The present invention relates to a friction welded product for rock drilling and a method for manufacturing such product by friction welding.

Technical background

Conventional rods for rock drilling either have a thread machined directly in the rod or a thread machined in a rod end forged to a diameter bigger than the rod diameter. Instead of forging up the dimension of rods it is possible to friction weld end pieces or guiding pieces with diameters bigger than the rod diameter. Conventional rock drilling rods are most often manufactured from holed rods and adapters from solid rods. For threaded rods at least one thread is often machined in a bumped up (forged) end with a diameter bigger than the rod diameter while shank adapters often are machined from rounds.

Instead of forging up the dimension or using extensive machining it has been more and more common to friction weld together pieces with big differences in diameter. However, drill equipment manufactured from conventional drill steel get weak "soft" zones Z on both sides of the interface. These zones have lower hardness than the unaffected rod and are thus the weakest parts of the rod (Figs. 1 A and 1 B). To avoid of the soft zones it is therefore necessary to alternately carburize, normalize or harden this type of rod after friction welding (Figs. 2A and 2B).

Another way is to compensate the weaker strength in the soft zones by bumping up the rod end before friction welding (Fig. 3).

Objects of the invention

One object of the present invention is to provide a friction welded rock drilling product wherein the heat affected zone is as strong as the product itself.

Another object of the present invention is to provide a friction welded rock drilling product having a high strength weld.

Still another object of the present invention is to provide an effective method for manufacturing rock drilling products by friction welding without softening the element.

Still another object of the present invention is to provide an effective method for manufacturing rock drilling products such as rock drill rods and shank adapters combined from two or more pieces which are friction welded together without subsequent carburation or other heat treating process.

Description of the figures

Fig. 1 A shows a core hardness distribution graph in the longitudinal direction of a prior art friction weld of two drill steel ends shown in Fig. 1 B before heat treatment. Fig. 2A shows a core hardness distribution graph of the prior art friction weld of two drill steel ends shown in Fig. 2B after heat treatment. Fig. 3 shows a prior art bumped up rod before friction welding in a longitudinal cross-section. Fig. 4A shows a core hardness distribution graph in the longitudinal direction of a friction weld of two drill steel ends according to the present invention shown in Fig. 4B without heat treatment. Fig. 5 shows a core hardness distribution graph of a friction weld of two drill steel ends according to the present invention in comparison with the conventional friction weld. Fig. 6 shows a schematic Fe-C phase diagram of a material used in the element according to the present invention.

Detailed description of the present invention

The drill tool preferably a drill or a rod for percussive drilling to be manufactured usually includes a stationary or non-rotatable component 25 and a rotatable component 22. The components 22,25 are made of steel and before friction welding starts, the core material of each component is uniform and is as such called parent material.

The function of the friction welding device, not shown, will be described hereinafter. A chuck means is opened and the shank 22 of the drill bit is inserted therein. The chuck means is closed to grip the shank in an aligned position. Then clamp means is opened and the end of the drill rod 25 is inserted therein. The clamp means is closed to grip the rod in an aligned position. Then the drill bit starts to rotate and the clamp means feeds the free end of the rod towards and into contact with the free end of the bit. The heat produced during friction welding make abutting ends possible to forge. The relative rotation of the components is stopped and the components are pushed further together and cooled, as in Fig. 4B. The method of friction welding is more specifically described in SE-A-95021 53-1 and is hereby incorporated by reference into the present description.

Conventionally the friction weld will comprise soft zones Z about the interface as shown in Figs. 1 A and 1 B. The interface can be defined as the bonding zone between two components. To make the heat affected zone

(HAZ) as strong as the rod itself, we have found it possible to use a steel that has such chemical composition that the hardness in the most tempered part of the HAZ close to the weld is so high that it after a short heating and pressure time during friction welding, the hardness will not decrease below

the core hardness of the rod with normal hardness, for instance 400 HV1 (HV1 is Vickers hardness with a load of 1 kp) when cooled to room temperature (Fig. 4A). The core hardness profile is shown with a dotted line I and the hardness substantially increases from the parent material towards the interface.

