US 3303691 A
Description (OCR text may contain errors)
z d rllrlrllrlllldrlfim f 1967 L. BEAUJARD ETAL ULTRASONIC INSPECTION METHOD AND APPARATUS Filed Feb. 19, 1963 Fig. 2
Jac ues New T Mme/mu kn FL 29K www United States Patent I 3,303,691 ULTRASONIC INSPECTION METHOD AND APPARATUS Louis Beaujard, Jacques Mondot, and Marian Kapluszak, Saint Germain-en-Laye, France, assignors to Institut de Recherches de la Sidrurgie Francaise, Saint Germainen-Laye, France, a professional institution of France Filed Feb. 19, 1963, Ser. No. 259,659 Claims priority, application France, Feb. 23, 1962, 888,981, Patent 1,323,002 10.Claims. (Cl.73-71.5)
The present invention relates to improvements in ultrasonic inspection systems, and more particularly to a novel method of ultrasonically detecting flaws in test pieces at an elevated temperature above the point of depolarization or the Curie point of the usual piezo-electric transducer used in such systems.
In most such coventional systems, a refractory material is interposed between the piezo-electric crystal, which is cooled, and the hot test piece so that the transducer is shielded from the heat. This causes considerable difficulties in assuring the required acoustic contact between the test piece and the transducer, and the effective passage of the ultrasonic beam through the refractory heat shield. Fused salts have been used in an effort to overcome these difficulties but such use has caused other and sometimes even greater ditficulties. None of the known systems of this type has proved altogether satisfactory.
The disadvantages of these conventional systems are further increased when the surface area of the test piece to be inspected is rough or irregular. These disadvantages include difliculty of operation and of handling the system and, most of all, considerable irregularities in the acoustic contact due to the presence of oxide surface coatings on metallic test pieces and a considerable absorption of ultrasonic vibrations in the couplant.
It is the primary object of the present invention to overcome these disadvantages and to permit ready ultrasonic inspection of test pieces whose temperature is considerably above that to which the electro-mechanical transducer of the system may be safely subjected.
With this and other objects in view, this invention provides an ultrasonic inspection method wherein an electro-mechanical transducer face is placed adjacent an area of the test piece to be inspected and a stream of a cold liquid couplant is continuously delivered between the transducer face and this test piece area to couple the transducer acoustically to the area. The liquid of the couplant has a boiling point below the temperature of the test piece and at least a portion of the stream of fresh liquid couplant entering the space between the transducer face and the test piece surface first sweeps the transducer face before more closely approaching the test piece to prevent the transducer face from being unduly heated by the test piece. The liquid couplant stream forms a flowing liquid film in the space and moves at all points at such high speed that the liquid does not reach its boiling point in the space. A high-speed stream of a cooling liquid, which may be the liquid couplant stream, reduces the surface temperature of the area rapidly and fieetingly.
In one embodiment of the invention, the high-speed stream of the cooling liquid is delivered to a region of the test piece having an area at least equal to the area of the test piece to be inspected, and the transducer face is immediately thereafter placed adjacent the cooled region while its surface temperature is still reduced, with a separate stream of a cold liquid couplant being delivered in the above-described manner. In this embodiment, the transducer face may be continuously moved over successive cooled regions of the test piece.
In another embodiment, the cooling liquid and liquid 3,303,691 Patented" Feb. 14, 1 967 couplant stream are the same, the high-speed liquid streamserving the function of cooling liquid and liquid couplant.
Either the liquid couplant or the cooling liquid, or both, are preferably water but if the temperature of the test piece is so high that water cannot be utilized, liquids with a higher boiling point are used. Merely by way of example, such liquids include oils of different boiling points or fused salts or metals. When such liquids are used, the transducer must be enclosed in a cooled, for instance water-cooled, container, the cooling liquid simultaneously establishing the necessary acoustic contact between the transducer and the container, while the container is in acosutic contact with the test piece through the liquid couplant with the high boiling point. It will be realized, however, that the number of separating interfaces through which the ultrasonic beam must pass in such a system is increased, which unfavorably influences the accuracy of the flaw detecting system. Therefore, water is definitely the preferred fluid medium.
