FR2695339A1 - Plasma jet surface treatment of cut surface - esp. of granite slab, to eliminate flaws and irregularities - Google Patents

Abstract

Thermal surface treatment of the cut faces of massive pieces, esp. granite slabs, comprises displacing a plasma jet across the cut face to cause fragmentation of minerals at the surface, without directing any cooling fluid onto the face. Appts., for carrying out the above process, comprises a workpiece support (10); a plasma arc torch (16) operating a 100 kW power and fed with Ar/air in the ratio 1:9; and a device (14) for X-Y displacement of the torch over at least the surface of the workpiece. ADVANTAGE - Allows surface flaw and irregularity elimination from a wide variety of rock compsns. ranging from soft granite to hard granite, as well as sandstone, allows treatment of thin slabs, avoids the need for simultaneous cooling and allows simple equipment to be used.

Description

The present invention relates to a method and a device for surface treatment of tail faces of massive parts, more particularly granite slabs.

US Pat. No. 2,655,909 discloses a process for treating the surface of stone blocks which makes it possible to give this surface a softened state.

Indeed, it is known that the mechanical tools of flow and machining cause grooves and splinters on the cut faces of the blocks of stone.

Thus the method described in this US patent provides the use of an oxyacetylene torch to heat the surface to be treated so as to cause a bursting of the parts of the protruding block under the effects of thermal stresses.

Only the surface of the stone is heated, thanks in particular to the fact that the torch is inclined relative to the plane of the surface to be treated. This makes it possible to cause only surface stresses to a depth corresponding substantially to that of the defects and irregularities of the block to be removed.

The surface condition obtained is satisfactory because the coarse appearance of the production surface is refined and the particles after bursting on the surface give the surface of the block a shiny appearance, viewed from a certain angle.

Such a method is applicable to the treatment of stone blocks of a certain nature because the caloric density provided is low and many stones have compositions that do not lend themselves to such treatment.

On the other hand, the flow of stone blocks is no longer carried out by juxtaposed holes but rather by means of animated diamond son which leads after flow to very smooth surface conditions.

US-A-2,781,754 discloses a device and method for heat treating rough surfaces of stone slabs which prevents cracking of the slab. It is a question of arranging the plate in a tray so as to immerse the plate with covering of this plate by a thin film of water. Also, the thermal stresses diffusing in the plate are reduced by the absorption of the calories resulting from immersion in the water of said plate.

It can be concluded from this patent that heating with an oxyacetylene torch actually heats the stone to a certain thickness and causes differential expansion between the two opposite faces, heated and cold, or warping of the plate.

The heat treatment of a thin stone plate is therefore impossible by means of an oxyacetylene torch, except to provide an immersion device and, in this case, there is in any case a loss of the calorific value of the torch. which decreases the power of the thermal effect.

In all cases, the small thicknesses are nevertheless 20 mm minimum with the disadvantages caused by the implementation of the method using the cooling tank device.

Another US Pat. No. 2,675,993, more particularly applied to cutting rock and more particularly granite, raises another disadvantage due to the oxyacetylene torch. Indeed, if the rock is heated to the melting of certain zones, pockets of minerals such as mica in particular, the cutting progresses because the thermal effect leading to progressive cracking is concentrated on the same area.

Also, the oxy-acetylene torch is combined with a jet of high-pressure water whose mechanical power completes the thermal effect, breaking the bonds of the heated mineral particles and ensuring a progression of the cut, including in the melting zones.

Thus, heating certain areas for relatively long periods of time has drawbacks and reduces thermal effects.

The patent application EP-A-471 692 describes a method and a device for treating the faces of a body of hard material, in particular stone.

The method consists in generating a high-temperature plasma stream with a power of the order of 30 kW directed towards the face to be treated, combined with the projection of a jet of high-speed cooling fluid immediately downstream of the flow of plasma, more particularly at a distance of the order of 2 cm.

The combined effect of plasma heating and cooling causes surface degradation of the surface of the cut stones.

This solves a problem due to oxyacetylene torches, namely, the cracking of thin plates by heat diffusion over too great a surface thickness.

Indeed, the heat density provided by the plasma streams, as is well known, in particular by the patent application FR-A-2 251 178, is much higher than that obtained by means of an oxyacetylene torch.

