GB1564926A

GB1564926A - Heat treatment of metal

Info

Publication number: GB1564926A
Application number: GB2533376A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1976-06-18
Filing date: 1976-06-18
Publication date: 1980-04-16
Also published as: AU2622177A

Description

(54) HEAT TREATMENT OF METAL (71) We, BOC LIMITED, an English company of Hammersmith House, London, W6 9DX, England, do hereby declare the invention, for which we pray that a patent may be granted to us, to be particularly described in and by the following statement:- This invention relates to the heat treatment of metal, (typically, ferrous metal).

Examples of processes of heat treatment include carburising, carbonitriding, carbon restoration, case hardening, neutral hardening, decarburising, malleabilising, annealing and normalising.

In order to heat treat metal it is standard practice to provide in the furnace in which the treatment is performed a non-oxidising atmosphere (except when on occasions it is desired to oxidise the metal.) For all types of heat treatment listed above it may be necessary at some stage in the treatment to provide in the furnace an atmosphere of chosen "carbon potential". The term "carbon potential" applied to an atmosphere in a heat treatment furnace indicates the percentage carbon content that the metal in the furnace would have were the metal and atmosphere to come to equilibrium. It is to be appreciated that in heat treatment the atmosphere in the furnace is generally not in equilibrium with the metal.

There are three well known ways of forming a suitable atmosphere within the furnace. The most commonly practised is to pass fuel gas and air through a so called "endothermic generator". In this generator a catalyst is provided and by keeping the temperature therein at a suitable value the fuel gas and air react catalytically to form a gas mixture typically consisting of about 24% by volume of carbon monoxide, about 30% by volume of hydrogen, up to 1%, by volume of each of carbon dioxide, methane and water vapour, and a balance of nitrogen. The carbon potential of such gas mixture can then be increased by adding a chosen proportion of fuel gas to the mixture. Thus, the carbon potential of the gas mixture supplied to the furnace may be regulated by regulating the addition of fuel gas (typically methane or propane) to the gas mixture emanating from the endothermic generator.

A second well known method of forming an atmosphere suitable for heat treatment of metals involves burning fuel gas in a socalled "exothermic generator". This procedure results in the formation of greater proportions of carbon dioxide and water vapour than are produced in an endothermic generator. At least most of the carbon dioxide and water vapour is then stripped from the gas mixture produced in the exothermic generator. The carbon potential of the stripped gas mixture may then be regulated by regulating additions of fuel gas to this gas mixture.

The third well known way of forming a furnace atmosphere having a chosen carbon potential is to mix together regulated proportions of nitrogen, hydrocarbon, and oxygen-containing gas or vapour and to pass the mixture into the furnace. Once inside the heated furnace the mixture reacts to form the carbon monoxide that is necessary for carburising to take place.

If a carbonitriding atmosphere is required, ammonia is mixed with the gases that are being passed into the furnace irrespective of the method chosen to provide those gases.

A disadvantage of using an endothermic generator to produce the furnace atmosphere is that unnecessarily large proportions of carbon monoxide and hydrogen are formed. This makes the atmosphere particularly hazardous both from the point of view of its explosive tendency and from the point of view of its toxicity. It has therefore become the practice of some operators of heat treatment furnaces to dilute gas from an endothermic generator with pure nitrogen.

This reduces the proportion of carbon monoxide in the resulting gas mixture and therefore reduces the quantity of fuel gas that has to be added to give an atmosphere of chosen carbon potential.

It is possible to control the concentration of carbon monoxide in gas produced in an exothermic generator by appropriately regulating the relative proportions of fuel gas and air that are admitted to the generator. Analagously, it is possible to select the relative proportion of nitrogen, hydrocarbon and oxygen-containing gas or vapour when forming a carburising gas in situ in a furnace so as to keep down the concentration of carbon monoxide in the furnace.

In order to give a good and reproducible heat treatment it is essential to control or regulate the carbon potential of the atmosphere in the furnace. The carburisation reaction may be represented as follows:- (M)+2COrC(M)+C02 where C (M) represents carbon alloyed with a metal M. When this reaction is at equilibrium the percentage of carbon in the metal (or at least in its surface) is dependent upon the ratio of the square of the partial pressure of carbon monoxide in the furnace atmosphere to the partial pressure of carbon dioxide in the furnace atmosphere. As the gas from endothermic generators typically contains around 24% by volume of carbon monoxide and in the order of 0.2 by volume of carbon dioxide, small additions of gases (e.g. fuel gas) to the furnace will make relatively little difference to the value of the square of the partial pressure of carbon monoxide. Accordingly, this figure has been treated by furnace operators as being sufficiently near to a constant to enable them to use a measurement of the proportion by volume of carbon dioxide in the furnace as an indicator of the carbon potential in the furnace. Alternatively, furnace operators have used the dewpoint of the furnace atmosphere as an indicator of the carbon potential in view of the following reaction that takes place in the furnace.

