GB2402401A

GB2402401A - Coated pistons

Info

Publication number: GB2402401A
Application number: GB0312961A
Authority: GB
Inventors: Christopher J Green
Original assignee: Halco Drilling International Ltd
Current assignee: Halco Drilling International Ltd
Priority date: 2003-06-05
Filing date: 2003-06-05
Publication date: 2004-12-08
Also published as: GB0312961D0

Abstract

A down-the-hole hammer piston in rock drilling apparatus is provided with a crack-inhibiting coating of wear-resistant material with anti-galling qualities, such as a refractory metal carbide applied by high pressure/high velocity oxygen fuel spray (HP/HVOF). The coating extends the life of the piston through increased resistance to surface particle ingress and reduced wear on the sliding outside surface of the piston and through reduced occurrence of thermal cracking from micro welding spots on the sliding outside surface of the piston. The coating also prevents thermal cracks from propagating through the coating boundary into the original (metal) piston material and thus from continuing into the piston causing catastrophic failure.

Description

P 1 7667GB-ICC/lp Title: "Hard facing/hard coating pistons" THE PRESENT

INVENTION relates to pistons and is particularly concerned with reducing the effect of damage such as galling and thermal cracking originating from galling at the working surface of a piston relative to a cylinder, in conditions where lubrication between the piston and cylinder is poor or unreliable. The invention is particularly, but not exclusively, applicable to the treatment of pneumatic down-the-hole hammer pistons in rock drilling equipment.

Lubrication of a down-the-hole hammer in rock drilling equipment can be poor, for example because the lubricant used is of a low viscosity or has low load- bearing properties or simply because the volume of lubricant supplied to the piston and its cylinder becomes reduced or is interrupted for one reason or another. The problem may be exacerbated by the need to use biodegradable oils for environmental reasons. Where, for any of the above reasons, lubrication is inadequate, galling may take place between the piston outside diameter and the cylinder bore and/or catastrophic failure of the piston may occur as a result of breakage originating in surface cracks produced from such galling.

Galling occurs from micro-welding of two surfaces sliding relative to one another, for example the co-operating surfaces of a pneumatic down-thehole hammer and its complementary cylinder. Such galling typically occurs when the lubricating fluid used in such a situation is displaced from between the surfaces in question, allowing direct contact between them. In such cases, metal-to-metal contact occurs and micro-welding between the surfaces can take place, which leads to the initiation of thermal cracks which can propagate towards the centre of the piston until material separation and hence piston failure occurs. s

It is an object of the present invention to avoid or mitigate this problem.

According to the invention there is provided a piston, for example, a down-the- hole hammer piston in rock drilling apparatus, provided with an anti- galling or crack-inhibiting coating of wear-resistant material.

The coating on the piston may be a metal carbide.

According to another aspect of the invention there is provided a method of protecting a piston, for example, a down-the-hole hammer piston in rock drilling apparatus, with a crack-inhibiting coating, comprising coating the piston with a crack-inhibiting coating of wear-resistant material with reduced galling qualities.

In a preferred embodiment of the invention, to reduce the effects of thermal cracking, a sacrificial barrier is applied to the outer, sliding surface of the piston, this barrier or coating being mechanically bonded to the piston surface.

The barrier is preferably a hardmetal cemented carbide alloy applied using a technique known as high pressure/high velocity oxygen fuel spraying, (abbreviated as HP/HVOF). HP/HPVOF spraying is advantageous when used to apply the barrier to the piston because there is no detrimental rise in the surface temperature of the pre heat-treated piston. Hence there is no reduction in the bulk mechanical properties of the piston particularly with regard to its impact properties, which have to be maintained. A coating thickness of 0.10 to 0.75 mm has proven to give a good ductile structure and reduces premature delamination of the coating from the piston surface.

In this particular application a tungsten carbide based alloy with a cobalt binder is currently preferred but other metal carbides could be used such as vanadium, chromium, titanium, or combinations thereof with binders of the iron group metals such as iron, and nickel.

Coating the piston with this extra hard structure also gives the added benefit of reducing the wear on the sealing outside diameter surface. This hard surface also protects the piston outside diameter surface from penetrating hard abrasive micro particles such as particles of silica. With a piston with an ordinary metal outer surface, these particles are able to penetrate that outer surface and raise the surface structure to create 'high spots', causing a micro welding action between the sliding piston outside diameter surface and the mating cylinder bore surface due to reduced running clearance, resulting in possible seizure or catastrophic failure as a result of thermal cracks propagating to the centre of the piston.

With a piston provided with a protective coating as described, when surface thermal cracks appear on the HP/HVOF coating of the sliding piston outside diameter surface, they only penetrate to the boundary between the protective coating and the piston material to which the coating was applied and stop at that boundary, or continue to travel under the coating along that boundary, protecting the base material from the crack propagation. This phenomenon is due to the mechanical boundary created by the fact that the coating and the base material of the piston are two dissimilar materials and that during application there is no significant temperature rise in the base material and hence no diffusion occurs between the coating and the base material.

The HP/HVOF coating also acts like a heat sink absorbing heat which may be generated at what might otherwise have been a potential micro weld site, avoiding any high temperature rise at such site, and the coating does not readily conduct this heat through to the base material of the piston.

To summarise, in preferred embodiments of the invention, a hard coating comprising a carbide, preferably of a refractory metal such as tungsten, titanium or tantalum, with a binder metal of the iron group (iron, nickel or cobalt) is applied to the outside diameter of a pneumatic down - the hole hemmer piston by the high pressure/high velocity oxygen fuel (/HVOF) technique, which does not reduce the impact properties of the pre - heat treated base material and reduces the need for oil based lubrication. Such a coating extends the life of the piston through increased resistance to surface particle ingress and reduced wear on the sliding outside surface of the piston and through reduced occurrence of thermal cracking from micro welding spots on the sliding outside surface of the piston. The coating also prevents thermal cracks from propagating through the coating boundary into the original (metal) piston material and thus from continuing towards the centre of the piston causing catastrophic failure.

In the present specification "comprises" means "includes or consists of" and "comprising" means "including or consisting of".

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A piston, for example a down-the-hole hammer piston in rock drilling apparatus, provided with a crack-inhibiting coating of wear-resistant material with anti-galling qualities.

2. A piston according to Claim 1 wherein the coating material is selected from the group comprising refractory metal carbides.

3. A method of protecting a piston, for example, a down-the-hole hammer piston in rock drilling apparatus, with an anti-galling or crackinhibiting coating, comprising coating the piston with an anti-galling or crack-inhibiting coating of wear-resistant material with anti-galling qualities.

4. A method according to Claim 3 wherein the coating material is selected from the group comprising refractory metal carbides.

5. A method according to any of Claim 3 or Claim 4 wherein the coating is applied by high pressure/high velocity oxygen fuel spray (HP/HVOF).