DE10003526A1 - Process for the carbonitriding of high-carbon and high-alloy steels - Google Patents

Abstract

Prior art methods exist for carbonitriding metallic workpieces by treating the workpieces in a carbonitriding atmosphere produced by using ammonia and a substance which contains carbon. Based on the prior art, the aim of the invention is to provide a cost-effective method for producing a carbonitriding atmosphere for high-carbon and/or high-alloy steels which prevents or reduces the formation of a covering layer or damage to the edge area. In order to carbonitride metallic workpieces that are comprised of high-carbon and/or high-alloy steels, the invention provides that nitrogen, ammonia and propane are used as the substance which contains carbon for producing the carbonitriding atmosphere.

Description

The invention relates to a method for carbonitriding metallic workpieces by treating the workpieces in a carbonitriding atmosphere using them a carbonaceous substance and ammonia is generated

When carbonitriding metal parts made of austenitic steel alloys diffuse carbon and nitrogen from the gas phase at high Temperatures in the face centered cubic and are in the Surface layer of the metal parts stored. After quenching the Metal parts remain in the diffusion zone and carbon and nitrogen cause residual compressive stress, whereby the surface layer is hardened.

In practice, carbonitriding uses so-called "endogas" into which ammonia is added. The ammonia content is in the range from 4 to 15% by volume of NH ₃ . Endogas is usually generated by the reaction of air with a hydrocarbon-containing gas such as natural gas, methane or propane. It is also known to produce suitable gas mixtures using an oxygen-containing organic liquid, such as methanol.

The carbon and nitrogen transfer from the gas phase into the material takes place via the following reactions:

CO + H ₂ (C) _{γ -Fe} + H ₂ O (carburizing) (1)

NH ₃ ⇒ (N) _{γ -Fe} + 1.5H ₂ (embroidery) (2)

The carburizing process is reversible when using endogas, so that there is the possibility of carburizing or decarburizing the materials. Atomic nitrogen is required for nitriding. However, the molecular nitrogen present in the endogas does not dissociate sufficiently in atomic nitrogen at the treatment temperatures. Ammonia is therefore added for embroidery, which at the high treatment temperatures partially decomposes into atomic nitrogen, which can be absorbed by the metal. The ammonia embroidery process is irreversible. The denitrification only takes place via the molecular nitrogen formation on the material surface:

2 [N] _dissolved → N _{2, gaseous} (3)

Particular problems arise with carbonitriding and / or high-carbon high-alloy steels. The following are the carburizing and nitriding conditions explained in more detail:

If carbon-rich steels are to be oversaturated with carbon in the surface layer, the carbon level of the gas phase must be set to high values. This is only possible if a carbon activity of 1 (graphite) or greater is set. When using endogas and at treatment temperatures of e.g. B. 850 ° C, the contents of carbon dioxide and water in the gas phase must be kept very low in order to obtain a high carbon activity. The decarburizing chemical reaction of these components (H ₂ O and CO ₂ ) can be seen from the following reaction equations.

2CO (C) _{γ -Fe} + CO ₂ (Boudouard _reaction ) (4)

CO + H ₂ (C) _{γ -Fe} + H ₂ O (heterogeneous water gas _reaction ) (5)

In order to maintain a given carbon activity, it is therefore required, water and carbon dioxide created by the carbon absorption to implement with a hydrocarbon. Usually propane is used for this used because propane reacts at relatively low carbonitriding temperatures (the but should not be increased due to alloy reasons) better than natural gas (Methane).  

The propane conversion with carbon dioxide or with water can be described stoichiometrically as follows using chemical equations:

3CO ₂ + C ₃ H ₈ 6CO + 4H ₂ (6)

3H ₂ O + C ₃ H ₈ 3CO + 7H ₂ (7)

Despite the better reactivity of the propane, a large excess of propane must be added to the furnace so that a high proportion of hydrocarbons leaves the system unused. Got through leaks or through that Treating metal can add uneven additional oxygen to the furnace space the marginal carbon contents and carburization depths in their uniformity and Reproducibility fluctuate widely.

In particular with regard to high-alloy steels, the use of endogas has as Carburizing carrier gas has another disadvantage. For example, have to be rich in chromium Carbonitrided steels, the alloyed chromium is oxidized and it forms According to the chromium content of the alloy, a closed oxide cover layer or it occurs a so-called "internal oxidation". A closed oxide cover layer works for the further diffusion of carbon and nitrogen as a barrier, since they do not Can release carbon. An internal oxidation, as is the case with tempered steels lower levels of chromium, manganese and silicon are observed, this affects Hardness structure negative. When quenching, it does not form martensite, but it does the result is a fine-streaked pearlite that is softer than martensite. The wear resistance is significantly reduced.

