Gear Hardness Technology

8/12/2019 Gear Hardness Technology

1/11


2/11

'Qu en ch tim e to500 of.,

, Qu e n c h . .. .. .._ _ s_e c _o . , . . n_d _s_ -IE ff e c t i v e -

nen

Noles:

1 ,. A s tr u ct u re n o t q u e n c h e dcuuc l u ll mar tens it ew ill n o tbe fi xe d u p b y t e m pe rin g .S t .ru c tu re t obe expected140 43401----1-----'1----'1--------------------1 2. M ate ria l in th e F' situ ation c ou ld be te m -pe re d to m ee t a bou t 300 Brlnell m i n i m u m .(T he a sq u e n c h e d h a r d n e ss w o ul dbe a b ov e' 3 00 H B. )

Excel lent( ov e r 9 0% m a r t e ns it e )

A 25

R e a so n ab ly g oo d(m a r te n sit e a n d s om e o th e r t ra n sf orm a t io n p ro ou c ts )

B 80 2003. M a te ria l in th e P' s it ua t io n w ou ld p ro ba b ly

fa ll to m e e t C ha rp y V n otc h a n d d u ctilit y re qu ire -m en ts n orm ally e xpe cte d for a good ste el. Ina d dit io n. th e fa tig ue st re n gt h w ou ld b e p oo r.

e ss good . b ut m a y b e a cc epta ble(m a r te n sit e . b a in it e. p ea rl it e. a n d p e rh a ps s om e f re e fe rri te )

c 20 0 600

,4. Poor qu en ch in g resu lts ca n re su lt Iromth ings like a n Im prope r prior stru c tu re orth e ' w ro ng a u st en it iz in g' t em p era tu re . (AWq u en c h i s n o tt he o n ly re a so n f or a p oo r S l ru C 1u re . )

P oo r. u su a lly n ol a c ce pt ab le fo r h ig h pe rfo rm a n ce pa rt s(lo w in m a rte n sit e. w it h m u c h b ain ite , p ea rlite , a n d tre s fe rrile )

300 1000

Ve ry p oo r. u su a ll y n o t a c c ep ta b le(p ea rlit e. fre e fe rrite . s om e b ain ite , m a y be so m e m a rte n sn s)

F 80 0 7000

it is often necessary to increase the hardness of the

stee l. A cco rd in g to A GM A stan da rd s,I a gear w ith

a hardness of400 BH N , w hich has a design life of

10 7 cycles can handle as m uch as 20% m ore load

than a gear w hichis hardened to 30 0 BH N . For

ha rdnesse s above 40 0 B HN th e c ap acity inc rea ses

with respect to pinin g resista nc e. bu t th e ca pacity

decreases w ith respect to bending strength , w hich

de terio rates b eca use the to oth b eco mes b rittle,

Though a greatdeal ofa tte ntio n is g iv ento th e

ha rd ne ss o f th e m aterial, i.t i s im po rtan tto under-

stan d th at the m ic ro stru ctu re, u po n w hich th e ha rd -

n e ss d e pe n ds , is w hat really m atters. A lthough in-

d e pt h d is cu ss io n of microstructure is beyond the

scope of th is article . it is w onh m entioning that thedegree of m artensitic structure is one of the prim e

indicators of a m aterial's quality. A GM A 2004-

B892 does a good job of identifying other m icro-

s tr uc tu ra l a sp e ct s t ha t m u st 'b e c o ns id e re d .

U nlik e m ost g ear h ea t trea tm en ts, th ro ug h h ard -

ening is a process w hich can be perform ed either

prior to orafter th e g ea r te ethare c ut. T he h a rd n es s

is ach iev ed b y h eatin g th e m aterial to th e a uste nitic

range (usually to about 1500-1600 F) and than

quenching and tem pering . For ituarions w hen the

teeth are cut after the m aterial has been hardened.m achinability becom es a consideration in deter-

m ining the hardness. For the m ostpart, conven-

tio na l g ea r c uttin g p ro ce sse s (n ab bin g. sh ap in g.or

milling) a re c ap ab le of cutting materials with

hardnesses of up to 400 BH N ..Though 400 BH N

is m achinable, gear teeth are m uch easier to

m achine w hen theh ardn ess islo we r. T he re will

be d i to rti on ifthe hardening is d on e after th eteeth

are cut T he teeth m ay have to be fin ish-m achined

to achievethe re qu ir ed a cc ur ac y.TIle Process. T o harden a part. by th is process,

