GB1577179A - Heat treatment of metals - Google Patents

Description

(54) HEAT TREATMENT OF METALS (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, and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to method and apparatus for heat treatment of metals. In particular, it relates to method and apparatus for bright annealing metals.

It is necessary to perform a bright annealing process in a protective atmosphere which is reducing in chemical character. It is well known to produce such an atmosphere by cracking ammonia. This forms a gas containing 75viz by volume of hydrogen and 25 /" by volume of nitrogen.

This atmosphere is sufficiently reducing to prevent oxidation of the metal being annealed by leaks of air into the furnace.

The reducing power of the furnace is monitored by measuring the dew point of the atmosphere. Bright annealing of certain metals such as copper, and low carbon steel also be performed in an atmosphere generated by an exothermic generator.

Attempts have been made to simulate cracked ammonia atmospheres by mixing hydrogen and nitrogen. This has economic advantages. Again, for economic reasons it is desirable to keep down the amount of hydrogen used. However, keeping the hydrogen content down increases the reducing power of the furnace atmosphere.

According to the present invention there is provided a method for bright annealing metal, which method comprises passing reducing gas from a source thereof into a heat treatment furnace which contains metal to be bright annealed and which at an annealing temperature passing into the furnace a diluent gas (as hereinafter defined) from a source separate from that of the reducing gas, monitoring the oxygen potential of the atmosphere in the furnace, and controlling the relative proportions of the reducing gas and diluent gas entering the furnace so as to maintain its atmosphere reducing (with respect to the metal) as indicated by the monitored oxygen potential.

The invention also provides apparatus for bright annealing metal, comprising a source of reducing gas; a source of diluent gas (as hereinafter defined), the sources being separate from one another; pipe lines placing the respective sources in communications with the atmospheres of a heat treatment furnace, a flow-control valve associated with each source, an oxygen potential monitoring device in communication with the furnace atmosphere, and means for automatically controlling the setting of one or both the flow-control valves so as to maintain the furnace atmosphere reducing with respect to the metal.

The oxygen potential indicates whether the atmosphere in the furnace is reducing or oxidising, that is whether the reversible reaction: 2M+02 < 2MO proceeds in the forward or reverse direction, where M represents the metal being bright annealed, It is necessary during bright annealing to maintain a reducing atmosphere in the furnace. Graphs of the oxygen potential required for the formation of various metal oxides versus temperature are well known. With the oxidation potential being expressed in kilocalories or kilocalories per mole, at a given temperature, of the oxygen potential is greater (less negative) than that required for the formation of metal oxide, the metal will be oxidised, where if the oxygen potential is less (more negative) there will be no oxidation.

It is conventional for the atmosphere in a bright annealing furnace to be maintained at a relatively constant dew point. We have discovered that the oxygen potential of the atmosphere at constant temperature and constant dewpoint increases slowly with increasing concentrations of diluent gas (and decreasing concentrations of reducing gas) in the atmosphere until the proportion of diluent gas reaches about 90%, after which, the increase in oxidation potential becomes much more rapid with increasing diluent gas concentration. Generally it is possible to keep down or to a minimum the amount of relatively expensive reducing gas generally hydrogen and/or hydrocarbon (eg methane or propane) that is required. The required amount of reducing gas will depend on the composition of the alloy, and the temperature at which it is to be treated.

If the alloy is stainless steel as much as 40 /a by volume of reducing gas may be required.

Since the response times of commercially available oxygen potential measuring devices are relatively rapid it is possible to work at an oxidation potential only a little below that at which the metal being bright annealed is oxidised. In other words, it is possible to work with hydrogen concentrations only a little above the theoretical minimum. When the furnace door is opened to enable more work to be put in the furnace the oxidation potential may tend to rise rapidly. The method and apparatus according to the present invention enables this effect to be counteracted by the supply of a greater proportion of reducing gas in the incoming gas mixture or by increasing the rate of flow of the incoming gas mixture into the furnace. By the term 'diluent gas' as used herein is meant as gas which, excluding impurities, present in small or trace amounts only is neither oxidising nor reducing, though the impurities themselves may be oxidising or reducing. Nitrogen is typically used as the diluent gas, although it is possible to use argon instead. Commercial nitrogen, obtained by the separation of air, inevitably contains traces of oxygen.

Typically nitrogen obtained from a cryogenic air separation plant will contain up to 500 ppm of oxygen. In practice, the oxygen concentration is likely to be much less than 500 ppm around 5 ppm is typical.

