WO2014009727A1

WO2014009727A1 - Method for treating elongated metal product by heating and oxidizing the surface in a controlled environment

Info

Publication number: WO2014009727A1
Application number: PCT/GB2013/051835
Authority: WO
Inventors: Maximus Akuh; Guido Plicht; Kris WILOCH; Brian CASHMORE
Original assignee: Kts Wire Ltd; Air Products Gmbh
Priority date: 2012-07-10
Filing date: 2013-07-10
Publication date: 2014-01-16
Also published as: EP2872657A1; US20150203950A1; MX2015000461A; GB201212251D0

Abstract

A method and apparatus are provided for making a metal product with improved surface characteristics. The method provides for treating the metal product through a method comprising: heating the metal product; exposing the outer surface of the metal product to a controlled environment; and oxidising the outer surface of the metal product. The heating may take the metal product to an elevated temperature, the elevated temperature of the metal product being at least 725°C. The method further comprises a controlled environment around the metal product which is controlled with respect to the atmosphere around the metal product, and in particular the nitrogen and/or oxygen and/or hydrogen content thereof. The result is a more consistent surface for elongate metal products, having a more consistent colour to the surface, a more durable and hard wearing surface and/or a surface which is not prone to flaking.

Description

METHOD FOR TREATING ELONGATED METAL PRODUCT BY HEATING AND OXIDIZING

THE SURFACE IN A CONTROLLED ENVIRONMENT

This invention concerns improvements in and relating to products and methods for making them, particularly with reference to the surface properties thereof. Elongate products, such as wire products, are of particular interest.

In existing techniques for the production and treatment of extremely elongate metal products, such as wire products, there are problems with the colour and consistency of the surface produced.

Attempts have been made to improve the position through the use of chemical treatments and the additional of salts and the like. However, such approaches have either been unsuccessful, for instance due to the resultant surface flaking off) or undesirable for other reasons (for instance because of the cost of the extensive modifications to the conventional process that they require to implement).

The present invention has amongst its possible aims to address such shortcomings. The present invention has amongst its possible aims to provide benefits to the products without expensive or extensive alteration to the conventional processes.

According to a first aspect of the invention we provide a method for treating a metal product, the method comprising:

a) heating the metal product;

b) exposing the outer surface of the metal product to a controlled environment; and c) oxidising the outer surface of the metal product.

The method of treating may include a first process stage. The first process stage may provide the heating for the metal product.

The heating may be provided in one or more furnaces. One or more or all of the furnaces may be induction furnaces. The heating may be provided by the temperature profile within the furnace. The heating may take the metal product from an ambient temperature to an elevated temperature. The elevated temperature of the metal product may be at least 725°C, possibly at least 750°C, preferably at least 850 °C, more preferably at least 875 °C, ideally at least 890°C. The elevated temperature may be at most 1400°C, preferably at most 1380°C. The elevated temperature of the metal product may be in the range 700°C to 1400°C, 13 051835 preferably the range 725°C to 950°C, more preferably 820°C to 900°C or still more preferably 825°C to 855°C

The controlled environment may be controlled with respect to temperature, for instance such that a temperature range is maintained.

The controlled environment may be controlled with respect to the atmosphere around the metal product, for instance the nitrogen and/or oxygen and/or hydrogen content may be controlled.

The controlled environment may provide one form of controlled environmental conditions provided in it. Preferably the controlled environment provides a consistent temperature ranges and/or mixtures of gases within it.

The controlled environment may be provided within a container provided in the first process stage. The container may be a tube or other elongate container. The wire may pass through the container, for instance from a container entrance to a container exit. The container entrance and/or exit may be profiled to correspond to the metal products profile, for instance with a small tolerance, for instance so as to minimise gas leakage from the controlled environment.

Separate controlled environments may be provided for separate strands of the metal product or a common controlled environment may be provided for multiple strands of the metal product.

