Abstract:
A double carboaustempering combined with a martensite-producing quench provides plain-carbon and low alloy steel power transmission shafts with a carbon-rich exterior having a martensite and bainite microstructure and a substantially bainite interior. The shafts offer increased fatigue resistance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to torque transferring low carbon steel shafts, for example, drive shafts for motor vehicles, and to a process for their preparation by carboaustempering with grain refinement. 
     2. Background Art 
     It has been known for many centuries that the physical characteristics of steel are strongly dependent upon its thermal and mechanical history. Frequently, steel parts are provided in their final shape or near net shape, and thus mechanical operations such as forging, rolling, etc., cannot be further used to alter physical properties. However, thermal treatments are still available for use with such parts. 
     It is modernly understood that the microstructure of metals and metal alloys can be quite complex. Even in plain-carbon steel and other low alloy steels where carbon is the principal non-ferrous ingredient, a variety of phases are known to exist, for example, the cubic face centered structure of austenite and the body centered tetragonal structure of martensite, as well as ±-ferrite, cementite, ledeburite, pearlite, and bainite. Transformation of one phase to another may take place within certain temperature ranges, and often, the degree to which a transformation takes place will be markedly affected by quench rates. For example, if the quench rate is high, the steel may be “frozen” in a morphology which is incapable of being formed in samples which are slow cooled. For these reasons, there are a myriad of possible heat treatment processes, each of which generates its own combination of physical characteristics, such as hardness, tensile strength, elongation, ductility, etc. In addition, samples of steel having had different thermal histories can exhibit markedly different fatigue resistance. 
     In addition to heat treatments which can, depending upon part size and geometry, affect the entire structure, there are heat treatments which affect mainly the outside of the structure, for example, carburizing which may be used to surface harden parts (“case hardening”) to achieve a more wear resistant and harder exterior combined with a more ductile interior. 
     Power transmission shafts must be strong and fatigue resistant. The stresses imparted to such shafts is rarely constant, and even in “constant speed” devices, the loads are generally cyclical. In the vehicle sector, loads can vary widely. Moreover, power transmission shafts often have features such as splines, holes for lubrication, etc., which often lower fatigue resistance at these points. The strength and resistance to fatigue for such parts can be increased by choosing a stronger alloy steel, but this solution involves considerable extra expense. A larger section shaft can also be used, but this solution uses more space, often restricted by design, and also involves a considerable weight penalty. 
     Typically, such shafts are induction hardened, as illustrated in U.S. Pat. Nos. 6,319,337 and 6,390,924, which employ induction hardened low alloy steel. However, with ever increasing loads coupled with the desire to keep weight as low as possible, induction hardening has not proven satisfactory in providing the fatigue resistance desired. 
     It would be desirable to provide steel power transmission shafts which offer improved fatigue resistance without employing highly alloyed steels, and without increasing the size and weight of the shaft. 
     SUMMARY OF THE INVENTION 
     It has now been surprisingly discovered that a multi-stage heat treatment process, in which an intermediate dwell which allows transformation into a mixture of bainite and martensite, and which separates two high temperature carboaustempering treatments, provides greatly improved fatigue resistance. The last of the carboaustempering treatments is followed by quenching to an additional low temperature regime whereby the carbon-rich case transforms to a mixture of bainite and martensite, while the interior, or “core” is substantially bainite. The shafts are then tempered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a photo micrograph of an interior of a torque transmission shaft of the invention, illustrating the grain structure; and 
         FIG. 2  is a schematic of the heat treating regime of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to a dual carboaustempering of torque transmission shafts of low alloy carbon steels or plain-carbon steels. Both of the latter types of steel are well known to the skilled artisan. Plain-carbon steel generally contains no other major alloying element other than carbon, and may be exemplified by SAE/AISI 10xx series steels. Low alloy steels have only minor amounts of alloying elements, and by the ISO definition, contain between 1% and 5% of elements deliberately added for the purpose of modifying properties. Non-limiting examples of low alloy steels include SAE/AISI 41xx, 43xx, 51xx, and 86xx series steels. Thus, the steels suitable for use in the present invention are carbon steels containing less than about 5 weight percent of purposefully added alloying ingredients. Typical alloying ingredients used in these minor amounts include, but are not limited to, silicon, vanadium, chromium, manganese, nickel, titanium, cobalt, and the like. 
     Torque transmitting shafts are likewise well known to the skilled artisan, and may be found, for example, in rigid axles, as well as in substantially exposed shafts in vehicles having independent suspension. The shafts are generally splined on at least one end, and often on both ends. One end may be equipped with a fork for a universal joint or other attachment means. Examples of power transmission shafts may be found in U.S. Pat. Nos. 6,319,337; 6,390,924; and 4,820,241, which are herein incorporated by reference. Such shafts may also be used in other applications such as large water pumps, stationary electrical generators, and the like. 
     The shafts are formed by conventional forging and machining steps, and are then heat-treated by the process of the invention. Some machining steps may be left until after heat treatment, if desired, but such steps generally do not include those which remove large amounts of surface material, since the microstructure is profiled and not constant throughout the part. 
     The steel shafts are first heated in a furnace to a temperature which will cause austenite transformation, e.g., 925° C.±50° C. A carbon-rich environment is provided by conventional methods. This is a carboaustempering process, and is well known. The shafts are held at this temperature for a period sufficient to develop a carbon-rich case on the shaft, preferably for a period of from 60 min. to 720 min., more preferably min. 360 to 600 min. The actual time of treatment necessary can be found by analysis of treated parts, and in general will vary with the type of steel, and also with part geometry, particularly thickness. The shafts are then rapidly quenched in molten salt to a temperature at which a phase transformation of austenite to a mixture of bainite and martensite occurs, e.g., 200° C.±50° C. The duration of this intermediate “dwell” is preferably from 30 min. to 120 min., more preferably 30 min. to 45 min., and as with carboaustempering, is both substrate and shape dependent. 
     Following the intermediate dwell, the shaft is again carboaustempered, for example, at 925° C.±50° C., but for a shorter time than before, for example, a period of 60 min. to 240 min., preferably from 60 min. to 120 min., and then again quenched and maintained at 200° C.±50° C., causing the case to transform to a microstructure containing both bainite and martensite, and the core to transform primarily to bainite. The shaft is held in the media for a time sufficient for this transfer to occur, for example, from 120 min. to 480 min., preferably 240 min. to 360 min. 
     The shafts are then tempered, for example, at 225° C.±50° C., preferably from 30 min. to 150 min., more preferably 60 min. to 120 min. A schematic of the overall process is illustrated in  FIG. 2 . 
     Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. 
     A series of identical power transmission output shafts of SAE/AISI 8620 steel and 18 inch length as used in the General Motors 4L70E transmission were carboaustempered in a single stage (Comparative Examples C1-C6) and subjected to cyclical torque loadings at a frequency of 5 Hz as indicated in Table 1. 
     Also tested were shafts doubly carboaustempered by the process of the subject invention. The first carboaustempering took place at 925° for 495 min., followed by quenching in media to 200° and held for 30 min. The shaft was then replaced in the furnace and carboaustempered for the second time at 925° for 60 min., followed by quenching to 200° and holding for 260 min. Tempering then took place at 225° for 110 min. The results of the fatigue test are presented in Table 1. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Exam- 
                 Max 
                 Min 
                   