The diluent nitrogen is thus slightly oxidising.

Hydrogen or cracked ammonia may be used as the reducing gas.

The metal to be bright annealed may, for example, be stainless steel, nickel, alloys of nickel, (such as those sold under the registered trade mark 'Monel'), and alloys of copper. The invention is particularly suited to the bright annealing of chromium steels.

The oxygen potential monitoring device or instrument may, for example, be that sold under the trade mark 'Thermox'.

The diluent gas and reducing gas are preferably premixed before being passed into the furnace.

The source of diluent gas may be a heatinsulated vessel containing liquid nitrogen, the vessel having an evaporator associated therewith. Alternatively, the source of diluent gas may for example be a 'pressureswing adsorption' or 'low temperature' plant for the separation of air.

Preferably, the reducing gas is supplied to the furnace at a chosen rate, and the rate of supply of diluent gas adjusted in accordance with the sensed oxidation potential. For example, the rate of supply of diluent gas may be adjusted to keep the oxidation potential within chosen limits. By this means it is possible to keep down the total flow of gas through the furnace.

The means for controlling automatically the setting of one or both the flow control valves may be analogous to such means as are conventionally used for using a variable electrical signal for controlling the setting of a flow-control valve.

Typically, each flow-control valve may be a solenoid valve, a motorised valve or variable orifice.

The furnace may be a batch furnace or a continuous furnace. It may have one or more chambers.

The invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1, is a schematic drawing showing apparatus according to the invention: Figure 2 is a graph illustrating how the oxygen potential of an atmosphere consisting of commercial hydrogen and commercial nitrogen varies with the composition of the atmosphere.

Referring to Figure 1 of the drawings a source 2 of commercial nitrogen communicates with a pipeline 6, and a source 4 of commercial hydrogen (and/or hydrocarbon) communicates with a pipeline 8. The pipelines 6 and 8 terminate in a common outlet pipe 10 which also serves as an inlet pipe to a heat treatment furnace 12.

In the pipelines 6 and 8 are non-return valves 14 and 16 respectively. Downstream of the non-return valve 14 is a solenoid-operated flow control valve 18, and downstream of the non-return valve 16 is a solenoid-operated flow control valve 20. In turn, a flow meter 22 is downstream of the valve 18, and a flow meter 24 downstream of the valve 20.

Located in communication with the atmosphere in the furnace 12 is an oxygen potential measuring instrument 26. This is connected in electrical circuit with a programmer 28 which is operative to control the actuation of the solenoid of the valve 18 so as to keep the oxygen potential of the atmosphere in the furnace 12 within limits which may be selected by the operator of the furnace.

In operation, the temperature of the furnace is raised to a bright annealing temperature (eg. 1050 to 11000C the work to be bright annealed therein. A nitrogen, nitrogen-hydrogen or nitrogen-hydrocarbon hydrocarbon atmosphere may be maintained during this period. Once the bright annealing temperature has been reached the oxidation potential of the atmosphere (formed by the gas mixture entering the furnace from the pipe 10) is kept within preselected valves, by virtue of the operation of the oxygen potential measuring instrument 26.

Typically, the temperature of the furnace atmosphere may be kept at a bright annealing temperature for up to 24 hours.

The work may then be allowed to cool to ambient temperature in a nitrogen, nitrogen-hydrogen or nitrogen-hydrocarbon atmosphere.

In Figure 2 is shown the relationship between hydrogen content of a hydrogennitrogen mixture and oxidation (oxygen) potential at various different, but constant dew points. The graph also shows how the oxidation potential of the oxidation reaction of various metals differs with temperature.

Thus, if the dew point of the gas mixture at the working temperature is known, suitable relative proportions of hydrogen and nitrogen can be selected so as to give a gas mixture which is reducing having regard to all the metals in the work being treated.

Moreover the oxygen potential sensor 26 and temperature controller 28 can be set so as always to keep the atmosphere reducing during the treatment.

WHAT WE CLAIM IS: 1. A method of bright annealing metal, which method comprises passing reducing gas from a source thereof into a heat treatment furnace which contains metal to be bright annealed and which is at an annealing temperature; passing into the furnace a diluent gas (as hereinbefore defined) from a source separate from that of the reducing gas; monitoring the oxygen potential of the atmosphere in the furnace, and controlling the relative proportion of the reducing gas and diluent gas entering the furnace so as to maintain its atmosphere reducing (with respect to the metal) as indicated by the monitored oxygen potential.

