FR2613650A1 - Nonsacrificial welding electrode based on tungsten - Google Patents

Abstract

Nonsacrificial electrode for TIG welding. The electrode consists of a tungsten matrix in which are embedded thorium oxide nodules representing 6 to 12 % by weight of thorium, the average distance between the nodules, measured perpendicularly to the electrode generatrices, being smaller than or equal to 5.5 microns. This electrode has a remarkable service behaviour.

Description

NON CONSO + 2ABLE WELDING ELECTRODE
TUNGSTEN BASED
The present invention relates to the technique of so-called non-consumable welding electrodes used in the welding process.
TIG and relates more particularly to the tungsten-based electrodes used to implement said method.

The TIG welding technique conventionally uses electrodes consisting of a rod, generally circular in pure tungsten.

In order to improve the service life of these electrodes, it is known to add thorium to the tungsten in the form of thorium oxide (thorine). This thorin, in the electrodes existing on the market, is present at a rate of approximately 2 to 6% by mass in tungsten.

despite the resulting increase in performance, it can be seen, for given operating conditions (amperage, arc length, material to be welded, geometry of the electrode) that the service life of these electrodes is very variable from one batch to another. This variation is all the more important as the electrode is used for severe service conditions: this is the case when one seeks to increase, for example in automatic welding processes, beyond the limit ordinarily accepted the density. of current passing through the electrode this to increase the welding speed or / and to weld materials with high melting point or / and relatively very conductive. Such is the case for example of the welding of ferritic stainless steels for which it is known that the energy input per unit of time must be around 50% higher than the energy input required for a conventional 18-8 austenitic steel, all parameters being equal.

These problems manifest themselves in a particularly acute manner from currents on the order of 250 amperes, or welding energies on the order of 1.5 KJ / cm.

The electrodes then wear out randomly and generally very quickly. The tip of the electrode often deforms in an uncontrolled manner from 1 to 2 hours and it becomes impossible to control the arc coming from the tip of the electrode and to ensure good geometric precision of the weld bead. This is particularly troublesome when using the TIG technique to manufacture from thin strip on a continuous line of thin welded tubes of high precision and subject to severe quality requirements.

Regardless of the quality problems, it becomes impossible with such electrodes to make a programmed workshop management in terms of monitoring and changing the electrodes.

The present invention seeks to define a TIG welding electrode of structure and composition such that it does not have the drawbacks described above.

In particular, efforts have been made to obtain an electrode which is resistant for a maximum and regular duration to very high current density.

The welding electrode according to the invention consists of a tungsten matrix in which thorium is added in the form of finely distributed thorium oxide ThO2 nodules (called thorine), the thorine content being between 6 and 12% by mass, the average distance between the thorinic nodules measured in any plane perpendicular to the generatrices of the electrode being less than or equal to 5.5 microns.

Preferably the nodules have an elongated shape.

Preferably the maximum average dimension of the thorinic nodules measured along the planes perpendicular to the generatrices of the electrode is less than or equal to one micron.

Preferably also, the maximum dimension of the thorin nodules measured in any plane parallel to the generatrices of the electrode is less than or equal to 10 microns.

We will also seek to obtain as the base matrix of the electrode a tungsten matrix as pure as possible and having a grain structure as fine as possible. Advantageously, the electrode will have undergone, during its manufacture a deformation in the direction of stretching parallel to its generatrices and the grains will have an elongated structure in the direction of the generatrices of the electrode, the dimension ratio parallel to the generatrices on di mension perpendicular to the generators being at least 3.

Advantageously, the use of the electrode according to the invention will comprise, during the first start-up, a preheating phase with a current density reduced compared to the current density in service.

The example below, given without limitation, illustrates a particular embodiment of the invention.

For the automatic welding of ferritic stainless steel type Z2CTl8 an electrode according to the invention is used having the following characteristics
diameter 3.2 mm service angle 300
tungsten matrix containing 7.6% by mass of thorine
maximum dimensions of thorium nodules
- parallel to the generators: 6 microns
- perpendicular to the generators: 1 micron
mean distance separating thorinic nodules in a plane
perpendicular to the generators: 5 microns.

Such an electrode held in service, at an intensity of 280 amperes at a voltage of 12 volts corresponding to a welding energy of 1.5 KJ / cm for a period of 15 hours without retouching and giving a quality of welding and a regularity of arc compatible with a very high level of quality of the products obtained.

 Under similar conditions, that is to say the same dimension, same welding parameters, a tungsten-based electrode having simply 5.5% thorine by mass in the form of thorin nodules distributed not regularly, simply held 1 li 30 hours at the end of which it was necessary to change the electrode to maintain the qualities of product and welding required.

It can thus be seen that, thanks to the invention, the percentages of thorin associated with the distribution of thorin nodules make it possible to obtain very good performance in service and to solve the problems underlying the invention.

Claims (5)

1) Non-consumable welding electrode consisting of a tungsten matrix and thorine nodules characterized in that the quantity of thorine is between 6 and 12X by mass, the average distance between the thorium oxide nodules measured in a any plane perpendicular to the generatrices of the electrode is less than or equal to 5.5 microns.  2) An electrode according to claim 1 characterized in that the maximum average dimension of the thorin nodules measured along the planes perpendicular to the generatrices of the electrode is less than or equal to one micron. 3) Electrode according to claim 1 or 2 characterized in that the maximum dimension of the thorin nodules measured in any plane parallel to the generatrices of the electrode is less than or equal to 10 microns. 4) Electrode according to one of claims 1 to 3 characterized in that the tungsten matrix has an elongated structure parallel to the generatrices. 5) Electrode according to claim 4 characterized in that the dimension dimension parallel to the generators to dimension perpendicular to the generatrices of the tungsten grains is at least 3.