1、13FTM25 AGMA Technical Paper Press Quenching and the Effects of Prior Thermal History on Distortion during Heat Treatment By A.C. Reardon, The Gleason Works 2 13FTM25 Press Quenching and the Effects of Prior Thermal History on Distortion during Heat Treatment Arthur C. Reardon, The Gleason Works The
2、 statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract Precision components such as industrial bearing races and automotive gears often distort unpredictably during heat t
3、reatment due to the deleterious effects of free or unconstrained oil quenching. Press quenching is a method that can be utilized to minimize the distortion of these complex components during heat treatment. This is accomplished in a quenching machine by utilizing specialized tooling for generating c
4、oncentrated forces to constrain the movement of the component during oil quenching. When performed correctly, this method of quenching can often achieve the relatively stringent geometrical requirements stipulated by industrial manufacturing specifications. It can be performed on a wide variety of s
5、teel alloys. These include high carbon through-hardening grades such as AISI 52100 and A2 tool steel, as well as low carbon carburizing grades such as AISI 3310, 8620, and 9310. The relevant aspects of this specialized quenching technique will be presented together with a case study of the effects o
6、f prior thermal history on the distortion that is generated during press quenching. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 September 2013 ISBN: 978-1-61481-082-7 3 13FTM25 Press Quenching and the Effects of Prior Thermal Hi
7、story on Distortion during Heat Treatment Arthur C. Reardon, The Gleason Works Introduction Press quenching is a specialized quenching technique that may be utilized to minimize the distortion of complex geometrical components during heat treatment1. Distortion is routinely encountered in industrial
8、 heat treating operations, and is an especially important consideration where high accuracy, precision components are concerned. It can result from a wide variety of independent contributing factors. In press quenching these can include, among others, 1. The quality and prior processing history of t
9、he material from which the part in question has been manufactured; 2. The prior thermal history and residual stress distribution contained within the part; 3. The generation of unbalanced thermal and transformation stresses induced by the quenching operation; 4. The grade of material and austenitizi
10、ng temperature that is used; 5. Transfer time between the austenitizing furnace and the quenching machine; 6. The type, condition, quantity, and temperature of quenchant used; 7. Direction and selective metering of quenchant flow over the component; 8. Duration of the quench at various flow rates; 9
11、. Locations of contact points on the component for applying external loads; 10. Magnitude of the forces applied for maintaining the required part geometry; 11. Proper quench die tooling design, set-up, and maintenance; 12. Pulsing methodology. Distortion issues during quenching High precision compon
12、ents such as automotive spiral bevel gears and aerospace quality bearing races can often distort appreciably during open tank oil quenching. Press quenching can help to minimize the distortion of such components by utilizing specialized tooling for generating concentrated forces at key locations to
13、constrain the movement of the component in a carefully controlled manner. It can be performed on a wide variety of components manufactured from both ferrous and non-ferrous based alloys. For example, a number of aluminum alloys are routinely press quenched. Common steel alloys that are press quenche
14、d include high carbon through-hardening grades such as AISI 52100 and A2 tool steel. Press quenching is particularly well suited for the processing of carburizing steel grades such as AISI 9310, 8620, and 3310. Ideally, the transformation temperature should be the same throughout the entire cross-se
15、ction of the component during quenching so that the material is capable of transforming in a uniform manner. However, in case carburized parts the martensite start transformation temperature (Ms) is not uniform throughout the entire cross-section of the part. During carburizing a composition gradien
16、t is produced as carbon is diffused into the part surface. This results in a corresponding gradient in the transformation temperature near the surface that can promote or aggravate distortion issues in such components during quenching. Non-uniformities in the base material microstructure due to segr
17、egation or improperly normalized material can also contribute to this type of distortion. Thin walled components such as large diameter bearing races are generally more susceptible to these distortion related issues than are relatively massive, compact geometries. Although press quenching cannot eli