the part is h ea te d to the austenitic range, a tem -

perature that varies, depending on the carbonand

alloy content, w ith in the range of about1500-

1600 P (81S-870C). In th is state the steel be-

c om e s a uste nite , which is a term for the so lid

so lu tion of carbon in fcc iron .' Then thepart is

r ap id ly q ue nc he d in oil (or som etim es w ater) to

transform the austen ite in to m artensite .uf the

quench is too slow , the structure w ill no t be fu lly

transform ed to m artensite. The resulting m icro-

structure will then contain w hat are called tran for-

m ati o n p ro duc ts, suc h as ferrite, b ain ite, pe arlite.

an d cem en tite . T he p ro perties o f h ard ne ss, tou gh-

ness, ductility , and strength are dependent on

the transform ation products w hicharepresent,The rate of cooling which m ust be achieved to

p ro pe rly tran sfo rm th e stee l to m arten site an dmini-

m ize the percentage of transform ation products is

d ep en de nt o n th e c he mitry of th e a llo y b ein g u se d.

The am ount and type of alloy ing elem entsin th e

s te el d ete rm in e its h ard en ab ility,

H ard en ab ility is a m easu re of the re lativ e d ep th

to w hic h h ard ne ssi achieved for a given quench

ra te a nd se ctio nth ickn es . I nother w ords, a m ate-

ria l w ith a h ig h h ard en ab ility , w hic h1 Squenched at

the sam e rate as a part of the sam e size. but w ith low

h ard en ab ility , w ill h av e ha rd m aterial de ep er.

T he a llo yin g e le me nts w hic hh ave a nimpact on

t h . eh ard en ab ility o f th esteel a re m an gan ese, ch ro -

m ium , nickel, and m olybdenum . TableJ is a t ab le

sh ow in g sev eral allo yreelsw hich are co mm on ly

used for through hardened gears. A m aterial such

as A ISl4140 is considered to be a low alloy teel

and has rather poor hardenabil ity . A m aterial such

a s A IS I 4340 i s co n sid er ed to be rich alloy steel

and has m uch. better hardenability ..Once th e p art h as b ee n q ue nc he d,it needs to be

M ..J B r o g l i eis the P residen t o fDudley TechnicalGroup. Stili Diego.

CA. DTO is aconsulting firm

specia lizing ingearing and re la ted

machine elements.

D I.F . S m i thi s a p ro je ctengineer wll h

Dudley TechnicalGroup.

MAR CHI APR I L I' 9 9 2


3/11

Table 3 - Approximate Carbon Content to Develop Maximum Hardness inI

Carburized Case of Nickel Alloy Steels

I Carbon Content Maximumfor Mall.

I

Rockwell CStee'IType Ouenchin91Cycle Hardness, % HliIf 'dness

2315 DO O . B O I 632515 DO O . .B O 623120 DO 0.90 654320 DO 0.90 674320 AH 0.85 66.5

Kruppb DO 0.60 61Kruppb AH 0.60 63

4620 DO O . B O 654620 AH 0.85 654626

I DO 0.85 654817 DO 0.70 ,65

4817 (+0.23% er l DO 0.70 655 Ni,-0.25 Mo (SAE EX-1) DO 0.70 63

6620 DO 0.90 65I

8620 RH 0.87 659310

I

DQ 0.80 63.5 I

9310 RH I 0.80 65 I

Carburlzed Case

tem pered to reduce the brittleness and toughen the

steel, since quenched m artensite is hard , but also

b rittle . Tem p erin g th ro ug h h ar de ne dp ar tsi gener-

ally done at40 0 to lOOOF (205to 450C)~or a

period of one or m ore hour. depending on the size

of the gear. Higher tempering tem peratu res in-

crease the toughness, but also low er the hardness.

LimitsOil t he P roces s. The quench and tem per

process is lim ited only by thes iz e o ffu rn ace s an d

qu en ch tan ksava i l ab le ,Today, this is as large as

several m eters. From a practical standpoin t. the

m ajor lim itation com e from the ability to

quench gears fat enough to obtain an accept-a b le m i cr os tr uc tu re . In som e cases, particu-

larly w :ith lean alloy steels, it is just im possib le

to quench large gears fast enough to obtainan

a cc e pt ab le m i cr os tr uc tu re .

