GB2051881A - Carbonitriding ferrous materials - Google Patents

Abstract

The object of the invention is to provide a process for carbonitriding carbon-free or low carbon ferrous materials which is capable of producing a substantially uniform distribution of carbides and/or carbonitrides and is achieved by forming a body of ferrous material either carbon free or with a carbon content lower than that required in the final product, and with a substantially uniform distribution of inter-metallic compounds throughout the body, forming on the surface of the body a coating of metal oxide, and subjecting the body to a heat treatment in the temperature range 720 DEG C to 900 DEG C in a carbonitriding environment.

Description

SPECIFICATION Carbonitriding ferrous metals This invention relates to a method of carbonitriding ferrous materials and in particular relates to the formation of alloy carbides and/or carbonitrides to develop optimum material properties in ferrous materials manufactured by either atomising a suitable melt to powder and subsequently producing the shape required from the powder, or casting the melt into ingots and suitably mechanically working the ingots to produce the shape required. By ferrous material is meant one containing one or more of any of the carbide forming elements employed in the production of alloy steels, e.g., chromium, molybdenum, tungsten and vanadium, in those proportions conventionally used in commercial grades of high speed steel, tool steels and other alloy constituted steels.

In many instances, it is highly desirable to produce a product with the ferrous material with a low carbon content or in a substantially carbon free condition, and when mechanical working process (cutting, shaping, stamping and the like) can be effected with relative ease. However, with conventional carbonitriding techniques, it has not been possible hitherto to produce a substantially uniform distribution of carbides and/or carbonitrides throughout the carbonitrided depth without the production of embritteling grain boundary net works, even when the product has a relatively small depth such as a hack-saw blade.

It is the object of the invention to provide a process for carbonitriding ferrous materials capable of producing a substantial!y uniForm distribution of carbides and/or carbonitrides throughout the carbonitrided material with the process being applicable to carbon free o, carbon containing materials.

According to the present invention, a method of carbonitriding a ferrous material comprises forming a body of ferrous material either carbon free or with a carbon content lower than that required in the final product, and with a substantially uniform distribution of intermetallic compounds throughout the body, forming on the surface of the body a coating of metal oxide, and subjecting the body to a heat treatment in the temperature range 7200C to 9000C in a carbonitriding environment.

With the body produced by either a powder route or an ingot, it is necessary to effect a mechanical working and thermal treatment to ensure the production of the substantially uniform dispersion of the inter-metallic compounds. It is therefore necessary to mechanically work the body at a temperature below the re-crystallisatiori and inter-metallic solid solution temperature. Whilst the presence of inter metallic compounds is essential to the process, the precise constitution of those inter-metallic compounds is not fully understood although analysis would suggest that they are predominantly complex iron tungstides with a minor proportion of other complex iron compounds containing, chromium, vanadium and molybdenum.

The metal oxide coating on the surface of the body can readily be achieved simply by elevating the temperature of the body in a treatment furnace in which the carbonitriding step is subsequently to be performed.

When the treatment furnace has been provided with its carbonitridiny atmosphere, it is preferable to operate the process in the temperature ranye of 7800C to 875or and still further preferably when the body is a hack-saw blade to operate the process at a temperature of approximately 8000 C.

The precise manner of creating the carbonitriding environment is not critical nascent nitrogen can be created by utilising cracked ammonia which is enriched with a suitable carbon containing gas such as propane and passed over a furnace retort.

As has been indicated above, the body of ferrous material can be formed by either a powder route or by an ingot route, and within either technique, the invention is applicable to bodies of mono- or bimetallic construction. The latter is particularly useful when the invention is applied to the production of bodies such as hack-saw blades where it is useful to employ one ferrous material or grade of ferrous material in the body and a second ferrous material or grade of ferrous material at the cutting edge.

Thus, a primary melt of required chemical specification but carbon free or of low carbon content may first be produced in, e.g., an induction or arc furnace, and either atornised to powder or cast into ingot form. The powder route for the production of the body may, for example, involve a first roll compaction of the powder (mono- or bi-metallic) at room temperature, sintering compacting at, e.g., 1 200 C1 400 C and cooling from this temperature at such a rate, e.g., 1 000C/sec. from sintering temperature to 4000C and 300C/sec. to room temperature as to prevent the precipitation of intermetallic compounds within the material body.The sintering process is carried out in hydrogen to a density lower than theoretical full density (e.g. 9096%) etc., cold rolling the sintered compact at room temperature to mechanically work the sintered compact and compress it to as close to theoretical full density as possible. The compact may then be placed in a furnace and heated in an oxidising atmosphere for a short period, e.g., 6500C for 4 hours and cooled in the furnace to room temperature.

