PM Materials for Gear Applications

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P/M Materials for Gear Applications Senad Dizdar and Pernilla Johansson, Höganäs AB, 263 83 Höganäs, Sweden Abstract P/M materials for gear applications have been reviewed based on results of own investigations of gear tooth bending strength of simple spur gears and rolling contact fatigue resistance of rollers. It was concluded that surface densified casehardened P/M gears reached gear tooth bending strength of reference casehardened machined wrought steel gears. Surface densified casehardened
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  Presented at EURO PM2007 in Toulouse, France on October 16,2007 1 P/M Materials for Gear Applications Senad Dizdar and Pernilla Johansson, Höganäs AB, 263 83 Höganäs, Sweden Abstract P/M materials for gear applications have been reviewed based on results of own investigationsof gear tooth bending strength of simple spur gears and rolling contact fatigue resistance of roll-ers. It was concluded that surface densified casehardened P/M gears reached gear tooth bend-ing strength of reference casehardened machined wrought steel gears. Surface densified case-hardened P/M rollers reached RCF-resistance of casehardened wrought machined rollers. Sur-face densification plays the main role in reaching high performance of P/M components. Chro-mium low alloyed P/M materials show a promising potential for a high performance/cost ratio. Introduction Until approximately one or two decades ago, P/M materials have been associated with low costand low performance gear applications, such as pump– or transmission gears for hobby andhousehold applications. However, introduction of high density technologies improved gear den-sity levels and by this their mechanical strength. Warm compaction [1] improved density levelsof parts such as power tool gears up to 7.2–7.3 g/cm 3 , and offered a lower cost alternative todouble–press double–sinter (DPDS) route. High velocity compaction [2] offered possibilities tocost effectively press large single level P/M parts such as parking gears to densities levels up to7.2-7.3 g/cm 3 . Finally, selective surface densification (SSD) techniques [3] open possibilities tofully dense the part surface to a depth so that Hertzian contact stresses and bending stress gra-dients are kept inside of it. The selective surface densification techniques include a number of radial and axial rolling techniques even combined with shot peening, and a part’s particularitywill decide the most appropriate technique.Demands on gear materials are continuously increasing. This is mainly because of develop-ments in automotive industry toward high performance vehicles with low fuel consumption andlow environmental impact. A good example can be to look on car gear transmissions as verydemanding ones. In the period from World War II to present, their gear module decreased frombetween 4 and 5 mm down to between 2 to 3 mm, while effective torque transmitted increasednearly twice. In automatic gearboxes for passenger cars one can even find high effective plane-tary gear stages with modules between 1.5 and 2 mm.The gears that meet automotive industry demands are made of case hardening steels such as16MnCr5, 15CrNi6 or 21NiCrMo2, manufactured by a process route consisting of three principaloperation steps: soft machining, case hardening and hard finishing. Soft machining includes anumber of machining steps from a forged/cut blank to hobbed optionally hobbed/shaved gear ready for case–hardening. After case hardening it is often needed to compensate for gear hard-ening distortions by using gear grinding/honing and bore grinding. Gears manufactured in thatway achieve more than 1000 MPa in gear tooth bending strength σ FE and 1500 MPa in gear pit-ting resistance σ Hlim as written in ISO 6336 and DIN 3990.The reason that case hardening is the main hardening technique today may be illustrated in Figure 1 which shows how specific gear tooth bending strength and