GB2268270A - Determining temperature of a ferrous metal object - Google Patents

Abstract

The temperature of a ferrous metal object is determined and controlled from its magnetic properties. A predetermined magnetic field is produced by a transmitting coil 21 fed with current housing variable pulse frequency from pulse generator 23. A receiving coil detects changes in the magnetic field due to changes in an elongated ferrous component passing through the magnetic field, and the resulting signals are analysed for magnitude, shape and frequency. Both coils are randomly wound on a former 22. The signals may be used to control the speed at which an elongate member is moved through a heat treatment furnace. <IMAGE>

Description

MAGNETIC INTERFERENCE DETECTOR This invention relates to a device for detecting magnetic interference caused by the presence of a ferrous metal object in a magnetic field. The invention relates particularly, but not exclusively to the control of temperature in a furnace adapted to heat ferrous metal material prior to the quenching of the material.

It is well known that a ferrous metal component may have various states having different proportions of, for example, pearlite, martensite or iron carbides etc. The proportions of these materials determine the hardness of the component.

On way of achieving a desired hardness of a ferrous metal component, is to heat it up in a furnace to a predetermined temperature before quenching it. The final state of the component, and hence its hardness depends critically on the temperature to which the component is heated prior to quenching. Thus by controlling the temperature of the component prior to quenching, it is possible to control the hardness of the quenched component.

It is known that structures present in the ferrous metal component, such as pearlite, martensite and iron carbides contribute to the combined carbon content of the component.

The proportions of these structures present vary with temperature.

According to a first aspect of the invention there is provided an apparatus for detecting the magnetic interference to a magnetic field comprising: magnetic means for producing a predetermined magnetic field, and detecting means for detecting a change in the magnetic field caused by the presence of a ferrous metal component positioned within or passing through the field.

The structure of a ferrous metal component depends on the temperature of the component. In addition different structures of the material will have different magnetic responses. Thus, by means of the present invention it is possible to detect the magnetic interference caused by a ferrous metal component to a magnetic field. This information may be used to determine the structure of the component.

The interference caused by particular ferrous metal components varies according to the mass of the component, the material composition of the component and the molecular state of the component. If any two of these variables are known, the third one may be measured. Thus if the material composition and the mass of a component are known, the molecular state may be deduced. Similarly if the material composition and the molecular state are known the mass of the component may be measured.

Preferably, the magnetic means comprises a transmitting coil randomly wound on an electrically discontinuous non-ferrous former. The magnetic means thus produces a changing magnetic field of torus shape.

Preferably the detecting means comprises a receiving coil randomly wound around the former. The receiving coil senses disturbance in the magnetic field if a ferrous object is placed in or passed through the torus field.

Advantageously, the apparatus further comprises input means for supplying a current having a frequency variable pulse to the transmitting coil. The size of the coils, the ratio of input to output turns and the frequency of the supply are all optimised for the type of test required.

Conveniently, the apparatus further comprises analysing means for analysing the signal received from the transmitting coil, in terms of magnitude, shape and frequency.

Preferably the apparatus further comprises recording means for recording the results from the analysing means, and for displaying the results in any convenient form.

According to a second aspect of the invention there is provided apparatus for controlling the temperature of a ferrous metal elongate member heated in a continuous heat treatment line comprising: apparatus for detecting a magnetic interference to a magnetic field according to the first aspect of the invention; and control means responsive to the signal produced from the analysing means, for controlling the speed at which the elongate member moves through the furnace thereby controlling the temperature to which the member is heated.

An embodiment of the invention will now be further described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of an apparatus for controlling the temperature of a component according to the second aspect of the invention incorporating an apparatus for detecting the magnetic interference to a magnetic field according to the first aspect of the invention; Figure 2 is a schematic representation showing the signal path through the apparatus of Figure 1; and Figure 3 is a cross-sectional representation of the transmitting and receiving coils, included in the apparatus of Figure 1; Figure 4 is an iron-carbon equilibrium diagram; and Figure 5 is an isothermal transformation diagram for EN 43 steel.

Referring to figures 1 to 3 an apparatus for controlling the temperature to which a ferrous metal bar is heated in a furnace is shown, designated generally by the reference numeral 10 (Figure 2). The apparatus 10 is connected to a furnace 11 in which a ferrous metal elongate member 12 will be heated prior to quenching. In order to produce a ferrous metal component having the desired hardness, it is necessary to accurately control the temperature to which the component is heated prior to quenching. The apparatus 10 comprises a detector 13 according to the first aspect of the invention and shown in more detail in Figure 2. The detector comprises magnetic means 21 comprising a transmitting coil, (shown in more detail in Figure 3) also wound randomly around an electrically discontinuous ferrous former 22.The magnetic means 21 further comprises detecting means comprising a receiving coil (Figure 3), also wound randomly around the former 22. A pulse generator 23 applies a current having a variable pulse frequency to the transmitting coil. The transmitting coil is thus able to produce a magnetic field of torus shape of desired magnitude, depending. on the voltage and current supplied to it by pulse generator 23. The receiving coil detects any interference to the magnetic field caused by the presence of a ferrous metal component, positioned in or passing through the magnetic field. Analysing unit 24 is connected to the receiving coil and converts signals produced by the receiving coil into a suitable form for display, on the display means 25.

The display means 25 is connected to a programmable logic controller (PLC) 26 which compares the signal produced by the receiving coil, with a known value of signal.

