US3462602A - Infra-red flaw detector - Google Patents

Description

Aug. 19, 1969 w. R. APPLE 3,462,602

INFM-RED FLAw ETEcToR Filed Au-g. 16, 1967 Difference AmDllfef Amplifier 72 Alarm Trigger Marker Hff eso/:3. 76 30 '6 'i1/L fr@ @1M ATTORNEY.

United States Patent O 3,462,602 INFRA-RED FLAW DETECTOR Wayne R. Apple, Boulder, Colo., assignor to Automation Industries, Inc., El Segundo, Calif., a corporation of California Filed Aug. 16, 1967, Ser. No. 661,022 Int. Cl. G01t 1/16; G01k 1/08; H013 39/00 U.S. Cl. Z50-83.3 7 Claims ABSTRACT F THE DISCLOSURE The present invention relates to the manufacture of rolled steel stock, such as sheets and plates, and to means for insuring that the rolled stock is free from any internal defects. This is accomplished by providing an infrared inspection system which is effective to scan the stock while it is being rolled and/or it is still at an elevated temperature. The inspection system is effective to locate and identify discontinuities or defects, 'such as pipe inclusions and/or edge laminations by detecting variations in the surface temperature of the rolled hot stock.

-Background of the invention In one steel manufacturing process a hot bloom or billet is rolled into thinner stock, such as a sheet or plate, hereinafter collectively referred to as rolled stock. The fully rolled stock may have a width on the order of up to 90 or 100 inches or more and a length up to 30 feet or more. After being rolled to the required thickness, the stock is trimmed to the desired dimension, sorted and allowed to cool. Subsequently the stock may be fabricated into more complex structures, such as by cold rolling into thin sheets, cutting, stamping, bending, etc.

It has been found under some circumstances the stock may possess hidden defects. In one type of defect the center portion of the stock separates for some reason, such as the presence of an inclusion. Since this makes the stock hollow, this type of defect is normally referred to as a pipe inclusion. Under other circumstances an edge portion separates. This type of defect is commonly called an edge lamination. If the stock contains defects of the foregoing variety, during the rolling operations the defect tends to grow in size and eventually cover a considerable area. If this defect is not discovered during subsequent processing of the stock, a costly failure may occur.

Since defects of this nature are normally buried within the stock, it is virtually impossible to detect the defects by a visual examination, particularlyat the speeds required in a modern rolling mill. Accordingly, numerous testing systems have been proposed to locate the defects automatically. IFor example, it has been proposed to utilize ultrasonics, dye penetrants, magnetic, eddy current systems etc. Such systems have not been entirely satisfactory for several reasons. Among other things, it has been necessary to allow the rolled stock to cool to ambient temperatures before making the inspection. This, of course, it a time consumming operation. Moreover, this requires the inspection being delayed until after the completion of the rolling, final trimming and cropping operations. As a consequence, when a defect has been located it has not been possible to salvage the rolled stock by judicious trimming, cropping, etc. Moreover, if additional rolling were required it was necessary to reheat the stock.

Summary of the invention The present invention provides means for overcoming the foregoing difficulties. More particularly the presice ent invention provides means for inspecting the stock while it is still hot and before it is trimmed or cropped. This is accomplished by providing a rolling mill having an infrared inspection station therein which reliably locates internal defects in the rolling stock by remotely sensing the elevated surface temperature of the rolled stock. Variations in the surface temperatures beyond predetermined limits indicates a defect below the surface.

In the limited number of embodiments disclosed herein the infrared inspection station includes a pair of radiometers for scanning the rolled stock at two different areas so as to be responsive to the difference between radiations from these two areas. The two scanning areas are relatively close to each other whereby any naturally occurring temperature changes will be substantially equal at both scan areas. However, if there are any defects, such as an edge lamination or pipe inclusion, there will be a corresponding variation in the difference between the temperatures and the two scan areas whereby the defect will be reliably located.

