INDUCTION-HARDENED DRILL STRING COMPONENTS BACKGROUND OF THE INVENTION

1. Field of the Invention THIS INVENTION relates to induction-hardened drill string components; and to drill strings incorporating such components.

The invention is particularly suitable for, but not limited to, drill string components, and drill strings, for single-pass blast hole drilling.

2. Prior Art A large proportion of production blast hole drilling worldwide is single pass. Single-pass refers to the technique of drilling a hole down, usually 15m - 19m deep, without having to add another drill steel to the string to get to the required depth.

Single-pass drill rigs generally have longer masts than multi-pass drill rigs to fit the drill string into. A single-pass drill string will normally consist of a top sub, two drill steels and a bit sub. The top sub screws into the rotary head that provides pull-down and rotary drive for the drilling operation. The first drill steel screws into the top sub and the second drill steel screws into the first steel. The bit sub screws into the second drill steel. The rotary drill bit, or down-the-hole hammer screws into the bit sub.

A large amount of air is passed through the inside of the drill string to force the cuttings, generated from the rotary drill bit or down-hole hammer, past the outside of the drill string. Normally the drill string is 25 mm-75mm smaller in diameter than the bit. The cuttings blown through the annular

cavity around the drill string may be travelling at 2000-15,000 feet per

minute. The abrasiveness, velocity and size of the cuttings passing the outer

diameter of the drill string are the main factors affecting wear rates on the drill string from this type of wear.

There is also another factor affecting wear rates of the first 2.5m-3.5m of the drill string connected to the drill bit. This factor is the requirement of drilling through 2.5m-3.5m of sub drill. Sub drill is set by how far you drill past the level of a new bench that will be worked to when the ground is

blasted. Sub drill is required due to the fact that when a pattern is drilled, and then blasted, it does not leave a completely flat surface. If the required

bench height is 18m, then the holes may be 20.5m-21.5m deep. This leaves

a 2-4m of shot ground that can be bulldozed or graded flat after the shovel has come through taking out the 18m of ore or waste.

The sub-drill is particularly hard on the drill string as it consists of broken rock that rubs against the drill string, while the bit works its way down

to unblasted material. Air is lost into the sub drill laterally due to the porous nature of the material, and does not create a sufficiently strong uphole blast

of air to clear the broken rock, until the bit is engaged in the unblasted

material. A single-pass drill string does not wear out threads, as the string is

coupled together until one of the components wears out or breaks.

At present, the method of extending the life of the drill string is by applying "hardfacing" using a welding process. This process normally

consists of a device to rotate the drill string while it is in the horizontal plane,

and a wire feeder that welds a run of hardfacing onto the component. The widths of these welds vary from approximately 6mm-40mm, and the entire length of the drill string can be hardfaced in this manner and has been done in the past. However, this process is expensive and time consuming. At present, most drill steels are manufactured using AISI 4140 for the tool joints and A106GB (mild steel pipe) or AISI 4140 for the connecting pipe.

The wear properties of the AISI 4140 connecting pipe are normally not that different than the A106GB. However, I have observed on one mine site 30% to 40% less wear on the 4140 tool joint adjacent to A106GB connecting pipe. The AISI 4140 drill steels normally cost around 30% more to manufacture due to the more expensive pipe and therefore it is not economic or at best marginally economic to utilise 4140 connecting pipe. The 4140 tool joints have a hardness of approximately 280-320 Brinell, where the A106GB connecting pipe is around 1650 Brinell. The difference in wear observed on the site appears to be directly related to the hardness of the material. A major component of the wear in single-pass drilling is caused by the drill string rubbing on the side of the hole and this mainly happens in sub drill or ground that has been blasted already. The 4140 material tends to not be gouged as deeply by the sharp rock compared to the A106GB connecting pie and therefore wears less. The 4140 material used for the tool joints has the same chemical make-up as the available 4140 connecting pipe, but the 4140 tool joint material is supplied quenched and tempered. The 4140 connecting pipe is not quenched and tempered, it is supplied as hot rolled and the hardness is around the 190-230 Brinell.

Single-pass drilling is normally used in metalliferous mining, Multi¬ pass drilling is used more in coal mines, where the seams are up to 120m below the surface, and at times, up to 7 drill steel sare coupled together to drill down to 85m. Normally, coal mines are not as abrasive as metalliferous mines, and the drill steel s do not wear as quickly. The wear, also, tends to be caused more by the sandblasting effect of the cuttings passing the outside diameter of the steel at up to 12000 feet/minute. The 4140 tool joints and A106GB connecting pipe tend to wear at the same rate. There is a tendency for the pipe to be 3-4mm smaller on the bottom of the pipe and this is normally a linear wear from bottom to top as the bottom of the pipe is in the ground subject to sandblasting longer than the top.

SUMMARY OF THE PRESENT INVENTION It is an object of the present invention to induction harden complete drill steels. It is a preferred object to harden such steels to a depth of at least

6mm and preferably to a hardness of >600 Brinell.

It is a further preferred object to effect the hardening using induction hardening and quenching heads which are hollow and passed along the drill steels. Other preferred objects will become apparent from the following

description.

In one aspect, the present invention resides in a method of induction

hardening a drill steel, including the steps of: supporting the drill steel to be hardened in a support member;

placing a hollow induction heating head and a hollow quenching head about the drill steel; advancing the induction heating head along the drill steel to heat an outer layer on the drill steel; and advancing the quenching head along the drill steel, to quench the outer layer of the drill steel to above the desired hardness and depth of hardness.

