FR2560090A1 - CAST IRON ARTICLE AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

THE INVENTION RELATES TO A CAST IRON ARTICLE INCLUDING IN A DETERMINED PART 11A OF ITS SURFACE LAYER A LAYER TREATED BY RESUMPING FORMED BY PARTIAL SOAKING TREATMENT INCLUDING AN ARC DISSOLUTION OPERATION OF PLASMA 12 OF ONE OR MORE THAN ONE SPECIES HIGH-HARD METAL CHOSEN FROM THE GROUP INCLUDING CR, MO, NI, W, V AND NB FOR INCORPORATION INTO THE SUBSTRATE OF ARTICLE 11 AND A COOLING OPERATION FOR SUBSEQUENT SOLIDIFICATION. DEPENDING ON THE PROCESS FOR MANUFACTURING THE CAST IRON ARTICLE, THE PLASMA ARC 12 IS PROJECTED BY THE PLASMA TORCH 1 ON THE DETERMINED PART 11A OF THE SURFACE LAYER TO FORM IN IT A POOL 13 OF MELTED METAL, ALL BY DELIVERING THE POWDERED HIGH HARDNESS METAL INTO THE PLASMA ARC 12.

Description

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  CAST IRON ARTICLE AND MANUFACTURING METHOD THEREFOR.

  The present invention generally relates to a cast article and its method of manufacture. More particularly, it relates to a quenched cast iron article for camshaft and rocker arm, for an internal combustion engine, in front of

  to resist abrasion, as well as its manufacturing process

cation.

  In an internal combustion engine, rotation

  the camshaft and the opening movement-

  valves are connected to each other by means of a combination of camshaft and rocker fingers so that the valves open and close at appropriate times to ensure the admission of a valve. fuel-air mixture and the exhaust of the burned gases, respectively. The camshaft and rocker pusher of an overhead cam type engine directly engage sliding contact with each other by some of their parts. In one

  overhead valve type engine, a cul-

  the slider part makes sliding contact with the camshaft and other parts thereof with a part of the rocker arm finger and therefore both the camshaft and

  tumbler have in one part of a layer

  perfective that they show a stronger resistance

  abrasion only in their other parts.

  Following a classical method, in the manufacture

  tion of a cast article such as a camshaft to have in one of its parts a superior resistance to abrasion, is added to the molten metal, during the molding of the article, a metal of great hardness

  such as chromium or molybdenum, and a

  cooler on a portion of the metal mold to achieve at the same time as the molding a hardened layer with excellent abrasion resistance in the - 2 -

contact with the cooler.

  To provide the hardened layer with greater hardness to further increase its abrasion resistance, the amount of high hardness metal added to the molten metal is simply increased. However, this increase in the added amount also increases, by the addition of the hard-hard metal, the hardness of the part of the article, such as

  trunnion, other than that in contact with the cooling

  This makes the cutting work difficult to perform after molding. In some cases, a hardened structure may also appear in the non

  contact with the cooler, which makes it very difficult

the subsequent cup.

  For this reason, in view of the subsequent cutting work, a high limit of 1.0% by weight is added to the addition of high hardness metal when a single hardness metal species of 1.4 is added. % by weight in case of addition of metals of several species, limits beyond which the addition of metal with high hardness makes the work of cutting difficult. Therefore, a current cast article has in one of its parts an abrasion-resistant layer

insufficiently trained.

  To overcome this disadvantage, we have already

  proposed a method according to which, during an operation

  molding, a molded article is produced in an ordinary manner, without adding metal with a high degree of hardness.

  make the work of cutting the article difficult

  and without placing a cooler on a portion of the metal mold and, after the molding operation, only the part to resist abrasion is subjected to quenching treatment; such a process is described for example in the patent application JA n SHO 53-115203

  deposited on 21 September 1978 under the benefit of the

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  of the patent application DE No. 27 42597.4 (filed

  September 22, 1974) and published as a patent

JA No. SHO 54-57010 May 8, 1979.

  According to this prior method, the

  electronic element the part to resist abrasion

  to remelt it before allowing it to cool

  even so that it forms within it a soaked layer.

  However, according to this reflow-quenching process,

  the quenched layer finally formed has the same constitution

  than the body of the molded article and, therefore, a soaked layer with a

  hardness or a very satisfactory abrasion resistance

  compared to those of a hardened layer formed

by means of a cooler.

