GB2148166A

GB2148166A - A process for treating the surface of a metallic material

Info

Publication number: GB2148166A
Application number: GB08423101A
Authority: GB
Inventors: Toshihiko Saga; Tsuyoshi Makita; Hisao Hirono
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 1983-09-14
Filing date: 1984-09-13
Publication date: 1985-05-30
Also published as: FR2551770B1; FR2551770A1; CA1249038A; DE3433698A1; GB8423101D0; JPS6070136A; JPH0353388B2; DE3433698C2; GB2148166B

Abstract

A molten portion 11 is generated, preferably by means of a plasma arc torch, at the surface of the metallic material 1, an additive 12 which is a metallic material different in kind from the metallic material being treated, or is a non-metallic material is introduced into the molten portion and the treated material is subsequently cooled. The surface treated metallic article which may be a valve cam has improved surface wear properties. The additive may be Ni, Cr, Mo, WC,SiC, Mo2C, Cr3C2, B4C, BN, TiB, MoS2, WS2, FeS, Cr2S3, Al2O3 and SiO2. <IMAGE>

Description

SPECIFICATION A process for treating the surface of a metallic material This invention relates to a process for treating the surface of a workpiece made from a metallic material, such as a valve cam for an engine.

Conventionally, a workpiece made from a metallic material such as cast iron is formed at its surface with a treated layer. This layer is produced by generating, at the surface of the metallic material, a molten portion using a plasma arc; the molten portion is then solified by cooling. The treated layer is only a hardened layer of chilled structure obtained by deliberately increasing the rate of cooling at the time of solidification of the molten portion. This technique is defective in that, for example, in the case of application of this process to a valve cam, it is not always easy to apply the technique to give workpieces having sufficiently high wear-resisting properties.

A workpiece having, at a surface intended for sliding contact with another surface, wear-and-bite resisting properties has previously been manufactured by a powder metallurgy process in which a metallic material in powder form as a mother material and a sulphide powder are mixed together and the resultant mixture is moulded under pressure and sintered. Alternatively, such workpieces have been manufactured by a casting process in which a sulphide powder is added to a molten mete and the resultant mixture is agitated and cast. Workpieces manufactured by such processes are disadvantageous in terms of cost because the sulphide, which is comparatively expensive, must be mixed as an unnecessarily high proportion of the product.Additionally, since the sulphide is kept at a high temperature for a comparatively long time during the sintering step in the powder metallurgy process and during the time from the changing step to the solidifying step in the casting process, the particles of the sulphide tend to decompress and the remaining amount of useful sulphide becomes very small. It is, thus, very difficu It to obtain a product having the desired wear-resisting and bite-resisting properties.

According to a first aspect of the present invention there is provided a process for treating the surface of a metallic material which process comprises: generating, at a surface region of the metallic material, a molten portion; introducing, into the molten portion, an additive which is a metallic material different in kind from the metallic material being treated, or is a non-metallic material; and subsequently cooling the treated metallic material. This process allows the foregoing defects to be overcome and there is obtained a surface treated layer which has improved wear-resisting properties when compared with workpieces by conventional processes.

The metallic material may be, for example, cast iron or an alloy such as aluminium alloy.

In preferred embodiments of this aspect of the invention, the molten portion is generated by a plasma arc, and, in these embodiments, the additive may be introduced into the molten portion by means of the plasma arc, thereby forcibly mixing the additive into the molten portion.

The additive is preferably a powder and may be selected from metals such as Ni, Cr and Mo, alloys thereof, carbides such as WC, SiC, Mo2C, Cr3C2 and B4C, borides such as BN and TiB, sulphides such as MoS2, WSP, FeS and Cr2S3, and oxides such as A1203 and SiO2.

