CA1242409A

CA1242409A - Cast iron article and method of making same

Info

Publication number: CA1242409A
Application number: CA000475028A
Authority: CA
Inventors: Tsuyoshi Makita; Toshihiko Saga; Hisao Hirono
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 1984-02-24
Filing date: 1985-02-25
Publication date: 1988-09-27
Also published as: DE3506302C2; JPH0353389B2; FR2560090A1; DE3506302C3; GB2155495A; GB2155495B; JPS60187660A; DE3506302A1; GB8504617D0; FR2560090B1

Abstract

ABSTRACT OF THE DISCLOSURE

A cast iron article having in a predetermined portion (11a) of a surface layer thereof a remelt-treated layer formed by way of a partial hardening treatment having a step adding, from among Cr, Mo, Ni, W, V, and Nb, one or more species of high-hardness metal through a plasma arc (12) into a substrate of the article (11) and a cooling step for solidification subsequent thereto. A method of making the cast iron article is also disclosed wherein the plasma arc (12) is discharged from the plasma torch (1) to the predetermined portion (11a) of the surface layer to form therein a molten pool (13), while supplying the high-hardness metal as powdered adjacent the plasma arc (12).

Description

CAST IRON ARTICLE AND METHOD OF MAKING SAME
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Bacl<ground of the Invention 1. Field of the Invention The present invention relates generally to a cast iron article and a method of making same. More particularly, the invention relates ko a cast iron article as hardened for a cam shaft and a rocker arm for internal combustion engines required to be abrasion-resistant, and to a method of making same.

2. Description of Relevant Art In the internal combustion engine, the rotation of a crankshaft and the open-close action of valves are interlocked with each other by means of a combination of a cam shaft and a rocker arm, so that the valves are timingly opened and closed to admit a fuel-air mixture and exhaust burned gases, respectively. The cam shaft and the rocker arm of an O~IC ~over-head camshaft) type engine are directly brought into slidable contact with each other at certain parts thereof~ In an OHV (over-head valve) type engine, a push rod is brought into slidable contact at a part of a tappet thereof with the cam shaft and at another part thereof with a part of the rocker arm and, therefore, the cam shaft : as well as the rocker arm is needed at ! a part of a surface layer thereof to have a higher abrasion resistance than the remaining part thereof.
According to a conventional method, in the manufacture of a cast iron article such as the cam shaft needed at a part ,, ....,~ .

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thereof to be superior of -the abrasion resistant, a metal of high hardness such as Cr (chromium) or Mo (molybdenum) is added to a molten metal when casting the article, and a chilling block is set on a part of a metal mould to form concurrently with the casting a chilled layer excellent in abrasion resistance in the part in contact with the chilling block.
To render higher the hardness of the chilled layer to ~ further raise the abrasion resistance, the quantity of the ; 10 high-hardness metal to be added to the molten metal is simply increased. However, such increase in the quantity to be added also increases, as a result of addition of the high-hardness metal, the hardness of other portions such as a journal portion of the article, thus making difficult a cutting 15 process after the casting. In a certain case, a chilled structure may be formed also in the part out of contact with the chilling block, thus rendering quite diFficult the cutting thereafter.
For such reason, in consideration of a subsequent cutting 20 process, the proportion of high-hardness metal to be added has an upper limit of 1.0 wt% when a single species oF
high-hardness metal is added or 1.4 wt% even in the case of addition of a plurality of metal species, over which limit the addition of high-hardness metal is impractical to the cutting 25 process. Consequently, in a conventional cast iron article, the abrasion-resistant layer in a part thereof is insufficiently formed.

