WO2001055471A1

WO2001055471A1 - Method for carbonitriding high-carbon and high-alloy steels

Info

Publication number: WO2001055471A1
Application number: PCT/EP2001/000838
Authority: WO
Inventors: Hans-Peter Schmidt
Original assignee: Messer Griesheim Gmbh
Priority date: 2000-01-27
Filing date: 2001-01-26
Publication date: 2001-08-02
Also published as: DE10003526A1

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

Process for the carbonitriding of high-carbon and high-alloy steels

The invention relates to a method for carbonitriding metallic workpieces by treating the workpieces in a carbonitriding atmosphere which is produced using a carbon-containing substance and ammonia

When carbonitriding metal parts made of austenitic steel alloys, carbon and nitrogen from the gas phase diffuse at high temperatures into the face-centered cubic lattice and are embedded in the surface layer of the metal parts. After the metal parts have been quenched, carbon and nitrogen remain in the diffusion zone and cause residual compressive stress, as a result of which 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 . 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:

(1) CO + H ₂ = (C β + H ₂ 0 (carburizing)

(2) NH ₃ => (N) _r-Fe + 1, 5 H ₂ (embroidery)

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 at the treatment temperatures sufficient in atomic nitrogen. For this reason, ammonia is added to the embroidery, which partially decomposes into atomic nitrogen at the high treatment temperatures, 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:

(3) 2 [N] dissolved "^ N ₂ , gaseous

Particular problems arise when carbonitriding high-carbon and / or high-alloy steels. The carburizing and nitriding conditions for this are explained in more detail below:

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. The levels of carbon dioxide and

Water of the gas phase must be kept very low in order to obtain high carbon activity. The decarburizing chemical reaction of these components (H ₂ 0 and C0 ₂ ) can be seen from the following reaction equations.

(4) 2 CO z> (C) γ-Fβ + C0 ₂ (Boudouard reaction)

(5) CO + H ₂ «• (C) _γ-Fe + H ₂ 0 (heterogeneous

Water gas shift reaction)

In order to maintain a given carbon activity, it is therefore necessary to order water and water generated by the carbon uptake

Implement carbon dioxide with a hydrocarbon. Propane is usually used for this, because propane reacts at relatively low levels Carbonitriding temperatures (which 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:

(6) 3 C0 ₂ + C ₃ H ₈ > 6 CO + 4 H ₂

(7) 3 H ₂ 0 + C ₃ H ₈ o 3 CO + 7 H ₂

Despite the better reactivity of the propane, a large excess of propane has to be added to the furnace, so that a high proportion of hydrocarbons leaves the plant unused. If additional oxygen reaches the furnace chamber unevenly due to leaks or through the metal to be treated, the marginal carbon contents and carburization depths can fluctuate greatly in their uniformity and reproducibility.

In particular with regard to high-alloy steels, the use of endogas as a carburizing carrier gas has a further disadvantage. If, for example, chromium-rich steels have to be carbonitrided, the alloyed chromium is oxidized and, depending on the chromium content of the alloy, a closed oxide cover layer or a so-called "internal oxidation" occurs. A closed oxide cover layer acts as a barrier layer for the further diffusion of carbon and nitrogen Since it cannot dissolve carbon, an internal oxidation, as is observed in tempered steels with low contents of chromium, manganese and silicon, has a negative effect on the hardness structure. When quenching, no martensite forms, but rather a fine-streaked pearlite, which is softer is a martensite, which significantly reduces wear resistance.

Chromium-rich alloys (chromium content> 5% by weight) and tempered steels can therefore only be carburized or carbonitrided to a limited extent in endogas. The invention is therefore based on the object of specifying 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 zone.

This object is achieved in that nitrogen, ammonia and propane are used as the carbon-containing substance to generate the carbonitriding atmosphere.

Instead of endogas mixtures in the invention

Process propane and nitrogen used to create the carbonitriding atmosphere. This lowers the oxygen activity in the treatment gas and reduces the formation of decarburizing components such as C0 ₂ and H ₂ 0. 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 higher nitrogen activity and a higher carbon activity in the carbonitriding atmosphere with a nitrogen / propane / ammonia mixture compared to the use of endogas. 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 C0 ₂ content, the C0 ₂ emissions are also reduced in this process. In addition, operating costs can be saved by replacing endogas with the cheaper nitrogen. The process according to the invention is particularly easy to reproduce if only propane is used as the carbon-containing substance to generate the carbonitriding atmosphere.

