Valve for internal combustion engines and method for obtaining a valve

Abstract

A valve for an internal combustion engine may include a ferrous body having a neck. At least one area of the neck may include a surface layer with a thickness of up to 300 m. The valve may also be provided with at least 17% solubilized metallic chromium, which may be obtained by an application of a surface melting treatment.

Claims

1. A method for obtaining a ferrous valve containing chromium for internal combustion engines comprising: cutting steel bars, friction welding of a valve body with a valve tip, beating the valve body, forging of at least one area of the valve and, applying a heat treatment, applying a surface melting treatment for solubilizing metallic chromium in the at least one forged area of the valve, and wherein the surface melting treatment reaches a temperature of up to 500 C and results in the at least one forged area of the valve having a hardness of approximately 100 Hs.

2. A method according to claim 1, wherein the surface melting treatment takes place by laser remelting.

3. A method according to claim 1, wherein the surface melting treatment occurs in a thickness of up to 300 m.

4. A method according to claim 3, wherein the surface melting treatment occurs in a thickness of up to 200 m.

5. A method according to claim 4, wherein the surface melting treatment occurs in a thickness of up to 100 m.

6. A method according to claim 1, wherein the at least one forged area is at least one area of a neck of the valve.

7. A method according to claim 6, wherein the at least one area of the neck contains at least 17% chromium.

8. A method according to claim 1, wherein the at least one forged area contains at least 17% chromium.

9. A method according to claim 8, wherein the at least 17% chromium is solubilized metallic chromium.

10. A method according to claim 1, wherein the at least one forged area is provided with a dendritic structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The valve for internal combustion engines may be better understood from the following detailed description which is based on the figures listed below:

(2) FIG. 1side view of a valve with all its component parts.

(3) FIG. 2schematic drawing representing the mechanism of intergranular corrosion by chromium depletion in the grain boundary areas and carbide precipitation in the grain edge areas.

(4) FIG. 3graph representing the mechanism of intergranular corrosion by chromium depletion in the grain boundary areas and carbide precipitation in the grain edge areas.

(5) FIG. 4photograph depicting intergranular corrosion in a prior art valve.

(6) FIG. 5photograph depicting intergranular corrosion and the occurrence of fractures in a prior art valve.

(7) FIG. 6photograph in perspective of a prior art valve in which part of the head area has been lost due to a major fracture (substantially parallel to the perimeter) and secondary fractures (substantially orthogonal to the perimeter).

(8) FIG. 7photograph depicting intergranular corrosion and the removal of some grains in section AA of the valve illustrated in FIG. 6.

(9) FIG. 8photograph depicting intergranular corrosion and the removal of material in section BB of the valve illustrated in FIG. 6.

(10) FIG. 9photograph depicting the stem area of a prior art valve after hot gas corrosion.

(11) FIG. 10photograph depicting the stem area of a prior art valve after hot gas corrosion and consequent removal of material.

(12) FIG. 11photograph depicting the head area of a prior art valve after corrosion through the passage of hot gases by combustion and consequent removal of material in the seat area.

DETAILED DESCRIPTION

(13) FIG. 1 discloses a valve 1 for use in any internal combustion engine, such as for example, an exhaust valve or an intake valve. This figure illustrates a side view, where the existence of the various parts composing the valve 1 can be clearly seen, consisting of a disc-shaped head 10 comprising a seat area 2 and a neck area 3 that acts as an area of transition to the stem 4, with the tip 5 of the valve 1 being located at the opposite end of the stem 4 to the head 10.

(14) Some examples that affect the durability of valves can be seen in FIGS. 2 to 11. Thus, as pointed out earlier, one of the phenomena that most affects the durability of prior art valves arises from intergranular corrosion (IGC).

(15) FIGS. 2 and 3 illustrate intergranular corrosion, which may be described as corrosion that begins at the grain edge 6. Due to exposure to high temperature, the chromium of the alloy migrates to the grain edge 6, i.e. the formation occurs of a precipitate of chromium (Cr.sub.23C.sub.6) in the grain edge 6 area.

(16) As a result, the loss of chromium as an element of the alloy, essential to corrosion resistance, leads to the dissolution of the grain edges 6 and of the adjacent areas 7, (see FIGS. 4, 5, 7 and 8).