H2+CO+(M)H20+C(M) In DOLS 2 450 879 and "Nitrogen - based Carbon Controlled Atmosphere-as alternative to Endothermic gas (A Cook), Heat Treatment of Metals, 1976, 1, pp 15 to 18 it has been stated that the abovementioned indicators of carbon potential may be used satisfactorily when carbon monoxide is formed in situ in the furnace by reacting a mixture of nitrogen, hydrocarbon and oxygen In these documents there are disclosed processes in which the concentration of carbon monoxide in the furnace atmosphere is reduced to well below 20% by volume. We have found, however, that at the desirable lower concentrations of carbon monoxide, monitoring the concentration of carbon dioxide or the dewpoint of the furnace atmosphere no longer makes possible satisfactory and reproducible carburisation of ferrous metal. There are two possible explanations of this phenomenon.

First, the proportions of carbon dioxide and water vapour in the furnace atmosphere are reduced to very low values indeed. For example, with a concentration of carbon monoxide of six per cent by volume the concentration of carbon dioxide is in the order of 0.03% by volume. Although conventional infra-red detectors are capable of accurately measuring such low concentrations of carbon dioxide there is we believe an unacceptably large risk of unrepresentative readings being caused by air seeping into the furnace and/or faulty sampling.

Second, we have found that the effect of adding hydrocarbons to increase the carbon potential of the furnace atmosphere may cause a dilution of the concentration of carbon monoxide by 1% or more. Such a change in volume is relatively large when the concentration of carbon monoxide is in the order of six per cent by volume. We thus believe that at low concentrations of carbon dioxide it is no longer sound to view the square of the partial pressure of carbon monoxide as a constant.

We have now discovered that the concentration of methane in the furnace atmosphere may give a usable indication of the carbon potential of the furnace.

According to the present invention there is provided a method of heat treating metal in a heat treatment furnace, including the steps of admitting to the furnace gases which include carbon monoxide, one or more hydrocarbons in gaseous or vapour state, and nitrogen, or which react in the furnace to form a gas mixture containing carbon monoxide, methane and nitrogen, monitoring the concentration of methane in the atmosphere of the furnace, and regulating the carbon potential of the atmosphere in the furnace by so regulating the passage of the gases into the furnace that the monitored concentration of methane in the furnace atmosphere kept equal or close to a chosen value. The metal is typically ferrous.

Preferably, the relative rates at which the respective gases are passed into the furnace are such that the concentration of carbon monoxide in the furnace atmosphere does not fall below 5% by volume when the atmosphere in the furnace is decarburising or neutral. The typical practice in a batch furnace is first to establish an atmosphere which is neutral (ie. neither carburising nor decarburising) and then pass into the furnace hydrocarbon at a greater rate so as to establish a carburising atmosphere. The addition of such extra hydrocarbon dilutes the carbon monoxide in the furnace and may take its concentration down to below 5% by volume. We believe that an optimum balance between the quality of heat treatment and keeping to a minimum the quantity of hydrocarbon used in the treatment, when performing a carburisation, is to establish in the batch furnace an approximately neutral atmosphere containing about 6% by volume of carbon monoxide and then to dilute this atmosphere with additional hydrocarbon(s) so as to give the required carbon potential.

If the gases passed into the furnace are provided by a gas mixture from an endothermic generator diluted with nitrogen, the correct amount of nitrogen to be used in dilution may readily be calculated. For example, it is necessary to dilute one volume of endothermic gas typically containing 24% by volume of carbon monoxide with 3 volumes of nitrogen to give a mixture containing 6% by volume of carbon monoxide.

If the gases to be admitted to the furnace come from an exothermic generator the ratio by volume of air to fuel gas passed into the generator may be chosen such that after carbon dioxide and water are removed from the ensuing gas a mixture containing the chosen percentage (6%) by volume of carbon monoxide is left.

It is also possible to select the proportion of carbon monoxide in the furnace atmosphere if this gas is formed in the furnace by the reaction of a hydrocarbon with oxygen or a vaporous or gaseous compound containing oxygen, the reaction taking place in a "carrier gas" or nitrogen.

The number of molecules of carbon monoxide that are formed is dictated by the stoichiometry of the following reaction ny X(O)n+n C x Hyonx CO + - H2 2 where (0) is oxygen or a compound containing oxygen e.g. CO2, H2O, an alcohol or any other oxygenated organic compound, and Cx Hy is a hydrocarbon (typically 'methane or propane). Suppose a gas mixture of air, propane and nitrogen is passed into the furnace. The reaction between the oxygen in the air and the propane is as follows:- 302+2C3H8o6CO+8H2 In order to provide 3 volumes of oxygen, approximately 15 volumes of air are required. Thus approximately 74 volumes of nitrogen, 15 volumes of air and 2 volumes of propane are required to give an atmosphere in the furnace containing approximately 6 volumes of carbon monoxide, 8 volumes of hydrogen and 86 volumes of nitrogen. There will also be traces of water vapour, carbon dioxide and methane in the atmosphere.