Brief description of drawings These and other features and advantages of the present invention will become readily apparent from the following detailed description of a limited number of embodiments thereof, particularly when taken in connection with the accompanying drawings wherein like reference numerals refer to like parts and wherein;

FIGURE l is a block diagram of an inspection system embodying one form of the present invention;

FIGURE 2 is a fragmentary side view of a portion of the rolled stock being inspected by the system of FIGURE l;

FIGURE 3 is a side view of a portion of a rolling mill showing an inspection station embodying the inspection system of FIGURE 1;

FIGURE 4 is a block diagram similar to FIGURE l but showing an inspection station embodying a different form of the present invention, and

FIGURE 5 is a side view of an inspection station embodying another form of the present invention.

Description of preferred embodiments Referring to the drawings in more detail and particularly to FIGURES 1 and 2, the present invention is particularly adapted to be embodied in a system 10 for rolling blooms into steel sheets or plates. This system A10 includes a rolling station 12, a trimming or cropping station 14, a rolling table 16 extending therebetween and an inspection station 18 disposed on said rolling table 16 between the rolling station 12 and the cropping station 14.

The rolling station 12 includes a pair of enlarged rollers 20 adapted to receive a billet or bloom from the blooming mill. These rollers 20 are effective to compress or roll the billet or bloom into rolled stock 22 having a substantially uniform thickness. The rollers 20' are normally about 2 to 3 feet in diameter and may be on the order of up to 5 or l0 feet long whereby the finished stock 22 may have a corresponding width.

The lengths of the fully rolled stock 22 vary over a considerable range but they frequently have lengths on the order of up to about 20 or 30 feet or longer. If the stock 22 is rolled to a thickness on the order of about 0.060 to about 0.25 it is normally referred to as a sheet. Whereas, if it is rolled to a thickness in a range from about 3756" up to about 1% or more, it is usually referred to as plate. However, throughout this application the expression rolled stock shall be used to indicate all types of rolled material without regard to the thickness.

Prior to, during and following the rolling operation the rolled stock has an elevated temperature which, under some circumstances, may be in the red hot region. The stock 22 is normally hot worked during the rolling operation and the surface temperature remains in an elevated region. In a typical rolling mill the temperature of the fully rolled stock 22 as it leaves the last set of rolls is frequently in the region of about 1800 (Fahrenheit). However, in some mills it may be in a region extending from below l000 (Fahrenheit) to about 2000 (Fahrenheit) or higher.

It' the original ingot is cast with any inclusions such as slag, clay, sand, air bubbles etc., the inclusions tend to remain in that portion of the ingot which is still in the liquid phase. Since the center of the ingot is the last to solidify many of the inclusions are at or near the center of the ingot. As a consequence as the bloom is formed from the ingot and the bloom is rolled into the stock 22, the inclusions tend to remain concealed below the surface. They are, therefore, extremely difficult if not impossible to visually observe. Typically in the fully rolled stock 22 the inclusions are normally centered approximately midway between the top and bottom surfaces of the stock 22, as seen in FIGURE 3. Moreover, as best seen in FIGURE l, the inclusions 24 are frequently located near an end of the stock in the region of the longitudinal centerline of the rolled stock 22.

During rolling the thickness of the stock 22 is decreased and the material spread over a wider area. At the same time any inclusions 24 are also rolled flat and spread. As a consequence an inclusion of this type may expand to cover several square feet by the time the stock 22 is fully rolled. Since this tends to produce a hollow region in the stock 22 this type of defect is frequently called a pipe inclusion.

Under some circustances a small crack or inclusion may be present in the stock 22 immediately adjacent to the edge 28 thereof. This may cause the edge of the stock 22 to begin developing a laminar separation during the successive rolling operation. Since these separations are normally at or near the edge 28 they are frequently referred to as edge laminations 26. During the repeated rolling and working this separation or edge lamination 26 gradually grows inwardly from the edge 28 of the plate and may also eventually cover up to several square feet, Edge laminations may be at or near one end of the plate, as at 26A, or near the middle, as at 26B.