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To overcome such shortcoming, there has been already proposed a method in which in a casting step a cast article is cast in an ordinary manner, without adding a high-hardness metal to an extent that makes cutting of the article difficult and without setting a chilling block on a part of a metal mould, and after the casting step no more than a part required to be abrasion-resistant is subjected to a hardening treatment; as disclosed, for example, by Japanese Patent Application No. SHO 53-115203 filed on September 21, 1978 based on the priority of German Patent Application No.
P-2742597.4 (filed on Sep-tember 22, 1974) and laid open as Japanese Patent Laid-Open No. SHO 54-57010 on May 8, 1979.
In the method according to the above-described prior art, an electronic beam is irradiated onto the part needing abrasion resistance, to remelt this part, before leaving same as it cools down by itself to form therein a chilled layer.
; According to such reme].t-hardening method, however, the constitution of the chilled layer as finally formed is same as that of the cast.article as the mother body thereof and, therefore, no chilled layer thus obtained is particularly excellent of the hardness nor of the abrasion resistance, when compared with a chilled layer formed by using a chilling block.
The present invention has been ach!ieved to effectively solve such conventional problems of a cast iron article and in a method of making same.
; Summary of the Invention According to the present invention, there is provided a

- 3 -., . . ~, cast iron article including, from among Cr (chromium), Mo (molybdenum), Ni (nickel), W (tungsten), V (vanadium), and Nb (niobium), at least one species of high-hardness metal dispersed or present as a solid solution in the form of a simple substance, an alloy, or a compound, wherein in a predetermined portion of a surface layer of the article is formed a remelt-treated layer by way of a partial hardening treatment having a step of dissolving to mix the high-hardness metal through a plasma arc into a substrate of the article and a cooling step for solidification subsequent thereto.
Moreover, according to the invention, there is provided a method of making a cast iron article including, from among Cr (chromium), Mo (molybdenum), Ni (nickel), W (tungsten), V
(vanadium), and Nb (niobium), at least one species of high-hardness metal dispersed or present as a solid solution in the form of a simple substance, an alloy, or a compound, comprising a first step of setting a predetermined portion of a surface layer of the article opposite -to a plasma torch, a second step of connecting an electrode of the plasma torch and the article to a negative terminal of a direct current power source and a positive terminal thereof, respectively, a third step of discharging a plasma arc from the plasma torch to the predetermined portion of the surface layer to form therein a molten pool, while supplying the high-hardness metal as powder in the plasma arc, a fourth step of having the plasma torch travel within a scope of the predetermined 3 r portion of the surface layer, while continuing discharging the plasma arc as well as supplying the powdered high-hardness metal in the plasma arc, a fi-fth step of stopping discharging the plasma arc as well as supplying the powdered high-hardness metal in the plasma arc, a sixth step of cooling the molten pool, and a seventh step of taking out the article.
Accordingly, an object of the present invention is to provide a cast iron article, which has at a predetermined portion thereof a remelt-treated layer markedly excellent of the abrasion resis-tance in comparison with a chilled layer formed using a chilling block, and a method of making such cast iron article.
The above and further objects, details and advantages of the present invention will become apparent from the Following detailed description of a preferred embodiment of the inven-tion when read in conjunction with the accompanying drawings.
Brief Description of the Drawings Figure l is a side view, partly in section, of an essential part of a plasma torch as applied to the manufac-ture of a cast iron article according to the invention.
Figures 2A, 2B, and 2C are EPMA (electron probe microanalyzer) analysis charts 7 showing the distribution of essential elements in a remelt-treated layer of the cast iron article.
Detailed Description of the Preferred Embodiment Referring first to Figure l as a partial sectional view showing the inner structure of a plasma torch 1 adapted for the manuFacture of a cast iron article 11 according to the present in~ention, the plasma torch 1 has, inside of a hollowed shield cap 2, a single nozzle 3 made of copper, defining between the cap 2 and the nozzle 3 an axial path 4 for admitting a shield gas such as of an inactive gas. At the center of the nozzle 3 is formed another axial path 5 for admitting a working gas such as of an argon gas to be changed into a plasma gas, and about this path 5, an end-closed path 6 for circulating a coolant. A tungsten electrode 7 is axially provided in the path 5 of the working gas, which path 5 is reduced at the lower end thereof to define an orifice 8 as a plasma jet hole for discharging the plasma gas.
Moreover, -the shield cap 2 has a plurality of tubular guides 9 obliquely provided therethrough to be arranged around the nozzle 3 at an equi-angular pitch, the guides 9 each respectively supporting one of a plurality of metal powder feed tubes 10 inserted therein to be so set that an extension of the axis of each tube lû passes a poin-t on an extension of the axis of the orifice 8.
There will be described below a method for manufacturing the cast iron article 11 according to the present invention, which employs the above-described plasma torch 1.
The cast iron article 11 is now assumed to be in a state as-it has been cast in an ordinary manner, without adding a high-hardness metal to an extent that might interfere with a cutting process and without using a chilling block, and ~2~
cut through the cutting process.
Firstly, as shown in Figure 1, the plasma torch 1 is set opposite to a predetermined portion lla, that is, a region required to be resistant to both abrasion and pitching, of a sur-Face layer of the cast iron article 11 as finished oF the cutting.
Then, the tungsten electrode 7 is connected to a nega-tive terminal (not shown) of a direct current power source (not shown), and the cast iron article 11, to a positve ter-minal (not shown) of the power source. The shield gas is admitted through the axial path 4, and the working gas, for example an argon gas, through the axial path 5. As a result, the electrode 7 discharges, driving the working gas of the path 5 into a plasma state to create the plasma gas, which has a flow path area thereof reduced at the oriFice 8 and rushes out therefrom in the Form of a plasma arc 12, that is a plasma jet of a high temperature and high speed, to be discharged from the nozzle 3. The discharged plasma arc 12 is directed to the predetermined portion lla of the surface layer of the cast iron article 11 which has a positive poten-tial with respect to the tungsten electrode 7, and with the arc heat there develops a molten pool 13 in the portion lla.
: Concurrently with these operations, in the plasma arc 12, a powder 14 consisting of one or more species of high-; 25 hardness metal is fed through the metal powder feed tubes 10.
The high-hardness metal is properly selected from among Cr (chromium), Mo (molybdenum), Ni (nickel), W (tungsten), V

