Constrained hardening method and apparatus for deformed bar workpieces

Abstract: Hardening by heating a longitudinal section of a workpiece (1) of a deformed-shape bar and quenching while constraining the workpiece section in dies (11,12), wherein the quenching is executed while the workpiece (1) being pressed toward a direction perpendicular to its longitudinal direction by a pressing means (17,18,19,20) which is installed adjacent to the workpiece section constrained in the dies (11,12). Warping by quenching can be minimized in deformed-shape workpieces as an automobile steering rack bar, wherein warping is difficult to prevent completely in prior constrained quenching process.

Description: The present invention relates to constrained hardening of workpieces of bar or so suitable for preventing warping. Especially this invention is directed to a hardening method and apparatus suitable for deformed shape workpieces such as steering rack bars for automobiles which have a tendency toward warping during hardening. Hardening of bar shaped machinery parts are very popular, in them for axially symmetrical parts such as a body of simple revolution, rotation around the axis during quenching for equalization of cooling is adopted for prevention of warping. Moreover prevention of warping by exerting constraining force is also adopted, such as a round bar is rotated among three rolls extending therealong during quenching as shown in JP-A-54-67504. However, bar workpieces of deformed shape such as a bar workpiece having a rack gear along its side tend to bend by hardening because a cooling rate does not uniform about their axes. Moreover to these workpieces above mentioned rotation method during quenching cannot be adopted, therefore press quenching is usually used for prevention of warping during hardening. In the press quenching, all or a part of a workpiece heated to an appointed hardening temperature is held in a set of dies having determined cavities, then the workpiece is cooled down under pressure by such a method as immersing into a cooling liquid tank. JP-A-10-158742 provides a method for hardening an overall length of a workpiece having a cross-section comprising a complete roundness part and a deformed part, and moreover, a short complete-roundness part contiguous to the deformed part. In this method, the hardening is performed in two steps. In the first step, the deformed part and the short complete-roundness part are heated to a hardening temperature and quenched while being constrained between split dies. Then in the secondary step, the complete roundness part is heated to a hardening temperature and quenched while being constrained among plural rotating rollers. Simultaneously, the short complete-roundness part is constrained among another plural rollers or the extended parts of the aforesaid rollers, for preventing warping by the secondary hardening step effectively and straightening the workpiece in its overall length. Recently there is a trend of converting into pipe parts for cutting weight of automobiles by making hollowed such an aforementioned bar parts with a rack gear along its side. However, the pipe parts are very liable to arise bending in hardening process comparing with solid bars, and moreover, the pipes could be collapsed by a pressing force in the aforementioned press quenching. If the pressing force is limited for preventing collapse, warping might not be suppressed and already strain might arise in the state of being pressed in the dies after cooled down. EP-A-0754771 provides a method and an apparatus for quenching a tubular workpiece, which is difficult to be constrained by an enough force for preventing warping during quenching, because of danger of collapse. In this method, warping is prevented by applying a constraining force which is increased from the beginning of the quenching to the termination thereof, corresponding to an increase of the elasticity limit stress in accordance with temperature decrease of the workpiece. Split dies, which are mainly applied to workpieces of deformed shape crosssection, and also plural rollers, which are applied workpieces of completely round crosssection, can be adopted for the constraining means in this method. Moreover even if the pressing force is considerably increased and the workpiece is made straight in the dies, warping might arise by existence of residual stress as the workpiece is taken out from the dies. Such residual stress is usually controlled by regulating a cooling rate by changing partly a cooling liquid amount for blowing against a workpiece depending on the workpiece shape. However, occurrence of warping is not so simple as it changes depending on steel constituents in relation to transformation temperature, because strain in hardening process is a sum of thermal strain and transformation strain. As explained above in some cases especially in hollowed deformed-shape workpieces warping cannot be suppressed