Table 2 show s tile com parison of tim erequired

to a ch ie ve d .iffe re nt le ve ls o f m eta llu rg ic al q ua lity

between AfSI 4140, a leanalloy steel withpoor

h ard en ab ility , a nd A I;S ~4340. a ric h a llo y ste el.In

order to com pare the hardenability of a m aterial.

e nd q ue nc h (Jominy)values are w id ely u se das an

in dic ato r o f a st ee l's h ard en ab ility,

Since the qu en ch is so criticalto the resulting

microstructure, it i s n ecessa ry to v er ify th e r es ults

w ith an appropriate sam ple. Too oftena te st c ou -

pon is used w hich is quite sm all as com pared to the

gear's sections. The sm all couponis rapidly

qu en ch ed, p ro duc in g go odresults, w bile the co ol-ing rate in the actualpart is too slow and produces

apoorresult, (a nd th isis w here it needs tobe good).

Carburizing

A s m entioned above, the alloy ing elem ents in.< 1

steel have an effect on the hardenability of the

3 0 1G E ... R T E . C H N 0 LOG Y

m aterial In earlier years, it w as know n that in-

cre asin g the h ard ness of th e m aterial inc rea se d

the strength of the gear. This relationship held

true up to a hardness of about 40 HRC At

hardnes es above this le ve l, th ematerial becomebrittle and the gears fal led in breakage faster than

gears w ith low er hardnesses. T he idea behind case

hardening is to keep the core ofthe tooth at a level

w hich w ould not betoo m uch b ey on d40 H R C,. to

avoid toothbreakage, but to h ard en th e o ute rsurtace,

o r ca se , t o i nc re a se p it ti ng r es is ta n ce .

Of the m ethods for case hardening gears,

carbu rizin g is th e p ro ce ss w hich is m ost o ften u sed .

The idea behind carburizingis to start w itha gear

b lank w hich has a low am ount of carbon in the base

m aterial. and then to add carbon to the outer sur-

face. A properly carburized gear w ill handle be-

tw een 30 and 50% m ore loadthan a th ro ugh h ard -

ened gear. C as e h ar de nin g is done prim arily to

iocrease the pi.tting resistance of tooth surface,

H ow ever, because of the residua] com pre sive

stre s: w h ic h is p re se nt in th e c ase a fte r c arb uriz in g,

there is also an in crease in be nd in g stre ng th .

The Process.Carburizedg ea rs a ch ie ve h ard ne ss

by quenching as do through hardened gears. The

difference is t ha t a c ar bu ri ze dgear h as a n in cre as ed

am ount of carbon in the surface, causing th is

area to become a hard case after quenching . w hile

the low er carbon core reaches a low er hardness.

C arb uriz in g ste els a realloy s te el s w ith a pp ro xi-m ately 10 to 20points of carbon .. T heprocess

involves heating the gears to a relatively high

tem perature and then rapid ly quenching to obtain

th e h ard ne ss . This heating and quenching w ill

resu lt in d istortion of the gear b lank . The am ount

of distortion w ill depend on the m assand con-

fig uratio n o fthe gear and can vary from a s lig ht

am ount to so m uch that the gear ro ll tbe scrapped .

Since the hard caseis re la tiv ely th in , g rin din gto

restore tooth accuracy m ay be so deep on one tooth

side that the rem ain ing case is too th in .

D ue to the propen ityto distort, it is reco m-

m ended to stress relieve the gearblankbefore

m achin ing and, possib ly , again one or m ore tim es

b efo re carb u riz in g .In really criticaljo bs,it m aybe

neces ary to put the b la nk s th ro ug h a mock

carburizing cycle. A m ock carburizm g cycle ex-

poses the blank to the tem peratures and cycles it

w ill see, and the blank still rem ains m achinable,

since no diffusion of carbon takes place.

The actual carburiz .ing i done by heating thegear b lanks to above the critical tem perature and


4/11

exposing the u rf ac es to c ar bo n .The c ar bo n c anbe

a solid, liquid. or gas. As most carburizingis gas

earburizing, the discu sion here deals with th i

method. The carburizing is done ina fumace which

contains a carbon atmosphere, such as natural gas,

Above the critical temperature, the carbon dif-

fuses into the materialon the surface.The amount

of carbon in the atmosphere must be controlled.Too much will cause carbide networks to form at

the tooth tips and too little will produce shallow

case depths, particularly in the root areas. The

amount is mea ured in terms of percent and i

referred to as the carbon potential. The optimum

carbon pote.ntiaJ~ which leads to the highest surface

hardness will vary, depending on the alloy being

used, Table 3 shows the carbon potentials which

give the optimum results for several alloy steel's .

