GB2183255A - Local remelting and resolidification - Google Patents

Abstract

It is known that a hardened chilled layer with a fine metallic composition having excellent abrasion resistance can be obtained if a sliding surface layer of an Fe- containing alloy component, such as a cam shaft used in a valve mechanism of an internal combustion engine, is rapidly melted by irradiating the surface layer with high- concentration energy such as a plasma jet, laser beam, etc. and is thereafter hardened by self-cooling. The abrasion resistance may be greatly enhanced if this remelting and hardening treatment is applied to a hardened chilled layer formed during casting the component. Furthermore, if a carbide-stabilizing agent such as Cr or Mo is added to the molten pool produced on the surface during the remelting and hardening treatment, a double carbide is produced in the hardened chilled layer and a high hardness can be obtained. For the sliding surface of a shaft member which is not used in as severe conditions as a cam of a cam shaft, even if the remelting and hardening treatment is performed in a spiral shape, the required abrasion resistance can be obtained, resulting in high productivity and good economy. <IMAGE>

Description

SPECIFICATION Methods for enhancing the abrasion resistance of alloy components This invention relates to a method of enhancing the abrasion resistance of a chill-cast Fecontaining alloy component such as a camshaft, and to components produced by that method.

In the present description, an "Fe-containing alloy" refers to cast iron, cast steel, and steel alloys.

In a cam shaft used for a valve mechanism of an internal combustion engine, even a minor change in the curvature of a cam surface can have substantial adverse effect on engine performance by changing the opening and closing times of corresponding valves or changing the degree of valve opening. For this reason, it is common to employ cast iron cam shafts made of JIS FC25 through FC30 materials, alloy cast iron material or the like, which are highly resistant to wear and bending, even if used for a long time. It is also known to form a chilled layer on a cam surface portion by using a chill when casting, or to form a hardened chilled layer by performing remelting and hardening treatment (self-cooling after rapid melting) on a cam surface portion after casting.

In recent years, the size and power of internal combustion engines for automobiles has been increasing, and the sliding surface pressure on cam surfaces has increased accordingly. As a result, it is necessary for a cam surface to have high abrasion resistance. However, sufficient abrasion resistance is difficult to obtain even if a chilled layer is formed on a cam surface portion when casting. On the other hand, when a chilled layer is formed by remelting and hardening after casting, although a comparatively high abrasion resistance is obtained compared with when a chilled layer is formed when casting, such a chilled layer is not completely satisfactory for the following reasons.

1. During remelting and hardening, pin holes and craters sometimes develop due to the cam material or the casting composition, i.e., a composition in which coarse graphite is crystallized, which results in an unsatisfactory cam shaft after finishing and a poor yield.

2. When remelting and hardening treatment is performed over all of a cam surface portion, the shoulder portions at both ends thereof are removed, and it is necessary to compensate for this by reforming the ends of the cam surface by machining. However, machining a hard chilled layer is difficult and makes mass production impossible. In addition, the effective width of the cam varies from cam to cam. If one avoids the removal of the shoulder portion at on both ends, untreated portions remain at both ends, which cause the cam to improperly abut against a rocker arm when the cam is installed in an engine. Accordingly, when one side of the cam abuts the rocker arm due to positional displacement, the pressure on the cam locally increases, producing pitting, peeling, and the like, and decreasing the durability of the cam and rocker arm.

3. The remelting and hardening treatment is performed by irradiating the component with a highly concetrated form of energy such as a plasma jet, a laser beam, or the like. Uniform treatment is usually performed on the entire surface of the essential portion of the component.

Thus, this treatment is time-consuming, and often results in poor productivity. Therefore, such a remelting and hardening method is economically disadvantageous and it is also disadvantageous to employ the method to treat portions such as a journal portion of a cam shaft which are not subjected to such severe conditions.