18、minate these effects, its use can help to minimize their contribution to the overall distortion that is produced during heat treatment. The amount of distortion encountered depends strongly on the nature of the heat treating process that is used. In order to minimize distortion related issues during
19、 quenching, heat should be extracted from the 4 13FTM25 component in as uniform a manner as possible. For parts that are designed with sudden changes in geometry with heavy or thick sections located adjacent to relatively thin sections on the same component, this can be difficult to achieve. A good
20、example of this is the teeth on a spiral bevel gear or pinion. The teeth have a greater surface area to volume ratio than the body of the gear or pinion, and due to their symmetrical nature and distribution the teeth have a tendency to distort by unwinding during quenching. As the work piece is subm
21、erged into the quenching medium, the teeth tend to cool and contract much more rapidly than the adjacent heavier sections. As a result of this varying quench rate, the teeth harden more rapidly and contract while the balance of the component is still in an expanded state. The outcome of quenching su
22、ch components is the generation of temperature gradients and non-uniform transformation induced stresses. This particular issue can be addressed in press quenching by selectively directing the quenchant flow toward the thicker sections and baffling it away from the teeth in order to promote a more u
23、niform quench. By implementing this important technique, lower levels of transformation induced distortion can be achieved. Press quenching machines A representative version of a standard quenching machine is depicted in Figure 1, and examples of the numerous parameters that may be adjusted during t
24、he course of a typical quenching cycle are shown on the machines main control screen in Figure 2. During operation, the component to be quenched is removed from a separate furnace (usually a box, continuous rotary, or pusher type furnace) and is placed onto the tooling of the lower die assembly. A c
25、lose up view of this die assembly is shown in Figure 3. Figure 1. Example of a modern quenching machine (Photograph courtesy of The Gleason Works, Rochester, NY) 5 13FTM25 Figure 2. Control screen showing the various parameters that may be adjusted over the course of a typical quenching cycle (Photo
26、graph courtesy of The Gleason Works, Rochester, NY) Figure 3. Hot bearing race positioned on the lower die assembly of a quenching machine just before it is retracted into the machine for quenching. Note the segmented die tooling and individual slotted rings. The slotted rings may be independently a
27、djusted to control the flow of oil over the part being quenched. (Photograph courtesy of The Gleason Works, Rochester, NY) 6 13FTM25 After the part is successfully loaded onto the lower die assembly the machine is actuated and the part is retracted into the machine where it is centered below the upp
28、er hydraulic ram assembly. As the assembly descends the center ram actuates one or more internal expanders that make contact with the inner diameter of the component at the specified points to maintain roundness at these locations (see Figure 4). Each component of the ram assembly (the center expand
29、er, inner and outer dies) is controlled independently through three separate proportional valves. A predetermined pressure level is usually maintained by the expander throughout the quench cycle. The inner and outer dies are lowered to make physical contact with the upper surfaces of the component b
30、eing quenched in order to control alignment, dish, and part flatness during the course of the quenching cycle. The flow of quench oil is then activated to quench the part. Figure 4 illustrates an example of a quench oil circulation path that can be established within the quenching chamber. Oil is pu
31、mped into the quench chamber through apertures around the outside diameter of the lower die. As the chamber fills up surrounding the component, oil flows out of the top. If the tooling is properly designed, the direction of oil flow over the component can be adjusted to obtain the best overall resul
32、ts. The elongated apertures at the exit may be adjusted to restrict oil flow, or may be fully opened to maximize flow depending upon the requirements for the part in question. The lower dies are constructed from several different concentric slotted rings that may be rotated to provide full flow or t
33、o restrict oil flow to the underside of the part. These particular features can be finely adjusted to help minimize the degree of distortion attributed to uneven heat removal during quenching. Timed segments during the quenching cycle can also be utilized to vary both the oil flow rate and duration
34、in order to establish a well-defined quenching recipe for a specific part design. Figure 4 Schematic cross-sectional diagram illustrating the contact of the center expander and the inner and outer dies with the part during quenching. The various components labeled in the diagram are: (1) the machine
