1、91 FTM 15AvGear Hardness Technologyby: M. J. Broglie and D. F. Smith, Dudley Technical GroupAmerican Gear Manufacturers AssociationA- TECHNICALPAPERGearHardnessTechnologyM. J. Broglie and D. F. SmithDudley Technical GroupTheStatementsandopinions contained hereinarethoseof the authorandshouldnotbe co
2、nstrued asan official action oropinion of the American Gear ManufacturersAssociation.ABSTRACT:As progress continuallyplaces demands on the geardesignerto makegearing that is smaller, lighter and more reliable,the demand for bettergear materials and heat treatprocesses increases. Proper hardnessof a
3、gear, both inthe tooth and inthe body is becoming increasingly critical since load carrying capacity is dependent on hardness.The scope of this paper is limited to the most common methodsof heat treatmentof steel gearing, yet within this areathere are still many methods of heat treatment in wide use
4、 throughoutthe industry. The fact that there are so manymethods in use is testimony to the shortcomings of each process. When a designer selects the material and heattreatment to be used, among the considerations are the size of the gear and the gear tooth, facilifes available for heattreatment, the
5、 overall cost associated with the material and process, and the reliability which is required by theapplication of the gear.Copyright 1991American Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia, 22314October, 1991ISBN: 1-55589-612-XGEAR HARDNESS TECHNOLOGYMichael J. Br
6、oglie and Danny F. SmithDudley Technical Group IncorporatedSan Diego, California1.0 Introduction. the machinability. Intercritical annealinginvolves heating the part to a temperatureIn a very general sense, increasing the above the AI point but below the A_ point.- hardness of a steel gear increases
7、 the Finally there is subcrltical annealihg whichstrength of the gear. However, for each heats the part to just below the firstv process there is a limlt to its effective- transformation temperature (AI), as inness. This paper contains background infor- tempering, and it is slow cooled; just as inma
8、tlon on each of the processes covered. In full annealing. Subcritical annealing iseach section what is desired and what is often done to stabilize the structure priorachievable is discussed. Typical processes to carburizing.are presented along with comments on vari-ables which affect the result. By
9、reviewingthe capabilities and processes it is pos- Normalizing. Normalizing is a process whichsible to determine the limits to each pro- involves heating the part to above the uppercess. critical as in annealing, but it is cooledoutside the furnace in still or agitatedThroughout this paper several h
10、ardness air. Normalizing is done to relieve resid-scales are mentioned. The abbreviations for ual stresses in a gear blank and for dimen-these scales are as follows: sional stability. A normalized part is verymachinable but will be harder than if itBHN Brinell hardness number were annealed.KHN Knoop
11、 hardness numberHRC Rockwell “C“ scaleHV Vickers hardness number Stress relieving. Stress relieving isheating to below the lower transformation2.0 Prellminary Heat Treatment Processes. temperature, as in tempering, and cooling inair. This is done primarily to relieveThere are several heat treatments
12、 performed internal stresses. This process isduring the manufacturing process which are sometimes called process annealing.intended to condition the metal for manufac-turing. Since these are essential processesthey will be described briefly. 3.0 Through Hardening.Through hardening refers to heat tre
13、atment_nnealing. Annealing is a process in which methods which do not produce a case. Thisa part is heated and then slowly cooled in term does not imply that the hardness isthe furnace to 600 oF (316 C). Full anneal- uniform throughout the gear tooth. Sincev ing involves heating to a temperature abo
14、ve the outside of a gear is cooled faster thanthe upper critical (A_ point). This will the inside, there will a gradient in theresult in softening t_e part and improving hardness. The achievable hardness is basedon the amount of carbon in the steel. The The Process. To harden a part by thisdepth of
15、hardness depends on the hard- process, the part is heated to the aus- _enability of the steel, tenitic range, a temperature that varies,depending on the carbon and alloy content,For the purposes of this paper we will within the range of about 1500-1600 F (815- vconcentrate on the quench and temper p
16、ro- 870 C). In this state the steel becomescess. This method is used to obtain the austenite, which is a term for the solidfinal core properties of the material for solution of carbon in fcc iron (ref. 3).gears which are either cased or not cased. Then the part is rapidly quenched in oil (orWhen thi
17、s process is used to develop the sometimes water) to transform the austenitecore properties for nitrided gears, it is into martensite. If the quench is too slowdone prior to the nitriding cycle. When it the structure will not be fully transformedis used to harden a carburized gear, it is to martensi
