FR2685923A1 - THERMAL SPRAY COATING PROCESS FOR PISTON RINGS. - Google Patents

Abstract

A method of thermal spray coating for piston rings, which comprises the initial phase of anchoring a plurality of rings (1) on a base structure (2), to prevent them from moving relative to each other, so that their external contact faces form a cylindrical surface, and further comprising the phase consisting in spraying, in a progressive and continuous manner, by a forced gas stream and on the cylindrical surface of the basic structure (2), a mass of droplets which are brought to fusion by an oxyacetylene torch, from at least one molybdenum supply wire (4), and at least one supply wire (4) of a softer metallurgical bonding material, the mass of droplets consisting of approximately 30 to 60% molybdenum, the remainder being a bonding material.

Description

THERMAL SPRAY COATING PROCESS

FOR PISTON RINGS

  The present invention relates to a coating process.

  by thermal spraying for piston rings, which uses different materials, is of low cost and

  keeps the average coating properties unchanged.

  A thermal spray coating process

  for known piston rings uses supply wires

  99.9% pure molybdenum which is melted by cha-

  respective lumens of oxygen and acetylene In this

  process, molten molybdenum droplets are sprayed

  seated against the external contact face of the piston rings

  tone, said face being at a relatively low temperature

  ble by comparison with the droplet temperature

  reaching said external contact face, compressed air

  me being used as a propellant The adhesion between

  droplets and the external contact face is obtained

  canonically, by the solidification of said droplets on said rough external contact face, given the

  strong contraction tension during solidification.

  Besides the high cost of this process, which only uses

  molybdenum, the coating layer obtained turns out to be

  gile, because the welding of coating layers suc-

  cessive is done via interfaces rich in oxides This

  fragility has the effect that the coating may crack

  surer and flake when the piston ring is

  subject to mechanical stress The integrity of the coating

  molybdenum is reached by the high operating temperatures of said piston ring near the

  combustion chamber, mainly due to the diff-

  substantial reference of coefficient of thermal expansion

  that between molybdenum and the base metal of the alloy

  iron, cause said cracks during stresses.

  In addition, the spray coating process using yarns lacks flexibility in use.

  to the combination of different materials.

  Another known method uses powder spraying, which allows the association of materials, but is

  very high cost, mainly in the case of a pulp

  verification by means of a plasma arc.

  Thus, the present invention aims to propose

  ser a thermal spray coating process

  for piston rings, which produces a coating which

  resists cracking and chipping when subjected to high mechanical stress, and which is of low cost.

  Another object of the present invention is to provide

  ser a thermal spray coating process

  for piston rings, which has greater

  flexibility in the use of different combinations

  materials, which makes it possible to combine different properties

férentes.

  These objectives are achieved by a coating process.

  thermal spraying for piston rings,

  which includes the initial phase of anchoring a

  reality of piston rings on a basic structure to prevent them from moving relative to each other, so that their external contact faces form a cylindrical surface, and a complementary phase consisting in spraying, in a progressive and continuous manner, by a

  forced gas stream, and over the full extent of said sur-

  cylindrical face, a mass of droplets, which are

  fusion tees by an oxyacetylene torch, from at least one molybdenum supply wire, and from at least one supply material of a bonding material

  softer metallurgical, the mass of droplets

  both 30 and 60% molybdenum, the remainder being a bonding material, said spraying being carried out so as to completely coat the cylindrical surface with

  the mass of droplets in at least one layer.

  The present invention will now be described, rela-

  tively to the accompanying drawings, in which: FIG. 1 is a view in longitudinal section through

  tial of a piston operating in a cylinder and provided with seg-

  piston elements, the gas pressure being indicated by the arrow; and a detail on a larger scale of one of said piston rings, highlighting the coating of its external contact face; Figure 2 schematically illustrates the segments of

  piston being sprinkled by means of supply wires,

  the present invention; and Figure 3 is an enlarged cross-sectional view of part of a piston ring having

  of a coating on its external contact face.

  According to the present invention, seg-

  piston elements 1, intended to be mounted on a piston piston

  ternative P and which work, during combustion, near

  of the inner wall of a cylinder C Each segment of pis-

  tone 1 has an external contact face with a

  non-abrasive and non-stick coating, which protects

  so-called piston rings 1 when they are subjected to high pressure and temperature during the

combustion.

  In one embodiment of the present invention, the external contact face 1a of each piston segment 1 is first produced by machining said piston segment 1, so as to form a surface annular groove at the circular central portion of said segment of piston 1, the said groove being subsequently filled with the coating, until the diameter of the external contact face la

  of piston ring 1 is recovered.

  In another embodiment of the present in-

  vention, the covering layer is arranged outside-

  lying on the external contact face la, thereby increasing the

  workpiece diameter in the area where the coating is applied

  pleated, until a layer of coating is obtained

  ment of a sufficient annular width to obtain, after final machining, a coating thickness of 2 to 7% of the base annular width of the segment.

  As shown in Figure 2, a base structure

  supports and retains, along its length, using fixing means such as bolts and nuts, a plurality of piston rings 1, which are prevented from becoming

  move relative to each other, and which are available

  sés so as to form a cylindrical surface having a rotary movement around a rotation shaft, which passes through the center of the segments and which coincides with the geometric axis of the basic structure 2, and a movement of

  translation, towards said geometric axis.