It remains the case that the device remains complex because it is necessary to jointly control the simultaneous movements of the plasma torch and the cooling fluid which must be very close. The combination of all the new parameters is very difficult to master. As for the presence of cooling means, it is made mandatory to obtain the desired effect of superficial degradation.

The industry of manufacturing stone facing parts aims to reduce the thickness of the worked plates in order to reduce the weight which leads to easier installation and to reduce sizing support. Another consequence is the economy of matter.

As for the cost price, it must be as low as possible in order, on the one hand, to remain competitive with other materials and, on the other hand, to allow a wider distribution.

The present invention provides a method and a device that go in this direction and that overcome the disadvantages of the prior art by allowing the treatment of many rocks of different compositions ranging from soft granite to hard granite, as well as sandstones, which allows to treat plates of very small thickness, which eliminates any simultaneous cooling of the heated zone and simplifies the device necessary for the implementation of the method.

For this purpose, the present invention proposes a method of surface treatment of the cut faces of massive pieces, in particular of granite slabs, by means of a heating flux which is characterized in that only a jet of plasma is projected on the surface. face of the piece while performing a displacement in translation to cover the entire face, so as to cause the bursting of minerals to the surface of the room to the exclusion of any other projection of cooling fluid.

According to a preferred embodiment of the method, a plasma jet is generated from a blown arc torch whose parameters are a current intensity (I) of between 250 and 350 A, the air flow rate plasma (D) is between 250 and 350 Nl / min, the distance between the nozzle exit and the surface of the plate (h) is between 20 and 50 mm, the angle of inclination (a) between plasma jet axis and the horizontal is between 35 and 600, the speed (v) displacement is between 10 and 25 cm / sec and the pitch (d) between two adjacent passes is between 25 and 50 mm.

More particularly, for "hard" granites of small thickness, the parameters of the process are
I = 275 A
D = 300 Nl / min
a = 450
h = 35 mm
v = 15 cm / sec
d = 35 mm and for a granite "hard" thick, the parameters are
I = 300 A
D = 300 Nl / min
a = 450
h = 30 mm
v = 18 cm / sec
d = 45 mm
In the case of thick "soft" granites the method provides the following parameters
I = 300 A
D = 300 Nl / min
a = 55C
h = 30 mm
v = 14 cm / sec
d = 25 mm.

and for thin "soft" granites
I = 275 A
D = 300 Nl / min
a = 500
h = 35 mm
v = 15 cm / sec
d = 25 mm
As for the device, it comprises a workpiece support, a blown-arc plasma torch operating with a power of the order of 100 kW, an argon / air supply with a ratio of 1 to 9, means for moving the torch X, Y covering at least the area of the rooms.

More particularly, according to a preferred embodiment, the device is characterized in that the lengths of the channels of the nozzle-anode are 40 and 60 mm for respective diameters of 8 and 12 mm.

The invention is described below according to a preferred nonlimiting embodiment with reference to the accompanying drawings which represent - Figure 1, a schematic view of the device, and - Figure 2, a schematic view of a plasma torch such used.

FIG. 1 shows a schematic view of the device according to the invention, namely a plate support frame, thermally insulating shims 12, and a X, Y displacement table 14 which supports a plasma torch 16 .

The plasma torch is argon / air type with more than 90% air volume and operating with an electric power of 100 kW.

The plasma flux obtained is about 10 cm long, 3 cm in diameter and the average temperature is 4,000 OC.

Surprisingly, it has been found that, for such powers, the smooth surface obtained after sawing flow can be altered by treatment by means of the method according to the invention which consists of moving the plasma stream in the immediate vicinity of the surface. excluding any projection of a complementary cooling fluid.

The surface condition is sufficiently rough to prevent slippage, especially when the treated plates are used on the ground or to avoid any mirror effect when the plates are used in facing, mirror effect quickly altered non-homogeneously by atmospheric conditions and pollution.

The process is essentially conditioned by the following parameters, the intensity of the supply current of the torch I, the flow rate of the plasma gas in jected D, the angle of inclination of the gas torch the distance between the nozzle outlet- anode and the surface to be treated h, the speed of displacement of the torch u, as well as the distance separating two successive passes d.

An optimization of these parameters led to the following results depending on granite types and thicknesses.

Hard granite, very thick (about 4 cm)
B hard granite, thin (less than or equal to 1 cm)
C soft granite, very thick (about 4 cm)
D soft granite, thin (less than or equal to 1 cm).