specific flank Hertzianstresses depend on gear module respective gear module/flank radius. The trends to more com-pact and more effective gearboxes lead to smaller modules and that causes higher and higher stresses in tooth root fillet and on tooth flanks. For modules smaller than appreciative 3 mm, thestresses increase sharply and the critical gear stresses shift from the gear flank to the gear   Presented at EURO PM2007 in Toulouse, France on October 16,2007 2tooth root. This phenomenon impact design of P/M gears, and shows that high performancegears need to be surface densified and case hardened in order to meet requirements fromautomotive industry.A long life of compact automotive gearboxes demands a stable full fluid film lubrication of gear teeth contact. Two main factors that establish and maintain such lubrication under high loadingare oil viscosity and surface roughness. Figure 2 shows viscosity–density curves for three typi-cal oils used in automotive gearboxes, gear oils SAE80W for manual gearboxes, automatictransmission fluid (ATF) according to Dexron III specification and small engine oil SAE10W30.Full fluid film lubrication gets benefits from a high oil viscosity, but commercial oils keep the vis-cosity as shown in Figure 2 due to several mechanical and chemical reasons. Low surfaceroughness on other side is beneficial to full fluid film lubrication i.e. the lower roughness thehigher film thickness–to–roughness ratio lambda. Gear shaving and gear grinding as two of teeth surface generating operations achieve ten–points–depth R  z  lower than 4 µm while P/Mgears surface densified by gear rolling/burnishing can achieve lower then 2 µm. For the proto-type gears presented later in the paper, gear pitting resistance increases as much as 5%(through the roughness factor  Z  R in ISO 6336). This is in fact an opportunity for surface densi-fied P/M gears. 00.20.40.60.811.2012345678910Gear module m n (mm)    E  q .  r  o  o   t   b  e  n   d   i  n  g  a  n   d   f   l  a  n   k  s   t  r  e  s  s  e σ F0 ~ F t /m n s H0 ~ √ (Ft/ ρ c ) 1101001000020406080100120Temperature (°C)    K   i  n  e  m  a   t   i  c  v   i  s  c  o  s   i   t  y   (  s  q .  m  m   /  s   ) Gear oil SAE80W, GL-4ATF DexronSmall engine oilSAE 10W30 Figure 1. An approximation of equivalent gear toothroot bending and Hertzian flank stresses as a functionof module.Figure 2. Viscosity vs. temperature for three typical oilsin automotive applications. Prototype gears and RCF-rollers Performance levels that P/M gears achieve were evaluated on simple spur gears from an auto-motive application and rollers from rolling contact fatigue (RCF) test defined by ZF Friedrichsha-fen in 1960’. Figure 3 shows the prototype gear, basic facts about the gear tooth root bendingtesting and a brief schedule of gear bending stress calculations according to ISO 6336. Figure 4illustrates rollers’ contact of the ZF-RCF test and lists some facts bout the RCF-testing. Materials and process routes experimentally evaluated The P/M materials used in this study are the prealloyed powders Astaloy 85Mo and Astaloy CrL,see Table 1 and Table 2. Astaloy 85Mo is prealloyed with 0.85% Mo while Astaloy CrL is preal-loyed with 1.5% Cr and 0.2 Mo. The manufacturing process route for each prototype gear androller is described in Table 1 and Table 2 by using codes listed in Nomenclature chapter. Thereference gears and rollers were machined from wrought round bars and case hardened ac-  Presented at EURO PM2007 in Toulouse, France on October 16,2007 3cording to common practice for components made of these materials. The reference gears wereadditionally grind to achieve quality DIN 7. The P/M prototype gears and rollers were firstpressed as rings and sintered to achieve the nominal core density, then machined to gear re-spective roller blanks with rolling–ready–geometry. The gear and roller blanks were then rolledusing a radial rolling machine and respective gear and roller die. All P/M prototypes were finallycase hardened in order to