Chromium-rich alloys (chromium content <5% by weight) and tempering steels can can only be carburized or carbonitrided to a limited extent in endogas.

The invention is therefore based on the object of an inexpensive method for Production of a carbonitriding atmosphere for high-carbon and / or high-alloys Specify steels that form a top layer or damage the Border zone prevented or reduced.  

This object is achieved in that the generation of Carbonitriding atmosphere nitrogen, ammonia and propane as carbon-containing substance can be used.

Instead of endogas mixtures, propane and nitrogen are used in the process according to the invention to produce the carbonitriding atmosphere. This lowers the oxygen activity in the treatment gas and reduces the formation of decarburizing components such as CO ₂ and H ₂ O. The lower oxygen activity of the treatment gas produced according to the invention has the result that the edge oxidation is lower in high-alloy steels and the formation of an oxide cover layer or an “internal oxidation” is prevented or reduced.

This is accompanied by a in the carbonitriding atmosphere Nitrogen / propane / ammonia mixture compared to the use of Endogas a higher nitrogen activity and a higher carbon activity. This in turn makes it easier to carburize high-carbon steels.

The dosage of the fresh gases nitrogen, propane and ammonia can be easily measured and adjusted, so that a predetermined, constant composition of the carbonitriding atmosphere and thus a defined and reproducible nitrogen and carburization can be achieved. Due to the comparatively low CO ₂ content, the CO ₂ emissions are also reduced with this process. In addition, operating costs can be saved by replacing endogas with the cheaper nitrogen.

The inventive method is particularly easy to reproduce Process if only to generate the carbonitriding atmosphere Propane is used as a carbonaceous substance.

The water content of the carbonitriding atmosphere is preferably set to less than 0.1% by volume. This can be achieved by using a dry nitrogen / propane / ammonia mixture. This avoids the decarburizing component H ₂ O and achieves high carbon activity in the carbonitriding atmosphere.

It is equally advantageous if the carbon dioxide content of the carbonitriding atmosphere is less than 0.1% by volume. Because even by avoiding the decarbonizing component CO _2, high carbon activity in the carbonitriding atmosphere is achieved.

Carbonitriding for high-carbon and high-alloy steels has proven itself a carbonitriding atmosphere proved to be particularly favorable, which between 8 up to 15 vol.% hydrogen, between 5 and 8 vol.% methane, between 0.5 and contains 1.5% by volume of ammonia and a maximum of 1% by volume of carbon monoxide.

Methane is formed by thermal decomposition of the invention used propane. The levels of carbon dioxide and water are included such a carbonitriding atmosphere at less than 0.1% by volume.

The invention is explained in more detail below using an exemplary embodiment:

Test execution

Tests on the carbonitriding of metal Components made of hardenable, chrome and manganese steel. For The carbonitriding atmosphere was produced using nitrogen and propane used.

The flow rates of nitrogen, propane and ammonia were each about Flow meter measured and a compared to the usual method below Endogas changed feed point metered. The sample gas for the analysis of itself furnace gas atmosphere was pumped off via a sampling pipe.

After quenching and cleaning the treated components, some of examined them metallographically. One sample, which was also treated, served in each case as an analytical sample. The contents of this sample were measured at two penetration depths  dissolved nitrogen and carbon determined. The measuring depths were standard at 50 µm and 100 µm from the surface.

Embodiment

In the nitrogen / propane / ammonia tests, the amounts of gases specified in Table 2 were used (data in liters per hour):

Table 2

In these tests, the required marginal nitrogen content was 50 µm partially and with a penetration depth of 100 µm always reached except for one sample. Also the The analyzed marginal carbon contents were in the corresponding two depths in the required range at 1.4-1.7% by weight C. For some of the samples, the Analysis values of the determined nitrogen and carbon contents as well as the Process parameters listed in Table 3.

Table 3

Reactions of the individual gas components resulted in contents in the furnace chamber of approx. 10% hydrogen, 6-7% methane, 1% residual ammonia and 0.5% Carbon monoxide so that the furnace exhaust burned with yellow-colored paint. The content of water and carbon dioxide was below that in all experiments Detection limit of the measuring device (less than 0.05%).  