According to one aspect of the invention there is provided a method of enhancing the abrasion resistance of a chill-cast Fe-containing alloy component comprising remelting the surface by application thereto of high-concentration energy and then resolidifying the surface. Preferably, the high-concentration energy is supplied by a laser beam or by a plasma torch.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a sectional view of the main portion of a casting mould for moulding a cam shaft acccording to one embodiment of the present invention; Figure 2 is a perspective view showing the main portion of a cam shaft having a cast chilled layer obtained using the casting mould of Fig. 1; Figure 3 is a perspective view illustrating remelting and hardening treatment using a plasma torch on a cast cam shaft; Figure 4 is a perspective view showing the main portion of a cam shaft subjected to the remelting and hardening treatment; Figure 5 and Figure 6 are sectional views taken along lines V-V and VI-VI, respectively, of Fig. 4;; Figure 7 is a sectional view showing the main portion of an overhead cam type valve mechanism of an internal combustion engine, the cam shaft of which was subjected to remelting and hardening treatment; Figure 8 is a photograph showing the micrometallic composition of a cam lift portion corresponding to that shown in Fig. 6; Figure 9 is a photograph (enlarged by 100 times) showing the metal composition of a hardened chilled layer which is the cam surface layer shown by arrow S in Fig. 8; Figure 10 is an enlarged photograph (100x) of the main portion of Fig. 8, showing the metal composition of a layer located below the hardened chilled layer where the remelting and hardening treatment was not applied; Figure 11 is a sectional view showing the main portion of a plasma torch for carrying out remelting and hardening treatment;; Figure 12 is a graph showing the results of an abrasion test performed on cam shaft I having a simple cast chilled layer and cam shaft II having a chilled layer hardened by remelting and hardening treatment; Figure 13 is a perspective view showing a shaft member according to another embodiment of the present invention, the surface of which was subjected to remelting and hardening treatment in a spiral shape; Figure 14 is a sectional view taken along line XIV-XIV of Fig. 13; Figure 15 is a partial cross-section of a portion of a cam shaft for an internal combustion engine according to a further embodiment of the present invention, the surface of which was subjected to remelting and hardening treatment in a spiral shape; Figure 16 is a schematic cross-sectional view of a test piece cut from the cam shaft of Fig.

15; Figure 17 is a photograph showing a micrometallic composition of a cross section of a test piece cut from a cam shaft along its axis; Figure 18 is an enlarged photograph (100x) showing the micrometallic composition of a journal portion surface layer of a test piece which was subjected to the remelting and hardening treatment; Figure 19 is an enlarged photograph (400x) showing the micrometallic composition of the hardened chilled layer of the journal portion surface layer of Fig. 18; Figure 20 is likewise an enlarged photograph (100x) showing the micrometallic composition of an oil seal portion surface layer of the test piece of Fig. 17 which subjected to remelting and hardening treatment; Figure 21 is an enlarged photograph (100x) of the micrometallic composition shown in Fig.

20; Figure 22 is a graph showing the abrasion loss and galling loss of the journal portion of the cam shaft of Fig. 15 in a durability test using an actual engine; Figure 23 is a graph showing the abrasion loss and galling loss of the journal portion of a cam shaft which was not subjected to remelting and hardening treatment in a durability test using an actual engine; Figure 24 is a graph of the relationship between the amount of added Mo and the hardness (HRC) of a cam sliding surface of a cast cam shaft which was not chilled during casting, and which was subjected to remelting and hardening treatment and which a carbide-stabilizating element was added;; Figure 25 is a graph as a result of the relationship between the amount of added Mo and the balance (HRC) of the cam sliding surface which was not chilled during casting, which was subjected to treatment performed during remelting and hardening treatment and to which was added a carbide stabilizing element; Figure 26 is a graph of the relationship between the amount of Mo and Cr which were added when remelting and hardening treatment was performed on a cam sliding surface which was not chilled during casting of a cast cam shaft and the abrasion loss of the cam sliding surface in a test using an actual engine;; Figure 27 is a graph showing the relationship between the depth below the sliding surface of a rocker arm cam follower and the hardness thereof, the rocker arm slipper being used together with a cam shaft in the test using an actual engine which was described with respect to in Fig.

