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Nitrogen hardening of creep-resistant G-NiCr28W alloy

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Pirowski Z. , Wodnicki J. , Gwiżdż A.

Archives of Foundry Engineering

2010

tom Vol. 10, iss. 3

79--84

In the group of creep-resistant materials, most important are heat-resistant nickel-based alloys. The G-NiCr28W alloy subject to detailed examinations was observed to have two different austenite-like phases. In the interdendritic spaces of alloy matrix, the presence of another phase, also characterised by paramagnetic properties, was detected. Inside this interdendritic phase, local areas of a lamellar structure, composed of both of the above mentioned phases, were present. Nitrogen treatment was observed to raise the microhardness of both these phases. The presence of nitrogen made the regions of a lamellar structure disappear completely. Their place was occupied by precipitates dispersed in the matrix, and occasionally forming large clusters. It has been observed that cold work can harden the G-NiCr28W alloy to a very small degree only, in spite of the fact that hardness is increasing systematically with the increasing degree of cold work. The said alloy, when enriched with nitrogen added as an alloying element, is characterised by hardness higher than the hardness of its nitrogen-free counterpart. The value of hardness is increasing even more under the effect of low-degree cold work, although increasing further the degree of cold work seems to have no effect on hardness increase. The problem faced in nickel-based materials is the possibility of making defect-free castings from alloys with high nitrogen content. Alloys investigated in the present study were remelted, cast and subject to solidification under high nitrogen pressure in the furnace chamber. However, melting carried out under these conditions could not prevent the occurrence of non-metallic inclusions which, while being unable to pass to a riser, formed local clusters or even thin films, resulting in numerous microcracks or discontinuities encompassing large regions of the casting. This problem seems to be of major concern and is the first one to require prompt solution in the currently executed large research project.