The above and other objects and features of the present invention will become more apparent in the following detailed description of two embodiments of the method, taken in conjunction with the accompanying drawing showing useful apparatus for carrying out the method and wherein FIG. 1 is a vertical section along line I-I of FIG. 2 of a searching unit for carrying out the method according to one embodiment;
FIG. 2 is a bottom view of the unit of FIG. 1;
FIG. 3 is a sectional view along line IIIIII of FIG. 4 similar to that of FIG. 1 of another embodiment; and
FIG. 4 is a bottom view of FIG. 3.
Referring now to the drawing and first to FIGS. 1 and 2, there is shown a searching unit useful for the continuous inspection of hot flat stock, for instance rolled metal sheets.
In the illustrated embodiment, the metallic body 1 of the searching unit has a flat end face 10 designed to be placed parallel to, and at a slight distance of, for instance, a few tenths of a millimeter from, the fiat surface of the test piece 2 to be inspected. In the illustrated embodiment, the searching unit is hydraulically supported on the test piece surface in a manner more particularly described and claimed in our copending application Serial No. 259,660, now Patent No. 3,159,756, filed on even date herewith and also entitled Ultrasonic Inspection. This support forms no part of the present invention and will, therefore, be described only sketchily herein. As shown, the end face 10 defines three circular orifices at the end of hydraulic liquid delivery conduits 3, 4, and 5 constituted by bores in the searching unit body 1 and permitting three supporting colums of a hydraulic liquid to flow therethrough to support the body end face 10 in spaced relation to the test piece surface.
A reservoir 6 holding a liquid under a suitable pressure is bolted to the top of the searching unit body 1 and receives the liquid through the supply conduit 19, tho liquid being pumped or otherwise delivered to the reservoir under the desired ressure from a source (not shown). With water used as hydraulic liquid, we have found a head of 4 kg./sq. cm. to be satisfactory but this may obviously vary according to operating conditions and dimensions of the searching unit and delivery conduits.
As more fully explained in our above-mentioned copending application, whose disclosure is herewith made a part of the description, the hydraulic fluid is fed into the delivery conduits through suitably restricted passages of calibrated nozzles 7, 8 and 9, respectively, in such a manner that the liquid fiow is controlled substantially independently of the conditions of support of the searching unit on the test piece and maintained substantially constant regardless of such conditions. In such a hydraulic support, the searching unit floats on the test piece and the distance on the end face 10 from the test piece surface is self-regulated in sole dependence on the head of the hydraulic liquid in the supporting columns.
Ultrasonic inspection iseffected by an annular piezoelectric crystal 11 pierced by a central hole 12 in alignment with conduit 3 to permit passage of the hydraulic liquid to the surface of the test piece. The piezo-electric transducer is connected by a coaxial cable in the manner illustrated in FIG. 3 to an ultrasonic generator (not shown) to constitute an otherwise conventional ultrasonic flaw detection system as described, for instance, in Ultrasonic Flaw Detection, issued by the U8. Department of Commerce, November 1958, and available through the Superintendent of Documents, U.S. Government Printing Oflice. Since the invention is not concerned with this aspect of the system, the electrical circuit elements connected to the transducer have not been illustrated.
In the illustrated embodiment, the hydraulic liquid fed through conduit 3 simultaneously serves as a high-speed stream of coolant and as a liquid couplant between the transducer face and the area of the test piece to be inspected. The stream of fresh liquid coming from reservoir 6 radially flows outwardly from conduit 3 and sweeps the transducer face to prevent the face from being heated. The stream fills the space between the transducer face and the test piece area to be inspected to form therein a liquid couplant, and the liquid film moves at such high speed that the liquid does not reach its boiling point in this space. As clearly shown, the liquid flow in the space is unencumbered and there are no dead spaces or eddies to interfere with the rapid and continuously even outward flow of the liquid over the entire transducer face and test piece area to be inspected.
