Method of flaw detection in billets

Abstract: A method for detecting flaws and surface defects in metal workpieces, particularly in non-magnetic steel workpieces. The surface of the workpiece (2) is heated by passing the workpiece is in its longitudinal direction through an induction coil (3), scanning the surface of the workpiece with an infrared camera (4) transversely across the surface of the moving workpiece immediately after induction heating and recording a streaked pattern temperature profile in which the streaked pattern repeats itself where there are flaws in the surface. An induction coil (3) is used which has a current frequency which gives a penetration depth of the induced current in the workpiece which is greater than the depth of the flaws which are to be detected. The flaws and surface defects are detected by recording a lower surface temperature than the rest of the surface of the workpiece by scanning the temperature profile immediately after induction heating.

Description: This invention relates to a method for flaw detection in metal members such as blooms or billets of steel or aluminium. The method of the present invention is particularly suited for detecting surface flaws in non-magnetic steel. It is well known that surface flaws occur in the formation of steel workpieces such as blooms or billets, and many different methods have been developed in order to detect these surface flaws so that they may be eliminated by grinding or the like. From US patent Re 32166 a method is known for the detection of flaws and surface defects in metallic workpieces where the workpiece is heated by passing it through an induction heater for induction heating with a high frequency current, immediately after which the workpiece is scanned with an infrared camera to determine the temperature profile of the scanned portion of the workpiece. The output signal from the infrared camera is used to generate a streaked temperature profile in which the streaked pattern repeats itself where there are flaws in the surface of the metallic workpiece. According to US patent No Re 32166 an increase in temperature is found adjacent surface irregularities, such as cracks. The increase in temperature due to a crack will, when the patterns from repeated temperature scannings are made, form a streaked pattern indicating a longitudinal surface crack in the workpiece. Cracks in the workpiece which do not run at a too steep angle to the longitudinal axis of the workpiece can thus be detected by the method described in US patent Re 32166. From US patent No 4.109.508 a similar method is known for detecting surface flaws in metallic workpieces. According to this method, the temperature across the workpiece is scanned before and after the workpiece is heated by passing it through a high-frequency induction coil. If the measured increase in temperature due to the induction heating for certain points on the surface of the workpiece exceeds the temperature increase for a crack free surface, this indicates flaws in the workpiece. Both the method according to US patent No Re 32166 and US patent No 4.109.808 are thus based on the fact that a crack is indicated by a higher increase in the surface temperature in the area adjacent the crack than the rest of the surface of the workpiece, after the surface of the workpiece has been heated by passing it through a high frequency current coil. It has been found that these known methods can be used with good results on magnetic materials such as magnetic steel. However, for non-magnetic material such as non-magnetic stainless steel it has been difficult to detect flaws by the above mentioned methods. When these known methods are used for detecting flaws in non-magnetic steel, it is found that the increase in surface temperature adjacent a flaw will not be higher than the increase in temperature of the crack-free part of the workpiece. It is believed that the reason for this lies in the differences in electric resistivity and permeability for magnetic and non-magnetic steels. Thus, when using a high frequency current coil with a frequency of 30,000 to 40,000 Hz the penetration depth of the induced current in the workpiece for magnetic steel will normally be substantially lower than the depth of the surface cracks which are to be detected. This low penetration depth of the induced current results in a much higher temperature increase in areas with flaws than in a flaw-free part of the surface. Due to the differences in electrical resistivity and permeability, for non-magnetic steels, it would be necessary to increase the frequency of the current coil by about 40 times in order to obtain the same low current penetration depth for non-magnetic steels as for magnetic steels. Such high frequencies would thus be necessary that with presently available high frequency induction coils, it would neither be technically nor economically viable to use such high frequencies for the detection of flaws. It is therefore an object of the present invention to provide a method for the detection of surface flaws in metallic workpieces, particularly in non-magnetic steel workpieces, which will overcome the above mentioned drawbacks of the known methods. Accordingly, the present invention relates to a method for detecting flaws and surface defects in metallic workpieces, particularly in non-magnetic steel workpieces, in which the surface of the workpiece is heated by passing the workpiece in its longitudinal direction through an induction coil; the surface of the workpiece is scanned with an infrared camera transversely across the surface of the moving workpiece immediately after induction heating; and recording a streaked pattern temperature profile in which the streaked pattern repeats itself where there are flaws in the surface, characterised in that an induction coil is used with a current frequency which gives a penetration depth of the induced current in the workpiece which is greater than a preselected depth of flaws which are to be detected. The flaws and surface defects can then be detected by recording a lower surface temperature than the rest of the surface of the workpiece when the temperature profile is scanned immediately after induction heating. By the method according to the present invention a current frequency is used which gives a penetration depth of the induced current which exceeds the depth of the flaws when the workpiece is passed through the induction coil. Thus the current will flow down and under flaws in the workpiece. It has surprisingly been found that the areas of the surface at the edges of a flaw will thus be heated less than the flaw free part of the workpiece. By scanning the temperature profile immediately after the workpiece has left the induction coil, the part of the workpiece having flaws will thus show a lower temperature than the flaw free part of the workpiece. It is, however, important that the temperature profile is recorded before the temperature has been equalised, and in practice it has been found that the temperature profile should preferably be scanned less than 0.1 second after the workpiece has left the induction coil. More preferably, the temperature profile is recorded less than 0.05 seconds after the workpiece has left the induction coil. When the method according to the present invention is used for detecting flaws in non-magnetic steel, it is preferred to use an induction coil with a frequency of up to 50,000 Hz. This is satisfactory for obtaining safe detection of flaws of normal depth. When used for magnetic steel, frequencies in the range between 100 Hz and 1000 Hz are used. The frequency needed for a certain material can easily be found by a person skilled in the art based on the known relations between the penetration depth of the current and electric resistivity and permeability. The invention may be carried into practice in various ways and one embodiment will now be further described by way of example with reference to the accompanying drawing in which the single figure is a schematic side view of an apparatus which can be used in the present invention. The apparatus shown in the figure is generally similar to the apparatus which is used in the invention of US patent No Re 32,166 and comprises a track 1, along which a billet 2 is progressing in the direction of the arrow. The billet passes through an induction heater 3 which operates at a frequency between 100 and 50,000 Hz, depending on the electrical characteristics of the billet. Downstream of the induction heater 3, and positioned immediately after the induction heater 3, there is an infrared thermo camera 4 which scans the workpiece and forms a temperature profile. When the temperature across the workpiece is measured, a temperature decrease will be found adjacent surface irregularities such as cracks. The temperature profile will form a streaked temperature pattern across the surface and the surface irregularities will be indicated by the fact that the streaked patterns repeats itself. The data obtained from the infrared camera can be recorded as a hard copy, but is preferably fed directly to a data processor 5, which in turn controls downstream marking equipment 6, such as a marking pen or grinding equipment. Since the size, shape and depths of the crack can be found from the temperature profile, the exact grinding necessary to remove the cracks can be established. After grinding, the workpiece can be subjected again to flaw detection apparatus 7, 8 to ensure that the correct grinding has been effected. The above described apparatus was used for detecting flaws in a non-magnetic stainless steel having the following chemical compositions: Max 0.08% C, max 2% Mn, max 0.045% P, max 0.03% S, max 1% Si, 18-20% Cr, 8-10.5% Ni, balance Fe. An induction coil was used with a frequency of 40,000 Hz, which gives a current penetration in the steel of 2.2 mm. The temperature profile was recorded 5 mm downstream of the coil. The velocity of the workpiece through the coil was 0.5 m/s. For cracks with a depth of 0.8 mm in the above steel, a decrease in temperature of 5C° compared to the temperature of crackfree surface was observed.

