_------------HEATTREATING FOCUSi. _ Austempered Gears and Shafts: Tough Solutions Kristin Brandenberg, Ka,thy L Havrynen, Ph.D. and John R"Keough•.P.E. Definitions, Acronvms and Abbreviations Abstract TEMPERATURE I-T DIAGRAM Austempered irons and steels offer ADI: austempered ductile iron. 'c 'f, .Ir1r'I~:t)::1,"" .'1.'. AGI: austempered gray iron. 800 1 p FCC AUSrE NI LE ...... ~_ the design engineer alternatives to con- I 14() .... Ausfenite: acicular ferrite and austenite. 700 ~~r~ - ~. ~ ventional material/process combinations. Austempering: a special, isothermal heat 1200 / Depending on the material and the appli- treatment process that can be applied to fer- rous materials. Austempering consists of 600 1000 " I' F+r"-tlt-+++-H--t--+-t-+-H cation, austempering may provide the 500 I \ PAl. TE austenitizing, followed by rapidly quenching producers of gears and shafts with the fol- 800 toa temperatura where the material is then 400 lowing benefits: ease of manufacturing, transformed isothermally to form either aus- 600 ferrite in cast iron or bainite in steel. ~oo increased bending andior contact fatigue Austenite: a face-centered, cubic, non-mag- '~oo 400 strength, better wear resistance or netic phase found in iron and steel alloys .. ~ ilstT Mh~T N I EI Austenitizing:forming austenite by heating 100 200 enhanced dampening characteristics ILl I I' "'l!l 1 ~OJR 'f,f WE,~. a ferrous alloy above its critical tempera- 0 resulting in lower noise. Austempered 0.51 2 51G J02 W 104 105 1(11 ture-to within the austenite (steel) or materials have been used to improve. the austenite + graphite (cast iron] phase region TI M E-S ECONOS from the phase diagram. performance of gears and shafts in many Fig. 1-Schematic I-T diagram iillustrating the Bainite: an austenitic transformation prod- austempering !red) and quenching &Iemperingi applications in a wide range of industries. , uct of acicular ferrite and' carbide found in (green) processes. some steels and cast irons. Upon cooling, it Introduction forms at temperatures between those at Austemperingis a special, isother- which pearlite and martensite transforma- I ksi, %, ft-lb MPa mal heat treatment process that can be tions occur. 250 1700 ' Carburizing: the process by which the sur- applied to ferrous materials to increase face carbon concentration of a ferrous alloy I strength and toughness. Figure 1 shows a is increased by diffusion of carbon from the i schematic isothermal (I-T) diagram with surrounding environment. I Fatigue: failure of structures that are' sub- both the austempering (green line) and jected to fluctuating and cyclical stresses. the quenching and tempering (red Line) Isothermat that which is at a constant tarn- perature. processes outlined .. Austempering con- Isothermallransforrnation!I~ I-lor ii-II dia- sists of austenitizing, followed by rapid- 'gram: a plot of temperature versus the loga- rithm of time for an alloy of definite composi- ly quenching to a temperature in the tion; used to determine when transforma- range of 260-38SOC (500-72S°F), tions beg.in and end for an isothermal heat where the material is then transformed treatment. 29!l307o~-------------~~324342358376394413430448465484500 isothermally to form either ausferrite Martensite: a metastable iron phase super- BHN s~tur~ted in carbon thatis the product of HI , (acicular ferrite and carbon stabil:ized dlffuslonless transformation from austenite. I Fig'.2-Ivpical properties of lIustempered ductile austenite) in cast iron or bainite (acicular Residual stress: a stress that persists in a imn as II function of Brinell hardness. material that is free of external forces or ferrite and carbide) in steel. The quench temperature gradients. 360 Stress corrosion cracking: a failure that Si:lm~les Shot Peened results from the combined action of a tensile ;; JOO ~ ... "'"1"--- and ",Ho",iia""d'----I stress and a corrosion environment; the cor- ~~r---------------------~W ~ 250 :--........... rosion environment lowers the stress levels ' 15:: .~~:::=414 I for cracking due to tensile stress alone. l:fi,lDl ~ 200 1--'-'-"'--i''''''--+~--~·-''::--1 I~!II,[QI ~-~...... <, ......., lensile strength: the maximum engineering e"''''~IO.(G) 8 150 t---t----"'<l----+---j stress, in tension, that may be sustained w;l5,lXIl CJ .... ,.... ~.,.... :g HJO 1----1---+-':... -f--- without fracture. ~,OOD Yieldl strength: the stress required to pro- .2(J..tlOO 118 ~ C;=!rld.ZUlI -Q'14M5ADll duce a very slight yet specified amount of «50 I I-t-- , plastic strain; a strain offset of 0.,002is com- 100,000o~-------------------~I.IXXI,I))) !O,OOC,OOl 100,000,000 !,.m:D,DIXl,OOO . manly used. NLimber Df CydliS il'ig.3--Tensile .strengthsof .austemperedgray iron Fig',4-Contacl fatiglle (90% confidence limits), 8S a function ,oflIustemperingtemperature (F). from the ASME Gear Research Institute. 