,, (vanadium) 9 Nb (niobium), and the like, and may be suitably given in the form of a simple substance of any element from thereamong, an alloy of two or more from thereamong or of one or more from thereamong with another metal or other metals, and/or a compound of one or more from thereamong with C
(carbon) and/or the like. The quantity of the powder 14 to be fed in the plasma arc 12 is limited, in terms of a weight proportion of high-hardness me-tal to the molten pool 14, to be preferably within a range of l.û to 15.0 wt%, when the number of species of high-hardness metal to be fed is one, or within a range of 0.7 to 15.0 wt% for each species and a range of 1.4 to 16.0 wt% in total, when it is more than one.
In this respect, at quantities of the powder 14 insufficient to reach the lower limit of corresponding one of the foregoing weight proportion ranges, the portion lla of the cast iron article 11 as remelt-treated to be partially hardened may be ineffectively different in the abrasion resistance from what would otherwise be given through a conventional chilling process, thus failing to be sufficient in the hardness. To the contrary, in the case where the upper limit is exceeded, the hardness may become too large to avoid undesired brittleness, resulting in a lowered pitching strength, and cracks may be likely to d!evelop when cooling after remelting by plasma as well as when grinding.
In the above-described remelt treatment, the metal powder 14 as fed in the plasma arc 12 is forcibly confined in the arc 12, whereby it is accelerated and heated, and thrown at high speeds and high temperatures onto the surface of the molten pool 13, to be mixed in the pool 13, while the molten pool 13 has a surface area thereof recessed with the pressure of the plasma arc 12 exerted thereon, which recessed area is caused to ripple and run about along with movements of the plasma torch 1, so that the molten pool 13 is effectively stirred. Accordingly, the powder 14 of high-hardness metal is mixed as in the molten pool 13 is evenly distributed therein by the stirring effect. As a result, where the metal powder 14 has a sufficiently low melting point or is dissolvable to the molten pool 13, it becomes evenly mixed with a substrate of the pool 13, forming an alloy and/or educing a compound.
Where the powder 14 is refractory to the pool 13, it becomes evenly dispersed therein, without changing the chemical composition.
By cooling the molten pool 13, the cast iron article 11 has in the surface portion lla a remelt-treated layer containing a homogenized alloy with one or more high-hardness metals and/or abrasion-resistant particles evenly dispersed, 2û which remelt-treated layer is thus excellently resistive to both abrasion and pitching.
There will be described below a number of essential operational conditions to form a remelt-treated layer high in the abrasion resistance and anti-pitching quality, according to the present invention.
For carrying the metal powder 14 there is employed a powder carrier gas to be let through the metal powder feed tubes 10, the flow speed of which gas may preferably be set at 0.5 m/sec or more to firmly confine the powder 14 in the plasma arc 12. With respect to the working gas to be admitted through the axial path 5, the flow rate may pre-ferably be limited within a range of 0.3 to 3 lit/min to be greatly reduced from a range of an ordinary plasma-melting, that is 30 to 60 lit/min, in order to prevent the powder 14 from being scattered out of the molten pool 13. Moreover, to reduce the working gas flow, the particle size of the powder 14 may favorably be limited within a range of 1 to 200 micrometers or, preferably, within a range of 1 to 100 micro-meters. Further, the arc current, which is required to be properly set in accordance with the material, dimensions, and configuration of the cast iron article 11 as the substrate to be remelted and the area and depth to be remelted as well as the quantity of the metal powder 14 and the travel speed of the plasma torch 1, may preferably be regulated substantially within a range of 30 to 200 amperes, by applying a voltage within a range of 0 to 30 volts.
In the foregoing embodiment of the invention, the powder 14 may comprise one or more species of high-hardness metal and a simple substance of S (sulfur) or a sulfide of high-hardness metal, whereby the high-hardness metal is to be evenly dispersed or dissolved in the form of a sulfide in a remelt-treated layer, thus increasing the lubricating ability 9 thereby further improving the abrasion resistance.
The quantity of sulfur to be added is limited, in terms of a ?~

