Hot Cracking Phenomena in Welds
Four austenitic steels and alloys of the composition given in Table 1 were examined. Test specimens were 6mm diameter by mm long. The results, given in Table 2, have shown a substantial difference in the behaviour of Alloy and H, and they also showed a good agreement with the hot cracking susceptibility data available from the modified Varestraint test and from practical observations .
Alloy BTR o C. Thus, for the applied strain rate representative to the shrinkage rate of the weld thermal cycle ductility dips were observed at o C and o C for alloys H and respectively, while for slower strain rates these dips were absent. This last observation calls for metallographic verification of the tests, in particular for comparing the microstructures of test specimens with these of real welds containing hot cracks and for identifying the micromechanisms of cracks formation at the conditions of the hot tensile tests. What follows, is a part of metallographic verification of the specimens from the mentioned IIW testing programme, which revealed various microstructural features coinciding with the microstructures of the real welds as well as some discrepancies further used to modify the Gleeble testing conditions.
In a more susceptible to hot cracking alloy H, a rapid grain growth occurred near to the NST, Fig. Also chains of equiaxial voids appeared along segregation bands, however at the highest testing temperatures these voids did not easily coalesce to cause the failure — the weakest link of the microstructure appeared to be the grain boundaries, Fig.
Hot Cracking Phenomena in Welds II
At temperatures lower than NST, i. The behaviour of alloy H indicated that various micromechanisms could operate at different testing temperatures and that the hot crack susceptibility is strain rate dependent. It was possible to find on the longitudinal sections of the hot tensile specimens the regions at which the highest density of intergranular transverse cracks appeared and using the formula:. An example below, Fig. In another case, of alloy AC66, the loss of ductility due to liquation was quite severe and the recovery of ductility on-cooling delayed.
Here, the liquation caused a permanent precipitation of a carbide eutectic, which evidently hampered deformability of the alloy.
The precipitates appeared in segregation bands, Fig. In Fig. These precipitates in alloy AC66 visibly assisted the hot cracking for more than o C down from the NST, mainly by the formation of voids around them. Fracture of the hot tensile specimens near NST occurred mainly by joining of these voids in the transverse direction thus indicating additional weakness of grain boundaries at this temperature, Fig.
The eutectics appeared elongated in the rolling direction of the tested material, Fig. Nevertheless, at lower temperatures a substantial recovery of ductility appeared, due to thermal-mechanical grain refinement, Fig.
Hot Cracking Phenomena in Welds II
Microfissuring in Multi-Bead Welds. Microfissures are reheat-type fine cracks of length about 1mm or less when visible on transverse sections of welds and up to a few millimeters in the length direction of the weld. They are often of a ductility-dip origin and form in inter-bead heat affected zones of multi-layer, multi-bead welds, almost exclusively in the upper layers of these welds, i. The microfissures are related to thermal-mechanical history of the weld manufacturing and assisted by the primary segregation of weld metal solidification and the secondary reheating liquation as well as by solid-state embrittling processes resulting in ductility dips.
They form in the multi-pass welds in zones of adjacent underlying weld beads, which constitute the heat-affected zone of a subsequent pass; an example is given in Fig. To explain the formation of microfissures the following model  can be used, Fig. The a and b zones of the interbead HAZs can be also treated as the upper one a in which the diffusion healing and the nil-ductility appear, and the lower one b in which the ductility has recovered substantially however strain hardening may occur.
In general, the microstructure of the as-solidified weld metal beads is far from thermodynamic equilibrium by containing large amounts of crystalline lattice defects, such as vacancies and dislocations as well as planar defects, which are formed to compensate shrinkage effects and which tend to annihilate during subsequent thermal cycling. Thus, when during the laying down of the bead 6 such microstructure in the bead 5 is heated up to a temperature near to solidus, a large number of defects annihilate and this process is affected at first by thermal expansion of the whole interbead HAZ, and then by shrinkage restrained in some particular directions.
An intensive annihilation of vacancies and grain growth causing local reduction of volume is expected to occur during heating in the upper layer a 6 of the HAZ by the overlying bead 6 , i. Then on cooling, this zone shrinks and may be particularly prone to nucleation of cracks if its ductility is low. However, the simultaneous shrinkage of the already solidified weld metal in the bead 6 acts to compensate this effect.
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The expansion and its resulting compressive strain may have a positive effect that is the acceleration of homogenization and partial healing in the upper layer a 6 of the HAZ, and these may reduce the sensitivity to liquation cracking. Additionally, the shrinkage after solidification of the weld metal bead 6 may close partly the liquation cracks in the upper layer a 6 of the HAZ. Depending on which effect prevails, the sensitization occurs or the microfissures nucleate in the upper or in the lower layer of the interbead HAZ.
The critical sites for their formation are intersections with the coarse grained part of the HAZ a 5 of the previous bead 5 where laying down of this bead caused the liquation. The location of microfissures at the intersection with the HAZ of previous beads and frequent propagation along boundaries of primary columnar grains into the underlying bead, indicate that the segregation and liquation are important factors for their formation. To study the susceptibility of the multi-bead weld joints to microfissuring as well as the influence of microfissures on hot strength and ductility of the weld metals, SICO testing can be used on Gleeble.
The simulated weld heating cycle is then applied and the sample can be compressed immediately at the peak temperature or at any temperature on cooling from the peak. To check for the presence of microfissures and their influence on hot ductility SICO tests were run on samples taken from the upper half and lower half of an austenitic stainless steel multi-bead weld, and the cylindrical bar samples were machined transversely to the weld length, like schematically presented in Fig.
For the SICO test specimens cut from the top layer of the multi-pass weld metal, critical strains were lower at each temperature at the same testing condition than that from the bottom layer with rare microfissures, as shown in Fig. Moreover, it was found that all the cracks appeared at the specimen surface directed to the top of the weld, which indicates the presence of microfissures in the top layers of the multi-pass weld joint. The microfissures simply act as nuclei or microcracks for crack opening during compression under secondary tensile stresses.
Cracking phenomena in welding
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Buy Hardcover. Buy Softcover. FAQ Policy. About this book Although the avoidance of hot cracking still represents a major topic in modern fabrication welding components, the phenomena have not yet been fully understood. The chart shows the evolution of the average number of times documents published in a journal in the past two, three and four years have been cited in the current year. Evolution of the total number of citations and journal's self-citations received by a journal's published documents during the three previous years.
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