According to a second aspect of the present invention there is provided a surface-treated metallic material which has been produced by a process as described above for the first aspect of the present invention. In one embodiment of the surface treated metallic material the molten portion, after solidification, contains one or more chromium sulphide in a volume ratio of from 0.2% to 12%. In another embodiment, the molten portion, after solification, contains one or more iron sulphide in a volume ratio of from 0.5% to 20%.

The additive powder to be used is usually below about 200 micron in particle diameter and is preferably below 100 micron. When the powder particles are added to the molten layer formed at the surface of the workpiece to be treated, the particles are converted into liquid particles and are applied, in the liquid state, with the current of the plasma arc, so that the molten layer is agitated violently. At the same time the liquid particles contained therein are finely divided by the turbulence in the molten metal and the finely divided particles are solidified by the cooling action of the cold mass of the mother metallic material. Accordingly, there is obtained a surface treated layer that has finely divided solid particles dispersed throughout the solidified metallic layer.It is preferable that the dispersed particles contained in the solidified layer are in the range of from 1 - 20 micron in diameter, so that the internal stress concentration ratio is decreased and so that the product is excellent in pitching-resisting properties, bite-resisting properties, and other such properties. In addition, the sulphides, if present, give certain lubrication properties to the surface of the workpiece. Additionally, if a frictional movement is repeated during operation of the product, the particle content of the workpiece is increased at the surface, and a coating of the sulphide of up to several hundred angstroms in thickness may be formed at the surface.

According to the present invention, the remelting treatment time is usually only about 1 second or below, so that substantial loss of the additive caused by a thermal decomposition thereof is not experienced.

Thus, according to the present invention, during the time when a workpiece is being melted at its surface by a plasma arc, powders of any kind of material other than the material of the workpiece are introduced, along with the arc, into the resultant molten portion for being mixed therein, so that the powders can be dispersed in the molten portion layer. After solidification thereof there is obtained a remelted treatment layer which has improved wear-resisting properties.

Therefore, when this process is applied to, for example, a slide surface of a valve cam for an engine, a product having excellent wear-resisting properties is produced. The operation of the process is comparatively simple and, in addition, the additive powders are not subject to heat for a prolonged time and, therefore, hardly any thermal decomposition loss of the efficiently distributed particles occurs.

For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 illustrates one example of a plasma-arc torch for carrying out the process of this invention; Figures 2 to 6 illustrate respective steps of the process of the present invention; Figure 7 shows the relationship between the chromium sulphide content % by volume of a surface treated metal according to the present invention and the abrasion loss amount; Figure 8 shows the relationship between the iron sulphide content % by volume of a surface treated metal according to the present invention and the abrasion loss amount.

Referring to Figures 1 to 6, the workpiece 1, such as a valve cam, is made of a metallic material, for example, cast iron or aluminium alloy. A plasma arc torch 2 faces the workpiece 1. As shown in Figure 1, the torch 2 is provided at its centre with an electrode 3. Around the electrode 3 is a nozzle 5 defining an operation gas passage 4. Around the nozzle 5 is a shield cap 7 defining a shield gas passage 6. The nozzle 5 is provided, at its forward end portion thereof, with a plasma gas passage 8 which is in communication with the operation gas passage 4. The nozzle 5 is also provided at an interior space thereof with a cooling water passage 9. Thus, a plasma gas jet is blown against the workpiece 1 through the plasma gas passage 8, and at the same time a plasma arc 10 is generated therethrough between the workpiece 1 and the electrode 3 to be applied to the workpiece 1.A molten portion 11 is thereby formed at the surface of the workpiece 1. Thus, if the torch 2 is caused to scan along on the workpiece 1, the molten portion 11 is formed continuously and extends along the scanning line. The extended molten portion gradually solidifies, from the starting end, by cooling so that a remelted treatment layer is produced.

Powder of a material different in kind from the metallic material of the workpiece 1 is prepared as an additive 12 and is introduced, along with the plasma arc 10, into the molten portion 11 so as to be forcibly mixed in the molten portion. A mixing tube 13 made of, for example, a ceramic material is provided on the torch 2 so as to project, at its forward end, toward the plasma arc 10, and the additive agent 12 is conveyed by, for example, argon gas, through the interior of the mixing tube 13 so as to be supplied into the arc 10.

