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Powder meta{ ~.-oo series stai~s~ess steels are becoming popL4iar with automobi{e makers for exh sust components, but there gs [imited pu.blished data on some of these relatively new mateFia{s° A group of researchers set out to stc~dv ~wo of them and came up with some

I T IS o[tcn the case when ncw mater ia ls arc in t roduced tha t it

takes some t ime for all propert ies to be formal ly p romu lga t -

ed in an ISO s tandard . Such is the case wi th P M stainless

steels 409LE and 409LNi. which were developed for

CARBOLITE will be showcasing its expertise in the design and manufac tu re of electric

ovens and furnaces for P M and other mater ia ls processing at the Thermprocess show to

be held in Dusse ldor f ir June. Carbol im equ ipmen t is used in sintering powder gold

wedding rings and the c o m p a n y has recently developed rheir range of control led-atmos-

phere furnaces for refining precious metals , including p la t inum, pa l l ad ium and rheni-

ran. In ano thcr h ighly spccialiscd application. Carboli tc is involvcd in the dcvclopmcnt

of a n u m b e r of specialiscd t-urnaces for the nuclear indus t ry One is dcsigned ~or recov-

ering u ran ium, another for hear-treat ing fuel p ins and a third. 10 of which are now in

operat ion, for evaluat ing low-level radiat ion in waste materials.

au tomobi l c exhaus t f lange applicat ions. M o s t of the l imited

mechanica l p roper ty da ta publ ished have been derived from limit-

ed product ion experience, and the goal of the research team -

Suresh O Shah. J ames R McMil len , Prasan k Samal and Leander

F Pease I i I - was to conf i rm static

mechanical propert ies and develop data

oil fat igue and mlpac t s t rengths of these

two materials , processed under commer -

cial m a n u f a c t u r i n g condi t ions . Thei r

f indings were presented ar SAE2003, held

last m o n t h in Detroit .

Fat igue s t rength is a critical p rope r ty

for s t ruc tura l c o m p o n e n t s subiec ted to

cyclic loading. Cyclic load ing can be a direct resul t o f the c o m p o n e n t ' s fnnc-

r ion. such as appl ica t ions involving

ro ta r lng shaf t s , cams and gears.

In these s i tuanons , the cyclic stress

impar ted is deliberate, and is usual ly of

s i gn i f i can /magn i tude . In some other situ-

at ions a par t may suffer s t ructural stress-

es, even t h o u g h it is no t directly subjected

to cyclic toadmg. Cyclic stress m ay orig>

hate fLOln secoudarv sources snch as

v ibra t ion or thermal expans ion and con-

t rac t ion of var ious c o m p o n e n t s in an

a s sembly Even t h o u g h the d a m a g c duc

to fa t igue a lone m a y n o t be s ignif icant .

it may still be a con t r ibu to r re so m e

o ther m o d e of failure, such as cor ros ion

o r w c a r ,

Flanges used in an to ine bile e x h a u s t

sys tems are nor directly subjec t to cyclic

loading , bu t they do exper ience cyclic

s t rcsscs duc to v ib ra t ion and t h e rm a l

expans ion / con t r ac t i on of var ious

2 0 ~:~ Apri| ~oo3 oo26-o657/o3 02003 Elsev~er Science Ltdo A|| rights reserved.

componen ts of the subassembly tha t

sonae would describe as thermal shock in

severe road conditions.

Exhaus t f langes are also subjected to

thermal fatigue as a result of al ternate

heat ing and cooling dm-ing operation of

the vchide. PM stainless steel exhaus t

flanges made of 409L. 409LE~ 409LNi and

4034L have met the static mechanical

slrength and thermal fatigue requirements

of the application, based on a number of

original equ ipment manufac turers '

OEM specified qualification tests.

It has been determined tha t these PM

materials n u l s [ p o s s e s s 57 I l l h ' l i l nu ln sin-

tercd density of 7.2 g/cm 3. and be pro-

duced via h igh- tempera ture hydrogen sin

fet ing in order to meet the static mechan-

ic properties and thermal fatigue require-

ments , as well as corrosion/oxidat ion

resistance and elevated tempera ture

strength retention (leak t ightness ' .

