DE10321259B4 - Process for the surface treatment of dynamically loaded metal components and use of the method - Google Patents

Abstract

Process for the surface treatment of dynamically loaded components made of metal, in particular of springs, characterized in that the component is subjected at least once to a hot-dip galvanizing and a surface treatment is carried out by shot peening after hot-dip galvanizing.

Description

The invention relates to a method for the surface treatment of components made of metal, which are provided for a dynamically loaded insert, in particular of springs. The invention further relates to a use of the method.

Screw-shaped springs, in particular axle springs for motor vehicles, which are also referred to as coil springs, body springs or lifting springs are manufactured today by cold or by hot winding processes. The steel grades used here have been continuously improved in recent years, which has led to a lower ductility and thus to an increased notch sensitivity of the spring steels used. This results in higher requirements for corrosion protection to prevent corrosive attacks and premature component failures. Mechanically stressed components are particularly subject to corrosion attacks by stress cracking and vibration cracking corrosion, in which by a combination of electrochemical attack and strong mechanical stress due to internal stress in the component corrosion is enhanced.

Vehicle springs (for example axle springs) are particularly exposed to corrosive attacks by salt water and mechanical loads due to falling rocks and are therefore subject to particularly stringent requirements for corrosion protection. Currently, vehicle springs are usually protected by powder coatings from corrosive attack, which adhere to the spring surface due to electrostatic charge and polymerize by subsequent heat treatment at about 180 ° C. As a carrier layer here is a zinc phosphating, which has a layer thickness of a few micrometers. However, the resulting plastic powder coatings often suffer damage to the surface during transport, during installation or by falling rocks during driving, which subsequently lead to corrosion damage and spring failure.

A better resistance to corrosion protection is basically given by applying metallic coatings on component surfaces. A serious problem in the metal coating of dynamically loaded components, in particular springs, whose elasticity must be preserved, lies in the high temperatures occurring in most metal coating process. Thus, steel springs achieve their long service life by introducing residual compressive stresses in the near-surface area by means of shot peening. However, these residual compressive stresses are reduced by the influence of elevated temperatures. Known methods, which have the aim of galvanizing springs or spring wire, therefore avoid elevated temperatures or perform the spring winding only after the coating process.

For example, is from the DE 100 26 044 C1 a method for coating of feathers is known in which a surface finish of the wound spring is first carried out by shot peening, then a mechanical and / or chemical pretreatment, such as sandblasting or lasers, and finally a coating of zinc, aluminum, copper or alloys of this is done by a thermal spraying process, in particular by flame spraying, at temperatures of 160 ° C. However, this method is associated with very high costs and also has the disadvantage that the applied layer is not materially connected in the sense of an alloy with the spring steel, but only a mechanical adhesion of the coating takes place on the surface roughened by sandblasting.

There are also known galvanic galvanizing, but also characterized by an insufficient connection of the metal layer to the base material. In addition, higher layer thicknesses are difficult to represent galvanically. For example, this describes WO 97/42352 A1 the galvanic coating of spring wire with zinc-aluminum alloys, wherein hard drawing and winding of the wire is carried out only after the galvanic coating. However, this method is only suitable for cold-drawn pearlitic spring steels with small wire diameters of 0.6 to 5.5 mm, which are unsuitable for a downstream surface treatment, for example by shot peening. In addition, the achievable with this method layer thicknesses in the range of 10-20 microns for use in axle springs in value not sufficient.

From the JP S59-110727 It is also known to coat a spring steel wire by flame spraying with nickel, aluminum and / or chromium and subsequently thermoformed into a spring. Subsequently, the surface is tempered by shot blasting. This method has the disadvantage that the coating is not materially connected to the spring wire and can be easily damaged by the downstream hot forming, whereby a safe corrosion protection is not given.

Another problem that is generally opposed to the use of high temperatures in the galvanizing of springs is the high silicon contents in the range of 1.5 to 1.8% in typical spring steels which result in the so-called sanding effect, and thus With Accelerated iron-zinc reaction and difficult to control layer growth go along. Critical silicon contents at which this problem occurs are in the range of 0.03 to 0.13% and above 0.28%.