When very deep cases are needed, the carbon

potential is held at a slightly higher level (up toI.] % carbon) in an initial portion ofthe carburiz-

in g cycle to give a bo o t to the diffusion.

The temperature in the furnace, the time in the

furnace, and the carbon potential ar e variables

which have an impact on the case depth. The alloy

content does not have an influence on carbon

diiffu ion. Fig. I is a chart showing the relation

between temperatureand time and case depth.It is possible to directly quench parts from the

carburizing temperature. This method minimizes

the distortion. but does not result in a rnicrostruc-ture which is capable of long life (10 8 to 10 9

cycles). The case often contains excessive car-

bides and retained austenite. The core structure is

unrefined, This method is used in the automotive

field. since automotive gears rarely see more

than 10 8 cycles. Also, ince the production .i

high, and the facilitiesand tooling used for auto-

motive gearing are highly developed, it i5P08-

sibleto obtain acceptable results.

Applications which require a high level of ma-

terial quality are cooled and then reheated prior to

quenching, In some cases it is also necessary to

deep freeze the gears so that transformation to

martensite is complete.

Limits to the Process. When the specifica-tions are correctly chosen by the engineer and

properly acme ved by the heat treater, a carburized

gear wiU be able to re i tpitting and also have

good bending trength, order to achieve this

capability, three things need to be in good order:

I) The surface and core hardness need to becorrect; 2)The ca e depth needs to be deep enough

in two areas and not too deep in one other place;

and 3) the m.icrostructure needs to be good enough

for the level of loading,

I . . .Hardness. The required surface and core

hardne sshould be selected ba ed on the applica-

tion. Depending on the alloy used, the hardnes

can be as high as 760 KHN (62 HRC). Long life

power gears which see high loads for omething

like 10 9 to 1010 cycles need to be up to abouI730

K .H N (6 0 H R C),and the Corehardness should be

in the range of36o. to 400 KHN (35 to 40 HRC).

Gears which are subjected to hock loading and

do not see too many cycles m ay be betteroffw.ith

a surface hardness which has been tempered

back:to 55 H RC in order to gain more toughness.

Once the desired hardness has been deter-

mined, the drawing or specifications need to be

specific asto what isrequired.' For instance when

hardness is checked on a mounted tooth sample,

it is typically checked by taking a microhardness

traverse, The microhardne s is taken either by

Knoop. a method using a 500- or lOOO-gram

load, or sometimes Vickers, using a kg load. Yet

nearly all drawings specify surface and core

hardness in values of Rockwell C , a method

which uses a 150 kg load. For this reason. a

conversion must be made from either the Knoop

number or the Vickers number to determine

whether the part. met the specified Rockwell num-

ber. Conversion is not simply a mathematical rela-tionship. Since the structureand cold working prop-

erties vary for different materials and hardnesses,

I10 I I I

J 0.400--,

'0.320.0 1 I I II

I =:::J II 6.0 0.2405.0 II I I 0.200 ,

I1 I : I l o o~4.0 0.1160

I I I~ ) I ~E 3.0 C a rb u ri zi n g Te m p e r a tu r eE

~ ~ 1 10,120

.51750 F-I /: J : 2.0 17ooF. ::iI- 0.080 H :;.. 11600F~t'-. NI

~

,.I II

L U

C ~ ~~ ClL U j . . . . . - - -1-k o o , 1 LUe n I I U-e 1.0 I I 0.040 ~1 1 '-----1 1UJ 0.8

I I ~ ~ I0.032 ~J I i liT

i=0.6, .;' -

, :U c I i0.024 5UJ ./ .. J r I Iu. 0.5 i I 0.020 ~u. / , I 1 1w 0. 4 I I 0.016 W I

0.3 ~ I d I I II

I'

I

II I0.012 I

I

0.2 ~ I 1Ii

l I III

I 0.008I 1 I

II

I LI

II

0.1 I I1 2 3 4 5 6 8 10 20 30 405060 80100 I

CARBUAIZING TIME. h .Fig. 1 -Nominal time and temperature requirements for different case depths.

