GB2044804A

GB2044804A - Heat treatment method

Info

Publication number: GB2044804A
Application number: GB7909276A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1979-03-16
Filing date: 1979-03-16
Publication date: 1980-10-22
Also published as: AU5639380A; JPS55128576A; AU531424B2

Abstract

In order to be able to control accurately carbon potential in a heat treatment process, an atmosphere of predetermined carbon monoxide concentration and chosen carbon potential is formed in a working chamber of the heat treatment furnace by admitting to the furnace inert gas (typically nitrogen), hydrocarbon (typically methane) and at least one organic liquid (typically methanol) which decomposes under the action of heat to yield carbon monoxide and hydrogen. The flow rates of these fluids are selected so as to give the predetermined carbon monoxide concentration. The respective flow rates required to give the carbon monoxide concentration at a chosen carbon potential are determined empirically. In a typical procedure, nitrogen and methanol are admitted to the chamber to build up the carbon monoxide concentration until the predetermined value is reached; hydrocarbon is then admitted and the methanol flow rate increased or nitrogen flow rate decreased so as to maintain the predetermined carbon monoxide concentration at the chosen carbon potential.

Description

SPECIFICATION Heat treatment method This invention relates to a method of heat treating metal. In particular it relates to such a method in which the metal is held in an atmosphere of chosen carbon potential.

In order to form an atmosphere of chosen carbon potential it has been the common practice in the art to perform an incomplete oxidation reaction between a fuel gas, typically propane or methane, and air in a kind of catalytic reactor known as an "endothermic generator" so as to form carbon monoxide and hydrogen. A gas mixture formed in an endothermic generator tends to be mildly carburising. In order to increase its carbon potential fuel gas such as propane or methane is added to it, typically after the metal to be heat treated has been raised to the working temperature in a furnace chamber to which the gas from the endothermic generator is continuously admitted. The more propane or methane that is added to the "endothermic gas", (i.e. the gas from the endothermic generator), the greater the carbon potential of the atmosphere in the furnace.

The following reactions take place in the furnace atmosphere (assuming that methane is added to the endothermic gas).

CH4 4 2CO H2 + CO H20 + CO

C + 2H2 CFE + CO2 H20 + CFe C 2 + H2 In the heat treatment art it is commonplace to state that the carbon potential of the atmosphere is proportional to the inverse of the carbon dioxide concentration or to the inverse of the water vapour concentration in the atmosphere. Thus, it has been the practice to calibrate flow meters controlling the admission of the gases to the furnace in terms of carbon potential by monitoring the carbon dioxide concentration in or dewpoint of the gas leaving the treatment chamber of the heat treatment furnace and plotting the variation in these parameters against, say, the rate at which hydrocarbon is added to the endothermic gas.Alternatively, a value in the conduit through which is supplied the hydrocarbon to be mixed with the endothermic gas may be controlled automatically in accordance with the monitored dewpoint or carbon dioxide concentration so as to maintain a chosen carbon potential.

These control methods are however based on approximations. First it is assumed that the furnace atmosphere is at equilibrium with the metal to be (say) carburised. Thus, from the chemical equations given above: (C0)2 (CO)(H2) CFE= = ~~~~~~~ (cm2) (H20) The gas generated in an endothermic generator generally contains about 20% by volume of carbon monoxide and about 40% by volume of hydrogen. On the other hand, it generally contains well under 1% by volume of carbon dioxide and water vapour. Therefore, it has been held in the heat treatment art to be a reasonable approximation to treat the concentrations of carbon monoxide and hydrocan as constants.

In recent years endothermic generators have for a number of reasons become increasingly expensive to operate and there have consequently been several proposals concerning the generation of a heat treatment atmosphere of chosen carbon potential entirely within the furnace itself. One such proposal is set out in UK patent specification 1 471 880. In this proposal a gas mixture comprising an inert gas (typically nitrogen), a hydrocarbon (typically propane) and an oxygen bearing medium (typically air or carbon dioxide) is formed and then admitted to the heat treatment furnace where it reacts to form carbon monoxide and hydrogen. However, the proportions of carbon monoxide and hydrogen that are formed are generally less than those produced in an endothermic generator.In consequence, carbon dioxide concentration and dew point are less suitable control parameters than they are when an endothermic generator is used to generate the furnace atmosphere.

