ZA200405498B

ZA200405498B - Element for percussive rock drilling and method for its production.

Info

Publication number: ZA200405498B
Application number: ZA200405498A
Authority: ZA
Inventors: Johan Linden
Original assignee: Sandvik Intellectual Property
Priority date: 2002-01-21
Filing date: 2004-07-09
Publication date: 2005-08-31
Also published as: SE525378C2; SE0200176L; SE0200176D0; WO2003062484A1

Description

ELEMENT FOR PERCUSSIVE ROCK DRILLING AND METHOD FOR ITS

PRODUCTION

Background of the invention

The present invention relates to an element for percussive rock drilling with new and improved fatigue properties and production economy properties as well as to a method for manufacturing an element.

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 in bits, rods, tubes, sleeves and shank adapters, is during drilling subjected to corrosive attack. This applies in particular to underground drilling where water is used as flushing medium and where the environment in general is humid. In combination with pulsating stress, caused by bending and the above-mentioned shock waves, so-called corrosion fatigue arises. This is a common cause for failure of the drill steel. A low-alloyed, case hardened steel is normally used for percussive rock drilling. Case hardening requires carburization of a surface layer in order to create compressive stresses in the impact surfaces, which give certain impeding effects on fatigue and improved wear resistance for the threaded portions. Case hardening is a time consuming operation and takes a large part of the cost for manufacturing of the drill steel. i An abrasion resistant steel (balance Fe), as shown in EP-A1-0527276, contains as basic elements from 0.05 to 0.45 wt.% C, 0.1 to 1.0 wt.% Si, 0.1 to . 2.0 wt.% Mn, 0.05 to 1.5 wt.% Ti, these contributing to an ‘enhancement of abrasion resistance without excessively increasing the hardness of the steel.

According to that document it is not necessary to enhance the hardness of the abrasion resistant steel by transforming the microstructure of the steel to a martensite which is the conventional way to enhance the abrasion resistance of ] steel. . Objects of the invention

One object of the present invention is to provide an elongated element for percussive rock drilling, which further improves production economy at modern mining.

Another object of the present invention is to provide a method for manufacturing of percussive drill steels, which implies substantial reduction in time for heat treatment.

Brief description of the drawings

These and other objects are attained by an elongated element and a method for manufacturing of drill steels for percussive rock drilling such as defined in the appended claims with reference to the figures.

Fig. 1 shows a microstructure from the core of a low-alloyed steel in a magnification of 1000x viewed in light optical microscope.

Fig. 2 shows a microstructure from the core of a drill steel according to the invention in a magnification of 1000x viewed in light optical microscope.

Detailed description of the invention

The invention relates to steels for percussive rock drilling made in an alloy with primarily martensitic basic matrix. Through the martensitic structure sufficient strength and core hardness for the application are obtained. The tensile strength should be 1000-1700 MPa and the core hardness 375-500 Vickers.

By carbonizing the drill steel in gas atmosphere compressive stresses are . obtained and high hardness at the surface, which prevents corrosion fatigue and improves wear resistance. To impede grain growth during heat treatment in , known steels, which would lead to deteriorated toughness and fatigue strength, this is carried out at temperatures of about 900°C. To obtain a carburized layer of sufficient thickness treatment times of the order of 6-10h are required.

Steels according to the invention are carburized at higher temperatures, . such that shorter carburization times become possible. Carburization is carried out in a steel according to the present invention at 960-1050°C, preferably about . 1000°C. Sufficient time for carburization can be estimated according to the following formula 16915 t=94.1075.p%2.e T where t is the carburization time in hours, D is the desired case depth in mm and

T is the temperature in Kelvin (K). The case depth is the measure used for the carburized depth and is defined as the depth where the hardness is lower than 550 Vickers. The desired case depth D depends in turn on the dimension of the rod, according to the formula

D=21-42.-R7% where R is the diameter of the rod in mm.

