EP2627795A1

EP2627795A1 - Method and arrangement for carburizing and carbonitriding metallic materials

Info

Publication number: EP2627795A1
Application number: EP10793142.0A
Authority: EP
Inventors: Hendrik Grobler; Bernd Edenhofer
Original assignee: Ipsen International GmbH
Current assignee: Ipsen International GmbH
Priority date: 2010-10-11
Filing date: 2010-10-11
Publication date: 2013-08-21
Also published as: DE112010005929A5; WO2012048669A1

Abstract

The invention relates to a method for the carburizing and carbonitriding of metallic materials, which is performed rapidly and uniformly in atmospheric furnaces and using hydrocarbon gases and gases such as nitrogen, hydrogen and ammonia. Heating is performed in a first method step; a hydrocarbon gas is added in a second method step; and the gas flow is turned off in a third method step. The method can be completed with fourth to sixth method steps.

Description

Method and device for carburizing and carbonitriding metallic materials

Technical area

The invention relates to a method and a device for carburizing and

Carbonitriding of metallic materials, in particular for a rapid and uniform process in atmospheric furnaces in the temperature range from 800 to 1150 ° C using hydrocarbon gases and gases such as nitrogen, hydrogen and ammonia.

State of the art

In the US 4,049,472 already a generic method with a

Gas mixture with CO2 described in which the atmosphere a certain

Has oxidation potential. An edge oxidation free (RO-free) carburizing is therefore not possible. The gas mixtures fed into the furnace with CH 4 / CO 2 ratios of 1: 1 to 4: 1 give in the furnace gas atmospheres which have an oxidizing effect on iron and its alloying elements such as Cr, Mn, Si. such

Gas mixtures are therefore not sufficient for a RO-free carburizing.

In addition, the desired C-level control - based solely on the supplied gas mixture and not the analysis of the atmosphere in the furnace - is empirical and not reliable.

Identical gas mixtures introduced into the kiln therefore result in different C-levels in the kiln, depending on the condition of the kiln (leakage, soot accumulation, etc.).

An analysis of the furnace atmosphere, ie, the gas constituents, in particular CO and CO ₂ , although sought for a high process reliability, but is usually not provided or with this method.

In addition, EP 049 530 dealt with a method and a

Device in which the heating of the batch of materials to be treated

CONFIRMATION COPY in normal, retortless carburizing furnaces under N ₂ flow due to lack of reducing gas components such as H ₂ an oxidation of iron and its

Alloy elements causes. Again, this procedure is for a RO-free

Carburization not suitable.

The same applies to the disclosed carburization in atmospheres produced by N ₂ flow and CO ₂ with pulsed added enrichment gas (hydrocarbons). Here, too, sets in the oven a high oxidation potential, the

Edge oxidation produced.

In addition, there is a strong Überkohlungs- and

Danger of formation of carbide, since in the carbon phase the C-activity of the furnace atmosphere is not detected and controlled, but only empirically adjusted by indirect measured variables such as the amount of soot produced in the furnace (soot sensor). According to EP 0408511 a method with empirical quantity metering of N ₂ , H ₂ and propane for the rapid production of a carbide layer is disclosed, which however does not take into account the furnace and workpiece state and to

different carbide layer thickness leads.

An exact adjustment of the edge C content is thus not possible, and there is a strong Überkohlungs- and carbide formation danger.

Thus, even this is a purely empirical process that is not process-safe and reproducible.

DE 19523956 describes a method for carburizing and carbonitriding of steels, in which also the carburizing and carbide layer formation takes place uncontrolled, analogous to EP 0408511. The lack of measurement of the C-activity or the C-stream causes a completely uncontrolled carburization in the supercritical phase. Adherence to the saturation limit is theoretically (computationally), but practically impossible.