  The present invention has been designed to solve these traditional problems posed by a cast article

  and by its manufacturing process.

  According to the present invention, there is provided a cast iron article comprising at least one metal species of high hardness, selected from the group consisting of

  chromium, molybdenum, nickel, tungsten, vanadium

  dium and niobium, dispersed or present in solution

  lide as a single body, alloy or compound,

  characterized in that it has, in a particular part,

  of its superficial layer, a layer treated with

  overhaul and by a partial quenching treatment

  a dissolving operation intended to incorporate the metal of high hardness by means of a plasma arc

  to the subjective of the article, and a cooling operation

  drainage for later solidification.

  According to the invention, there is also provided a method of manufacturing a cast iron article comprising at least one metal species of high hardness, selected from chromium, molybdenum, nickel, tungsten,

  vanadium and niobium, dispersed or present in

  Solid form in the form of a single body, alloy or compound, comprising a first operation comprising

  to place a determined part of the surface layer-

  The article in front of a plasma torch, a second operation of respectively connecting an electrode of the plasma torch and the article to the negative terminal of a DC power source and the positive terminal. from this source, a third operation consisting in causing the plasma torch to project a plasma arc on the determined part of the surface layer in order to form a puddle of molten metal therein while causing the metal to reach a high hardness. the state of powder in the plasma arc,

  a fourth operation to move the trawl

  Plasma water between certain limits on the determined part of the superficial layer, while continuing to project the arc of plasma and to make arrive the metal with great hardness powder in the arc of plasma, a fifth operation consisting of stopping the projection of the plasma arc as well as sending the powdery high-hardness metal into the plasma arc, a sixth operation of cooling the molten pool and

  a seventh operation of removing the article.

  Accordingly, it is an object of the present invention to provide a cast iron article having a remelt-treated layer having a remarkable abrasion resistance in a particular portion.

  to that of a hardened layer obtained by means of a

  cooler, as well as a method of manufacturing a

such article in cast iron.

  The invention will be better understood from the

  detailed description given below, as an example

  ple, of one of its preferred embodiments with reference to the accompanying drawings, in which:

  FIG. 1 is a side view, partially

  5, in section, of an essential part of a plasma torch applied to the manufacture of a cast iron article according to the invention; and FIGS. 2A, 2B and 2C are electron probe microanalyzer diagrams showing the distribution of essential elements in a

  layer treated by recasting the cast article.

  We first refer to Figure 1 which is a

  partial sectional view showing the structure

  of a plasma torch 1 adapted for the manufacture of a cast article 11 according to the present invention.

  invention; the plasma torch 1 comprises, at the

  2 of a hollow protective cap 2, a single copper nozzle 3 defining with the cap 2 an axial path 4 for the admission of a protective gas such as a gas

  inert. In the center of the nozzle 3 is defined another work.

  an axial jet 5 for the admission of an active gas such as argon to be placed in the plasma state and this path 5 is surrounded by a path 6 closed at the end of the cooling agent circulation. A tungsten electrode 7 is disposed axially on the path 5 of the active gas, which path narrows at its lower end to

  define a plasma ejection port 8 for the

  jection of the gas to the state of plasma.

  In addition, the protective cap 2 has a

  series of tubular guides 9 which cross oblique-

  to surround the nozzle 3 at an equal distance

  each other, each guide 9 supporting one of several

  10 powder metaL feed tubes 10 inserted into the nozzle so that the axis of each tube 10 passes.

by the axis of the orifice 8.

  We will describe below a process of manufacturing

  of the cast article 11 according to the present invention.

  using the plasma torch 1 described above.

  It will be assumed that the cast iron article 11 has been routinely molded without the addition of high hardness metal to a degree which may make cutting difficult and without the use of a cooler.

  and that he has undergone the cutting operation.

  First, as shown in FIG.

  place the plasma torch 1 in front of a

  completed 11a, having to resist both abrasion and the appearance of cracks, a superficial layer

  of the cast article 11 in the finished state after cutting.

  The tungsten electrode 7 is then connected to the negative terminal (not shown) of a DC power source (not shown) and

  article 11, at the positive terminal (not shown

  felt) from the power source. The protective gas is fed along the axial path 4 and the gas

  active, eg argon, along the axial path 5.

