Compression springs , partly too Tension springs and Leg springs The subsequent treatment of their surfaces gives them additional properties – depending on the application, they become, for example, harder, more rustproof or more heat-resistant. Gutekunst feathers offers for his Catalog- and individual steel springs In addition to classic processes such as bright galvanizing, burnishing, electropolishing, pickling, phosphating, passivating, chrome-plating, nickel-plating, galvanizing and shot-peening, there are also a variety of special surface treatments. These include thick-film passivations, powder coatings, Delta®-Tone and Delta®-Seal corrosion protection systems, glass bead blasting, PTFE Teflon coatings, gold plating, copper plating or the chromating of galvanized steel springs.
All surface treatments for you at a glance:
Classic surface treatments for steel springs
Key Features: Burnishing is an optical surface treatment. As a result of the process, the steel springs have a uniform color, but are only slightly resistant to corrosion.
Burnishing is one of the classic surface treatments and is a non-layer-forming process in which oxygen is stored in the outermost layer. The treatment creates a black, thin and non-metallic coating of iron oxide on a low-alloy iron material such as steel springs. This covering, which is usually less than a micrometer thick, not only enhances the appearance of the material, but also protects it. The process takes place at low temperatures below 150 degrees Celsius. This means that there is no thermal load. The structure and roughness of the nib are only changed insignificantly. The dimensional accuracy of the coated workpieces remains the same.
Key Features: Electropolishing treats the surfaces of stainless steel. It is primarily used to optically refine the material – for example in medical devices.
Electropolishing consists of a single operation in which a very thin layer of the surface is removed by means of anodic dissolution. Impurities, particles, micro-cracks as well as structural disturbances and local stresses are also cleaned up. The treated surfaces are characterized by various properties: They are metallically pure and shiny, free of burrs and particles, closed in the microscopic range and have optimal properties of the base material. Another advantage: the process is particularly gentle and the steel spring is not subjected to thermal or mechanical stress.
Advantages over other processes are also good corrosion resistance, minimized micro-roughness, improved fatigue strength and a reduced coefficient of friction. Treated surfaces also impress with their low product adhesion and reduced build-up of deposits. They can therefore be cleaned very easily and quickly. The area of application ranges from the chemical and pharmaceutical industry to environmental technology and electronics industry to architecture and construction.
Key Features: The pickling process chemically removes all impurities from a surface.
Pickling is a process that removes all contamination from stainless steel surfaces chemically – i.e. by acid. The result is a metallically pure surface of the metal spring. Only then can the protective passive layer form in the next step.
Key Features: This process is one of the optical surface treatments and offers a high level of protection against corrosion.
Bright galvanizing is a galvanically generated protection system for steel surfaces. During the process, a very thin zinc metal layer is created; the layer thicknesses can be adjusted between 2 µm and 30 µm over the length of time spent in the zinc bath. In order to adequately protect the metal from corrosion, the layer is also passivated or chromated (see section Passivation or Galvanizing + Passivation). This makes the material much more durable. Traces of corrosion, also known as white rust, can be prevented with gloss galvanizing. The surface is also more adhesive, more thermally stable, impresses with its high functionality and looks more appealing.
Key Features: The special thing about phosphating is the temporary corrosion protection and the favorable sliding properties.
Phosphating, also known as bonding, atramentizing or Parkerizing, is one of the most common process technologies in the surface treatment of steel springs. Here, a conversion layer of firmly adhering metal phosphates is formed by means of chemical reactions between the metallic surface and a phosphate solution. These surfaces are interesting for applications in which value is placed on temporary corrosion protection, good adhesion for subsequent paintwork, friction and wear reduction or electrical insulation. Phosphating is also very economical and inexpensive.
Key Features: Passivated surfaces have a high resistance due to their oxide layer or a conversion layer (chromate, fluoride, etc.), can regenerate themselves and offer good corrosion protection.
Passivations arise as a natural reaction on almost every metallic surface and offer more or less good protection against corrosion. In surface technology, artificially created passivation often takes place on zinc, aluminum, magnesium, cadmium or stainless steels. The special form of passivation is the so-called chromating, which used to contain Cr VI compounds and provided very good corrosion protection on zinc, aluminum and cadmium layers. The classic chromating on zinc has been replaced in recent years by the development of Cr VI-free passivations, and the new developments such as blue, yellow, black and thick-film passivations are in no way inferior to the earlier processes in corrosion protection and optical properties. Today, passivations are based on harmless Cr-III compounds and other additives such as fluorides, zirconium compounds or organic complexes. In addition to zinc and aluminum layers, steels, especially stainless steels, can also be chemically passivated.
Key Features: Chrome plating is an optical surface treatment and offers good protection against corrosion.
Chrome-plated surfaces are built up in several layers. They protect the metal springs from corrosion and have low static friction. They can be mechanically reworked, are resistant even at high temperatures and do not tarnish. Chrome-plated surfaces are used across all sectors in many areas of the technical industry.