weight proportion of sulfur to the remelt-treated layer, to be preferably within a range of 0.2 to 1.5 wt%. ~here the weight proportion is less than 0.2 wt%, the lubricating ability does not become remarkably high and, on the contrary, where it is more than 1.5 wt%, the remelt-treated layer becomes brittle, thus lowering the pitching strength.
Incidentally, one or more species of high-hardness metal may be fed to the molten pool 13, in the form of a ferrous alloy or a carbide.
Figures 2A, 2B, and 2C are analysis charts plot-ting the results of an EPMA (electron probe microanalyzer) analysis of a remelt-treated layer formed by feeding respective simple substances of Cr (chrome) and Mo (molybdenum) as high-hardness metal and adding thereto S (sulfur). Each given chart covers a depth region of l.û to 1.1 mm from the surface of the remelt-treated layer. As can be seen from the analysis charts, the remelt-treated layer is substantially homogeneous with respect to S, Cr, and Mo.
There will be described below the results of a number of comparison experiments, in each of which a cast iron article according to the invention was compared with a conventional cast iron article with respect to the abrasion resistance.
Experiment I
A cam shaft for automobiles as an iron casting to FC 30 (grey iron casting, Grade 5) of the JIS (Japanese Industrial Standards), subjected to a rough cutting process, was remelt-treated by remelting the surface of a cam lift portion thereof with a plasma arc, while adding thereto a metal powder of Cr, under the following conditions.
Remelt treatment conditions:
Plasma arc current 85 amperes Working gas ~low 0.3 lit/min Added Cr powder quantity 1.4 g/min Plasma torch travel speed 1 m/min The cam lift portion had thereon a layer thus remelt-treated and chilled to be hardened, which layer was 1.6 mm in the depth and HRC (Rockwell C-scale, JIS) 63 in the hardness and substantially homogeneously contained approxima-tely 13 wt% of Cr. The cam shaft was then finished by grinding a cam part thereof, and labelled as test piece A.
On the other hand, another cam shaft containing 0.9 wt%
of Cr in the substrate thereof was cast and had no more than a cam lift portion thereof chilled by applying thereto a chilling block. Qlso this cam shaft was then finished by grinding a cam part thereof, and labelled as test piece B.
Both test pieces A, B were tested on an actual machine at an engine speed of 1,000 rpm and an oil temperature of 65 C, for a lasting period of 200 hours. The test results showed a maximum abrasion depth of 25 micrometers for the cam part of the test piece A, and 105 micro!meters for that of the test piece B, whereby the test piece A was proven to be remarkably superior in the abrasion resistance.
Experiment II
A cam shaft for motorcycles as an iron casting to FCD 55 (speroidal graphite iron casting, Class 3) of the JIS, sub-jected to a rough cutting process, was remelt~treated by remelting -the surface of a cam lift portion thereof with a plasma arc, while adding thereto a metal powder of Mo2C of particle sizes within a range of 10 to 50 mircometers, under the following conditions.
Remelt treatment conditions:
Plasma arc current 80 amperes Working gas flow 0.5 lit/min Added Mo2C powder quantity 0.3 g/min Plasma torch travel speed 1.2 m/min A remelt-treated layer thus obtained was chilled to be hardened, which layer was 1.8 mm in the depth and HRC 57 in the hardness and contained 1.5 wt% of Mo. The cam shaft was then finished by grinding a cam part thereof, and labelled as test piece C.
On the other hand, another cam shaft was cast to be an iron casting to FCD 55 of the JIS and chilled to be hardened.
Also this cam shaft was finished by grinding a cam part thereof, and labelled as test piece D.
Both test pieces C and D were tested on an actual machine under conditions similar to the experiment I. The test results showed a maximum abrasion depth of 80 micro-meters for the cam part of the test piece C, and 120 micro-meters for that of the test piece D, whereby the test piece C
was proven to be superior to the test piece D in the abrasion resistance.

