The present invention relates to a drill rod and a method for its manufacturing 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 on both sides of the weld. These zones have lower hardness than the unaffected rod and are thus the weakest parts of the rod. To avoid of the soft zones it is therefore necessary to alternately carburize, normalize or harden this type of rod after friction welding.

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

During top hammer drilling under ground e g production drilling or drifter drilling the most common method is to use one rod drilling. Water flushing is used to press out the cuttings from the hole. Earlier these types of drilling

methods often were performed via hand held machines equipped with integrals, i.e. a tool where a cemented carbide insert is brazed directly on a bumped up end of the rod. Nowadays it is mostly highly mechanized drilling where all the machinery is carried on drill rigs and one rig often have two machines running on the same rig. With the mechanized method it has been possible to increase the effects of the machines. Therefore the rods are subjected to higher forces both at collaring and at drilling. In tools for mechanized drilling, the rods are threaded and a separate bit is mounted on the rod. That means that one rod can be used much longer than an integral since the rod instead of being discarded when the bit is worn out, it can be drilled further with a new bit mounted. Water flushing result in a risk for corrosion fatigue, especially since the water for example in mines often is acid and therefore extra corrosive.

The most subjected part of this type of rods is the threaded end and especially the clearance portion between the full section rod and the thread.

Objects of the invention

One object of the present invention is to provide a friction welded drill rod wherein the end piece(s) has (have) a unique resistance against corrosion fatigue in combination with that the heat affected zone of the weld is as strong as the rod itself.

Another object of the present invention is to provide a friction welded drill rod having a high strength weld.

Still another object of the present invention is to provide a low temperature heat treatment method for producing a corrosion fatigue resisting drill rod without softening the rod at the most load subjected parts of the rod, i.e. at

the thread, the flushing hole of the end piece(s) and at the clearance between the thread and the rod body.

Still another object of the present invention is to provide an effective method for producing drill rods combined from two or more pieces which are friction welded together without subsequent carburation or other high temperature heat treatment process.

Description of the figures Fig. 1 shows an end of a threaded drill rod according to the present invention in a longitudinal cross-section. Fig. 2 shows a schematic Fe-C phase diagram of a material used in the drill rod according to the present invention. Fig. 3 shows the hardness variation, with a dotted curve, from the surface into the core after nitrocarburizing of a conventional rock drill steel (such as type SS2534) while the continuos curve in the same graph shows the hardness variation according to the present invention. Fig. 4A shows a core hardness distribution in the longitudinal direction of the nitrocarburized threaded end piece (to the right) and through the friction weld into the rod (left), the latter normally with no heat treatment after hot rolling of the rod. Fig 4B shows a steel weld according to the present invention in a longitudinal cross-section. Fig. 5 shows a continuous tempering curve for a steel suitable for the present invention, while the dotted curve describes an example of a tempering curve for a conventional steel for drill.

Detailed description of the present invention

The drill rod 10 for percussive drilling comprises a first component 25 comprising a threaded portion 1 1 connected to a clearance portion 12, which connects to a short rod portion 16. The threaded portion 1 1 forwardly connects to a striking surface 1 7. The first component 25 is connected to a

second component 22, which preferably is a long rod portion 13. A flush channel 26 is provided centrally in the drill rod 10. The short rod portion 16 is of substantially the same diameter as the rod portion 13.

The drill rod to be manufactured, usually includes the stationary or non- rotatable component 25 and the rotatable component 22. The components 22,25 are made of steel and before friction welding starts, the steel at the mating end of each component is uniform in the core and is as such called parent material.

Before friction welding, the threaded end of the component 25 is nitrocarburized to increase the corrosion resistance as compared to a carburized or normalized surface. A nitrocarburized layer is designated the numeral 14 in Fig. 4B and its depth is about 0.3 mm. Alternatively other types of low temperature surface heat treatments can be used, such as

Blacknite , Coloumite , Tenifer etc. Low temperature heat treatment here means that the steel has been heated below A1 temperature, see x in Fig. 2 i.e. no phase transformation to austenite (gamma-phase) is possible.

