Patent Application
20050039829
The present invention relates to a method of induction heat treatment. More specifically, the invention comprises a method for induction hardening certain metal components, particularly those having a shape that prevents uniform induction coupling over the entire surface to be hardened in a single step. Most particularly, the invention comprises a method for induction hardening the outer surfaces of the trunnions of a spider of a steel tripod-type constant velocity joint.
The tripod-type constant velocity joint is widely used in automotive vehicles. It is most frequently used to provide a plunging and angulating constant velocity joint for use with halfshafts, the name given to the two driveshafts or axle shafts that run from the transaxle to the wheels in front wheel drive vehicles. A plunging joint is one that permits axial movement between the shafts. Because it is widely used in automotive vehicles, the tripod joint is manufactured in relatively high volumes. A tripod joint consists of a spider which comprises a hub, often with a splined bore in its center, and three angularly-spaced shafts or trunnions extending from the hub, and three roller bearings. Each roller bearing is fixed on an end of a trunnion and is adapted to slide in a corresponding groove that is cut into the inside surface of a tube-like housing. The power to the driven wheels of a vehicle is transmitted through the trunnions and the point of contact between the three bearings and three grooves. Given the magnitude and cyclic nature of the loading of the trunnions, they have carefully defined requirements for strength, toughness and fatigue resistance.
While variations exist in the designs for and materials used in spiders of different manufacturers, a spider is typically formed by forging from a pearlitic/ferritic steel blank, and is then subjected to various metal forming, finishing and heat treatment steps to produce a finished article. In one example, the spider is specified as an AISI 1050 warm-forged steel material. The required properties comprise a surface hardness on the trunnions in the range of RC58-63, with a hardened case depth of approximately 1.0-2.0 mm effective at RC50, and a core hardness of RC15-30. The microstructure is required to show martensite, preferably tempered martensite, in the case with fine grains of pearlite and ferrite in the core. The required hardness is necessary to provide the strength in the load bearing areas of the outer surface, and the necessary toughness and fatigue resistance in the core.
Spiders of various configurations have previously been hardened by carburizing to provide the necessary microstructural properties, such as those described above. The use of carburizing for case hardening has a number of well-known limitations. These include the fact that the process treats the entire surface of the component, the material and processing costs associated with the process, the processing time necessary to heat the parts to temperature and produce the required carburized case depth, as well as limitations related to process control, batch processing, capital expense and facility requirements for large furnaces, environmental issues, and control of the finished part quality. Also, the carburizing process has the potential to form undesirable microstructures, which include carbides, grain boundary oxidation, decarburization and retained austenite that can each affect the functionality of the finished part. Therefore, it is desirable to develop an improved method of heat treatment that addresses the limitations mentioned above and that provides a method for surface or case hardening a part such as a spider.
Induction heat treatment is known to be an effective method of case hardening pearlitic/ferritic steels and avoiding many of the limitations mentioned above that are associated with carburizing. For example, induction hardening has been widely used for the case hardening of various types of steel gears. However, induction hardening has significant limitations in cases where the surface requiring heat treatment is irregular, such as gears having relatively larger teeth where the distance from the tip to the root of a tooth is such that the electromagnetic coupling, and hence induction heating, varies significantly from the tip to the root. While some solutions have been proposed to facilitate the use of induction hardening with articles having irregular surfaces, such as the use of a plurality of different coils and different induction frequencies to treat different portions of the surface, or the use of coil designs that are adapted to the contour of the irregularities in order to provide more uniform coupling, induction hardening has not been used for various types of irregularly shaped components, including spiders, perhaps because the use of the techniques described above are not applicable to provide a single pass induction heat treatment of the critical surfaces of a spider due to its irregular shape.
Therefore, it is desirable to develop an induction heat treatment method that can be utilized to provide induction hardening of irregularly shaped components, particularly those that comprise a hub and a plurality of trunnions extending outwardly therefrom, such as the various spiders utilized for tripod-type constant velocity joints.
