Strength Properties in all Weld Metal 9 Ni-joints, SAW ERNiCrMo 4

摘要

Horizontal joints of LNG storage tanks are welded by SAW 2.4mm wire ERNiCrMo-4 with the wire-flux combination Thermanit Nimo C 276/Marathon 104. All welding metal properties have been tested successfully. Problems occurred in qualification tests to fulfil the min. strength by longitudinal "all weld metal" specimen as imposed by API 620, whereas transverse tests passed without any problem and correspond to the all weld metal values. Results of all examinations which have been performed on the all weld metal-, and the API620 specimen have produced differences between both specimen. The metallographic tests on the API620 specimen have shown that quite isolated conditions are predominant at this location. The gauge section is placed to about 80% within one bead having an isotope microstructure. This specific metallurgical structure in the gauge section provokes the slight decrease of the tensile strength in comparison to the transverse and all weld metal tensile test. As it can be seen in the results of the chemical testing the increase in the Fe-content in the API620 specimen is another contribution to the loss in strength. Furthermore the surface condition of the specimen shall not be underestimated as the irregularities such as small "notches" favour an earlier break compared to a smooth surface. The application of the longitudinal tensile test acc. to API 620 is related to some uncertainties. These are on one hand problems in the definition "all weld" (there is no all weld metal at all). Furthermore the technical aspect of the isolated condition, macro/microstructure as well as the chemistry, has shown that the low strength values cannot be taken as a decisive for the acceptance.