Galvanization and ZINKEL® on cast iron
The cast iron (also called, until the nineteenth century, 'ferraccio' [bad iron] for the lower quality and the worst workability compared to mild steel) is a relatively high carbon alloy (2.11% < C < 6.67% that is the saturation limit); it is obtained by reduction or heat treatment of iron minerals.
The features that allow the use of cast iron in many applications are:
- Economy in production;
- Resistance to wear;
- Good workability with machine tools;
- Mechanical resistance comparable to steel;
- Possibility of creating very complex shapes,
by simple casting;
- High hardness and compressive strength;
- Excellent castability.
Gray or lamellar cast iron
It is the most widespread cast iron type, produced with the fusion of cast iron and steel scrap, with the addition of graphitizing elements (C between 2.5% and 4% in
weight, Si between 1% and 3% always by weight, P).
Because of the lamellae, silicon and phosphorus, there is often fragility; to reduce it the bath is inoculated with CaSi, which favors the heterogeneous nucleation and, therefore, the formation of short lamellae.
In any case, the fragility induced by the lamellar graphite overshadows the pearlitic or ferritic constitution of the metal matrix and makes useless to speak of yield, ductility and resilience.
Also called spheroidal cast iron or spheroidal graphite cast iron or nodular cast iron - in English ductile iron.
It is a cast iron in which the graphite, rather than in a lamella shape,
appears in nodules in a spheroid shape.
The nodules are in a metal matrix whose structure is a function of the chemical composition of the specific type of cast iron, of the cooling speed at the time of solidification and of any subsequent thermal treatments.
Malleable cast iron
It dates from the eighteenth century, contains graphite in a rounded shape thanks to the heat treatment of malleabilization: starting from the unalloyed white cast iron, into pieces of
small thickness (this is a considerable limitation in the production of malleable cast iron) the thermal process involves heating to 950 ∞C for many hours, so as to favor decomposition: Fe3C -> 3 Fe + Cg.
The formation of roundish graphite (in a ferritic or pearlitic matrix) gives greater yield and ductility, approaching in this to mild steel.
Sometimes considered unfairly as an old and surpassed material, malleable cast iron still has its own importance: it is the right choice for small-section products and for products that have to withstand low temperature.
The malleable iron is used in the railway field, in general, for valves, taps, fittings, electrical fittings, hand tools, washers, stirrups, agricultural tools, etc.
White cast iron
It is a particular variety of cast iron that does not contain graphite carbon, but exclusively cementite. (Iron carbide is indicated with the symbol Fe3C).
Treatments for cast iron
Cast iron, like all ferrous, needs to be protected from atmospheric oxidising agents.
Traditional treatments are:
It is a process of coating metal surfaces with an organic film, made for decorative purposes and / or for protection against corrosion and aggressive agents.
The pieces being processed are covered with paint powder based on synthetic resins that adheres by electrostatic effect, then passed into an oven where due to the temperature the paint first melts and then polymerises giving an adherent layer.
However, the coating has no resistance to mechanical wear.
The technology provides a high degree of protection against corrosion, using combinations of special base coatings, followed by a final layer or top coat.
Widely used by the fastener industry, these coatings are suitable to a variety of applications.
The range is very versatile: fasteners, hose clamps, clips or brake discs for the automotive industry, fasteners for wind power plants and for the construction industry and fittings for furniture.
Being a deposit not purely metallic has a low surface hardness, and unfortunately on the parts with complex geometry, cavities and threaded holes, creates residue that does not always allow good adhesion to the substrate.
THE NEW TREATMENTS PROPOSED BY GT GALVANOTECNICA:
It is the process by which a zinc coating is applied to a metal product, generally made of steel, to be protected from galvanic corrosion. In fact, it limits the formation of electrolytic micro-cells with an anodic action.
Zinc is less electronegative (ie less noble) than steel, therefore, in the event of breakage or porosity of the protective film, it becomes the sacrificial anode in electrolytic corrosion and is consumed if there is closure of the field lines.
It implements the performance of the other treatments:
- Compared to painting it has a high surface hardness (over 500 HV), so there is no risk that the deposit will be damaged by scratches and external abrasions. High resistance to corrosion (with thickness of 10 microns, which guarantees about 1000 h of resistance on the base metal, according to DIN 50021 SS).
- Compared to electrolytic zinc plating: it increases mechanical resistance and corrosion resistance, allowing an even higher corrosion resistance in areas that are difficult to reach with a lot of deposits.
- Compared to lamellar zinc (geomet, zintek, deltaprotect etc.): better surface hardness, better adhesion to the substrate and, consequently, no residue in threads and cavities, while guaranteeing the same corrosion performance.
- Bright finish, stable over time even under difficult environmental conditions.
- It can be further implemented with nano-technological products that allow to protect the deposit itself and also the parts with complex geometry (deep holes, chickpea holes and cavities, etc.), allowing an oxidation protection during the material storage phases.