Even furnace atmospheres formed by mixing hydrocarbon with endothermic gas may, we believe, be inherently difficult to control during carburising or carbonitriding if dew point or carbon dioxide concentration is relied upon as the control parameter. Conventionally, control graphs are plotted for endothermic gas containing 20% by volume of carbon monoxide when this gas is formed by reacting natural gas and air in the endothermic generator, and for endothermic gas containing 23% by volume of carbon monoxide when this gas is formed by reacting air and propane in the endothermic generator.We believe that such graphs are not fully suitable for use in the accurate control of carburising or carbonitriding and other processes of heat treatment as they neglect the dilution of the carbon monoxide that takes place when hydrocarbon and (in the example of carbonitriding) ammonia are added to the endothermic gas.

This dilution is enhanced by the tendency for the hydrocarbon to crack in the furnace to give hydrogen. For example, each volume of methane that cracks yields two volumes of hydrogen.

We calculate that dilution of endothermic gas (at 925"C) containing 20% carbon monoxide with respectively 10% by volume of methane and 20% by volume of methane has an effect on the concentration of carbon monoxide in the endothermic gas approximately as shown in the table below: CO H2 N2 CH4 (% by vol) (% by vol) (% by vol) (% by vol) Endothermic gas 20 40 40 90% by volume endothermic gas 1 7 47 34 2 1 0% by volume methane 80% by volume endothermic gas 14 53 28 5 20% by volume methane According to these calculations, the carbon monoxide concentration does not remain substantially constant at 20% throughout the heat treatment.We believe that such deviation in the carbon monoxide concentration accounts for many defects such as retained austenite and soft surfaces that occur in work which is case hardened by additions of hydrocarbon to endothermic gas.

It is an aim of the present invention to provide a method of heat treatment metal in which the carbon monoxide concentration in the heat treatment atmosphere is able to be maintained relatively constant.

The invention provides a method of heat treating metal in a working chamber of a heat treatment furnace, including the step of forming an atmosphere of chosen carbon potential in the chamber at a chosen temperature by admitting to the chamber at the chosen temperature, fluids comprising inert gas, hydrocarbon, and at least one organic liquid which under the action of heat decomposes to yield carbon monoxide and hydrogen, the relative proportions of the fluids being selected so as to give a chosen carbon potential and a predetermined concentration of carbon monoxide in the atmosphere, the relative flow rates of the said fluids required to give the chosen carbon potential at the predetermined concentration of carbon monoxide having been determined empirically by ascertaining the respective flow rates of the said fluids required to give different carbon potentials at the said carbon monoxide concentration.

According to the invention there is also provided a method of heat treating metal including the steps of passing inert gas and an organic liquid, which is capable of generating carbon monoxide and hydrogen, into a treatment chamber in which the metal to be heat treated is (or is to be) placed, heating the chamber and thereby causing the organic liquid to decompose to carbon monoxide and hydrogen, and at a chosen temperature and predetermined carbon monoxide concentration in the chamber: (a) admitting hydrocarbon to the chamber, and (b) performing at least one of the following steps: : (i) increasing the rate at which the organic liquid is admitted to the chamber; (ii) decreasing the rate at which inert gas is admitted to the chamber; (iii) admitting to the chamber a decarburising agent comprising one or more of air, carbon dioxide, water or oxygen; (iv) admitting to the chamber an additional organic liquid which at the temperature in the chamber decomposes to yield carbon monoxide and hydrogen; so as to compensate at least in part for the reduction in the carbon monoxide concentration in the chamber that tends to be caused by admitting the hydrocarbon to the chamber.

The method according to the invention may be used to carburise ferrous metal, or to harden ferrous metal at a chosen carbon potential. The method according to the invention may also be used to carbonitride ferrous metal, in which instance ammonia is desirably introduced into the chamber simultaneously with the hydrocarbon.