In the variant 3b set forth in this document, a CO-H ₂ gas mixture is used with which a reproducible carburizing with controlled C level can be carried out with appropriate gas analysis. But it has the disadvantage that in her due to the high CO content of the atmosphere generated

Carburizing layers are not RO free. The applicant has already developed a method and a device according to EP 1 160 349 (see, comparatively in FIG. 1), in which a heating with IM 2 and KW feed quantity control takes place in 4 steps 1, so that the C edge is in the predetermined range, 2. a compensation phase in the predetermined range is, 3. an enrichment and diffusion phase is set to an upper and lower limit of the C-rim by varying the N2-KW rivers and 4. cooling to room temperature. It has been found that the heating in N2 and KW to adjust the low C content only small amounts of reducing

Gas components (H2) leads in the furnace atmosphere. Thus, in conventional carburizing furnaces (chamber furnaces and continuous furnaces), a certain amount of RO is generated because of the naturally penetrating atmospheric oxygen (door openings, etc.).

The carburizing in N ₂ with KW to set a high edge C content and in the balancing phase of a low edge C content is inaccurate and not

reproducible. A possible counter-regulation with H ₂ for fast

Lowering the C activity is not mandatory.

Since the process always operates below the saturation limit, the cycle times are relatively long.

In the mentioned process control, only the control of

Gas flow rates used.

The determination of the edge C content based solely on the inflowing

Gas quantities and not on the basis of the direct analysis of the gas components in the oven is inaccurate and faulty and does not take into account the respective furnace condition.

The outlook for a remote process for case hardening of metallic materials according to EP 0080124 discloses a pulse-like addition of HC, followed by a short break. In this case, after several pulse / pause cycles a longer pause interval is inserted, which is 10 to 100 times longer than the pulse / pause cycle of the addition of N-containing gas, the amount of which increases during the HC pulse addition becomes. The duration of the interval interval increases with the increasing carburization time. The pulse / pause cycles comprise a constant short period of time. Inert gases, such as N ₂ and hydrocarbons, in particular propane, are used.

Analogously to EP 049 530, this method also has disadvantages. Although the newly introduced Intervallpausen should indeed eliminate the disadvantage of uncontrolled carbide formation and dissolve the carbides formed; but they act as a purely empirical instrument without knowledge of the amount and size of the carbides produced. The interval breaks prove to be unsuitable, because depending on the amount of carbide the length of the

Interval break must be set, but not without measurement of the

Carbide attack is feasible.

Thus, according to this analysis of the prior art, the criticism peels off

to the effect that the carburizing and carbonitriding of metallic

Intensive further development of materials for a fast and uniform process in atmospheric furnaces is necessary to achieve a largely RO-free overall

To reach carburization. In addition, it has been in publications, such as

• W. Göhring in "Gas mixtures introduced hot into the furnace chamber as

Atmosphere in the heat treatment of steel "(HTM 30 (1975) 2,107-1 1 1) the unregulated carburizing in a fission gas-natural gas mixture at 920 ° C, followed by a controlled carburizing in a natural gas

Air mixture (p 1 10) set forth;

• J. Wünning et al. in "controlled carburizing in CO-free atmospheres" (HTM 31 (1976) 3,132-137, the saturation compensation method control using a mathematical model for the determination of the time shares for

Saturation and compensation phase and the geometry influence of edges and peaks according to a formula for a C mass flow described; • F. Nestor et al. in "Carburization of a high manganese, low carbon steel by methane - nitrogen gas mixtures" (J. Heat Treating 1 (1979) 1, 58-63) developed a functional relationship of carburization in CHVNfe mixtures and

• ALNAT-HC in "Significant reduction in edge oxidation in the

Heat treatment "(Portal Edition 7 (2010)) described that acetylene is not suitable because of large carbon fouling as enrichment gas, related experiments were made with N2, H ₂ and propane, a gas mixture of N ₂ , H ₂ and propane together with Kern-Liebers to

Aichelin was converted to a new fumigation station with the gases N ₂ , H ₂ , acetylene and propane for the process of N ₂ - methanol and only the gas mixture was changed, all other treatment data remained the same.

Since in the state of the art analyzed in this way primarily information on the

Gas flow rates (rudimentary only in EP 0408511 treated) and the furnace pressure missing, but which gain in importance for the optimization of litigation, the market calls for operation more efficient

Further developments of methods and devices for carburizing and

Carbonitriding of metallic materials in atmospheric furnaces.