The rolling table 16 is disposed adjacent to the rollers and receives the fully rolled stock 22 as it emerges from the rollers 20. This table 16 includes a plurality of substantially parallel and horizontal side rails 30 with several relatively small diameter rollers 32 extending therebetween. These rollers 32 are adapted to support the weight of the stock 22 and allow it to be moved longitudinally on the table. Normally at least a portion of the rollers 32 are power driven whereby the operator can control the longitudinal position of the stock 22 on the table 16 and can even cause it to be repeatedly passed back and forth through the rollers 20 until it is rolled down to the desired thickness.

The cropping or trimming station 14 is disposed adjacent the end of the rolling table 16. After the stock 22 has been passed through the rollers 20 and reduced in thickness to the desired level, the rollers 32 are driven to carry the stock 22 through the trimming station 14. At this point the ends and/or edges 28 of the stock 22 are trimmed or cropped to reduce the stock 22 down to the desired width and/or length. Also, if there are any defects present in the stock 22 the defective portions may be cropped. This removes the defect and leaves entirely sound rolled stock. Since the stock 22 is still very hot at this time these trimming or cropping operations may be easily performed. Following the trimming or cropping operation the stock 22 may be sorted according to its intended future use and/or shunted into a storage area and allowed to cool to ambient temperature, etc.

In order to locate the various defects, such as pipe inclusions 24, edge laminations 26, etc. the inspection station 18 may scan the plate 22 as it travels across the rolling table 16, This inspection may occur between successive rolling operations. However, in the present instance it is made after the rolling is completed and the stock 22 is ready to be transferred to the cropping station 14.

The present inspection station 18 is of the so-called infrared variety wherein the radiations from the stock 22 are monitored. As a result the station 18 may be effectively separated by a considerable distance from the stock 22 and thereby protected from extremely high surface temperatures on the rolled stock 22.

The station 18 includes radiation IR sensing means 34 for receiving the infrared radiations and producing an electrical signal corresponding thereto. In the present instance the radiation sensing means is effective to sense the radiations at two separate and distinct areas. This may be accomplished by a single pickup which alternately scans the two separated areas. However, in this embodiment two separate pickups are provided for continuously scanning the two areas. Each pickup includes a device, such as a radiometer 36 and 38.

Each of the radiometers 36 and 38 includes an optical head 40 and42 having an infrared cell or similar device. The cell is disposed inside the head so as to be responsive to the radiations in the wavelengths naturally radiated from the surface 44 because of its elevated temperatures. Suitable electronics are coupled to each of the cells whereby electrical signals are provided having amplitudes that are functions of the intensity of the received radiations.

Lens system 46 and 48 are provided for the optical head 40 and 42. Each lens system 46 and 48 is focused onto a relatively small scan spot 50 and 52 respectively, on the surface 44. The radiations from these spots 50 and 52 are concentrated into the respective cell. It may thus be seen the signals produced by the radiometers 36 and 38 are functions of the surface temperatures at the respective scan spots 50 and 52 and the difference between the two signals corresponds to the difference between the two temperatures.

The lens systems 46 and 48 are arranged such that the two scan spots 50 and 52 are spaced a predetermined distance from each other. Although the direction and amount of spacing can be varied to satisfy any particular requirements, in this embodiment the scan Spots 50 and 52 are disposed laterally of the rolled stock with one of the scan spots 50 disposed near the center of the stock 22. As the stock 22 travels across the rolling table 16 the scan spot 50 follows a scan line 54 which extends through the region where a pipe inclusion is normally most apt to appear.

The second scan spot 52 is laterally displaced from the first spot 50 and will thereby follow a second sean line 56. This seocnd scan line 56 is preferbaly displaced from the region where the pipe inclusions 24 are most common. It will be seen if a defect, such as the pipe inclusion 24 passes through the inspection station 18, one radiometer 36 will receive radiatins corresponding to the temperature of the surface 44 adjacent to the defect while the other radiometer 38 receives radiations corresponding to the surface temperature of a defect free region.