Typically the chosen temperature is in the range 700 to 1 300 C. if the heat treatment is a "neutral" hardening, the chosen temperature may be in the range of 723 to 1250"C (typically 750 to 850"C). If the heat treatment is a carburisation, the chosen temperature may be in the range 850 to 1 000 C (typically 900 to 950"C), and if the heat treatment is a carbonitriding the chosen temperature may be in the range 800 to 900"C. The temperature of the chamber is preferably held at the chosen temperature during the heat treatment.

The inert gas is preferably nitrogen. Instead of or in addition to nitrogen, the inert gas may be a noble gas, for example argon. By the term "inert gas" as used herein is meant a gas which does not react chemically with the other gases or any vapour in the chamber atmosphere, or a mixture of such gases.

The hydrocarbon preferably contains three carbon atoms or less and is preferably an alkane.

Preferably the hydrocarbon is methane. Methane is readily available as natural gas. Alternatively, propane may be employed as the hydrocarbon.

The organic liquid introduced into the chamber before the said introduction of the hydrocarbon is commenced is preferably one that on being heated decomposes to carbon monoxide and hydrogen only. More preferably, each molecule of such organic liquid decomposes under the action of heat to yield one molecule of carbon monoxide and two molecules of hydrogen. The preferred example of such an organic liquid is methanol. Instead of methanol, it is possible, though not preferred, to use another organic liquid such as ethyi acetate or acetone. The organic liquid introduced into the chamber before the said introduction of the hydrocarbon is commenced may alternatively be a mixture of, for example, methanol and another organic liquid which decomposes under the action of heat to yield carbon monoxide and hydrogen.

Decomposition of the organic liquid to yield carbon monoxide and hydrogen requires the input of heat to supply first the enthalpy of vaporisation and then the enthalpy of decomposition.

Although it is generally possible to use the heater which raises the temperature of the chamber to supply this thermal energy, it is often preferred, particularly in large furnaces, to supply at least some of this heat from another source by vaporising the (or each) organic liquid outside the furnace. In, for example, some smaller sealed quench furnaces it is however acceptable to vaporise the or each organic liquid in the furnace itself. Typically, in such an instance, the or each organic liquid may be dripped into the chamber, preferably in the direction of a fan which will help to disperse the vapor.

After a heat treatment has been performed at a chosen carbon potential, another heat treatment on another batch of metal may be performed at the same or a different carbon potential without the need to stop the supply of hydrocarbon, and any decarburising agent or additional organic liquid that is introduced into the chamber during the first heat treatment and to readjust accordingly the rates of supply or organic liquid and inert gas. All that in general is needed if a different carbon potential is required is to readjust the rates of introduction of the various fluids into the chamber so as to give the required carbon potential at the chosen carbon monoxide concentration.

If starting with a chamber that is at ambient temperature, it may be desirabie to "condition" the atmosphere in that chamber before attempting to build up a large concentration of carbon monoxide. The chamber may be conditioned by passing through it at elevated temperature inert gas containing a small quantity of reducing agent until, say, the dew point of the atmosphere has fallen to a chosen value (e.g. - 20"C). The reducing agent may typically be selected from hydrocarbon, ammonia or an organic liquid which decomposes under the action of the heat to yield carbon monoxide and hydrogen.

The relative proportions of inert gas to the furnace chamber before the addition of hydrocarbon may be chosen in accordance with the desired concentration of carbon monoxide in the heat treatment atmosphere that is formed as a result of the decomposition of the organic liquid. Suppose, for example, it is desired to form an atmosphere containing 20% by volume of carbon monoxide. For each 40 moles of inert gas that are supplied it is therefore necessary to supply 20 moles of methanol.

Although it is within the scope of the invention deliberately to supply a decarburising agent at this stage (ie. before the said addition of the hydrocarbon), this is not a preferred procedure, unless it is desired to supply a mixture of organic liquid (eg. methanol) and water during the period of the treatment in which the hydrocarbon is supplied. In this latter example, it is often more convenient to premix the water and methanol in chosen proportions before the start of the treatment and supply this mixture than to mix the methanol with water continuously during the treatment. Moreover, it is sometimes difficult to avoid adventitious leaks of air into the furnace chamber. Elderly sealed quench furnaces are, for example, prone to such leaks.Any such decarburising agent will not substantially increase the concentration of carbon monoxide in the furnace atmosphere unless admitted to the chamber in undesirably large proportions.