The controlled environment may be provided with a gas inlet and/or gas outlet. The controlled environment may be provided with a gas inlet proximal to the inlet for the metal product to the controlled environment. The gas inlet and the inlet for the metal product may be separate from one another. The controlled environment may be provided with a gas outlet proximal to the outlet for the metal product from the controlled environment. Preferably a common outlet is provided.

The gas inlet may feed a gas mixture to the controlled environment from a gas feed system. The gas feed system may be provided with a nitrogen supply and/or oxygen supply and/or hydrogen supply. Preferably one or more or all of the gases are provided in single element form. Preferably separate supplies of nitrogen and oxygen are provided, but a common supply, for instance an air supply could be used. The supplies may be provided by pressurised storage tanks.

The nitrogen supply may be provided at a pressure of at least 10 BAR in the store, for instance gas cylinders. The oxygen supply may be provided at a pressure of at least 10 BAR in the store, for instance gas cylinders.

The hydrogen supply may be provided at a pressure of at least 10 BAR in the store, for instance gas cylinders.

The gas supplies may connected to a mixer. The mixer may control the proportions of the oxygen and/or nitrogen and/or hydrogen in the gas mixture.

The ¾ to 0₂ ratio in the total gas mixture may be between 1.5: 1 and 4: 1, preferably between 1.6:1 and 3.7: 1, more preferably between 1.65:1 and 3.5:1. The H₂ to 0₂ ratio in the total gas mixture may be at least 1.5: 1, preferably at least 1.6:1, more preferably at least 1.65:1. The H₂ to 0₂ ratio in the total gas mixture may be at most 4:1, preferably at most 3.7:1, more preferably at most 3.5:1.

According to one preferred form, for instance producing a black surface finish, the H₂ to 0₂ ratio in the total gas mixture may be between 1.5:1 and 2: 1, preferably between 1.6:1 and 1.85:1, more preferably between 1.65:1 and 1.8: 1. The H₂ to 0₂ ratio in the total gas mixture may be at least 1.5:1, preferably at least 1.6:1, more preferably at least 1.65:1. The ¾ to 0₂ ratio in the total gas mixture may be at most 2:1, preferably at most 1.85: 1, more preferably at most 1.8: 1.

According to another preferred form, for instance producing a blue surface finish, the H₂ to 0₂ ratio in the total gas mixture may be between 3.2:1 and 3.7:1, preferably between 3.3:1 and 3.6:1, more preferably between 3.35:1 and 3.5:1. The H₂ to 0₂ ratio in the total gas mixture may be at least 3.2: 1 , preferably at least 3.3:1, more preferably at least 3.35:1. The H₂ to 0₂ ratio in the total gas mixture may be at most 3.7:1, preferably at most 3.6:1, more preferably at most 3.5: 1.

The oxygen proportion may be between 0.7% and 2.3% of the total gas mixture, preferably between 0.8% and 2.2%, more preferably between 0.9% and 2.1%.

The hydrogen proportion may be between 3% and 3.9% of the total gas mixture, preferably between 3.2% and 3.7%, more preferably between 3.3% and 3.6%.

According to one preferred form, for instance producing a black surface finish, the oxygen proportion may be between 1.7% and 2.3% of the total gas mixture, preferably between 1.8% and 2.2%, more preferably between 1.9% and 2.1 %. The hydrogen proportion may be between 3% and 3.9% of the total gas mixture, preferably between 3.2% and 3.7%, more preferably between 3.3% and 3.6%.

According to another preferred form, for instance producing a blue surface finish, the oxygen proportion may be between 0.7% and 1.3% of the total gas mixture, preferably between 0.8% and 1.2%, more preferably between 0.9% and 1.1%. The hydrogen proportion may be between 3% and 3.9% of the total gas mixture, preferably between 3.2% and 3.7%, more preferably between 3.3% and 3.6%.

The nitrogen proportion may represent the balance after the oxygen proportion and hydrogen proportion.

The pressure in the gas mixer may be under 10BAR, for instance under 5 BAR.

The gas mixture may be fed from the gas mixer to a reactor. The gas mixture may be fed to the reactor directly or more preferably via a buffer tank. A catalytic reactor may be provided. The reactor may cause the oxygen and hydrogen to at least partially react. The reactor may cause the production of water. The reaction may consume all of the oxygen.