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Experiment III
A cam shaFt for automobiles as an iron casting to FC 30, subjected to a rough cutting process, was remelt-treated by remelting the surface of a cam lift portion thereof with a plasma arc, while adding thereto, by mixing at a proportion of 50% to 50% in the weight ratio, a Cr3C2 powder and a Mo powder of particle sizes within a range of 2 to 60 micro-meters, under the following conditions.
Remelt treatment conditions:
Plasma arc current 80 amperes Working gas flow 0.5 lit/min Added Cr3C2 -~ Mo quantity 0.3 g/min Plasma torch travel speed 1 m/min A remelt-treated layer thus obtained on the cam lift portion was chilled to be hardened, which layer was 1.7 mm in the depth and HRC 58 in the hardness and contained approximately 0.9 wt% of Mo and approximately 0.8 wt% of Cr.
The cam shaft was then finished by grinding a cam part khereof, and labelled as test piece E.
On the other hand, another cam shaft was cast to be an iron alloy casting of an FC 30 material containing 0.3 wt% of Mo and 0.6 wt% of Cr and chilled to be hardened. Also this cam shaft was finished by grinding a cam part thereof, and labelled as test piece F.
Both test pieces E and F were tested on an actual machine under conditions similar to the experiment I. The test results showed a maximum abrasion depth of 63 micro-, :

