Welding and post weld heat treatment of 2.25%Cr-1%Mo steel

Transcription

1 University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year 2005 Welding and post weld heat treatment of 2.25%Cr-1%Mo steel Benjamin King University of Wollongong King, Benjamin, Welding and post weld heat treatment of 2.25 This paper is posted at Research Online.

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3 25µm Figure mm final run CGHAZ after exposure to 550 C for 0.5 h 25µm Figure mm final run CGHAZ after exposure to 550 C for 4 h 90

4 25µm Figure mm final run CGHAZ after exposure to 550 C for 10 h The carbide structure in the CGHAZ of several welds was also investigated after holding at 550 C. Figures 58 and 59 show a relatively carbide-free structure in the as-welded condition. Further time at temperature (Figures 60-63) shows the presence of two distinct types of carbides. Relatively large Fe-rich carbides and smaller, Cr/Mo-rich precipitates. 91

5 Figure 58 - TEM image of carbon replica of 12 mm CGHAZ material, as-welded Figure 59 - TEM image of carbon replica of 12 mm CGHAZ material, as-welded 92

6 These carbide replica images of the 12 mm CGHAZ material in the as-welded condition show a structure relatively free of carbide particles. There were some small, isolated carbide particles with compositions rich in Cr. B A Figure 60 - TEM image of a carbon replica of a 12 mm CGHAZ after being held at 550 C for 2 h 93

7 Figure 61 - TEM image of carbon replica of 12 mm CGHAZ material after holding at 550 C for 2 h. The carbon replicas seen in Figures 60 and 61 show the effect of 2 h exposure to 550 C. It can be seen that there are large conglomerates of particles dispersed through the structure as marked A in Figure 60. The spherical particles, marked B in this image were rich in Ti and Mn and thus appear to be inclusions, similar particles, while not shown in Figures 58 and 59, also appeared in the as-welded CGHAZ sample. In addition to these large particles, this sample showed a fine-scale dispersion of particles in several areas as shown in Figure 61. The average analysis of two regions of these finer particles showed the composition of the metallic component to be approximately: 68% Cr, 24% Mo, 8% Fe (all values atomic %). 94

8 Figure 62 - TEM image of C-replica of 12 mm CGHAZ material held at 550 C for 10 h Figure 63 - TEM image of C-replica of 12 mm CGHAZ material held at 550 C for 10 h 95

9 The replicas of the sample heated to 550 C for 10 h again show two distinct sizes of particles. The larger (micron sized) particles in Figure 62 showed compositions of the metallic components of approximately; 93% Fe, 6% Cr,1% Mo. The smaller particles were Cr-rich similar to the finely dispersed particles in the 2 h sample (Figure 61), but were higher in Fe with the approximate composition of the metallic component; 61% Cr, 12% Mo, 26% Fe. C Figure 64 - TEM image of carbon replica of 26 mm CGHAZ material after being held at 550 C for 10 h 96

10 Figure 65 - TEM image of carbon replica of 26 mm CGHAZ material after being held at 550 C for 10 h. Image taken from the area in Figure 64 labelled C. The carbon replica from the 26 mm CGHAZ material which had been exposed to 550 C for 10 h also shows the presence of two distinct forms of carbide. The larger particles seen in Figure 64 were Fe-rich and similar to those observed in the other samples: 86% Fe, 12% Cr, 2% Mo. There were several distinct regions in this sample which showed a fine dispersion of carbides, as marked C in Figure 64 and shown in Figure 65. These particles also showed a Fe-rich composition of 78% Fe, 17% Cr, 5% Mo. 97

11 4.5 Effect of PWHT on microstructure The 12, 16 and 26 mm samples were also subjected to the standard PWHT (from which the 12 mm material is exempt under AS4458). The resulting CGHAZ microstructures are shown in Figures The hardness data recorded in Table 7 show reasonably consistent hardness values across the three thicknesses. Table 7 CGHAZ hardness after PWHT at 700 C for 1 h 12 mm 16 mm 26 mm HV µm Figure mm final run CGHAZ after PWHT at 700 C for 1 h 98