The present invention relates to a method of welding the links of a steel chain.

It is a common knowledge that the weakest point of a chain is the weld between the rod ends of the link which are bent together. The conventional joining methods are flash welding or resistance butt welding; SE-A 331 623 describes such well-known methods.

In the zone affected by the high temperature developed during the welding, a grain-coarsening takes place in the material which primarily reduces the toughness. The grain-coarsening depends mainly on the welding performance and on the analysis of the chain material. During the welding the temperature should be as low as possible and the temperature-influenced zone should be as small as possible. Nickel, in particular, in the link material improves the toughness of the welding zone. Alloying elements, however, make the steel considerably more expensive, and advantages from those additives can be derived only in the temperature- influenced zone and, in particular, in the area closest to the joint where the welding temperature is at its maximum. In other parts of the link, a cheaper steel could be used. Surprisingly, it appears now that one can increase the toughness in the weld by using friction welding; a method known from DE-OS 2915418. In that connection, the invention has been given the characteristics stated in the claims. The invention will be described more in detail by means of the enclosed drawing that, in a side view, show a chain link in production before being welded. A conventional way of making a steel chain will be described briefly. One cuts the link material 1 into suitable lengths, bends it into an oval shape so that the ends of the link material will coincide along one of the sides of the link. During this process one should also hook the link into the adjacent link already welded. The welding process is as follows: electric electrodes are connected to both sides of

the link ends, a suitable gap for starting the arc is adjusted between the link ends and, when there is an arc, the gap is widened until the desired arc appearance is reached. When the ends have received intended welding temperature, the ends are compressed so that the joint form a bead. The zone influenced by the high temperature during the welding is rather extended.

According to this invention, the link material 1 should be cut into somewhat shorter lengths than conventionally, so that an insert 5 with about the same diameter as the link material 1 can fit into the gap between the end surfaces 4 of the link material 1. The insert 5 is a uniform cylinder, whose length between the end surfaces suitably is between about half the insert diameter and one and a half times the insert diameter. It could also have a smaller length. The analysis of the insert 5 should contain toughness increasing alloying elements, e.g. nickel so that, during the welding, such alloying elements will be added to a sufficient extent to the zones affected by the high temperatures in the end surfaces 4 of the link material 1. In this way, one will have a desired structure in the most exposed zone without having to alloy all the link material higher. Then, the friction welding proceeds as usual by the rotation of the insert 5 while the surfaces are under a suitable pressure. The friction of the surfaces between the insert 5 and the link material 1 will bring about desired temperature and then the surfaces should be put under further pressure. Compared with flash welding, friction welding has the great advantage of reducing considerably the zone of link material 1 affected by the high temperature.