ELONGATED ELEMENT AND STEEL FOR PERCUSSIVE ROCK DRILLING

Abstract: The present invention relates to a martensitic, corrosion resistant steel for rock drilling with properties adjusted essentially with regard to resistance against corrosion fatigue. This has been obtained in that an elongated element for percussive rock drilling which includes at least a thread and a flush channel has been made in a corrosion resistant steel with a mainly martensitic structure.

Description: ELONGATED ELEMENT AND STEEL FOR PERCUSSIVE ROCK DRILLING Background of the invention The present invention relates to a martensitic, corrosion resistant steel for rock drilling, with new and improved properties, particularly with regard to resistance against corrosion fatigue. Prior art During percussive rock drilling shock waves and rotation are transferred from a drill machine via one or more rods or tubes to a cemented carbide equipped drill bit . The drill steel, i.e. the material m bits, rods, tubes, sleeves and shank adapters, is during drilling subjected to corrosive attack. This applies m particular to underground drilling where water is used as flushing medium and where the environment m general is humid. The corrosive attacks are particularly serious m the most stressed parts, i.e. thread bottoms and thread clearances. In combination with pulsating stress, caused by bending and above-mentioned shock waves, so-called corrosion fatigue arises (Figure 1) . This is a common cause for failure of the drill steel. A low-alloyed, case hardened steel is normally used for the drilling application. The reason is that abrasion and wear of the thread parts have generally been limiting for life. As the drill machines and tools have become more efficient, these problems have however diminished and corrosion fatiαue has become a limiting factor. The case hardening gives compressive stresses m the surface, which gives certain retarding effects on the fatigue. US-A-5, 496, 4f 1 relates to a high strength martensitic stainless steel. The steel contains: 0.06 wt- % or less C, 12 to 16 wt-% Cr, 1 wt-% or less Si, 2 wt-% or less Mn, 0.5 to 8 wt-% Ni , 0.1 to 2.5 wt-% Mo, 0.3 to 4 wt-% Cu, 0.05 wt-% or less N, and the balance being Fe and inevitable impurities; said steel having an area ratio of delta-ferrite phase of at most 10 ø. The known steel intends to solve the problem of stress corrosion caused by an acidic environment . Objects of the invention One object of the present invention is to provide an elongated element for percussive rock drilling which further improves the efficiency of modern mining. Another object of the present invention is to provide an elongated element for percussive rock drilling with increased life. Still another object of the present invention is to provide a drill steel with reduced corrosion rate. Still another object of the present invention is to provide a drill steel with reduced sensitivity for corrosion fatigue. Short description of the drawings These and other purposes are obtained through an elongated element and a drill steel such as they are defined m the enclosed patent claims with reference to the figures. Fig. 1 shows, at 25x, cracks m a thread bottom m a low-alloyed steel. Fig. 2 shows, at 500x, the structure of a drill steel according to the invention. Detailed description of the invention The invention relates to a steel for rock drilling made m a corrosion resistant alloy with a martensitic matrix where the resistance is obtained by additions of Cr as well as Mo, , Cu and/or . Tnrough the martensitic structure, (figure 2), the necessary strength and core hardness for the application is obtained. Preferably the martensite content is >50 wt-% but <100 wt-%, preferably >75 wt-%. Tne ultimate tensile strengtn shall be >800 MPa, preferably 1300-3000 MPa . By making the drill steel m a corrosion resistant alloy owing to the chromium addition a passive layer on the surface is obtained, whicn prevents corrosion or reduces the corrosion rate and thereoy the corrosion fatigue, especially m thread bottoms such as is shown m Fig. 1. In order for the drill steel according to the invention to be sufficiently corrosion resistant it is required that it has a chromium content of at least 11%. The total content of carbon and/or nitrogen (C+N) must be >0.05%, preferably 0.1-0.8%. Alternatively the chromium content can be lower than 11%, down to 5%, which then can De compensated for by the addition of preferably molybαenum (up to 5 %, preferably 0.5-2 wt-%), tungsten (up to 5 %, preferably 0.5-2 wt-%) and/or copper (up to 2 s -, preferably 0.1-1 wt- %) wherein the total content Mo+W+Cu>0 , 5% , preferably >1 wt-%. Still an alternative is that tne alloy has a composition which gives a PRE-number >10, preferably 12- 17. PRE means Pitting Resistance Equivalent and describes the resistance of an alloy against pitting corrosion. PRE is defined according to the formula PRE =Cr+3.3 (Mo+ ) +16N where Cr, Mo, and N correspond to the contents of the elements m weight percent . A steel according to the invention shall in addition have a surface hardness of more than 400 Vickers, preferably 500-800 Vickers m order to further increase its resistance against abrasion caused by e.g. movements m threaded joints, drill cuttings or contact with the surrounding rock (the bore wall) . Preferably tne steel has a 0.5-2.0 mm thick surface layer with increased hardness . Drill steels according to the invention are made by conventional steel rod production and machining. In order to obtain the desired martensitic structure the steel is hardened or cold worked. The wear resistance can be further improved by induction hardening of the surface or by applying surface treatment methods such as carburizmg and nitridmg. The invention also relates to the use of a steel according to the invention as a drill steel . Instead of performing the whole element m steel according to the invention one or both thread ends can be performed according to the invention and be welded or joined on to a rod or a tube of another material. EXAMPLE In so called drifter drilling about 4 long rods are used. The critical part of the rods are the bottoms on the male threads such as shown m Fig. 1 where the flushing water and pulsating stresses give rise to corrosion fatigue which frequently results m fracture. Drifter rods were made of three alloys with compositions according to the following: Test %C %Cr %Nι %Mo %W %Cu %N %Fe Martensite content l- 0.18 1 3 0 3 0 02 0 01 0.12 0.012 Rest 98% 5-8 0.50 1 4 3 0.15 0.02 0 01 0 06 0 011 Rest 89% 9-12 0.35 11 9 0.22 1 05 0 01 0 06 0 013 Rest 95% Drilling was performed m a rig for drifter drilling underground and the drilling was continued until fracture/wear. The following lengths of life, measured in drilled meters, were achieved: Test no 1 2 3 4 5 6 Drilled meters 3299 2904 303: 2876 2893 3121 Test no 7 8 9 10 11 12 Drilled meters 2976 2656 2628 2189 3222 2929 Normal life for drifter rods of conventional type, i.e. of low-alloyed, case hardened steel, is at the test site m question where the rock primarily consists of granite, about 2000 m, which shows that use of a drill steel according to the invention gives a remarkable improvement . In other words all steels according to the present invention contain the common feature of C+N > 0.1 wt-% such that a preferred steel is selected from one of the compositions listed below: • C+N > 0.1 wt-% and Cr > 11 wt-%, or • C+N > 0.1 wt-% and Cr > 5 wt-%, Mo < 5 wt-%, < 5 wt-%, Cu < 2 wt-%, Mo+ +Cu > 0.5 wt-%, or • C+N > 0.1 wt-% and Cr+3.3 (Mo+ ) +16N > 10 wt-%.

