Nickel, Cobalt and Silicon in Induction Furnace Melting

Charge calculation starts before the power is switched on. For alloy and stainless heats, three materials decide most of the outcome: nickel, cobalt and silicon. They are bought to different grade tables, they fail in different ways, and each one asks something different of the furnace.

This chapter takes them one at a time — the physical constants first, then the grades, then what the material actually does once it is in the bath. It continues the raw materials requirements for induction melting with the three charge materials that cause the most off-spec heats.

Start with the constants

Three numbers decide whether a material can be handled in the unit you have: melting point, density, and how the material couples to the coil. Grade, packing and price come after.

All three melt far below the working range of a medium-frequency induction furnace. The difficulty is never whether they can be melted. It is when they melt, where they sit while they melt, and what they carry into the bath with them.

1. Nickel: two forms, two impurity profiles

Pure nickel exists in two commercial forms, and they are not interchangeable on a charge sheet: pyrometallurgical nickel block and electrolytic nickel.

Pyrometallurgical nickel block

The block is made by carbon reduction of nickel oxide concentrate powder. The concentrate is blended with a dextrin binder, pressed into briquettes and reduced at high temperature. Impurities stay in the block. Because the reductant is oxidised away, the block develops a large volume of pores, and those pores adsorb gas.

So the block is lower in purity and higher in gas. Degassing has to be strengthened deliberately before use — this is not a step that can be skipped for a cheaper charge.

Its normal destination is low-alloy structural steel and ordinary stainless steel — heats where 0.5–1.0% cobalt in the charge is not a problem and the gas load can be handled.

Electrolytic nickel

Electrolytic nickel is refined nickel produced hydrometallurgically. Crude nickel is cast into anode plates and electrolysed in a nickel sulphate solution. Cell voltage is controlled so that nickel deposits on the cathode while impurities remain in the electrolyte or settle into the anode slime. Grades above Ni 99.9% are produced this way.

Grade assignment follows the application, not the price list:

• Special grade and No.1 nickel — superalloys and precision alloys.
• No.2 and No.3 nickel — high-nickel alloy steel and alloy cast steel.

The gas problem is a surface problem

Two plates of the same grade can carry very different amounts of gas. A dense, flat cathode and a cathode covered in nodules do not behave the same in the furnace. Electrolyte adhering to the plate surface is itself a source of both gas and impurity.

Handling therefore has a fixed order: wash the plate in clean water to remove adhering electrolyte, dry it, then shear it. Plates with heavy nodulation need closer selection rather than more washing. In an induction furnace this matters more than in an arc furnace, because there is no slag boil to strip hydrogen back out — whatever the charge brings in stays in the bath.

2. Cobalt: the grade table does the work

Cobalt is a small-volume addition. Its main destinations are cobalt-bearing superalloys, precision alloys and high-speed tool steel; in ordinary steel it is rarely used as an alloying element at all. In practice that means cobalt arrives as a weighed special addition for a specific heat, not as part of the bulk charge — and the grade table, not the price, should decide which one you buy.

Read the nickel row before you read the cobalt row. Co-2 permits up to 0.3% nickel, Co-3 up to 0.5%, and Co-4 up to 1.5%. On a heat with a nickel ceiling, a lower cobalt grade silently borrows against that ceiling — this is one of the most common ways a nickel specification is blown by an addition that was, on paper, only cobalt. The iron and manganese rows work the same way, less dramatically.

Melting is not the constraint for cobalt: 1495 °C sits comfortably inside the range of every unit we build. Charge it early with the nickel so both are fully dissolved well before the block is tapped, and sample before the final adjustment rather than after.

3. Silicon: three products, one element

Silicon enters the furnace for two different reasons. It has a strong affinity for oxygen, which makes it the standard deoxidiser; it also strengthens ferrite and improves electromagnetic properties, which makes it an alloying element. The same element is bought as three different products depending on which job it is doing.

Metallic silicon is produced at 1500–1800 °C by carbon reduction of quartz. Its density of 2.4 g/cm³ is the number that matters most in the furnace: against a steel bath at roughly 7.8 g/cm³ it floats. An addition that floats sits at the slag–metal interface, which is exactly where the oxygen potential is highest, so a part of the silicon is burned before it ever reaches the analysis.

The practical consequences are simple. Add silicon late, after deoxidation rather than before it. Add it submerged, not on top of the slag. And charge for the reaction, not only for the aim analysis — the recovery figure you carry in the charge sheet is a measurement of the furnace, not a constant.

4. What the induction furnace adds to the problem

Induction furnace melting works by eddy currents induced in the charge itself. A cold, loose, low-density charge couples poorly and melts slowly; the same metal, cut to the right size, couples well. That is why charge sizing has to follow power frequency rather than habit — our steel melting range runs from 2500 Hz at the 100 kg end down to 300 Hz at 10 t, and the heavier the unit, the coarser the charge has to be to absorb power efficiently.

There is no arc and no slag boil in the process, which means almost no refining ability. Gas, sulphur and tramp elements brought in by the charge are largely still there at tap. That is the real reason the handling rules in this chapter are stricter for induction melting than the equivalent rules for an arc furnace: the furnace will not fix a bad charge, it will only melt it.

5. Charging and handling rules

1. Weigh to the grade table, not to the estimate. Nickel and cobalt are the two additions where a 0.1% error is visible in the final analysis.
2. Put nickel and cobalt low in the furnace. Under the bath they dissolve while the rest of the charge is still melting, instead of sitting on top and oxidising.
3. Degas pyrometallurgical nickel block. The porosity that makes it cheap is the same porosity that holds gas. Damp briquettes are not a minor issue.
4. Wash, dry, then shear electrolytic nickel. The order matters. Shearing first and washing afterwards leaves electrolyte in the cut faces.
5. Add silicon last, and account for the burn. Submerged addition after deoxidation, with stirring, gives the most repeatable recovery.
6. Keep the three materials segregated in store. Grade confusion between nickel and cobalt grades is the single most common cause of an off-spec heat that looks like a melting problem and is not.

6. Selection checklist

If you are sizing a furnace around a specific charge mix — nickel- and cobalt-bearing alloys, stainless, or high-silicon electrical steel — send us the aim analysis and the charge you intend to use. We will match it against the induction steel melting furnace range (0.1–20 t, 40–60 minutes per heat) or the stainless steel melting furnace, and tell you where the charge, not the equipment, is the limiting factor.

FAQ

Can a medium-frequency induction furnace melt nickel and cobalt?

Yes. Nickel melts at 1453 °C and cobalt at 1495 °C, both well inside the operating range of a standard medium-frequency unit. What limits the heat is charge density and sizing — how well the solid charge couples to the coil — and the gas the charge brings in, not the melting point.

Why does electrolytic nickel have to be washed before charging?

Electrolyte adhering to the plate surface is a direct source of gas and impurity in the bath. Washing in clean water, drying and then shearing removes most of it; a plate with heavy surface nodules needs to be selected out rather than simply washed harder.

Which cobalt grade should be bought?

Decide from the heat, not from the price per kilogram. Co-1 (Co 99.98% min.) is for work where total impurities below 0.02% matter. Co-2 to Co-4 carry progressively more nickel, iron and carbon — so on a heat with a nickel ceiling, the cheaper grade can cost you the specification.