EP0662525B1 - Process for preventing surface oxidation during steel carburizing - Google Patents

Abstract

Preventing of surface oxidn. in the carburisation of steels under a carbon-contg. gas mixt. at carburisation temp. is novel in that the steels are heated up to the carburisation temp. under a nitrogen/hydrogen gas mixt. or with pure hydrogen and during the carburisation the nitrogen/hydrogen gas mixt. or the hydrogen is replaced by a carbon-contg. gas mixt with an oxygen activity smaller than that necessary for the formation of manganese (II) or chromium (III) oxide.

Description

The invention relates to a method for avoiding edge oxidation when carburizing steels according to the preamble of claim 1.

In case hardening, low carbon steels are annealed in carbon donors at temperatures between 800 and 950 ° C (see for example US-A-4 152 177 or US-A-4 643 402). The surface is enriched with carbon and hardens when quenched. Endogases which contain about 20% CO, 40% H ₂ , 40% N ₂ are mostly used as carbon donors. When these steels are carburized with endogas, the base alloy elements are oxidized in the edge zone of the steels, so that they are no longer present in the subsequent microstructure formation. An undesirable one forms in the edge zone of the steels Structure which has unfavorable properties and requires mechanical removal or blasting of this edge zone in order to achieve the required properties of the steels (workpieces).

Investigations have shown that this edge oxidation is essentially caused by the oxygen potential of the endogases used, although these gases have a strongly reducing effect and there is no "free oxygen" at the respective carburizing temperature. The oxygen activity is determined by contents of CO, CO ₂ , H ₂ O and the non-oxygen-containing components (H ₂ , CH ₄ ). The dominant partial carburization reaction in such CO-containing gas atmospheres is the decay of carbon monoxide on the workpiece surface:

CO _gas = [C] _dissolved + [O] _adsorbed

The released carbon, as well as the adsorbed oxygen, are released from the alloy and diffuse into the steel. The amount of dissolved oxygen is determined by the oxygen activity of the gas phase and the duration of the treatment time and is very much less than the amount of carbon that dissolves. The oxygen solubility in pure iron at 950 ° C. and a C level of 1% by weight carbon using an endogas from methane is approximately 0.0003% by weight oxygen (3 ppm oxygen).

If the oxygen partial pressure for the formation of a metal oxide is exceeded, the oxidation of the respective metal occurs.

Me + H ₂ O = MeO + H ₂



Me + CO ₂ = MeO + CO

The oxygen potential of the carburizing media used is usually so low that there is no oxidation of the iron. Alloying elements present in the steels, however, have a high affinity for oxygen, so that small amounts of dissolved oxygen in the alloy lead to what is known as internal oxidation.

Common alloying elements are: Cr, Mn, Si, Ti, V and others, which are present in low concentrations. Edge oxidation or internal oxidation means oxide precipitates of the above-mentioned metals within a metal grain or along the grain boundaries, which are formed by the diffused dissolved oxygen and are then dispersed in the matrix.

$${\text{X}}_{\text{t}}\text{=}\sqrt{{\text{k}}_{\text{p}}\text{. t}}$$

X_t

Eindringtiefe des Sauerstoffs

D_o

Diffusionskoeffizient des Sauerstoffs in der Legierung

C_o

Sauerstoffkonzentration aus der Legierungsoberfläche

C_Me

Konzentration des unedlen Metalls in der Legierung (z.B. Silizium)

ν

stöchiometrischer Faktor

The kinetics of oxygen uptake obey a diffusion-controlled time law and the depth of penetration thus increases parabolically with the carburization time. The penetration depth of the oxygen and the resulting edge oxidation depth can be calculated using the following equation: $${\text{X}}_{\text{t}}^{\text{2nd}}\text{=}\frac{\text{2}\left({\text{D}}_{\text{O}}{\text{. C.}}_{\text{O}}\right)}{{\text{ν. C.}}_{\text{Me}}}{\text{. t = k}}_{\text{p}}\text{. t}$$

$${\text{X}}_{\text{t}}\text{=}\sqrt{{\text{k}}_{\text{p}}\text{. t}}$$

X _t

Oxygen penetration depth

D _o

Diffusion coefficient of oxygen in the alloy

C _o

Oxygen concentration from the alloy surface

C _Me

Concentration of the base metal in the alloy (e.g. silicon)

ν

stoichiometric factor

The invention has for its object to prevent edge oxidation when carburizing steels.

Starting from the prior art taken into account in the preamble of claim 1, this object is achieved according to the invention with the features specified in the characterizing part of claim 1.

Advantageous developments of the invention are specified in the subclaims.

The invention provides a carburizing process with only low investment and operating costs because the annealing can be carried out in conventional industrial furnace systems at atmospheric pressure.

According to the invention, the edge oxidation of the steels is avoided by the heat treatment being carried out in gas phases which contain no or only little oxygen-containing molecules. When these steels are carburized, the partial pressure of oxygen in the gas atmosphere does not exceed the formation pressure of the oxides.

The gas components hydrogen and hydrocarbons of the gas mixture according to the invention, the oxygen activity of which is less than is necessary for the formation of manganese (II) or chromium (III) oxide, are not oxygen-containing (oxygen-free), so that almost none Partial pressure of oxygen is present. The carbon transfer from the gas phase into the steel during the initial and diffusion phase is large and the required carbon content in the edge of the material (approx. 1% C) sets in relatively quickly. At carburizing temperatures, the unstable hydrocarbon (C _x H _y ) on the alloy surface mainly breaks down into hydrogen, methane and atomic carbon, which quickly diffuses into the material. If propane is used, the decay can take place according to the following reaction equation: $${\text{C.}}_{\text{3rd}}{\text{H}}_{\text{8th}}{\text{→ 2 CH}}_{\text{4th}}{\text{+ C}}_{\text{ad}}$$

The resulting carbon activity in the gas phase is influenced by the amount of hydrocarbon added. Since the gas phase consists mainly of hydrogen, the C level is regulated by the methane / hydrogen ratio that arises. The carburization reaction via methane decay in hydrogen atmospheres is:

The hydrogen content and in particular the methane content that is established are continuously analyzed and the hydrocarbon supply is regulated to a desired marginal carbon content on the basis of the actual values recorded.