The present invention provides a method of induction heat treatment, comprising the steps of: (1) selecting an article, such as spider, for heat treatment comprising a hub having a hub surface and a plurality of angularly spaced trunnions extending from a corresponding plurality of trunnion shoulders formed in the hub surface, each trunnion shoulder having a trunnion shoulder surface, and each trunnion having a trunnion axis and a trunnion surface; (2) selecting an induction coil, which is adapted to receive a trunnion for heat treatment and apply a magnetic field to the trunnion surface and trunnion shoulder surface; (3) placing a trunnion within the induction coil with its corresponding trunnion shoulder adjacent to the induction coil; (4) rotating the trunnion within the induction coil about the trunnion axis at a selected speed; (5) energizing the induction coil to apply the magnetic field and produce induction currents within the trunnion shoulder surface and trunnion surface of the article for a time sufficient to induce heating them to a heat treatment temperature (TH) to at least a selected case depth; (6) withdrawing the trunnion from the induction coil at a selected rate; (7) cooling the trunnion surface and trunnion shoulder surface of the article to a temperature (TC) to the selected case depth; and (8) repeating steps (3)-(7) for a selected number of the trunnions.
More particularly, the present invention also provides a method of induction heat treatment of a spider having a barrel-shaped outer surface, a plurality of cylindrical trunnion shoulders formed in the outer surface of the hub and a corresponding plurality of angularly spaced cylindrical trunnions extending from the shoulders, each trunnion shoulder having a trunnion shoulder surface, and each trunnion having a trunnion axis and a trunnion surface, comprising the steps of: (1) selecting an induction coil, which is adapted to receive a trunnion for heat treatment and apply a magnetic field to the trunnion surface and the trunnion shoulder surface; (2) placing a trunnion within the induction coil with its corresponding trunnion shoulder adjacent to the induction coil; (3) rotating the trunnion within the induction coil about the trunnion axis at a selected speed; (4) energizing the induction coil to apply the magnetic field and produce induction currents within the trunnion shoulder surface and trunnion surface of the article for a time sufficient to induce heating them to a heat treatment temperature (TH) to at least a selected case depth; (5) withdrawing the trunnion from the induction coil at a selected rate; (6) cooling the trunnion surface and trunnion shoulder surface of the article to a temperature (TC) to the selected case depth; and (7) repeating steps (2)-(6) for a selected number of the trunnions.
The present invention also includes an article, such as: a steel spider comprising a hub, a plurality of angularly spaced trunnion shoulders extending from the hub, each having a trunnion shoulder surface, and a corresponding plurality of angularly-spaced trunnions extending from the plurality of trunnion shoulders, each trunnion having a trunnion axis and a trunnion surface, the trunnion surfaces and the trunnion shoulder surfaces comprising a hardened case, wherein the hardened case is formed by an induction heat treatment.
Certain difficulties associated with the inductive heat treatment of components having an irregular outer surface, such as the spider of tripod-type CV joints, have been overcome by the use of the method of heat treatment and induction coil described herein.
The present invention undertakes to improve the production of such components as compared to previous methods, such as carburizing, by enabling the use induction hardening, and thereby providing better control over the hardening process by hardening one component at a time, improving the metallurgical and mechanical and properties of the components, and allowing for a reduction in heat treatment cost.
The hardening operation will be simplified, and allow improved control, by the application of this invention because the components will be processed one at a time. The integration of the part location, heating, and quenching functions into a single, robust machine simplifies the heat treatment operation compared to previous methods, such as carburizing, by reducing the part handling requirements and reducing complex cycle parameters (e.g. adjusting the entire process for part-to-part variations in a batch of parts due to different temperature and environmental conditions that exist in a large heat treating furnace) to a small set of control parameters for each individual part (e.g. power, induction time, quench flow rates, etc.). Enabling the automatic control of process variables, such as the power level, total power delivered, quench temperature, quench flow rate, and cycle timing parameters, along with other process variables, will enable improved process control.
The mechanical properties of the components may also be improved by the selective application of heat in only the areas where high hardness is desired to give more precise control over the hardness and properties of the critical areas of the component while minimizing distortion from the hardening process.
Benefits from this invention include increased component strength (as compared to components processed by conventional methods such as carburizing), use of lower cost materials, shortened process times, reduced forging costs, reduced distortion, improved microstructures, improved tool life, deeper case depth capabilities, and the use of cellular process lines.
Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.
