1、99FTM6 The Submerged Induction Hardening of Gears by: D.W. Ingham, David Brown Special Products American Gear Manufacturers Association TECHNICAL PAPER COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesThe Submerged Induction Hardening of Gears - David W
2、. Ingham, David Brown Special Products The 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 With examples of field failures directly attributable to Tooth by Tooth In
3、duction Hardening, there has been a negative feeling against the use of this process. This paper shows successes of this process founded on Process Development and Quality Control. A case for and against Tooth by Tooth Induction Hardening is layed out for the reader to draw their own conclusion. Cop
4、yright O 1999 American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia, 22314 October. 1999 ISBN: 1-55589-744-4 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesThe Submerged Induction Hardening of Gears Authors D.W Ingha
5、m (David Brown Special Products Ltd) and G Parrich (Consultant) Viewing the Gearing Industry from our base in the UK we feel the Passion built up against the Tooth by Tooth induction Hardening Process for Gears. This Passion is particularly strong in the USA. We would like to strike a blow against t
6、his Passion and this paper is intended for this purpose. In staking our Claim for the Process we do understand the anti-feelings - which are based on numerous examples of field failures directly attributable to the Process. Our Claim is founded on Process Development and Quality Control, features wh
7、ich are absolutely essential to the integrity of Gears manufactured using the Tooth by Tooth Induction Hardening Process We would seek to encourage Engineers to embrace this impressive tool and thereby achieve notable Gear Rating Enhancements. By attainment of an in-depth understanding of the Proces
8、s, together with rigorous Quality Control these Enhancements are worthy of the effort. This paper lays out the case for and against Tooth by Tooth Induction Hardening for the impartial. and partial observers to draw their own conclusions. INTRODUCTION The tooth bv tooth submerged Induction Hardening
9、 Process for gear tooth surface hardening has been successfully carried out at David Brown, for weil over thirty years During that time Process experience, backed up by in depth Research and Development, has given David Brown Engineers a much greater understanding of, and confidence in, the results
10、obtainable from the Process Also, field experience and the refinement in gear design and manufacturing procedures specifically to accommodate the Induction Hardening Process now ensures that gears so treated are of guaranteed quality lhe purpose of the process is to produce a continuous hardened lay
11、er which extends along the entire length of the tooth, and from the tip of one tooth, down the flank, around the fillet and root area and up the opposite flank to the tip of the next tooth (Fig 1) and to ensure that the depth of the hardened zone is sufficient so that in service the subsurface high
12、tooth stresses are contained within the high strength regions In the submerged, tooth by tooth process, the inductor (Fig 2). which has essentially the same shape as the space between two adjacent gear teeth, is energised and traversed along the tooth space heating and austenitising the neighbouring
13、 tooth surfaces, including the root-fillets, as it goes. The heating operation, which is of a short duration, takes place beneath the surface of the quenchant so that as soon as the inductor has moved on it is replaced by the surrounding quenchant; thus, heating and quenching are very localised. pro
14、gressive, and of short duration. The zone being heated and quenched during this process is CO localised that the distortion and growth problems, which tend to plague carburize case hardening, are essentially avoided. By virtue of a high surface hardness and the presence of the surface compressive re
15、sidual stresses, imparted by the process, the contact and bending fatigue strengths are dramatically improved. This article deals with many aspects of the process itself, describes problem areas, considers applications. and discusses the properties and the quality of the product Fig i. Typical Harde
16、ning Pattern Fig 2. The Inductoi COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Services- The Submerged Induction Hardening of Gears - Fig 3. Schematic diagram of the gear handling machine for tooth-by-tooth submerged induction hardening - THE INDUCTION HARDE
17、NING PROCESS At David Brown the frequency used for Gear Induction Hardening is 9.6kHz. and the range of tooth sizes processed is 8 to 38 module. The Facility is shown schematically in Fig 3. Adjacent to this is a generator, water cooling tank, oil circulation tank and control console. The Gear Handl