Explanation of the essence of the invention

The invention is based on the new object, the procedural

To allow process control for carburizing and carbonitriding of metallic materials temporarily without gas flow and in particular to use the furnace pressure as an additional important process parameters to exclude the disadvantages of the above-cited method with different gas mixtures from the outset, to solve this task in detail so is that

• an edge oxidation-free (RO-free) carburizing or an exact adjustment of the edge C content and the carburization depth is achieved,

• information on the gas flow rates and the furnace pressure

be considered and

• the carburizing and carbide layer formation controlled / reproducible and not according to empirical findings, so that · the carburizing and carbonitriding of metallic materials in one

fast and even process can be done in the atmosphere furnaces.

According to the invention, this becomes functional by the method in FIG

resolved sub-steps, wherein in a first process step, a heating in N ₂ and / or H ₂ , advantageously in NH3-cracked gas, with a flow rate corresponding to a furnace chamber volume exchange of 1 to 10 times per hour takes place, second step after reaching the intended

Carburizing temperature of a hydrocarbon gas, preferably natural gas, ethane, propane, propylene, ethylene or acetylene is added, with a flow rate corresponding to a 1- to 10-fold change of the furnace chamber volume per hour until the residual CH ₄ content in the oven and / or the C-stream in the Workpiece surface has reached a predetermined value, which ensures a high C transfer rate in the oven, and third step, the HC gas flow and the N ₂ , H ₂ or NH ₃ - split gas flow off and in the furnace atmosphere, at least temporarily completely without gas flow in the Kiln is carburized until the CH ₄ content or the C-stream has reached a predetermined second value corresponding to a much lower C-transfer rate, in which phase the furnace pressure is controlled to a predetermined value between 1 and 10 mbar overpressure.

Further, the invention is embodied if, in a fourth process step, steps 2 and 3 calculate the C transfer rate several times using a mathematical model and controlling the edge C content at or near the maximum C saturation limit of the respective metallic alloy of materials are repeated, the fifth step, the C-activity of the furnace atmosphere by

Addition of H ₂ or an H2-N ₂ gas mixture, advantageously ammonia cracked gas, is lowered at a flow rate corresponding to a 1- to 10-fold change of the furnace chamber volume per hour, so that in the furnace a C activity, essentially defined by the partial pressure ratio of p (CH ₄ ) to p (H ₂ ) ² , which corresponds to the desired edge C content of the workpiece is held at the same time possibly necessary redispersion of the KW and / or sixth step, this gas atmosphere, until, according to a mathematical boundary C content model calculation, the desired edge C content and the desired carburization depth have been reached. It is essential that this inventive process control by a

Continuous gas analysis of H ₂ and CH ₄ and the measurement of the C-stream is supported in the workpiece surface. In this case, the C-current can be detected, for example, by measuring the change in the electrical resistance of an iron wire located in the furnace atmosphere. To protect the furnace against excessive sooting, it may also be useful to measure the amount of soot in the furnace with the aid of a corresponding soot sensor for improved process reliability.

For the process variant of carbonitriding, uncleaved ammonia is added to the process gas in an amount corresponding to a flow rate between 1 and 20% of the flow rate of the N ₂ -H ₂ mixture in the fifth and / or sixth process step.

With the invention, a highly efficient method is provided for the operator of atmospheric ovens, which is both an economical use of the

Process gases ensured as well as a randoxidationsfreies (RO-free) and also ensures very fast carburizing.

This makes carburization controllable and reproducible because it does not run according to empirical findings and largely independent of subjective influences. The carburizing and carbonitriding of metallic materials can - in the case of differentiated controlled individual steps - take place overall in a fast and even process in the atmospheric furnaces.

To carry out the method, a device is used, the first means for detecting and controlling the flow rate of the respective gas mixture according to the structural

Dimensions, b) second carburizing temperature detecting means c) third means for measuring the residual CH ₄ content. D) fourth means for measuring the H ₂ content e) Fifth means for C current measurement f) sixth means for furnace pressure measurementAregelung g) seventh means for detecting the amount of soot h) eighth means for closing the furnace exhaust gas i) ninth means for controlling the carburization process comprises, which means with a control / regulation for process control

are functionally linked according to the parameters of the program.