merely by pressing between dies because of limitation of a pressing force. Moreover even if deformation is thoroughly suppressed within the dies, warping might occur by residual stress as taking out from the dies. Therefore complete prevention of warping is difficult as matters now stand. The present invention is intended to minimize warping in hardening of deformed-shape bar workpieces which are difficult to prevent warping completely in prior constrained hardening processes by dies. These objects are achieved with the features of the claims. FIG. 1 is a cross-sectional side view of an equipment of the present invention and FIG. 2 is a slant view of a rack bar as an example of a workpiece. FIG. 3 and FIG. 4 are cross-sectional front views of the equipment shown in FIG. 1, respectively at a dies part and at a part of pressing means. FIG. 5 is a cross-sectional front view at a dies part of an equipment of this invention as a different example from FIG. 3. FIG. 6 is a cross-sectional front view at a part of pressing means of an equipment of this invention as a different example from FIG. 4. FIG. 7 is a side view of an another example of an equipment of this invention. From FIG. 8(a) to FIG. 8(d) are drawings for explaining the effect of constrained hardening method of this invention. In the present invention, as a constrained hardening method by heating a longitudinal section of a workpiece of a deformed-shape bar and quenching the section of the workpiece in dies, not only a constraining force by the dies but also another forces are exerted for preventing warping in hardening. Namely, this invention intends to minimize occurrence of warping by exerting stress to the workpiece for extinguishing strain at a constrained quenching process as a positive means for controlling strain. As a method for attaining this, quenching is executed while the workpiece being pressed toward a direction perpendicular to its longitudinal direction by a pressing means which is installed adjacent to one side or both sides of the workpiece section constrained in the dies. FIG. 1 is a cross-sectional side view of an example of an equipment of the present invention wherein a workpiece is shown in parallel to its length. FIG. 2 is a slant view of a rack bar as a workpiece 1 in FIG. 1, where the rack bar is in its length composed of a full-circle part 2 in which the cross section is a full circle, a deformed-shape part 3 in which the cross section is not a full circle, and more a short full-circle part 4 which extends beyond the deformed-shape part 3. FIG. 1 shows a situation of quenching by spouting coolant as water after the deformed-shape part being heated, wherein this part is pressed by the dies 11 and 12 in a position of the side formed rack facing downwards. Then, pressing means characteristic of this invention are connected to the dies 11 and 12 at a place adjacent to both sides of the workpiece part constrained in the dies. The pressing means comprise pressing members 17, 18, 19 and 20 which press the workpiece toward a direction perpendicular to its longitudinal direction, namely, vertical direction in this example. The pressing means are adjacent to pressing parts of the dies which can be said as substantial parts of the dies, so as a bending force to be exerted to the workpiece between the pressing means and the dies. Namely, this example has such construction by connecting the pressing members 17, 18, 19 and 20 to the dies 11 and 12 themselves. Each of the pressing member can move along an advance-and-retreat position and presses the workpiece depending on demand. The parts of the workpiece where the pressing members contact are a full-circle part 2 and a short full-circle part 4 (FIG. 2) which are out of the zone heated for hardening. Therefore its temperature is low and it has enough hardness, then dent will not occur on the workpiece by pressure of the pressing members. Besides, in the above example the pressing means are connected to both sides of the dies, however, one side of the pressing members as 19 and 20 can be omitted in the case that only one side is necessary depending on shape of a workpiece. FIG. 3 is a cross-sectional front view of the equipment shown in FIG. 1 wherein the workpiece is shown in perpendicular to its length. This drawing shows the substantial parts of the dies 11 and 12, however, the present invention does not restrict especially the construction of these parts. In this example the dies comprise a plurality of projections 13 defining a contour corresponding to a contour of the workpiece 1, and recesses 14 between the projections as shown in FIG. 1 and FIG. 3. By corresponding the inner face of the dies to a contour of a workpiece as shown above, the dies can deal with any direction of warping of the workpiece. Besides, if a workpiece is hollowed and thin which is easily collapsed by pressure, a method of prior invention of this inventors in EPC patent No. EP-A-0754771 is