The outputs from the two radiometers 36 and 38 are coupled to a suitable electronic system for processing the temperature signals. In the present instance this includes a differential amplifier 64 having two inputs 58 and 60 and a single output 62. The two inputs S8 and 60 are coupled to the outputs of the two radiometers 36 and 38 and receive the temperature signals therefrom. The signal present on the output 62 will be a function of the difference between the two signals. More particularly, if the ternperatures of the two scan spots 50 and 52 are identical the two temperature signals will be equal and the output will be zero. However, if one of the scan spots is hotter or cooler than the other scan spot, a temperature differential exists and accordingly there will be a corresponding difference signal present on the output 62. The amplitude of the difference signal is a function of the difference -between the temperature of the two scan spots 50 and 52.

The output 62 of the difference amplifier 64 is coupled to an amplifier 66 which amplies the difference signal to a more useful level and improves the signalto-noise ratio. This amplifier is in turn coupled to suitable output means for utilizing the difference signal. For example, the output means may include a meter 68 0r similar device to indicate the difference :between the temperature of the scan spots.

In addition, the output means may include an automatic device such as a reject or trigger circuit 70 which becomes hoperative when the temperature differential is outside of a predetermined range. For reasons that will be explained subsequently when an excessive temperature differential does exist, a defective area is present. The trigger 70 may be coupled to an alarm 72 and/ or marker 74 for indicating to the operator the presence of a defective area and its location. As a result these defective areas can be removed at the cropping or trimming station 14.

As the stock 22 is rolled to its final dimensions and carried across the table 16 it begins to cool. Because of convection cooling and for other reasons the naturally occurring heat losses are approximately twice as great from the top surface as from the bottom surface. As a result there is a general tendency for the heat to llow vertically through the stock with the top surface 44 normally being considerably colder than the bottom Surface 76.

It can be appreciated if there are any internal discontinuities within the stock 22 there will be corresponding variations in the thermal conductivity and the rate at which the energy flows upwardly. For example, if there is an air pocket or void present, the tiow of heat from the bottom 76 to the top 44 will Ibe reduced. This produces corresponding localized variations in the temperature of the surface 44. As the scan spots '50 and 4S2 at the foci of the radiometers 36 and 38 move along the scan lines l54 and 56, they will produce fluctuations in the temperature signals. These fluctuations correspond to the localized variations in the surface temperatures.

The temperature differences occurring around defects tend to be of relatively small magnitude if the stock 22 is merely allowed to cool naturally. As a consequence the radiometers 36 and 38 must be very sensitive to detect these variations and produce a signal having a satisfactory signal-to-noise ratio. It has also been found variations in emissivty etc. can produce variations that are significant compared to the temperature changes produced by naturally cooling defects. The temperature variations can 'be increased if the rate of cooling is increased and particularly if the increased cooling occurs on only one side of the stock 22.

If the amount of cooling is of sufficient magnitude there will be a very large difference between the temperatures on the top and bottom surfaces `44 and 76. By cooling the underside of the stock the thermal energy will flow downwardly from a region just below the top surface 44 toward the bottom surface 76. As a consequence the ilow of thermal energy will -be substantially entirely downwardly. If the rolled stock 22 has a uniform thermal conductivity the temperature of the top surface 44 will be uniform. However, if there is a discontinuity such as the inclusion 24 or the edge laminations 26 there will be a corresponding discontinuity in the thermal conductivity. This, in turn, will result in the temperature of the upper surface 40 directly over the defect cooling at a considerably slower rate than normal, i.e. it will Ibe relatively hotter.