26; Figure 28 is a graph of the relationship between the chrome content of a rocker arm cam follower and the sliding abrasion loss thereof, the rocker arm cam follower being used together with a cam shaft in a test using an actual engine; and Figure 29 is a graph showing operating time and sliding surface abrasion loss of cams and slippers in a test which three kinds of cam shafts and rocker arms were incorporated into an internal combustion engine.

If a slidable member in the form of a cam surface of a cast cam shaft is formed with a chilled layer by using a chill made of, for example, copper, a fine composition with deposited cementite (Fe3C) can be obtained, and satisfactory abrasion resistance and galling resistance are obtained when, in use, the cam surface is in sliding contact with a rocker arm or a valve lifter. By further increasing the fineness of the composition of this chilled layer, it is possible to increase the abrasion resistance. More specificly, the cam suface layer may be self-cooled after is has been rapidly remelted by being irradiated with high concentråtion energy such as a plasma jet, laser beam, etc.Since the composition before processing is already a stable cementite deposition composition, there can be obtained a high-quality remelted and hardened chilled layer in which there is almost no occurrence of pin holes, carbide deposition, etc. during remelting and hardening treatment. Furthermore, by carrying out the remelting and hardening treatment in such a manner as to leave both end portions in the widthwise direction untreated, the shoulder portions of both end portions can be prevented from becoming slender. In addition, since the untreated end portions are chilled layers having satisfactory abrasion resistance, cam shafts having a constant effective width and good durability can be obtained.Furthermore, in a remelting and hardening method in which the treatment is performed only on the essential portions and not on both widthwise ends, working time can be shortened and manufacturing costs can be reduced.

The method in accordance with the present invention can be applied not only to cam shafts but also to valve lifters, rocker arms, and other Fe-containing alloy components.

When the remelting is performed, it is preferred to add at least one kind of carbide stabilizing agent selected from the group consisting of Cr, Mo, V and Nb in an amount of preferably 0.5 to 4% by weight to the molten surface to produce an alloy and to produce a fine double carbide having high hardness, thereby giving the cam surface extremely good abrasion resistance.

Fig. 1 is a sectional view of the essential portion of a metal mould 10 for casting a cam shaft for an internal combustion engine. The metal mould 10 comprises an upper mould 12 and a lower mould 14. A chill 20 which is made of copper for example, and which corresponds to a cam lift portion is fitted inside the cam moulding hollow 18 of the moulding hollow 16 of the mould 10.

A portion of a cam shaft 34 obtained by pouring a hot melt of a material in accordance with JIS FC25 into mould 10 is shown in Fig. 2. This cam shaft 34 comprises a cam 36 and a journal portion 44. A chill-cast layer 40 which was formed by the chill 20 is formed on the lift portion 38 of the cam 36. After being remelted and hardened by a method which will be described later, the cam shaft 34 is incorporated into a valve mechanism portion of an internal combustion engine 10 shown in Fig. 7. The cam 36 of the cam shaft 34 is in slidable contact with a cam follower 28 which is soldered to the body 26 of a rocker arm 24 which is pivotally supported at its one end by the spherical operating end of an oil tappet 30 while the other end abuts against the valve stem end of an intake valve 32.

After casting the cam shaft 34, remelting and hardening treatment is performed on the lift portion 38 which has been formed with cast chilled layer 40. The remelting and hardening treatment is achieved by irradiating the cam surface with a plasma 48 which is discharged from a plasma torch 46 over an area (denoted by U in Fig. 5) extending from 10" to 90" forward and backward along the cam contour about the cam axis of the cam shaft 34, the portion 42 where abrasion resistance is particularly required. The plasma 48 is moved over a width of 2 mm or more so as to draw a snaking path and remelt the cam surface layer except for at both shoulder end portions A in the widthwise direction. The cam surface is then rapidly chilled (self-chilled) and hardened to form a remelted and hardened chilled layer 42 on the cast chilled layer 40 (see Figs. 3 through 6).

The thus-obtained cam lift portion 38 of the cam shaft 34 has a composition as shown in Fig.