  As a result, a discharge occurs from the elec-

  trode 7, putting the active gas present on the path to the plasma state to generate the plasma, which has only a reduced passage section at the orifice 8 and rushes out of it under in the form of a plasma arc 12, that is to say a plasma jet at high temperature and at high speed, to be projected by the nozzle 3. The projected plasma arc 12 is directed towards the determined part lla of the surface layer of the article 11, which is at a positive potential with respect to the tungsten electrode 7, and the heat of the arc appears in the part

11a molten pool 13.

  In doing so, we make arrive in the arc of

  plasma 12, through the metal powder feed tubes

  10, a powder 14 composed of one or more species of metal with high hardness. The high hardness metal may be selected from the group consisting of

  chromium, molybdenum, nickel, tungsten, vanadium

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  dium, niobium and the like and can be a body.

  a single one of these metals, an alloy of two or more of these metals or one or more of them with another metal or other metals and / or a compound of one or more of these metals with carbon and / or the like. The amount of powder 14 to be delivered in the plasma arc 12, expressed as a percentage by weight of metal with a high hardness relative to the puddle 14, is preferably between 1.0 and 15.0%, when the number of high-hardness metal species to arrive is equal to 1 or between 0.7 and 0% per species and between 1.4 and 16.0% in total, when this number is greater than 1. Indeed, when the amount

  14 is below the lower limit of

  corresponding percentage of percentages by weight

  For example, the abrasion resistance of the portion 11a of the remelted cast iron article 11 for partial quenching purposes can hardly differ from that which would otherwise be achieved by the conventional cooler method, the hardness being the same. thus proving insufficient. On the other hand, if the upper limit is exceeded, the

  hardness may become too great to avoid

  undesirable quality, which lowers resistance to

  cracks and creeks are likely to appear during cooling after recasting at

plasma as well as during grinding.

  In the remelting process described above, the metal powder 14 arriving in the plasma arc 12 is forcibly confined in this arc, so that it is accelerated and heated, then projected at high speed and at high temperature over the surface of the puddle 13, to mix with it, while the surface of the puddle 13 is hollowed by the pressure of the plasma arc 12 exerted on it, the hollow thus formed being made to move according to the movements of the plasma torch 1, which effectively ensures the agitation of the puddle 13. Therefore, the powder 14 of high hardness metal incorporated in the sheet 13

  it is regularly distributed by the effect of

  tion. Therefore, when the metal powder 14 has a sufficiently low melting point or is soluble in the slab 13, it regularly mixes with the substrate of the puddle 13, forming with it an alloy and / or a compound. When the powder 14 is not soluble in the puddle 13, it disperses uniformly, without changing

of chemical composition.

  By cooling the puddle 13, we make

  appear in the superficial part 11a of the article

  The cast iron core 11 comprises a recoat-treated layer containing a homogenized ally with regular dispersion of one or more metals of high hardness and / or regularly dispersed abrasion-resistant particles, which layer has thus been treated with excellent

  resistance to both abrasion and cracking.

  A number of essential operating conditions will be described above for obtaining, according to the invention, a remelt-treated layer having a high resistance to abrasion and to the appearance

of creeks.

  To entrain the metal powder 14, a carrier gas is used, through which the metal powder feed tubes 10 are passed, the flow velocity of this gas being preferably set at 0.5 m / s or more for better to confine the powder 14 in the plasma arc 12. As for the active gas to be made to arrive by the axial path 5, it is preferably maintained

  flow rate between 0.3 and 3 L / min, a very low

  than that of ordinary plasma melting, which is 60 L / min, in order to prevent the powder 14 from being dispersed out of the puddle 13. Moreover, to reduce the 9-flow of active gas, can advantageously limit the particle size of the powder 14 to the interval from

  1 to 200 microns or, preferably, from 1 to 100 microns

  cromètres. In addition, the arc current, which must be properly set according to the material, dimensions

  and the configuration of the article made of cast iron

  the sludge to be recasted and the area and depth to be remelt and the amount of metal powder 14 and the speed of movement of the plasma torch 1 can preferably be set within a range of 30. at 200 amps,

  by applying a voltage of 20 to 30 volts.