T h a t means s inter ing at above

2300c'F r~r 260'~C. These ma te rmls mus t

also conta in vcry low levels of carbon

and mtrogen to meet the weldability

requirement. Fatigue strengths of PM

steels are no t predictable using ;heir u]ti- materials. The process parameters and

mate tensile strengths, say the researchers.

For mos t wrougl~L steels the fa/igne

strcngth can be estimatcd by mult iplying

the ult imate tensile strength by a factor

known as the fatigue endurance ratio. For

most wrought steels, the fatigue ratio is

0.38 or higher.

But earlier work on a number of PM

steels has shown that this would be a17

unsafc assumpt ion as their endurance

ratios showed variance between 0.16 and

0.47. Al though MPIF Standard 35 lists the

fatigue strengths of several PM austenitie

and tcrritic stainless steels, but does not

list fatigue data on 409L, 409LE or

409LNi. since these are relatively new PM

sintered densities selected were based on

commercial practice aJrrently employed

for the manufac ture of automorlvc

exhaus t flanges and oxygen sensor bosses.

The sintered densities of the samples test

ed were between 7.2g/cm 3 and 7.3D cm 3.

Commercially available grades of

409LE and 409LNi were used and the

chemical composi t ions of the alloy pow-

ders are shown in Table 1.

The powders were lal-J ricated with

commcrcially available ctbylcne bisstcar-

amide solid lubricant. "Dog bone" tensile

test specimens and unoact test bars were

prepared by compact ing under a pressure

of 45 tons a square inch (620MPM. The

green specnnens were delubricated and sin-

feted in an industrial pusher furnace

metal-powder.net April 2oo3 ~r~ ~

Sintering took place at 2400°F (1315°C for 45 minutes. The sin-

termg atmosphere was 100 per cent hydrogen with a very low dcw

point.

Representative samples were tested for density, hardness and

interstitial contents nitrogeu oxygen and carbon . Dog bone

tensilc spceimcns wcrc tested in the as-sintered condition for

ultimate tensile strength UTS). yield strength and percentage

Reader Enquiry Service No 608. Go to www.metal-powder.net to make your enquiry.

elongation. Some of the impact bars were used for impact test-

ing in the as-sintcred un-notched condition as Charpy speci-

mens. while others were machined into fatigue-test specimens.

Fatigue specimens were prepared in ~ccordance with MPIF

Standard 56 and the procedure specified in ASTM E647 was fol-

lowed for machining. The surface finish of rhe machined speci-

mens was 0.2 macrons.

The results of tensile, impact, hardness and density are shown in

Table 2. The interstitial contents the samples, from both materials.

were in the ranges of 20_50 to 2200 ppm for oxygen, 50 to 100 ppm

for nitrogen and 0.005 to 0.012 per cent for carbon.

The researchers found that the tensile strength, yield strength

and elongation data obtained for 409LE and 409LNi are very

similar to those reported in earlier publications. Table 3 lists the

results of the fatigue test of the two materials. The fatigue

endurance limit is based on a fatigue life of 10 -7 cycles. It was

assumed that the low rate of material removal employed in

machining the fatigue specimens results in a minimal degree of

for both materials.

These results came as someth ing of a surprise. Thc fatigue

endurance limit of 409LE was determined ~.o be 29.4 ksi 203

MPa, , significantly higher than 0.38 X UTS, or 20.8 ksi 143

MPa J. The fatigue endurance limit of 409LNi is even higher, h

was dctcrmined to be 36.1 ksi (249Mpa which is much higher

that would have been predicted a fatigue endurance ratio of

0.38 - 31.8 ksi 218 MPa . The researchers concluded that high-

temperature hydrogen-sintered 409LE and 409LNi offer satis-

factory fat igue strengths with fatigue endurance ratios signifi-

cantly hi,~her than 0.38. Both materials also exhibit satisfacto-

ry impact strength.

Dynamic mechanical properties of PM 400 series materials

are strongly influenced by their sintered densities. Sintering m a

nitrogen-bearing atmosphere impairs their impact strength.

PM409LNi, sintered to a density of 7.2g/cm 3, utilising high

sintering temperature and a 100 per cent hydrogen dry sintering

atmosphere, offers a unique combination of both high static

and dynamic mechanical properties.

These properties make the material highly dcsirable for

flange designs, where flexing or bending of components is a

potential problem.

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