When the above mentioned atmosphere has been established in the furnace, (this may be done by flushing the furnace with nitrogen while the work inside is raised to a chosen temperature and then passing the gas mixture (or separate gas streams) into the furnace for a period of 15 to 30 minutes) the proportion of propane (or other hydrocarbon in the mixture) may be increased in order to achieve a carburising atmosphere. Thus the method involves passing into the furnace atmosphere stoichiometric proportions of oxygen or oxygen containing medium and hydrocarbon(s) to establish an approximately neutral atmosphere, and then increasing the ratio of hydrocarbon(s) to oxygen containing medium so as to provide a carburising atmosphere. We prefer to achieve this adjustment of the said ratio by increasing the volumes of hydrocarbon contained in the incoming gases rather than by decreasing the volumes of oxygen or oxygen containing medium in these gases.

It is to be appreciated that any propane or other hydrocarbon higher than methane will on entering the furnace will rapidly (and almost instantaneously) decompose into methane, hydrogen and carbon.

The method according to the present invention may be performed in the carburising (or carbonitriding) chamber or chambers of a continuous heat treatment furnace. In such chambers, an atmosphere of approximately constant carbon potential is maintained throughout the operation of the furnace. The concentration of methane in the atmosphere is thus monitored and the addition of hydrocarbon to the atmosphere is regulated so as to keep the monitored value of the methane concentration substantially constant at the chosen value.

Preferably a detector sensitive to infrared radiation is used to monitor the concentration of methane in the furnace atmosphere. It is possible to convert an existing detector adapted to measure carbon dioxide into one adapted to measure methane.

Standard gas regulating and mixing equipment may be used to control and regulate the introduction of the gases into the furnace. Thus each separate stream of gas, prior to being mixed with other(s), may be passed through a pressure regulator and flow meter or calibrated orifice. Adjustment of the rate at which each stream of gas is passed into the furnace or a mixing device upstream of the furnace may be effected by manual operation of valves or automatically by means of solenoid or motorised valves which are adapted to be actuated by electrical signals generated by the methane detector such that the carbon potential is kept as close as possible to a chosen value.

The method according to the invention is further illustrated by Figures 1 and 2 of the accompanying drawings. Figure 1 shows how the concentration of hydrogen. carbon dioxide, carbon monoxide and methane in a furnace atmosphere at 925" vary when different volumes of propane are admixed with 300 cubic feet per hour of nitrogen and 60 cubic feet per hour of air and the resulting gas mixture is then passed into the furnace. Figure 2 illustrates the relationship between the methane concentration and carbon potential at 9250C.

With reference to Figure 1, a gas mixture is formed by passing nitrogen at 300 cubic feet per hour, air at 60 cubic feet per hour and propane at a variable rate to form a gas mixture which is then passed into a heat treatment furnace containing a low carbon ferrous metal at a temperature of 925"C. It can be seen that as the percentage by volume of propane in the mixture is increased from 0 to 2% so the proportion of carbon monoxide and hydrogen in the furnace atmosphere increases. This is because the propane reacts with the oxygen of the air to form hydrogen and carbon monoxide. As the proportion of propane is increased the proportion of carbon dioxide in the furnace atmosphere decreased.

The proportion of carbon monoxide in the furnace atmosphere increases with increasing concentration of propane in the incoming gas until it reaches a maximum which coincides with the proportion of propane in the incoming gas reaching the stoichiometric quantity required for all the oxygen in the air to be converted to carbon monoxide. As shown in Figure 1 this maximum occurs when the proportion of propane in the incoming gas mixture reaches about 2.0 by volume. As the proportion of propane is increased beyond this level so the level of methane and hydrogen in the furnace atmosphere is increased as the volume of propane in the incoming gas mixture is now in excess of the stoichiometric quantity required to react with all the oxygen in the air and as this propane dissociates into methane and hydrogen. In fact, methane is found in the furnace atmosphere just before (at 2% by volume) the content of carbon monoxide in the furnace atmosphere reaches its maximum. The effect of the methane and hydrogen concentrations increasing per unit time without extra carbon monoxide being formed per unit time is to dilute the carbon monoxide in the furnace atmosphere. The volume of carbon monoxide therefore falls from about 6% at 2.0% by volume of propane to about 5.5% by volume at a propane concentration in the incoming gas mixture of 4% by volume.