  In the above embodiment of the invention, the powder 14 may comprise one or more species of metal of high hardness and sulfur in the state of simple body or of sulfide of a metal with great hardness, so that the metal with high hardness is to be dispersed or dissolved regularly in the form of a sulphide in

  layer that has been reprocessed, which increases the

  see lubricant and further improves the resistance to abrasion. The amount of sulfur to be added, expressed as a percentage by weight of sulfur relative to the layer

  treated by recasting, is limited and preferably

  taken between 0.2 and 1.5%. When the percentage by weight is less than 0, 2%, the lubricating power is not very great and, on the other hand, when it exceeds 1.5%, the

  layer that has been reprocessed becomes fragile, which

  minue the resistance to the appearance of cracks.

  It should be noted in passing that one or more species of metal with great hardness can be

  the puddle 13 as a ferrous alloy or carbide.

  Figures 2A, 2B and 2C are diagrams

  analyzing the results of the micro-analysis.

  one-layer electronic probe analyzer processed by

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  remelting formed by bringing chromium and molybdenum to the metals of high hardness and adding sulfur to them. Each given chart covers a depth of 1.0 to 1.1 mm from the surface of the remelt-treated layer. As can be seen from the analysis diagrams, the recast layer has

  substantially homogeneous in S, Cr and Mo.

  The results of a number of comparative tests are described below, in each of which a comparison between a cast iron article according to the invention and a cast iron article has been compared.

on the resistance to abrasion.

  Test 1: An automobile camshaft was subjected to a cast iron cast iron (gray cast iron, grade 5) according to the Japanese industrial standard (NI.I.J), having undergone a roughing treatment,

  refounding with the plasma arc the surface of one of its parts

  camming forces, by adding chromium metal powder, under the following conditions Reworking treatment conditions: Plasma arc current 85 amps Active gas flow 0.3 l / min Flow rate of powder Cr 1.4 g / min

Movement speed of the trawl

plasma water 1 m / min.

  The lifting of the cam having a thus reductively treated and cooled layer for quenching which was 1.6 mm deep and HRC hardness (Rockwell

  scale C, N. I. J.) of 63 and had a significant

  a uniform Cr content of about 13% by weight. The finishing of the camshaft was then performed by grinding its

  cam part and labeled "test-tube A".

  On the other hand, another camshaft containing 0.9% by weight Cr was molded into its substrate and

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  cooled only the cam lift part

  laying on it a cooler. This camshaft was also subjected to a finishing by grinding of

  its cam part and it was labeled "test-tube B".

  Both test pieces A and B were tested on a real motor running at 1000 rpm and an oil temperature of 65 C for a period of 200 hours. The test results indicated a maximum depth of abrasion of 15 microns by the cam portion of the specimen A and 105 micrometers for that of the specimen B, so that the specimen A was found to be very superior as for the abrasion resistance. Test 2: A motorcycle camshaft was submitted as cast iron FCD 55 (graphite cast iron)

  spheroidal, class 3) according to the N. I. J. to a skewed

  ge, it was reprocessed by refounding the surface of one of its cam lobes to the plasma arc, while adding a molybdenum carbide powder with particle sizes ranging from 10 to 50 micrometers, in the following conditions: Redesign treatment conditions: Plasma arc current 80 amps Active gas flow rate 0.5 L / min Mo2C powder addition rate 0.3 g / min

plasma torch 1.2 m / min.

  The reconditioned layer was cooled

  thus obtained to soak it, a layer which had a

  foundry of 1.8 mm and HRC hardness of 57 and contained 1.5% by weight of molybdenum. We then finished the camshaft by grinding the cam part, and we labeled it

"specimen C".

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  On the other hand, a camshaft was molded to FCD 55 molding according to N.I. J. and cooled for quenching. This camshaft was also finished by grinding its cam portion and labeled "Test D". Both test pieces C and D were tested on a real engine under the same conditions as in test I. The results revealed a maximum depth of abrasion of 80 micrometers for the cam portion of test-tube C and 120 micrometers for that of the specimen D, so that the specimen C proved superior to the specimen D as to the

abrasion resistance.

  Test 3: A roughed FC 30 cast iron cast camshaft was subjected to a remelting treatment by rebonding the surface of one of

  his cam lift to the plasma arc, while

  50/50 weight ratio, chromium carbide powder and molybdenum powder having particle sizes ranging from 2 to 60 microns, under the following conditions: Reducting treatment conditions: Current plasma arc 80 amps Active gas flow rate 0.5 l / min Addition rate of Cr3C2 + Mo 0.3 g / min

Speed of movement of the chain

plasma light 1 m / min.

