GB1559690A

GB1559690A - Treatment of steel products

Info

Publication number: GB1559690A
Application number: GB4640975A
Authority: GB
Original assignee: British Steel Corp
Current assignee: British Steel Corp
Priority date: 1976-11-10
Filing date: 1976-11-10
Publication date: 1980-01-23

Description

(54) IMPROVEMENTS IN AND RELATING TO THE TREATMENT OF STEEL PRODUCTS (71) We, BRITISH STEEL CORPORATION, a Corporation incorporated and existing un- der the Iron and Steel Act 1967 whose principal office is at 33 Grosvenor Place, London, S.W.l, 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 a method of carburising high alloy cutting steels. More especially, but not exclusively, the invention relates to a method ofcontinuouslycarburising high alloy cutting steel in strip or sheet form (hereinafter referred to simply as metallic strip).Examples of high alloy cutting steels are stainless steels, tungsten based steels and molybdenum based steels.

Previously, steel which has been carburised in a gas carbon atmosphee has been deleterious because of the presence within the carburised steel of a number of extremely stable continuous grain boundary carbide films. The present invention provides a method by which the formation of these carbide films is inhibited.

According to the present invention in one aspect a method of carburising a high alloy cutting steel comprises the steps of heating the steel in a non-carburising non-oxidising atmosphere to its solution temperature or to a temperature at which the solid solution phase is predominant, holding the steel at this temperature in a carbonaceous atmosphere for a period of time sufficient for the carbon content of the steel to be increased at least to the required level and heat treating the carburised steel in a non-carburising, non-oxidising atmosphere for a period of time sufficient substantially to homogenize the carbon content within the steel.

Preferably, the steel is cooled after homogenisation to a temperature below that at which the steel will oxidise within the ambient atmosphere.

The non-carburising, non-oxidising atmosphere may consists of cracked ammonia nitrogen, hydrogen or argon or mixtures of these gases. Both the heating and heat treating steps may be carried out under atmospheric or sub-atmospheric pressure conditions. The heating step may be effected within a vacuum of, for example, approximately 10- torr. The carbonaceous atmosphere may consist of a hydrocarbon gas such as methane or propane or may comprise a hydrocarbon gas diluted with an inert gas such as hydrogen, nitrogen or argon.

The steel may comprise metallic strip rolled from a cast ingot or produced by roll compacting metallic powder. Alternatively, the steel may be in the form of flat or shaped blanks or components.

According to the present invention in another aspect, a method of continuously carburising a high alloy cutting steel in strip form comprises moving the strip along a predetermined path and subjecting it as it moves along this path to the sequential steps of heating in a non-carburising non-oxidising atmosphere to the solution temperature of the steel or to a temperature at which the solid solution phase predominates, maintaining the strip at this temperature within a carbonaceous atmosphere for a period of time sufficient to raise the carbon content of the strip to the required level and homogenising the strip in a non-carburising, non-oxidising atmosphere.

The strip may be supported as it is conveyed along the predetermined path by means of a gas cushion, a continuous belt or by means of a number of spaced support members.

Portions of the steel strip may be masked before carburisation to produce a product having portions of differing carbon content Masking may be effected by means of copper or nickel plating or refractory oxide paste applied to the appropriate portions of the strip by means of rollers or brushes.

By solution temperature is meant the temperature range when the alloy is in the austentic condition without carbide present.

By a temperature at which the solid solution phase predominates is meant the temperature range, usually below the solution treatment temperature range, where a proportion of a second phase (usually carbide or mixed carbides) is present in the austenitic matrix phase.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Figare 1 is a schematic side elevation partly in section of a furnace in accordance with the invention and Figures 2, 3 and 4 are block diagrams which schematically illustrate three alternative methods of producing steel strip in accordance with the present invention.

The carburising furnace schematically illustrated in Figure 1 is refractory lined and includes a heating zone 1, a carburising zone 2, a homogenisation zone 3 and a cooling zone 4.

The zones 1, 3 and 4 are respectively connected through pipes 5, 6, 7 to a common or separate sources of non-carburising, nonoxidising gas. The gas is preferably cracked ammonia, hydrogen, nitrogen or argon or a mixture of these gases. The carburising zone 2 communicates via a pipe 8 to a source of hydrocarbon gas. Preferably, this gas is propane or methane. Entry and exit seals 9, 10 are respectively located at the inlet and outlet sides of the furnace. In addition, gas seals 11, 12 are provided to isolate carburising zone 2 from the adjacent heating and homogenisation zones.