Thus, the additive agent 12 is introduced, along the arc 10, into the remelted portion 11 for being mixed therein.

In more detail, the flowing speed of the plasma jet at the plasma torch 2 is 20 m sec., the flowing speed of the shield gas at the outer periphery of the plasma jet is 0.33 m'sec., and the conveying speed of the powder is, in this instance, 7 m7sec., which is about 1.5 - 3 times the flowing speed of the shield gas. Thus, the powder overwhelms the shield gas flow and is introduced into the plasma jet.

Respective operation modes of the process of the present invention are as shown in Figures 2 to 6. Firstly, as shown in Figures 2 and 3, the plasma arc 10, is generated between the plasma torch 2 and the workpiece 1 and there is thereby created, on the surface of the workpiece 1, a molten portion 11, as shown in Figure 3.

The additive agent 12 is conveyed by, for example, argon gas, through the interior of the mixing tube 13 and, as shown in Figure 4, the additive agent 12 is introduced into the plasma arc 10On an upstream side thereof and is accelerated by the arc 10, and is introduced, along with the arc 10, into the molten portion 11 where it is violently mixed.

The plasma torch 2 is scanned in one direction as shown in Figure 5 and the molten portion 11 is continuously elongated in the scanning direction, and solidifies, in sequence, from the starting end region, by being rapidly cooled by the large cold mass of the remainder of the workpiece 1. Thus, a treated layer 1 1a is produced. During this operation, every part of the elongated molten portion 11 is agitated violently and turbulently by the arc and the additive agent 12 contained therein is dispersed substantially uniformly through the molten portion. The resultant treatment layer 1 1a obtained has the additive agent 12 substantially uniformly dispersed through the solidified layer as shown in Figure 6, and the treated layer has improved properties, such as wear-resisting properties, corresponding to the additive agent 12.

The gas flowing rate of the plasma jet at the plasma arc 10 is preferably smaller than that used for normal plasma melting and is, for example, 0.3 - 3.0 llmin., and, for this gas flowing rate, the conveying speed of the additive agent 12 is, for example, 0.5 mlsec., and the electric current and the voltage of the arc 10 are, for example, 30 - 200A and 20 - 30V respectively. The powder of the additive agent 12 is usually below 200 micron in size and is preferably below 100 micron.

When powders of the additive agent 12 are mixed in the molten portion 11, they may be dispersed in their original powder form, or they may be made molten by the heat to yield an alloy or a compound in the molten portion 11.

Example 1 The apparatus shown in Figure 1 was used, and the workpiece was made of an FC30 Ohkoshi type abrasion test piece and treated as follows: The plasma electric current was 50 A, the plasma gas flowing rate was 0.8 1 #min., and the plasma torch scanning speed was 0.5 m'min. The molten portion was formed on the whole surface of the workpiece and chromium powder was used as the additive agent. The chromium powder was in the range of 5 - 100 micron in particle diameter, and the supply rate thereof was 0.2 g/min.

The molten layer formed had a depth of 1.8 mm from the surface, and solidified by rapid cooling to become a chilled structure. The treated layer produced contained chromium powder dispersed nearly uniformly throughout the whole region of the workpiece in a content ratio of about 1.2% by volume.

The resultant product was represented by A, and a product having a simple remelted treatment layer was represented by B. An abrasion test was carried out in respect of each product and the results are set forth in Table 1.

TABLE 1 CP/O (by volume) Specific abrasion amount A 1.2 8.6 x 10-8 mm2/kg B 0 2.2 x 10-7 mm2/kg The rotor used in the abrasion test was one prepared by subjecting a raw sample of SCM 420 to a carburizing treatment and then applying a hard chrome plating of 80 micron in thickness. The abrasion test speed was 1.36 m/sec., the final load was 3.1 kg, and the abrasion distance was 200 m.