The invention will be described with reference to an exemplary embodiment in an application variant (s).

Brief Description of the Drawings In the drawings show

1 is a graph of the carburizing process according to the prior art according to EP 1160349, and FIG. 2 is a graph showing the carburizing process on a

Material as a result of the steps according to the invention as well

Fig. 3 is a schematic representation of a block diagram of

inventive device.

Best way to carry out the invention

Fig. 1 shows the continuously flowing gas quantities of acetylene, nitrogen and hydrogen and the more or less empirically made temporal variations of these gas flow rates for adjusting various marginal C contents. In contrast, Fig. 2 shows the scheme of the process, according to the method of the invention, the supply of constant amounts of gas within the individual process steps and the temporary interruption of the gas flow in the furnace and the use of gas analysis (CH ₄ and H ₂ ) and the pressure control for the Control of carburizing.

In this way, a controlled and reproducible carburization with HC gases (without the use of CO and CO2 or H ₂ O) is achieved at the same time complete absence of edge oxidation of the carburized metallic surface layers.

These positive effects are achieved when, at a preferred

Process example in the process variant of a carburizing and hardening in an atmosphere furnace, such as multi-purpose chamber furnace with a volume of a

Heating chamber of 4 m ³ and

a case-hardening batch of gear parts made of 16MnCr5 steel on a CD of 0.8 mm the following steps are carried out:

I. Process step 1: heating the batch in the heating chamber of the chamber furnace under rinsing the

Chamber with a hydrogen-nitrogen gas mixture, eg 75% H ₂ and 25% N ₂ , with a flow rate of 15 Nm ³ / h.

II. Process step 2: After reaching the carburizing temperature of 930 ° C, feed natural gas into the furnace at a flow rate of 4 Nm ³ / h, until the concentration of methane in the furnace has risen to 6% by volume.

III. Process step 3: stopping the natural gas flow and closing the burn-off valve, ie carburizing substantially in "stationary" gas, the furnace pressure being maintained between 2 and 5 mbar by a temporary addition of H ₂ / N ₂ gas the methane content in the furnace has dropped to 0.5%. IV. Process Step 4:

Opening the burn-off valve and adding natural gas to the furnace at a flow rate of 4 Nm ³ / h until the calculated marginal carbon content of the 16MnCr5 steel reaches the maximum carbon saturation limit at 930 ° C.

V. Process Step 5:

Lowering the carbon activity of the furnace atmosphere by closing the flow of natural gas into the furnace and opening the H ₂ / N ₂ gas flow into the furnace at a flow rate of 10 Nm ³ / h while lowering the furnace temperature to 850 ° C. Duration of this step until both the furnace temperature of 850 ° C is reached and the carbon activity of the furnace atmosphere, defined by the ratio of the partial pressure of methane p (CH ₄ ) to the square partial pressure of

Hydrogen p (H ₂ ) has fallen to the value of 0.75.

VI. Process step 6:

Holding the batch at 850 ° C while controlling the carbon activity of the

Oven atmosphere to the value 0.75 by pulsed natural gas addition with simultaneous furnace pressure control to a pressure value between 2 and 5 mbar by temporarily adding H ₂ and N ₂ , until the calculated edge carbon content of the steel reaches its target value of 0.75 wt. % and the calculated carburization depth have reached their target size of 0.8 mm. This process according to the invention is accompanied by a continuous and integrated gas analysis of H ₂ and CH ₄ or the measurement of the C-stream in the respective furnace. In this case, the direct measurement of the C-current by means of the change in the electrical resistance of an iron wire. Also, the measurement of the amount of soot formed in the furnace can be used to increase the process reliability.

According to FIG. 3, a program for controlling / regulating the process control is integrated into the device for the atmospheric oven, which program is designed according to the Method steps 1 to 3 of FIG. 2, the parameters for receiving, regulating and output of at least one of the criteria, such as a) ratio or the flow rate of the

Furnace chamber volume exchange of the process gases per hour, b) furnace pressure values, c) carburizing temperature and addition of the HC gas according to the residual CH ₄ content or C current to a predetermined value, d) shutdown of the fission gas flow in the furnace atmosphere according to reaching of the CH ₄ content or the C-stream, e) calculating the C-transmission rate by means of a mathematical model to control the edge-C content and the carburizing depth, f) reduction of the C-type of furnace atmosphere, g) continuous gas analysis, h Measuring the C-current by changing the electrical resistance or i) adding uncleaved ammonia according to one

required flow rate controls.

