July 18, 2019
On this blogpost, we share the main findings of the study "Value-in-use of nickel products in the production of ferrous alloys" conducted by Rasoul Sadeghi and, Dr. Sebastian Kreft. The study investigated and compared the effects of using nickel metal and ferronickel on the total cost and quality of produced ferrous alloys. The main results help both foundry operations and purchasing departments for making more informed decisions in selecting the optimal raw material. Nickel is a very important alloying metal which finds application in many cast irons and steels. The main reason for adding nickel in ferrous alloys is to promote an austenitic microstructure. Nickel generally increases ductility, toughness and corrosion resistance. There are two main categories of nickel products available for foundries, nickel metal, which is available in a variety of shapes and sizes, and ferronickel.
In general, foundrymen favor pure metals over ferroalloys because of their ease of use. However, producing ferroalloys rather than pure metals has some advantages over refining them to highly purified metals. In fact, the refining and concentration process is costly and it is not logical to remove iron while it is the main element in ferrous melting.
Nickel is an essential and widely used alloying element in high-temperature-resistant superalloys and heat-, oxidation-, and corrosion-resistant irons and steels. Nickel is well-known for its solid solution strengthening and promoting of high toughness, mainly at low temperatures. Besides its main application in stainless steelmaking nickel is also widely used in low-alloys steels and irons.
The addition of higher amounts of nickel promotes austenite stabilization at room temperature together with loss of ferromagnetism and increasing resistance to corrosion. Because of its little or no affinity to react with carbon, nickel has graphitization effects in ferrous alloys which are needed for the characteristic graphite microstructure of cast iron. For steels, this phenomenon needs to be stopped by keeping carbon in low percentage amounts or adding carbide forming elements like chromium. Furthermore, nickel inhibits the grain growth and increases the hardenability in ferrous alloys.
Nickel products for foundries
According to the International Nickel Study Group (INSG) there are two classes of primary nickel products which are applicable for foundries. Class I, which contains products with a nickel content of more than 99 wt% and Class II, which contains products with a nickel content of less than 99 wt% as shown in Table 1.
Ferronickel and Nickel Pig Iron (NPI) are two alternatives to nickel metal. NPI typically contains around 4 to 10 wt% nickel. Its high amount of carbon (>3 wt%), silicon (>3 wt%) and manganese (>1 wt%) mean that its application in foundries is highly restricted. In fact, the main consumers of this product are stainless steel mills in China, because of their capacity to reduce carbon and other undesired elements . Ferronickel is a ferroalloy containing mainly iron and nickel. It is supplied in the shape of flowable granules with a typical particle size between 2 and 50 mm. Part of the production process is a refining step which reduces the amount of carbon, silicon, manganese, sulfur, and phosphorus to an acceptable level for foundries.
Table 2 shows the guaranteed chemical specification of the Brazilian ferronickel and the guaranteed chemical specification of 4”x4” LME grade cut nickel cathodes.
According to the Ellingham diagram, which is a graph showing the temperature dependence of the stability for compounds in melting and casting temperature range of ferrous alloys (1300 to 1600 °C), nickel has less affinity to oxidation than other available elements in the molten path. Therefore, there is not a significant amount of nickel loss during melting and in this regard, there are no differences between using ferronickel or nickel metal. However, there are three other differences that melting departments may be concerned about: the existence of iron, cobalt and other elements in ferronickel.
Main conclusion of the study
From a technical point of view, ferronickel and nickel metal both can easily be added without a major loss through oxidation.
Cobalt is in the center of consideration because it has been considered as the main reason for refusal of ferronickel. However, from a metallurgical point of view, it is not a harmful element for cast iron and steel except for nuclear power applications which are sensitive to the radioactive properties of cobalt. The presence of even small amounts of cobalt, as an alloying element, has been reported to significantly improve the mechanical, metallurgical and corrosion resistance properties of cast iron and steel. Other elements like carbon, silicon, sulfur, phosphorus, and copper are present in very low and acceptable amounts.
From a financial point of view, ferronickel delivers a clear cost advantage. The savings potential of around 10% per ton of nickel should create a strong incentive for other foundries to use ferronickel in their process. Furthermore, automated charging of the furnace saves time, which indirectly creates additional savings for the consumer depending on the specific operation.