TREATMENTS AND
SURFACE FINISHES

Anodising consists of applying a surface treatment to aluminium in order to coat it with a layer of oxide. Creation of the layer is controlled, and conducted by means of an electrolytic process. The oxide layer renders the aluminium tough and resistant, making it perfect for use in new industrial applications subject to a great deal of wear.

Blackening is a process whereby a layer of oxide is applied to a piece of metal, generally steel, to protect it against corrosion and improve its appearance. There are two kinds of blackening: hot and cold. Cold blackening is used on small parts and is very easy to apply. Hot blackening on the other hand is a process that takes much longer to complete.

Passivation is a process that chemically removes the metal pollution accumulated on the surface of stainless steel workpieces as a result of their handling and manufacturing.
Passivation is an essential process to ensure that the passive layer of stainless steel chromium oxide is correctly generated and that the resistance to corrosion is therefore optimal for the type of alloy.

Phosphate conversion is another of the steel coating processes. The metal phosphate conversion process consists of the application of a phosphoric acid and phosphate salt solution by immersion or aspersion, which chemically alters the metal surface to form a non-soluble phosphate film. Phosphate conversion can have a variety of functions, such as creating a rough surface so that the material can be painted. However, it can also function as an antioxidant if protective grease is applied after the bath.

Nickel plating is a metallic nickel coating applied to metals by means of an electrolytic bath to increase their oxidation and corrosion resistance or improve the appearance of the parts. There are basically two types of nickel plating: Dull nickel plating and Bright nickel plating.

Zinc plating is a zinc coating applied to metals to protect them against oxidation and corrosion. This surface coating also improves the visual appearance of the part. White zinc plating consists of chemically preparing the part by degreasing and stripping it before submerging it in the different zinc plating baths (acid, alkaline or non-cyanide) followed by an electrolytic bath providing a mean coating thickness of 10-12 micras. For greater protection against corrosion, a chrome finish is applied which will also determine the final appearance of the part, which can be white, yellow or green, depending on the desired protection and shade.

Hard chrome plating is an electrolytic treatment used to cover parts with an adhesive chrome film, of variable thickness, with excellent mechanical properties and very high corrosion resistance, which can subsequently be polished or rectified.
It is also used to repair parts and improve their final appearance. In fact, for many it is considered to be the decorative coating par excellence.

Tin plating is a process whereby a tin coating is deposited by means of an electrolyte bath onto metal parts in steel, brass, copper or Zamak thereby increasing their resistance to oxidation, corrosion and wear, improving their weldability, and enhancing their appearance in ornamental elements.

Brass plating is a process whereby a brass coating is deposited by means of an electrolytic bath onto metal parts, whether in steel, brass, copper or Zamak thereby increasing their resistance to oxidation, corrosion and wear, and enhancing their appearance in ornamental elements.

This is the most common heat treatment for improving the mechanical properties of the processed material. During this treatment, the steel is heated to very high temperatures followed by controlled cooling. An immense variety of hardnesses can therefore be achieved depending on the type of material, and the time and way it is heated. It is also important to stress that the hardening treatment is followed by tempering to relieve the stress and reduce the fragility of the workpieces.
Normally this type of treatment can be applied to almost any steel intended for industrial purposes: tool steels, stainless steel, hot-worked steel, and steels for bearings. Any of these steels can be treated.
Hardening and tempering treatments help to improve resistance to wear, hardness, ductility and traction.

With this metal heat treatment, the metal is heated to a certain temperature for a pre-set time before cooling it slowly. This softens the material, improves its toughness, recovers its ductility, eliminates residual stress, refines the grain size and reduces segregation. It can also be used to change a material’s mechanical, electrical or magnetic properties. The type of steel annealing used is determined by the desired results and the composition of the material to be treated.
Normalising
This is the metal heat treatment used to give steel a uniform and fine-grain structure, hence guaranteeing its mechanical properties. It is used above all in carbon and low alloy steels to normalise their structure after forging, hot lamination or smelting. The material is heated rapidly to a temperature similar to that required for hardening (800º-920°C), which is when new austenitic grains form, much smaller than the previous ferrite grains. Following the procedure, the steel is left to cool freely in the air, during which time new ferrite grains of refined size and uniform structure take shape.

