| Nickel-based alloy is a high temperature alloy, with high strength, good oxidation resistance and corrosion resistance at high temperature, it is widely used in the manufacture of key components in nuclear pressurized water reactor and steam generator. But Nickel-based alloy weld metal has high susceptibility to Ductility-dip cracking(DDC), there is no comprehensive understanding and common agreement on the mechanism of DDC, and there is no better way to control it. In this paper, two kinds of filler metals ERNi Cr Fe-7 and ERNi Cr-4 of 690 Ni-based alloy were studied. A new test method was used to evaluate DDC, combined with the finite element calculation design variable cross-section samples, and high temperature tensile test was carried out on Thermorestor-W simulation test machine. Put forward a new crack sensitivity criterion-threshold stress, from mechanical point understanding the DDC generation mechanism of the nickel base alloy welding wire ERNi Cr Fe-7 and ERNi Cr-4, and combined with threshold strain to evaluate crack sensitivity under various temperatures.To study the occurrence of cracks in the specimen s, high temperature tensile tests were performed by using variable cross-section specimens of two kinds of materials, the deformation temperature is 850 to 1200℃. The specimen surface will produce DDC when the specimen undergo a certain deformation, with the increase of temperature, the number of cracks increases first and then decreases, and the maximum number of cracks on the specimen surface at 1100℃. Threshold strain was used to evaluate the susceptibility of DDC, it is concluded that when the temperature is 900℃ to 1150℃, the plastic property of the material is relatively low, and th e threshold strain to produce crack is relatively small. Compared with ERNi Cr-4, the threshold strain of ERNi Cr Fe-7 cracking is smaller. Threshold stress was used to evaluate the susceptibility of DDC, when the deformation temperature within the range of 950~1150℃, the threshold stress value is small, in other words, the most sensitive deformation temperature range of two kinds of materials to produce crack is 950℃ to 1150℃.High temperature compression tests of two kinds of materials were performed by using the Thermorestor-W simulation test machine, hot deformation temperature is from 850℃ to 1200℃, and strain rate is 0.1s-1. Acquiring true stress-true strain curves of specimens during high temperature deformation, obtain the relationship between flow stress and temperature at the same strain. Analysis believe that the generation of DDC is closely related to the stress, when the flow stress exceeds threshold stress, it is easy to produce cracking. The temperature is below 900℃or higher than 1150℃, the threshold stress is greater than the flow stress, temperature is 900℃ to 1150℃, the threshold stress is less than the flow stress. Compared with ERNi Cr-4, the threshold stress of ERNi Cr Fe-7 cracking is smaller. This is because the flow stress of ERNi Cr Fe-7 during hot deformation is higher, the deformation resistance of materials increase and the deformation mainly concentrate on the grain boundaries, therefore, it is easy to produce intergranular crack. Under the same deformation temperature, two kinds of material specimens cra cking in the same way. There are more intergranular cracks and a small amount of transgranular cracks at 850℃ and 1200℃, but only appear intergranular cracks at 900~1150℃. The crack propagation direction and loading direction has certain relationship, it is found that the crack proliferate direction and the loading direction has an angle of 70 deg to 90 deg. The initiation position of DDC is in the interface between M 23C6 and austenite matrix(γ), and extends along the grain boundaries. Due to M23C6 and γ matrix is non-coherent relationship, the interface bonding strength is low and the stress concentration is larger, so the M23C6 can increase the susceptibility of the weld metal DDC. |
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