Object of the present invention is to provide a method for surface treatment of dynamically loaded metal components, in particular of springs for motor vehicles, which ensures improved corrosion protection in terms of chemical and thermal resistance and against extreme mechanical stress. The coating produced by the process should have as self-healing properties as possible. The process should also be inexpensive to carry out.

This object is achieved by a method having the features of claim 1. According to the invention, the component is subjected at least once to a hot-dip galvanizing and carried out a surface treatment by shot peening before and / or after, preferably before and after the hot-dip galvanizing. By the known method of hot-dip galvanizing arise in the boundary region of the component surface and the applied zinc coating alloys of the component material with zinc with an increasing zinc content in the direction of the outer layers. This material connection of the component material with its coating is characterized by a very high resistance to mechanical stress. Another advantage of zinc coatings lies in the so-called self-healing effect, are closed by the surface damage, for example in the form of cracks or notches, due to a passivation reaction again. According to the following equations, the primary corrosion product zinc hydroxide, under the action of carbon dioxide, forms a basic zinc carbonate, which forms an extremely resistant and well-protective top layer, which is responsible for the actual corrosion protection: Zn + 2H ₂ O → Zn (OH) ₂ + H ₂ 5Zn (OH) ₂ + 2CO ₂ → Zn ₅ (OH) ₆ (CO ₃ ) ₂ + 2H ₂ O

Usually, hot-dip galvanizing is used primarily for low-carbon steel grades, typically cold or hot strip steel. In the case of highly alloyed steel grades, in particular those with high silicon contents, as is customary in dynamically loaded components, hot dip galvanization is ruled out in the prevailing opinion due to the already described Sandelin effect. However, it has been found that with appropriately selected galvanizing parameters and zinc alloys, the hot-dip galvanizing of typical spring steels, for example 50 CrV 4-, 54 SiCr 6 or 60 SiCrV 7 steels, is possible and although the occurrence of the Sandelin effect is not completely prevented can be kept within controllable limits.

The achieved advantageous material properties according to the invention coated springs allow the use of axle springs, which are exposed to extreme mechanical stress by falling rocks, for example, in off-road vehicles or vehicles with poor road equipment. The axle springs according to the invention are also suitable for use in regions with moist, cold climatic conditions due to their good corrosion protection against chemical attack.

The already mentioned problem of the loss of residual compressive stresses of preformed springs due to the high temperature effects in hot-dip galvanizing can be overcome particularly advantageous by a downstream shot peening.

The process step of hot-dip galvanizing is carried out in a known manner by immersing the component at least once in a bath of molten zinc, which may optionally contain further alloying additives, such as aluminum. The zinc content of the melt is at least 98%. Zinc has a melting temperature of about 419 ° C. Possible temperatures of the galvanizing bath are accordingly in a range between 420 and 560 ° C. Preferably, the hot dip galvanizing is carried out at temperatures of 440 to 460 ° C. At these temperatures, as a result of mutual diffusion processes of the liquid zinc with the material surface of the component, in particular with the steel surface, a coating of differently composed iron-zinc alloy layers occurs. Depending on the process, a phase which is also referred to as a pure zinc coating and whose composition corresponds to the zinc melt is formed on the outermost alloy layer.

In order to achieve optimum galvanizing results, the component should be subjected to a surface pretreatment prior to hot-dip galvanizing, which may include a cleaning, in particular degreasing, pickling, in particular with dilute hydrochloric acid, and / or fluxing in a flux bath.

The shot peening, which preferably takes place both before and after hot-dip galvanizing, serves to densify and solidify the surface of the component, as a result of which mechanical properties of the material are purposefully improved. In the event of of screw-shaped springs, a suitable shot peening treatment causes the build-up of residual compressive stresses and the increase of the fatigue strength. The shot peening can be carried out in blast wheel, injector or compressed air systems with blasting media made of steel, stainless steel, glass or ceramic. The preferred spherical blasting agent may have average diameters between 20 microns and 6 mm, in particular between 100 microns and 1 mm.