2. A method as claimed in claim 1, in which the atmosphere contains at least 90 /,, by volume of diluent gas.

3. A method as claimed in claim 1 or claim 2, in which the diluent gas is nitrogen.

4. A method as claimed in any one of the preceding claims, in which the reducing gas is hydrogen.

5. A method as claimed in any one of the preceding claims, in which the rate of introducing the reducing gas into the furnace remains unaltered.

6. A method as claimed in any one of the preceding claims, in which the metal is a high chromium steel.

7. A method as claimed in any one of the claims 1 to 5, claims, in which the metal is stainless steel, nickel, on alloy of nickel copper or an alloy of copper.

8. A method of bright annealing substantially as herein described with reference to the accompanying drawings.

9. Apparatus for bright annealing metal, comprising a source of reducing gas; a source of diluent gas (as hereinbefore defined), the sources being separate from one another; pipelines placing the respective pipelines in communication with the atmosphere of a heat treatment furnace; a flow control valve associated with each source; an oxygen potential monitoring device in communication with the furnace atmosphere, and means for automatically controlling the setting of one or both of the flow control valves so as to maintain the furnace atmosphere reducing with respect to the metal.

10. Apparatus for bright annealing metal, substantially as herein described with reference to, and as shown in, Figure 1 of the accompanying drawings.

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

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. limits which may be selected by the operator of the furnace. In operation, the temperature of the furnace is raised to a bright annealing temperature (eg. 1050 to 11000C the work to be bright annealed therein. A nitrogen, nitrogen-hydrogen or nitrogen-hydrocarbon hydrocarbon atmosphere may be maintained during this period. Once the bright annealing temperature has been reached the oxidation potential of the atmosphere (formed by the gas mixture entering the furnace from the pipe 10) is kept within preselected valves, by virtue of the operation of the oxygen potential measuring instrument 26. Typically, the temperature of the furnace atmosphere may be kept at a bright annealing temperature for up to 24 hours. The work may then be allowed to cool to ambient temperature in a nitrogen, nitrogen-hydrogen or nitrogen-hydrocarbon atmosphere. In Figure 2 is shown the relationship between hydrogen content of a hydrogennitrogen mixture and oxidation (oxygen) potential at various different, but constant dew points. The graph also shows how the oxidation potential of the oxidation reaction of various metals differs with temperature. Thus, if the dew point of the gas mixture at the working temperature is known, suitable relative proportions of hydrogen and nitrogen can be selected so as to give a gas mixture which is reducing having regard to all the metals in the work being treated. Moreover the oxygen potential sensor 26 and temperature controller 28 can be set so as always to keep the atmosphere reducing during the treatment. WHAT WE CLAIM IS:

1. A method of bright annealing metal, which method comprises passing reducing gas from a source thereof into a heat treatment furnace which contains metal to be bright annealed and which is at an annealing temperature; passing into the furnace a diluent gas (as hereinbefore defined) from a source separate from that of the reducing gas; monitoring the oxygen potential of the atmosphere in the furnace, and controlling the relative proportion of the reducing gas and diluent gas entering the furnace so as to maintain its atmosphere reducing (with respect to the metal) as indicated by the monitored oxygen potential.

2. A method as claimed in claim 1, in which the atmosphere contains at least 90 /,, by volume of diluent gas.

3. A method as claimed in claim 1 or claim 2, in which the diluent gas is nitrogen.

4. A method as claimed in any one of the preceding claims, in which the reducing gas is hydrogen.

5. A method as claimed in any one of the preceding claims, in which the rate of introducing the reducing gas into the furnace remains unaltered.

6. A method as claimed in any one of the preceding claims, in which the metal is a high chromium steel.

7. A method as claimed in any one of the claims 1 to 5, claims, in which the metal is stainless steel, nickel, on alloy of nickel copper or an alloy of copper.

8. A method of bright annealing substantially as herein described with reference to the accompanying drawings.

9. Apparatus for bright annealing metal, comprising a source of reducing gas; a source of diluent gas (as hereinbefore defined), the sources being separate from one another; pipelines placing the respective pipelines in communication with the atmosphere of a heat treatment furnace; a flow control valve associated with each source; an oxygen potential monitoring device in communication with the furnace atmosphere, and means for automatically controlling the setting of one or both of the flow control valves so as to maintain the furnace atmosphere reducing with respect to the metal.

10. Apparatus for bright annealing metal, substantially as herein described with reference to, and as shown in, Figure 1 of the accompanying drawings.