This is the metal heat treatment used to give steel a uniform and fine-grain structure, hence guaranteeing its mechanical properties. It is used above all in carbon and low alloy steels to normalise their structure after forging, hot lamination or smelting. The material is heated rapidly to a temperature similar to that required for hardening (800º-920°C), which is when new austenitic grains form, much smaller than the previous ferrite grains. Following the procedure, the steel is left to cool freely in the air, during which time new ferrite grains of refined size and uniform structure take shape.

Cementation is an austenitic thermochemical heat process applied to steel workpieces, including those which are almost completely made in iron. During this process, the outer layer is enriched with carbon or carbon and nitrogen to enhance its properties.
The objective of cementation heat treatment is to improve the surface hardness of the workpiece, lending it much greater strength, but without going so far as to change the composition of the nucleus, thus creating a tough workpiece, resistant to the fatigue and wear caused by working in conditions of friction.
This is one of the processes most used for pinions, cams, pulleys, bushing… These are workpieces for use in cold working and in high-friction conditions, meaning that a hardened coating (up to 1.4 mm) will protect them against wear.
Also worth noting is that the best materials for cementation are alloy and non-alloy steels with a low carbon content, such as ST-52, F-154, F-155…

Nitriding is a heat treatment applied to steel. The process changes the composition by adding nitrogen as the steel is heated to a temperature of approximately 550ºC in either a salt bath or an ammonia atmosphere for a specific amount of time. As a stand-alone treatment, this procedure achieves an extremely high surface hardness with minimal deformations, given the low temperature involved. It also increases resistance to corrosion and fatigue.

This is a treatment midway between carburisation (cementation) and nitriding, where both are present at the same time, thus increasing the hardness of the steels treated by simultaneously permeating the surface with carbon and nitrogen.
The process involves placing the part to be treated in an atmosphere similar to cementation, composed of basic ammonia and hydrocarbons, and with temperatures in the region of 750-800ºC (lower than required for cementation and higher than nitriding).
The treated parts are steels similar to those used in cementation, with low carbon content (0.1-0.2%) and great thickness in the endeavour to obtain extremely hard surfaces and a tough nucleus, in addition to other mechanical properties such as fatigue resistance, wear resistance and torsion resistance. One important advantage is that they suffer very little deformation due to the fact that the nitrogen absorbed in the process reduces the critical speed of the steel hardening process.

Thanks to this treatment, a superficial layer of nitrogen, carbon and, above all, sulphur can be added to a metal or metal alloy, such as steel, by means of a salt bath that transfers said elements at a temperature of almost 565ºC. This generates iron sulphide and resides in the edges of the grain, causing embrittlement of the metal while making it harder.
This treatment considerably increases the wear resistance of metals on reducing their friction coefficient.
It is used on low-carbon steels. After sulfurization, the dimensions of the workpieces increase slightly, improving their resistance to wear, favouring lubrication and preventing seizure.

Induction hardening consists of exposing the steel workpiece to an AC magnetic field to induce heating of the component surface. The aim of this process is to obtain a hard surface coating without going so far as to change the structure of the material nucleus.
At this point the magnetic field is eliminated and the part is cooled in different ways, at controlled speed. The higher the speed the greater the hardness. This is a rather special process involving rapid heating and cooling to achieve high mechanical resistance of the steel. It is ideal for cases where the hardness of specific material surface is required.
Some of the materials to which induction hardening can be applied are F-125 and F-114.

Powder coatings are enormously interesting in the field of industrial painting, due to the fact that they give the metal workpieces a protective and decorative finish. The epoxy paint used in this process is a mixture of pigment particles and resin which is sprayed onto the surface of the metal workpiece. The material is electrostatically charged for improved adherence to the workpiece and chemically reacts during the curing process in special ovens, where it finally melts into a uniform layer. This procedure produces very satisfactory results.
It gives the workpiece an attractive, uniform and hard-wearing appearance. Given its excellent results, powder paint application is one of those to have experienced greatest growth in the industrial sector worldwide. Powder makes for a more compact, finer and higher quality appearance.

Traditional liquid paint is usually sprayed on, having first of all applied a primer to ensure strong adherence of the final coat to the workpiece. Although powder paint is increasingly used in industry, liquid paint continues to offer a number of advantages.
It can be a lot faster and cheaper to use on single parts. I t is also the most commonly used process for painting large-sized workpieces and assemblies given that the process does not need subsequent drying in an oven. The investment required for this process is also much lower.