Preferably, in performing the method according to the invention, the temperature of the furnace is raised, methanol and inert gas (and any decarburising agent) are passed through the furnace so as to establish an atmosphere of inert gas, carbon monoxide and hydrogen, the metal loaded into the chamber when a chosen carbon monoxide concentration in the furnace chamber and when a chosen temperature are achieved, the temperature of the gas in the chamber returned to the chosen temperature for at least 1/4 hour before supply of hydrocarbon (and any ammonia) is started.

Preferably, for each 20 moles of methanol that are supplied a quantity of hydrocarbon containing from 2 to 4 moles of carbon atoms are supplied, depending on the required carbon potential. If ammonia is added to the furnace chamber, this too will tend to reduce the concentration of the carbon monoxide in the atmosphere in the chamber, and it will be desirable to compensate for such addition of ammonia as well as compensating for the addition of hydrocarbon.

Preferably, if one or more decarburising agents are admitted to the furnace with the hydrocarbon, the relative rates of admission are such that up to 1 mole of oxygen atoms are provided by the decarburising agent for each 4 moles of methanol that are admitted. If there is any inleak of air this is preferably quantified in terms of the amount of carbon monoxide it will give rise to in the atmosphere of the chamber.

Preferably the proportion of carbon monoxide in the atmosphere is maintained at or close to a chosen value of at least 15% by volume of the atmosphere, and more preferably of at least 20% by volume of the atmosphere.

The additional organic liquid may, for example, be isopropanol, or methyl acetate. Both these compounds decompose on entering the furnace atmosphere to form carbon monoxide, hydrogen and carbon. The isopropanol or methyl acetate may be introduced into the chamber in the same state as the methanol.

The required reduction in the rate of supplying nitrogen to the chamber, the required increase in the rate of supplying methanol (and/or other organic liquid) to the chamber or, the required rate at which decarburising agent and/or additional organic liquid may be introduced into the chamber in order to keep the concentration of carbon monoxide in the atmosphere substantially constant may be determined preferably by simple experiment or alternatively by calculation.

Another alternative is for the required adjustment to be effected automatically. A suitable automatic control means may typically comprise a carbon monoxide measuring instrument in communication with the atmosphere in the chamber, the instrument being typically positioned in the outlet of the furnace, the instrument being capable of generating signals which are transmitted to a suitable valve-controller programmed to adjust the position of an automatic valve so as to maintain the sensed carbon monoxide concentration substantially constant, the automatic valve being positioned in a conduit for supplying methanol and/or other organic liquid, inert gas, decarburising agent, or additional organic liquid to the chamber. The rate of supply of hydrocarbon may then be related to carbon potential by means of simple experiment.

When carburising, the so-called "boost diffuse" method may be adopted. This involves subjecting the work to a first carbon potential, which is generally relatively high, and then reducing the carbon potential to a second, lower value to enable carbon build up in the outer layers of the work to diffuse into the interior of the work. In accordance with the method of this invention the change in the carbon potential is preferably effected without substantially altering the carbon monoxide concentration in the furnace.

The method according to the invention makes possible, we believe, accurate carburisation and accurate carbonitriding.

An example of how the method according to the invention can be performed in a sealed quench furnace will now be described.

A sealed quench furnace includes a working chamber and a quenching chamber (in which a bath of oil may for example be maintained). Work may be transferred from the working chamber to the quenching chamber through a purge chamber (sometimes referred to as a "vestibule").

There is an outer door which may be opened to enable work to be loaded from outside the furnace into the purge chamber, and to be removed from the purge chamber to outside the furnace. There is an inner door which provides acces to the working chamber from the purge chamber and vice versa. There is not a gas tight seal between the purge chamber and the working chamber so that gas can pass from the latter into the former in operation of the furnace.

It is required to produce in, say, two separate batches of steel gears a surface carbon of 0.8% by weight and an effective case depth of 0.03 inches (0.076 cm).

it is assumed that an Efco sealed quench furnace having a working chamber whose capacity is 9 cubic feet (approximately 0.3 cubic metres) is employed. It is also assumed that the working chamber is initially at ambient temperature.