The treatment ensures the production of an oxide coating on the compact and the precipitation of the intermetallic compound with the desired strip morphology. The material then may be carbonitrided in the range 7200C--90OOC. The carbonitriding conditions and period will naturally be dictated by the furnace capacity and type and by the amount of the charge, but may, for example, in a 1 5 litre furnace be for 3 hours with the atmosphere created by blowing in ammonia gas at 265 L/hr and methane hydrocarbon at 18 L/hr.

The ingot route for the production of the body may, for example, comprise cog rolling the ingot at not more than 1 5000 with an intermediate rolling step at not more than 10000C and a finish roll at not more than 8000C. The strip would then be oxidised and carbonitrided in much the same manner as has been exemplified above in relation to the powder metallurgy route.

To produce a bi-metallic product, two ingots would be prepared at least one of which is a ferrous material as hereinbefore defined, and both ingots may be mechanically worked by, e.g., cog rolling or forging at no more than 11 50 C, with an intermediate rolling step at no more than 1 00000 and a finished roll at no more than 8000C. At least the rolled ingot of the ferrous material should then be cold drawn, and following which the two products of the rolling and where required cold drawing steps are suitably welded together and subsequently machined (if required) to produce a required surface finish and (if required3 suitably heat treated, e.g., to precipitate intermetallic compounds. The bi-metallic product is then oxidised and carbonitrided as has been exemplified above.

The even distribution of carbides and/or carbonitrides brought about by the invention may be enhanced by the introduction of, e.g., a diffusion step into the heat treatment of the carbonitrided material, either as a separate step subsequent to carbonitriding or as a stage in the carbonitriding step itself.

Witch bodies such as hack-saw blades, or indeed where any forming, shaping or machining step is required to produce the body in its final required form, it will be self evident that this takes place prior to carbonitriding. Following carbonitriding, hardening and tempering of the body may take place as required in conventional manner. It is preferred to include in the carbonitriding a diffusion step, in which the temperature is elevated to increase diffusion rates and reduce the operation time.

The invention therefore allows the production of a body of low carbon content or even (for practical purposes) carbon free with the distinct advantage of the ease of mechanical working of a body in the production of complex shapes or with shapes requiring mechanical deformation such as hack-saw blades with set teeth, with the subsequent introduction into the body of sufficient carbon to bring the carbon content up to that which is required in the body for its effect on mechanical properties and to allow the body to serve its intended function, the method of the invention ensuring that at least over the depth of carbonitrided region the carbides and/or carbonitrides are substantially uniformly distributed.

To further exemplify the invention, it will now be described in greater detail in relation to the production of hack-saw blades measuring 300 mm x 13 mm x 0.60 mm of both the mono-metallic and bi-metallic type, the resultant structures being shown in the accompanying drawings in which: Figure 1 is a photomicrograph of a conventionally produced hack-saw blade; Figure 2 is a photomicrograph to the same magnification as Figure 1, showing a hack-saw blade produced by the ingot route according to the invention; and Figure 3 is a photomicrograph to the same magnification by the powder route according to the invention.

For the hack-saw blades of the invention, primary melting to the basic chemical specification can be in an induction or electric arc furnace. Typically, the chemical specification for the primary melt can be: C Si %Mn %W %V %Mo %Cr Cutting edge low carbon M2 8--.35 .15 .80 6.4 2.0 5.0 4.10 Backing, low carbon O.35 .50 .50A80 1 - - 1.0/1.6 For the ingot route, Table I shows typical mechanical working data.

TABLE I

MONO METAL BI-METAL Geometric Temperature Geometric Temperature Process Parameter of Operation Parameter of Operation Operation cms C cms C Forging Input 10 x 10 x 91 1150 Output 5x5x183 1.000 Cog Roll Input 15 x 5 x 61 1150 3 x 3 x 46 1000 Output x 1 x 183 1000 2 qb x 51 800 Ins roll Input 36 x 1 x61 11000 Output 71 x3 x 71 650 Finish Input 71 x 3 x 20 800 2 + x 51 900 Roll Output 71 x .6 x 101600 0.6 + x 457 650 Cold Input 0.6 + x 457 R.T.

Draw Output 0.06 X .15 x 152M R.T.

