EXPERIMENTAL STUDY OF EDDY-CURRENT SENSORS FOR THE INSPECTION OF HEATED CYLINDRICAL BODIES

A. M. PANASYUCHENKO, V. K. ZHUKOV, B. A. DOBNER

(Presented by the scientific seminar of the Department of Information-Measuring Technology)

In flaw detection of heated metals, high temperature becomes a serious obstacle, often leading to a considerable complication of the sensor design and to a sharp decrease in its sensitivity. In this connection, the development of sensors that are simple in design and have high sensitivity is an urgent task. Sensors developed earlier for inspection at high temperatures were used either without cooling or with air or water cooling.

Sensors without cooling [4] are used at comparatively low temperatures (up to 500°C), are structurally complex and not durable. The advantage of the sensors is their high sensitivity. A sensor with water cooling [1] makes it possible to extend the temperature range to 700°C. But the sensor design is complex. In addition, when cylindrical products are inspected with surface-mounted sensors, certain difficulties arise.

Fig. 1. Feed-through sensor with water cooling.

In the present work, several variants of sensors with different types of cooling are considered, intended for the inspection of cylindrical nonmagnetic products heated to high temperature.

The design of a feed-through sensor with water cooling is shown in Fig. 1. The sensor is made from a tube that is at the same time a conductor creating the magnetizing field and a jacket through which water flows. The advantage of such a sensor lies in the simplicity of manufacture. As a certain drawback of the design, limiting the field of its application, one may note the absence of a frame. To ensure sufficient rigidity, the number of turns must be limited; the consequence of this is the low inductance of the sensor. Nevertheless, sensors of this type are used in a number of cases for inspecting the dimensions of products whose temperature does not exceed 800°C [3].

For the inspection of geometric dimensions and structural inhomogeneities of cylindrical heated bodies, in a number of cases it is convenient to use

slotted sensor, which can operate with either air or water cooling.

Figure 2 shows a possible version of a sensor with water cooling. The wall of the sensor facing the article being tested has a water jacket 3, by means of which the entire sensor housing 2 is cooled, as well as the ferrite 1 and winding 4. A disadvantage of such a cooling system is that the ends of the ferrite half-rings, despite the low temperature of the shield, heat up to a temperature on the order of 300°C, since ferrite has low thermal conductivity. At the same time, the Curie point for ferrite oxides lies within the range of 120°C. Consequently, this type of cooling is not sufficiently effective.

Fig. 2. Slotted sensor with water cooling:
I — ferrite half-rings, II — sensor housing, III — water jacket, IV — measuring winding, V — article.

Fig. 3. Slotted sensor with water cooling:
I — water reservoir, II — drain slots, III — guide planes.

Fig. 4. Slotted sensor with air cooling:
I — fan, II — air duct.

The second possible version of a sensor with water cooling is shown in Fig. 3.

A reservoir with water 1 is placed above the sensor. Through slots 2, along inclined planes 3, water freely drains onto the walls of the sensor and, wetting them, rolls downward, simultaneously cooling both the walls and the ends of the ferrite. This type of cooling is the most effective, since the energy released by the rod is completely absorbed by the water flow and does not reach the sensor walls or the ends of the ferrite half-rings. In this case neither the ferrite nor the shield heats up. Individual drops of water during the inspection process may fall onto the red-hot rod, but this does not intro-

errors into the operation of the measuring device and does not disturb the structure of the bar being inspected.

In the case of air cooling (Fig. 4), the air jet blows from below both the bar and the side walls of the sensor and its ends. Energy transfer from the bar to the sensor occurs by thermal radiation and by air convection. With such a cooling system, heating of the sensor due to

Fig. 5. Dependence of the temperature of the pole of the ferrite half-ring on the temperature of the product for:
a) different bar diameters; b) different fan capacities.

Fig. 6. Feed-through sensor with air cooling:
a) sensor assembly; b) sensor coil.

air convection is practically absent and takes place only by thermal radiation. At the same time, the sensor is cooled by the air flow. Experiments showed that, at a bar temperature of 1100°C, a fan capacity of 10 l/s, and the maximum bar diameter, equal to 30 mm, the temperature of the end of the pole does not exceed 50°C. If the pole ends are polished and a shiny coating is applied to the inner walls of the sensor, the temperature of the pole end can be reduced still further.

The relationship between the pole temperature \(t^\circ_{\text{pol}}\) C, the bar temperature \(t^\circ_{\text{pr}}\) C, the fan capacity, and the bar diameter for one and the same sensor is shown in Fig. 5.

In those cases where the sensitivity of the split sensor is insufficient, it is necessary to turn to a design with a feed-through coil. Below is a description of two versions of a feed-through sensor developed and studied by the authors.

The sensor shown in Fig. 6a consists of two magnetizing coils 1 and one measuring coil 2. The coils are wound with copper wire of the MGTf type in heat-resistant insulation. The coil frame (Fig. 6b) is made of copper in order to increase heat resistance and to provide better heat removal. To avoid a sharp decrease in sensitivity due to a short-circuited turn, the frame has a slot 3. The coils are fastened to the walls and to one another by means of insulating bushings 4. Air supplied from below by pump 6 blows over the article being inspected and the sensor coils. At a bar temperature of 800°C, a fan capacity of 10 l/sec, a bar diameter of 30 mm, and an internal coil diameter of 40 mm, the temperature of the frame under the winding did not exceed 60°C. The disadvantages of this sensor include the complexity of the design and the impossibility of using it at temperatures exceeding 1000°C.

Fig. 7. Feed-through sensor with water cooling.

These disadvantages are partly eliminated by the water-cooled sensor shown in Fig. 7. The sensor is intended for quality inspection of bars during rolling and consists of coil 1 and cup 2. The coil and the cup are made of nonmagnetic steel and have a longitudinal slit 3. The cup is connected to the coil by means of a thread. It is necessary that, in the screwed-together state, the slit in the coil and the slit in the cup coincide. The cup is used to protect the coil windings from external damage. Two windings are placed in the coil groove: magnetizing and measuring; in this case MGTf wire is also used, since moisture can reach the wire through the slit. In order for the moving bar 5 to enter the sensor, the latter has a conical lead-in. Cooling is carried out by a jet of water supplied directly into the sensor onto the article. At an article temperature exceeding 1000°C and a high speed of motion, the water falling on it does not lead to the appearance of defects or other structural inhomogeneities. At the same time, the windings practically do not heat up.

The type of sensor and the method of its cooling must be selected in each specific case, taking into account the temperature of the article, the required sensitivity, and the dimensions of the article being inspected.

REFERENCES

1. A. I. Nikitin. Measurement of the wall thickness of pipes during hot rolling. Nondestructive Testing Methods, Part I, ONTI, 1965.

2. V. K. Zhukov, V. E. Dreizin, I. G. Leshchenko. An induction impedance sensor with an annular magnetic core. Defektoskopiya, No. 2, 1966.

3. Electromagnetic sensors for inspection at elevated temperatures. KIT No. 30, 1966.

4. V. E. Shaternikov and V. A. Denisov. High-temperature eddy-current sensors. Defektoskopiya, No. 3, 1968.