recommended, wherein the pressure by the dies 11 and 12 is varied during cooling in hardening process. There are cooling liquid spouting holes 15 in many places of the above-mentioned recesses 14 of the dies, to which cooling liquid is supplied from cooling liquid supplying rooms 16. Besides, if more than one cooling liquid spouting hole 15 is provided in the recesses 14 of each die 11 and 12, spouted cooling liquid will spread over the entire surface of the workpiece. Then a cooling rate can be controlled by proper allocation of the cooling liquid spouting holes. Namely, cooling is fast at the places exposed directly to cooling liquid jets, and contrarily cooling is relatively slow at the places cooled by flowing liquid dissipated through the recesses. By controlling a cooling rate partly in the workpiece as above explained, cause of warping can be eliminated to some extent even in a deformed-shape workpiece as a rack bar, before the step of prevention of warping by constraining force. FIG. 4 is a cross-sectional front view of the equipment shown in FIG. 1 in perpendicular to the length of the workpiece, wherein the workpiece is shown at a part of pressing means which is a characteristic part of this invention. In this drawing the pressing means are composed of two pressing members 17 and 18 and other parts. These pressing members are connected to the upper die 11 and the lower die 12 respectively and move together with the dies. In this example a rack bar as the workpiece 1 is placed in a position of the rack part facing downwards in the quenching process. Consequently warping is presumed to arise only as bend toward vertical direction because of symmetry of the workpiece. Therefore per one pressing means, there are two pressing members at the upper and lower sides of the workpiece, making possible to exert a bending force only toward a vertical direction. Namely, in this example there are four pressing members as a total, where two of them are shown in both FIG. 1 and FIG. 4 as 17 and 18, and the other two are shown only in FIG. 1 as 19 and 20. Control of a pressing force is executed by varying pressing stroke of the above-explained pressing members 17,18,19 and 20, namely by varying their protruding lengths in the state of being closed the dies and held a workpiece. By this reason the pressing member should be equipped with an advance-and-retreat position regulating means for regulating a pressing stroke. Because a range of the protruding length to be controlled is small as within 1 mm, it is recommended that the pressing member is threaded as mechanism for regulating small distance, so as the pressing member to move in advance and retreat by its rotation. The equipment shown in FIG. 1 and FIG. 4 has such a mechanism wherein the pressing members themselves are composed of hexagon headed bolts and can move in advance and retreat by its rotation. Besides, 21, 22, 23 and 24 in the drawings are lock nuts which are attachments of the pressing members and fasten the pressing members in their appropriate rotation positions. The equipment shown in FIG. 1, FIG. 3 and FIG. 4 is one example of this invention, therefore this invention is not restricted to such an embodiment. For instance instead of two dies of the upper die 11 and the lower die 12 in FIG. 3, four dies of 31, 32, 33 and 34 as shown in FIG. 5 which press along vertical and horizontal directions can be adopted. Because this equipment can control a pressing force along not only a vertical direction but also a horizontal direction, warping can be prevented more efficiently. Moreover in the equipment of two dies as in FIG. 3, a workpiece may happen to be difficult to take out by fitting in the dies, however, the equipment in FIG. 5 has not such problem. Besides, in the equipment shown in FIG. 5, a cross-sectional side view is practically same as FIG. 1, therefore it is not shown again. Also similar to FIG. 1 and FIG. 3, 35 are projections, 36 are recesses, 37 are cooling liquid spouting holes and 38 are cooling liquid supplying rooms in FIG. 5. Moreover in a workpiece of some shape, a direction of warping is not only vertical because of its symmetry unlike the aforementioned rack bar. In this instance pressing members should push the workpiece along another direction within a direction perpendicular to the length of the workpiece. In the case where dies are composed of the vertical and horizontal four dies 31, 32, 33 and 34 as shown in FIG. 5, pressure can be exerted toward any direction by providing a pressing member to each of the four dies as similar to FIG. 4, though they are not shown in a drawing again. Namely, pressure can be exerted not only toward vertical and horizontal directions which are pressing directions of each pressing member, but also toward a slant direction by a combination of pressing by the vertical and horizontal pressing members. As well in the case that the dies are composed of upper and lower two dies as