create compressive residual stresses by creating a hard martensiticsurface with a softer core. ρ F α en s Fn = 3.03 mmh Fa = 2.95 mm ρ F = 1.0 mm α en = 29.6°Tip contact ISO 6336 Teeth number z  = 18 mm Modul m  n = 1.5875 mm Pressure angle  α n = 20° Face width b  = 10 mmGear dataElectromagnetic resonance machine -VibrophoreTest frequency f = 80..120 Hz  Stress ratio R =  σ F0_min  /  σ F0_max = 0.1 Test stop criteria:- 3*10 6 load cycles (run-out) or 5 Hz frequency dro p  Fatigue testing σ F0 =Y Fa Y Sa F p m n bY Fa =6 (h Fa /m n ) cos α en (s Fn /m n ) 2 cos α n Y Sa = (1.2 + 0.13 s Fn / h Fa ) * q s(1.2 + 2.3 s Fn /h Fa)-1 = 1.50(q s = s Fn /2 ρ F = 1.52)Tooth root bending stress ISO 6336= 2.83 d b F p h  F  a     s F   n     C    r     i    t     i   c   a     l    s   e   c    t     i   o    n    3  0   °        3       0       ° F  p F  p Test gear Vibrophore jawsd b ZF-RCF test variant used in brief - R 1 / R 2 = 30/70 mm- Line contact between cylinders- Rotational velocity 3000 RPM- Full fluid film lubrication- Relative sliding 24%-T  est lubricants:- Gear oil SAE 80W-ATF Dextron III- Lubricant temperature 80°C-Test stop criteria:- Run-out at 50*10 6 load cycles- Width-through crater on conact surface- RCF Hartzian resistance is highest Hertzian stress atwhich test rollers survuve at least 50 50*10 6 load cycles.This is estimated by tests at two load levels with limitedfatigue life.Counter-roller  R 2 Test roller  R 1 Figure 3. The prototype gear, basic facts about the gear tooth root bending testing and a brief schedule of gear bending stress calculations according to ISO 6336Figure 4. Illustration of the rollers’ contact of the ZF-RCF test and some basic test data. Figure 5, top row, shows the microstructure in case hardened Astaloy 85Mo materials. The mi-crostructure is plate martensite that has high carbon content at the surface and in the core it islath martensite with low carbon content. Between the surface and the core there will be a gradi-ent of the carbon content were the martensite gradually will have lower carbon content.Figure 5, bottom row, shows the microstructure in the case hardened Astaloy CrL. The micro-structure is plate martensite with high carbon content at the surface and further in the materialthe carbon content will decrease and the plate martensite will be mixed with more and lathmartensite. Eventually the lath martensite will be mixed with more and more bainite so in thecore the microstructure will be bainite mixed with lath martensite. The gears made from Asta-loy CrL were low pressure carburized but this process was not optimized for the gears.  Presented at EURO PM2007 in Toulouse, France on October 16,2007 4     N   o . Material     D   e   n   s    i    t   y     /   c   m     3      P   r   o   c   e   s   s   r   o   u    t   e    S    D    D     0 .    9    8     /    E    h    t     5    5    0    H    V   1 16MnCr5(SAE5115)7.9 M,CQT,GG-/0.192 Astaloy CrL+0.2C(Fe-1.5Cr-0.2Mo+0.2C)7.1 P,S,M,GR,CQT0.20/0.253 Astaloy CrL+0.2C(Fe-1.5Cr-0.2Mo+0.2C)7.4 DPDS,M,GR, CQT0.20/0.254 Astaloy 85Mo+0.2C(Fe-0.85Mo+0.2C)7.1 P,S,M,GR,CQT0.20/0.305 Astaloy 85Mo+0.2C(Fe-0.85Mo+0.2C)7.4 DPDS,M,GR, CQT0.20/0.326 Astaloy 85Mo+0.2C(Fe-0.85Mo+0.2C)7.6 DPDS,M,CQT-/0.25Table 1. Material and process route of tested prototype gears.     N   o Material     D   e   n   s    i    t   y    (   g    /   c   m     3     ) Processroute     S    D    D     0 .    9    8     /    E    h    t     5    5    0    H    V    m   m    /   m   m    ) 1 21NiCrMo2(JIS NCM220H, SAE8620)7.9 M,CQT-/0.82 Astaloy CrL+0.2C(Fe-1.5Cr-0.2Mo+0.2C)7.1 P,S,M,R,CQT0.7/0.73 Astaloy CrL+0.2C(Fe-1.5Cr-0.2Mo+0.2C)7.6 P,S,M,RCQT1.4/0.94 Astaloy 85Mo +0.3C(Fe-0.85Mo+0.3C)7.0 P,S,M,R,CQT1.0/1.05 Astaloy 85Mo +0.3C(Fe-0.85Mo+0.3C)7.0 P,S,M,CQT-/1.0Table 2. Material and process route of tested ZF-RCF rollers.Figure 5. Case and core microstructure photographs of case hardened Astaloy 85Mo (top row)) and Astaloy CrL materials(bottom row)  
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