With conventional rock drill steel type SS 2534 for drifter rods nitrocarburizing (Blacknite etc.) gives too low core hardness due to a tempering effect during heat treatment, see the dotted curve in Fig. 3. The steel to be heat treated by nitrocarburizing (or Blacknite , Colournite etc.) shall have high tempering resistance (hot hardness) and secondary hardening peak around the nitrocarburizing temperature (550-610°C, normally at 580°C) such that the core hardness does not fall below 440 HV1 (kg) (Vickers Hardness) and preferably not below 450HV and will thus give the nitrocarburized layer 14 a good support to withstand the impact loads from

the shock waves during percussive drilling, see the continuos curve in Fig.3 and the continuos tempering curve in Fig. 4.

The function of the friction welding device, not shown, will be described hereinafter. A chuck means is opened and the threaded portion 25 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-9502153-1 and is hereby incorporated by reference into the present description.

Conventional drill steels get too soft core hardness after nitrocarburizing, about 300 HV1 , and are not suitable for percussive drilling, see the dotted curve in Fig. 3. At percussive drilling, the rod surface as well as the rod core are subjected to high fatigue stresses from the shock wave and from torsion and bending forces. This means that it is very important that the surface and the core have enough strength to stand those fatigue stresses. Furthermore, the high pressure on the threads during percussive drilling, demands enough core hardness to support the nitrocarburized layer. We have found it possible to get sufficient hardness and fatigue strength in the core by using a steel that has a chemical composition such that the steel after hot rolling and cooling down to a normal rod hardness about 400 HV1 can be nitrocarburized at about 580°C and at the same time increase the hardness due to secondary hardening to at least 440 HV1 , as disclosed by the continuous curve in Fig.

3. The steel type also gives a hardness in the soft zones about the interface that is at least 390 HV1 , i.e. about the same as in conventional rod steels. Core hardness profiles are shown in Fig. 4A. It is also possible to nitrocarburize in full rod length to get a core hardness of at least 440 HV1 .

Compared to a conventional drill steel, a proper steel shall have higher hot hardness and 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 HV1 at 600°C compared to a conventional steel wherein the hot hardness is about 100 HV1 at 600°C.

The continuous tempering curve in Fig. 5 describes an example of secondary hardening range S for a steel suitable for the present invention, while the dotted curve describes an example of a tempering curve for a conventional steel for drill rods having a tempering time about an hour.

The core hardness of the softest part of the HAZ in the drill rod according to the present invention is about (i.e. ± 5%) equal to the hardness of the parent material.

The components 22, 25 shall have a hardness in the HAZ that is comparable with the core hardness of the rod and normally higher than 400. For at least the component with the weakest cross-section the following shall be valid. The strength in the heat affected zone is comparable with that of the parent, not heat affected steel material. The hardness, shown by a line I in Fig. 4A, in the normally soft zone is at least at the same level as the normal core hardness in a conventional normalized drill rod, i.e. higher than 390 HV1 .

The main elements, normally Cr, Mo and V are held at a level that gives a core hardness after nitrocarburizing of at least 440 HV1 and a HAZ hardness of at least 390 HV1.

The steel type used in the threaded component 25 or in both components 22, 25 has a chemical composition which lies in the interval in weight-% of: 0.1 5-0.50 C; max. 0.1 5 Si; min 0.2 Mn; 0.5-1 .5 Cr; 0.5-4 Ni; 0.5-2 Mo; max. 0.5V; max. 0.5 W; 0.5 Ti; max. 0.1 Nb and max. 0.05 Al, the balance being Fe. An example of such a steel is 0.32 C; 0.9 Si; 1.0 Cr; 0.5 Ni; 1.0 Mo; 0.1 V, the balance being Fe.

Also small amounts of other elements forming carbides, nitrides or nitro- carbides such as Ta and Zr is possible to use in the steel as those elements give a secondary hardening effect and also slows down the grain growth. Al and B are other elements 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 a parent material having a high hot hardness, heat treating the threaded portion 1 1 , the inner passage 26 and clearance portion 12 of the first component 25 against corrosion fatigue preferably by nitrocarburizing the first component 25 separately, 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 and 25 may be stationary while the other component is rotatable during

friction welding. Preferably the free ends of rotatable 22 and non-rotatable 25 components that are to be connected, are free from joint preparation and have end surfaces substantially perpendicular to a rotational axis of the components.

Thus the main features of the present invention is to provide a drill rod wherein the risk of corrosion fatigue is decreased in the threaded portion 1 1 and in the clearance portion 12 by heat treating, preferably, nitrocarburizing the separate first component 25 and then friction weld the first component to a rod 13 that might be carburized, normalized, nitrocarburized etc.