18、ing Machine rigidly supports the gear, accurately rotates aligns and indexes it during processing On this plant the water-cooled inductor is secured to a workhead transformer which itself IS mounted onto a carriage within the gear handling machine ( Fig 4 ) The workhead transformer can be set to tra
19、verse a distance of over 1 metre on linear bearing tracks The actual length of inductor travel is controlled by pre-set limit switches Because the plant is intended for the submerged version of the process with the inductor at the bottom centre position, much of the handling equipment is contained i
20、n an open tank which is filled with quenchant during processing, and drained for loading and setting up. The generator which provides up to 75 Kw of energy for the process, converts the mains power supply of 380volts. 50Hz to a medium frequency (9.6kHz.1 supply at a nominal voltage of 500v. This is
21、transformed to a supply of 50v. energy by the 14:l workhead transformer in the gear handling machine. The water cooling tank supplies three recirculatory lines: a 1 To the inductor, which is capable of some heating via its own resistance, and by radiation from the workpiece during processing. b To t
22、he quenchants heat exchanger. c )To the generator and the workhead transformer. COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesD.W. Ingham and G. Parrish The control console manages the induction hardening process by control of the inductor traverse s
23、peed, inductor energising and de-energising, quenchant flow, cooling water supplies etc. Over many years David Brown have performed Research Projects on the Process in addition to Production Hardening. O Consequently relationships between hardening parameters and hardened depth/pattern have been est
24、ablished. As a result the need to establish parameters on separate test pieces has been eliminated. The Process is controlled by several significant parameters. these being: 1) The inductor Workpiece Gap This is the space between the inductor and the gear tooth and is quite critical. The surface to
25、volume ratio differences at locations around the tooth profile, demand different energy requirements. Consequently the shaping of the inductor (Fig 5 I is important to optimise the coupling. The inductor is designed for rigidity to ensure accurate geometrical positioning. Fig 5 Typical inductor-to-w
26、orkplace coupling Research has shown that the heating effect is controlled by the design of the Inductor The David Brown design incorporates two copper sides connected by a copper bridge along the root Strategically placed thermocouples within the body of a tooth being hardened have shown a typical
27、temperature profile Fig 6 On the mid flank position two temperature peaks are experienced. coinciding with the passage of the copper sides of the inductor In the root position a single peak is found, associated with the copper bridge The practice at David Brown involves the exclusive use of Numerica
28、lly Controlled Machine Shaping of Inductor Blanks The use of accurate shaping means that it is only necessary for the operator to ensure that the inductor is aligned. central to the tooth space, and that the root gap is correct When this is done, the inductor to work-piece gaps at other positions ar
29、ound the inductor will be correct 5 mad 35kW power 150rnrnmin traverse speed 0.5 150rnrn:min 2 I 6 6 10 12 Ia 16 18 x) 22 II 26 26 30 32 Tim. rnomr Fig 6. An example of the temperature distribution within a gear tooth during an inductor pass. The depth and location of the temperature sensor is indic
30、ated against each temperature curve. 2) The Power As the Power is increased so the depth of heating is increased for a given tooth size. It naturally follows that the larger the tooth size the larger will be the Power requirements. 3) Inductor Traverse Speed Traverse SQeed also determines the depth
31、of heating by allowing more time for heat diffusion. Sufficient time should be available to allow transformation to Austenite and Research by a dilatometry study showed that for an 81 7M40 (4340) steel in the quenched and tempered condition, three seconds were required to achieve carbon solution, an
32、d that a degree of coarsening with a slight reduction of hardness took place after nine seconds. Therefore, heating durations within the three to nine second range are normal for the process, which means that if the inductor has an effective length (time above AC3) of 18mm. the traverse speed range
33、will need to be in the approx range of 125 - 350mm/min. 4) Quenching and cooling jets Surrounding the mounted inductor are a) the fore and aft quenchant curtain jets which help to stabilise the vapour phase which occupies the coupling space, and also hasten and control the quenching b) the side spra
34、ys, also curtain jets. which play on the tooth top edge and adjacent flank so as to control the heating pattern on the top of the tooth, and the amount of back-tempering on the adjacent tooth addendum The settings for these jets. and the quantities of quenchant flowing through them is important 5) P