meters for the cam part of the test piece E, and 110 micrometers for that of the test piece F, whereby the test piece E was proven to be extremely superior in the abrasion resistance.
Experiment IV
A cam shaft for automobiles as an iron casting to FC 30, subjected to a rough cutting process, was remelt-treated by remelting the surface of a cam lift portion -thereof with a plasma arc, while adding thereto, by mixing at a proportion of 65% to 35% in the weight ratio in this order, a Cr3C2 powder and a Mo powder of particle sizes within a range of 2 to 60 micrometers, under the following conditions.
Remelt treatment conditions:
Plasma arc current 80 amperes Working gas flow 0.5 lit/min Added Cr3C2 ~ Mo quantity 1.6 g/min Plasma torch travel speed 0.5 m/min A remelt-treated layer thus obtained on the cam lift portion was chilled to be hardened, which layer was 1.5 mm in the depth and HRC 64 in the hardness and contained approximately 5.6 wt% of Mo and approximately 9.4 wt% of Cr.
The cam shaft was then finished by grinding a cam part thereof, and labelled as test piece G.
ûn the other hand, another cam shaft was cast to be an iron alloy casting of an FC 30 material containing 0.3 wt% of Mo and 0.6 wt% of Cr and chilled to be hardened. Also this cam shaft was finished by grinding a cam part thereof, and labelled as -test piece H.
Both test pieces G and H were tested on an actual machine under conditions similar to the experiment I. The test results showed a maximum abrasion depth of 38 micro-meters for the cam part of the test piece G, and 110 micro-meters for that of the test piece H, whereby the test piece G
was proven to be extremely superior in the abrasion resistance.
Experiment V
A cam shaft for automobiles as an iron casting to FC 30, subjected to a rough cutting process, was remelt-treated by remelting the surface of a cam lift portion thereof with a plasma arc, while adding thereto, by mixing at a proportion of 50% to 50% in the weight ratio, a Cr3C2 powder and a MoS2 powder of particle sizes within a range of 1 to 10 micrometers, under the Following conditions.
Remelt treatment conditions:
Plasma arc current 80 amperes Working gas flow 0.5 lit/min Added Cr3C2 + MoS2 quantity 0.8 g/min Plasma torch travel speed 0.9 m/min A remelt-treated layer thus obtained on the cam lift portion was chilled to be hardened, which layer was 1.6 mm in the depth and HRC 63 in the hardness and contained, approxi-mately, 3~4 wt% of Mo, 4.8 wt% of Cr, and 0.82 wt% of S. The cam shaFt was then finished by grinding a cam part thereof, and labelled as test piece I.

On the other hand, another cam shaft was cast to be an iron alloy casting of an FC 30 material containing 0.3 wt% of Mo and 0.6 wt% of Cr and chilled to be hardened. Also this cam shaft was finished by grinding a cam part thereof, and labelled as test piece J, of which the chemical composition is same as that of the test piece F of the experiment III.
Both test pieces I and J were tested on an actual machine under conditions similar to the experiment I. The test results showed a maximum abrasion depth of 26 micrometers for the cam part of the test piece I, and 110 micrometers for that of the test piece J, whereby the test piece I was proven to be extremely superior in the abrasion resistance.
As will be concretely understood from the foregoing description, according to the present invention, a cast iron article such as a cam shaft or a rocker arm, as it is once cast, has newly formed in a predetermined portion of a surface layer thereof a remelt-treated layer containing one or more species of high-hardness metal such as Cr and Mo evenly dispersed or dissolved therein, thus permitting no more than the predetermined portion of the surface layer to be hardened to a high degree, while leaving the rest sufficiently low of the hardness to faclilitate associated cutting work, and therefore there can be achieved a cast iron article not only superior at a prticular portion thereof in both abrasion resistance and anti-pitching quality, but also excellent as itself in the cutting workability.