8 (corresponding to Fig. 6). When Fig. 9, which is a 100x enlargement of the composition of the hardened chilled layer (the black portion of Fig. 8) is compared with Fig. 10, which is a 100x enlargement of the composition of the cast chilled layer 40 (the layer where remelting and hardening treatment is not performed), it can be seen that the composition of the hardened chilled layer 42 is made extremely fine by rapid chilling (the black spots of the enlarged photograph denote pearlite).

As is apparent from Fig. 8, the shoulder portions at both end portions A in the widthwise direction of the cam surface are unaltered. It is known that by remelting only the central portion, the shapes of both end portions A can be maintained the same as at the time of and variation in the cam effective width can be prevented.

Further, in the above-mentioned embodiment, the surface portion of the cast chilled layer 36 was simply remelted. Alternatively, when remelting, metals such as Cr, Mo, V, Nb, or alloys thereof which differ from the base material or metal compounds such as Cr3C2 or MoS2 may be added in the form of a powder to a molten pool. In this case, the hardened chilled layer becomes an alloy composition or a diffused and reinforced composition having excellent abrasion resistance. Furthermore, a metal powder such as Ni, Cu or Mn in elemental form can be added to the molten pool, resulting in the strengthening of the matrix. Detailed examples thereof are shown hereinafter.

1) 0.2to by weight of a 25Fe-75Si alloy was inoculated into a hot melt, the composition of which was C 3.46%, Si 1.81%, Mn 0.57%, Cr 0.42%, P 0.09%, S 0.087% and a remainder of Fe (all by weight %). The hot melt was poured into a metal mold with a chill fitted into a cam lift portion, and the cam lift portion surface layer was chilled.

2) The resulting cam shaft was desanded and annealed, a bore was formed in the shaft end portion, a key groove was machined, and the like. The cam shaft was then heated to approxi mately 450 C, the top portion of the cam lift portion was remelted using a plasma torch as shown in Fig. 3. At the same time, a powder having a composition of 50 weight % or carbonized chrome and 50 weight % of disulfuric molybdenum was added to the molten pool, which was rapidly chilled by self-chilling.

Fig. 11 schematically illustrates the state during remelting treatment. A plasma torch 46 comprises a tungsten electrode 50, a tip 52 which encloses the tungsten electrode 50 and has a gas conduit 54 and a cooling water passage 56, and a shield cap 58 which encloses the tip 52 and defines a shield gas conduit 60. Two dissimilar metal powder inlet pipes 62,62 pass through and are secured to the front end portion of the shield cap 58. A gas such as argon, hydrogen or nitrogen is discharged from the gas conduit 54 in the form of a jet and forms a plasma 48 which is directed onto the surface of the cam lift portion 38A. A resulting molten pool P is supplied with the carbonized chrome powder and the disulfuric molybdenum B in the above-mentioned proportions through the dissimilar metal powder inlet pipes 62,62.

Test Example Cam shaft I which was made of an FC material (Comparative Example), which had a cast chilled layer as shown in Fig. 2, and which was subjected to remelting and hardening treatment, and cam shaft II (example of the present invention) which was made of an FC material and which was subjected to remelting and hardening treatment on the upper surface of a cast chilled layer as shown in Fig. 4 was incorporated into an engine employing a rocker arm which was made of 18% Cr steel and which had been subjected to soft nitriding treatment. A durability test of 2000rpmx300 hrs. was performed on the engine. Fig. 12 shows the abrasion loss of the cam lift examined after the durability test.As seen from the figure, the abrasion loss of cam shaft il was extremely small when compared with that of cam shaft I, and the maximum abrasion loss (abrasion loss at the most worn part) of cam shaft II generally corresponded to the minimum abrasion loss (abrasion loss at the least worn part) of cam shaft I.

For a slidable shaft member made of an Fe-containing alloy which must be abrasion resistant although not to the same extent as a cam surface of a cam shaft, it is not necessary for its sliding surface to be as uniformly remelted and hardened as a cam 36 Thus, for such a sliding member, the required abrasion resistance can be obtained even if the sliding surface is more or less remelted and hardened with some gaps left between the treated portions. This remelting and hardening method with some gaps between the treated portions is highly efficient and economically advantageous. More specifically, a sliding surface of a sliding shaft member made of an Fe-containing alloy may be subjected to remelting treatment in a spiral form by being irradiated with high concentration energy, and then hardened and chilled to form a hardened chilled layer having a spiral shape.The remelting treatment is practiced by moving a high concentration irradiation means and the shaft member with respect to one another in the axial direction.