35、ower switching When a tooth space is to be hardened the inductor is automatically advanced into the tooth space to a distance equal to about half the inductors length. At that point the inductor is energised, and after a short dwell at the entry the traverse of the inductor along the tooth space com
36、mences. Similarly at the exit end of the tooth, the inductor stops, dwells, and is theq de-energised. This generally ensures a satisfactory hardening pattern at the tooth ends. However, experience has shown that on occasions the exit pattern could be improved by cancelling the dwell, and running thr
37、ough on full power. or by running through and de-energising during the exit. These, of course, are minor adjustments aimed to ensure a good product. COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesThe Submerged Induction hardening ot beats short term t
38、empering The end of the hardened zone denotes where the temperature, due to induction heating, had attained the Al value of say 725C , but if the steel had been previously tempered at 650C the core immediately beneath the case will have experienced heating to within the 650C to 725C temperature rang
39、e, and hence some additional tempering 2) Microstructures O STEELS FOR INDUCTION HARDENING At David Brown we have adopted the policy of using only Medium Carbon Alloy Steels of the 4340 type composition for Induction Hardened Applications 4140 is also used in limited quantities. These type of materi
40、al will produce a pre- tempered surface hardness of over 57HRC. and a tempered surface hardness of typically 55HRC. In these days of inherently clean steels the basic quality of the material is not a problem for the hardening process. The blanks from which the gears are cut, are through hardened and
41、 tempered either as forgings. or after rough machining. One aim of tempering should be to eliminate residual stresses in the gear therefore high tempering temperatures (600 deg C) should be employed. The tempered martensitic microstructure so produced is the most suitable one for induction hardening
42、. this being because the structure is homogeneous with respect to carbon, and the particle size of the carbides is small which favours easy solution during the short induction heating period, Le., 3 to 10 seconds. The as-hardened and tempered strength need not exceed about 1000 N/mm2. and therefore
43、gear cutting and other machining operations are not difficult to perform. 700 160 6oo t Fig 7. Typical Hardening pattern PROPERTIES RESULTING 1) Hardness A typical hardness distribution is presented in Fig 7. Induction hardened surfaces, for which the carbon content is nominally O 40%c usually have
44、hardness values of over 55HRC and UP to GOHRC as hardened. Tempering at 200/250C reduces lhe hardness slightly to about 54-57HRC. Two features of interest will be noted: an added plateau of hardness ( broken line i, and a trough in the curve just below the case-core junction. The first feature, whic
45、h is occasionally observed, may relate to the extent of carbon solution, and to the degree of carbon hornogenisation within the austenite phase, noting that for a steel such as 4340 it will take about three seconds to dissolve the carbides but more time than that to achieve a modest degree of homoge
46、nisation. Solution and homogenisation are better served by having the fine carbide characteristic induced by previous hardening and tempering. The trough at the end of the hardened zone is attributed to The hardened layer of an induction hardened and low temperature tempered material usually consist
47、s of fine tempered martensite, and the structure has a very much refined austenitic grain-size; though this is not usually apparent. Process parameters are selected such as to avoid the development of coarse martensitic microstructures which can have a negative influence on the toughness of the hard
48、ened layer. The microstructure of an induction hardened layer does not always appear to be martensitic, but sometimes tends to resemble the original quenched and tempered structure, though very much finer. Nevertheless, the hardness values attained by induction hardening are typical of the martensit
49、ic condition. 31 Residual Stresses Heating of a steel surface by induction currents will be accompanied by a thermal expansion. and a superimposed contraction when the material passes through the austenite transformation temperature range. As a result of this. there is the possibility that yielding will occur somewhere wiihin the heated layer: probably close to the eventual case/core junction. and this will contribute to the residual stress distribution. However, the main contribution to the development of residual stresses will be that due to the martensitic transformation. T