Fans or similar devices may be located within one or more of the zones 1 to 4 to promote gas circulation within the individual zones.

A high alloy cutting steel strip 'S' is unwound from a coil 14 and is passed through the furnace in the direction indicated and recoiled at 15. As the strip passes through the furnace it is supported in a conventional manner. For example, the strip may be supported within each zone on a cushion of gas the same as that circulating within the zone, or on a continuous belt or by means of a number of spaced support members. The strip passes between adjacent zones through narrow slots formed in the separating walls of the individual zones.

The pressure existing within each zone may be atmospheric or sub-atmospheric. In the former case the gas atmosphere within the carburising zone may consist of methane or propane or may comprise one of these gases diluted with an inert gas. The gas contained in the other three zones may be supplied from a common source and may consist of argon, hydrogen, nitrogen or cracked ammonia. Where the pressure existing within the furnace is sub-atmospheric, for example, 5 to 500 torr, the gas within the carburising zone 2 will consist of methane or propane.

The pressure existing within the heating zone may be a vacuum of approximately 10-3 torr and the gas present in this zone and in the homogenisation and cooling zones may comprise an inert gas such as argon, hydrogen, nitrogen or cracked ammonia.

In one typical example of operating the method and apparatus described above, a stainless steel strip 0-3mm thick and containing 13% chromium and 0 3 carbon is heated to 1200"C in an argon atmosphere for approximately one minute. The heated strip is then carburised at this temperature in a methane atmosphere for 50 seconds to raise its bulk carbon content to 080X. Following carburisation the strip is homogenised in an argon atmosphere for approximately two minutes to ensure that the carbon within the strip is substantially uniform and then cooled in an argon atmosphere for approximately one minute, to reduce its temperature to prevent oxidation of the strip as it enters the ambient atmosphere.The carburised strip contains no continuous carbide grain boundaries, the carbon gradient being substantially uniform throughout.

In a second typical example an M2 tool steel strip 0-625mm thick is heated for ap proximately one minute in a vacuum of 10-3 torr to raise its temperature to 1250C . The strip is then carburised at this temperature in a methane atmosphere at a pressure of 200 torr for a period of approximately three minutes to raise its carbon content to 08v0.

The carburised strip is homogenised in a hydrogen atmosphere for approximately three minutes prior to cooling in either an inert gas atmosphere or by oil quenching for one minute. Again the carburised strip contains no continuous carbide grain boundaries, the carbon being substantially uniform throughout.

In a third typical example an M2 tool steel component is heated to 1 1000C in a vacuum of 101torr for a period of five minutes. The component is then carburised at this temperature in a methane atmosphere at a pressure of 500 torr for a period of approximately three minutes to raise its carbon content. The carburised component is homogenised in a hydrogen atmosphere for a period of three minutes at a pressure of 10-1 torr. The carburisation and homogenisation steps are then repeated three times to raise the carbon content of the component to the required level. Following final homogenisation the component is cooled rapidly in a protective atmosphere or is vacuum quenched.

In a further typical example an M2 tool steel is heated to 1 1000C in an inert atmosphere, carburised for approximately three minutes in a methane atmosphere diluted with argon (eg. a 40% methane, 60% argon atmosphere), homogenised for approximately three minutes in an inert atmosphere, recarburised and rehomogenised and then rapidly cooled.

Previous attempts to carburise steel in gas carbon atmosphere have included the step of heating the steel to solution or treatment temperatures within the gas carbon atmosphere. It has now been established that the inclusion of this step produces steel in which the carbon is present as continuous grain boundary carbide films and is not uniformally distributed throughout the steel. Such a steel is disadvantageous in that further cold working or shearing is made extremely difficult due to crack formation along the grain boundary films and the material docs not respond correctly to heat treatment.

Carbon starts to diffuse into the steel at temperatures well below solution treatment temperatures, i.e. between 500 and 600"C.

Because of the more rapid penetration of carbon along the surface connected grain boundaries of the steel, thin layers consisting of continuous grain boundary carbide films are formed as the temperature of the steel progresses through the range where alloy carbides are stable under equilibrium conditions. These films are extremely stable and are not easily taken into solution even at high treatment temperatures; in addition, their presence at the steel surface slows down the rate of carbon diffusion during subsequent carburisation at solution temperature. Thus, truly homogenised steel cannot be produced.