Example 2 The apparatus shown in Figure 1 was used, and the workpiece was made of an S50C Ohkoshi type abrasion test peice and treated as follows :- The whole surface of the test piece was treated under the following conditions: the plasma electric current was 100 A, the plasma gas flowing rate was 0.8 llmin., and the plasma torch scanning speed was 0.5 mimin., MO2C powder was used as the additive agent. The powder was in the range of from 2 - 30 micron in size, and the the convenying rate thereof was 0.6 glmin. The resultant molten layer was formed to a depth of 1.2 mm from the surface, and solidified to give a martensite structure.The treated layer produced contained the Mo dispersed substantially uniformly throughout the whole region thereof in a content ratio of about 3, 6% by volume.

The resultant product was represented by C, and a product having a simple remelted treatment layer was represented by D. An abrasion test was carried out in respect of each product and the results are set forth in Table 2.

TABLE 2 Mo % Iby volume) Specific abrasion amount C 5.2 7.8 x 10-7 mm2/kg D 0 8.5 x 10-6mm2/kg Example 3 The apparatus shown in Figure 1 was used, and the workpiece was made of a Ni-10% Cu alloy Ohkoshi type abrasion test piece which was treated as follows: The whole surface thereof was scanned by the plasma torch under the following conditions: the plasma electric current was 100 A, the plasma gas flowing rate was 0.8 1/min. and the plasma torch scanning speed was 0.5 mimin., TiB powder was used as the additive agent. The supply rate thereof was 0.4 g/min.The resultant layer was formed to have a depth of 1.0 mm, and solidified to give a treated layer which contained TiB powders dispersed substantially uniformly throughout the whole region thereof in a content ratio of about 2.6% by volume.

The resultant product was represented by E, and a product composed only of the remelted treatment layer was represented by F. An abrasion test was carried out to give the results set forth in Table 3.

TABLE 3 TiB% (by volume) Specific abrasion amount E 2.6 4.0 x 10#6mm2/kg F 0 7.2 x 10 6mm21kg Example 4 The apparatus shown in Figure 1 was used, and the workpiece was made of a FC30 Ohkoshi type abrasion test piece which was treated as follows: The whole surface was treated under the following conditions: the plasma electric current was 50 A, the plasma gas flowing rate was 0.8 I!min., and the plasma torch scanning speed was 0.5 m!min., FeS powder was employed as the additive agent. The powder was from 5 to 30 microns in size, and the conveying rate was 0.3 g/min. The molten layer was formed to have a depth of 1.6 mm and solidified by cooling to give a treated layer of chilled structure.The added FeS particles and the resultant (FeMn)S particles generated by the reaction between part of the FeS powders and the manganese component of the mother material were dispersed nearly uniformly throughout the whole region thereof in a content ratio of about 20% by volume.

The resultant product was represented by G, and a product having a simple remelted treatment layer was represented by H. An abrasion test was carried out in respect of each workpiece and the results are set forth in Table 4.

TABLE 4 FeS(FeMn)S % (by volume) Specific abrasion amount G 2.0 6.9 x 10-8mm2;kg H 0 2.2 x 10-7mm2rkg Example 5 The apparatus shown in Figure 1 was used, and the workpiece was made of an Al alloy AC2B Ohkoshi type abrasion test piece which was treated as follows: The whole surface was treated under the following conditions: the plasma electric current was 100 A, the plasma gas flowing rate was 0.8 1/min., and the plasma torch scanning speed was 0.8 mimin. The resultant molten layer was mixed with Al203 powder as the additive agent. The powder was 0.5 - 10 microns in size, and the conveying rate was 0.6 g/min. The treated layer solidified by cooling and was formed to a depth of 0.8 mm. The layer contained the A1203 powder in a substantially uniformly dispersed state throughout the whole region of the solidified layer in a content ratio of about 6.0% by volume.

