Upon drilling with a drill string connected to a top hammer, a gap is formed between co-operating shoulders of the tubes or rods included in the drill string after each shock wave. Water is collected on the surface of the shoulder between each shock wave so that each impact provides a jet stream of water away from the impact surface. The part of the jet stream that then travels radially inwards will give rise to damage on associated male portion, see Figs. 1A and 1 B, and finally the male portion will break so that the drilling cannot be contin- ued.

In US-A-6,334, 493, a male portion is shown where a threaded portion has been provided with a metallic material, which is more precious than the steel in the male portion. Thereby, it is intended to solve the problem of corrosion fatigue in the threads by coating at least the parts of the thread of the male portion that co-operate with other parts of the threaded joint.

Objects of the Invention An object of the present invention is to substantially decrease the damage on a male portion for percussive rock drilling.

Another object of the present invention is to substantially improve the resistance to erosion corrosion in sections having a reduced cross-section in a male portion for percussive rock drilling.

Brief Description of the Drawings These and other objects have been attained by a male portion that has received the features according to the characterizing clause of the appended independent claim, reference being made to the drawings. Fig. 1A shows a conventional male portion after erosion corrosion. Fig. 1 B shows a cross-section of a threaded joint having the conventional male portion. Fig. 2A shows a

shows a threaded joint having a male portion according to the present invention in longitudinal cross-section. Fig. 2B shows an enlarged cross-section of the male portion according to the present invention. Fig. 2C shows a cross-section of the male portion according to the present invention.

Detailed Description of the Invention The threaded joint shown in Figs. 2A and 2B for percussive drilling com- prises a sleeve-shaped portion or a female portion 11 having a cylindrical female thread or cylindrical internal thread 12 in co-operation with a pin or male portion 13 according to the present invention provided with a cylindrical male thread or cylindrical external thread 14. The male portion 13 and the female portion 11 constitute integrated parts of identical drill elements or drill tubes 10A, 10B. The shown thread is a so-called trapezoidal thread but other thread shapes may be possible, e. g. a rope thread. Furthermore, the male portion has a through flush channel 15, through which a flush medium, usually air or water, is led. The threads 12,14 comprise thread flanks and thread roots arranged between the flanks.

The male thread 14 is terminated, in the direction of the other end of the tube, in a clearance surface 16. The clearance surface 16 connects via a con- cave portion 17 to a shoulder 18, which is substantially perpendicular to the centre line CL of the threaded joint. The shoulder 18 connects in turn to the envelope surface of the tube 10A. The concave portion 17 is preferably softly rounded, such as via a radius.

The female portion 11 has an end surface 19 arranged on the free end thereof, which is substantially perpendicular to the centre line CL of the threaded joint. When the joint is tightened according to Fig. 2A, the end surface 19 and the shoulder 18 abuts against each other. During drilling, the water that is deposited on the surfaces 18 and 19 will provide a jet stream of water in a direc- tion shown by the arrow in Fig. 1 B. The part of the jet stream that travels radially inwards has an angle of action of about 90° and when it meets the surface in the concave portion, a protective layer of oxide on the surface is broken. The exposed pure metal surface then oxidizes (corrodes) again before the oxide

thereof again is torn away. This course of events gives rise to accelerated so- called erosion corrosion.

The joint is also shown in US-A-4,968, 068, which patent hereby is incorpo- rated in the present description, where there are shoulder and/or bottom abut- ment between the tubes 10A, 10B.

According to the present invention, the concave portion 17 of the male por- tion is provided with a coating consisting of at least one surface-modifying, ero- sion-protecting layer 20. The layer is continuous in the direction of circumfer- ence. The layer follows the curvature in the portion 17. The maximum thickness of layer is 0,002-5 mm, preferably 0,02-2 mm. A plane P, which coincides with the shoulder 18, substantially touches or intersects the layer 20. The layer 20 is more resistant to erosion corrosion than the supporting or subjacent steel.

Examples of such erosion corrosion-resistant materials are nickel, chromium, copper, tin, cobalt and titanium as well as alloys of these, preferably erosion- resistant steels or Co or Ni base alloys. Additional layers may be made in order to increase the binding between coating and steel. The material in the layer 20 has at least 50 mV higher electrode potential than the steel, preferably at least 100 mV, most preferably at least 250 mV higher electrode potential than the steel.

A number of different surface-coating methods may be used in order to apply the layer, e. g. chemical or electrolytic plating, thermal spraying or tool-tip- ping, preferably tool-tipping by means of laser.

Thus, the invention relates to a male portion for percussive drilling having a concave portion, which is coated with a protective layer in order to substantially improve the resistance to erosion of a low-alloy steel body.