Referring to Figure 1, the ferrous metal elongate member 12 which is to be heated passes through a furnace 11. On emerging from the furnace the component is pressed in press 14 before passing through detector 13. The transmitting coils are supplied with a current from pulse generator 23, and thus a magnetic field is created through which the elongate member 12 passes. The receiving coils detect interference caused by the elongate member 12 to the magnetic field. The analysing unit 24 analyses the signals and the signal is displayed on display means 25. The PLC 26 compares the signal with a known signal, which is related to desired temperature which is to be achieved. If the indicated temperature is too great then the PLC 26 speeds up the feed rollers 15, and hence the elongate member 12 thus reducing the amount of time the bar 12 spends in the furnace.Its temperature is reduced accordingly. If however the indicated temperature is too low, and the PLC 26 slows down the speed of the feed rollers 15 and hence the elongate member 12.

Referring to Figure 4 an iron-carbon equilibrium diagram is shown. The diagram indicates the different structures which an iron-carbon steel may take depending on its temperature.

Below 910-C pure iron has a body centred cubic crystal structure; but on heating the metal to a temperature above 910-C its structure changes to one which is face centred cubic.

Face centred cubic iron will take up to 1.7% of carbon into solid solution, whereas body centred cubic iron will dissolve scarcely any carbon (a maximum of only 0.03%). Since the solid solubility of iron-carbon alters in this way, changes in the structure will occur on heating and cooling.

Any solid solution of carbon up to a maximum of 1.7% in face centred cubic iron is called austenite, whilst the very dilute solid solution formed when up to 0.03% carbon dissolves in body centred cubic iron is called ferrite. For all practical purposes ferrite may be regarded as pure iron, since less than 0.03% carbon will have little effect on its properties. Thus, in a carbon steel at say, 1000"C all of the carbon present is dissolved in the solid austenite. When this steel cools the austenite changes to ferrite, which will retain practically no carbon in solid solution. Assuming that the cooling has taken place fairly slowly, the carbon will be precipitated as the hard component called cementite.

Referring to Figure 3, the equilibrium diagram for iron carbon alloys is shown. The upper critical temperature for ferrous material having a particular carbon content is determined by line 31. The upper critical temperature varies according to the carbon content of the particular material. The lower critical temperature is determined by line 32 and is substantially constant, and does not vary with the carbon content of the ferrous material. Above the upper critical temperature a ferrous material having a carbon contentof less than 0.88 will be entirely austenite. Below the upper critical temperature, but above the lower critical temperature the ferrous metal material will be a mixture of austenite and ferrite for carbon contents of less than 0.8%, and below the lower critical temperature the ferrous material will be a mixture of ferrite and cementite.It has been found that the magnetic response of a ferrous material changes dramatically as it passes through the upper critical temperature, and again as it passes through the lower critical temperature.

By means of the present invention it is thus possible to identify the upper and lower critical temperatures of the ferrous metal component, irrespective of steel composition.

Once the appropriate temperature has been identified, the material may be quenched. The temperature from which the material is quenched and the rate of cooling determines the final composition and thus the hardeness of the quenched material.

The device does not come into contact with the component, which reduces problems due to contamination etc. In addition the system is able to operate at high speeds, ie to values in excess of 300 metres per second component movement.

Once the elongate member 12 has emerged from the former it is plunged into a quench medium 16. The bar is then tempered at tempering unit 17. The elongate member 12 is tempered at a tempering temperature determined by referring to time temperature transformation diagram (Figure 5). It is then passed through tension rollers 18 before being stored at a stacking point 19.

Referring to Figure 3 the transmitting and receiving coils are shown in more detail. In order to obtain electrical stability of the device, the coils are maintained at a constant temperature by cooling cavities within the former 22.

Although the invention according to the first aspect of the invention has being predominantly described in terms of an apparatus for controlling the temperature to which a ferrous metal component is heated in a furnace, it is to be understood that the invention according to the first aspect of the invention, may be applied widely to different situations. For example, the invention could be used on a production line of, for example, to determine whether bolts being produced are of an appropriate mass (size).

Claims (8)

1. Apparatus for detecting the magnetic interference to a magnetic field comprising: magnetic means for producing a predetermined magnetic field, and detecting means for detecting a change in a magnetic field caused by the presence of a ferrous metal component positioned within or passing through the field.

2. Apparatus according to claim 1 wherein the magnetic means comprises a transmitting coil randomly wound on an electrically discontinuous non-ferrous former.

3. Apparatus according to claim 1 or claim 2 wherein the detecting means comprises a receiving coil randomly wound around the former.

4. Apparatus according to any one of the preceding claims further comprising input means for supplying a current having a frequency variable pulse to the tranmitting coil.

5. Apparatus according to any one of the preceding claims further comprising analysing means for analysing the signal received from the transmitting coil in terms of magnititude, shape and frequency.

6. Apparatus according to claim 5 further comprising a recording means for recording the results from the analysing means and for displaying the results in any convenient form.

7. Apparatus for controlling the temperature of a ferrous metal elongate member heated in a continuous heat treatment line comprising: apparatus for detecting the magnetic interference to a magnetic field comprising: magnetic means for producing a predetermined magnetic field, and detecting means for detecting a change in a magnetic field caused by the presence of a ferrous metal component positioned within or passing through the field; and control means responsive to the signal produced from the analysing means, for controlling the speed at which the elongate member moves through the furnace, thereby controlling the temperature to which the member is heated.

8. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.