The resultant product was represented by I, and a product having a simple remelted treatment layer was represented by J. An abrasion test was carried out in respect of the workpieces and the results are set forth in Table 5.

TABLE 5 Al203 % Fby volume) Specific abrasion amount 6.0 8.3 x 1 0-6mm21kg J 0 6.2 x 10-5mm2/kg It is clear, from each of the foregoing Examples, that the workpiece in each example is extremely improved in its abrasion resisting properties by the additive agent mixed therein.

Example 6 The apparatus shown in Figure 1 was used, and the workpiece was made of a FC30 Ohkoshi type abrasion test piece which was treated as follows. A slide surface of the workpiece was treated under the following conditions: the plasma arc electric current was 80 A, the plasma gas flow rate was 0.8 Ilmin., and the plasma torch scanning speed was 0.3 mimin. The additive was Cr2S3 powder of 2 - 10 microns in size conveyed at a rate of 1.2 gamin. by argon gas.The resultant molten layer formed had a depth of 1.2 mm from the surface of the workpiece and, after solidification, the treated layer contained dispersed particles of a mixture of chromium sulphides comprising (CrFe)2S#, (CrFeMn)2S2, (CrFe)3S4, (CrFeM n)3S4 generated by the reaction of the added Cr2S3 with Fe of the mother material and an alloying element Mn of the mother material. The chromium sulphide mixture was 7.5% in volume ratio and the particle size was about 1 - 8 microns. In this process, the molten portion was rapidly solidified, by the cold mass of the remaining portion of the mother metallic material, into a chilled structure of ledeburite deposition, in which (CrFe)2S3, (CrFe)3S4, (CrFeMn)2S3 and (CrFeMn)3S4 were dispersed therein.

The resultant product was subjected to a grinding treatment at its slide surface to obtain a test piece K. For comparison purposes, a product L having a simple remelted treatment layer of chilled structure but with no Cr2S3 powder was made. In a similar manner to Example 1, an abrasion test was carried out and the results are set forth in Table 6.

TABLE 6 Dispersed material containing ratio Specific abrasion % by volume amount K Crsulphide 7.5 8.0 x 10 9mm2/kg L - 2.2 x 10-7mm2/kg Example 7 The apparatus shown in Figure 1 was used, and the workpiece, which was made of a binary alloy of Fe and Cr, was treated as described below. A slide surface of the workpiece was subjected to the following conditions: the plasma arc current was 80 A, the plasma arc argon gas flowing rate was 1 I/min., and the plasma torch zigzag scanning speed was 0.3 m/min. The additive agent was 50 weight % of Cr3C2 powder of 2 - 10 micron, and 50 weight % of MoS2 powder of 5 - 60 micron at a supply rate of 0.1 g/min. by argon gas.

After solidification the treated layer had dispersed therein particles of chromium sulphides comprising Cr2S3 and Cr3S4 produced by the reaction between the components of the mixture powders. The depth of the treated layer was 1.4 mm from the surface. The chromium sulphides were about 0.5% by volume in content ratio and about 1 - 9 micron in particle size. The resultant product was subjected to a grinding treatment at its slide surface and used as test piece M. For comparison purposes, a product having a simple remelted treatment layer was prepared and used as test piece N. In almost the same manner as in the Example 1, an abrasion test was carried out to yield the results tabulated in Table 7.