Key Features: Nickel-plated surfaces are, on the one hand, corrosion-resistant and, on the other hand, have favorable sliding properties. Nickel plating is one of the optical surface treatments.
Nickel is suitable for a variety of different applications and is part of electroplated metal coatings. In order to nickel-plate steel springs, they are immersed in a nickel electrolyte – an aqueous solution containing nickel salts – after a special pretreatment. When an electrical voltage is applied, a fine layer is formed on the surface of the spring. The coating is resistant to air and water as well as diluted acids and some alkalis. On the other hand, nickel is unsuitable in contact with nitric acid, concentrated hydrochloric acids and ammonia. The transition metal has a light silver color with a slightly yellowish hue. Due to its characteristic color, it differs very well from chrome-plated surfaces. However, nickel is not resistant to tarnishing and can take on a dark color over time.
Key Features: Galvanized surfaces offer particularly good protection against corrosion.
When galvanizing, steel springs are provided with a metallic coating made of zinc. Frequent processes are continuous and discontinuous hot-dip galvanizing, thermal spraying with zinc and galvanizing. The processes differ on the one hand in the production of the protective layer and in their layer thickness. The duration of protection and the mechanical load-bearing capacity of the workpiece also depend on this. Hot-dip galvanizing and thermal spraying produce layer thicknesses of well over 50 µm, with electro-galvanizing the layer thickness of up to 30 µm can be selected individually. Electroplated zinc layers are also only offered with an additional passivation layer, on the one hand these serve to increase the corrosion protection and are offered in different colors (transparent / blue, yellow, black, olive) for optical reasons.
In addition to its shielding effect, zinc is corrosion-resistant. Galvanized workpieces are often used as a sacrificial anode against contact corrosion. This makes them suitable for applications in which more noble metals with different potential for dissolution are conductively connected. The galvanized material prevents the metals from corroding up to a distance of five millimeters. Exposed cut edges and imperfections in the zinc coating can be optimally protected.
Key Features: Shot peening increases the dynamic service life of steel springs significantly.
Shot peening is a special type of surface treatment. In this process, compressed air or centrifugal wheels shoot the abrasive at high speed onto the springs. The abrasive consists of round or rounded grains that give the process its name.
The aim of shot peening is to increase the surface volume of the material and to generate residual compressive stress. The result is a compression of the material and thus the solidification of the surface. With this process, the fatigue strength of the metal spring is significantly improved.
Special surface treatments for steel springs
Key Features: The thick layer passivation creates a clear, iridescent surface that offers good corrosion protection, much better than a zinc layer alone.
The thick-film passivation is a passivation layer on a zinc layer that has been galvanically applied beforehand. The process is also known as transparent passivation. Here, a non-metallic protective layer is created on a metallic material such as a steel spring. The coating is absolutely free of chromium VI and is mainly used where conventional chromating cannot be used. This type of surface treatment is very suitable for higher coatings.
Key Features: Powder coating is also one of the optical surface treatments. Powder-coated materials have an excellent gloss retention and color fastness of the 180 different RAL colors.
In order to process steel springs in this way, the corresponding colored powder is electrostatically charged and applied to the material using a so-called powder gun. The powder coatings are based on various binders such as acrylate, epoxy and polyester resin. They are offered in all RAL colors and in any desired intermediate shade. The only requirement: the spring must be electrically conductive. This is true if it is made of steel or aluminum, for example. The powder is then burned into the material in a furnace at temperatures between 140 and 200 degrees Celsius. This creates an evenly dense coating. This impresses with its high light and weather resistance and is also shock and scratch resistant.
Delta®-Tone and Delta®-Seal coating
Key Features: The Delta® micro-layer corrosion protection system consists of a two-layer structure. Delta-Tone is the basecoat, Delta-Seal the subsequent topcoat. Both together form a very high level of corrosion protection for metal springs.
Due to the structure of the base and top coat, tailor-made solutions can be achieved with regard to the functional properties, the friction behavior and the corrosion resistance of surfaces.
In the Delta®-Tone process, an inorganic and micro-layer-forming base layer is applied to the steel springs, which contains zinc and aluminum flakes. The subsequent baking process creates a silver-metallic, glossy coating with a uniform thickness. This has excellent corrosion protection properties and at the same time ensures very good electrical conductivity. This enables cathodic protection and avoids the risk of hydrogen embrittlement.
The right basecoat depending on the requirements:
- Delta®-Tone 9000 is especially suitable for springs, spring band clamps and stamped and bent parts.
- Delta-Protekt® KL 100 was specifically developed for threaded parts and is often used in the automotive industry. The base layer can withstand constant temperatures of 96 hours at 180 degrees Celsius.
- With its integrated lubricant, Delta-Protekt® KL 105 makes the use of a topcoat superfluous. The coating is adjusted to the specific coefficient of friction and thus enables cost savings as a 2-in-1 system.