  The treated layer was cooled to soak

  recasting thus obtained on the lifting of the cam,

  which had a depth of 1.7 mm and an HRC hardness of 58 and contained approximately 0.9% by weight of

  molybdenum and approximately 0.8% by weight chromium.

  The camshaft was then finished by grinding its

  cam part and labeled "test-piece E".

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  On the other hand, another FC cast iron camshaft containing 0.3% by weight molybdenum and 0.6% by weight chromium was molded and cooled for quenching. This camshaft was also finished by grinding its cam portion and labeled "specimen F". Both test pieces E and F were tested on a

  true engine under the same conditions as in test 1.

  The test results revealed a depth of abra-

  63 micrometers by the cam portion of the test piece E and 110 micrometers for that of the specimen F, so that the test piece E was found to be

  extremely superior in abrasion resistance.

  Test 4: A roughed FC 30 cast iron automotive camshaft was subjected to a remelting treatment by re-melting the surface of one of the

  his cam lift to the plasma arc, while adding

  as a mixture in a weight ratio of 65/35 in order, a chromium carbide powder and a particle-size molybdenum powder ranging from 2 to micrometers, under the following conditions: Reducting treatment conditions: Current plasma arc 80 amps Active gas flow rate 0.5 l / min Addition rate of Cr3C2 + Mo 1 6 g / min

Speed of movement of the chain

plasma light 0.5 m / min.

  The resurfacing layer thus obtained was cooled to harden on the cam lift, which layer had a depth of 1.5 mm and an HRC hardness of 64 and contained approximately 5.6% by weight of molybdenum and 9.4 % by weight of chromium. We then finished

  the camshaft by grinding its cam part and it was

quit "specimen G".

  On the other hand, we molded another camshaft

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  FC cast iron at 0.3 wt.% Mo and 0.6 wt.% Cr and cooled for quenching. We also finished this camshaft by grinding its cam part and we

labeled it "test tube H".

  Both test tubes G and H were tested on a

  true engine under the same conditions as in Test 1.

  The test results revealed a depth of abra-

  maximum of 38 micrometers by the cam portion of the specimen D and 110 micrometers for that of the specimen H, so that the specimen G proved to be

  extremely superior in resistance to abrasion

  if we. Test 5: A rough-cast FC 3 cast iron camshaft was subjected to a remelting treatment by re-melting the surface of one of its cam lifts to the plasma arc, while adding, as a mixture in a ratio by weight of 50/50, a powder

  of chromium carbide and a molyb-

  dene at particle sizes between 2 and 10 micrometers, under the following conditions: Redesign treatment conditions: Plasma arc current 80 amps Active gas flow rate 0.5 l / min Cr3C2 + MoS2 0 addition flow rate , 8 g / mn

Movement speed of the trawl

plasma water 9.9 m / min.

  The so obtained recasting layer was cooled to soak on the cam lift, which layer had a depth of 1.6 mm and an HRC hardness of 63 and contained, approximately 3.4% by weight of Mo, 4. , 8% by weight of Cr and 0.82% by weight of S. The camshaft was then finished by grinding its cam portion.

  and labeled "test-tube I".

  On the other hand, another FC 30 iron alloy camshaft was molded containing 0.3% by weight of Mo

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  and 0.6% by weight of Cr and cooled for the tremor.

  per. This camshaft was also finished by grinding the cam portion and labeled "J specimen", having the same chemical composition as the test specimen F of Trial 3. The two specimens I and J were tested on a true engine under the same conditions as in test 1. The test results revealed a maximum depth of abrasion of 26 microns for the cam portion of the specimen I and 110 microns for that of the specimen J , so that specimen I proved to be extremely superior in abrasion resistance. As it is understood from the foregoing description, according to the present invention, a cast iron article such as a camshaft or rocker arm already molded in a given part of its surface layer is formed. a remelt-treated layer containing one or more of a high hardness metal species such as chromium and molybdenum regularly

  dispersed or dissolved within it, which increases

  the degree of hardening of only the determined part of the surface layer, keeping in the rest a hardness low enough to facilitate the associated cutting work, which makes it possible to obtain a cast iron article which not only presents in a certain part a

  superior resistance to both abrasion and

  creep, but which is also remarkable

well at cutting work.

