GB2076966A

GB2076966A - Apparatus and method for measuring layer thicknesses of a multilayered metal member

Info

Publication number: GB2076966A
Application number: GB8017068A
Authority: GB
Original assignee: Japan Steel Works Ltd
Current assignee: Japan Steel Works Ltd
Priority date: 1980-05-23
Filing date: 1980-05-23
Publication date: 1981-12-09
Also published as: GB2076966B

Abstract

A method and apparatus for measuring the thicknesses of a cladding layer 10 and a base metal layer 8 by applying pulses of ultrasonic waves (T) on the side of a sample opposite the cladding layer using a contact probe 1. The reflected waves are sensed by a crack detector and displayed on an oscilloscope. By eliminating noise and interference, the position of the interface 9 between the cladding layer and base metal layer are measured from a pulse (4) produced due to the discontinuity of acoustic impedance at the interface. <IMAGE>

Description

SPECIFiCATION Method and apparatus for measuring layer thistnesa ses of a multilayered metal member The present invention relates to a method and apparatus for measuring the separate thicknesses of the various layers of a multilayered metal member such as a base metal having a clad metal layer formed thereon, for example clad steel.

So-ealled clad steel is broadly used in various industrial fields because it is very economical and also durable. Clad steel is so formed with a material such as stainless steel, titanium, aluminium, copper or an alloy thereof, which is different from the base metal, metallurgically overlaid on one side or both sides of the base metal such as carbon steel or low alloy steel by hot rolling, explosive adhesion or welding.

Such clad steel is particularly well adapted for use in a corrosive environment. However, in such applications, it is particularly important to maintain a proper thickness of the cladding material which forms the corrosion resistant layer.

Various methods for measuring the thickness of the cladding material or layer have been proposed.

In a first such method, the thickness of the layer is mechanically measured with a measurement tool after etching away a peripheral portion. In a second prior art method, the overall thickness including that of the base metal is measured using a supersonic thickness gauge and the thickness of the cladding layer is calculated. In a third method, the thickness of the cladding layer is measured using an electromagnetic minute thickness detector to measure the variation of magnetic permeability caused by the cladding layer.

The above described conventional methods have the following drawbacks. The first mechanical method is only applicable for measuring a marginal portion of the clad steel. With this method, it is impossible to measure the thickness over the entire clad steel surface particularly at locally changed portions which may be formed due to forming compression. The second method in which a supersonic thickness gauge is employed may be used for measuring the thickness over the entire clad steel surface but it is impossible to use it to measure changes of the thickness due to variations in the forming rates of the cladding layer and the base metal. The third method wherein a electromagnetic minute thickness detector is used is applicable only to measurement of the cladding layer and its measuring precision is very low.In addition, the method is not applicable to the measurement of a cladding layer having magnetic properties.

It is thus an object of this invention to provide a method and apparatus for measuring the thickness of various layers of a multilayered metal member in which the above mentioned drawbacks are overcome or reduced.

According to one aspect of this invention, there is provided a method for measuring the thickness of various layers of a multilayered metal membercom- prising the steps of: providing a pulsed source of ultrasonic waves at a position adjacent a first surface of said metal member; producing electrical signals in response to ultrasonic waves reflected from a second surface of said member; displaying on an oscilloscope a graphical representation of said electrical signals; and performing measurements on said graphical representation to determine the position of an interface between tNo of said layers relative to at least one of said surfaces.

According to another aspect of the invention, there is provided a method for measuring the thickness of various layers of a multilayered metal member comprising the steps of: providing a wide-band and high-damping contact detecting terminal means and a supersonic wave crack detector having a wideband amplifying means at a position adjacent a surface of said metal member; coupling said crack detector to an oscilloscope having a continuously adjustable time scale; maintaining a displayed pulse width and a rejection of signals produced by said crack detector at a zero level and increasing the gain thereof so that an interface echo can be seen between first and second layers of said metal member on said oscilloscope; increasing said pulse width so that said interface echo increases in amplitude together with a noise echo; and increasing said rejection to thereby remove said noise echo from said interface echo whereby said interface echo is displayed as a clear wave shape.

According to a further aspect of this invention, there is provided an apparatus for measuring thicknesses of various layers of a multilayered metal member comprising; wide-band and high-damping contact detecting terminal means having a wide bandwidth and a high damping effect; supersonic wave crack detecting means; wide-band amplifier means for amplifying signals produced by said crack detecting means and rejection filter means coupled to said amplifying means; and a cathode-ray tube oscilloscope having an input connected to said crack detecting means.