In order to produce the foregoing type of accelerated cooling, a cooling device such as a jet 7S may be provided below the rolled stock 22 in the region where it leaves the rollers 20. This jet 78 directs a stream [t0` of coolant, such as cold air, water, etc., against the bottom 76 of the stock 22. This coolant absorbs large quantities of thermal energy and lowers the temperature of the bottom surface 76.

In a practical application it is normally preferable t0 direct a fairly large stream 80 of Water at about room temperature against the underside of the stock 22. This stream 80 is directed against a substantial area that extends across the width of the stock and is aligned with the regions containing the types of defects which are of interest. For example, if pipe inclusions 24 are of primary interest the stream 80 covers the central portion of the stock 22. If edge laminations 26 are of primary interest the stream 80 covers the edge portions of the stock. If both types of defects are of interest both portions of the stock are covered =by the stream. In addition, for reasons that will become apparent subsequently, the stream 80 also covers an area which is generally free of the foregoing types of defects.

The spots 50 and 52 are spaced a considerable distance, for example several feet, away from the area 82 cooled by the stream 80. By the time a particular part of the rolled stock 22 has traveled over the jet 78 and reaches the scan spots 50 and 52, a large quantity of thermal energy will have flowed downwardly toward the -bottom surface 76 and the temperature of the top surface will have been greatly reduced. The two scan spots 50 and 52 are laterally spaced but both of the scan lines 54 and -56 pass over the cooled region 82. The radiometers 36 and 38 will thereby produce signals that are functions of the reduced temperatures.

The temperatures at the scan spots 56 and 52 will be a function of several factors, such as the initial temperature of the stock, its rate of travel, the amount of heat absorbed by the coolant, the time delay between the cooling and the scanning etc. In addition the temperatures of the scan spots 50 and 52 are a function of the thermal conductivities of the rolled stock 22 underlying the scan spots 50 and 52. If the stock 22 is uniform and free of defects, the conductivity is substantially uniform and the temperatuers of the scan spots 50 and 52 will be substantially identical. However, if a defect is present as pointed out above, the heat ilow is reduced and the surface temperature above the defect is greater than normal, i.e. a hot spot is present. Under these circumstances a large temperature differential will be present between the two scan spots 50 and 52.

It might be expected the two temperatures, the resultant infrared radiations and the signals from the radiometer would be substantially constant (assuming there are no defects present). However, it has been found as a practical matter the stock 22 is frequently unevenly heated, there are irregularities in the rolling process, the speed of the stock varies, the amount of heat absorbed by the coolant varies, the emissivty varies, etc. These factors result in significant variations in the temperature of the surface. However, these variations occur primarilyin the longitudinal direction over extended distances. The changes which occur in directions transverse of the stock are relatively small. As a consequence although the temperatures and/or the radiations from the two laterally spaced scan spots 50 and 52 may vary, they both vary in similar manners whereby the temperature differential is relatively small (assuming there are no defects present). Moreover, if there is a defect present under only one of the scan spots, the temperature differential will be very large compared to the slowly varying factors. Accordingly, the reject level for the trigger can be set above the naturally occurring variations and below the defect produced variations.

In order to utilize this system the unrolled material is fed between the two rollers and onto the table 16. As the rolled stock 22 enters the inspection station 18 and jet 78 directs a stream 80 of coolant against the underside of the hot stock 22. This absorbs large quantities of thermal energy from the bottom surface 76 and causes substantial quantities of the thermal energy to ow downwardly from the region of the upper surface 44. This, in turn, will cause the top surface 44 to cool at an accelerated rate. The two radiometers 36 and 38 will receive the radiations resulting from the temperatures of the two scan spots 50 and 52 and will produce signals which correspond to these two temperatures.

