The present invention relates to a method and a device for controlling carbon potential of an atmosphere in a heating chamber used for a heat treatment of metals or metal alloys.

Heat treatment is the process of heating and cooling metals using specific predetermined methods to obtain desired properties of the resultant metal feedstock. Both ferrous and non-ferrous metals and/or their alloys undergo heat treatment before putting them to use.

Various gases are used during the heat treatment to protect the metal part from unwanted reactions, such as decarburizing or oxidizing, as well as to facilitate desired reactions, at the elevated temperatures in the heat treatment process cycle.

For most carbon steel materials, the gases used during the heat treatment are usually the mixtures of carbon monoxide (CO), hydrogen (H2), nitrogen (N2), carbon dioxide (CO2), and trace amounts of water vapour. These gases are produced using either endothermic generators (employing natural gas or propane as the carbon source) or are injected directly into the furnace using nitrogen-methanol mixtures.

In the first case, in order to generate a suitable atmosphere, typically, a source of air and a source of hydrocarbon are supplied into a heated retort comprising a catalyst that is used to crack the mixture. When the reacted mixture leaves the retort, it is cooled down to avoid the carbon reformation reaction (where CO decomposes into CO2 and carbon in the form of soot). The cooled mixture is then supplied into the furnace where the metal part is treated.

In the second case, in order to supply a nitrogen-methanol mixture for the heat treatment, three different methods can be employed. In the first method, methanol is introduced into a cracker, where CO and H2 are produced by a pyrolysis reaction, and the resultant carbon monoxide and hydrogen are then fed into the furnace. In the second method, methanol is firstly vaporized in a vaporizer and then fed into the furnace along with the nitrogen gas. In the third method, methanol in liquid state and nitrogen gas are injected directly into the furnace. At present endo gas and nitrogen-methanol gas atmospheres are widely used in the neutral processes, e.g. neutral hardening, annealing or tempering, mostly for carbon steel materials. However, there are limitations to the use of both atmospheres. For example, the use of a retort or generator requires additional investment into the plant equipment, furthermore additional maintenance is required. The cost of producing such atmospheres is another point of concern.

A simpler and less expensive protective atmosphere has been developed, such atmosphere being the mixture of nitrogen and hydrocarbon. It is understood that methane or propane are normally used as a source of hydrocarbon, with propane being preferred. An oxygen probe is employed to monitor residual oxygen in the atmosphere and regulate the hydrocarbon injection which can prevent the oxidation of the part. However, the oxygen probe cannot detect the C-level in the atmosphere which is induced by the direct hydrocarbon injection. This will lead to the presence of over-stoichiometric hydrocarbon in the mixture, and, in turn, to undesired soot formation, corrosion etc.

It is an object of the present invention to provide a method and a device for controlling carbon potential of an atmosphere that at least partially mitigates the problems described above.

One or more of these problems are solved by a method according to independent claim 1 . Advantageous embodiments are defined in the sub-claims.

According to an aspect of the present invention a method of controlling carbon potential of an atmosphere in a heating chamber used for a heat treatment of metals or metal alloys is provided, said method comprising the steps of: providing an inert gas and a hydrocarbon gas into the heating chamber; providing a flow of a fluid into the heating chamber configured to react with the hydrocarbon to create a content of carbon monoxide inside the heating chamber; wherein the fluid is one or more of carbon dioxide, water, oxygen or air and measuring the oxygen content, the carbon monoxide content and the temperature inside the heating chamber, and calculating the carbon potential from the measured oxygen content, the measured carbon monoxide content and the measured temperature, wherein in a first control loop if the carbon monoxide content deviates from a predetermined threshold, the flow of the fluid is adjusted in order to reach the predetermined threshold, and wherein in a second control loop if the calculated carbon potential differs from a predetermined carbon potential, the flow of hydrocarbon is adjusted in order to reach the predetermined carbon potential.