                 Life, 
                 Increase 
               
               
                 ple 
                 (Nm) 
                 (Nm) 
                 Description 
                 Cycles 
                 % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 C1 
                 1762 
                 −1762 
                 singly Carboaustempered 
                 97489 
                 — 
               
               
                 1 
                 1762 
                 −1762 
                 double heat treated 
                 169245 
                 74 
               
               
                 C2 
                 1762 
                 −1762 
                 singly Carboaustempered 
                 118198 
                 — 
               
               
                 2 
                 1762 
                 −1762 
                 double heat treated 
                 156887 
                 33 
               
               
                 C3 
                 1545 
                 −1545 
                 singly Carboaustempered 
                 200003 
                 — 
               
               
                 3 
                 1545 
                 −1545 
                 double heat treated 
                 440972 
                 120  
               
               
                 C4 
                 1545 
                 −1545 
                 singly Carboaustempered 
                 447540 
                 — 
               
               
                 4 
                 1545 
                 −1545 
                 double heat treated 
                 509131 
                 14 
               
               
                 C5 
                 1328 
                 −1328 
                 singly Carboaustempered 
                 1290084 
                 — 
               
               
                 5 
                 1328 
                 −1328 
                 double heat treated 
                 2000000 
                 554+ 
               
               
                 C6 
                 1328 
                 −1328 
                 singly Carboaustempered 
                 1301786 
                 — 
               
               
                 6 
                 1328 
                 −1328 
                 double heat treated 
                 2950000 
                 127+ 
               
               
                   
               
               
                 * Test stopped 
               
             
          
         
       
     
     The results indicate that a very sizable increase in fatigue resistance occurs due to the heat treatment of the subject invention. On average, the fatigue resistance, as indicated by the number of cycles before failure, was greater by about 70%. Failure generally occurred, as expected, at a 4 mm lubricant hole or near a spline. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.