If the rolled stock 22 has substantially uniform characteristics over its entire width and is free from any discontinuities such as the pipe inclusions 24 etc., the two scan spots 50 and 52 will have substantially identical temperatures. These temperatures will depend upon various characteristics, such as the initial temperature of the rolled stock, the amount of heat absorbed by the cooling jet, the rate of travel of the stock across the rolling table, the radiating characteristic of the surface, etc. Normally all of these factors vary at a relatively slow rate. As a consequence if the two scan spots 50 and 52 are fairly close together, for example a few feet apart, the temperatures and the radiations will be substantially identical. The two temperature signals are in turn coupled through the difference amplifier 64 whereby an amplifier signal is provided that is a function of the difference between the two temperatures at the two scan spots 50 and 52.

If there are no discontinuities aligned with the scan spots the temperature differential will be zero or very small, i.e. less than the threshold level of the trigger 70. As a consequence no defects will be indicated.

However, if there is a discontinuity, such as the pipe inclusion 24 or edge laminations 26, there will be a corresponding localized variation in the thermal conductivity through the thickness of the stock 22. Normally this produces a decrease in the conductivity, a corresponding decrease in the rate of heat flow toward the bottom and an elevated temperature on the top surface immediately adjacent to the discontinuity. Under these circumstances as the scan spot 50 travels over the hot spot the temperature signal from the radiometer 36 increases. Normally the second scan spot 52 is laterally displaced from the discontinuity and will continue to travel in a cooled region. The difference amplifier will now produce a large signal. The amplitude of this signal normally far exceeds the reject level and as a consequence an indication or alarm will be produced.

It can be appreciated that the present embodiment is primarily adapted for locating pipe inclusions disposed somewhere around the center line of the stock. In the event it is desired to identify edge laminations one of the radiometers may be positioned to place the scan spots near the edge of the stock whereby it will travel over any edge lamination. If it is desired to identify pipe inclusions and/or edge laminations on both sides of the plate, more than two radiometers may be provided for producing scan spots which are located at strategic areas on the stock. Moreover, a single radiometer may be provided for scanning laterally across the stock 22 and then gating the signal to provide signals corresponding to the lateral areas of the stock 22.

Although the foregoing arrangement is effective it has been found desirable, under some circumstances, to .utilize the embodiment 90 of FIGURE 4. This embodi ment 90 is substantially identical to the preceding embodiment in that it is disposed over the rolling table 16 between the rolling station 12 `and the trimming or cropping station 14. In addition a cooling jet is disposed beneath the rolling table 16 so as to direct a stream of coolant against the underside of the rolled stock. This jet absorbs large quantities of thermal energy and cools the under side of the stock whereby a cooled region 92 is formed substantially the same as in the preceding embodiment.

A pair of radiometers 94 and 96 are provided and focused upon the rolled stock so as to define scan spots 98 and 100 in substantially the same manner as described in the preceding embodiment. However, in this embodiment the two scan spots 98 and 100 are disposed in substantially longitudinal alignment with each other whereby both radiometers will follow the same scan line 102 and scan substantially identical materials. The two scan spots 98 land 100 are disposed on opposite sides of the cooled region 92. Thus the two radiometers will now produce signals which-represents the difference between the temperatures before and after cooling.

If the stock 22 is sound and free from any discontinuities the top surface will cool at a rapid rate. Accordingly, the temperature difference will normally exceed some predetermined level. However, if there is a discontinuity there will bea decreased rate of heat transfer and the surface temperature will not cool as fast as normal. This results in a temperature between the scan spots and 100 which is smaller than normal.

The two radiometers 94 and 96 are in turn coupled to a difference amplifier`104, an amplifier 106 and a trigger circuit 108. This embodiment functions essentially the same as the preceding system except the trigger 108 now causes an alarm when the temperature difference is too small.

As an alternative the embodiment of FIGURE 5 may be employed. This embodiment 116 is substantially the same as the two preceding embodiments in that it is normally located between the rolling station 12 and trimming or cropping stations 14. Also the rolled stock 22 is cooled by a jet 118 which directs a stream 120 of coolant against the bottom surface 122 of the stock 22. In this embodiment a pair of radiometers 124 and 126 are provided. However, they are positioned on the opposite sides of the stock so as to form a scan spot on the upper surface 128 and a scan spot on the bottom surface 122.