Advantageously, providing the additional flow of fluid that would react with the hydrocarbon causing the formation of the carbon monoxide allows estimation of the content of the CO formed inside the heating chamber. This is important in order to calculate the C-level of the inert gas - hydrocarbon atmosphere in the heating chamber, also known as carbon potential. In C-level measurements, three parameters have to be measured: temperature, CO concentration and oxygen concentration. As the inert gas - hydrocarbon atmosphere contains no CO, it is therefore not possible to measure the amount of carbon monoxide in the heating chamber. Introduction of the additional fluid that can react with the hydrocarbons and form CO allows all three parameters needed for the C-level calculation to be obtained and, in turn, to precisely measure the amount of carbon-containing species in the atmosphere inside the heating chamber. This helps avoiding decarburization of the metal or metal alloy. Furthermore, it is also possible to adjust the amount of carbon monoxide in the atmosphere. Advantageously, it allows control of the atmosphere in the heating process and make sure the appropriate quality of the metal part is achieved.

In an embodiment, the predetermined threshold for the content of carbon monoxide is between 1 ,000 and 50,000 ppm. The predetermined threshold is selected such that sufficient CO is created in the heating chamber to make the atmosphere controllable. Advantageously, various levels of carbon content inside the heating chamber can be achieved.

The fluid is any one or more of carbon dioxide, water, oxygen or air. Advantageously, a wide selection fluids allows conversion of the hydrocarbon present in the heating chamber into CO in a variety of ways. The conversion can follow a variety of paths, such as, using methane as an example:

2CH4 + 02 (also from air) 2CO + 4H2

CH4 + H2O - CO + 3H2

It is understood, that if higher hydrocarbons are used, the reactions will follow similar paths and respect similar stoichiometry.

In yet another embodiment, the inert gas is any of nitrogen, argon, helium, or mixtures thereof.

In an embodiment, the hydrocarbon is any of methane, propane, butane, propylene, or mixtures thereof. Advantageously, by using various gases and/or their mixtures it is possible to create an optimal inert atmosphere with the most suitable characteristics that are tailored to the material to be heat treated.

In another embodiment, the metals or the metal alloys are ferrous. In another embodiment, the metals or the metal alloys are non-ferrous. Advantageously, by using various metals and/or their alloys it is possible to cover a wide range of metals that can be used in a variety of applications.

In an embodiment, the method further comprises measuring an oxygen content by means of an oxygen sensor. The presence of the oxygen sensor allows information to be received on the quantity of the oxygen gas and thus allowing calculation of the carbon potential value of the heating process.

In another embodiment, the method further comprises measuring a moisture content of the atmosphere in the heating chamber. Advantageously, the presence of the moisture, or dewpoint, sensor allows estimation of the moisture inside the heating chamber, and, in turn, to estimate carbon potential via the gas shift reaction (H2O + CO -> CO2 + H2). It is understood that the lower the dewpoint or oxygen level, the higher is the carbon potential. In the other words, when water reacts with CO the carbon potential is decreased.

In an embodiment, the heating chamber is heated to 500 - 1 ,050 °C. Advantageously, it allows heat-treatment of a wide variety of metals or metal alloys. The choice of the temperature is dictated by the nature of the metal or metal alloy to be heat treated. The inventive method might be carried out in an apparatus comprising: a heating chamber for receiving the metals or the metal alloys; at least one conduit for supplying an inert gas, a hydrocarbon and/or an inert gas - hydrocarbon mixture into the heating chamber; at least one carbon monoxide sensor configured to determine a content of carbon monoxide inside the heating chamber; at least one oxygen sensor configured to determine a content of oxygen in the heating chamber; at least one temperature sensor configured to determine the temperature in the heating chamber; a controller configured to calculate the carbon potential based on the content of oxygen, the content of carbon monoxide and the temperature in the heating chamber, and a first controller configured to adjust the flow of the fluid if the content of carbon monoxide in the heating chamber deviates from a predetermined threshold, and a second controller configured to adjust the flow of hydrocarbon if the calculated carbon potential differs from a predetermined carbon potential.

In an embodiment, the at least one conduit comprises a first conduit for supplying the inert gas and a second conduit for supplying the hydrocarbon. Advantageously, the presence of separate conduits allows the introduction of the inert gas and hydrocarbon through an individual conduit, thus allowing better control of the carbon species introduced into the heating chamber, and, in turn, better control over the carburization during the heating process.

In another embodiment, the apparatus further comprises a moisture sensor for determining moisture content of the atmosphere in the heating chamber.