As shown in Figure 1 the concentration of methane in the furnace atmosphere increases from 0 up to about 1% by volume approximately linearly as the concentration of propane in the incoming gas mixture increases from 2.0% by volume to just over 3.2 /n by volume. Moreover, as shown in Figure 2 the concentration of methane also increases linearly with the carbon potential until this parameter has reached around 1.0% by weight. The rate of increase in the concentration of methane subsequently tends to decrease with a further increase in the concentration of propane in the incoming gas mixture.

At a carbon potential of about 1.4% at 925"C ferrous workpieces become saturated with carbon and as a consequence carbon is deposited as soot on the walls of the furnace and components. It is generally important to control the carburisation such that the metal being carburised does not become saturated with carbon. Monitoring the methane concentration in the furnace atmospheres affords an easy way of doing this.

It is to be appreciated that the relationship between methane concentration and carbon potential as illustrated in Figure 2 is temperature dependent. In general, as the temperature decreases so the slope of the methane v age carbon curve decreases, and similarly, as the temperature increases so the slope of the % by volume methane v % by weight carbon curve increases. The invention is further illustrated by the following example: Example Iron to be carburised is loaded into a batch heat treatment furnace. Nitrogen is passed through the furnace over the iron as the temperature of the iron is raised to 925"C. A gas mixture is formed by mixing 300 standard cubic feet per hour of nitrogen with 60 standard cubic feet per hour of air and 8 standard cubic feet per hour of propane. This mixture is passed into the furnace for a period of 30 minutes. The rate at which propane is supplied is increased manually to 15 standard cubic feet per hour so as to give a carbon potential of 0.8%.

Signals generated by a methane detector in the furnace are recorded on a chart recorder which is in turn set to adjust the rate of flow of propane into the furnace by controlling a motorised valve so as to maintain the carbon potential at 0.8%. The carbon potential is maintained at this value for 3 hours. The supplies of propane and air are then turned off as is the input of heat into the furnace. The carburised iron is then allowed to cool to near ambient temperature in an atmosphere of nitrogen.

This produced a carburised case depth of 0.035 inches.

WHAT WE CLAIM IS: 1. A method of heat treating metal in a heat treatment furnace, including the steps of admitting to the furnace gases which include carbon monoxide, one or more hydrocarbons in gaseous or vapour state, and nitrogen, or which react in the furnace to form a gas mixture containing carbon monoxide, methane and nitrogen, monitoring the concentration of methane in the atmosphere of the furnace, and regulating the carbon potential of the atmosphere in the furnace by so regulating the passage of gases into the furnace that the monitored concentration of methane in the furnace atmosphere is kept equal or close to a chosen value.

2. A method as claimed in claim 1, in which the relative rates at which the respective gases are passed into the furnace are such that the concentration of carbon monoxide in the furnace atmosphere does not fall below 5% by volume.

3. A method as claimed in claim 1 or claim 2, in which there is established in a batch furnace an approximately neutral atmosphere containing about 6% by volume of carbon monoxide and then this atmosphere is. diluted with additional hydrocarbon(s) to give the required carbon potential.

4. A method as claimed in any preceding claim, in which the gases passed into the furnace consist of a gas mixture produced in an endothermic generator diluted by nitrogen.

5. A method as claimed in any of claims I to 3, in which the furnace atmosphere is formed by reacting hydrocarbon with oxygen or a vaporous or gaseous compound containing oxygen, the reaction taking place in a 'carrier gas' of nitrogen.

6. A method as claimed in claim 4 or claim 5 when dependent upon claim 1 or claim 2, in which a carburising treatment or a carbonitriding treatment is performed in the chamber or chambers of a continuous heat treatment furnace, and an atmosphere of approximately constant carbon potential is maintained throughout the operation of the furnace.

7. A method as claimed in any preceding claim in which a detector sensitive to infrared detection is used to monitor the concentration of methane in the furnace atmosphere.

8. A method of heat treating metal, typically ferrous, according to claim 1, substantially as herein described with reference to the drawings.

9. A method of heat treating metal, according to claim 1, substantially as described in the Example.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. rate of flow of propane into the furnace by controlling a motorised valve so as to maintain the carbon potential at 0.8%. The carbon potential is maintained at this value for 3 hours. The supplies of propane and air are then turned off as is the input of heat into the furnace. The carburised iron is then allowed to cool to near ambient temperature in an atmosphere of nitrogen. This produced a carburised case depth of 0.035 inches. WHAT WE CLAIM IS:

1. A method of heat treating metal in a heat treatment furnace, including the steps of admitting to the furnace gases which include carbon monoxide, one or more hydrocarbons in gaseous or vapour state, and nitrogen, or which react in the furnace to form a gas mixture containing carbon monoxide, methane and nitrogen, monitoring the concentration of methane in the atmosphere of the furnace, and regulating the carbon potential of the atmosphere in the furnace by so regulating the passage of gases into the furnace that the monitored concentration of methane in the furnace atmosphere is kept equal or close to a chosen value.