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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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