Microstructural evolution and mechanical properties of high strength 3–9% Ni-steel alloys weld metals produced by electron beam welding

• The nickel has a great effect on the microstructures of the weld metals.
• Grains were rapidly coarsened with increasing Ni content.
• The nickel has a great effect on content and morphology of retained austenite.
• The hardness slightly increased with increasing the rotation speed.
• Results have shown a surprising effect of Ni content on absorbed energy.

The microstructures and mechanical properties of weld metals of high strength steels having 3–9% Ni content have been investigated with getting a better insight into the role of retained austenite. The weld metals were produced autogenously by electron beam welding (EBW) process. The results showed that once Ni content exceeded 4% prior austenitic grains of the weld metals were rapidly coarsened and solidified into a cellular dendritic structure. The content of retained austenite increased with Ni addition and was preferentially distributed along the lath boundary, edges of coalesced bainite, cellular dendritic boundaries and at prior austenite grain boundaries. Retained austenite morphology was also changed on increasing nickel content from a discontinuous film into a continuous one. The impact toughness for half-size specimens has shown a significant drop from 126 J to 40 J when Ni content increased from 3% to 5%, while further addition of Ni partially recovered the toughness. Thorough investigation of fracture surface of weld metals, after impact test, elucidated that retained austenite was beneficial and has an effective role in reducing the detrimental effect of coalesced bainite that formed in the microstructure.