By means of the method and apparatus described above in which the steel to be carburised is heated to its solution treatment temperature in a non-carburising non-oxidising atmosphere prior to carburisation, homogenous diffusion of the carbon occurs up to the carbon levels required without grain boundary carbide films being present.

The methods and apparatus described above have particular application in the manufacture of high alloy steels which include relatively high carbon contents. Various process routes for producing such steels which include the carburisation stage described herein are detailed below with reference to Figures 2 to 4.

In Figure 2, the various stages in producing a high alloy cutting steel strip from a powder material and including a carburising step as described above are set out. A melt having a carbon content substantially less than that required in the finished product, but containing other alloy constituents in substantiallythe same amounts as required in the finished strip is atomised by a water spray to produce a powder of approximately the required sieve size. The carbon content of the powder is preferably between 0-01 and 030v0 by weight and the required carbon content of the finished product may be above 0-80% by weight.

The powder is passed through the nip of a pair of contra-rotating compaction rolls to produce a green strip which is sintered for a short time in a hydrogen atmosphere before being rolled to final gauge by cold rolling.

Because of the low carbon content of the strip, it is relatively unhardened and can be worked relatively easily. The rolled strip is then carburised in a furnace by a method as described above to raise its carbon content to the required level. In an alternative arrangement, blank components of the required size and shape are cut from the rolled strip and are then through-carburised as mentioned above.

In Figure 3, the process steps for producing high alloy steel product from a cast ingot are set out. The ingot is cast from a low carbon melt similar to that described with reference to Figure 2. The ingot is hot rolled to approximate shape, descaled and then cold rolled to final strip gauge; blank components produced from the rolled strip are carburised in a carbonaceous atmosphere to raise their carbon contents to the required levels.

As in the previous embodiment, due to its low carbon content the ingot is relatively unhardened and contains no semi-continuous carbide films; thus, it can be worked to shape relatively easily without high losses in product yield.

In the method illustrated in Figure 4, a melt having a carbon content substantially less than that required in one or more portions of the finished strip product (for example hacksaw or razor blade strip with hardened cutting edges), but containing other constituents in substantially the same amounts as required in the finished products as a whole, is atomised by a water spray to produce a powder of approximately the required sieve size. As in the previous embodiments, the carbon content of the powder is preferably between 0-01 and 0 30% by weight and the required carbon content of the high carbon portion (s) of the finished product may be above 0 80% by weight.

The powder is passed through the nip of a pair of contra-rotating compaction rolls and sintered for a short time in a non-oxidising atmosphere, such as hydrogen or argon atmosphere before being rolled to the final gauge. As mentioned previously, because of the low carbon content of the strip, it is relatively unhardened and can be rolled to final gauge relatively easily.

In an alternative embodiment, the initial steel strip may be hot and/or cold rolled from a cast ingot instead of from a powder compact as described above.

A masking fluid or material is then applied to those portions of the rolled strip (or blank components produced from the rolled strip) which are to constitute relatively low-carbon portions of the finished product and the strip (or blank components) placed in a furnace such as described with reference to Figure 1 for carburisation.

Masking may be effected by means of a copper or nickel plating fluid or refractory oxide paste applied to the appropriate portions of the strip by means of rollers, brushes or by submersion of the appropriate portions within a bath of the masking fluid.

In an alternative embodiment, the masking fluid is applied to the strip prior to rolling to final gauge.

The embodiment described with reference to Figure 4 has particular application to the production of flexible blades, such as hacksaw blades and to razor blade strip. Conventional through-hardened tool steel strip blades frequently fail due to breakage before the cutting edge has fulfilled its useful life. This breakage is due to brittleness caused by the overall high carbon content of the blades.

Hacksaw blades produced in accordance with the present invention are manufactured from a relatively low carbon alloy steel which is comparatively soft and ductile. The blades are then carburised so that only the or each cutting edge and the adjacent strip portion is subjected to the carburising media; consequently, on heat treatment, only the or each edge portion of each blade is hardened and hence suitable for cutting, whilst the remainder of the blade is still ductile and less likely to break during use.

Razor blades do not normally fail in service due to brittleness, but the high cost of producing razor strip is in some part due to the presence of carbon in the material during processing resulting in yield losses due to cracking. In the present invention, carbon is added only to the cutting edge of the finished strip or blades which results in relatively easier and cheaper production.