The invention is explained below with the aid of an embodiment shown in the drawings which show:

Figure 1 -a schematic view of a prior art system for the precise metering of nitrogen and propane;

Figure 2 -a schematic view of the system for supplying and controlling of the process gas; and Figure 3 -a flow chart illustrating a method of heat-treating a metal or a metal alloy in an embodiment of the present invention.

Figure 1 shows a prior art apparatus 100 for heat treatment of metal and/or metal alloys. The apparatus 100 comprises a supply that provides a material 101 into the system. It is understood that the material 101 can be metal or metal alloy. It is also understood that the apparatus can be without limitation a roller hearth furnace apparatus, although other types of heat treatment apparatuses can also be employed, such as pusher furnaces, conveyor furnaces, tubular strand furnaces, chamber furnaces, sealed quenching furnaces or multiple purpose furnaces.

The material 101 is then supplied into a heating section 103 or heating chamber where it is heated to a predetermined temperature. An external supply of nitrogen 104 and an external supply of hydrocarbon 105 are also provided within the apparatus 100. The supply of nitrogen carrier gas and a hydrocarbon are then pre-mixed to be supplied into the heating chamber as a nitrogen - hydrocarbon mixture 106.

The heating chamber 103 is further provided with a thermocouple 107 to measure the temperature inside the heating chamber 103 and an oxygen sensor 108 to measure the quantity of oxygen present inside the heating chamber 103.

The apparatus 100 is further provided with a controller 109 which controls the supply of the nitrogen - hydrocarbon mixture 106 into the heating chamber 103, based on the reading of the temperature and oxygen content from the thermocouple 107 and the oxygen sensor 108 respectively.

The heat-treated material 101 is then supplied into an oil bath 110 containing agitated mineral oil where it is quenched to achieve the desired physical properties. It is understood by the skilled person that any other means for cooling/quenching can be used at this step. After this step, the material 101 can also be further optionally subjected to washing and/or tempering processes (not shown).

Figure 2 shows an apparatus 200 according to the invention for supplying and controlling of the process gas atmosphere during the heat-treatment process. The apparatus 200 comprises a furnace, or a heating chamber 201 , where a metal or metal alloy can be heated. The temperature in the heating chamber 201 is selected dependent on the type of metal or metal alloy to be heated. It is understood that the source of metal and/or metal alloy can be ferrous or non-ferrous. It is also understood that the heating chamber can be heated to 500 - 1 ,050 °C.

The apparatus further comprises at least one conduit for supplying an inert gas - hydrocarbon mixture into the heating chamber 201. In an alternative embodiment, inert gas and hydrocarbon can be supplied via different conduits. In this embodiment, the apparatus 200 comprises a first conduit 202 for providing a source of inert gas, and a second conduit 203 for supplying a source of hydrocarbon.

The inert gas can be any of nitrogen, argon, helium, or mixtures thereof. In turn, the source of hydrocarbon can be any of methane, propane, butane, propylene, or mixtures thereof.

The apparatus 200 further comprises a third conduit 204 for providing a flow of fluid that is configured to react with the hydrocarbon to create a content of carbon monoxide inside the heating chamber. The fluid can be any one or a mixture of carbon dioxide, water, oxygen or air.

It will be understood by a skilled person that the choice of the inert gas, the hydrocarbon and the reactive fluid is dictated by the composition of the metal part to be heat-treated.

The apparatus 200 comprises a carbon monoxide (CO) sensor 205. The carbon monoxide sensor is used to detect the presence of carbon monoxide inside the heating chamber 201 . In an exemplary embodiment, the carbon monoxide sensor can be an IR (infrared) sensor. The carbon monoxide is formed inside the heating chamber 201 as a rection product between the hydrocarbon and the reactive fluid. The chemical reactions that correspond to the transformation of the hydrocarbon into CO are described above. It is understood that the amount of carbon monoxide inside the heating chamber 201 is expected to be in the range between 1 ,000 and 50,000 ppm.

The apparatus 200 further comprises an oxygen sensor 206. The oxygen sensor 206 is configured to measure the amount of oxygen present inside the heating chamber. In an exemplary embodiment, the oxygen sensor 206 can be a zirconia oxygen sensor, however other types of oxygen sensors are also contemplated. These sensors can be, without limitation, A-probes, electrochemical sensors, optical sensors, galvanic sensors and the like.