1. Stufe

Hauptaufkohlungsphase

2. Stufe

Diffusionsphase

However, if large carburization depths are required, ie long carburization times (greater than 8 hours), the hydrogen / hydrocarbon gas mixture can be brought to the end by a dilute nitrogen-methanol cracking gas Carburizing phase. This carburizing variant is therefore a two-stage carburizing process:

1st stage

Main carburizing phase

2nd stage

Diffusion phase

During the diffusion phase (approx. 1 to 2 hours), the hydrogen dissolved in the workpiece during the main carburization phase is greatly reduced, so that hydrogen embrittlement can be excluded.

For the one-step process, the carbon-containing gas mixture is replaced by nitrogen after carburizing and the hydrogen dissolved in the steel is thus broken down. The steel is kept in the nitrogen atmosphere for between 5 and 15 minutes.

If hardening is carried out at a lower temperature than the carburizing temperature, nitrogen can be purged during the cooling phase to harden the temperature in order to break down the dissolved hydrogen. The carburizing phase can thus be used 100%.

Examples:

The steel 16 MnCr 5 (1% Mn; 1% Cr; 0.20% Si) was carburized in an industrial furnace system. The furnace system was formed with endogas at approx. 1,000 ° C before the first carburization. During the formation, the temperature and thermal voltage of the oxygen probe or the dew point or the CO ₂ content were measured and registered, and a clear statement was made about the quality of the furnace formation. The carburizing process was carried out as follows:

One-step procedure

Step 1:

Drive the steels into the oven and flush them with nitrogen (N ₂ ) to make them oxygen-free.

2nd step:

Heating the steels to the carburizing temperature under a nitrogen / hydrogen atmosphere.

3rd step:

From a temperature of 750 ° C, feed in a hydrogen / propane gas mixture.

4th step:

Carburizing the steels at a specified holding time and temperature in the hydrogen / propane furnace atmosphere.

5th step:

Approximately 1 to 2 hours before the holding time expires, the C level of the furnace atmosphere is regulated by adding propane to the value that sets a desired marginal carbon content in the steel.

6th step:

Flush the furnace chamber with nitrogen (large flushing volume) and keep the steels at temperature for about 10 minutes or cool them down to hardening temperature.

7th step:

Harden steels.

Two-step process

Step 1:

Drive the steels into the oven and flush them with nitrogen (N ₂ ) to make them oxygen-free.

2nd step:

Heating the steels to the carburizing temperature under a nitrogen (N ₂ ) / hydrogen (H ₂ ) atmosphere.

3rd step:

From a temperature of 750 ° C, feed in a hydrogen / hydrocarbon gas mixture.

4th step:

Carburizing the steels at a specified holding time and temperature in the hydrocarbon furnace atmosphere.

5th step:

About 1 to 2 hours before the holding time (carburizing time) expires, the gas atmosphere is replaced by a nitrogen-methanol cracked gas.

6th step:

Approximately 1 to 2 hours before the holding time expires, the C level of the furnace atmosphere (C level control via oxygen probe, CO ₂ or water content) is regulated by adding propane or other hydrocarbons to the value that sets a desired marginal carbon content in the steel.

7th step:

Cool steels to hardening temperature.

Step 8:

During the cooling to hardening temperature, the C level is kept constant at the desired value.

Step 9:

Harden steels.

The furnace gas composition was continuously analyzed for H ₂ , CH ₄ , CO, CO _{2 and} H ₂ O in both process variants throughout the entire process. The temperature profile was also measured and registered. Carbon and oxygen activity were continuously determined and corrected for their target values.

Claims (7)

1. Process for avoiding surface oxidation in the carburization of steels with alloy elements which are present in a low concentration under a carbon-containing gas mixture at the carburization temperature, characterized in that the steels are heated to the carburization temperature under a nitrogen/hydrogen gas mixture or with pure hydrogen and the nitrogen/hydrogen gas mixture or the hydrogen is replaced during the carburization by a carbon-containing gas mixture having an oxygen activity smaller than that required for the formation of manganese(II) oxide or chromium(III) oxide, and the carbon-containing gas mixture yields carbon which diffuses into the steel in dissolved form.
2. Process according to Claim 1, characterized in that the carbon-containing gas mixture is a hydrogen/hydrocarbon gas mixture, preferably a hydrogen/propane gas mixture.
3. Process according to Claim 1 or 2, characterized in that the carbon-containing gas mixture is replaced towards the end of the carburization by a cracked nitrogen/ methanol gas.
4. Process according to one of Claims 1 to 3, characterized in that the composition of the gas mixture or of the cracked gas is detected during the heating and carburization and, as a function of the detected actual values, the carbon content is adjusted to a desired surface carbon content by adding hydrocarbons.
5. Process according to one of Claims 1 to 4, characterized in that the carbon-containing gas mixture or the cracked gas is replaced by nitrogen after the carburization and the hydrogen dissolved in the steel is thereby reduced.
6. Process according to Claim 4, characterized in that the steel is held at the carburization temperature for between 5 and 15 minutes in the nitrogen atmosphere.
7. Process according to Claim 4, characterized in that the steel is cooled in the nitrogen atmosphere to a temperature below the carburization temperature.