A fourth conduit 13 connects the reservoir 6 with a V-shaped orifice 14 in end face 10 of the searching unit body 1. The orifice 14 is located between the crystal 11 and the line which connects the orifices of the conduits 4, 5, and the apex of the V-shape is directed toward the afore-mentioned line. The conduit 13 delivers a wave or sheet of cooling liquid to the test piece surface, the flow of this cooling liquid being controlled by another nozzle 15 which is threadedly mounted for ready replacement in conduit 13, as nozzles 7, 8 and 9 are mounted in conduits 3, 4 and 5, respectively. Thus, suitable calibration of the nozzle passages controls the liquid flow through all conduits.
This searching unit operates in the following manner:
The searching unit and the test piece are moved relatively to each other in the direction of arrow A, either by maintaining the test piece at rest and moving the searching unit over it in the indicated direction or by maintaining the searching unit at rest and moving the test piece past it in the opposite direction. In either case, the cooling liquid wave delivered through V-shaped orifice 14 rapidly and fleetingly reduces the surface temperature of the [hot test piece before the transducer moves past the cooled region.
The hydraulic liquid delivered through conduits 3, 4 and flows as a film under the end face of the searching unit, the even distribution of the liquid being facilitated by the circular enlargements 16, 17 and 18 forming the outlet orifices of the delivery conduits. The cold hydraulic liquid flowing out of delivery conduits 4 and 5 causes a first intensive cooling of the test piece surface. This cooling is completed by the cooling liquid flowing out of V-shaped orifice 14. The inclination of the two branches of the V-shaped orifice causes the cold wave of liquid emerging therefrom like a jet to sweep before it the liquid film produced by the hydraulic liquid coming from conduits 4 and 5, and which has been heated by contact with the hot test piece surface, as the relative movement in the direction of arrow A proceeds. This V-shaped cold liquid jet thus sweeps the hot liquid laterally outwardly andout of the path of the immediately following transducer which will inspect the thusly cooled region of the test piece while the surface temperature is still reduced.
Simutaneously, the liquid flowing through conduit 3 is distributed radially and flows under centrifugal force and in a homogeneous stream under the transducer face with a speed sufiicient to prevent the cooled surface from being reheated by the internal heat 'of the test piece and the rapidly flowing liquid from reaching the boiling point in the path of the ultrasonic beam. At the same time, this rapidly flowing liquid couplant cools the face of the transducer.
The embodiment of FIGS. 3 and 4 is a simple searching unit useful for carrying out the method of this invention when a fixed point of a test piece is to be inspected. The piezo-electric transducer 11a is similar to that of the embodiment of FIG. 1 and is suitably arranged in a mounting supported on a test piece (not shown) by three points of support 22, 23, 24 forming a tripod support for the transducer on the test piece, the face of the annular transducer being adjacent an area of the test piece to be inspected. The transducer defines a central hole 12a in alignment with an axial conduit 3a in the transducer mounting. As conventional, the transducer is connected with conventional elements of an electric generator and indicating circuit (not shown) by a coaxial cable 21 coupled to the mounting at 20. In op eration, the transducer mounting is placed on its tripod support over the area of the test piece to be tested and a suitable cold liquid, such as water, is supplied under pressure to the delivery conduit 3a through connection 19a leading to a supply of water under the required head, similarly to the embodiment of FIG. 1. If the test area is hot, the jet of cold liquid flowing through the hole 12a cools the surface and is radially distributed in an outward fiowof considerable speed between the face of the transducer and the test piece area adjacent thereto. liquid pressure is high enough to assure a sufliciently rapid outward flow, the liquid film constituted by the rap idly flowing stream of liquid will simultaneously cool the transducer face and the test piece area adjacent thereto whileacoustically coupling the transducer to this area. The speed of the liquid flow is so chosen that the liquid will not boil in the space traversed by the ultrasonic beam.