  In addition, by adding sulfur in addition to the metal with high hardness, the resistance can be further increased.

abrasion.

  It is conceivable that the present invention can be applied to any article of cast iron to have, in one of its parts, resistance to abrasion and,

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  in its remaining part, a good machinability such as a cutting ability, such as for example, a

  camshaft or rocker arm.

  Of course, we can bring to the realization

  described various modifications and variants without

  to go out of the scope of the invention.

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Claims (15)

  1. Cast iron article comprising at least one metal species of high hardness selected from the group   including chromium, molybdenum, nickel, tungsten   medium, vanadium and niobium, dispersed or present in solid solution in the form of a single body, alloy or compound, characterized in that it has, in a determined part of its surface layer, a treated layer (11a) by recasting and by partial quenching treatment comprising a dissolving operation for incorporating by a plasma arc the high hardness metal to the substrate of the article, and a cooling operation for subsequent solidification. 2. The article of cast iron according to claim 1, characterized in that a species of said high hardness metal in the form of a single body or compound is dissolved to be mixed with said substrate and in that the proportion of said metal species to high hardness compared to the recoat layer is included between 1.0 and 15.0% by weight.   3. Cast iron article according to claim 1, characterized in that at least two species of said high hardness metal in the form of a single body or   compound are dissolved to be mixed with   trat and in that the proportion of these two or more species of high hardness metal referred to the recoat layer is between 0.7 and 15.0% by weight for each of the metals with high hardness and between 1.4 and 16.0% by weight in total.   4. Cast iron article according to claim 1, characterized in that the compound of said species of   High hardness metal is a sulphide.   5. Cast iron article according to claim 4, characterized in that the percentage by weight of sulfur present in said sulfide, rDporté said diaper - 18 -   processed by recasting, is between 0.2 and 1.5%.   6. Cast iron article according to claim 1, characterized in that the compound of said species of   High hardness metal is a carbide.   7. Cast iron article according to claim 1, characterized in that it is a camshaft for an internal combustion engine and in that the remelt-treated layer is formed on the surface of a cam of said shaft. with cams.   8. Cast iron article according to claim 1, characterized in that it is a tumbler finger for   internal combustion engine and in that the   reshaped is formed on the camming surface of the tumbler finger.   9. A method of manufacturing a cast iron article comprising at least one metal species of high hardness,   selected from the group consisting of chromium, molybdenum   nickel, tungsten, vanadium and niobium, dispersed or present in solid solution in the form of a single body, alloy, or compound, comprising a   first step of placing a deter-   mined the surface layer of the article opposite   of a plasma torch (1), a second operation con-   able to connect respectively an electrode of the trawl-   plasma and the article (11) to the negative terminal of a DC power source and to the terminal   positive from this source, a third operation con-   being able to project by the plasma torch (1) a plasma arc (12) on the determined part of the surface layer to form a molten metal puddle (13) therein while causing the metal to reach the   high hardness (14) in powder state in the plasma arc   ma (12), a fourth operation of moving the plasma torch (1) between certain limits on the determined portion (11a) of the surface layer, 25600u90 - 19 -   while continuing to project the plasma arc (12) and to make the powdery hard metal (14) arrive in the plasma arc, a fifth operation of stopping the projection of the plasma arc and the sending the powdery high hardness metal into the plasma arc, a sixth operation of cooling the molten pool and a seventh operation of remove the article.   10. Process according to claim 9, characterized   characterized in that said powdery hard metal (14) is a powder of said simple body, said alloy, a carbide or a sulfide of said metal species with great hardness.   11. The method of claim 9, characterized   rised in that the particle size of said metal to   high hardness in powder is between 1 and 200 cromètres.   12. The method of claim 9, characterized   in that the carrier gas conveying said metal to   High hardness flows at a flow rate of not less than 0.5 m / s.   13. The method of claim 9, characterized   in that an active gas is generated to generate said plasma arc at a speed of between 0.3 and 3 l / min.   14. Process according to claim 9, characterized   in that the voltage and the intensity of establishment of said plasma arc are respectively between   and 30 volts and between 30 and 200 amperes.   15. The method according to claim 9, characterized   in that said metal is made to a high hardness at a speed of between 0.3 and 1.6 g / min and in that   said plasma torch travels at a speed between 0.5 and 1.2 m / min.