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Moreover, by adding 5 besides such high-hardness metal, the abrasion resistance can be still more raised.
It will be easily understood that the present invention may be applied -to any cast iron article needed, at a part thereof, to be resistant to the abrasion and, in the remaining part thereof, to have a good workability such as to a cutting process, including a cam shaft and a rocker arm as a matter of course.
Although there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cast iron article having as at least part of the surface thereof an integral hardened surface layer containing one or more hardening elements each at a concentration of 0.7 to 15% by weight, the total concentration of said hardening elements in said surface layer being from 1 to 16% by weight, said hardening elements being chosen from chromium, molybdenum, nickel, tungsten, vanadium and niobium and being incorporated in said surface layer as a dispersion or solid solution of the hardening element or an alloy or compound thereof, the concentration on a percentage by weight basis of said at least one hardening element in said article being higher in said hardened surface layer than beneath said hardened surface layer, and said hardened surfaced layer being formed by melting a surface of said article and introducing said at least one hardening element into the melted layer.

2. An article as claimed in claim 1 wherein at least one said hardening element is included in said integral hardened surface layer as a sulfide.

3. An article as claimed in claim 2 wherein the proportion of sulfur in said sulfide to said integral hardened surface layer is from 0.2 to 1.5 wt%.

4. An article as claimed in claim 1, 2 or 3 wherein said at least one hardening element is introduced into the melted layer formed by means of a plasma arc.

5. An article as claimed in claim 1 wherein at least one said hardening element is included in said integral surface layer as a carbide.

6. An article as claimed in claim 1, 3 or 5 in the form of a cam shaft for an internal combustion engine, and wherein said integral hardened surface layer is formed on a cam surface of said cam shaft.

7. An article as claimed in claim 1, 3 or 5 in the form of a rocker arm for an internal combustion engine, and wherein said integral hardened surface layer is formed on a cam-follower surface of said rocker arm.

8. A method of producing a cast iron article having as at least part of the surface thereof an integral hardened surface layer containing at least one hardening element chosen from chromium, molybdenum, tungsten, vanadium and niobium incorporated therein as a dispersion or solid solution of the hardening element or an alloy or compound thereof, said method compising melting at least part of the surface layer of a cast iron article, introducing into the melted layer through a plasma arc at least one said hardening element in the form of the element or an alloy or compound thereof and resolidifying said melted layer thereby to produce a hardened surface layer wherein the concentration on a percentage by weight basis of said at least one hardening element is higher than in the part of the article beneath said hardened surface layer, said at least one hardening element being introduced in sufficient quantity that said hardened surface layer contains said hardening elements at individual concentrations of 0.7 to 15% by weight and at a total concentration of 1 to 16% by weight.

9. A method as claimed in claim 8 comprising the steps of:
(a) setting a predetermined portion of a surface layer of a cast iron article opposite to a plasma torch;
(b) connecting said article and an electrode of said plasma torch to a positive terminal and a negative terminal respectively of a direct current power source;
(c) discharging a plasma arc from said plasma torch to said predetermined portion of said surface layer to form therein a molten pool while supplying said at least one hardening element in powder form in said plasma arc;
(d) causing said plasma torch to travel over said predetermined portion of said surface layer, while continuing the discharge of said plasma arc and the supply of said hardening element to said plasma arc;
(e) stopping the discharge of said plasma arc and the supply of said hardening element to said plasma arc; and (f) cooling said molten pool.

10. A method as claimed in claim 9 wherein the working gas for said plasma arc is supplied at a rate of from 0.3 to 3 1/min.

11. A method as claimed in claim 9 or 10 wherein the voltage and the amperage for creating said plasma arc are from 20 to 30 volts and from 30 to 200 amperes respectively.

12. A method as claimed in claim 9 or 10 wherein said at least one hardening element in powder form is supplied at a rate of from 0.3 to 1.6 g/min., and wherein said plasma torch has a travel speed of from 0.5 to 1.2 m/min.

13. A method as claimed in claim 9 or 10 wherein said at least one hardening element is supplied in the form of a powder of the element, of an alloy, of a carbide or of a sulfide.

14. A method as claimed in claim 9 or 10 wherein said at least one hardening element is supplied in the form of a powder having a particle size of from 1 to 200 micrometers.

15. A method as claimed in claim 9 or 10 wherein said at least one hardening element is supplied in a carrier gas having a flow rate of at least 0.5 m/sec.