Normally these two scan spots are in direct alignment with each other. Thus the first radiometer 124 will produce a temperature signal Corresponding to the top surface temperature while the second radiometer 126 will produce a signal corresponding to the bottom temperature. The two radiometers 124 and 126 are coupled to a difference circuit for activating an alarm when the temperature difference exceeds a predetermined level.

It can be appreciated if the stock 22 is of uniform acceptable quality and free from any discontinuities, the temperature difference will be below a predetermined level. However, if there is a discontinuity present so as to impede the fiow of thermal energy between the two surfaces 122 and 128 the temperature difference will exceed a predetermined level whereby the alarm will be actuated.

While only a limited number of embodiments of the present invention have been disclosed herein it will be readily apparent to persons skilled in the art that numerous changes and modifications may be made without departing from the invention. Accordingly, the foregoing drawings and description thereof are for illustrative purposes only and do not in any way limit the scope of the invention which is defined only by the claims which follow.

I claim:

1. A nondestructive tester for inspecting rolled stock following a rolling operation, said tester including the combination of cooling means for reducing the temperature of at least a portion of said rolled stock whereby said portion is allowed to change its temperature for a predetermined time interval;

a pair of radiometers for receiving infrared radiation from the surface of said rolled stock and providing signals corresponding to the temperatures of said surface,

focusing means for focusing said radiometers onto a pair of separated scan spots whereby said signals are functions of the temperatures at said scan spots, said scan spots being disposed at two different points which are cooled before being scanned by the radiorneters;

circuit means coupled to said radiometers and responsive to said signals to produce a difference signal that is a function of the difference between the temperatures of said surface at two separate locations corresponding to dilerences in the rate of cooling; and

utilizing means coupled to said circuit means and responsive to said difference signal, said utilizing means being effective to perform an operation whenever the difference signal Varies beyond predetermined limits.

2. A nondestructive tester for inspecting rolled stock following a rolling operation and while said stock is cooling, said tester including the combination of temperature sensor means adapted to be spaced from the stock for scanning said stock along at least one scan line, and

circuit means coupled to said sensor means being effective to produce an electrical signal that is a function of the difference between the temperatures of said surface lat two separate locations.

3. The nondestructive tester of claim 2 including utilizing means coupled to said circuit means and responsive to said electrical signal, said utilizing means beng effective to perform an operation whenever the electrical signal varies beyond predetermined limits.

4. The nondestructive tester of claim I1 wherein said means includes a jet for directing a stream of coolant against a surface of the workpiece.

5. The nondestructive tester of claim 2 wherein the temperature sensor includes a pair of radiometers for receiving infrared radiations from the surface of the cooling rolled stock and providing signals corresponding to the temperatures of said surface, and

focusing means for focusing said radiometers onto a pair of separated scan spots whereby said signals are functions of the temperatures at said scan spots.

6. The nondestructive tester of claim 5 wherein the scan spots defined by said radiometers are disposed longitudinally of the workpiece on the opposite sides of the cooled portion, one of said scan spots being positioned before the area cooled by the jet and the other being positioned after said area whereby the difference signal corresponds to the amount of cooling produced by the jet.

7. The nondestructive tester of claim 5 wherein the radiometers and the scan spots are disposed on the opposite sides of the workpiece whereby said signal corresponds to the difference between the temperatures on the opposite surfaces of the workpiece.

References Cited UNITED STATES PATENTS 3,044,297 7/ 1962 Hanken.

3,188,256 6/1965 Shoemaker.

3,216,241 11/1965 Hansen.

3,245,261 4/1966 Buteuy et al.

3,295,842 1/ 1967 Stelling et al 73-351 3,206,603 9/ 1965 Mauro.

RALPH G. NILSON, Primary Examiner MORTON J. FROME, Assistant Examiner U.S. C1. X.R. 73--351

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