After the reaction and/or after exiting the reactor, the gas mixture may have a temperature of between 100°C and 280°C, preferably between 150°C and 230°C, more preferably between 165°C and 210°C. The temperature and/or other conditions are preferably such that no liquid water is present.

The gas mixture may pass through a heat exchanger before reaching the controlled environment. The heat exchanger may be used to control the temperature of the gas mixture, for instance to maintain the gas mixture below a temperature limit.

The gas mixture may have a pressure of less than 1 BAR when entering the controlled environment, for instance between 0.3 and 0.6 BAR.

The method of treating may provide that the surface of the metal product oxidises in the first process stage, for instance in a first reaction, preferably within the controlled environment. The oxidation may be to the form Fe₃0₄ and/or according to the reaction form 3Fe + 4H₂0 -> Fe₃0₄ + 4H₂. This oxidation may be completed in the controlled environment.

The method of treating may provide that the surface of the metal product is reduced in the first process stage, for instance in a second reaction, preferably with the second reaction occurring after the first reaction, preferably within the controlled environment. The reduction may be to the form FeO and/or according to the reaction of form Fe₃0₄ + H₂ -> 3FeO + H₂0. This reduction may be completed in the controlled environment.

The metal product may be pre-formed before the use of the method of treating. The method of treating may alter one or more surface properties and/or characteristics of the preformed metal product. The method of treating may alter one or more internal properties and/or characteristics of the pre-formed metal product. The method of treating may alter one or more properties and/or characteristics for the whole of the pre-formed metal product. The metal product may be fed to the method of treating from a storage location. The storage location may be one or more coils.

Preferably multiple strands of the metal product are fed through the method of treating in parallel with one another.

The method of treating may include a quenching stage. The quenching stage may be provided after the first process stage. The quenching stage may be provided immediately after the first process stage. The quenching stage may harden the metal product. The quenching stage may increase or provide Martensite content for the metal product. The quenching stage may be provided with oil. The oil may be circulated and/or may be cooled. The metal product may be immersed in the oil as it passes through the quenching stage.

The temperature of the metal product on entering the quenching stage may be at an exit elevated temperature. The exit elevated temperature of the metal product may be at least 725°C, possibly at least 750°C, preferably at least 850°C, more preferably at least 875°C, ideally at least 890°C. The exit elevated temperature may be at most 1400°C, preferably at most 1380°C. The exit elevated temperature of the metal product may be in the range 700°C to 1400°C, preferably the range 725°C to 950°C, more preferably 820°C to 900°C or still more preferably 825°C to 855°C. The temperature of the metal product on leaving the quenching stage may be less than 50°C, more preferably less than 35°C.

The method of treating may include a further immersion stage. The further immersion stage may be provided after the first process stage and/or after the quenching stage. The further immersion stage may be provided immediately after the quenching stage. The further immersion stage may treat the metal product, for instance to temper the metal product. The further immersion may increase the ductility and/or remove internal stresses in the metal product. The further immersion stage may be provided with molten lead. The molten lead may be circulated and/or may be heated. The metal product may be immersed in the molten lead as it passes through the further immersion stage.

The temperature of the metal product on entering the further immersion stage may be less than 50°C, more preferably less than 35°C. The temperature of the metal product on leaving the further immersion stage may be in the range 370°C to 650°C, more preferably in the range 390°C to 610°C.

The method of treating may include a further quenching stage. The further quenching stage may be provided after the first process stage and/or after the quenching stage and/or after the further immersion stage. The further quenching stage may be provided immediately after the further immersion stage. The further quenching stage may harden the metal product. The further quenching stage may be provided with water and/or oil, for instance as oil dissolved in water. The oil may be circulated and/or may be cooled. The metal product may be immersed in the oil as it passes through the further quenching stage.

The temperature of the metal product on entering the further quenching stage may be in the range 350°C to 650°C or more preferably 390°C to 610°C. The temperature of the metal product on leaving the further quenching stage may be less than 80°C, more preferably less than 65°C.