MAR C H f APR I L 1 9 9 2 3 1


5/113 G EAR TEe H N a LOG V

The 1.50kg load used [or a Rockwell C check

is inappropriate to check the hardness close to the

surface orelsewhere in the case . Th isisbecause the

size of the indention made by the 15 0 kg load

homogenize the conditions over a large area.This

can mask local deficiencies.

Since it is appropriate to check a pa:rt with a

microhardness method, the drawing and specifi-

cations should state the hardness number terms

of a microhardness method. The equivalent

Rockwell value could also be noted on the draw-

in g for reference. An example of what is meant is

shown below:

Case Hardness: 58 -62 H RC

(Poor Practice)

Case Hardness: 6 90 -776 K H N (5 8-62 H RC ,ref)

(Good Practice)

2. Case Depth. Fig. 2 shows the shape of a

typical carburized case. Note that the thickness at

the lip is thicker than the case at the pitch diam-

eter, while the case at the root fillet isthinner than

at the pitch diameter, Though this shape is typi-

cal, most drawings only specify one value for

ca e depth. Many drawings also fail W be clear as

to how the case depth should be determined,

The effective casedepth isusually d efin ed a s th e

depth of hardness to 50 HRC. Since there i room

for misunderstanding this statement, a

microhardness value similar to the one below

would al 0 appearon the drawing of specification:

Effective Case Depth: Determined by 542 K.HNcutoff point (5 0 HRC, ref.)

The case depth at the pitch line (and in the

dedendum just below the pitch line) is critical,

since this a rea is most susceptible to pitting. The

case depth should be deep enough for the case-to-

A FLANK CASE THICKNESS8 ROOT CASE THICKNESSC T IP CASE THICKNESSD CO RE HA 'RO N ES S T AKENHERE

Fig. 2 - Carburized case pattern. Shaded area isall 510 HV (50 HRC) or higher in hardness.Unshaded area is less than 510 HV (50 HRC).

core interface to be deep enough to avoid cracking

due to subsurface hear stresses. The depth of case

needs to be determined by the transmitted load

and not by any relationship to the diametral pitch.

A minimum value of case depth at the pitch line

can be determined from the following rela tion-

ship, which is based on the Hertzian band width:

hee= s din 0t mflmg +1)7.0 X 108 cos ub

where,

se ' maximum contact stress in the region of

106 - 107cycles

d = pinion pitch diameter, in.

0t ;;;;; ressure angle, transverse

ub ' base helix anglemG ;;;;; ooth ratio

For situations where the ratio is high and the

lowest point of single tooth is deep in the

dedendum, the case depth may also need to bepecified at a point in this region.

As mentioned above, carburized gear teeth

gain in bending stress because of the residual

stress in the case. This gain can only be realized

if the case depth in the critical bending area nearthe root is deep enough. A minimum value fo r

this case depth can. be based on the diaraetral

pitch, since the bending stress is related to the

tooth size, If the teeth are sized properly forbending stress, then the following relationship

should be valid for effective case in the toot

he ;;;;;O.6inormru d iametra lpitch

Such a value should appear on the drawing.

Many gears used today are operating at pres-

sure angles of 22.5 to 25 Also, it is very

common for designs to make the pinion long

addendum. Though there are many advantages

to these tooth form, the drawback is that this

tends to make the top land quite narrow. To avoid

the risk of tooth tips breaking off, the maximum

ca e depth atthe tooth tip should be limited to .40divided by the normal diamenal pitch,

Getting the case depth right at all the e points

becomes unmanageable when the teeth are very

small Twenty-pitch teeth are difficult and 28-

pitch is the practical Jimir. With extreme care,

finer pirches can be done. The difficulty in getting

the case depth right on small gears is that the

portion of dille in the carburizing cycle during

which the temperature is not stable (coming up to

temperature and cooling) is large, compared to

the overall cycle. Since the temperature is avariable affecting carbon diffusion. it is hard to


6/11


7/11


8/11

r es id u al - t re sspa tt er n , Fo r th esere aso ns, co ntro l o f

th e w hite layer is a concern w h e n n it ri di nggears.

