GB2030481A - Push-rod for internal combustion engine - Google Patents

Abstract

A valve push-rod for an internal combustion engine is formed by electric resistance welding a case-hardened end piece 2 on the end of a steel tube 1 and controlling the rate of cooling following cessation of the welding current. This may be effected by heating the weld zone so as to form in the zone a transition layer 6 comprising a mixed ???- martensite and primary troostite structure 6b, which bridges the annular weld zone boundary around the tube wall in the region of the end of its base. The heating current is applied simultaneously with cessation of the welding current. There is a reduction in the tendancy to generate cracks in the weld zone and the mixed transition layer adds toughness to the welded joint. <IMAGE>

Description

SPECIFICATION A push rod for internal combustion engines This invention relates to a push rod for internal combustion engines and to a method of making the same.

A conventional push rod is usually made solely by welding using electric resistance welding, an end piece having a concave or convex arcuate end surface and having a carbon-hardened layer formed on the peripheral wall surface to the peripheral edge of a port at one or each end of a steel tube material. Examples of such push rods are described in, for example, U.S. Patents No. 2,975,775 and No. 2,960,080. However, with conventional push rods the end piece sometimes becomes detached during running of the engine.

According to one aspect of this invention we propose a valve push rod for internal combustion engines comprising a steel tube and, on at least one end thereof a case-hardened end piece welded to the tube, wherein the weld zone extends into the case hardened end piece from an annular weld zone boundary around the tube wall in the region of the end of its base and wherein the weld zone includes a transition layer havng a mixed a-martensite and primary troostite structure, bridging a radially outer or radially inner part of the weld zone boundary. Preferably, there is an end piece on each end of the tube, each having a curved surface remote from a substantially flat surface to which the tube is secured by welding. The curved surface of one end piece may be convex and the other concave.

According to another aspect of this invention we propose manufacturing such a valve push rod for internal combustion engines by securing the tube and end piece together by electric resistance welding and controlling the rate of cooling following cessation of the weld current so as to form the mixed transition layer.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings of which: Figure 1 is an elevation showing one embodiment of the push rod according to the present invention.

Figure 2 is an enlarged sectional view of a part of the rod shown in Figure 1; Figure 3 is an enlarged sectional view of a part of a conventional push rod, showing a section corresponding to that in Figure 2. This conventional push rod appears the same in elevation as the push rod of Figure 1; Figure 4 is a vertically sectioned view of an end piece and an end part of a steel tube prepared for joining by electric resistance welding.

Figures 1 and 2, show a steel tube 1 and two end pieces 2 one having an end 2' with a concave arcuate surface 2a and the other having an end 2' with a convex arcuate surface 2b. Each end piece 2 has a flat surface 2" to which the tube is attached. The end piece 2 is secured by electric resistance welding to the peripheral edge 1 a' of the port at the end of the steel tube 1.

The end piece 2 shown in Figure 2 has a case-hardened layer 4 consisting of a ss-martensite structure.

During electric resistance welding, a transition layer 6 forms in the weld zones extending into the hardened layer 4 and delimited by the annular weld zone boundary 5 around the tube 1. This transition layer 6 is subdivided into layers 6a, 6b and 6c having different metallurgical structures. The layer 6a has a very hard brittle a-martensitic layer of hardness Hv of 750 to 800 formed to be segmental in the section on the intermediate part of the weld zone boundary 5 and made annular along the weld zone boundary. The layer 6b is tough and has a hardness Hv of 450 to 550. It has a mixed structure of an a-martensite and primary troastite, and bridges the layer 6a from the radially outer Sb to the radially inner part 5a of the annular weld zone boundary 5.The thin layer 6c consisting of a tough secondary troastite having a hardness of Hv 480 intervenes between the mixed layer 6b and the hardened layer 4. Notches 6' may form in the radially inner or radially outer end of the transition layer 6 as the parts are pressed together during resistance welding.

If the structure illustrated in Figure 2 showing a push rod according to the present invention is compared with the structure illustrated in Figure 3 of the conventional push rod it will be seen that, with the present invention, the end piece 2 and the end of the tube are welded together through the tough mixed layer Sb, whereas, in the weld zone 3 of the conventional push rod, no such tough mixed layer 6b is formed, the very hard brittle a-martensite layer 6a providing a weld layer. The push rod produced according to the present invention is therefore stronger than a conventional push rod.

Both the push rod of the present invention and the conventional push rod, are made by bringing the end edge la' of the tube into contact with the fixed side surface 2" of the end piece as illustrated in Figure 4 and resistance-welding by instantaneously passing through the point of contact to fuse the parts together. The next step in the production of a conventional push rod is cooling in air to produce in the region 3 at the end of the tube 1 and at the surface 2" of the end piece 2 at the interface therebetween layers as illustrated in Figure 3 of an a-martensite which is hard and brittle and in which fine cracks 6" inevitably develop.During welding the temperature of the region 3 rises to about 1400C at the onset of the current but quickly drops to be below 1000 C and continues to fall so quickly that the generation of the cracks is unavoidable. The cracks develop as the temperatures fall between 900 and 1 000C particularly near 700 and 250 C.

The cooling velocity near 2500C is reduced by the residual heat of the push rod and the chuck which also acts as an electrode but no means of reducing the rate of cooling immediately following the onset of current flow.