The carbon and nitrogen activities in the respective treatment atmosphere were calculated from the measured values for the gas components H ₂ , CO, CO ₂ , CH ₄ , H ₂ O, NH ₃ . These result as follows:
_{Stick reaction} : NH ₃ ⇒ (N) _{γ -Fe} + 1.5H ₂ (steady state)
Nitrogen activity (a _N ): a _N = kp _(N) . P _NH3 / P _H2 ^3/2
Nitrogen solubility (C _N ): C _N = f _(i) . a _N

The nitrogen concentration in the carburized alloy is defined by the stationary contents of hydrogen and residual ammonia in the furnace gas and by the treatment temperature. These concentrations can be measured with gas analyzers and used to calculate nitrogen activity (a _N ). The nitrogen activity (a _N ) is a direct measure of the expected nitrogen concentration (C _N ) in the alloy. The correction factor f _(i) takes into account all parameters that have a positive or negative influence on the solubility of nitrogen. It contains the alloy, gas and plant-specific influencing variables and must therefore be determined empirically.

Due to the high carbonitriding temperature and the low stationary Ammonia content in the furnace atmosphere, no γ'- Form nitrides or ε-nitrides.

Carburization reaction: C ₃ H ₈ ⇒ 2 (C) _{γ -Fe} + CH ₄ + 2H ₂ (steady state)
Carbon activity (a _c ): a _c = kp _(c) . P _CH4 / P _H2 ²
Carbon solubility (C _c ): C _c = f _(i) . a _c

The carbon concentration in the carburized alloy becomes through the stationary contents of hydrogen and methane in the furnace gas and through the Treatment temperature defined. The methane forms during thermal Cleavage of the propane and by reaction from the ammonia dissociation resulting hydrogen with carbon.  

Methane formation: (C) _graphite + 2H ₂ = + CH ₄ (steady state)

These concentrations, which are not in chemical equilibrium, can be measured with gas analyzers and used for the calculation of the carbon activity (a _c ). The carbon activity (a _c ) is a direct measure of the expected carbon concentration (C _c ) in the alloy. The correction factor f _(i) takes into account all parameters that influence the solubility of carbon. The alloy, gas and plant-specific influencing variables are contained in the constant and must therefore also be determined empirically.

Table 4 lists the gas concentrations measured in the furnace chamber according to the method of the invention (data in vol.%):

Table 4

When using a dry nitrogen / propane / ammonia mixture, almost no decarburizing components (H ₂ O and CO ₂ ) are contained in the treatment gas. An additional propane additive to avoid the risk of decarburization is therefore generally not necessary. The use of nitrogen / propane / ammonia increases process reliability.

Since the edge oxidation depth of the alloy elements, such as silicon, manganese and Chromium, which increases with increasing carbon monoxide content, is also out Edge oxidation reasons the use of nitrogen / Popan instead of Prefer endogas. Because as can be seen from the above information, in the process according to the invention in the treatment gas low carbon monoxide content. - compared to the usual procedure under endogas, in which the carbon monoxide content is between 20 vol .-% and is about 23% by volume. The depletion depth of alloying elements and the resulting hem from fine-streaked pearlite can by the  method reduced or avoided and the quality of the treated components can be improved.

In addition, operating costs are saved when replacing endogas with the cheaper nitrogen. And since CO ₂ emissions are reduced, carbonitriding with a nitrogen / propane / ammonia mixture is also advantageous for environmental reasons.

Claims (5)

1. A method for carbonitriding metallic workpieces by treating the workpieces in a carbonitriding atmosphere which is produced using a carbon-containing substance and ammonia, characterized in that for carbonitriding metallic workpieces made of high-carbon and / or high-alloy steels for the generation of the carbonitriding atmosphere nitrogen, ammonia and Propane can be used as a carbon-containing substance. 2. The method according to claim 1, characterized in that for generating the Carbonitriding atmosphere exclusively propane as a carbon-containing substance is used. 3. The method according to claim 1 or 2, characterized in that the Water content of the carbonitriding atmosphere is less than 0.1% by volume. 4. The method according to any one of the preceding claims, characterized characterized in that the carbon dioxide content of the carbonitriding atmosphere is less than 0.1% by volume. 5. The method according to any one of the preceding claims, characterized characterized in that the carbonitriding atmosphere is between 8 to 15% by volume Hydrogen, between 5 and 8 vol.% Methane, between 0.5 and 1.5 vol.% Contains ammonia and a maximum of 1 vol .-% carbon monoxide.