Thus, there is provided a method which takes into account the differences in acoustic impedances of the base material and the cladding material thereon.

A metallic compound at the interface between the cladding material and the base material forms has crystals of different crystal grain. Because of the difference in crystal grain arrangement at the interface, upon applying ultrasonic waves, an echo is gener ated at the interface due to the difference in structure of the two chemical compounds. The signal from the echo may be amplified and rectified and from the resulting signal the thickness of the cladding material can be precisely and positively measured over the entire surface of the material. Such a measurement has hitherto been considered impossible.

With the present invention, it is possible to measure the thickness of clad steel which may in fact vary during manufacture or may change due to aging so that such clad material, once it passes such inspec tion, may be used with complete confidence and safety.

This invention will now be described in more detail, by way of example, with reference to the accompanying drawings in which: Figures 1A and 1 B are diagrams for illustrating the principle of measurement of using ultrasonic waves employed in the invention; Figures 2A to 2D show successive states of measuring clad steel according to the method of the present invention; Figures 3A and 3B show another method according to the present invention; Figure 4 is a block diagram of an input circuit used with the invention; and Figure 5 is a block diagram of an output circuit used with the invention.

Figures 1 A and 1 B illustrate the principle of meas urement of thickness of a test piece using supersonic waves according to the invention. With the assumption that the speed of sound is constant in a uniform material, a supersonic wave 2 is emitted from a contact detecting terminal 1,travels through the material, and is reflected back to the transducer 1 from the lower surface of the material. Figure 13 shows an image on cathode-ray tube oscilloscope corresponding to Figure 1A. The position of a bottom surface echo B is indicated on the oscilloscope tube in which the total thickness t of the material is proportional to atimet'.

The relationship between the scale of the time axis and the actual thickness measured may be calibrated by using a test piece having a known thickness in which the speed of sound is constant from which the thickness t is measured by reading out on the time axis the position of the bottom surface echo B of the test material being measured.

Supersonic waves have the property that when they pass through materials different in acoustic impedance, a part or most of the supersonic waves is reflected at the interface therebetween. The grea- teethe difference in the acoustic impedance, the greater is the amount of such reflection.

Reflections often occur due to differences in the size of the crystal grain or its arrangement or they may occur because of differences in chemical composition. Taking advantage of the above-noted property of supersonic waves, the present invention provides a method and an apparatus utilizing this supersonic wave property to thereby measure the thicknesses of the cladding layer and the base metal ofthe clad steel.

An embodiment for measuring the respective thickness of the cladding layer and the base metal will be described in which the base metal is made of carbon steel having a small difference in its acoustic impedance from that of a cladding layer thereon which is made of stainless steel with accordingly only a very weak echo generated at the interface therebetween. It should be noted that in case of other cladding materials such as aluminium, copper and alloys thereof, the differences of their acoustic impedances from that of the base metal are much greater than in this example. That is, it is easier to measure the thicknesses.

The method of the present embodiment differs significantly from those described before. According to the present invention, a weak interface echo is clearly separated just in front of the bottom surface echo with a precision of at least 10 times that previ ously obtainable by amplifying the grain, enhancing the pulse output, and displaying the shape of the echo signal on an osciBloscope tube. Therefore, it is possible to precisely read out the thicknesses of the base material and the cladding material.

With the present embcdiment, a wide-band high damping contact detecting terminal having a wide frequency spectrum and a damping constant so as to make the signals highly resolvable and a supersonic crack detector are employed. The supersonic crack detector has the following properties.

(1) An echo having a narrow width is faithfully amplified by means of a wide-band amplifying circuitry having a bandwidth above 0.5MHz and in which the gain is constant over a wide frequency range.

(2) In regard to the oscilloscope employed, the gain for input signals should be continuously variable.

(3) Also, the time axis position on the oscilloscope should be capable of moving an image point beyond a scale length corresponding to the thickness of the material to be measured. For example, for an oscilloscope in which the measurable range is limited to 10.0 mm, the minimal scale units of the time axis should correspond at least to 0.2 mm.

(4) Amplifying rectilinear propagation is not affected by the rejection with respect to an echo producing a signal amplitude higher than the maximum scale ofthe oscilloscope. The adjustment of the rejection is continuously variable.