For the powder mute, Table II shows typical mechanical working data: TABLE II

MONO METAL I BI-METAL Geometric Temperature Geometric Temperature Parameters of Operation Parameters of Operation Process Operation cms C cms C 1. Roil Compact 3 x .25 x coil R.T. 3 x ,25 x coil R.T.

2. Sinter (Hydrogen) 3 x .25 x coil 1200--1400 3 x .25 x coil 1200-1400 3. Cold Roll Input 3 x .25 x coil R.T. 3 x .25 x coil R.T.

Output 3 x.06 xcoil 3 x .06 x coil 4. Intermetallic Pre cipitation treatment 600-750 600-750 After mechanical working, the sheet or strip material can be cold sheared to the desired size, the teeth produced along the cutting edge, and the teeth given the desired degree of set. Following this, the blades are subjected to the carbonitriding process of the invention, and are finally hardened and tempered.Typically, the carbonitriding schedule can be the creation of a carbonitriding atmosphere by flowing an admixture of ammonia bottle gas at 265 litres/hr. and methane at 18 litres/hr. is passed through the furnace, and over a retort to form nascent nitrogen. The furnace is preconditioned for two hours, and with the carbonitriding step having a duration of two hours. Naturally, the precise carbonitriding conditions are dependent on the furnace type and capacity. Hardening and tempering can be effected in any conventional manner. Thus, they can be hardened in a vacuum furnace which allows the convenience of incorporating a diffusion period of 20 to 30 minutes at 11 000C prior to hardening (solution treatment) at 1180 C for four minutes. This can then be followed by a nitrogen quench to room temperature.There is however no reason why a diffusion period should not be included during the carbonitriding cycle, with hardening effected in a muffle, salt bath or another single treatment.

Tempering can be effected in an air circulatory furnace e.g., at 5400C for one hour, for a single or multiple temper.

The photomicrographs of Figures 1, 2 and 3 show a conventionally produced hack-saw blade (Figure 1) in comparison with hack-saw blades in accordance with the invention produced by the ingot route (Figure 2) and by the powder route (Figure 3). It is immediately apparent on considering Figures 2 and 3 in relation to Figure 1 that a considerably better distribution of carbides and/or carbonitrides results from the invention.

The invention thereby allows all mechanical working to be effected on a substantially carbon free stock material including the cutting and setting of teeth on a hack-saw blade and carbonitriding of the blade with considerably better distribution of carbides and/or carbonitrides than has hitherto been possible.

Claims (14)

1 . A method of carbonitriding a ferrous material comprising forming a body of ferrous material either carbon free or with a carbon content lower than that required in the final product, and with a substantially uniform distribution of inter-metallic compounds throughout the body, forming on the surface of the body a coating of metal oxide, and subjecting the body to a heat treatment in a carbonitriding environment.

2. A method as in Claim 1, wherein a mechanical working and thermal treatment is effected prior to carbonitriding to ensure the production of the substantially uniform dispersion of the inter-metallic compounds.

3. A method as in Claim 2, wherein the body is mechanically worked at a temperature below the re-crystall isation and inter-metallic solid solution temperature.

4. A method as in any of Claims 1 to 3, wherein the metal oxide coating on the surface of the body is achieved simply by elevating the temperature of the body in a treatment furnace in which the carbonitriding step is subsequently to be performed.

5. A method as in any of Claims 1 to 4, wherein the carbonitriding content environment is provided by utilising a dissociated ammonia atmosphere.

6. A method as in Claim 5, wherein the atmosphere is enriched with a suitable carbon containing gas.

7. A method as in any of Claims 1 to 6, wherein prior to the carbonitriding step the body to be carbonitrided is formed from a powder.

8. A method as in any of Claims 1 to 7, wherein prior to the carbonitriding step the body to be carbonitrided is formed from an ingot.

9. A method as in any of Claims 1 to 8, wherein the body has a mono-metallic construction.

10. A method as in any of Claims 1 to 9, wherein the body has a bi-metallic construction.

1 A method as in any of Claims 1 to 10, wherein as pdrt of the carbonitriding a diffusion step is included in which the temperature of the body is elevated to increase diffusion rates and reduce the operation time.

12. Hacksaw blades when produced by the method of any one of Claims 1 to 11.

1 3. A method of carbonitriding ferrous materials substantially as herein before described with reference to Figure 2 or Figure 3 of the accompanying drawings.

14. Hacksaw blades when produced by the method of Claim 1 substantially as hereinbefore described with reference to Figure 2 or Figure 3 of the accompanying drawings.