The inventive method is particularly suitable for the surface treatment of dynamically loaded components made of metal, in particular springs made of steel. The material properties produced by the method are particularly advantageous for extremely mechanically and / or chemically stressed components, such as axle springs for motor vehicles, for carrying.

Further preferred embodiments of the invention are the subject of the remaining dependent claims.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

1 a flow diagram of the method according to the invention;

2 a sectional view of a Feuerverzinkungsschicht on a low alloy steel member according to the prior art and

3 a sectional view of a hot dip galvanizing layer on a steel spring after a shot peening treatment.

The method according to 1 begins in step S1 with the production of the component to be treated according to the invention, for example an axle spring for motor vehicles. In this case, a spring steel wire, for example made of 54 SiCr 6 steel, wound in a conventional manner in a cold or warm winding process to form a spring.

In the subsequent step S2, the preformed spring is tempered by tempering and / or stress relief annealing, wherein both tempering steps can also be combined with one another. During tempering, the component is heated to a material- and hardness-specific temperature up to its core area and then cooled again to room temperature. Tempering serves to increase the toughness of the material, so that a ductility which is necessary due to its application is guaranteed. Also multiple treatments are possible. Stress relief annealing has the sole purpose of reducing the inherent stresses in the component caused, for example, by previous heat treatments or mechanical processes. Usually, the component is heated in air or under a protective gas atmosphere and brought to room temperature while cooling the furnace. The annealing increases the dimensional accuracy of the components and avoids later distortion.

The compensation steps depend primarily on the production method of the spring. In the case of a cold-rolled spring, the wire is typically already tempered before winding, so a subsequent tempering is no longer required. But in order to reduce the inherent stresses introduced during cold winding, a stress relief annealing should take place. This leads to a better dimensional accuracy and also allows hot setting for a better construction height accuracy. If, on the other hand, it is a hot-rolled spring, it is already free from residual stresses as a result of the hot winding, but it still has to be tempered. The spring is quenched for this purpose after winding in oil and then tempered. Both heat treatments typically take place at temperatures of 300 to 400 ° C. For the subsequent hot-dip galvanizing it is irrelevant whether the spring was previously cold or hot rolled.

Subsequently, in step S3, a first shot peening treatment is carried out, in which the spring is treated in centrifugal, injector or compressed air systems with a spherical blasting agent made of steel, stainless steel, glass or ceramic, wherein the surface is compacted and solidified. In particular, in dynamically loaded components, such as axle springs of motor vehicles, the introduction of necessary compressive residual stresses in the surface area, which owe the steel springs their long life by shot peening.

Before the hot-dip galvanizing, a surface pretreatment of the steel spring, which may comprise several steps, takes place in step S4. Preference is given to first cleaning, with residues of fats and oils are removed in a degreasing bath or otherwise. Usually aqueous, alkaline or acid degreasing agents are used. After an intermediate rinsing step, a pickling treatment may be used to remove inherent contaminants, such as scale or rust. The pickling is usually carried out in dilute hydrochloric acid. Optionally, after a further rinsing step, a fluxing of the spring can follow in a flux bath in order to achieve a final fine cleaning of the steel surface. The flux usually consists of an aqueous solution of zinc chloride and / or ammonium chloride and increases the wetting ability of the steel surface for the molten zinc.

After an optional intermediate drying station takes place in step S5, the hot dip galvanizing of the spring. In this case, the spring is immersed in a liquid zinc melt, which may contain other alloying additives, such as aluminum. As a result of mutual diffusion processes, alloy layers of iron and zinc with varying compositions are formed.