Preferably, the induction heating head and the quenching head are provided in a combined unit. Preferably, the drill steel is manufactured from AISI 4140 material, and the outer "layer" of the material is hardened to a depth of at least 6mm, and to a hardness of at least 600 Brinell.

The support member may be a lathe or other suitable support structure. Preferably, the temperature of the drill steel is closely monitored by optical pyrometers during the heating step.

Preferably, where region(s) of the drill steel are to be engaged by tools, eg., for uncoupling, an outer layer of the material in the region(s) hardened to a lower Brinell hardness. In a second aspect, the present invention resides in a drill steel, or other drill string component, hardened by the method, of the first aspect, hereinbefore described.

In a third aspect, the present invention resides in a drill string incorporating one or more drill steels or drill string components, hardened by

the method of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS To enable the invention to be fully understood, preferred embodiments will now be described with reference to the accompanying drawings in which:

FIG. 1 is a side view indicating the general appearance of a single- pass mining drilling string;

FIG. 2 is a sectional side view of a typical drill string hardened in accordance with the present invention; FIG. 3 is a schematic side elevation view of a drill string being hardened in accordance with the invention; and

FIG. 4 is an end sectional view of the induction heating head/ quenching head unit taken in line 4-4 on FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates an example of a typical single-pass drill string 10, where a cushion sub 11 and saver sub 12 operably connect at least one drill steel 13 to a rotary drilling head 14. A bit sub 15 connects the, or lowermost, drill steel 13 to the rotary drill bit or down-hole-hammer (not shown).

The drill string 10 is stabilised by a deck bush 16 on the drill rig (not shown).

FIG. 2 illustrates a cross-sectional side view of a drill string 13, manufactured of 4140 material, where an induction-hardened outer layer 13a, preferably 6.0mm-10.00mm thick, and of at least 600 Brinell hardness, is formed along the string.

NB: (i) The screw threadsi 3b (of the pin tool joint 13c) are not hardened.

(ii) The outer layer of the recesses 13d, engageable by uncoupling tools, are preferably hardened (eg to a 6.0mm thickness) to a hardness greater than 4140 pipe, but less than the at least 600 Brinell hardness of the 5 layer 13a.

The hardness of the outer layer of the recesses 13d may be in the range of 280-500 Brinell.

The method of effecting the hardening of the layer 13a will now be described. o The drill steel 13, of 4140 material for the tool joints and the connecting pipe, is supported in a long (eg. 12m bed) lathe 20 by gripping one end with a chuck 21 and supporting the other by a chuck or a tailstock 22.

The induction heating and quenching unit 30 is hollow and passes s about the drill steel 13.

The induction heating and quenching unit 30 is mounted on the lathe carriage 23 and its rate of advance along the lathe 20 is controlled by a feed screw 24.

An induction heating head 31 is connected to a high frequency o electrical power source 32 via suitable power cables 33, and closely surrounds the pipe 13.

The quenching head 34 is located adjacent to, and downstream of, the induction heating head 31 , and is connected to a source of quenching liquid (not shown) by suitable pipes 35.

The drill steel 13 is heated by electrical induction generated by the induction heating head 31. A current is oscillated at high frequencies through the pipe wall or tool joint. The resistance of the material to the electron flow converts the electrical energy into heat, which is closely regulated and monitored by optical pyrometers 36 mounted on the unit 30. The relative speed of the drill steel 13 passing through the induction heating head 31 , and the power applied, is precisely controlled to achieve heat balance at the required temperature.

The drill steel 13 is then subjected to quenching in the quenching head 34 to achieve the desired hardness and depth of hardness. The induction head 34 has quenching sprays 37, integral to the head 34, directing quenching liquid onto the drill steel 13.

A potential problem in the process could be the drill steel 13 bending as it is heated and quenched. To limit this bending, the drill steel 13 can be rotated as the induction heating and quenching unit 30 is passed over it - this will tend to evenly distribute the heating and quenching around the drill steel 13. If the drill steel 13 does bend, it is proposed to either hydraulically straighten the drill steel or spot heat the drill steel to straighten it.

The induction hardening method need not be limited to drill steels, as other drill string components, such as bit subs and stabilisers may be hardened as well.

One of the problems of hardening complete drill strings is that when drill string components are uncoupled, using a hydraulic pipe wrench fitted standard to the drill, the break-out dies in the wrench that grip the drill steel

need to be harder than the drill steel it is attempting to grip. If the dies are softer or the same hardness, they will not bite into the steel and the drill steel will slip on the dies. This would make uncoupling very difficult. It is proposed to overcome this problem by induction hardening the region(s) eg recesses 13d, on the drill steel 13 that are gripped by the pipe wrench. The hardening process will be modified for these region(s) to ensure they are softer than the rest of the induction hardened material 13a. The softer outer layers of the recess 13d will still be harder than standard quenched and tempered 4140 material, but not too hard that the dies will not grip. (The "softer" outer layers of the recess 13d are preferably hardened, eg., in the range of 280-500 Brinell.)

It will be readily apparent to the skilled addressee that the extended service life of the drill string components, and the drill strings incorporating same, manufactured by the method of the present invention, whilst possibly having a higher initial capital outlay, will result in considerable savings over the life of the drill strings.

Various changes and modifications may be made to the embodiment described and illustrated without departing from the present invention.