shown in FIG. 3, pressing means can be designed to exert not only toward a vertical direction as shown in FIG. 4, but also toward another directions. FIG. 6 is a cross-sectional front view of an example of such a pressing means wherein a workpiece is shown in perpendicular to its length. Namely, 41 and 42 in this drawing are an upper die and a lower die respectively, then respectively two pressing members among 43, 44, 45 and 46 are connected to each of the dies. Because these pressing members can press the workpiece 1 toward different directions 90 degrees apart, they can press toward any direction by exerting one or two of the pressing members. In the every aforementioned constrained hardening apparatus of the present invention, the pressing means are connected to the dies, however, the pressing means can be provided apart from the dies. FIG. 7 is a side view of an example of such an apparatus where pressing means are installed adjacent to both sides of the workpiece section constrained in the dies 61 and 62, wherein the pressing means comprise pressing members 63, 64, 65 and 66 and other parts which press the workpiece toward a direction perpendicular to its longitudinal direction, namely, toward a vertical direction in this example. The pressing members 64 and 66 which are located in a lower side among them are screwed into a supporting base 67 to which the lower die 62 is mounted, so as to be controlled their vertical positions. Besides, 68 and 69 in the drawing are rock nuts. On the other hand, the pressing members 63 and 65 which are located in an upper side are connected to hydraulic cylinders 71 and 72 so as to be pressed against the workpiece 1. Accordingly for controlling a pressing stroke in use of the above pressure regulating means, in case of pressing the workpiece downwards, the lower pressing member is lowered below a position corresponding to the dies, and then the upper pressing member is pressed by the hydraulic cylinder 71 or 72. On the other hand in case of pressing the workpiece upwards, the lower pressing member is raised above a position corresponding to the dies, and then the upper pressing member is retracted to be free. Thus an upward pressing force is yielded as a reaction force by the pressing force of the upper die 61. Besides, 70 in FIG. 7 is a hydraulic cylinder of the dies. The apparatus as FIG. 7 where the pressing means and the dies are separated is more favorable from a standpoint of performance than the aforementioned apparatus where the pressing means are connected to the dies, although the apparatus becomes complicated. That is, in the apparatus where the pressing means are connected to the dies, rigidity of the dies should be sufficiently high not to be distorted, because the pressing force by the pressing means is sometimes considerably higher than the pressing force of the dies themselves, namely than the pressing force against the hardening part. However, the apparatus as FIG. 7 where the pressing means and the dies are separated has not such a problem. Moreover because the pressing means and the dies are separated, degree of freedom in operating condition is high as the pressing means can be operated independently from the dies. In the above explanation of FIG. 7, the pressure regulating means act to control pressing stroke of the pressing members as similar to the aforementioned example in FIG. 1. However in case of pressing the workpiece downwards in FIG. 7, a pressing force can be controlled as an another pressure regulating method, wherein the lower pressing member 64 or 66 is retracted to be free, and then the upper pressing member 63 or 65 is pressed by the hydraulic cylinder 71 or 72 in an appointed force. Moreover at the apparatus in FIG. 7, an another pressure regulating means can be adopted wherein hydraulic cylinders are provided to all of the pressing members including the lower pressing members 64 and 66 for controlling pressing forces by hydraulic pressure. Consequently in this case, at least one of the pressing stroke and the pressing force is controlled by the pressure regulating means at each of the pressing member. Besides, in the example shown in FIG. 7, the pressing members exert only toward vertical direction, however it is a matter of course that pressing members of horizontal direction can be added in case of necessity. On the other hand, any of the four pressing members 63, 64, 65 and 66 shown in FIG. 7 can be omitted from the apparatus under some conditions. Moreover though the dies in FIG. 7 are a set of an upper and lower two dies, it is a matter of course that a set of four dies can be adopted as shown in FIG. 5. As for an operating method of the pressing means in the constrained hardening apparatus of this invention, occurrence of warping is minimized by exerting stress for extinguish strain arisen in the workpiece, as mentioned before. Namely, when completely straight workpiece is held in the dies and heads of the