Figs. 13 and 14 show examples of a hardened chilled layer 72 formed in the shape of a spiral on the peripheral surface of a shaft member 70 which is in sliding contact with a bearing 76.

Non-hardening portions 74 are left between the spiral hardened chilled layers 72. These portions 74 are protected by the hardened layers 72 when in contact with the bearing 76, and the remelted and chilled area C as a whole has adequate abrasion and galling resistance. If a deposition having a lubricating function such as a carbide is present in the non-hardened portion 74, the durability of the shaft member 70 is further improved in area C. This method is easy to practice as a surface processing method for a shaft member, and it is not necessary to uniformly chill the entire surface which is in sliding contact with other members, the method can be performed in a short time.

There will now be described on example in which the above-mentioned treatment method is applied to a cam shaft of an internal combustion engine.

Fig. 15 shows a one end portion (the end portion driven by a timing belt or chain) of a cam shaft 80 made of cast iron (JIS FC25 material: gray cast iron) for an internal combustion engine, the cam shaft 80 including a cam 82. A journal portion 84 supported by a bearing and a peripheral surface of an oil seal portion 88 are subjected to remelting and chilling treatment using constant pitches.

The conditions for the remelting and hardening treatment were as follows: 1) The cam shaft 80 was preheated to a temperature of 400 to 450"C prior to treatment and was rotated at 35 rpm during remelting treatment.

2) The peripheral surface of the journal portion 84 and the oil seal portion 88 were remelted using a plasma arc melting apparatus as a treatment means under the following conditions: arc current=150A, plasma argon gas flow rate=0.6 1/min., shield argon gas flow rate 7 1/min., plasma arc torch velocity=90 mm/min., and pitch (sprial gaps)-4mm.

Test Example 2 In order to examine the change of composition along the depth of the cam shaft 80 subjected to remelting and chilling treatment under the above-mentioned conditions, a test piece as shown in Fig. 16 was cut from the cam shaft 80 along line XVI of Fig. 15. The test piece was polished and etched and composition photographs as shown in Figs. 17 through 21 were obtained. Fig.

18 shows a boundary portion between the hardened chilled layer 86 and an internal non-chilled layer in the journal portion 84, while Fig. 19 shows only the hardened chilled layer 86. Similarly, Fig. 20 shows a boundary portion between the hardened chilled layer 90 and an internal nonchilled layer in the oil seal portion 88, while Fig. 21 shows only the hardened chilled layer 90.

A duration test using an 1800 c.c. gasoline engine was carried out on a cam shaft 80 which was subjected to remelting and hardening treatment and an untreated cam shaft of the same material and the same size. The rotational speed of the cam shaft was 2000 rpm, the lubricating oil temperature was 50 to 60"C, and the duration of testing was 300 hrs. Abrasion loss and galling loss of the peripheral surface of the journal portion 84 of the cam shaft 80 were checked after the duration test. The results are shown in graphing form in Fig. 22. Abrasion loss and galling loss of the peripheral surface of the journal portion of the untreated cam shaft were also checked after the duration test. The results are shown in Fig. 23. In the figures, D denotes the surface level before the test.

Evaluation of Test Results 1) Figs. 16 through 21: On the peripheral surfaces of the journal portion 84 and the oil seal portion 88, hardened chilled layers 86 and 90 were formed with equal intervals, and the depths thereof were 0.2 to 0.3 mm, respectively. In Fig. 18 and Fig. 20, it is seen that many pieces of graphite are dispersed within the pearlite base in the untreated layer, while the hardened chilled layers 86 and 90 have a fine eutectic composition of cementite (white) and austenite. The hardness of the untreated layer was HRB93 to 95, while that of the hardened chilled layers 86 and 90 was HRA 71 to 75. Thus, improved durability due to an increase in hardness can be expected.