The sealed quench furnace is lit in the normal way, and nitrogen is passed from outside the furnace directly into and through the working chamber at a rate of about 1 50 cubic feet per hour (approx. 5 cubic metres per hour). The working chamber is heated so as to raise its chamber. When the temperature of the working chamber has reached 650"C methane is passed from outside the furnace directly into the working chamber of the furnace at a rate of, say, 5 cubic feet per hour (approx. 0.1 5 cubic metres per hour) so as to condition the atmosphere in the furnace by rendering it reducing.The dew point or carbon dioxide content of the gases leaving the working chamber may be monitored and when it has fallen to a value which indicates that the atmosphere is suitably reducing, the supply of methane to the furnace is stopped and a supply of methanol to the working chamber is initiated. The methanol may be dripped into the working chamber, or alternatively, may be vaporised just outside the furnace and fed into the working chamber as a vapour. The methanol is supplied to the working chamber of the furnace at a rate of 2.5 litres per hour. The temperature of the working chamber is then raised to 925"C.

In the working chamber of the furnace, methanol vapour decomposes to form carbon monoxide and hydrogen. Typically, the concentration of carbon monoxide in the atmosphere in the working chamber will reach 20% by volume within a quarter of an hour of a temperature of 925"C being achieved. The work may then be loaded into the purge chamber after opening the outer door. The outer door is then closed and the work held in the purge chamber for up to about 5 minutes. The inner door is then opened and the work transferred to the working chamber. Since the work is initially at ambient temperature, this causes a large drop in the temperature of the atmosphere in the working chamber. Thus, after loading the working chamber with the work, the temperature of the atmosphere is allowed to return to 925"C.

Although it is possible to start to build up the carbon potential in the atmosphere once the temperature of the atmosphere has reached 925"C, it is preferred to hold the work at this temperature for at least fifteen minutes to allow the work to achieve a uniform temperature of 925"C.

In order to build up the required carbon potential without substantially altering the carbon monoxide concentration in the furnace, methane is supplied to the working chamber at a rate of 30 cubic feet per hour (approx. 1 cubic metres per hour) and the rate of introduction of nitrogen and/or methanol adjusted appropriately. For example, the rate of introduction of methanol may be increased to 2.95 litres per hour or the rate of introduction of nitrogen reduced to 1 28 cubic feet per hour (approx. 4 cubic metres per hour).

The gases may be supplied to the furnace at such rates for typically a period of 4 hours in order to achieve the required case depth. At the end of this four hour period, with the rates of supply of fluids to the working chamber not being altered, the inner door is opened and the work transferred to the purge chamber. The inner door is then closed again. From the purge chamber the work is transferred to the quench chamber.

The second batch of work to be case hardened is then loaded into the purge chamber after opening the outer door. The outer door is then closed and the second batch of work held in the purge chamber for up to 5 minutes. If desired, the supply of methane to the working chamber may be stopped, and the rates of supplying methanol and nitrogen returned to their original values (ie. 2.5 I per hour and 150 cubic feet per hour respectively). After the second batch has been held in the purge chamber for a suitable period the inner door may then be opened and the second batch loaded into the working chamber. The inner door is then closed again. The temperature of the working may then be allowed to return to 925"C and the temperature of the work to come to equilibrium with that of the chamber.Carburising may then begin again by admitting methane to the working chamber at the rate of 30 cubic feet per hour with rate of supply of methanol or nitrogen being adjusted, as aforementioned, so as to maintain a substantially constant concentration of carbon monoxide in the atmosphere.

It is not necessary to adjust the supply of fluids to the working chamber after transferring the first batch from the working chamber to the quenching chamber and before introducing the second batch into the working chamber. The supply of fluids may remain unaltered thus allowing carburisation to begin once the second batch of work has been introduced into the working chamber and has acquired a temperature sufficient for carburisation to take place.

After the second batch of work has been positioned in the working chamber and the first batch has been quenched to ambient temperature, the first batch of work is transferred from the quenching chamber to the purge chamber, and then unloaded from the furnace.