pressing members are just touching to the workpiece, a pressing force is exerted only if the workpiece bends toward a direction to be pushed by the pressing members. Then if the workpiece is completely straight, a pressing force is no longer exerted. However from a standpoint of effect, it is same as inner faces of the dies are extended along a length of the workpiece instead of the pressing members. Such a method is not denied in this invention because this method may be effective in some conditions depending on a shape of the workpiece. However, effect of this invention is exhibited by protruding the pressing members so as the workpiece to be bent elastically, even if a straight workpiece is held between the dies. That is, although the workpiece is bending in the state of still being held between the dies at the end of a quenching process, a straight product can be obtained by being open the dies and released the constraining force, being canceled strain caused by residual stress. FIG. 8(a) to FIG. 8(d) are drawings for explaining the effect of constrained hardening method of this invention, wherein a workpiece is shown schematically as viewing from its side. Here the workpiece 51 represents a imaginary deformed-shape workpiece not restricted to that shown in FIG. 2, wherein the workpiece is quenched while being constrained by the dies 52 and 53. For instance, when upward pressing forces are exerted by the pressing members 55 and 57 at both sides of the dies 52 and 53 as shown in FIG. 8(a), a bending force as shown by the arrows in FIG. 8 (b) acts to the workpiece. Naturally, if the pressing forces are inverse to the above by using the pressing members 54 and 56, the direction of bending is inverse to the above. The method for using pressing members as in FIG. 8 (a) and FIG. 8 (b) is effective for the deformed-shape workpiece as shown in FIG. 2, because this workpiece has a tendency toward warping to curve for one direction. On the other hand, when mutually reverse pressing forces are exerted by the pressing members 55 and 56 at both sides of the dies 52 and 53 as shown in FIG. 8(c), a bending force as shown by the arrows in FIG . 8(d) can be worked to the workpiece. Moreover, of course a pressing member only at one side of the dies can be exerted. Therefore, bending forces suitable in accordance with a shape of deformed-shape workpieces can be exerted by such combination of directions of pressing forces by the pressing members. As explained above the pressing means in the constrained hardening apparatus of this invention can be controlled in pressing conditions variously in accordance with a tendency for occurrence of quench warping in the workpiece. Then here arises a problem how to determine pressing stroke and so on in practical constrained quenching procedures. This invention has a premise to deal with many workpieces of a same shape, as can be convinced by the matter to use the dies which have a contour of the inner faces corresponding to a contour of the workpiece. Accordingly, by executing quenching while successively varying conditions of the pressing means by trial and error, conditions for minimizing warping of the products can be determined as a result. However as a more rational method, it is recommended that test quenching is executed with all pressing means being retracted from the workpiece and constraining only by dies, and then proper quenching is executed with setting the pressing means in a pressing direction and in a pressing stroke or pressing force so as to reform bend occurred in the test quenching. Though bend in the workpiece after hardening cannot be always minimized by one test quenching, an optimum condition can be found by slight additional control. EXAMPLE Hardening of the rack part of a workpiece having a rack gear along a part of pipe side as shown in FIG. 2 was executed. Material was equivalent to JIS S40C which is carbon steel for mechanical structures. As for approximate dimensions of the workpiece, an outer diameter and an inner diameter of the pipe were respectively 23 mm and 19 mm at the short full-circle part 4 and the deformed part namely being formed the rack 3 to be a waned circle in a cross section, and lengths of the short full-circle part and the deformed part were 30 mm and 270 mm respectively. Besides, a length of a part where a rack is actually formed was 180 mm. With respect to the full-circle part 2, it was 25 mm in outer diameter, 21 mm in inner diameter and 450 mm in length. The deformed part 3 of the above-mentioned workpiece was heated to hardening temperature 950 °C by an induction heating coil. Besides, some portion near the end of the short full-circle part 4 was not included in heating region. Then it was quenched by jet of cooling water while exerted a pressing force between the dies shown in FIG. 1 and FIG. 3. Besides, the pressing force was equal to an elasticity limit stress at 950 °C of the workpiece so as not to be