- Delta-Protekt® KL 130 is dark gray and predestined as a substrate for black covering shades, because impact points are no longer visible.
- Delta-Protekt® KL 120 is particularly suitable for connecting elements with threads and for springs or spring band clamps. The red and white corrosion resistance goes far beyond the requirements of other surface finishes.
Delta®-Seal is a top coat made of organically highly cross-linked, micro-layer-forming material. It optimally complements the previously applied base layer. The coating and baking process leaves a firm, chemical-resistant covering. Excellent resistance to external influences is achieved through the use of certain resins such as acrylate, epoxy and polyurethane resin with an adapted curing.
The Delta®-Seal variants, the black Delta-Protekt® VL 450 or Delta-Protekt® VT 600 and the silver Delta-Protekt® VL 411 GZ.
Glass bead rays
Key Features: Glass bead blasting is a special form of shot peening.
With glass bead blasting, compressed air, wheel or injector blasting systems throw very small glass beads at high speed against the surface of the steel spring. The process creates very homogeneous surfaces with excellent optical properties. Glass bead blasting is therefore used for iron-free cleaning and for decorative surfaces.
Teflon coating (PTFE)
Key Features: Teflon-coated steel springs have very good sliding properties.
Coatings made of Teflon, also called PTFE (polytetrafluoroethylene), have various properties: In addition to very good corrosion resistance and optimal non-stick properties, they also include low coefficients of friction and resistance to high temperatures. They are used, among other things, in the food industry, in clean room applications, in the textile and paper industry as well as in plant and mechanical engineering.
Key Features: Gilding is also one of the optical surface treatments. Gold-plated materials are not only particularly conductive, they also prevent excessive switching sparks. Mechanically and chemically, however, they are not very resistant.
With chemical gold plating, the steel nib is covered with a thin layer of pure gold. This means that the solderability of the material is retained and is further improved. Gold-plated metal springs are therefore preferably used in the electrical industry. The areas of application also include decorative areas.
Key Features: Copper-plated metal springs are, on the one hand, corrosion-resistant and, on the other hand, are characterized by good conductivity.
With copper plating, a thin layer of copper covers the metallic springs. This coating is a popular basis for many corrosion protection systems – including nickel or chrome coatings. Because the covering ensures permanent protection. The layer is usually 5-15 µm. Copper-plated springs also offer improved thermal and electrical conductivity. Another advantage: They can be painted without any problems.
Galvanized + Passivated (Chromated)
Key Features: The passivation (previously chromating) of galvanized components creates a functional and optical surface treatment. It offers a high level of corrosion protection and is available in blue, yellow, black and olive.
Before passivation, the steel springs must be galvanized. Only in the next step do they immerse in a solution of passivation solutions with various additives (Cr-III, fluorides, acids). This creates a very thin chemical conversion layer, also known as a conversion layer – without the application of an electrical voltage. The different passivations can be differentiated according to color. Blue (transparent), yellow, black and olive are possible. Most passivations today are Cr-VI-free and RoHS-compliant. RoHS stands for Restriction of Hazardous Substances, i.e. the restriction of the use of certain hazardous substances. Passivations / chromates containing Cr-VI are therefore not suitable, for example, in sectors such as the automotive industry.
A particularly good alternative to chromium (VI) -containing chromates are thick-film passivations. This process creates a clear, iridescent surface with good protection against corrosion.
The following colors and properties are offered galvanized and passivated:
- Blue: The hue turns slightly bluish. There is usually a smooth transition to the transparent. The corrosion protection of blue passivated surfaces is relatively good, the layer is Cr (VI) -free and thus conforms to the RoHS 2002/95 / EC directive.
- Yellow: The color is similar to brass or an impure gold tone. This type of passivation provides corrosion protection comparable to that of blue passivation and is often used when the appearance of the yellow chromating, which was often used in the past, is desired. The layer is Cr (VI) -free and therefore conforms to the RoHS 2002/95 / EC directive.
- Black: With this coloring, the metallic character of the surface is retained in places. Because the color is not as strong as with a paint job. The corrosion protection of the layer is medium to good. The coating is Cr (VI) -free and therefore conforms to the RoHS 2002/95 / EC directive.
- Olive: The color olive is very strong, which means that the metallic character of the upper layer is largely lost. This layer contains chromium (VI) and is therefore not RoHS-compliant.
- Thick film: Thick-film passivation is the latest development in passivation layers on galvanized surfaces and, with regard to corrosion protection, is a worthy replacement for the yellow chromating containing Cr-VI, which was often used in the past. The thick-film passivation is transparent to slightly iridescent, it is Cr-VI-free and therefore conforms to the RoHS 2002/95 / EC directive.
Would you like more information about surface treatments for compression springs, tension springs and torsion springs? You can contact the Gutekunst Federn technical department by phone (+49) 035 877 227-11 or by email firstname.lastname@example.org .
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