With a piezo-electric transducer 11a, 30 millimeters When thetemperature of the test piece was 800 C. a rate of flow of water of 3.2 gallons per minute was necessary. In that case the transducer was puton the test piece and it wasnecessary to wait about two seconds be-' fore theapparatus started to operate.
It is easy to understand that it is always'necessary to have a high flow speed in order that the layer of steam Of course the speed of the be carried away by 'water. cooling fluid that is necessary depends on many factors; shape and dimensions of the transducer, distance between the latter and the test piece, said distance varying as the wave length, nature of the surface, etc.
The considerable advantages in the ultrasonic inspection of hot test pieces obtained by the method of the pres'-' ent invention will be obvious from the above description of certain specific embodiments thereof and they will be most pronounced when the test piece surface is rough, irregular or covered by a coating of oxide, as for instance in metallurgically produced objects. In the conventional systems, wherein a refactory material is interposed between the transducer and the test piece, the temperature gradient between the transducer and the test piece is If the 1 formed in the refractory material and the temperature of the test piece is practically unchanged.
According to the invention, the area of the test piece to be inspected is rapidly and fleetingly cooled down by a localized cooling jet, the surface of this area assuming a low enough temperature for the time that the transducer face is opposite it to prevent the liquid couplant from boiling although its boiling point is lower than the general temperature of the test piece. This is accomplished by interposing between the transducer and the test piece surface a liquid film moving at great speed so that the thermal inertia of the liquid produces a non-boiling zone in front of the transducer face. In this manner, ultrasonic testing is possible because the liquid couplant can fulfill its function of acoustically coupling the test piece with the electro-mechanical transducer while the transducer remains protected from the high temperature of the test piece.
Due to the rapid flow of the liquid interface, the only heating of the transducer face can be effected by convec tion. To accomplish this, it is most important to avoid any dead or eddy zones in the space between the transducer and the test piece so that the flow speed of the liquid is nowhere reduced but proceeds at all points so rapidly as to prevent the liquid from boiling. A relatively homogeneous stream of liquid in all directions to form a rapidly moving liquid film meets this condition.
At the same time, this fleeting surface cooling of a limited area of the test piece does not substantially modify the physical characteristics of the test piece, which rae due to its elevated temperature. The considerable internal heat of the test piece suflices to reestablish the surface temperature rapidly to at least nearly its original degree at the tested area as soon as the inspection is completed and the searching unit is moved on and away from this area. Therefore, the ability of the tested piece to be subjected to such finishing operations as rolling, clipping, shaving, stamping or any other operation requiring a certain plasticity of the hot meta-l, is in no way impaired.
Water will obviously be the most useful and practical cooling and couplant liquid, and it will be used wherever feasible. Preliminary experiments have shown that water can be successfully used in the method of the present invention for test piece temperature up to about 500 C. or 600 C. and it is quite likely that further experiments may establish its usefulness even beyond these operating temperatures. Depending primarily on the control of the water pressure and the corresponding speed of flow of the water, a non-boiling zone may still be maintained in the path of the ultrasonic beam. Outside of this Zone, the water will boil and evaporate but the vapor production is favorably influenced by the heating phenomenon. In effect, the vapor volume is not considerably larger at 1000 C. than it is at 300 C. The only essential condition is the avoidance of vaporization of the couplant in front of the transducer because this would gravely disturb the acoustic contact and testing would become impossible if the ultrasonic beam would have to pass through vapor and bubbles.
Another advantage of the invention in the inspection of iron metal test pieces resides in the fact that the sudden contact of a cold liquid jet ahead of the transducer with the hot test piece surface tends to cause a thermal shock on the oxide layer generally present on such pieces at high temperatures. This thermal shock will tend to crack and detach the oxide pellicle and to sweep it away by the dynamic force of the liquid before the transducer reaches the test area.
While the invention has been particularly described in connection with a preferred embodiment, it will be clearly understood that many variations and modifications may occur to the skilled in the art without departing from the spirit and scope of this invention as defined in the appended claims.