(17) The result of such an effect leads to the fracture of the valve as depicted in FIG. 6, which discloses a valve where part of the head area was lost due to a main fracture (substantially parallel to the perimetersee FIG. 7), as well as displaying secondary fractures (substantially orthogonal to the perimetersee FIG. 5).

(18) After chemical etching and 500 times magnification, a detailed illustration of intergranular corrosion is provided in FIGS. 7 and 8, as well as the removal of some grains in sections AA and BB of FIG. 6, respectively.

(19) The surface melting treatment by laser remelting applied to the area of the neck 3 of the valve 1 can desensitize steels and improve resistance to intergranular corrosion, through the formation of fine dendritic structures.

(20) The improvement in resistance to intergranular corrosion is attributed to the dissolution of chromium carbides (Cr.sub.23C.sub.6) and to the homogenization of chromium depleted areas. Since the surface melting treatment, e.g. by a laser remelting process occurs in a short space of time and only on the surface of the component, a sudden cooling occurs, producing a new solidification with a high temper level, sufficiently rapid to prevent the resensitization of the surface.

(21) Compressive stresses are also developed in this surface melting treatment, which leads to resistance to IGSCC (InterGranular Stress Corrosion Cracking) also being improved.

(22) The fine dendritic cell structure obtained through surface melting treatment by laser remelting causes the valve 1 to be capable of resisting intergranular corrosion and keeping its properties throughout the useful life of the internal combustion engine. On the other hand, this change in surface structure does not critically affect the mechanical properties of the valve 1, such as fatigue strength.

(23) Laser surface remelting treatment can be applied to any area of the valve, preferably, but not necessarily, applied in the area of the neck 3 which, as already discussed, is the area where corrosion preferentially occurs. As a result, a valve is obtained for internal combustion engines provided with a ferrous body comprising at least in the area of the neck 3 of the valve 1 a surface layer with a thickness of up to 300 m, with at least 17% solubilized metallic chromium.

(24) Optionally, any method may be applied that ensures sensitization of the metallic chromium on the surface of the valve so as to ensure the same technical effect described above.

(25) If the method of solubilization of metallic chromium is applied by laser remelting, the area where such a method is applied will result in a dendritic structure.

(26) The present invention thus manages to anticipate and provide the characteristics of mechanical strength initially specified when a steel is selected that is stainless in behaviour, thermal and chemical for the operating parts of the valve 1 by enabling, after the plastic transformation process to which the valve is subjected, a valve to be produced that is capable of offering the necessary resistance to corrosion, in particular by hot gases.

(27) In a first preferred configuration, such a method for obtaining a ferrous valve 1 containing chromium (17% or more) for internal combustion engines comprises the following steps: step i) cutting steel bars, friction welding of a valve body with a valve tip, process of beating (up-setting), forging of at least one area of the valve 1 and heat treatment, the method 10 being characterized in that it further comprises the following step: step ii) applying a surface melting treatment for solubilizing metallic chromium in at least one forged area of the valve 1.

(28) It should be further noted that the valve derived from the present invention will receive the treatment described above up to 300 m from the surface. In an alternative configuration, the thickness of the treatment will be up to 200 m. In another configuration, the thickness of the treatment will be up to 100 m. It should be noted that the solubilized metallic chromium with a content of 17% or more will thus be found in surface thicknesses of up to 300 m, preferably in a thickness of up to 200 m, preferably in a thickness of up to 100 m.

(29) Thus, the focus of the present invention will always be the solubilization of metallic chromium in the areas of the valve 1 that have undergone plastic transformation, so that such areas contain at least 17% chromium for offering the necessary corrosion resistance. Naturally, the valves of the present invention will have a lower unit cost than the prior art solutions capable of achieving equivalent results.

(30) This stems from the fact that the valve of the present invention is based on a cheaper raw material and a simple, fast and economic method, which at the same time improves the properties of the valve at a reduced cost.

(31) While one example of a preferred embodiment has been described, it should be understood that the scope of the present invention includes other possible variations, with the purpose of improving the characteristics of resistance to wear and to corrosion and the reduction of frictional forces, being limited only by the content of the appended claims, including the possible equivalents therein.