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Compositional factors controlling solidification cracking in C-Mn steel laser welds

 

TWI Industrial Member Report Summary 623/1997

By G I Rees

Background

The possibility of using lasers to weld steel of greater than lOmm thickness is attractive for many reasons. The speed with which laser welding can be performed, due to the high power delivery and the deep penetration characteristic of the process, offers the possibility of significant increases in productivity over conventional welding, as does the ease with which laser processes can be automated. Many of the characteristics of laser welds are advantageous in themselves. The deep penetration effect produced by 'keyhole' welding and the small weld volume produced result in significantly lower levels of distortion than would be the case for arc welding. Low hydrogen levels are also easy to maintain. Laser welds are, however, prone to solidification cracking and porosity. Recent work at TWI, investigating the relative susceptibility of various steels to solidification cracking, has revealed that the relationship between the cracking susceptibility and the composition of the steels is different from that seen in arc or EB welds.

The task of the present research project's first year was to identify suitable test apparatus for ranking the relative solidification crack susceptibility of C-Mn and low-alloy steels. It was concluded that the Russell test gave results' which were in accordance with those obtained from laser butt welds and which were a significant improvement on those produced by the test previously used at TW. The test involves producing melt-runs, rather than actual welds, and can therefore be used only to rank material crack sensitivity, rather than determining suitable,parameters for real welds. However, the morphology of the cracks formed in the Russell test matches that of cracks in actual laser butt welds. The cracks are short, typically <5mm, and are intermittent, located at the plate mid-thickness. This report details hrther work canied out to rank the solidification cracking susceptibility of a range of steels of different compositions, using the Russell test.

Objectives

  • To determine the typical degree of scatter for repeated tests using the Russell test apparatus.
  • To rank the solidification cracking susceptibility of a range of C-Mn steels, using this test.

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