collapsed. At this time the pressing members 17, 18, 19 and 20 which were at both sides of the dies were retracted to be separated from the workpiece. After above-mentioned hardening, bending was measured to be 2.3 mm, whereby the workpiece was rotated with both ends being supported on knife edges, and deflection was measured by a dial gauge at a point midway between both ends. The reason of this bending are that strain could not be suppressed because of restriction of the pressing force within an elasticity limit stress at 950 °C and that residual stress was occurred by an asymmetric cooling rate owing to the deformed-shape workpiece. An appearance of the bending at that time was the side with a rack being inside. Then, hardening was executed by using the pressing means shown in FIG. 1 and FIG. 4, whereby protrusion of lower pressing members 18 and 20 were 0.2 mm and 0.3 mm respectively at the longer full-circle side 18 and the shorter full-circle side 20. The other conditions as heating temperature and a pressing force by the dies were same as the former case. As a result bending was so small to be 0 . 04 mm, as can be seen the remarkable effect of this invention. As above explained the constrained hardening method of deformed-shape bar workpiece in this invention is able to prevent occurrence of warping in quenching which is difficult to prevent completely in prior constrained quenching process by dies, because of residual stress, limitation of a pressing force to a hollowed workpiece and so on. Therefore warping by quenching can be almost prevented in mass production of deformed-shape members as an automobile steering rack bar. This invention is not restricted within the embodiments above explained with the drawings. This invention may be changed or modified in other forms without diminishing its attendant advantages. Therefore such changes and modifications are covered by the appended claims.

Claims: A hardening method by heating a longitudinal section of a workpiece (1) of a deformed-shape bar and quenching while constraining said workpiece section in dies (11, 12 ; 61, 62), wherein the quenching is executed while the workpiece being pressed toward a direction perpendicular to its longitudinal direction by a pressing means (17 to 20 ; 63 to 66) which is installed adjacent to at least one side of said workpiece section constrained in the dies. The method according to claim 1, wherein test quenching is executed while the pressing means (17 to 20 ; 63 to 66) being retracted from the workpiece (1), and then proper quenching is executed while setting the pressing means in a pressing direction, and in at least one of a pressing stroke and a pressing force, so as to reform bending occurred in said test quenching, whereby bending in the workpiece is minimized after hardening. A constrained hardening apparatus for quenching a longitudinal section of a workpiece (1) being constrained in dies (61, 62), comprising a pressing means which is installed adjacent to at least one side of said workpiece section constrained in the dies, said pressing means comprising at least one pressing member (63, 64, 65, 66) which press around the workpiece toward a direction perpendicular to its longitudinal direction, and each of said pressing member comprising a pressure regulating means for regulating at least one of a pressing stroke and a pressing force independently on pressing conditions by the dies. A constrained hardening apparatus for quenching a longitudinal section of a workpiece (1) being constrained in dies (11, 12), comprising a pressing means which is connected to the dies at a place adjacent to at least one side of said workpiece section constrained in the dies, said pressing means comprising at least one pressing member (17, 18, 19, 20) which presses around the workpiece toward a direction perpendicular to its longitudinal direction, and each of said pressing member comprising an advance-and-retreat position regulating means for regulating a pressing stroke. The apparatus according to claim 4, wherein the advance-and-retreat position regulating means comprises the pressing member (17, 18, 19, 20) being threaded whereby the pressing member moves in advance and retreat by rotation. The apparatus according to claim 3, 4 or 5, wherein the dies have a plurality of projections (13) defining a contour corresponding to a contour of the workpiece, wherein recesses (14) are defined between saidprojections, and wherein a cooling liquid spouting hole (15) is open into at least one place of said recesses of each die.

Inventors: FUKUHARA TETSUKAZU, KATO JUNICHI, SETO YOSHIKI, YAO YUGO, FUKUHARA, TETSUKAZU, KATO, JUNICHI, SETO, YOSHIKI, YAO, YUGO, Fukuhara, Tetsukazu, c/o Neturen Co., Ltd., Kato, Junichi, c/o Neturen Co., Ltd., Seto, Yoshiki, c/o Neturen Co., Ltd., Yao, Yugo, c/o Neturen Co., Ltd.
Applicants: NETUREN CO LTD, NETUREN CO., LTD.
Application year: 1999
Main classification: B21D3
IPC codes: C21D9, C21D1