For carrying out the method, the atmosphere furnace is assigned the configuration of a device according to FIG. 3, which essentially comprises: a) first means for detecting and controlling the flow rate of the gas mixture in question according to the structural dimensions, b) second means for recording the carburizing temperature, c) third means for measuring the residual CH ₄ content d) fourth means for measuring the H ₂ content e) Fifth means for C current measurement f) sixth means for furnace pressure measurement g) seventh means for detecting the amount of soot h) eighth means for closing the furnace exhaust gas i) ninth means for controlling the carburization process which means with the control / regulation for process control are functionally linked according to the parameters of the program.

Industrial Applicability The process and device according to the invention execution provides the furnace operator in the heat treatment of metallic workpieces an efficient edge oxidation free (RO-free) carburizing with precise adjustment of the edge C content and the carburizing ungestiefe ready, wherein the carburizing

controlled / reproducible and not according to empirical findings, so that the carburizing and carbonitriding of metallic materials can be done in a fast and even process. Reference number list

1 = atmosphere oven

2 = First means of measuring and controlling gas flow rates 3 = Second means of measuring temperature

4 = Third means for measuring the residual CH ₄ content

5 = Fourth means for measuring the H ₂ content

6 = fifth means for C current measurement

7 = Sixth means of furnace pressure measurement / control

8 = Seventh means for detecting the amount of soot

9 = Eighth means for closing the furnace exhaust gas line

10 = Ninth means for controlling the carburization process

I = process step 1

11 = method step 2

III = process step 3

IV = process step 4

V = method step 5

VI = method step 6

Legend

KW = hydrocarbon

NH3 = ammonia gas

H3diss = dissociated ammonia

H ₂ = hydrogen gas

N ₂ = nitrogen gas

Ti = temperature of the carburization process

T ₂ = temperature of the diffusion process (before quenching)

V _H2 = volumetric flow H ₂ (m ³ / h)

H2 / N2 = volume flow H ₂ / N ₂ mixture (m ³ / h)

VC2 H2 = volume flow C ₂ / H ₂ mixture (m ³ / h) V _N 2 = volume flow N ₂ (m ³ / h)

VNH3 = volume flow NH ₃ (m ³ / h)

Wc = carbon level (wt%)

V «w = volumetric flow of a hydrocarbon gas (m ³ / h) CCH ₄ = concentration of methane in the furnace atmosphere (vol%)

CRand = concentration of carbon at the edge of the metallic

Workpiece (% by weight)

P = furnace pressure (mbar)

t / h = time in hours

Claims

claims

Method of carburizing and carbonitriding of metallic

Materials, in particular for a rapid and uniform process in atmospheric furnaces (1) in the temperature range from 800 to 1150 ° C using hydrocarbon gases and gases, such as nitrogen, hydrogen and ammonia, characterized in that in a) first method step (I) a Heating with at least one of the gases N ₂ or H _{2 is carried out} with a flow rate corresponding to a required furnace chamber volume exchange, b) second process step (II) after reaching the intended carburizing temperature, a HC gas is added until the residual CH ₄ content in Furnace or the measured C-stream has reached a predetermined value in the workpiece surfaces, which ensures a high C-transfer rate in the furnace and c) third process step (III) the HC gas flow and the N ₂ , H ₂ or NH ₃ gap gas flow turned off and carburized in the furnace atmosphere at least temporarily completely without gas flow into the furnace until the CH ₄ content the furnace atmosphere or the measured C-current in the workpiece surfaces has reached a predetermined second value, which corresponds to a significantly lower C transfer rate.

Method according to Claim 1, characterized in that, in a fourth method step (IV), steps b) and c) are calculated by calculating the C transmission rate using a mathematical model and

Control the edge C content at or near the maximum C saturation limit of the respective metallic alloy of the materials are repeated several times.