After the second batch has been case hardened for a period of, say 4 hours, it is transferred to the quenching chamber via the purge chamber. After it has been quenched, the second batch of work is unloaded from the furnace.

It is to be appreciated that various flow rates of methane, methanol and nitrogen required to produce different chosen carbon potentials at a chosen concentration of carbon monoxide may readily be determined by simple experiment.

The flow rates quoted in the example above assume that there is no leak of air into the working chamber. If there is such a leak it will tend to reduce the rate at which methanol is required to be supplied to provide a chosen carbon monoxide concentration.

Claims

1. A method of- heat treating metal in a working chamber of a heat treatment furnace; including the step of forming an atmosphere of chosen carbon potential in the chamber at a chosen temperature by admitting to the chamber at the chosen temperature fluids comprising inert gas, hydrocarbon, and at least one organic liquid which under the action of heat decomposes to yield carbon monoxide and hydrogen, the relative proportions of the fluids being selected so as to give a chosen carbon potential and a predetermined concentration of carbon monoxide in the atmosphere, the relative flow rates of the said fluids required to give the chosen carbon potential at the predetermined concentration of carbon monoxide having been determined empirically by ascertaining the respective flow rates of the said fluids required to give different carbon potentials at the said carbon monoxide concentration.

2. A method of heat treating metal including the steps of passing inert gas and an organic liquid, which is capable of generating carbon monoxide and hydrogen, into a treatment chamber and thereby causing the organic liquid to decompose to carbon monoxide and hydrogen, and at a chosen temperature and predetermined carbon monoxide concentration in the chamber: (a) admitting hydrocarbon to the chamber, and (b) performing at least one of the following steps: (i) increasing the rate at which the organic liquid is admitted to the chamber; (ii) decreasing the rate at which inert gas is admitted to the chamber; (iii) admitting to the chamber a decarburising agent comprising one or more of air, carbon dioxide and water or oxygen; (iv) admitting to the chamber an additional organic liquid which at the temperature in the chamber decomposes to yield carbon monoxide and hydrogen; so as to compensate at least in part for the reduction in the carbon monoxide concentration in the chamber that tends to be caused by admitting the hydrocarbon to the chamber.

3. A method as claimed in claim 1, in which the organic liquid decomposes to carbon monoxide and hydrogen only.

4. A method as claimed in claim 3, in which the organic liquid is methanol.

5. A method as claimed in claim 2, in which the organic liquid admitted to the furnace chamber to achieve the predetermined carbon monoxide concentration decomposes to carbon monoxide and hydrogen only.

6. A method as claimed in claim 5, in which the organic liquid is methanol.

7. A method as claimed in claim 2, 5 or 6, in which the additional organic liquid decomposes to carbon monoxide, hydrogen and carbon in the chamber.

8. A method as claimed in claim 7, in which the additional organic liquid is isopropanol, or ethyl acetate, or a mixture thereof.

9. A method as claimed in any of the preceding claims, in which the organic liquid is vaporised before being admitted to the chamber.

1 0. A method as claimed in any one of claims 1 to 8, in which the organic liquid is dripped into the chamber.

11. A method as claimed in any of the preceding claims, in which the predetermined carbon monoxide concentration is at least 15% by volume.

1 2. A method as claimed in claim 11, in which the predetermined carbon monoxide concentration is at least 20% by volume.

1 3. A method as claimed in any of the preceding claims, in which the hydrocarbon is methane.

1 4. A method as claimed in any of the preceding claims, in which the inert gas is nitrogen.

1 5. A method as claimed in any of the preceding claims, in which the heat treatment is a carburisation which is performed at two carbon potentials, the first being higher than the second.

1 6. A method as claimed in any of the preceding claims, in which the heat treatment is a carburisation and is performed at a temperature in the range 900 to 950"C.

17. A method as claimed in any of claims 1 to 14, in which the heat treatment is a carbonitriding and is performed at a temperature in the range 800 to 900"C.

1 8. A method as claimed in any of claims 1 to 14, in which the heat treatment is a neutral hardening and is performed at a temperature in the range 750 to 850"C.

1 9. A method of heat treating metal in a working chamber of a furnace, substantially as described in the example herein.