(5) The time axis scale on the oscilloscope is divided into at least fifty equidistant scale units.

(6) A measurement range of 10 mm or less can be sufficiently enlarged.

Figures 2A to 2D shown oscilloscope displays for the embodiment in which by using thus constructed contact detecting terminal and supersonic crack detector, the clad steel plate is measured from the base material surface side. The base material and the cladding material for this example are AST 7A 440 and AlSi 304, respectively, and have thicknesses of 7.8 mm and 2.0 mm, respectively. The total thickness thereof is thus 9.8 mm.

Figure 2A shows a state in which the pulse width from the interface echo and the rejection of the crack detector are maintained at zero levels and the gain is set to B = 100% + 26dB so that an initial echo 4 can just be seen. In Figure 2A, T designates the transmitted pulse and B a bottom echo from the outside face ofthe sample. When the interface echo is seen, the pulse width thereof is increased by changing the time axis scale while the time delay of the oscillo- scope is adjusted to maintain the position of the pulse constant on the display screen. Then, while the state shown in Figure 2B is maintained, effectively only the rejection is increased to eliminate the noise echo. The resulting interface echo 4 can be clearly seen in the form of a line in Figure 2C. As described above, by matching gain, pulse width and rejection, an interface echo having a sharp initial rise can be displayed on the display screen of the oscilloscope.

The time axis is in advance determined by accurate calibration using a known test piece. The measurable range is established in order to include the total thickness of the material to be measured.

Since a surface 5 of the material which the contact detecting terminal contacts is coincident with the zero point, the distance from this surface to the position 6 of the rise of the interface echo 4 represents the thickness of the base material and the distance from the rising position 6 of the interface echo 4 to the rising position 7 of the bottom surface echo B represents the thickness of the cladding layer 10.

Also, the distance from the surface 5 to the position 7 of the bottom echo B represents the total thickness.

To obtain thickness measurements, the distance between these positions are scaled in accordance with the velocity of sound of the various materials.

Figure 3 shows a case where a thickness of 10 to 20 mm is measured in substantially the same manner as in the previous example. In this case, if the above-described rising position of the bottom surface echo B' which has been already calibrated is moved to the zero point 5 by using the time axis adjustment feature of the oscilloscope, the real measurable range is 10 to 20 mm. The thickness can be obtained by adding 10 mm to the direct scale readout at the position 6 to 7 corresponding to the interface echo or the bottom surface echo. In the same manner, a much greater thickness or depth can be measured in order to maintain the range of measurement within 10 mm at all times. Therefore, a high accurate measurement can uniformly be achieved.

As described above, according to the present invention, it is possible to measure the thickness of the cladding material layer of clad steel which has hitherto been considered impossible. According to the invention, it is possible to very precisely measure the thickness at any position of the clad steel within +1 mm with ease and also to measure the thickness from either surface side or backside.

If the total thickness of the clad steel is more than 2.5 mm and that of the cladding material is more than 0.4 mm, it is quite easy to measure both thicknesses. It is also possible to measure a thickness of a material having a curvature of more than 1.5 times that of the contact detecting terminal diameter which has a cylindrical or arcuate shape.

Therefore, according to the present invention, the thickness of the cladding layer processed through forming can be precisely known. This is economical bcause the use of expensive material can be minimised.

Referring next to Figure 4 there is shown therein a schematic diagram of the contact detecting terminal and associated circuitry used for generating ultrasonic pulses. A resistor 20 is connected between a source of DC voltage, for example 500 volts, and one terminal of a switch 22, the other terminal of which is connected to ground. Switch 22 is preferably a silicon controlled rectifier or other semiconductor switch. Switch 22 is operated periodically at a convenient rate for the oscilloscope. One terminal of a capacitor 21 is connected to the junction between resistor 20 and switch 22 while the otherterminal is coupled through a series combination of variable resistor 23 and fixed resistor 24 to ground.

Adjustment of variable resistor 23 determines the pulse width or pulse energy of pulses through capacitor 21. The junction of capacitor 21 and variable resistor 23 is coupled to coil 25, the other terminal of which is coupled to ground. The common point of capacitor 21, variable resistor 23 and coil 25 are connected to the input terminal of wide-band transducer 28 of contact detecting terminal 27. Contact detecting terminal 27 also includes a piece of damper material 26 disposed upon the backside of transducer 28 to prevent ringing thereof. Contact detecting terminal 27 may, for example, be a model CLF-5 contact detecting terminal manufactured by Krautkraemer Company of West Germany. Preferably, transducer 28 is ultrasonically coupled to test piece 30 by the use of a contact medium 29 such as a gel or the like, a number of types of which are commercially available.

