calculated by the FEM software ANSYS 10.0. As the width of the welded joint is not sufficiently large, their effect on residual stress distribution is generally ne￾glected. Thus, the three-dimensional (3D) thermal-stress coupled field analysis can be simplified as 2D axisymmet￾ric problem. The 2D FEM model (after being meshed) is shown in Fig. 11a by using PLANE 13 2D coupled field solid element. Also, birth–death element was applied in the weld seam with 21 layers to simulate the welding procedure, seen in Fig. 11b, of all the layers that were initially dead, the lowest layer was firstly activated when being welded, then the rest ones would be activated layer by layer above the former ones sequentially. Displace￾ment in horizontal and vertical directions of the left and the right borders of the welded joint set zero were the boundary conditions. The physical properties of T92 and HR3C base materials as well as ERNiCr-3 welding wire used in calculation are listed in Fig. 12 and Table 7,12,38 among which the shear modulus is commonly regarded as one-tenth of the elastic modulus. The results of the computational analysis are presented in Fig. 13, which shows the Von Mises equivalent stress distribution across the welded joint. It can be obviously observed from Fig. 13a that residual stress mainly ac￾cumulates at the two HAZ regions. The largest resid￾ual stress occurs at the HAZ of T92 region, seen in Fig. 13b, which has already reached around 330 MPa. Mean￾while, the residual stress of the HAZ of HR3C is also up to around 250 MPa. This could be ascribed to the preferable ductility of austenite HR3C, as it is able to off￾set stresses through plastic deformation. The FEM result verified the fact that residual stresses usually accumulate at HAZ of the undermatched part, i.e. the ferrite HAZ, in ferrite/austenite dissimilar steels welded joints. A conclu￾sion can then be put forward that the selection of welding wires with similar strength of ferrites could effectively re￾lieve the residual stresses of the joints after welding; in other words, it can increase the comprehensive strength of the joints. Comprehensive analysis In terms of creep voids, they are commonly generated un￾der the interaction among high temperature, load stress and ageing time. Till date, there are several classic con￾troversial theories of the emergence of creep voids.39,40 By Greenwood,41 Argon,42,43 Raj and Ashby,44 the initia￾tion positions of void nucleation were argued; by Hull and Rimmer,45 Needleman46 and Hancock,47 the controlling factors of void growth were debated; and by Stowell,48 Nicolaou and Semiatin,49,50 and Chokshi,51 the coales￾cence procedures of voids were discussed. In this paper, based on their works, a four-stage ‘nucleation to crack’ mechanism of creep voids was put forward to clearly un￾derstand the mechanisms of creep voids generation for the T92/HR3C dissimilar steels welded joints. 1 Nucleation In this stage, the creep voids with irregular shapes are generated from the effect of grain boundaries sliding and/or grain matrix deformation under specific load stress and temperature in creep process. Accordingly, the voids commonly nucleate at the grain boundaries of the weakest part or the part with largest residual stress of tested material. As for the T92/HR3C dissimilar steels welded joints, the rupture positions i.e. the voids nucle￾ation positions varied under different creep conditions, seen in Table 5. This may be accounted for the different properties of the five regions across the joint. 2 Growth Creep voids continuously grow under constant temper￾ature and load stress in this growth stage. However, the c 2010 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 34, 83–96