To obtain a carburized layer of sufficient thickness treatment times of the order of 2-5 hours are required according to the invention in comparison with conventional carburization times of 8-9 hours.

The drill steel according to the present invention shall in carburized condition have a surface hardness of more than 400 Vickers, preferably 500-800

Vickers in order to give good resistance against abrasion caused by for example the motion of the thread joints, drill cuttings or contact with the surrounding rock (the bore wall). Preferably, the carburized layer with increased hardness has a thickness of 0.5-2.0 mm.

Steel adapted for drill steel according to the present invention has the following composition in wt.%:

N 25 :

Cc 0.1-0.5, preferably 0.21-0.35 ) Si <2, preferably 0.1-0.5

Mn <2, preferably 0.5-1.5

Cr <5, preferably 1.0-2.0

Ni <5, preferably 2.5-3.5

Mo <2, preferably 0.1-0.5

R Ti 0.01-0.05, preferably 0.015-0.030 . as well as one or more of the following intentionally added grain growth impeding substances of the following range(s) in wt.% \% <1,

Nb <1,

Al < 0.5 and

Ti <1 fulfilling the following relationship:

Ff > 0.07, preferably >0.1 and

Ff < 5, preferably <3 and most preferably <1 where the fine grain factor

Ff= 1.8x%Al + 0.9x%Ti + 1.8x%V + 4.9x%Nb

Preferably, at least one of the grain growth impeding substances shall be present in the intervals below:

V: 0.05-0.15

Nb: 0.01-0.10

Al: 0.035-0.065

Ti: 0.05-0.10

Rather than making the whole element in the steel according to the . invention one or both threaded ends can be made according to the invention and be welded or be joined on a rod or a tube of another material.

The present invention also relates to a method for manufacturing an elongated element for percussive rock drilling comprising at least one connection means, such as a thread and flush channel. At least the connection means is . made in a steel with a structure comprising primarily martensite. The method comprises the following steps: . - providing the steel with a structure comprising martensite in the range of 55-98 volume-%, - manufacturing the steel according to melt metallurgical production technique, wherein at least one substance intended to impede grain growth during heat- treatment is intentionally added, - performing conventional rod manufacturing and machining, whereafter - carburizing and hardening is made at temperatures of 960-1050°C, preferably about 1000°C.

The invention also relates to an elongated element for percussive rock drilling comprising at least one connection means, such as a thread, and a flush channel, wherein at least the thread is made in a steel with a structure comprising primarily martensite (i.e. 55-98 by volume% martensite), where the steel comprises at least one intentionally added substance such as Al, Ti, Nb and V, to impede grain growth during heat-treatment.

With "intentionally" is meant that the substances do not occur in steel as impurities from the melt raw material but rather are added during the steel production. The content of impurities for a substance in this type of steel is about 0.01 wt.% or less. The intentionally added substances can in too high total content have negative influences on the toughness and machinability of the steel.

Example

Extension rods were manufactured from five steel alloys with compositions as disclosed below, where alloys 1-4 correspond to steels according to the invention and alloy 5 corresponds to prior art. The alloys 1-4 each have an intentional addition of one of the substances Al, Ti, V, and Nb, respectively, whereas the remaining three lie at impurity levels. Fig. 1 shows the microstructure of the core of Alloy 5 carburized for 5 hours at 860°C and Fig. 2 shows the microstructure of the core of Alloy 2 according to the present invention carburized for 5 hours at 960°C. %C %Cr %Ni, %Mo %N %Al %Ti %V %Nb %Fe Ff

Alloy 1 021 126 259 021 0.029 0.01 0.001 0.001 rest 0.108

Alloy 2 022 134 262 027 0.020 0.021 0.001 0.001 rest 0.117

Alloy 3 023 121 292 028 0.026 rest 0.257

Alloy 4 022 129 276 0.25 0.019 0.017 0.01 0.001 rest 0.482

Alloy 5 023 130 2.82 028 0.007 0.020 0.005 0.001 0.001 rest 0.047

Heat-treatments were performed in a furnace for carburization of extension rods where a gas with the carbon potential 1.0 % was obtained by means of a mixture of nitrogen, methanol and LP-gas. At metallografic investigation the following results were obtained, where K refers to the grain size of the austenite determined in accordance with ASTM E112 and D refers to the case depth in mm:

At temperatures above 925°C unwanted grain growth occurs in alioy 5. In alloys 1-4 grain size values K according to ASTM are larger than or equal to

ASTM 7.5 at all heat treatment tests, which show that drill steels according to the invention obtain sufficient case depth without unwanted grain growth up to 1000°C.

The disclosures in Swedish patent application No. 0200176-6, from which this application claims priority are incorporated herein by reference.

Claims

Claims

’ 1. Elongated element for percussive rock drilling comprising at least one connection means and a flush channel, wherein at least said connection means is performed in a steel, characterized in thatthe steel has a structure comprising martensite in the range of 55-98 volume-%, and at least one intentionally added substance intended to impede grain growth during heat-treatment.
2. The element according to claim 1, characterized in that said at least one substance is any of Al, Ti, Nb and V.
3. The element according to clam 2, characterized in that at least one of the substances Al, Ti, Nb and V is present and then in the following range(s)

(wt. %): \Y <1, Nb <1, Al <0.5 and Ti <1, wherein the range fulfills the following relationship:

0.07<Ff<5b where Ff= 1.8x%Al + 0.9x%Ti + 1.8x%V + 4.9x%Nb.
4. The element according to claim 3, characterized in that at least one of . the substances Al, Ti, Nb and V is present and then in the following range(s)

(wt. %): \Y 0.05-0.15, Nb 0.01-0.10, Al 0.035-0.065 and Ti 0.05-0.10
5. The element according to anyone of the preceding claims 2-4, characterized in thatthe steel in addition has the following composition ’ in wt.%: Cc 0.1-0.5, preferably 0.21-0.35, Si <2, preferably 0.1-0.5, Mn <2, preferably 0.5-1.5, Cr <5, preferably 1.0-2.0, Ni <5, preferably 2.5-3.5, Mo <2, preferably 0.1-0.5 and

N 0.01-0.05, preferably 0.015-0.030
6. Method for manufacturing an elongated element for percussive rock drilling comprising at least one connection means and a flush channel, wherein at least said connection means is performed in a steel, wherein the method comprises the following steps: .- providing the steel with a structure comprising martensite in the range of 55-98 volume-%, - manufacturing the steel according to melt metallurgical production technique, wherein at least one substance intended to impede grain growth during heat- treatment is intentionally added, - performing conventional rod manufacturing and machining, whereafter - carburizing and hardening is made at temperatures of 960-1050°C, preferably about 1000°C.
7. The method according to claim 6, using one or more of Al, Ti, Nband V as a . substance to impede grain growth.
8. The method according to claim 7, using one or more of the substances Al, Ti, Nb and V and then in the following range(s) (wt.%): \Y <1, Nb <1,

Al <0.5 and Ti <1, fulfilling the following relationship:

0.07<Ff<5 where Ff= 1.8x%Al + 0.9x%Ti + 1.8x%V + 4.9x%ND.
9. Method according to claim 7 or 8, using one or more of the substances Al, Ti, Nb and V and then in the following range(s) (wt.%): Vv 0.05-0.15, Nb 0.01-0.10, Al 0.035-0.065 and Ti 0.05-0.10
10. The method according to anyone of the preceding claims 7-9 using a steel, which in addition has the following composition in wt.%:

C 0.1-0.5, preferably 0.21-0.35, Si <2, preferably 0.1-0.5, Mn <2, preferably 0.5-1.5, Cr <5, preferably 1.0-2.0, Ni <5, preferably 2.5-3.5, Mo <2, preferably 0.1-0.5 and

N 0.01-0.05, preferably 0.015-0.030