DE102004021043A1 - Surface treatment method - Google Patents

Abstract

The invention relates to a method for treating a metallic surface (10), in particular a spring steel wire for producing compression coil springs, wherein the metallic surface (10) is subjected to a nitriding or a nitrocarburizing process. In this case, those regions (14) of the metallic surface which are not to be nitrided or nitrocarburized are subjected to cold working before the nitriding or nitrocarburizing process is carried out.

Description

The The invention relates to a method for treating a metallic surface and its use according to the preamble of the independent claims.

It is known, metallic surfaces harden by nitriding or Nitrocarburierverfahren. The hardening is based on a diffusion of nitrogen, for example in the Steel of the metallic surface to be treated. This leads to storage from nitrogen to interstitials and formation of nitrides and for nitrogen attachment to carbides to form carbonitrides. Nitriding produces hard surface layers, resulting in the hardness, the wear resistance and the fatigue strength of the metallic surface can be increased.

A first possibility for nitriding metallic ear surfaces is to subject the corresponding surface to a so-called gas nitriding. Such a method is for example the DE 101 47 205 C1 refer to. In this case, a workpiece is heated in a nitriding in a first step to a temperature of about 400 ° C. This heating is preferably carried out under an ammonia atmosphere. Thereafter, in a second step, the workpiece is heated to a nitriding temperature which is approximately between 500 ° C and 600 ° C. The actual nitriding of the workpiece takes place in an atmosphere of ammonia and an oxidizing agent, an atmosphere of ammonia and a carbon carrier or in a pure ammonia atmosphere.

Another possibility for nitriding metallic surfaces is the so-called plasma nitriding. In this case, the workpiece is exposed to a gas mixture at an elevated temperature, the nitration being assisted by the action of a plasma. Such a method is for example the DE 100 568 142 A1 refer to.

nitrided component surfaces However, due to their layer characteristics and hardness after a nitriding treatment only limited mechanical machinability. In order to improve processability, nitriding often produces a partial heat treatment carried out. This will be areas that become mechanical at a later date processed and thus should not be nitrated, covered locally. This is done, for example, in gas nitriding by the previous one Order appropriate pastes, creating an interaction of nitrogenous the atmosphere with the covered surface areas is prevented. In plasma nitriding, a masking of the non-nitriding surface areas made, whereby the occurrence of a Glimmsaumes at the masked component surface avoided and thus prevents the generation of reactive nitrogen becomes.

Both Methods for covering non-nitriding surface areas are however on small surface areas limited and it can especially in plasma nitriding only areas with low geometric complexity be effectively masked.

From the US 5,399,207 Furthermore, a gas nitriding process is known in which the corresponding workpiece is subjected to a mechanical load in the form of a rubbing action of particles in order to improve the nitriding effect.

task The present invention is a method of treatment metallic surfaces to provide during which the implementation nitriding or nitrocarburizing a nitriding or Nitrocarburieren predetermined surface areas is prevented.

Advantages of invention

The inventive method with the characterizing features of claim 1 has the advantage that in a simple and cost effective way certain areas of a surface to be nitrided the action of a nitrating or Nitrocarburierbehandlung can be effectively removed, so that this one later mechanical processing are accessible. These are the ones Areas of the metallic surface, which should not be nitrided or nitrocarburised before the implementation of the Nitriding or Nitrocarburierverfahrens subjected to cold working.

The pretreated surface areas can thereafter without further coverage of both gas and plasma nitriding be subjected. this makes possible the partial treatment also of component surfaces with high geometric Complexity. Another advantage is that cold working is easy can be automated and thus carried out inexpensively. About that In addition, a pollution of the components or furnace systems by Paste residues prevented.

With in the subclaims listed activities Advantageous developments are possible.