Claims: A method for detecting flaws and surface irregularities in metal billets (2), in which the surface of the billet (2) is heated by passing it through an induction coil (3), and immediately downstream of the coil (3), the workpiece is scanned with an infrared camera (4) to determine a temperature profile across the billet (2), and a streaked pattern temperature profile is recorded, characterised in that a current frequency is used which gives a penetration depth of the current induced in the billets (2) which exceeds a preselected depth of flaws which are to be detected, and in that flaws and surface irregularities are detected by a lower surface temperature adjacent the flaws than in the rest of the surface of the billet when the temperature profile is scanned immediately downstream of the induction coil (3). A method as claimed in Claim 1, characterised in that the temperature is recorded by scanning the temperature profile less than 0.1 second after the billet (2) has passed the induction coil (3). A method as claimed in Claim 2, characterised in that the temperature is recorded by scanning the temperature profile less than 0.05 seconds after the billet (2) has passed the induction coil (3). A method as claimed in any preceding claim, characterised in that for billets (2) of non-magnetic steel, a current frequency of up to 50,000 Hz is used. A method as claimed in any of Claims 1 to 3, characterised in that for billets (2) of magnetic steel a current frequency of 100 to 1000 Hz is used.

Inventors: HOVLAND HELJAR, NILSEN JAN, STROM YNGVE, HOVLAND, HELJAR, NILSEN, JAN, STROM, YNGVE
Applicants: ELKEM TECHNOLOGY, ELKEM TECHNOLOGY A/S
Application year: 1989
Main classification: G01N25
IPC codes: G01N25