18、te. The resulting microstructuredone after the gear has been carburized, will then contain what are called transfor-For gears which are not cased the load mation products, such as ferrite, bainite,carrying capacity of a gear is dependent on pearlite and cementite. The properties ofthe core hardness
19、of the material (The hardness, toughness, ductility and strengthcapacity of case hardened gears is primarily are dependent on the transformation productsdependent on case hardness). It is general- which are present.ly accepted to use the hardness value mea-sured at the root diameter in the center of
20、 The rate of cooling which must be achievedthe tooth when making comparisons to properly transform the steel to marten-site and minimize the percentage ofDepending on the loading the gear must transformation products is dependent on thehandle, it is often necessary to increase chemistry of the alloy
21、 being used. The a-the hardness of the steel. According to mount and type of alloying elements in theAGMA standards (ref. 1), a gear with a steel determine its hardenability.hardness of 400 BHN, which has a design lifeof 10 cycles, can handle as much as 20% Hardenability is a measure of the relative
22、more load than a gear which is hardened to depth to which hardness is achieved for a300 BHN. For hardnesses above 400 BHN the given quench rate and section thickness. Incapacity increases with respect to pitting other words, a material with a high hard-resistance, but the capacity decreases with ena
23、bility, which is quenched at the samerespect to bending strength, whlchdeterior- rate as a part of the same size, but withares because the tooth becomes brittle, low hardenability, will have hard material Adeeper.Though a great deal of attention is given tothe hardness of the material, it is import-
24、 The alloying elements which have an impactant to understand that the microstructure, on the hardenability of the steel are man-upon which the hardness depends, is what ganese, chromium, nickel, and molybdenum.really matters. Although in-depth discus- Table 1 is a table showing several alloysion of
25、microstructure is beyond the scope steels which are commonly used for throughof this paper, it is worth mentioning that hardened gears. A materlal such as AISIthe degree of martensitic structure is one 4140 is considered to be a low alloy steelof the prime indicators of a materials and has rather po
26、or hardenability. A mater-quality. AGMR 2004-B89 (ref. 2) does a good ial such as AISI 4340 is considered to be ajob of identifying other mlcrostructural rich alloy steel and has much better hard-aspects that must be considered, enability.Unlike most gear heat treatments, through 7ASLE1 _mmonThrou0h
27、H_denadO_ee_hardening is a process which can be per- _i _ Mn S Pformed either prior to or after the gear sl _ Mo N!teeth are cut. The hardness is achieved by m_ ,_,heating the material to the austenitic range _1046 0._ 0._ o.o5oo.o4o(usually to about 1500-1600 OF) and then _41_ o._ 0._ 0.04o0.0_ 0._
28、 o._ 0._ -quenching and tempering For situations _4140 0._ 0._ o.o4oo.o350._ 0._ 0._ - AIS141_ 0._ 0._ 0.040 0.0_ 0._ 0._ 0._ -when the teeth are cut after the material _m4_40 0._ 0.700.040o.0,_0._ 0.800._ _._has been hardened, machinability becomes a _slse40 o._ 0._ o.o4o0.0_ 0._ 0._ 0._ 0._conside
29、ration in determining the hardnessFor the most part, conventional gear cuttingprocesses (bobbing, shaping or milling) are Once the part has been quenched it needs tocapable of cutting materials with hardnesses be tempered to reduce the brittleness andof up to 400 BHN. Though 400 BHN is machin- tough
30、en the steel, since quenched martensiteable, gear teeth are much easier to machine is hard but also brittle Tempering throughwhen the hardness is lower, hardened parts is generally done at 400 to1000 oF (205 to 540 C) for a period of oneThere will be distortion if the hardening is or more hours, dep
31、ending on the size ofdone after the teeth are cut. It may be gear Higher tempering temperatures in-necessary to finish machine the teeth in crease the toughness but also lower theorder to achieve the required accuracy hardness.Limits on the Process. The quench and 4.0 Carburizing.temper process is l
32、imited only by the size- of furnaces and quench tanks which are As mentioned above, the alloying elements inavailable. Today, this is as large as a steel have an effect on the hardenabilityv several meters. From a practical stand- of the material. In earlier years it waspoint, the major limitation c
33、omes from the known that increasing the hardness of theability to quench gears fast enough to material increased the strength of the gear.obtain an acceptable microstructure. In This relationship held true up to a hardnesssome cases, particularly with lean alloy of about 40 HRC. At hardnesses above