The double stage ga .n itriding process has the

ad antage of producing le s white layer than the

single tage process. It is also a m o re e ff ic ie nt

process. T he d ouble stag e process uses tw o n itrid -

ing cycles with the first being imilar to the single

t ag e p ro ce ss , except for duration . N orm ally the

gear i first ni t rided at a 151030% diSliOCiat i on r at e

fo r 4 to 12 hours. T he econd stage of nitriding

then takes place at a temperature equal to or

g re at er t ha nthe first stage, b ut w it h a d is so ci at io n

rate of 80 to 85%. To get this di sociarion rate an

external di oeiaror i required. Som e typicaldouble tage cycles and achieved case hardnesses

and depths areshown in T able 5.

I on itr id in g. l ou itr id in g, a s m e ntio ne dabove, is

an exten ion of con ven tio nal g asnitriding which

uses the method o f p la sm a discharge physics to

deliver nitrogen ions to' th e workpiece surface.

The general m ethod involves useofhigll voltage

electric energy in a vacuum v es se l c on ta in in g

nitrogen gas. The mechani m which cracks the

nitrogen gas into m onatom ic nitrogen ionsi

sim ilar to th atwhich takes place in a fluorescent

la mp . E le ctric al c on ne ctio ns c ha rg ethe workpiece CORE

an d the nitriding ve sel so that the workpiece

becom es acathode, and the v acuu m v essel 00-

i s t er m ed the white layer because it etches out

white in a micrograph. Typical th ickn esses o f th e

white layer a re b elo w .0 0 I (.025mrn).

B ecause th e w hitelayer is a brittle structure, it

i s o ft en required that i t thickne s be minimized.

Although one can grind th e brittle white layer

off after the nitriding process, this is a costly

operation that is n ot a lw a ys practical. There isno

guarantee that grin din g w illbe uniform (espe-cially inthe root fillet region) and. i f it i s, t ha t the

case will be uniform at different location. on the

gear tooth ..A to oth th athas required hardnes and

ca e depth at th e 0.1[). w ill not alw ays have the

required hardness and c as e d ep th at the form diam-

eter or other locations. Grinding of a un iform

amount of stock can lead to im balance of the

comes an anode. Electrons accelerating towards

the anode impact with the diatom ic nitrogen gas

and dis ociate the ga into nitrogen ions, These

ions, in tum , accelerate tow ards the cathode and

since the cathode is the workpiece, the nitrogen

ions actually impinge upon the workpiece.

limits on the Proce s.. T he p rim ary a dv an ta gethat n itriding ha overother case hardening tech-

niques that i nv ol ve q ue nc hin g processes is the

m all com parative distortion of treated parts, G e-

ometry and tolerances of certain gears m ake ni-

trid in g 'th e o nly v ia ble c ase h ard en in g a lte rn ativ e.

Ring g ea rs a ndother gearing that have th in -wa ll ed

sections that would distort too much during a

quen ch in g process are o ften n itrided,In addition,

nitridingis u se d s om e tim e sw hen th e s ize of agear

makes quench di rortionand th e s ub se qu en t g rin d.ing p ro bl em s u n ac ce pt ab le .

Reproducibility of the nitriding process is an -

other advantage it has over other com m on ca e

hardening methods. Given parts ofidentical ge-

om etry andsimilar metallurgical quality and u -

ing identical n itridin g cycles,case depth. ca e

hardness. and case com position w illbe compa-

rable. [1 1 addition, parts be tween batches will

distort in exactly the arne w ay. Th ismean that

machining ca n be biased b ef or e n it ri di ng to com-

pen ale fo r expected di tortions.

Ion itrid in g rates are betterin both am ount of

d isto rtio n a nd reproducibility than conventional

g as n itridin g. M uch ofthis has todo with the degree

to w hich each o f th eseprocesses can be co ntrolled.

Conventional gas n itrid in g, th ou gh a v ery c on tro l-

lable process, doe' not lend itself as w ell to pro-

I I C,o;SE

A BO U T

0..10DEEP(.25mm)

FRE 'E FERRITE

Fig. 5 Undesirable carburized structure (case, core)


9/11

Table 4 - Common Ni tri ding Gear Steel s

St.eel C Mn Si C AI Mo Ni

I Nitralloy 135M 0.41 0.55 0.30 1.60 1.00 0.35I Nitralloy N 0.23 0.55 0.30 1.15 1.00 0.25 3.00

AISI4340 0.40 0.70 0.30 0.80 0.25 1.83

AISI4140 0.40 0.90 0.30 0.95 0.20I 31 CrMoV 9 0.30 0.55 0.30 2.50 0.20

100 8 0

0 60E 0 50E 0 40

.l:

a. O 30III1:1III

0 2001oQ) 0 15:> Q) O 10t:

LU O 08

GE R

Ii

I , i I

I I ' , II I r 'J,i U

, . . ;1 I

I I )/ I Ii I i i-

..IiI I I

Il.H'

.,i 1

V I I I I,

I I ': 1 I I:

, ,II

I . III I I I

0 0 4 0O 032

0 0 2 4.S

0 0 2 0 ~0 0 1 6 '5 .