TABLE 7 Test Dispersed Containing Specific abrasion amount Piece Material ratio % by volume M Crsulphide 0.5% 3.6 x 10-5mm2/kg N - 8.5 x 10#6mm2/kg Example 8 The apparatus shown in Figure 1 is used, and the workpiece which was a cam lift portion of a cam shaft made of FC30 for use in an engine of a motor car was subjected to the treatment as described below. The surface was treated under the following condition: the plasma are electric current was 60 A, the plasma arc Ar gas flow rate was 0.5 I/min., and the torch scanning speed was 1 mimin. Cr2S3 powder of 2 - 10 micron in size as the additive agent was supplied to the molten layer at a supply rate of 0.6 g/min.After solidification thereof, there was formed a treated layer, that is, a chilled hardened layer of 1.8 mm in thickness, containing therein various kinds of chromium sulphide such as (CrFe)2S3, (CrFeMn)2S3, (CrFe)3S4 and (CrFeMn)3S4 which were uniformly dispersed therein. The chromium sulphides were 2.2% by volume in content ratio and were 1 - 8 micron in particle size, and the hardened layer thereof was HRC 58 in hardness.

The resultant cam shaft was subjected, at its cam surface, to a grinding treatment to obtain a test piece 0 to be used for a service test, that is, an operational suitability test. A cam shaft of the same material as above was subjected to simple remelting treatment under the same conditions as above, but with no additive agent, to yield a cam shaft test member P. The hardened layer thereof was 1.9 mm in thickness and was HRC 51 in hardness.

Service test condition Engine speed 1,000 r.p.m Oil temperature 650C Testtime 200 hours As a result of the tests, the abrasion loss amount of the cam top of the test member 0 was 10 micron in depth, while the abrasion loss amount of the cam top of the test member P was 120 micron in depth.

Example 9 The apparatus shown in Figure 1 was used, and the workpiece, which was a valve rocker arm made of SCM420 for use in a motor car engine, was treated, at its sliding surface, as described below. Thus, the plasma arc current was 45 A, the plasma arc Ar gas flowing rate was 0.5 1/min., the torch scanning speed was 0.8 m/min., and the supply rate of Cr2S3 powder (2 - 10 micron in size) was 0.4 g/min. After solidification of the molten portion, the treated layer, was of a chilled hardened structure containing evenly dispersed particles of chromium sulphides comprising a mixture of (CrFe)2Ss, (CrFeMn)2S3, (CrFe)3S4 and (CrFeMn)3S4 generated by the reaction of the additive Cr2S3 with the main component, Fe, and a partial component, Mn, of the mother material.The hardened layer was 1.0 mm in thickness and the chromium sulphide content was 3.4% by volume. The product was subjected to carburizing and was then subjected to grinding to obtain a test piece Q. Additionally, the sliding surface of a valve rocker arm of the same material as above was subjected to carburizing, but not the remelting treatment, to obtain a test piece R. Aservice test was carried out in respect of these two test pieces under the conditions listed below. As a result it was found that the abrasion loss amount, in depth of the sliding surface of the test piece Q, was 3 micron, and, that of the sliding surface of the test piece R, was 50 micron.

Test conditions: Engine speed 1,000 r.p.m. Oil temperature 65 C Test operation time 200 hours Example 10 The apparatus shown in Figure 1 was used, and the workpiece, which was a stator made of a FCD55 Ohkoshi type abrasion test piece, ws treated, at its sliding surface, as described below. Thus, the plasma arc electric current was 80 A, the plasma arc Ar gas flow rate was 0.8 Ilmin. and the torch scanning speed was 0.3 mimin. The molten layer formed was supplied with FeS powder (5 - 40 micron in size) at a supply rate of 1.5 g/min. conveyed by argon gas. The molten layer was 1.2 mm in thickness.During this operation, some of the FeS powders added reacted with the main component, Fe, and an Mn component of the mother material to generate (FeMn)S and, as a result of the solidification, a hardened layer containing uniformly distributed particles of iron sulphides comprising a mixture of FeS and (FeMn) S was produced. The distributed particles were 1 to 9 microns in size, and present in an amount of 15% by volume. The resultant product was subjected to grinding treatment at its slide surface to obtain a test piece S. For comparison purposes, a workpiece of the same material as above was subjected to simple remelting treatment under the same condition as above, but without the addition of any additive agent, to obtain a test piece T. An abrasion test was carried out in the same manner as in Example 1, in respect of each of the two test pieces S and T, to obtain the following results set forth in Table 8.