2) Fig. 22 and Fig. 23: It is seen that the abrasion loss and galling loss of the cam shaft 80 were extremely small compared with the abrasion loss and galling loss (Fig. 23) of the untreated cam shaft. Thus, durability can be greatly improved by remelting and hardening treatment.

In carrying out the above-mentioned remelting and hardening treatment, it may be preferred to add a powder of at least one kind of carbide stabilizing agent selected from the group consisting of Cr, Mo, V, and Nb added to the molten pool in the amount of 0.5 to 4 weight % of the composition of the treated portion of the object. According to this method, a double carbide is produced in the remelted and hardened chilled layer, thereby improving the surface hardness and providing excellent abrasion resistance. I However, if the amount of the carbide stabilizing agent which is added is less than 0.5 weight %, the amount of the double carbide which is formed becomes so small that the abrasion resistance cannot be improved extensively.If the added amount exceeds 4 weight %, there is little further improvement in abrasion resistance, and as the added metal powder is so expensive, its addition in large amount is economically disadvantageous.

The effect of remelting and hardening treatment by adding a carbide stabilizing metal agent powder was confirmed for a cam shaft (non-chilled cast article) made of cast iron of the same configuration and the same material as the cam shaft 34 shown in Fig. 7. Various kinds of powders may be used as the added metal powder such as a simple metal powder selected from the group consisting of Cr, Mo, V, and Nb, or a powder of a compound composed of two or more of such metal elements, or a powder consisting of a compound of such metal elements and simple elements, etc.

The remelting and hardening treatment is carried out by using the plasma arc (see the plasma torch 46 of Fig. 11) as a heating source with a gas supply rate of 0.75 1/min, a torch velocity of 47 mm/min., a powder feeding rate of 1.8 g/min. from a powder inlet pipe (see the powder inlet pipe 62 of Fig. 11), and an arc current of 145A.

Using this remelting and hardening treatment, it is possible to obtain a hardened chilled layer with fine double carbides, thereby increasing the hardness of the cam sliding surface and greatly improving the abrasion resistance.

Test Example 3 Fig. 34 shows the alteration in hardness of a cam sliding surface when the added amount of Cr is held constant (0.08 weight %) and the added amount of Mo is varied. The compositions of the cam sliding surfaces of samples a, b and c are shown in Table 1; a JIS FC 25 material was used as a base material and Cr and Mo were added. T.C. denotes the total amount of carbon.

Table 1 wt% SAMPLE T.C. Si Mn P S Cr Mo a 3.15 2.15 0.69 0.03 0.05 0.08 0.04 b 3.15 2.15 0.69 0.03 0.05 0.08 1;1 c 3.15 2.15 0.69 0.03 0.05 0.08 1.6 From Fig. 24, it is apparent that when the added amount of Cr is held constant, the hardness of the cam sliding surface increased as the added amount of Mo increases.

Test Example 4 Fig. 25 shows the change of hardness of a cam surface when the added amount of Mo is held constant (0.04 weight %) and the added amount of Cr is changed. The compositions of the cam sliding surfaces of samples a, b and c are shown in Table 2. A JIS FC 25 material was used as a base material and Cr and Mo were added.

Table 2 wt% SAMPLE T.C. Si Mn P S Cr Mo a 3.15 2.15 0.69 0.03 0.05 0.08 0.04 b 3.15 2.15 0.69 0.03 0.05 0.95 0.04 c 3.15 2.15 0.69 0.03 0.05 1.8 0.04 From Fig. 25, it is apparent that when the added amount of Mo is held constant, the hardness of the cam sliding surface increases as the added amount of Cr increases.

Test Example 5 Fig. 26 shows the results of a durability test using an overhead cam type (OHC) 4 cylinder internal combustion engine with a displacement of 1800 c.c. having a cam shaft (see the cam shaft 34 of Fig. 34) including a cam having a cam sliding surface having the composition shown in Table 3 in which JIS FC25 was used as a base material and Mo and Cr were added thereto.

The test conditions were an engine speed of 2000 rpm, an oil temperature of 50 to 60 C, zero load, and operating time of 200 hours.