In an alternative embodiment, the apparatus 200 can comprise a dew point sensor instead of the oxygen sensor 206. The dew point sensor can be used to determine the carbon potential value through the water shift reaction described above.

The apparatus 200 further comprises a temperature sensor 207 that is configured to measure the temperature inside the heating chamber 201 . The temperature sensor 207 can be, without limitation, any of thermocouples, RTDs (resistance temperature detectors), thermistors, or semiconductor based integrated circuits (IC).

The apparatus 200 further comprises a controller 208 The controller 208 is configured to receive the information from the oxygen, carbon monoxide and temperature sensors (205, 206, 207) and calculate the carbon potential, or C-level, using known conventional calculation methods. If the carbon potential deviates from a predetermined carbon potential, the controller is configured to adjust the flow of the hydrocarbon gas or the flow of the inert gas to reach the pre-determined carbon potential.

Two control loops are implemented. A first control loop ensures that the CO content is the atmosphere is always sufficient high to make the atmosphere controllable. Therefore, the CO content in the heating chamber is measured and compared to a predetermined threshold. If the carbon monoxide content deviates from the predetermined threshold, the flow of the fluid is adjusted in order to reach the predetermined threshold. A second control loop is provided in order to control the carbon potential of the atmosphere in the heating chamber. If the calculated carbon potential differs from a predetermined carbon potential, the flow of hydrocarbon is adjusted in order to reach the predetermined carbon potential.

The first control loop has priority over the second control loop. In case of conflicting controls the first control loop has priority. Thereby, it is always guaranteed that the carbon monoxide content of the atmosphere is such that the atmosphere is controllable.

For example, when the calculated C-level or carbon potential is too high for the material to be heat-treated, the controller is configured to increase the amount of inert gas to decrease the carbon potential. Inversely, when the carbon potential is too low, the controller is configured to increase the flow of hydrocarbon to increase the carbon potential. Advantageously, this allows provision of the appropriate amount of hydrocarbon and thus avoids oxidizing, decarburizing and sooting. As the amount of hydrocarbon is carefully controlled, secondary effects such as corrosion are also avoided. Furthermore, a corresponding apparatus for the present invention is easy to set up and requires less maintenance compared to the known conventional set ups. This allows the minimization of production costs when compared to the generated or methanol based atmospheres.

Figure 3 shows a method 300 which is implemented by a controller. In step 301 a supply of a nitrogen - hydrocarbon mixture occurs via at least one conduit. It is understood by the skilled person, that separate or a single conduit can be used to supply the nitrogen and the hydrocarbon into the furnace. In step 302 the controller is arranged to supply a source of metal or metal alloy into the furnace. In step 303 the metal is subjected to heating at a predetermined temperature for a predetermined time. Optionally, in step 304 another control loop is present which determines whether the required CO level is reached. If it is reached, the process proceeds to step 304. If it is not reached, the flow of fluid that is configured to react with the hydrocarbon needs to be adjusted. In step 305 the controller is configured to measure the amounts of oxygen, CO and the temperature inside the heating chamber and calculate the C-level based on these three parameters. In step 306 the controller estimates the C-level, and if it is as required (which is dependent on the type of metal/metal alloy used), the process proceeds to step 307, where the material is quenched/cooled and, optionally, subsequently tempered. If, however, the C-level exceeds the predetermined threshold, or is insufficient, the controller is configured to adjust the amount of nitrogen and/or hydrocarbon supplied to the furnace and re-measure the C-level as per step 305. Once all the previous steps 301 -307 are completed, the process ends 308. It will be appreciated that embodiments of the present invention can be realised in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims. Reference sions

100 prior art apparatus

101 material

103 heating section

104 external supply of nitrogen

105 external supply of hydrocarbon

106 nitrogen-hydrocarbon mixture

107 thermocouple

108 oxygen sensor

109 controller

200 apparatus

201 heating chamber

202 first conduit

203 second conduit

204 third conduit

205 sensor

206 oxygen sensor

207 temperature sensor

208 controller