Compared to conventional drill steel a suitable steel shall have higher hot hardness and/or a secondary hardening between 550-610°C. With "higher hot hardness" is here meant that the steel used in connection with the present invention has a hot hardness about 200 HV0.5 (HV0.5 is Vickers hardness with a load of 0.5 kp) at 600 °C compared to a conventional steel wherein the hot hardness is about 100 HV0.5 at 600 °C.

The core hardness of the softest part of the HAZ in the drill rod according to the present invention is higher or about equal to the hardness of the parent material but not less than 3% lower than the hardness of the parent material.

Preferably at a core hardness of 400 HV1 in the parent material of a steel rod according to the present invention, the minimum hardness in the "soft zone" after friction welding shall be higher than or about 400 HV1 and not less than 390 HV1 . For a conventional steel rod with a core hardness of 400 HV1 the minimum hardness in the soft zone Z after friction welding is much lower than 400 HV1 and normally about 325 HV1 .

The component and the rod shall have a hardness in the HAZ that is higher than or about 400 HV1 and not less than 390 HV1 . The strength in the HAZ is comparable with that of the parent, not heat affected steel, material. The hardness, shown by a line I in Fig. 5, in the normally soft zone is at least at the same level as the normal core hardness, shown with a dotted line P in Fig. 5, in a conventional normalized drill rod, i.e. it shall be higher than 400

HV1 . The steel in the presently claimed product shall have high hot hardness. The end piece might be normalized or carburized or may have been subjected to any other high temperature heat treatment. High temperature heat treatment here means that the steel has been heated above A1 temperature i.e. the temperature above which austenite (gamma phase) can be formed, for example as depicted by E in the Fe-C diagram in Fig. 6. The heat treatment may be case hardening (carburizing), carbonitriding, boriding, hf surface induction hardening etc. It might also be some sort of surfacing like laser surface welding, powder spraying (fused coating) etc.

The main elements, normally Cr, Mo and V are held at a level that gives a HAZ hardness of at least 390 HV1 .

The chemical composition range for the steel type used in a rod according to the present invention is in weight-% 0.1 5-0.40 C; max 1.5 Si; min 0.2 Mn; 0.5-1 .5 Cr; 0.5-4 Ni; 0.5-2 Mo; max 0.5 V; max 0.5 W; max 0.5 Ti; max 0.1 Nb and 0.05 Al; the balance being Fe. Example of such steel is 0.18C; 0.9Si; 1 ,2Cr; 1.8Ni; 0.75Mo and 0.1 V.

Also small amounts of other elements forming carbides, nitrides or nitro- carbides such as Ta and Zr are possible to use in the steel as those elements give a secondary hardening effect and also slow down the grain growth. B is another element that can be used as grain refiner together with nitrogen.

More specifically the method for producing a friction welded product for rock drilling comprises the steps of providing components 22 and 25 of parent material, each having a high hot hardness, providing clamp means for clamping the first component 25, providing rotation means for rotating the second component 22, putting free ends of the first and second components

together and rotating the first and second components relative to each other so as to form a weld 27 and cooling the weld to room temperature thereby keeping the lowest core hardness of the heat affected zone above 390 HV1 . It is however understood that any of the components 22 or 25 may be stationary while the other component 25 or 22 is rotatable during friction welding. The free ends of the rotatable 22 and non-rotatable 25 components can be joint prepared or free from joint preparation and have end surfaces substantially perpendicular to a rotational axis of the components.

Thus the method according to the present invention provides a welding process wherein no heat treatment is necessary after friction welding. It is possible to apply corrosion protection substantially directly after friction welding and cooling. It is furthermore not neccessary to bump up the end of the product since there will be no soft zones in the vicinity of the interface. Costly finishing operations are thus avoided since the friction welded product instantly will obtain a high strength weld.