It will be appreicated that the methods described above with reference to Figures 2, 3 and 4 permit the production of high-carbon steel products without the high yield losses experienced previously due to working of a material having high hardenability and, with particular reference to the method described and illustrated in Figure 4, steel products having one or more selected portions of highcarbon content and others of low carbon content.

WHAT WE CLAIM IS: 1. A method of carburising a high alloy cutting steel comprising the steps of heating the steel in a non-carburising, non-oxidising atmosphere to its solution temperature or to a temperature at which the solid solution phase is predominant, holding the steel at this temperature in a carbonaceous atmosphere to increase the carbon content of the steel and heat treating the carburised steel in a non-carburising, non-oxidising atmosphere for a period of time sufficient substantially to homogenize the car'oon content within the steel.

2. A method as claimed in claim 1 wherein the steps of carburising and homogenising the steel are repeated one or more times.

3. A method as claimed in claim 1 comprising the additional step of cooling the steel after homogenisation.

4. A method as claimed in claim 2 comprising the additional step of cooling the steel after final homogenisation.

5. A method as claimed in any one of the preceding claims in which the non-carburising non-oxidising atmosphere consists of cracked ammonia or nitrogen or hydrogen or argon or mixtures of these gases.

6. A method as claimed in any one of the preceding claims in which the heating and heat treating steps are carried out at subatmospheric pressure.

7. A method of continuously carburising a high alloy cutting steel in strip form comprising the steps of moving the strip along a predetermined path and subjecting it as it moves along this path to the sequential steps of heating in a non-carburising, non-oxidising atmosphere to the solution temperature of the steel or to a temperature at which the solid solution phase predominates, maintaining the strip at this temperature within a gas carbon atmosphere to raise the carbon content of the strip to the required level and homogenising the strip in a non-carburising, non-oxidising atmosphere.

8. A method as claimed in claim 7 in which the strip is supported as it is conveyed along the predetermined path by means of a gas cushion.

9. A method as claimed in claim 7 or claim 8 in which portions of the steel strip are masked before carburisation to produce a product having portions of differing carbon content.

10. A method of carburising high alloy cutting steels substantially as herein described with reference to the accompanying drawings.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. furnace such as described with reference to Figure 1 for carburisation. Masking may be effected by means of a copper or nickel plating fluid or refractory oxide paste applied to the appropriate portions of the strip by means of rollers, brushes or by submersion of the appropriate portions within a bath of the masking fluid. In an alternative embodiment, the masking fluid is applied to the strip prior to rolling to final gauge. The embodiment described with reference to Figure 4 has particular application to the production of flexible blades, such as hacksaw blades and to razor blade strip. Conventional through-hardened tool steel strip blades frequently fail due to breakage before the cutting edge has fulfilled its useful life. This breakage is due to brittleness caused by the overall high carbon content of the blades. Hacksaw blades produced in accordance with the present invention are manufactured from a relatively low carbon alloy steel which is comparatively soft and ductile. The blades are then carburised so that only the or each cutting edge and the adjacent strip portion is subjected to the carburising media; consequently, on heat treatment, only the or each edge portion of each blade is hardened and hence suitable for cutting, whilst the remainder of the blade is still ductile and less likely to break during use. Razor blades do not normally fail in service due to brittleness, but the high cost of producing razor strip is in some part due to the presence of carbon in the material during processing resulting in yield losses due to cracking. In the present invention, carbon is added only to the cutting edge of the finished strip or blades which results in relatively easier and cheaper production. It will be appreicated that the methods described above with reference to Figures 2, 3 and 4 permit the production of high-carbon steel products without the high yield losses experienced previously due to working of a material having high hardenability and, with particular reference to the method described and illustrated in Figure 4, steel products having one or more selected portions of highcarbon content and others of low carbon content. WHAT WE CLAIM IS:

1. A method of carburising a high alloy cutting steel comprising the steps of heating the steel in a non-carburising, non-oxidising atmosphere to its solution temperature or to a temperature at which the solid solution phase is predominant, holding the steel at this temperature in a carbonaceous atmosphere to increase the carbon content of the steel and heat treating the carburised steel in a non-carburising, non-oxidising atmosphere for a period of time sufficient substantially to homogenize the car'oon content within the steel.