Hard and soft granites consist mainly of cardinal minerals (quartz, feldspar), but soft granites have large size heterogeneities, and traces of weathering.

<Tb>

<SEP> I <SEP> I <SEP><SEP> D <SEP> a <SEP> h <SE> N <SEP> d
<tb><SEP> (A) <SEP> (Nl / min) <SEP> (O) <SEP> (mm) <SEP> (cm / sc) <SEP> (mm)
<tb> A <SEP> 300 <SEP> 300 <SEP> 30 <SEP> 45 <SEP> 18 <SEP> 45
<tb> B <SEP> 275 <SEP> 300 <SEP> 45 <SEP> 35 <SEP> 15 <SEP> 35
<tb> C <SEP> 300 <SEP> 300 <SEP> 55 <SEP> 30 <SEP> 14 <SEP> 25
<tb> n <SEP><SEP> 275 <SEP> 300 <SEP> 50 <SEP> 35 <SEP> 15 <SEP> 25
<Tb>
The entire plates thus treated have a surface state with roughness and relief granite as desired to make the plates anti-slippery.

Furthermore, the plates do not have cracks, cracks or residual deformations, including for very thin plates.

It turns out that the present method uses only a plasma torch excluding any coolant.

Also, the intense surface thermal shock is caused by the heating density which makes the surface of the ambient temperature rise to a very high temperature, the flow of plasma gas ensuring the entrainment of the particles and the scanning of the treated surface. This is perfectly visible, especially with the help of an ultra-fast camera.

 Nevertheless, a complementary brushing allows a perfect finish.

We can think that this surprising phenomenon is due to the vaporization of the water of constitution of the grains composing the rock.

The plasma torch 16 comprises a cathode 18 and an anode nozzle 20 as shown in FIG. 2.

The anode nozzle has a diameter 01 over a length L1 and a diameter 42 over a length L2.

between the nozzle and the cathode, a diaphragm 22 has been interposed.

The channels of the anode nozzle have more particularly diameters 41 = 8 mm and 02 = 12 mm and lengths L1 and L2 respectively of 40 and 60 mm.

Such a torch provides the above results.

Claims (8)

 1. A method of surface treatment of the cut faces of massive pieces, especially granite slabs, by means of a heating flux characterized in that it projects only a plasma jet on the face of the piece while moving in translation to cover the entire face, so as to cause the minerals to burst at the surface of the room to the exclusion of any other projection of cooling fluid.  2. Method according to claim 1, characterized in that generates a plasma jet from a blown arc torch whose parameters are a current intensity (I) of between 250 and 350 A, the flow of air plasma (D) is between 250 and 350 Nl / min, the distance between the nozzle-anode outlet and the surface of the plate (h) is between 20 and 50 mm, the angle of inclination (a) between the axis of the plasma jet and the horizontal is between 35 and 60, the speed (v) of displacement is between 10 and 25 cm / sec and the pitch (d) between two adjacent passes is between 25 and 25 cm / sec. and 50 mm.  3. Method according to any one of the preceding claims, characterized in that for a granite "hard", thin less than 10mm the parameters are  I = 275 A  D = 300 Nl / min  a = 450  h = 35 mm  v = 15 cm / sec  d = 35 mm  4. Method according to claim 1 or 2, characterized in that for a "hard" granite of great thickness, of the order of 5 cm, the parameters are  I = 300 A  D = 300 Nl / min  a = 450  h = 30 mm  v = 18 cm / sec  d = 45 mm  5. Method according to claim 1 or 2, characterized in that for a "soft" granite of great thickness (4 cm) the parameters are  I = 300 A  D = 300 Nl / min  a = 550  h = 30 mm  v = 14 cm / sec  d = 25 mm.  6. Method according to claim 1 or 2, characterized in that for a soft granite of small thickness, less than 10 mm, the parameters are  I = 275 A  D = 300 Nl / min  a = 500  h = 35 mm  v = 15 cm / sec  d = 25 mm  7. Device for surface treatment of cut faces of massive parts, especially granite slabs for carrying out the process according to any one of claims 1 to 6, characterized in that it comprises a part support, a blown-arc plasma torch operating with a power of the order of 100 kW, an argon / air feed with a ratio of 1 to 9, XY torch moving means covering at least the surface area of the parts.  8. Device according to claim 7, characterized in that the lengths of the channels of the nozzle-anode are 40 and 60 mm for respective diameters of 8 and 12 mm.