2 shows a scanning electron micrograph of a hot dip galvanizing layer on a low alloy steel component according to the prior art in a sectional view. Clearly visible is the multi-phase layer structure of different alloy phases, which ensures a firm and permanent connection of the zinc layer with the base material. In 2 Below is the innermost layer, the α-Fe phase of the component with an Fe content of over 94%, followed by three iron-zinc alloy phases with decreasing iron content to the outside. Specifically, these are the Γ phase (FeZn ₃ , Fe ₃ Zn ₁₀ , 21-28% Fe), the δ phase (FeZn ₇ , FeZn ₁₀ , 7-11% Fe) and the ξ phase (FeZn ₁₃ , 5-6% Fe). The uppermost layer, the so-called pure zinc coating (η-phase, <0.03% Fe), corresponds in its composition substantially to the molten zinc.

After cooling the galvanized steel spring in air or in a water bath according to 1 as the last step S6 another shot peening treatment of the galvanized steel spring. Due to the material connection of the zinc layer with the spring steel, the downstream shot peening treatment is possible with a suitable layer structure and suitable layer thickness. As a result, the reduced compressive stresses due to the high galvanizing temperatures can be re-introduced. This is possible because an inherent stress maximum in steel springs is usually present at 200 to 300 microns below the material surface. With correspondingly adapted shot blast treatment parameters and zinc layer thicknesses in the range of 60 to 100 μm, it is possible to rebuild the internal compressive stresses in these layer depths.

3 shows a scanning electron micrograph of a hot dip galvanizing layer on an axle spring made of 54 SiCr 6 steel after the final shot peening process according to the invention. Due to the sandeline effect, the classical phase structure (cf. 2 ) no longer recognizable here, although a multi-phase structure is still present. The zinc layer is still closed. The supporting Γ phase (FeZn ₃ , Fe ₃ Zn ₁₀ ) ensures a firm and permanent connection to the spring steel.

The spring thus produced is well protected against mechanical and chemical corrosion attacks and is therefore particularly suitable as an axle spring for motor vehicles, for example for off-road operation.

LIST OF REFERENCE NUMBERS

• S1

  spring coiling

  S2

  Tempering / stress relieving

  S3

  Shot peening

  S4

  Cleaning / Pickling

  S5

  Hot-dip galvanizing

  S6

  Shot peening

Claims (14)

Process for the surface treatment of dynamically loaded components made of metal, in particular of springs, characterized in that the component is subjected at least once to a hot-dip galvanizing and a surface treatment is carried out by shot peening after hot-dip galvanizing. A method according to claim 1, characterized in that the shot peening takes place before and after the at least one hot dip galvanizing. A method according to claim 1 or 2, characterized in that the hot dip galvanizing by at least one time immersing the component in a bath of molten zinc, which may optionally contain other alloying additions, is performed. Method according to one of the preceding claims, characterized in that the hot-dip galvanizing at temperatures between 420 and 560 ° C, in particular between 440 to 460 ° C, is performed. Method according to one of the preceding claims, characterized in that by galvanizing a substantially consisting of zinc or of an iron-zinc alloy layer having an average layer thickness of 40 to 150 .mu.m, in particular from 60 to 100 microns, is applied. Method according to one of the preceding claims, characterized in that before the hot-dip galvanizing and optionally after the hot-galvanizing preceding shot peening treatment of the component is subjected to a surface preparation comprising at least one of the steps - cleaning, especially degreasing, - pickling, in particular with dilute hydrochloric acid, or - Fluxing in a flux bath. A method according to claim 6, characterized in that the surface pretreatment comprises the steps of cleaning followed by pickling. Method according to one of claims 6 or 7, characterized in that after cleaning and / or pickling the fluxing is carried out. Method according to one of the preceding claims, characterized in that prior to the cleaning treatment, the component is subjected to an annealing process and / or a Spannungsarmglühprozess. Method according to one of the preceding claims, characterized in that a steel, in particular 50 CrV 4-, 54 SiCr 6 or 60 SiCrV 7 steel is used as the material of the component. Method according to one of the preceding claims, characterized in that the component is preformed. Method according to one of the preceding claims, characterized in that the component is a cold or warm wound spring, in particular an axle spring for motor vehicles. Use of the method according to one of claims 1 to 12 for the surface treatment of dynamically loaded components made of metal. Use according to claim 13 for the surface treatment of springs, in particular axle springs for motor vehicles.

Images