  At a certain distance from said base structure 2, two gas siphons 3 are arranged, which are simultaneously supplied via their respective inputs a, b, with oxygen and acetylene, so as to form a fusion flame capable of melting the elements chemicals used in the present spraying process Each gas siphon 3 has a flame setting, which is obtained by varying the proportions of oxygen and acetylene in the flame. In addition to the gases which are distributed in the siphons 3, so as to form the flame described above, each siphon 3 receives, via a third and a fourth inlet c and d, respectively, chemical elements intended to be melted and used in the coating of said piston rings 1, and a volume of compressed air which transports,

  via an output "s" of each of said siphons 3, the drops

  telets of each molten chemical element, towards the external contact face of the cylindrical surface of the structure

basic 2.

  In a preferred embodiment of the present invention,

  vention, the coating is obtained by the simultaneous thermal spraying of two chemical elements, which are

  different but have the same concentration, and which pre-

  feel in the form of rolls of feed wires 4,

  passing respectively into a siphon 3.

  In the preferred solution described, one of the chemical elements used in the coating is molybdenum, which makes the coating anti-abrasive and non-stick, and which is used at a purity of 99.9%. The other material is

  stainless steel This element, less hard than molybdenum-

  does, allows the metallurgical bonding of molybdenum to the face

  of external contact the piston rings 1, and the cohesion

  Several layers of coating applied to the previous layers For each feed wire that is used, the oxyacetylene flame has a specific setting, obtained because an adequate proportion of oxygen and acetylene defined according to the melting point of each element

  for the composition of the supply wires.

  Stainless steel, which has a lower melting point

  high, allows a better metallurgical bond, augnen-

  both the resistance to cracks in the coating under the effect of mechanical stresses, improving its adhesion

  to the base metal of the piston ring 1, as well as the cohesion

dust particles.

  The adhesion of the coating to the base metal is mainly

  metallurgical properties In this adhesion, the tastes

  molybdenum pellets, when sprayed on the external contact face of the piston rings, reach this surface, which is at a lower temperature, then solidifying in an anchoring condition to the metal of

base previously roughened.

  According to the present invention, the coating is obtained

  using an alloy, in this case being a metall alloy

  metallurgical chemical molybdenum and stainless steel, so as to allow better adhesion of molybdenum to

  the external contact face la of the piston rings,

  thus guaranteeing its resistance to cracks and breaks

  during high operating temperatures at which

  the piston rings 1 are submitted near the chamber

combustion.

  The spraying takes place without interruption during a determined number of cycles of movements of the base structure 2, until the external contact face la of each piston segment 1 of said base structure 2 has a homogeneous coating layer and continues from

  molybdenum / stainless steel mixture with an an-

  as previously mentioned.

  In the present solution, the coating is obtained

  by simultaneous spraying of molybdenum and stainless steel-

  dable, taking into account the points that are moved longi-

  tudinally over the entire extent of the base structure 2, so as to form, on the cylindrical surface of said base structure 2, helical paths resulting from the

  rotational and translational movements of said structure

  basic re 2, relative to gas siphons 3, and therefore relative to the mass of droplets which

are melted by spraying.

  Stainless steel not only guarantees the adhesion of molybdenum to the base metal of the external contact face la of each piston ring 1, but also allows the bonding

  molybdenum droplets to each other.

  Each roll of feed wire 4 has its own screw

  distribution size for its respective siphon 3, defined according to the concentration required for mixing each

  chemical element used in the coating.

  In addition, the speed of rotation of the basic structure 2 remains proportional to the speed of rotation of the supply wire rollers 4 supplying the gas siphons 3, which is defined according to the number of layers and the annular width of each layer. coating intended to adhere metallurgically to the external contact face la of each piston ring 1 The modulus of said speed

  of rotation of the basic structure 2, relative to the mo-

  dule of the rotation speed of each wire roll

  supply 4 can be, for example, zero.

  Once spraying of the molybdenum / stainless steel alloy on the cylindrical surface is complete,

  each piston ring 1 is subjected to machining at its

  that of external contact, so as to complete his profile

of work.

Claims (4)

  1 Thermal spray coating process for   piston rings, which includes the initial phase consisting   both to anchor a plurality of secrnments (1) on a base structure (2), to prevent them from moving relative to each other, so that their faces   external contacts (la) form a cylindrical surface,   characterized in that it further comprises the phase consisting in spraying, gradually and continuously, by a   forced gas current and over the entire extent of said over-   cylindrical face of the basic structure (2), a mass of droplets which are brought to fusion by an oxyacetylene torch, from at least one supply wire   made of molybdenum (4), and from at least one supply wire   tion (4) of a more tensile metallurgical bonding material   dre, the mass of droplets consisting of about 30 to 60%   molybdenum, the remainder being a bonding material, said spraying being carried out so as to completely coat said cylindrical surface with the mass of   droplets in at least one layer, said cylindrical surface   drique being subjected, during spraying, to a movement   rotation around its geometric axis, simultaneously   with a translational movement around said geo-axis   metric, so as to produce helical displacements   dale of spray points over the entire extent of the cylindrical surface.   2 The method of claim 1, wherein the material   metallurgical bonding line is stainless steel.   3 The method of claim 1, wherein the mass of droplets consists of substantially equal parts of molybdenum and stainless steel.   4 The method of claim 3, wherein the coating   The external contact face of said piston rings (1) has a finished final thickness of 2 to 7%. of the annular width of the segment.