The method of treating may provide a metal product cleaning stage, for instance after the further quenching stage and/or before a product storage stage. The metal product cleaning stage may remove material from the metal product. The metal product cleaning stage may be provide an air wipe.

The method of treating may provide a rust preventer application stage, for instance after the further quenching and/or metal product cleaning stage and/or before a product storage stage. The rust preventer may be applied by spraying and/or wetting and/or immersion in the rust preventer.

The method of treating may fed the metal product, after one or more treatments, to a metal product storage stage, such as one or more coils.

The method of treating may provide that the characteristics and/or properties of the surface do not alter after the quenching stage, particularly with respect to the oxidisation of the surface. The method of treating may provide that no or substantially no, for instance less than 1%, of the oxidation of the surface of the metal product occurs after the further quenching stage, preferably after the tempering stage and most preferably after the quenching stage.

The first aspect of the invention may include any of the features, options and possibilities set out elsewhere in this document, including in the other aspects of the invention.

According to a second aspect of the invention we provide apparatus for treating metal product, the apparatus including

a) a heating stage for heating the metal product;

b) a controlled environment, to which the outer surface of the metal product is

exposed;

c) an oxidising stage for oxidising the outer surface of the metal product. The second aspect of the invention may include any of the features, options and possibilities set out elsewhere in this document, including in the other aspects of the invention.

According to a third aspect of the invention we provide a method for treating a metal product, the method comprising:

a) heating the metal product;

b) exposing the outer surface of the metal product to a controlled environment;

c) oxidising the outer surface of the metal product;

d) quenching the metal product; and

e) tempering the metal product.

The third aspect of the invention may include any of the features, options and possibilities set out elsewhere in this document, including in the other aspects of the invention.

According to a fourth aspect of the invention we provide apparatus for treating a metal product, the apparatus including

a) a heating stage for heating the metal product;

b) a controlled environment, to which the outer surface of the metal product is

exposed;

c) an oxidising stage for oxidising the outer surface of the metal product;

d) a quenching stage for quenching the metal product; and

e) an tempering stage for tempering the metal product.

The fourth aspect of the invention may include any of the features, options and possibilities set out elsewhere in this document, including in the other aspects of the invention.

According to a fifth aspect of the invention we provide a metal product, the metal product being a heated, quenched and tempered metal product.

The metal product may be formed of plain carbon steel and/or steel alloy. The metal product may be a wire. The metal product may be a rolled wire. The metal product may be a shaped wire. The metal product may be a flat wire. The metal product may have a width of between 0.2mm and 50mm, more preferably between 1.5mm and 22mm. The metal product may have a thickness of between 0.2mm and 10mm, more preferably between 0.4mm and 8mm.

The metal product may have round edges or square edges. The metal product may have a Hardness Rockwell C Scale RC of 20 to 60, preferably 35 to 60, more preferably 40 to 55, for instance with a 5 point range. The metal product may have a tensile strength of 1000 to 2000N/mm², more preferably 1250 to 1750N/mm².

The surface of the metal product may not form any flakes, for instance when scratched with a knife.

The surface of the metal product may be free from scales, for instance when visually inspected.

The surface of the metal product may be consistent in colour, for instance greater than 90% of the surface being of the same colour, preferably greater than 98%, more preferably greater than 99% and ideally greater than 99.9%. The colour may be black. The colour may be dark blue. The colour may be brown or straw.

The metal product, particularly the surface of the metal product may have a shelf life of at least 5 days, preferably at least 8 days, more preferably at least 12 days and ideally at least 20 days, before visible rust forms on the surface, for instance when stored at 15°C and 80% humidity.

The fifth aspect of the invention may include any of the features, options and possibilities set out elsewhere in this document, including in the other aspects of the invention.

Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying figures in which:

Figure 1 is a schematic illustration of the process stages in a conventional wire treatment process;

Figure 2 is a schematic illustration of the process stages of a wire treatment process according to an embodiment of the invention;

Figure 3 is a schematic detailed illustration of the controlled environment of an embodiment of the invention;

Figure 4 is a schematic illustration of a gas feed system; and Figure 5 is a schematic illustration of an alternative gas feed system.

Various techniques are known for the production and treatment of extremely elongate metal products, such as wire products. A consistent property of these production and treatment techniques is that the surface which forms on the metal product is inconsistent. This can manifest itself in terms of the colour of the surface in particular. The surface is frequently uneven in colour and with a sheen similar to that seen when oil spreads on water. The inconsistency of the surface can also manifest itself in terms of the surface being a heavy scale which has a tendency to flake off with use and/or time.

The uneven properties of the surface are undesirable in a number of respects. The uneven colour impairs the visibility of any markings applied to the metal product, such as the distance markings on a tape measure. The uneven properties can also shorten the shelf life of the product before visible weathering or even rusting has occurred. This is an issue with respect to the storage of such metal products between production and subsequent use in a further stage.

The present invention has amongst its possible aims to provide a more consistent surface for elongate metal products. The present invention has amongst its possible aims to provide a more consistent colour to the surface of elongate metal products. The present invention has amongst its aims to provide a more durable and hard wearing surface and/or a surface which is not prone to flaking. The present invention has amongst its possible aims to provide a surface which is more resistant to oxidation and/or rusting and/or weathering.

In a conventional wire treatment process, Figure 1, a series of parallel wires 1 are feed through the same treatment stages to give maximum wire throughput for the process stages provided.

The pre-produced wire 1 is received coiled. The coiled wire 1 is fed from the coil 3 into the first process stage 5 and carries on through all the stages and then on to a coil 27 after the last process stage. Periodically new coils 3 are spliced to or otherwise fed to form the wire strand going to the first process stage 5. Periodically full coils are cut from the wire strand after the last process stage and new coils of finished product started.

The first process stage 5 is an induction heated furnace 7. This is used to raise the temperature of the wire strand to the desired level. The temperature profile within the furnace may be used to raise the temperature to the ultimate level needed. The temperature within the first process stage 5 prevents oxidation of the wire 1. Upon leaving the first process stage 5, the hot wire 1 is quickly quenched in a quenching stage 9. This is done by immersing the wire 1 in a bath 11 of cooled oil 13. The oil 13 prevents oxidation of the wire 1 and the exposure of the wire 1 between leaving the furnace 7 and entering the oil 13 of the oil bath 11 is short enough for there to be no material oxidation of the wire 1.

The combination of heating and quenching is used to give the desired internal structure for the wire 1, namely a martensite.

From the quenching stage 9, the wire 1 proceeds into a further immersion stage 15 in the form of a lead bath 17. The molten lead 19 in this bath 17 is used to heat the wire 1 and cause tempering. This prevents the brittleness which would otherwise be present. The heat applied to the wire 1 in the bath 17 together with moisture cause the oxidation of the surface of the wire 1 before it reaches the next stage.

From the further immersion stage 15, the wire 1 proceeds into a final quench stage 21. The final quench stage 21 uses water and a soluble oil in combination to reduce the wire temperature quickly.

The cooled wire 1 then passes to an air wipe cleaning stage 23, a rust preventer application stage 25 and then onto the final coil 27.

Oxidation for the wire occurs primarily after the further immersion stage 15 and before the final quench stage 21.

In the wire production process of the present invention, the minimal changes possible are made to the overall process, so as to minimise the capital and operating costs for the revised process. As a result, many of the stages are the same as in the conventional method and the sequence in which they are provided is the same. However, the environment provided and materials added to certain of the stages is greatly changed and with very significant results.

Once again, the pre-produced wire 1 is received coiled and is fed to the first process stage 5. The first process stage 5 is again an induction heated furnace 7.