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. As a result, the decarburizing component H ₂ 0 is avoided and a high carbon activity in the carbonitriding atmosphere is achieved.

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 decarburizing component C0 ₂ we achieve a high carbon activity in the carbonitriding atmosphere.

A carbonitriding atmosphere which has between 8 and 15% by volume of hydrogen, between 5 and 8% by volume of methane and between 0.5 and 1.5% by volume of ammonia has proven to be particularly favorable for carbonitriding of high-carbon and high-alloy steels and contains a maximum of 1 vol .-% carbon monoxide. Methane is formed by thermal decomposition of the propane used in the invention. The contents of carbon dioxide and water in such a carbonitriding atmosphere are less than 0.1% by volume.

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

Experimental Procedure

Tests on the carbonitriding of metal components made of hardenable, chromium and manganese steel were carried out on a retort furnace. Nitrogen and propane were used to create the carbonitriding atmosphere.

The flow rates of nitrogen, propane and ammonia were measured in each case using a flow meter and using a method which was lower than that used in the conventional method Endogas changed feed point metered. The sample gas for the analysis of the furnace gas atmosphere that was established was pumped out via a sampling tube.

After quenching and cleaning the treated components, some of them were examined metallographically. A sample that was treated together served as an analytical sample. The contents of dissolved nitrogen and carbon were determined on this sample at two penetration depths. The standard measuring depths were 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 contents in 50 μm and in 100 μ penetration depth were always met except for one sample. The analyzed marginal carbon contents in the corresponding two depths were in the required range of 1.4-1.7% by weight C. For some of the samples, the analytical values of the nitrogen and carbon contents determined and the process parameters are listed in Table 3.

Table 3

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

The carbon and nitrogen activities in the respective treatment atmosphere were calculated from the measured values for the gas components H ₂ , CO, C0 ₂ , CH, H ₂ 0, NH ₃ . These result as follows:

Stick reaction: NH ₃ => (N) _γ-Fe + 1, 5 H ₂ (steady state)

Nitrogen activity (a _N ): a _N = kp _(N ) ^* P _N H3 PH2 ^{3 ~}

Nitrogen solubility (CN): CN = f _(i) ^* aN

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 (SN) is a direct measure of the expected nitrogen concentration (CN) in the alloy. The correction factor ₎ 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 steady ammonia content in the furnace atmosphere, no γ'-nitrides or ε-nitrides can form on the alloy.

Carburizing reaction: C ₃ H ₈ = 2 (C) _γ - _Fe + CH ₄ + 2 H ₂ (stationary

Status)

Carbon activity (a _c ): a _c = p _(C ) ^* PcH4 PH2 ² Carbon solubility (C _c ): C _c = f _(i) ^* a The carbon concentration in the carburized alloy is defined by the stationary contents of hydrogen and methane in the furnace gas and by the treatment temperature. The methane forms during the thermal cracking of the propane and through reaction of the hydrogen resulting from the ammonia dissociation with carbon.

Methane formation: (C) graphite + 2 H ₂ - + CH (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 takes into account all parameters that affect 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 invention (data in% by volume):

Table 4

When using a dry nitrogen / propane / ammonia mixture, almost no decarburizing components (H ₂ 0 and C0 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, increases with increasing carbon monoxide content, the use of nitrogen / popan instead of endogas is also preferable for edge oxidation reasons. Because as can be seen from the above information, a very low carbon monoxide content occurs in the treatment gas in the process according to the invention. - In comparison to the conventional process under endogas, in which the carbon monoxide content is between 20 vol .-% and about 23 vol .-%. The depletion depth of alloy elements and the resultant hem of fine-streaked periite can be reduced or avoided by the method according to the invention 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 C0 ₂ emissions are reduced, carbonitriding with a nitrogen / propane / ammonia mixture is also advantageous for environmental reasons.

Claims

claims

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 only propane is used as the carbon-containing substance for generating the carbonitriding atmosphere.

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 in that the carbon dioxide content of the carbonitriding atmosphere is less than 0.1 vol .-%.

5. The method according to any one of the preceding claims, characterized in that the carbonitriding atmosphere between 8 to 15 vol .-% hydrogen, between 5 and 8 vol .-% methane, between 0.5 and 1.5 vol .-% ammonia and maximum Contains 1 vol .-% carbon monoxide.