TABLE 8 Test Dispersed Containing Specific abrasion amount Piece Sulphide ratio % by volume S FeS and 15% 4.5 x lO-8mm2/kg (FeMn)S T - 2.2 x 10-7mml2akg Example 11 The apparatus shown in Figure 1 was used, and the workpiece which was a stator made of a S50C abrasion test piece was treated, at its sliding surface, as described below. The plasma arc electric current was 80 A, the plasma arc argon gas flowing rate was 1 I/min., and the torch zigzag scanning speed was 0.3 mlmin. The resultant molten layer, which had a depth of 1.4 mm was supplied with MoS powders of 10 - 40 micron in size at a supply rate of 0.15 g/min. conveyed by argon gas.Consequently, a hardened layer containing uniformly dispersed particles of FeS and (FeMn)S generated by reaction of the additive MoS with the component Fe and Mn of the mother material was obtained. The resultant particles were about 1 - 7 micron in size, and represent about 0.8% by volume in content ratio.

The resultant powder was ground at its sliding surface to obtain a test piece U. For comparison purposes, a workpiece of the same material as above was subjected to a simple remelting treatment under the same condition as above, but without the addition of any additive agent, to obtain a test piece V.

The same abrasion test as above was carried out in respect of these test pieces to obtain the following results set forth in Table 9.

TABLE 9 Test Dispersed Containing Specific abrasion amount Piece Sulphide ratio % by volume U FeS and 0.8% 4.2 x 10-mm26/kg (FeMn)S V - 8.5 x 10-6mm2/kg Example 12 The apparatus shown in Figure 1 was used, and the workpiece, which was a cam lift portion of a cam shaft of FC30 for use in a motor car engine was treated, at a surface thereof, as described below. Thus, the plasma arc electric current was 60 A, the plasma arc Ar gas flowing rate was 0.51/min., and the torch scanning speed was 1 m/min. The additive agent was WS2 powder (2 - 10 micron in size) and was supplied to the molten portion at a supply rate of 0.6 g/min. conveyed by gas. The resultant molten layer was 1.8 mm in depth and was allowed to solidify.Consequently, a hardened layer containing uniformly dispersed particles of FeS and (FeMn)S generated by reaction of the additive WS2 with composition elements Fe, Mn of the mother material was formed. The dispersed particles were about 1 - 10 microns in size, and represented 2.8% by volume in content ratio. The hardened layer was HRC53 in hardness. This cam shaft was subjected to grinding treatment to obtain a test piece W. A cam shaft of the same material as above was subjected to a simple remelting treatment to obtain a test piece X. The hardened layer thereof was HRC51 in hardness. A service test was made in respect of each of these test pieces, under the following conditions: the engine speed was 1,000 r.p.m., the oil temperature was 65 C and the test operation time was 200 hours.As a result thereof, the abrasion loss amount of test piece U was 30 micron in depth, and that of the test piece V was 120 micron in depth.

Example 13 The apparatus shown in Figure 1 was used, and the workpiece, which was a valve rocker arm made of SCM420 for use in a motor car engine was treated, at a sliding surface thereof, as described below. Thus, the plasma arc current was 45 A, the plasma arc Ar gas flowing rate was 0.51/min., and the plasma torch scanning speed was 0.8 mimin. The additive agent of FeS powder was conveyed by argon gas and was supplied to the resultant remelted portion at a rate of 0.4 g/min. After solidification a hardened layer containing dispersed (FeMn)S particles, generated by the reaction of some of the added FeS powders with a component Mn of the mother material, and also containing the added FeS particles was produced.The particles of FeS and (FeMn)S were about 1 to 8 micron in size, and represented about 3.2% by volume in content ratio. The workpiece thus treated was then subjected to carburizing to further improve the hardness of the sliding surface thereof. The hardened layer was about 1.2 mm in thickness, and had a wear-resisting structure layer wherein the foregoing sulphides were mixed into the carburized layer. The product was subjected to grinding to obtain a test piece W. For comparison, a workpiece of the same material as above was subjected to a simple remelting treatment, and was then carburized as above to obtain a test piece X. In respect of each of these test pieces W and X, a service test was carried out under the following conditions: the engine speed was 1,000 r.p.m., the oil temperature was 65 C, and the test operation time was 200 hours.