In the furnace 7 the wire is heated according to the temperature profile provided. Unlike in the previous approach, as shown in Figure 3, the wire 1 enters a tube 53 whose axis 55 is aligned with the direction of movement arrow A of the wire 1. Only one tube 53 is shown in Figure 2 for clarity reasons. The tube 53 is provided with an inlet 57 profiled to receive the wire 1 and minimise gas mixture 59 loss from inside the tube 53. A similar structure is provided for the outlet 61 at the end of the first process stage. The wire 1 is inside the tube 53 throughout its time in the furnace 7.

On entry into the furnace 7, the temperature of the wire 1 is quickly raised from an ambient temperature, for instance 20°C, to the controlled treatment temperature of between 725°C and 1450°C.

As the wire 1 is passing along the axis of a tube, as shown in Figure 3, the wire 1 is surrounded by the gas mixture 59 fed to the tube through a gas inlet 65. The gas inlet 65 is connected to the gas feed 67 system described in more detail below. The role of the gas mixture 59 is also described in detail below.

The tube 53 and wire 1 pass through the furnace and are maintained at the desired temperature there and prior to reaching the bath 11. As an option, it would be possible to provide the controlled conditions within a tube 53 provided after the furnace 7 and prior to reaching the bath 11, or combinations thereto.

The controlled conditions within the tube 53 mean that oxidation occurs here. The temperature and the controlled conditions control the oxidation. This is a fundamental difference compared with the conventional approach where oxidation occurs in the much later stages and the furnace is just used for heating.

Upon leaving the first process stage, and the tube 53, the hot wire 1 is quickly quenched. This is done by immersing the wire 1 in a bath 11 of cooled oil 13. The oil 13 and temperature reduction prevent further oxidation. During the short exposure between leaving the furnace 7 and entering the oil 13 some further oxidation may occur due to the conditions the wire 1 has already been exposed to and/or the reactions already underway.

The combination of heating and quenching is used to give the desired internal structure for the wire 1 , namely a martensite, as in the conventional process. However, the surface has been materially altered relative to the surface at this stage in the conventional process.

From the quenching stage 9, the wire proceeds into a further immersion stage 15 in a lead bath. The molten lead 19 in this bath 17 is used to heat the wire 1 and cause tempering. This prevents the brittleness which would otherwise be present.

From the further immersion stage 15, the wire proceeds into a final quench stage 21. The final quench stage 21 uses water and a soluble oil in combination to reduce the wire temperature quickly.

The cooled wire 1 then passes to an air wipe cleaning stage 23, a rust preventer application stage 25 and then onto one of the final coils 27. Unlike in the conventional process, the oxidation for the wire occurs primarily in the earlier stages of the process and no, or substantially no, oxidation occurs after the final quench stage 21.

As mentioned above, the gas inlet 65 receives a gas mixture 59 from a gas feed system 67, as shown in Figure 4. The gas feed system 67 is provided with a nitrogen storage tank 69, a hydrogen storage tank 71 and an oxygen storage tank 73. The three storage tanks 69, 71, 73 are connected to a gas mixer 75. The nitrogen fed to the gas mixer 75 from the compressed gas cylinders, reduces in pressure from around 300BAR (from a full cylinder) to 9.5 BAR in the gas mixer 75. The hydrogen fed to the gas mixer 75 from the compress gas cylinders, reduces in pressure from 172BAR to 9.0 BAR.

The gases are mixed in the gas mixer 75 to give the gas mixture 59 in the form of 2.7% H₂ + 1.2% 0₂ + 96.1% N₂. The gas flow is reduced to 10-20m³/h and introduced, via a buffer tank 77, to a catalytic reactor 79. The gas mixer 75 is connected to the buffer tank 77 to ensure a steady supply of the gas mixture 59 to the controlled environment in the tube 53.

In the catalytic reactor 79, the oxygen is reacted with the hydrogen. As a result of this reaction, H₂0 + H₂ + N₂ forms the gas stream and the temperature average of 204°C is produced.

If higher temperatures for the gas stream are desired, then a heat exchanger (not shown) can be used. The gas valves and gas regulators are unable to handle the gas stream as hot as 230°C and so the heat would be introduced after it had passed through those pieces of equipment, but before entering the tubes 53.