Claims: ms 1. Steel for an elongated element for percussive rock drilling including at least a thread and flush channel c h a r a c t e r i s e d m that tne steel m at least the thread is corrosion resistant and has a mainly martensitic structure. 2. Steel according to claim 1, wherein the martensite content is >50 wt-% but <100 wt-% and that the steel contains : C+N > 0.1 wt-% and Cr > 11 wt-%, or C+N > 0.1 wt-% and Cr > 5 wt-%, Mo < 5 wt-%, W < 5 wt-%, Cu < 2 wt-%, Mo+ +Cu > 0.5 wt-%, or C+N > 0.1 wt-% and Cr+3.3 (Mo+W) +16N > 10 wt-%. 3. Steel according to claim 2 , wherein the martensite content is >75 wt-%. 4. Steel according to any of the preceding claims, wherein the Cr content is >5 wt-%. 5. Steel according to claim 4 which m addition contains <5 wt-% Mo, <5 wt-% W and/or <2 wt-% Cu. 6. Steel according to any of the preceding claims wherein the composition gives a PRE-number >10 where PRE is defined according to formula PRE =Cr+3.3 (Mo+W) +16N and where Cr, Mo, W and N correspond to the contents of the said elements m weight percent. 7. Elongated element for percussive rock drilling including at least a thread and flush channel c h a r a c t e r i s e d m that at least the thread is made of a corrosion resistant steel with a mainly martensitic structure. 8. Element according to claim 7, wherein the martensite content is >50 wt-% but <100 wt-% and that the steel contains: C+N > 0.1 wt-% and Cr > 11 wt-%, or C+N > 0.1 wt-% and Cr > 5 wt-%, Mo < 5 wt-%, W < 5 wt-%, Cu < 2 wt-%, Mo+W+Cu > 0.5 wt-%, or C+N > 0.1 wt-% and Cr+3.3 (Mo+W) -rl6N > 10 wt-%. 9. Element according to claim 8, wherein the martensite content is >75 wt-%. 10. Use of a steel for an elongated element for percussive rock drilling including at least a thread and flush channel c h a r a c t e r i s e d m that the steel m at least the thread is corrosion resistant and has a mainly martensitic structure.

Inventors: LINDEN JOHAN, LUNDELL LARS-GUNNAR, LINDEN, JOHAN, LUNDELL, LARS-GUNNAR
Applicants: SANDVIK AB
Application year: 2001
Main classification: C22C38
IPC codes: C22C38, E21B17