34、thissteels, it is just impossible to quench level the material became brittle and thelarge gears fast enough to obtain an accept- gears failed in breakage faster than gearsable microstructure, with lower hardnesses. The idea behind casehardening is to keep the core of the toothTable 2 shows the comp
35、arison of time re- at a level which would not be too muchquired to achieve different levels of metal- beyond 40 HRC, to avoid tooth breakage, hutlurgical quality between AISI 4140, a lean to harden the outer surface, or “case,“ toalloy steel with poor hardenahility, and increase pitting resistance.A
36、ISI 4340, a rich alloy steel. In order tocompare the hardenability of a material, end Of the methods for case hardening gears,quench (Jominy) values are widely used as an carburizing is the process which is mostindicator of a steels hardenability, often used. The idea behind carburizing isto start w
37、ith a gear blank which has a lowSince the quench is so critical as to the amount of carbon in the base material and,resulting microstructure, it is necessary to then, to add carbon to the outer surface.verify the results with an appropriate A properly carburized gear will handlesample. Too often a t
38、est coupon is used between 30 and 50% more load than a throughwhich is quite small as compared to the hardened gear. Case hardening is donegears sections. The small coupon is rapid- primarily to increase the pitting resistancely quenched producing good results, while of tooth surface. However, becau
39、se of thethe cooling rate in the actual part is too residual compressive stress which is presentslow and produces a poor result (and this is in the case after carburizing, there is alsowhere it needs to be good), an increase in bending strength._ The Process. Carburized gears achievehardness by quen
40、ching as do through hardenedv gears. The difference is that a carburizedgear has an increased amount of carbon inTABLE2 ApproximateQuenchlngBehavtor the surface causing this area to become aof 4140 and 4340 Steelhard case after quenching, while the lowerq_ch timeto SOOF, carbon core reaches a lower
41、hardness.Quench secondseffectiveness Structure to be expected4z4o 4340 Carburizing steels are alloy steels with- approximately i0 to 20 points of carbon.A 23 80 E_eZZ_.t The process involves heating the gears to aslight amount to so much that the gear musthe scraped. Since the hard case is rela-Poo_
42、._u_Zly,or,_eptablefo_hi_ tively thin, grinding to restore toothD 300 i000 performanceparts accuracy may be so deep on one tooth side(IOW In msrtenslte, _Ith much bainite,pearlite, andfreeferrite) that the remaining case is too thin.w_yp_zy,otprize Due to the propensity to distort it isF 800 7000 (p
43、earllte, free ferrite, some bainite,mayb. termite) recormnended to stress relieve the gear blankbefore machining and, possibly, again one ormore times before carburizing. In reallyNotes: critical jobs it may be necessary to put theI. A structure not quenched out to full martenslte will sot be fixed
44、up blanks through a “mock“ carburizing cycle.hyt=p,rl,g. A mock carburizing cycle exposes the blank2. Material in the “F“ situation could be tempered to meet about 300 tO the temperatures and cycles it will seegrlnell mlnlm_m. (The “as quenched“ hardness would be above 300 EB.) and the blank sti 11
45、remains machinable since3. Materlal in the “F“ situation would probably fail to mee_ Charpy V no diffusion of carbon takes place.notch and ductlllty requirements normally expected for a good steel.In addltlonp the fatigue strength would be poor. The actual carburizing is done by heating4. Poor quenc
46、hl.g results can result from things lfke an improper prior the gear blanks to above the critical tem-srueture or the wrong sustenltizlng temperature. (A slow quench isnot the only re_son for a poor structure.) perature and exposing the surfaces to car-bon. The carbon can be a solid, liquid or tained
47、 austenite. The core structure isgas. As most carburizing is gas carburiz- unrefined. This method is used in the auto-ing, the discussion here deals with this motive field singe automotive gears rarely -method. The carburizing is done in a fur- see more than 10“ cycles. Also, since thenace which con
48、tains a carbon atmosphere such production is high and the facilities and vas natural gas. Above the critical tempera- tooling used for automotive gearing areture the carbon diffuses into the material highly developed, it is possible to obtainon the surface. The amount of carbon in the acceptable res
49、ults.atmosphere must be controlled. Too muchwill cause carbide networks to form at the Applications which require a high level oftooth tips and too little will produce material quality are cooled and then reheat-shallow case depths, particularly in the ed prior to quenching. In some cases it isroot areas. The amount is measured in terms also necessary to deep freeze the gears soof percent and is referred to as the carbon that transformation to martensite is com-potential. The optimum carbon potential plete.which leads to the highest surface hardnesswill vary depend