So it is advantageous if as cold deformation Shot peening, embossing or rolling over is done since these cold working methods are effective and yet easy to perform.

drawing

An embodiment of the invention is illustrated in the drawing and explained in more detail in the following description. 1 shows a flow chart of the method according to the invention, 2 a plot of the hardness of a Vickers surface cold-worked before the nitriding process over the distance of the measuring point from the component surface, 3 For comparison, a plot of the hardness of a Vickers non-cold-worked surface before nitriding versus the distance of the measuring point from the component surface according to FIG DE 100 56 842 A1 and

4 a microsection of a according to the inventive method before the nitriding cold-formed component surface after the nitriding process.

embodiment

In 1 a flow chart of the method according to the invention is shown according to a first embodiment. In this case, in a first step, the component 10 partially cold deformation 12 subjected, as a cold-forming shot peening, embossing or rolling can be performed. After that, the component points 10a superficially the cold-formed areas 14 on. The cold forming process alters the crystalline structure of the steel to prevent nitrogen from diffusing. In a further step, the nitration takes place 16 the component surface. The resulting component 10b in the form of a longitudinal section along the line A - A of the component 10a has a nitrogen-containing diffusion layer 18 on, with the previously cold-formed areas 14 are resistant to nitriding and have no nitrogen-containing diffusion layer of any kind.

A hardness distribution of the cold-formed areas 14 after the nitriding process is in 2 shown. It can be seen that the measured hardness according to Vickers practically does not change independently of the distance of the measuring point to the component surface. This is an indication that occurs during the nitriding process in the cold-formed areas 14 no nitrogen-containing diffusion layer has formed.

For comparison, in 3 the measured hardness of a component surface has been subjected to a nitriding process according to the prior art without prior cold deformation. It can be seen that the Vickers hardness increases sharply, in particular in an edge region of 0.1 mm at the component surface and thus can be concluded on the existence of a nitrogen-containing diffusion layer.

In 4 is still a thousand times magnification a microsection of the component surface in the area of the cold-formed surface areas 14 imaged after nitriding, it can be seen that the component surface consists of a uniform material layer and no nitrogen-containing surface layer exists.

The nitriding of the component surface can be carried out, for example, by gas nitriding, wherein the component is exposed at elevated temperatures to an atmosphere containing nitrogen or a nitrogen-containing compound such as ammonia, and may additionally contain oxygen-containing compounds such as oxygen or water. The gas nitriding can be carried out, for example, as the DE 100 56 842 A1 can be seen.

Preferably However, the nitriding of the component surface is done by means of a plasma nitriding process. The nitriding process takes place at a negative pressure of 250 Pa in a gas atmosphere, the 30 to 40 vol.% nitrogen, 15 to 25 vol.% hydrogen, 15 to 25% by volume of argon and 25 to 35% by volume of methane. When Process temperature is selected a temperature of 350 to 450 ° C, the Nitrierdauer amounts approximately 24 hours. The set voltage is between 400 and 600 volts, preferably using a pulsed AC voltage which is a pulse / pause ratio from 1: 1 to 1: 3.

the Nitriding process or cold deformation can optionally be a heat treatment precede the workpiece surface to be nitrided, which is referred to as so-called tempering. Furthermore, the to nitriding workpieces a cleaning treatment, for example by treatment with alkaline-aqueous Detergents or alcohol-based cleaners. In particular, in the plasma nitriding can continue to fine cleaning the workpiece surface through Sputtering in a hydrogen or hydrogen-argon plasma before the actual plasma nitration done.

In addition, the nitriding process can be followed by aftertreatment of the nitrided or partially nitrided components. This post-treatment can take the form of a mechanical post-processing, for example by microstructuring the work pieces are made.

Claims (9)

Process for treating a metallic surface, in particular a spring steel wire for producing compression coil springs, wherein the metallic surface is subjected to a nitriding or nitrocarburizing process, characterized in that those regions ( 14 ) of the metallic surface which are not to be nitrided or nitrocarburized are subjected to cold working prior to the nitriding or nitrocarburizing process. Method according to claim 1, characterized in that that as cold deformation is shot blasting, embossing or rolling over. Method according to claim 1 or 2, characterized that the nitration or Nitrocarburierbehandlung is carried out at a temperature of 360 to 480 ° C. Method according to one of claims 1 to 3, characterized that the nitriding or nitrocarburizing treatment over a period of 5 to 24 Hours. Method according to one of the preceding claims, characterized in that the metallic surface ( 10 . 14 ) is subjected to plasma nitriding or plasma nitrocarburization. Method according to claim 5, characterized in that that plasma nitration is based on a pulsed high voltage. Method according to one of the preceding claims, characterized in that the metallic surface ( 10 . 14 ) is tempered before nitriding or nitrocarburizing. Method according to one of the preceding claims, characterized in that the metallic surface ( 10 . 14 ) is microstructured after nitriding or nitrocarburizing. Use of a method according to one of claims 1 to 8 for the production of compression coil springs for injectors.