Table 3 wt% SAMPLE T.C. Si Mn P S Cr Mo f 3.21 2.38 0.71 0.04 0.06 0.04 0.03 9 3.21 2.38 0.71 0.04 0.06 0.31 0.22 h 3.21 2.38 0.71 0.04 0.06 0.88 0.78 3.21 2.38 0.71 0.04 0.06 1.27 1.05 3.21 2.38 0.71 0.04 0.06 1.54 1.29 k 3.21 2.38 0.71 0.04 0.06 1.89 1.55 3.21 2.38 0.71 0.04 0.06 2.31 1.79 The rocker arm (see the rocker arm 24 of Fig. 7) which the cam contacted was a conventional one. It has a body comprising a chrome steel for carburizing and a cam follower made of a sintered Fe-containing alloy having the composition of Table 4. After the cam follower was soldered to the body, both of them were carburized. The hardness of the cam follower after heating was HRC56 to 58.

Table 4 wt% C Si Mn P Cr Mo W V 1.7 0.8 0.3 0.34 5.7 0.41 1.72 0.14 From Fig. 26, it is apparent that the abrasion loss of the cam sliding surface decreased as the added amounts of Mo and Cr increased.

In this case, if the added amount of a mixture of Mo and Cr is less than 0.5 weight %, the abrasion resistance cannot be improved extensively, while if it exceeds 4 weight %, there is little further improvement of the abrasion resistance and the costs are increased. Therefore, the added amount of the mixture is preferably in the range shown by hatching in Fig. 26, i.e., 0.5 to 4 weight %.

Test Example 6 In this example, the abrasion losses of a cam follower and a cam surface were checked. The cam shaft had the composition shown in Table 5, and the cam sliding surface of the cam was subjected to remelting and hardening treatment in accordance with method of this invention. The rocker arm had a body made of a usual carburized steel, and the cam follower thereof which was in sliding contact with the cam was formed of a high-Cr cast steel. The hardness of the cam sliding surface was HRC53 to 56.

Table 5 wt% T.C. Si Mn P S Cr Mo 3.23 2.36 0.69 0.04 0.05 1.45 1.15 The cam follower may be carburized and polish lapped after being soldered to the body of the rocker arm, or it may be nitrided after being carburized. In the former case, the Cr content is preferably 10 to 25 weight % in view of improving the abrasion resistance. In the latter case, since a hard diffusion layer and a nitride layer are formed in sequence on the base material, even if the Cr content is as low as 8 to 10 weight %, sufficient abrasion resistance can be obtained, and material costs can be reduced. Of course, the Cr content in the latter case may be generally the same as in the former case in which nitriding is not performed.

Fig. 27 shows the relationship between the depth below the surface 5 of a cam follower which is in sliding contact with a cam and hardness (micro Vickers hardness). The average hardness of the nitride compound layer forming the surface 5a was a high as HmV 1300 to 1400.

Test Example 7 Fig. 28 shows the results of a durability test carried out using an overhead cam type (OHC) 4 cylinder internal combustion engine with a displacement of 1800 cc having a rocker arm whose body was made of JIS SCr 420 (Cr steel) and a cam follower made of a high-Cr cast steel having the composition of Table 6 which was soldered to the body. The test conditions were an engine speed of 2000 rpm, an oil temperature of 50 to 60"C, zero load, and an operating time of 200 hours.

Table 6 wt% Sample T.C. Si Mn P S Cr Mo mo,m 2.3 1.8 0.65 0.1 0.05 5.1 0.48 no,n 2.51 1.86 0.71 0.11 0.04 9.2 0.51 00,0 3.41 1.56 0.68 0.1 0.05 17.6 0.52 po,p 3.46 1.48 0.72 0.1 0.05 21.6 0.55 Cam follower mo, no, oo and po were not subjected to nitriding treatment, while n, o, and p were subjected to nitriding at 570"C for 60 minutes.

From Fig. 28, it is apparent that even if nitriding treatment is not performed, the abrasion loss of the cam follower decreases as the Cr content is increased. Furthermore, if nitriding treatment is performed, even when the Cr content is the same, the abrasion loss is significantly decreased.