What is claimed is:
1. In a method of ultrasonically inspecting a test piece having an elevated temperature, the steps of placing an electro-mechanical transducer adjacent a test piece to be inspected so that a face of the transducer is oppositely spaced from a surface area of the test piece continuously delivering a stream of a cold liquid couplant between the transducer face and said area to couple the transducer acoustically to said area, the liquid of the couplant having a boiling point below the temperature of the test piece, at least a portion of the stream of fresh liquid couplant first sweeping said face of the transducer before more closely approaching said area of the test piece to prevent the transducer face from being heated by the test piece area, said liquid couplant stream forming a flowing liquid film in said space moving at all points of said space at such high speed-that the liquid does not reach its boiling point in said space.
2. In the method of claim 1, said stream being water.
3. In the method of claim 1, said portion of said stream being water.
4. In the method of claim 1, a portion of said stream being water.
5. In a method of ultrasonically inspecting a test piece having an elevated temperature: the steps of delivering a high-speed stream of a cooling liquid to a surface region of the test piece to reduce the temperature of said region, immediately thereafter, and while said surface temperature is reduced, placing a face of an electromechanical transducer oppositely and spacedly adjacent said region, continuously delivering a stream of a cold liquid couplant between the transducer face and said region to couple the transducer acoustically to said test piece, the liquid of the couplant having a boiling point below said elevated temperature, at least a portion of the stream of fresh liquid couplant first sweeping said face of the transducer to prevent the transducer face from being heated before the stream more closely approaches said region of the test piece, the liquid couplant stream forming a flowing liquid film in said space and moving at all points of said space at such high speed that the liquid does not reach its boiling point in said space.
6. In the method of claim 5, the liquid couplant and the cooling liquid being water.
7. In a method of ultransonically inspecting a test piece having an elevated temperature: the steps of placing an electro-mechanical transducer face oppositely adjacent a surface area of the test piece to be inspected, continuously delivering a high-speed stream of a cold liquid couplant between the transducer face and said area of the test piece whereby the transducer is acoustically coupled to said area, the liquid having a boiling point below said elevated temperature, at least a portion of the stream of the fresh liquid first sweeping the transducer face to prevent the transducer face from being heated before the liquid more closely approaches said test piece area, the liquid stream forming a flowing liquid couplant in said space and moving at all points of said space at such high speed that the liquid does not reach its boiling point in said space.
8. In the method of claim 7, the liquid being water.
9. A method of ultrasonically inspecting a test piece having an elevated temperature which comprises:
(a) placing an eletromechanical transducer adjacent a surface portion of the test piece to be inspected in such a manner that a face of the transducer and said surface portion define a space therebetween;
(b) passing a stream of a liquid couplant through said space so as to fill the same,
(1) the boiling point of said liquid couplant being lower than said elevated temperature, and
(2) the rate of movement of said stream in said space being sufficient to hold the temperature of each portion of said couplant in said space below said boiling point; and
(c) passing an ultrasonic beam between said transducer and said test piece throughthe liquid in said space.
10. An apparatus for ultrasonicallyinspecting a test piece havingan elevated temperature comprising, in combination:
(a) a supporting body having a face;
(b) an electromechanical transducer mounted on said body and exposed on said face,
(1) said body being formed with a plurality of openings in said face; and
(0) means for discharging a liquid from each of said openings,
(1) two of said openings defining a line in said face spaced from said transducer,
(2) a third opening in said face being V-shaped and interposed between said transducer and said line, the apex of the V-shape being directed toward said line.
References Cited by the Examiner UNITED STATES PATENTS 2,873,391 2/1959 Schulze 7367.9 X 2,992,553 7/1961 Joy 7367.8 3,121,325 2/1964 Rankin et al 7367.8 X 3,171,047 2/1965 Bergman et a1 3108.7 3,242,723 3/1966 Evans 7371.5
FOREIGN PATENTS 1,282,845 12/1961 France.
RICHARD C. QUEISSER, Primary Examiner.
J. B. BEAUCHAMP, Assistant Examiner.