IV

0 012 1:1Q)

10 0 0 8 o

Q)

:: >0 0 0 6 :u

Q)

:t:

O 004 LU

0 0 0 30 06

2 3 4 5 678 10 15 2 0 30 40 5 0 60 8 0 10 0Nitriding Time, h

cess control. One example of this is the fact that

the thickness and composition of the compound

(white) layer can be successfully and repeatedly

co n tro lled w h en io n ir rid ln g .One even has tbeoption

of requiring no white layer. As controversy exists

over whether the brittle compound layer is an initia-

lion site for cracking, this is an attractive option.Of the disadvantages of the nitriding process,

the main one isthat ittakes much longer than:other

common case hardening techniques. The diffu-

sion rate being exponentia lly dependent on tem-

perature, nitriding takes place much slower than

typical case carburizing or induction hardening

procedures. The unpleasant side effect of this

time dependence is that practical nitrided case

depths are shallower than other case depths.

Fig. 6 is a chart showing typical nominal gas

nlrriding times for different case depths.

Other disadvantages include the dependence

A . F la n k c a se th ic kn e ssB . H oo t c a se t hic kn e ssC. No case at center, top landD. Core hardn ess ta ken h ere

A

B

Fig. 7 - Induction hardened case pattern by thescanning method of tooth heating, I

TE HNOLOGY

on and sensitivity of the achievable case hard-

ness to the metallurgy of the base material, and

the tendency of nitrided cases to be less ductile

than other cases. Lower case hardnesses and less

ductility, in general, result in lower allowable

stresses for nitrided gears.

Ca r bo n i tr id in g Ga seo u s .Carbonitriding as a

process is related to both carburizing and nitriding.

Typically carried out within the temperature rangeof 1550 to 1650 0 P (845-900C), earbonitriding

utilizes temperatures above transformation tem-

peratures, Diffusion of carbon from a carbon-

aceous atmosphere is part of the process as well.

However, like nitriding, diffusion of nitrogen is also

involved. This is usually accomplished by additionof anhydrous ammonia to the carbon atmosphere,

The advantages of this process are related to

the fact that it is essentially a compromise be-

tween the two parent processes. Taking place at

lower temperatures than straight carburizing, the

process has reduced distortion. Having a more

favorable diffusion rate, the process produces a

case faster than straight nitriding,

Carbonitriding is used for small gears with

finer pitches than could be controllably carburized.

InductionHardening. Induction hardening is a

heat treating process which uses alternating cur-

rent to heat the surfaces ofa gear tooth. The area

is then quenched resulting in an increase in

hardness ofthe heated area. The hardness patternwhich is achieved varies, depending on the type

and shape of the inductor. An inductor which is

circumferential will harden the teeth from the

tips downward. While this pattern may be ac-

ceptable for splines and some gearing, heavily

loaded gears need a hardness pattern which is

more like a carburizedcase. This type of induc-

tion hardening is known as contour hardening. A

typical case for a contour induction hardened

tooth isshown inFig. 7. ALsoshown in this figure

are the three critical places to check the case on

an induction hardened part The discussion in

this section deals with gears which are hardened

by this method.

Since the area below the surface remains cool,

it acts as a fixture minimizing distortion, In

order to achieve high surface hardness, an induc-

tion hardening material usually has from 40 to

50 points of carbon. The resulting surface hard-

ness is generally 53 to 58 HRC. The core hard-

ness is developed by quenching and temperingthe blank prior to the induction hardening.


10/11

Table 5 - Typical Double Stage Gas Nitriding Cycles

EffectiveCase Depth

(Re50)teel CycleMaximum Minimum Surface

W h it e , l. a ye r Hardness Core Hardness

Rc 49-54 Rc 2735

.01Sitra.lloy 135M 1 1 [email protected]% diss.