35、 guard attached to the upper die assembly; (2) outer upper die; (3) inner upper die; (4) component undergoing quench; (5) lower die assembly; and (6) center expander cone. The oil flow path through the quench chamber is depicted by the flow line arrows. (Image courtesy of The Gleason Works, Rocheste
36、r, NY) 7 13FTM25 The oil quenching process itself may consist of up to three general stages: (1) the initial vapor blanket stage where the first oil to come in contact with the part is instantly vaporized and forms a vapor barrier that surrounds the part and acts as an effective thermal insulting la
37、yer; (2) the vapor transport stage where oil breaks through the vapor blanket resulting in more rapid heat transfer; and (3) the liquid stage where heat extraction occurs predominately by convective heat transfer. In order for uniform heat extraction to occur during the initial stages of quenching,
38、the oil flow rates must be sufficient to prevent the formation of a vapor blanket. If vapor bubbles are allowed to form in areas around the surface of the component, uneven heat extraction will result that can lead to unacceptable hardness variations and distortion. After this initial quenching stag
39、e has been successfully eliminated, lower quenchant flow rates may be safely tolerated. The quenchant flow rate profile that is ultimately established for the part in question must be carefully selected so that the hardness and geometry requirements are satisfactorily met. Too slow of a quench rate
40、will result in a slack quench, undesirable transformation products, and hardness variations. Too rapid of a quench rate could result in cracking and/or unacceptable part distortion. The establishment of an oil flow path around the part and selection of the proper oil flow rate are often determined u
41、sing a trial and error process. Success frequently depends upon the knowledge, experience, and skill of the machine operator. The average oil temperature for most press quenching operations typically falls somewhere within the range of approximately 75F to165F, depending upon the material in questio
42、n, the nature of the quenching operation, the type of quench oil being used, and post heat treat property requirements. Average quench oil temperatures exceeding 140F should generally be avoided as a precaution to prevent damage to the machine seals that are used to contain the quench oil. Proper an
43、d routine maintenance of the quench oil bath is an often neglected aspect of the press quenching process, and can lead to unexpected variations in the hardening response of the materials processed in these types of systems. As the quench oil continues to be used the oil additives gradually break dow
44、n, and fine particulates can accumulate over time even if the oil is continuously filtered. If left undetected, this can lead to accelerated quench rates which can compromise the integrity of the oil quenching process. A specific die tooling design configuration and machine set-up is required for ea
45、ch component that is press quenched. The use of expanding segmental dies are often employed to maintain bore size and roundness in bearing races and gears. If a component possesses a bore diameter that is physically too small to accommodate these segmental dies, a solid plug could be used instead to
46、 control the diameter and taper of the bore. The plug would simply be removed after quenching. When there are different locating surfaces on the lower die assembly it is imperative that the dimensions between these surfaces be held to a close tolerance from piece to piece. Failure to adhere to this
47、rule may result in unwanted distortion and/or inconsistent results. Contracting dies are also available to maintain the geometrical tolerances for the outside diameter of components where this is a critical factor. A good example of this are gears which incorporate thin web sections in conjunction w
48、ith relatively heavy sections for gear teeth, bosses, and bearing diameters. Gears used in aerospace applications such as helicopter gear assemblies often incorporate several of these features which may cause them to contract unevenly during quenching. This problem can be effectively remedied by the
49、 application of compressive loads on the outside surface of the component during press quenching. It should also be noted that the inner and outer dies are typically pulsed during quenching to maintain the geometry of the part and to minimize distortion. The pulse feature periodically eases the applied pressure exerted by the inner and outer dies, allowing the component to contract normally as it cools while still maintaining the desired part geometry. If this feature was not incorporated (and on some of the older machines it isnt available), the stresses that would be induced from