The present invention will become more fully understood from the detailed description given below, the appended claims, and the accompanying drawings in which:
Referring to
With regard to the step of selecting 10 an article 100, this method of induction heat treatment is ideally suited for the induction hardening heat treatment of articles 100, such as spider 200, comprising a hub 210 having a plurality of angularly-spaced trunnions 220 or shafts extending from the hub surface 215, as illustrated in
While it is believed that the method of the present invention may be used for the induction heat treatment 1 of a number of articles 100 of the type described above,
Referring to
Tripod joints are presently made by a number of manufacturers. This being the case, there are many variations in the particular features and details of tripod joints and their associated spiders 200, including variations of the size, including the thickness and diameter, the degree and type of curvature of hub surface 215, the shape and size of trunnions 220, the composition of the material and methods used to form spider 200, and other features. However, while some differences exist, most spiders comprise pearlitic/ferritic steels, and it is believed that the present invention is applicable to many of the spiders currently being manufactured for tripod CV joints.
Having selected 10 article 100, such as spider 200, the method of heat treatment 1 comprised the additional step of selecting 20 an induction coil 300. Referring to
Induction coil 300 is also adapted to apply a planar magnetic field to the trunnion surfaces 235 and trunnion shoulder surfaces 245 of trunnions 220. By planar, it is meant that the centerline of the magnetic field that results when induction coil 300 is energized, which roughly corresponds to the centerline of the tube, defines a plane. Referring to
Referring to
Referring to
Referring to
Referring to FIGS. 7D-F, the next step of method 1 comprises withdrawing 60 the trunnion 220 from the induction coil 300 at a selected rate along its longitudinal axis 306 so as to scan trunnion 220 within induction coil 300 and gradually withdraw trunnion 220 from induction coil 300. In the case of spider 200, the withdrawal or scan rate of the trunnion was about 0.098 cm/sec. Further, it is preferred that the steps of energizing 50, withdrawing 60 and cooling 70 be coordinated to provide a dwell at the initial placement position, in order that that trunnion shoulder surface 245 be heated above the austenite transition temperature prior to initiating the step of withdrawing 60 and cooling 70. For example, in the case of spider 200, the dwell was about three seconds, after which trunnion 220 was withdrawn 60 to the position shown in
The next step of method 1 comprises cooling 70 trunnion surface 235 and trunnion shoulder surface 245 of article 100 to a temperature (TC) to the selected case 250 depth. This temperature (TC) can be any temperature that is lower than the heat treatment temperature (TH), but typically will be selected to produce certain desired transformation products within case 250. In the case of spider 200, the desired transformation product in case was martensite, hence, TC was selected to be below the martensite transformation temperature, which in the case of AISI 1050 was about 200° F. Cooling 70 comprised quenching trunnion in an aqueous quenchant comprising 3-5% of a commercially available polymer quenchant additive, Aqua Quench 251, for a time sufficient to cool trunnion surface 235 and trunnion shoulder surface 245 below TC. Quenching was accomplished by pumping a large volume of the quenchant through inductor coil 300 and quench ring 308 onto the trunnion surface 235 and trunnion shoulder surface 245. Quenching 70 was accomplished using quench ring 308 having a plurality of spray holes 310 in the lower surface of quench ring that were directed radially inwardly and downwardly towards longitudinal axis 306 of induction coil 200 as shown in
Referring to
Applicants believe that it is also possible to use method 1 to also produce a tempered martensite structure in case 250 by controlling the step of cooling 70 so that trunnion surface 235 and trunnion shoulder surface 245 are cooled by quenching such that TC is in a range that is below the martensite start temperature (about 610° F.) but greater than the martensite finish temperature (about 200° F.), and then permitting the part to cool under ambient conditions, such that the martensitic structure is tempered by the residual heat in article 100 by means of the reduced cooling rate. Quench composition and concentration, temperature, flow rates and time are adjusted to allow the use of residual heat to sufficiently temper (stress relieve) the part, thereby eliminating the need for secondary tempering processing. It is believed that this will reduce the residual stresses in case 250, as well as the hardness to a range of about RC58-63. Applicants believe that this can be accomplished by shielding previously heat-treated trunnions from subsequent quenching 70, such as by use of a quench shield 320 as shown in phantom in
Following the step of cooling 70, method 1 comprises repeating steps 30 through 70 for a selected number of trunnions 220. In the case of spider 200, it is preferable to perform these steps on all three trunnions 220. However, is it possible to apply method 1 to some or all of trunnions 220. Further, it is will be appreciated that the heat treatment for each trunnion 220 need not be the same, if the design criteria for trunnions 220 are not the same. Further, the heat treatment can be varied along the length of each of trunnions 220 and trunnion surfaces 235 should the design criteria so require.
Following induction heat treatment 1, spider 200 may optionally be hard turned to produce the finished dimensions of article 100.
The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.