We have found that, if the rate of cooling of the region 3 is reduced by feeding heat to said region when the current is cut off not only is the generation of the cracks prevented but also the structure formed within the above mentioned transition layer 6 will be the mixed a-martensite and troostite structure 6b not present in the conventional push rod. To achieve this, the peripheral end edge 1a' of the tube is brought into contact with the surface 2" of the end piece 2 and the instantaneous current is passed while the area of contact is heated (e.g. using gas) to a desired temperature the intensity of heat then being gradually reduced. High frequency or induction heating may also be used, or a smaller current than the welding current may be passed between the parts, heating being initiated the instant the welding current is cut off.

Notches 6' shown in Figures 2 and 3 are produced wheM the parts are pressed together during welding. In the conventional production of push rods it is likely that cracks 6" will develop from these notches. The generation of such cracks 6" is avoided in the production of push rods in accordance with the present invention by reducing the rate of rapid cooling as described above. If the width W of the transition layer 6 is less than the width of the weld zone boundary 5 as illustrated in Figure 2, the cooling effect is improved. The results depend largely upon the means chosen for reducing the rate of cooling, the state of the peripheral edge 1 a' at the end of the tube and the control of the pressing force. Favourable conditions are determined experimentally.

If as shown in Figure 3 the transition layer 6, of a hard, brittle and most undesirable a-martensite structure, products beyond the fusing boundary surface 5 both inside 5a and outside Sb of the tube cracks 6" are produced at the tip of each notch 6'.

Tests will now be described exemplify the reduction of the rate of cooling and the effects which occur.

A steel tube material 1 of ASTM A512-66 MTlolo, a tube diameter of 8.0 mm, thickness of 1.2 mm and length of 186.0mm was mounted in a chuck, an end piece 2 of ASTM A-575-73 Glo120, a carbon-hardened depth of 0.Smm (full cementation), surface hardness Hv of 700, outside diameter R of 10.5 mm, thickness t of 10 mm and a bottom thickness t' of Smm having the end surface 2' made a concave arcuate surface la and having the flat surface 2" was mounted on the end edge la' of the steel tube 1.Next a current at a voltage of 8 volts, SO cycles and a current density of 1 6,000Aicm2 was passed through the tube 1 and end piece while they were pressed together with a force of 1,100kg for 2/100 second, then a current of the same voltage and frequencey but having a smaller current density of 4,500alum2 was synchronously passed with the parts under the same pressure for 6/100 second simultaneous with cutting off the current to reduce the quick cooling and then they were naturally air-cooled to obtain an expected product.

Fifty samples were prepared in the manner described from which thirteen were selected at random. Three of the selected samples were subjected to the section test of the welding zone 3 and the remaining ten were tested to ascertain the strength of weld.

The width W of the transition boundary range 6 was found to be less than the width of the weld zone boundary 5 and a layer of the mixed structure 6b having notches 6', accuated over outside 5b from inside end part 5a of the weld zone boundary 5 were observed within the transition layer 6. The results of the strength test of the weld zone 3 were as mentioned in Table 1.

The sample referred to in the following were prepared in the same manner except that no control of the rate of cooling was applied in the production at the conventional samples.

Table 1 Sample push rods according to Conventional push rod the present invention samples No. Strength No. Strength No. Strength No. Strength 1 1200kg 6 1230kg 1 1060kg 6 820kg 2 1210 7 1200 2 1120 7 1030 3 1220 8 1140 3 860 8 970 4 1160 9 1250 4 1030 9 1010 5 1180 10 1210 5 1080 10 890 x 1200 x 987 R 110 R 300 The strength indicated in the table is the tensile force necessary (in the axial direction of the tube) to separate the end piece and the end part of the tube.

Table 1 shows that the average strength x was 1200kg for a push rod produced according to the present invention but was 987kg for the conventional push rod. Also the fluctuation R was 11 Okg for a push rod according to the present invention as compared with 300kg for a conventional push rod.

Claims (9)

1. A valve push rod for internal combustion engines comprising a steel tube and, on at least one end thereof a case-hardened end piece welded to the tube, wherein the weld zone extends into the case hardened end piece from an annular weld zone boundary around the tube wall in the region of the end of its base and wherein the weld zone includes a transities layer having a mixed a-martensite and primary troostite structure, bridging a radially outer or radially inner part of the weld zone boundary.

2. A valve push rod according to claim 1 wherein the radial width of the weld zone is less than the radial width of the annular weld zone boundary.

3. A valve push rod according to claim' 1 or claim 2 wherein the end piece has a curved surface remote from a substantially flat surface to which tile tube is secured by welding.

4. A valve push rod according to claim 3 and having an end piece secured at each end of the tube.

5. A valve push rod according to claim 4 wherein the curved surface of one end piece is convex and the other is concave.

6. A method of manufacturing a valve push rod for an internal combustion engine and comprising securing a case-hardened end piece to the end of a steel tube by electric resistance welding and controlling the rate of cooling following cessation of the electric welding current so as to form in the weld zone a transition layer comprising a mixed a-martensite and primary troostite structure briding a radially outer and radially inner parts of an annular weld zone boundary around the tube wall in the region of the end of its base.

7. A method according to claim 6 wherein the rate of cooling is reduced by heating the weld zone following cessation of the welding current.

8. A valve push rod for internal combustion engines, constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

9. A method of manufacturing a valve push rod for internal combustion engines substantially as hereinbefore described with reference to the accompanying drawings.