42 MARCH/APRIL .2001' GEAR TECHNOLOGY' www.geartechnoJogy.com' www.oowenrensmssston.com _------------IHIEATTIRIEATINGiIF;OCUS------------_ and temper process consists of austenitiz- increases wi.thincreased Brinell hard- ing and then rapidly quenching below the ness. The different grades of Am- 2 8 martensite start temperature. The marten- achieved through a variation in the Conlact·fatigue str ngth KIl'Nlrnml site that form is very hard and brittle; au tempering temperature 3I1d time-can o 10 2Q 30 40 50 subsequently. it must undergo, a temper- create a range of properties in ADI ~; Ferriuc ing step to acquire the desired combina- applicable to the specific requirements of ru PeBrlilic tion of strength and toughness. the component design, as seen in Table L Pnrflllc-~nductiDn HnHOId D Because austempering is an isother- Figure 3 showsthe relation hip of as- Au_"" Illl5"C ~: mal process, it offers advantages versus cast gray iron to Am asa function of Austtmpertd Il32(rc H quenching and tempering. Since the for- austempering temperature. Increased ten- Kymomt. ~_I/I,DII ma mation of bainite or ausfernleoccul's sile strength can be achieved by austem- ~ Cast over minute or hours al a single temper- pering gray iron at various temperatures. ~ Through-Hardened ature, distortion is minimized and crack- Contact Fatigue. Austempered duc- Gas-Nitridad ing does not occur. Meanwhile. the for- tile iron lends itself to increased contact Case·Carburized mation of martensite occurs immediately fatigue strength and wear resistance. o 1000 1500 ....... "HI 2000- as the metal temperature drop below the Figure 4 compares the allowable contact o 1000 1500"'n .. ,lBOO Hel1Ilan compressIVe stress P • N/mm! martensite stan temperature. Because sires behavior of ASTM Grades 2 and 5 fig. s-comparison o~ ,contact fatilJlll strengths cooling is achieved at different rates in ,(ASTM 1050-700-07 and 1600-1300- 01AD I wi1btl10SII"of cou-vllntionalllucUle iron and various sections, there is, a non-uniform 00). Figure 5 demonstrates that the COIl- steals used fOJ gear applications. transformation, wllich can re ult in sig- tact. 'fatigue properties of various grades nificant distortion and/or cracking. of ABI are comparable to gas nitrided ."1.~~.. " 0..,... 1 ..... Carbo-Austemperingw iss heat treat steels and competitive with carburized process used on certain steels where !.he ami hardened steel surface of the part is carburized.fellcwed Figure 6 illustrates that ADI has by an isothermal quench at a temperature improved abrasion resistance when com- r that produce a high carbon, bainitic case. pared to reels and quenched and tem- Ii • • • When the proces is applied to low-car- pered ductile iron. AD] experience less -.. Fig" 6--Pin abrasion tast, com,pa.ring wolU-II'- loss bon steels, it results in the formation of a volume loss at similar hardness levels, It equiva'ient hardrllfsses·. bainiticcase and a low-carbon, tempered resulting in a. component with improved 150 - martensite core. For medium-carbon wear characteristics .. reel , bainite is formed throughout the Bending Fatigue_ ADI also presents 125 cross-section of ll1e part. an increase in bending fatigue for gear 100 Austempered Irons applications. Figure 7 shows the compar- Austempering can be applied! to duc- ative allowable tooth root bending stress- tile and gra.y iron casting to produce es for ADm Grades 2 and 5. Figure 8 beneficial propenie relevant to numer- shows a comparison of tooth root bend- ous application. In the cas e of gears and ing fatigue in various materials, That fig- shafts, austempering yields austempered ure demonstrates that ADI is competitive Ob---------------------~10!m 1110.00II 1.00II,00II ID.OOOOOll 1110.00II,00II Numb81 01 C \~ ductile irons (AD]) and austernpered gray with cast and through-hardened teels, It Fill'.1-5ingletooth bending faligH. (90%, 'confi- irons (AGI) with better strength, wear 811 0 shows that shot-peened Am ha ,dencllimitS) from ASME GearReulfch Institute. resistance, and noise dampening proper- improved fatigue strength that i compa- 'lies than either as-cast irons or other rable to gas-nitrided and case-earburized Fm.. ~ ",,,,.1ttKi S§l competitive materials. As seen in Figure steels. Shot peening can improve the ~~:-ti¥i!IMrII I allowable bending fatigue of carburized 2, the tensile and yield strength of AOI ~ ~K.ftiMI <;! ~ ... 2" 0-·,- _"wc ~ H--,- Grade Tensile Yield Ilmpact l~pic811 <_"""0011 ~ H"",·- Strength :Sneng,lh Elong. IEnergv IHa~lIness !MP8/ksil [IMPa/bil [%) [J/lb-HI IBHNI tu< ~ !~~ .......~ a ~ H 11 850/125 550/80 10 loons 26S-32.11 -..:- ~ 2 10501150 700/1100 7 80/60 302-363 o 100 .200 300400 500 600 700 3 12001175 BOO/l25 4 60145 341~-444 Tooth Joot b8nd,ng·l.uo"" s"ongth. Ntmm' 4 14001200 1110{)f155 1 35/25 366-477 l~lg. B-Comparison of bending faliguestrengms 5 1600/2311 1 300/185 N/A N/A 444 555 'ofADI wilhlhose ,al,conventionall ducti r 'Iiron ,andl ' S't8111 uled lor 'gear appHcations. ww .... poweflrllnsmls.sion.com ". www.ge8rteclmology.com • GEAR TECHNOLOGY" MARCH/APRIL 2001 43 _-------------HEATTREATINGIIFOCUS------------. 16 .-- ~. 14 x 21' ~ 180 I § " »: <.:I.