As a result it was found that the abrasion loss amount of the test piece W was 10 micron in depth and the abrasion loss amount of the test piece X was 50 micron in depth.

It is clear from the foregoing Examples 1 to 13 that if various kinds of additives such as Cr, Mo, TiB, Awl203, FeS and other sulphides are mixed in the molten layer of a workpiece being treated, the resultant remelting treatment layer is improved in its wear-resisting property as compared with workpieces which have rio additives. Chromium sulphides, in particular, are advantageous in that the chromium sulphides have high stability at high temperatures and are not, therefore, decomposed, even at temperatures in excess of 1,000 C, thus resulting in very stable slide surfaces. The chromium su Iphide also serves as a lubricant.

Figure 7 shows the relationship between a content ratio of chromium sulphide and the abrasion loss amount of a product, in respect of a cam shaft made of FC30 for a motor car engine. As will be clear therefrom, abrasion and wear-resisting properties are improved with a chromium sulphide content ratio of nearly 0.2% by volume which is remarkable for the addition of such a small amount of additive. If the amount of additive is above about 12% by volume, the product tends to be lowered in its toughness, and more additive increases the expense. Therefore, from the viewpoint of economy, it is preferable to limit the amount of additive to about 12% at the maximum.

Figure 8 shows the relationship between a content ratio of iron sulphide (FeS + (FeMn)S) and the abrasion loss amount of a product, in respect of a cam shaft of FC30 for a motor car engine. In this case, the effect of the sulphide is exerted when the amount of additive is above 0.5%. However, if the amount of additive is increased to more than about 20%, no more improvement is apparent and, therefore, it is preferable, from an economic viewpoint that the amount of additive is limited to about 20% at the maximum.

Claims

1. A process for treating the surface of a metallic material which process comprises: generating, at a surface region of the metallic material, a molten portion; introducing, into the molten portion, an additive which is a metallic material different in kind from the metallic material being treated, or is a non-metallic material; and subsequently cooling the treated metallic material.

2. A process according to Claim 1, wherein the molten portion is generated by a plasma arc.

3. A process according to Claim 2, wherein the additive is introduced into the molten portion by means of the plasma arc, thereby forcibly mixing the additive into the molten portion.

4. A process according to any preceding claim, wherein the additive is a powder.

5. A process according to any preceding claim, wherein the additive is chosen from one or more of the following materials: a metal; an alloy; a carbide; a chloride; a sulphide; and an oxide.

6. A process according to Claim 5, wherein the metal is nickel, chromium or molybednum.

7. A process according to Claim 5, wherein the carbide is chosen from WC, SiC, Mo2C, Cr3C2 and B4C.

8. A process according to Claim 5, wherein the chloride is chosen from BN and TiB.

9. A process according to Claim 5, wherein the sulphide is chosen from MoS2, WS2, FeS and Cr2S3.

10. A process according to Claim 5, wherein the oxide is chosen from Awl203 and SiO2.

11. A surface treated metallic material produced by a process as described in any one of Claims 1 to 10.

12. A surface treated metallic material according to Claim 11, wherein the molten portion, after solidification, contains one or more chromium sulphide in a volume ratio of from 0.2% to 12%.

13. A surface treated metallic material according to Claim 11, wherein the molten portion, after solidification, contains one or more iron sulphide in a volume ratio of from 0.5% to 20%.

14. A process according to Claim 1, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

15. A surface treated metallic material according to Claim 11, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.