When the gas mixture 59 is introduced to the tubes 53 which define the controlled environment in the furnace, the gas mixture 59 is at 80°C to 230°C and the gas mixture 59 has a dew point of around 20°C. The combination of the gas temperature and the dew point means that the H₂0 is maintained as steam within the tubes 53.

All the oxygen from the water is used in the first oxidation reaction. This reaction starts once the necessary temperature threshold is reached and given the presence of the gas mixture. This part of the process also provides some initial hardening. The reaction can take place in temperature range of 575 °C to 1377 °C and is of the form 3Fe + 4H₂0 -> Fe₃0₄ + 4H₂. A temperature of 900 °C may be used.

The H₂ in the gas mixture is used for the second reaction. This second reaction starts once the first reaction occurs and increases as the amount of the surface for which the first reaction has occurred increases. This part of the process also provides some further hardening and there is reduction of Fe₃0₄ to FeO. 2013/051835

A gas removal hood may be provided over the bath 11 of cooled oil 13 to capture the gaseous reaction products and unreacted part of the gas mixture. This can assist in drawing off the gas mixture and stopping any further reactions.

In an alternative form, the hydrogen storage tank can be used with an air storage tank to provide the gas feed. The gases when supplied in this form are less easily varied to provide some of the process control characteristics, however, and so the three separate gas stores are preferred. For instance, the dew point is materially different in air when compared with that for oxygen. In such a case, the compressed air (which is 20% 0₂ + 80% N₂) is fed to the gas mixer from the compress gas cylinders and the pressure reduces from 200BAR to 9.0 BAR.

In the alternative gas feed system shown in Figure 5, the operation is provided for in a similar manner to Figure 4, but with the oxygen or compressed air being fed direct from the storage tank 73 to the reactor 79.

In terms of the process conditions applying during these stages:

a) The wire 1 is uncoiled at 20°C or the ambient temperature in the process plant.

b) The induction furnace is used in the first furnace stage to heat the wire from to 20°C up to 550°C.

c) In the tubes, the wire is isolated from the ambient atmosphere and subjected to the controlled atmosphere. The tubes are also insulated to maintain the wire temperature at the desired level of 550°C.

d) The controlled gas mixture is introduced to the tubes with the gas at a temperature of between 20°C to 204°C.

e) In the second furnace stage, the wire and controlled environment is heated to 575°C. This provides the first oxidation phase and generates a consistent oxidation layer on the surface which is blue in colour. The surface is oxidised to Fe₃0₄.

f) In the third furnace stage, additional heat is used to raise the temperature to 900°C. This provides a reduction phase and generates a consistent oxidation layer on the surface which is black in colour. The surface is reduced from Fe₃0₄ to FeO by the ¾ reduction.

g) In the quench stage the wire temperature drops from above 725°C (typically 840 to 900°C) to 30-35°C and the quench oil causes the formation of the desired uniform martensite structure.

h) The lead bath stage is then used to temper the wire by heating the wire again to 400 to 600°C. i) The second quench stage uses a water and soluble oil quench to take the wire temperature back down to 30 to 60°C.

j) The air wipe stage is used to clean the wire and to cool it further to 20 to 25°C, or ambient conditions.

k) The rust preventative is applied at 20°C, or ambient conditions.

1) The finished wire in the coiling stage is at 20°C, or ambient conditions.

In the above method, the temperature and gases in the controlled environment are controlled so as to provide a uniform black surface for the wire product. Other surface colours are possible through alternative conditions.

The black colour is caused by the formation of FeO as the surface material. This is based upon the proportion of H₂ converted to H₂0 being correct for the oxidation to black FeO as the oxide form.

If the level of H₂ is lower, then the second phase will not occur and the oxide will be in the form after the first oxidising phase, namely Fe₃0₄.

If the level of H₂0 supplied to the first oxidation phase is not high enough then a brown or straw colour oxide will form. This is a result of the reaction 2Fe + 3H₂0 -> Fe₂0₃ + 3H₂ (instead of 3Fe + 4H₂0 -> Fe₃0₄ + 4H₂).