3. The method according to claim 1 or 2, characterized in that in a fifth method step (V) the C-activity of the furnace atmosphere is lowered by adding Hb or a H ₂ / N ₂ gas mixture at a flow rate corresponding to a replacement of the furnace chamber volume so that in the oven, a C-activity, essentially defined by the partial pressure ratio of p (CH) to p (H ₂ ) ² , adjusts, which corresponds to the desired edge C content workpiece, with possibly simultaneously becoming necessary replenishment of the

Hydrocarbon.

4. The method according to any one of claims 1 to 3, characterized in that in a sixth step (VI), this gas atmosphere is maintained until, according to a mathematical model calculation, the desired carburizing ungestiefe is reached.

5. The method according to any one of claims 1 to 4, characterized in that in the first process step (I) NH ₃ -gaping gas with a

Flow rate corresponding to a furnace chamber volume exchange of 1 to 10 times per hour is used.

6. The method according to any one of claims 1 to 5, characterized in that in the second process step (II) natural gas, ethane, propane, propylene, ethylene or acetylene is added at a flow rate corresponding to a 1- to 10-times replacement of the furnace chamber volume per hour ,

7. The method according to any one of claims 1 to 6, characterized in that in the phase of the third process step (III) the furnace pressure is regulated to a predetermined value between 1 and 10 mbar pressure.

8. The method according to any one of claims 1 to 7, characterized in that in the fifth method step (V) corresponding to a 1- to 10-times Replacement of the furnace chamber volume in the hour ammonia cracking gas is used.

9. The method according to any one of claims 1 to 8, characterized in that in the sixth method step (VI) the edge C content is calculated according to a mathematical model calculation.

10. The method according to any one of claims 1 to 9, characterized in that the process control of the steps by a continuous gas analysis of H ₂ and CH ₄ or measurement of the C-stream is carried out in the respective furnace.

11. The method according to any one of claims 1 to 10, characterized in that the C-current directly by a change of the electric

Resistance is measured by means of an iron wire.

12. The method according to any one of claims 1 to 11, characterized in that a soot sensor is used to detect a soot amount.

13. The method according to any one of claims 1 to 12, characterized in that for a process variant of carbonitriding in the fifth

Process step (V) and / or sixth process step (VI) the

Process gas is added uncleaved ammonia with an amount corresponding to a flow rate between 1 and 20% of the flow rate of the N ₂ / H ₂ mixture.

14. The method according to any one of claims 1 to 13, characterized by the use of a program for the process control in a respective atmospheric furnace, comprising at least one parameter for receiving / control / output a) of the ratio / the flow rate of the

Furnace chamber volume exchange of the process gases per hour, b) the values of the furnace pressure, c) the carburizing temperature and the addition of the HC gas corresponding to the residual CH ₄ content and / or the C flow to a predetermined value and, if necessary, including a signal value from the soot sensor. d) the shutdown of the fission gas flow in the furnace atmosphere

e) the calculation of the C transfer rate by means of a mathematical model for the control of the marginal C content and the carburizing depth f) of the reduction of the C activity of the furnace atmosphere, g) according to the achievement of the CH ₄ content or the C stream; h) the measurement of the C-current by changing the electrical

Resistance and i) the addition of uncleaved ammonia according to a required flow rate.

15. Device for carrying out the method, characterized by an atmosphere furnace (1) with a burn-off valve (9), first means for measuring and controlling the gas flow amounts of a gas metering and control device (2) for the process gases H ₂ / H ₂ and KW and NH3, second means to

Temperature measurement for carburizing temperature detection (3), third CH ₄ content measuring means (4), fourth H ₂ content measuring means (5), fifth C current measuring means (6), sixth Means for measuring furnace pressure (7), seventh means for detecting the amount of soot formed in the furnace by means of a soot sensor (8), eighth means for

Close the furnace exhaust pipe (9), ninth means for controlling the

Carburization process and control of the process parameters (10) including the calculation of the C activity and the C flow as well as for the model calculation of the edge C content and the carburization depth.