NO761064L - - Google Patents

Description

Device for heating a workpiece.

When welding steel in certain qualities, the steel must first be heated to a certain temperature before the welding takes place. After welding, the steel must often be heated further and then cooled again. Warm-up, holding times and cool-down must. happen within certain time intervals. During the process, the temperature should be kept close to the calculated one.

The steel is usually heated by wood

using asbestos mats fitted with electrical resistance wires. The regulation of current for these is usually done manually. This makes it difficult to achieve a uniform heating, to keep the temperature within reasonable limits during holding times and to achieve a uniform cooling.

According to the present invention, all control takes place

and regulation of the entire heat treatment process automatically.

The mat or the mats are supplied with electric current from a transformer, and the invention is characterized by a circuit breaker arranged on the primary side of the transformer, at least one sensor body connected to the mat for sending control signals to the circuit breaker via a regulator, which contains means for programmed indication of changes in the electrical current's nominal value during the treatment period.

An embodiment of the invention shall be described in more detail below with reference to the drawing,

■ there

fig. 1 schematically shows the constituent components,

and

fig. 2 shows a design of a cam included in the regulator.

In fig. 1 is a workpiece designated by 10.

Before and after a welding operation, the workpiece is kept covered

of an asbestos mat 11 in which electrical resistance wires 12

is admitted. These are supplied with electricity from a transformer 13. This in turn is connected to a power supply line 14. The power supply is determined by a circuit breaker 15 which receives control signals from a regulator 16.

Before a welding operation, one may wish to heat the workpiece up to a certain temperature. In order to achieve proper heating through, which naturally depends on the shape and amount of material of the workpiece, a certain time is required during which the power supply can be constant. After welding, it may be desirable to maintain a higher temperature, which involves an increased current supply for a certain time, after which a slow cooling is obtained by reducing the current supply.

The regulator 16 contains a device which indicates predetermined changes in the nominal value of the temperature.

In the mat 11, there is at least one sensor body 17 which emits signals to the regulator 16 with regard to the instantaneous value of the temperature. The nominal value or the instantaneous value is illustrated schematically in fig. 1 with hands 18 or 19-in a window 20.

The regulator is suitably of the fall bar type

and contains a profiled cam disc that rotates at a known speed, e.g. makes one revolution in 12 hours.

Such a cam disc 21 is shown in fig. 2. A part

22 of the cam disc's circumference coincides with the base circle, and as a cam follower 23 is touched by this part, the nominal value is kept constant.

The cam shows a steeply rising part 24 there. the power supply increases. Then follows a part 25 with an essentially constant radial distance, and thus a constant, but in relation to the part 22, increased nominal value. The circumference of the cam disc ends with a part 26 with a gradually decreasing radial distance where a controlled cooling takes place.

The shape of the cam disc can of course vary in many ways depending on how you want the temperature to vary during the treatment process. Alternatively, you can use an electronic memory unit that controls the nominal

value change.

By the fact that the circuit breaker 15 is located on the primary side of the transformer, several advantages are achieved. The power switch can be made considerably smaller than if it were placed on the secondary side, and the risk of the contact surfaces being burnt is reduced.

Claims (2)

Device for heating a workpiece by means of a mat (11) containing electric resistance elements (12) which are supplied with electric current from a transformer (13), characterized by a circuit breaker (15) arranged on the primary side of the transformer, as well as at least one sensor body (17) connected to the mat (11) for issuing control signals to the circuit breaker (15) via a regulator (16), which contains means (21) for programmed indication of changes in the nominal value of the electric current during the treatment period. 2. Device according to claim 1, characterized in that the regulator (16) is of the drop bar type and contains a profiled cam disk (21) which rotates at a known speed.