1 50 hr @ 1026 QFI 84%diss.

Nitralloy N 1 . 1 1 0hr@ 975F

28%d;ss,SOhr@ 975F84% dlss,

,014

1 0 h r @ '975F28% dlss,5 0 h r @ 9'75F84% diss.

AISI4140 .025e t c h e d

1 0 . I h r@975F28% diss.5 0 I hr @ 975 F84% diss.

AISI4340 .025e t c h e d

B y heatingthe outside layers the m aterial tries

to . expand while being restrained by the inner

m aterial. A s this Iayercools, there is an increase in

volum e due to the increased hardness. T he result,

if properly done, i an outer ca e with residual

compressive stre s at th e su rfa ce . The case-core

interface is a c ritic al a re aon in du ctio n h ar de ne d

gears. If n ot, p ro pe rly d on e, th is a rea is su sc ep tib le

to cracking, In thi region, there are highresidual

, t res es due to drastic differences of the case and

core tructures and the fact that the transition

occurs in a very short distance, (See Fig, 8.)

In du ctio n h arde nin g is done prim arily to in-crease th e pining resistance of a gear, Though the

load carrying capacity of induction hardened

gearing i not as high as the best carburized

gears, it is still quite high. And, in addition, this

process does have som e advantagesovercarbur-

[zing. such a s les s distortion on particularly thin-

rim med internal gears,

The Process .Through hardening m ate ria ls a re

u ed for induction hardened gears.The same

comments on hardne and hardenability as were

made in the through hardening section apply here.Simply put, th e am ount of carbon in the m aterial

determ ines the achievable hardness and the alloy

content determine thehardenahility. T his le ad s

to the sam e conclusion reached in the through

hardening section; thatis, if h ig h s ur fa ce h ar dn es s

and a deep ca e are required.a richalloy steel with

an adequate carbon content is needed.

A s w ith through hardening, the teeth canbe cu t

either prior to or after thequ en ch a ndtemper cycle

which develops the core properties,

W h en a g ea r is c arb uriz ed ,il is sa id to g o th ro ug h

,0007

,0007

.0007

.0007

Rc 62-65 Rc 32-36

Rc62-65 Rc3844

A c 48-53 R c 27-35

o ne c om p le te heattreat cycle, w hile a gear w hich

isinduction hardened is said to gothrough a

num ber ofheat treat cycles equal to three tim es the

nu rnber of teeth. T he inductor scans onetooth slot

at a tim e and. because the heat treating conditions

are different atthe tooth end. than ill the middle,

il ca n be said that three heat treatments occur per

tooth, O ne heat treatm entoccurs a t he in du ct or

enters the tooth slot, one occurs acrossthe middle

of th e slo t, a ndathirdas t he inductor pa ses o ff th e

tooth. Therefore, themore teeth there are, the

g re ate r th ecomplexity of the job.

The case depth isa function of the power andspeed of theinductor travel It i s diff icul t to verify

the case depth on an induction hardened partwith-

out sectioning anactualpart, C hecking on thee n d

is not practicalbecau e the ca e depth on the tooth

end is u uaUy not as deep as in thecenter area to

prevent heat dam age on the ends.

70

65

Ua: 55J:< IiII) 50. .:

Dlij 45J:

40

35

30

I I I I ' I I IJI I I II

II I

',- I l II I1- -'I

- Kr -.h l l- -I I I

I

I

II ... .d -Sub surtaCf IIK 1 1 1 0 1hereI, ... . . III - ,

r - - - h l .-1 :SUb s ur fac e str es s , III : still ralher high. so

1;]

,I

mducttcn hardened rcase needs 10 be i fi - . . t t l I 1 1 1 1 .1

I- I I I

0 ,0 00 0 .0 20 0 .0 40 0 .0 60 0 00 0 0 ,1 00 0 ,1 20 0 ,1 40 0 .1 60 0 .1 00 0 .20 0Case Dep1 h. Il l,

M R CH I A PR LL '8 II 2 3


11/11

G~EAIR'GE~NERAT IN'G TO O'l.S12 Pitch. FineI HO'BS ALI. BORE SIZES

TAU ..VOLUTE PVD GOLDTitanium-Nitride Coated Hobs & Cutters

, A - :. .. .. .... rp t range

Gear Hardness Technology

Documents

Transcript of Gear Hardness Technology