Hence, control of the hydrogen level and control of the oxygen level can be used to control the colour of the product formed. Other variations are anticipated to provide further colours or shades of colour.

In addition to the surface colour control, the position at which oxidation occurs in the overall process is changed. This means that whilst in the conventional process the wire can be exposed to air in two places late in the process sequence, in the new process all the wire surface is fully oxidised prior to reaching these locations and it is not possible for another oxidation to occur as all Fe is protected with by the FeO surface layer which has already fully formed.

Detection of any problems with the black oxide layer is apparent from the blue or straw colour forming should there be any surface damage in the lead tank, for instance, causing later air oxidation.

Claims

1. A method for treating a metal product, the method comprising:

heating the metal product;

exposing the outer surface of the metal product to a controlled environment; and oxidising the outer surface of the metal product.

2. A method according to claim 1, the method further comprising heating the metal product to an elevated temperature, the elevated temperature of the metal product being at least 725°C.

3. A method according to claim 1 or claim 2, the method further comprising the controlled environment being controlled with respect to the atmosphere around the metal product.

4. A method according to claim 3, the method further comprising the nitrogen and/or oxygen and/or hydrogen content being controlled.

5. A method according to any preceding claim, the method further comprising the controlled environment being provided within a container provided in a first process stage, the container being a tube or other elongate container, and the metal product passing through the container.

6. A method according to any preceding claim in which a gas mixture is fed to the controlled environment and in which the H₂ to 0₂ ratio in the total gas mixture is between 1.5:1 and 4:1.

7. A method according to any preceding claim in which a gas mixture is fed to the controlled environment and in which the H₂ to 0₂ ratio in the total gas mixture is between 1.65:1 and 3.5:1.

8. A method according to any preceding claim in which a gas mixture is fed to the controlled environment and in which the H₂ to 0₂ ratio in the total gas mixture is between 1.5:1 and 2:1

9. A method according to any preceding claim in which a gas mixture is fed to the controlled environment and in which the H₂ to 0₂ ratio in the total gas mixture is between 3.2:1 and 3.7:1.

10. A method according to any preceding claim, the method further comprising that the surface of the metal product oxidises in a first process stage.

11. A method according to claim 10 in which the oxidation is substantially completed in the controlled environment.

12. A method according to any preceding claim, the method further comprising that the surface of the metal product is reduced in a first process stage.

13. A method according to claim 12 in which the reduction is substantially completed in the controlled environment.

14. A method according to any preceding claim in which the method further comprises a quenching stage, the quenching stage being provided after the first process stage, the quench stage hardening the metal product.

15. A method according to any preceding claim in which the method further comprises a further immersion stage, the further immersion stage being provided after the first process stage and/or after the quenching stage, the further immersion stage tempering the metal product.

16. A method according to any preceding claim in which the method further comprises a further quenching stage, the further quenching stage being provided after the first process stage and/or after the quenching stage and/or after the further immersion stage, the further quenching stage hardening the metal product.

17. A method according to any preceding claim in which the method of treating provides that the characteristics and/or properties of the surface do not alter after the quenching stage, particularly with respect to the oxidisation of the surface.

18. A method according to claim 17 in which the method provides that less than 1% of the oxidation of the surface of the metal product occurs after the further quenching stage, preferably after the tempering stage and most preferably after the quenching stage.

19. Apparatus for treating a metal product, the apparatus including

a heating stage for heating the metal product;

a controlled environment, to which the outer surface of the metal product is exposed; an oxidising stage for oxidising the outer surface of the metal product.

20. A method for treating a metal product, the method comprising:

heating the metal product;

exposing the outer surface of the metal product to a controlled environment;

oxidising the outer surface of the metal product;

quenching the metal product; and

tempering the metal product.

21. Apparatus for treating a metal product, the apparatus including

a heating stage for heating the metal product;

a controlled environment, to which the outer surface of the metal product is exposed; an oxidising stage for oxidising the outer surface of the metal product;

a quenching stage for quenching the metal product; and

an tempering stage for tempering the metal product.