Referring next to Figure 5, there is shown therein a block schematic diagram of the supersonic crack detector and associated circuitry utilised with the invention. A transducer 31 of the crack detector 40 is ultrasonically coupled to test piece 30 by the use of contact medium 29. Preferably, transducer 31 of crack detector 40 and transducer 28 of contact detecting terminal 27 are physically located adjacent one another in a probe. Also similar to the case of contact detecting terminal 27, a piece of damper material 39 is disposed at the rear of transducer 31 of crack detector 40. The output of transducer 31 is fed through a high frequency amplifier 32, attenuator 33 and second high frequency amplifier 34 to the input of detector 35. The waveforms at various points along this circuit are indicated on the diagram of Figure 5. Detector 35 may be constituted by either a half-wave rectifier orfull-wave rectifier as desired.

The output waveforms from detector 35 are indicated for both types of rectifiers. Filters 36 and 38 are coupled to the output of detector 35. Filter 38 is a rejection filter which reduces the high frequency components in the output of detector 35. Preferably, rejection filter 38 is adjustable so that the operator can obtain the clearest possible output waveform.

The output of rejection filter 38 is coupled through a wide-band video amplifier 37 to the vertical input terminal of the cathode-ray tube oscilloscope with which the circuit is used. Crack detector 40 may, for example, be constituted by a model USL-32 crack detector manufactured by Krautkraemer Company of West Germany.

Claims

1. A method for measuring the thickness of various layers of a multilayered metal membercompris- ing the steps of: providing a pulsed source of ultrasonic waves at a position adjacent a first surface of said metal member; producing electrical signals in response to ultrasonic waves reflected from a second surface of said member; displaying on an oscilloscope a graphical representation of said electrical signals; and performing measurements on said graphical representation to determine the position of an interface between two of said layers relative to at least one of said surfaces.

2. A method for measuring the thickness of various layers of a multilayered metal membercomprising the steps of: providing a wide-band and high damping contact detecting terminal means and a supersonic wave crack detector having a wide-band amplifying means at a position adjacent a surface of said metal member; coupling said crack detector to an oscilloscope having a continuously adjustable time scale; maintaining a displayed pulse wide ancl a rejection of signals produced by said crack detector at a zero level and increasing the gain thereof so that an interface echo can be seen between first and second layersofeald metal member on said oscilloscope; increasing said pulse width so that said interface echo increases in amplitude together with a noise echo; and increasing said rejection to thereby remove said noise echo from said interface echo whereby said interface echo is displayed as a clear wave shape.

3. An apparatus for measuring thicknesses of various layers of a multilayered metal member com- prising: wide-band and kigh-damping contact detecting terminal means having a wide bandwidth and a high damping effect; supersonic wave crack detecting means; wide-band amplifier means for amplifying signals produced by said crack detecting means and rejection filter means coupled to said amplifying means; and a cathode-ray tube oscilloscope having an input connected to said crack detecting means.

4. Athickness measuring apparatus as claimed in Claim 3 further comprising pulse generating means coupled to said contact detecting terminal means, said pulse generating means comprising a first resistor and a periodically-operated electronic switch coupled between terminals of a DC voltage source, a capacitor having a first terminal coupled to a junction between said first resistor and said switch, a variable resistor and second resistor coupled in series with one another and coupled between a second terminal of said capacitor and a ground terminal of said DC voltage source and an inductor having a first terminal coupled to said second terminal of said capacitor and a second terminal coupled to said ground terminal, said first terminal of said inductor being coupled to an inputterminal of said contact detecting terminal means.

5. Athickness measuring apparatus as claimed in Claim 3 or Claim 4 further comprising detector means coupled to an output of said wide-band amplifier means, said rejection filter means being coupled to an output of said detector means.

6. A method for measuring the thickness of various layers of a multilayered metal member substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

7. An apparatus for measuring the thickness of various layers of a multilayered metal member substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

Amendments to claims filed on 20 March '81.

Claim 1 deleted, remaining claims re-numbered and appendancies corrected.