Since then it has become possible to reduce the content of trace elements in nickel drastically. For measurement and control devices, specific properties and their stability over time and different production lots is decisive. Both can be achieved by a strict limitation of trace elements.
Article by Theodor Stuth, hpulcas GmbH, Germany
Current technologies for producing nickel
Starting material - Nickel is initially produced in the form of cathode plates or powder, both having a purity of up to 99.98%. As cathode plates have a number of undesirable properties, they are cut into squares and either dissolved (by a galvanising process) or melted, to be eventually used in the production of pure nickel, nickel alloys or stainless steel. Melt metallurgy - It is extremely difficult to reach the highest purity of the starting material if nickel is processed by means of melt metallurgy.
If nickel is melted under an ambient air, carbon, aluminum, silicon and titanium are added to deoxidise the melt as well as manganese and magnesium to globularise sulphur. These elements are supposed to be slagged, but remain partially in the melt and thus deteriorate the degree of purity. When melting by industrial vacuum metallurgy processes, the vacuum is not high enough, which is why the addition of deoxidisers cannot be completely avoided. Vacuum-melted slabs have either high oxygen or carbon content.
To achieve high grades of purity, nickel made by melt metallurgy must be purified. This can be done by electroslag remelting (ESR) and zone refining (ZR). Powder metallurgy - Powder can be produced by the Mond process, which allows one to achieve high degrees of purity. The Ni 270 norm presupposes that such qualities are to be produced by means of powder metallurgy. Strip can be directly rolled from powder or pressed into slabs, which are sintered and hot-rolled. Large slabs are difficult to produce by powder metallurgy, however, resulting in density variations. Products made by means of powder metallurgy remain porous to some extent; fully dense products require massive subsequent reduction.
Production of high purity nickel directly from cathode plates
Because of the high degree of purity of cathode plates, it has frequently been tried to use them as starting materials for industrial nickel production. These trials have always failed because the negative characteristics of cathode plates could not be completely overcome. These are:
- The plates are covered with an orange peel, dotted with occasional warts. Direct rolling of cathode plates results in surface cracks;
- As a result of the electrolytic winning process, cathode plates show a columnar grain structure, which resists deformation against the axis of the grains;
- Cathode plates are produced by inserting a thin starter sheet into the electrolytic bath. Both sides of a sheet receive nickel layers built up by electrolytic deposition. The cathode plate therefore consists of three layers. Upon rolling, the surface of the plate, which is in contact with the rolls, flows faster than the material inside the plate. The strain thereby induced may split the plate horizontally during rolling, or at least loosen the cohesion of the layers, also resulting in splitting or the appearance of delaminations;
- The starter sheet is usually corrugated to hinder bending in the hot bath. As a result, a cathode plate takes this form as well. Corrugation cannot completely hinder twisting, thus the plates get out of plane. To use them in an industrial process, plates must be flattened;
- If the corrugation extends over the whole length of a plate, upon flattening, three-dimensional stresses are introduced into the plates, resulting in stress cracks upon hot rolling;
- There could be substantial thickness differences of up to seven millimetres cross- and longwise throughout the plate. Depending on the distribution of thickness differences, upon rolling, plates can take different shapes, e.g. a wedge shape of a cathode plate results in a sabre shape of a rolled plate. Randomly dispersed thickness differences result in different elongation upon rolling, resulting at best in a fishtail form of the end of a plate. Greater thickness differences may lead to stress cracks developing on the surface during rolling. In the worst cases, holes appear in a plate; and
- Plates are loaded with hydrogen, which may create bubbles during hot processing and render the welding of plates into strip impossible.
Strip & wire directly from cathode plates
Hpulcas has developed a range of technologies to overcome the deficiencies of cathode plates:
- Plates are scanned and data are transformed into computeraided design (CAD) processable data. By means of CAD, a new surface lying 0.2mm below the current surface is created. Extra surface is then removed by milling, and orange skin and warts are also removed;
- By the same technique, thickness differences surpassing one millimetre are also removed;
- The declining edges of the plates are trimmed in order to hinder the edges tearing during hot rolling;
- Hydrogen is driven out by heat treatment;
- A columnar grain structure is converted into a globular structure by heat treatment; and
- The three-layer structure is dissolved by promoting grain growth over the layer boundaries.
Thereby the plates are prepared for processing. They are heated to the hot rolling temperature, hot rolled, levelled, brushed to remove the oxide scale, cut into rectangles of uniform width and welded into a strip. By a sequence of reducing and annealing steps, grain structure of the weld seam and the heataffected zone are approximated with the grain structure of the plates. For the production of wire, the plates are cut into sticks, frontally welded and drawn to a desired size.