The Cr content is preferably in the range shown by hatching is Fig. 28, i.e., 8 to 25 weight %. If it is less than 8 weight %, abrasion resistance cannot be obtained. Even if it exceeds 25 weight %, there is no further improvement in abrasion resistance, the costs become high, and working performance is deteriorated, which is undersirable.

Test Example 8 Fig. 29 shows the results of a durability test carried out using an overhead cam type (OHC) 4 cylinder internal combustion engine with a displacement of 1800 cc having a combination of cam shafts A, B, and C and rocker arms Al, B1, and C1. The test conditions were an engine speed of 2000 rpm, an oil temperatire of 50 to 60"C, zero load, and an operating time of 200 hours.

Cam shaft A (example of present invention) was made of a JIS FC25 material. The cam sliding surface thereof was subjected to remelting and hardening treatment, in accordance with the invention, Cr and Mo powder were added, and the composition shown in Table 7 was obtained.

Table 7 wtWo T.C. Si Mn P S Cr Mo 3.21 2.34 0.65 0.04 0.06 1.41 1.12 The cam follower of the rocker are Al was formed of a high chrome cast steel having the composition of Table 8, and it was nitrided.

The hardness of the sliding surface thereof was generally HmV 1300. The body of the rocker arm was formed of a JIS SCr 420 material.

Table 8 wt% T.C. Si Mn P S Cr Ni 3.46 1.61 0.71 0. 12 0.06 17.0 0.56 The sliding surface of the cast cam shaft B which had the composition shown in Table 9 was subjected to chilling treatment during casting. The hardness thereof was HRC 49 to 52.

Table 9 wt% T.C. Si Mn P S Ni Cr Mo 3.45 2.62 0.71 0.11 0.08 0.45 0.46 0.2 The cam follower of the rocker arm B1 was formed of a sintered Fe-containing alloy having the composition shown in Table 10. It was subjected to a usual carburizing treatment. The hardness of its sliding surface was HRC 54 to 57. The body of the rocker arm was likewise made of JIS SCr 420 material.

Table 10 wt% C Si Mn P Cr Mo W V 1.72 0.82 0.31 0.32 0.58 0.40 1.71 0.13 Cam shaft C was made of a material having the same composition as cam shaft B and was subjected to the same treatment. The cam follower of the rocker arm C1 was generally the same as the cam follower of rocker arm Al.

From Fig. 7, it is apparent that form the combination A and Al, the abrasion loss of the cam and the cam follower sliding surface was extremely small compared with the other conbinations and excellent abrasion resistance was obtained.

The present invention is applicable not only to cam shafts and rocker arms but also to other components which are subject to abrasion through sliding contact with other components.

Claims (12)

1. A method of enhancing the abrasion resistance of a chill-cast Fe-containing alloy component comprising remelting the surface by application there to of high-conncentration energy and then resolidfying the surface.

2. A method according to Claim 1 in which the high-concentration energy is supplied by a laser beam on by a plasmo torch.

3. A method according to Claim 1 or 2 in which a carbide stabilizing agent is added to the molten surface.

4. A method according to Claim 3 in which the carbide stabilizing agent is selected from Cr, Mo, V and Nb or combinations, alloys or compounds formed therefrom.

5. A method according to Claim 4 in which the carbide stabilizing element is Cr3, C2 or MoS2.

6. A method according to any preceding claim in which Ni, Cu, or Mn or a combination thereof is added to the molten surface.

7. A method according to any preceding claim in which the component is heated to 400-450"L prior to remelting.

8. A method according to any preceding Claim in which the component is a cam, camshaft, cam follower, valve-lifter or rocker arm.

9. A method according to any preceding claim in which the remelting and resolidification is restricted to a desired portion of the surface of the component.

10. A method according to Claim 9 in which the portion defines a spiral form about the member.

11. A chill-cast Fe-containing alloy component produced by a method in accordance with any of the preceding claims.

12. A chill-cast Fe-containing alloy component constructed and arranged substantially as shown in any of the accompanying Figs. 2-7 and 13-17.