ABSTRACT
The low carbon 46Ni-35Cr-9Mo alloy is a solid solution strengthened cast Ni-Cr-Mo alloy additionally containing Cu and N. This alloy displays the best known mechanical properties and corrosion resistance among the existing cast low carbon Ni-Cr-Mo alloys. This work reports on the properties achievable for a centrifugally cast low carbon 46Ni-35Cr-9Mo alloy tube of 4500 mm length and 55 mm wall thickness. The phase formation after the solidification as well as the influence of heat treatment on the microstructure development is examined. The influence of the various heat treatment states on the mechanical properties as well as corrosion resistance is studied. An extensive testing program has been taken for the determination of mechanical properties on samples taken from different orientations within the centrifugally cast tube. The properties do not exhibit directionality and hence demonstrate the isotropic behavior of the cast product.
INTRODUCTION
Nickel alloys find applications as engineering materials since they can be used in a wide variety of environments ranging from both aqueous and high temperature corrosion resistance.1 These alloys are available in the wrought form as well as the cast form and have been widely used for applications. Whereas the high carbon containing alloys find applications for creep resistance and carburization resistance at elevated temperatures, the low carbon containing alloys predominantly find usage in the chemical process industry.2,3 Nickel is metallurgically compatible over a wide composition range with a number of alloying elements. This quality has made it possible to realize binary, ternary, quarternary alloy combinations. This has resulted in the development of various nickel based alloys which are alloyed principally with chromium and molybdenum. Chromium increases the corrosion resistance in oxidizing media while molybdenum increases resistance to reducing media.1 Table 1 shows the nominal chemical composition of various Ni-Cr-Mo alloys. The classification is based on considering the major alloying element as the solvent and is shown in increasing order of Cr+Mo. It is evident that the new alloys have been improved in metallurgy by additions of other alloying elements like tungsten, niobium and copper which further increase the corrosion resistance in terms of for e.g. pitting resistance, crevice corrosion resistance as well as resistance to high chlorine oxidizing solutions, non- oxidizing acids. The alloy UNS N06058 is an addition to the Ni-Cr-Mo alloys which is alloyed with nitrogen.4 The next kind of alloy systems based on Nickel as a matrix are the “G-type” alloys which are used for phosphoric acid resistance.5 These alloys are alloyed with copper, cobalt, niobium as well as tungsten and were essentially developed to fill the performance gap between the high molybdenum containing stainless steels and the Ni-Cr-Mo alloys. The next grouping considered is the Ni-Fe-Cr type alloys for e.g. Alloy 825, contains more iron than chromium. The addition of iron leads to less solubility for molybdenum. However, there is a cost advantage due to the higher amount of iron. Additionally, increasing molybdenum leads to increase in the localized corrosion resistance in an oxidizing chloride environment and the presence of sufficient amount of nickel makes it stress corrosion cracking resistant. The alloys in this group are also alloyed with copper to impart resistance to sulphuric acid. Table 1 also shows the cast equivalents for some of the Ni-Cr-Mo based alloys. It is evident that most of the commercially successful wrought Nickel based alloys have cast equivalents.
