Enhancement of Austenitic Stainless Steel Fatigue Performance by Gas Nitriding: Parametric Study and Optimization AbstractThe aim of the present work is to subject an austenitic stainless steel material type AISI 304 to a con ventional gas nit riding pr ocess thr ougho ut temper atu re range of 400 !00 o "# nitriding time of $0%&0 hrs and ammonia '()3 * flow rate of $00 % !00 litre+hr, and fatigue behaviour of nitrided specimens were then e-amined 'studied or investigated*. The corrosion rates 'in mm+year* were also detected 'noticed* in order to identify the corrosion resistance re duc tio n due to nit ridi ng pr ocess /ua nti tat ivel y. re vious wor k on thi s sub jec t was performed using the classical method of changing one factor at a time. This old methodre/uires a lot of specimens and e-tensive e-perimental work which is both costly and time consuming. 1urthermore# the classical method is not capable of detecting the interaction effects between the factors and also can not perform optimi2ation. All previous drawbacks are tackled by using the response surface methodology 'S* in the design of e-perimentstatistical methods. According ly# Sdesign of e-periment is created, samples were prepare d and gas nitrided. 1atigue tests were performed on an Av ery%5enison fatigue testingmachine at a stress ratio 6$. The corrosion tests were performed 'carried out* on A"instrument machine '7ill $8*. The corrosion rates were determined on the basis of cyclic sweeps corrosion tests in 39 (a"l solution. The fatigue strength of the teste d specimens has increased by about 8:9 as compared with un%nitrided specimens. The results of corrosion test s re vealed that corrosion re sist ance for nit rided mat erials was aff ected by nit ridingprocesses. The presence of nitrides in the nitrided layer s reduces their protective properties# but its value does not e-ceed !09 of an un%nitri ded material 'austen itic stainless steel AISI304* at the obtained point 'processing parameters set* of ma-imum fatigue limit. Therefore# the nitriding process is preferred process that can be used 'applied* on austenitic stainless steel type AISI 304 improve 'enhance* its properties 'fatigue# wear and hardn ess* withoutlarge degradation 'affecting* of its corrosion resistance. The nitrided layer thickness 'depth* was e-amined by light microscope after grinding and polishing then etching using 89 (italagent '8 ml nitric acid ; <= ml ethanol*. >ptimum settings for the time and temperature factors were obtained. ?eywords@ 1atigu e# "orrosion# 7a s nitriding# Stainless steel# Ammonia# espon se surface. 1!ntroductionMetals and alloys have a diverse application in the medical field, particularly as implantable internal (in vivo) struc tura l, load- bear ing materials in devices for partial and total joint replace ment, fractur e fixat ion, and instruments (see Figure (1)). hey have a successful history in the human body because of their appropriate mechanical properties, corrosion resistance, biocompatibility, and manufacturability. Metals have high elastic moduli and useful yield strengths such that components may be manufactured that !ill sustain significant loads !ithout large elastic deformations or any plastic deformations. hese metals also possess sufficient fatigue strength, ma"ing them suitable for cyclic load applications for joint replacements or fracture fixation plates. #fproperly fabricated, the fatigue strength of metal components is very predictable and can be designed using appropriate margins of safety. $ne restriction for implant metals is that they cannot have magnetic properties due to the use of magnetic resonance imaging (M%#) systems used as diagnostic tools in the medical field. he future development of ne! biomedical materials and the development of ne! manufacturing processes to improve the performance of traditional biomedical materials !ill increase the need for more materials science and meta llurgical engin eering rese arch , inclu ding meta llogr aphy and micr ostr uctur al characteri&ation. $ne group of most metals and alloys used for orthopaedic medical device are stainless steels '11
Transcript
7/23/2019 Fatigue and Corrosion for Nitrided 304 Stainless Steel
Enhancement of Austenitic Stainless Steel Fatigue Performance
by Gas Nitriding: Parametric Study and Optimization
Abstract
The aim of the present work is to subject an austenitic stainless steel material type AISI 304
to a conventional gas nitriding process throughout temperature range of 400 !00 o"#
nitriding time of $0%&0 hrs and ammonia '() 3 * flow rate of $00 % !00 litre+hr, and fatigue
behaviour of nitrided specimens were then e-amined 'studied or investigated*. The corrosion
rates 'in mm+year* were also detected 'noticed* in order to identify the corrosion resistance
reduction due to nitriding process /uantitatively. revious work on this subject was
performed using the classical method of changing one factor at a time. This old method
re/uires a lot of specimens and e-tensive e-perimental work which is both costly and time
consuming. 1urthermore# the classical method is not capable of detecting the interaction
effects between the factors and also can not perform optimi2ation. All previous drawbacks
are tackled by using the response surface methodology 'S* in the design of e-periment
statistical methods. Accordingly# S design of e-periment is created, samples were
prepared and gas nitrided. 1atigue tests were performed on an Avery%5enison fatigue testing
machine at a stress ratio 6$. The corrosion tests were performed 'carried out* on A"
instrument machine '7ill $8*. The corrosion rates were determined on the basis of cyclic
sweeps corrosion tests in 39 (a"l solution. The fatigue strength of the tested specimens has
increased by about 8:9 as compared with un%nitrided specimens. The results of corrosion
tests revealed that corrosion resistance for nitrided materials was affected by nitriding
processes. The presence of nitrides in the nitrided layers reduces their protective properties#
but its value does not e-ceed !09 of an un%nitrided material 'austenitic stainless steel AISI
304* at the obtained point 'processing parameters set* of ma-imum fatigue limit. Therefore#the nitriding process is preferred process that can be used 'applied* on austenitic stainless
steel type AISI 304 improve 'enhance* its properties 'fatigue# wear and hardness* without
large degradation 'affecting* of its corrosion resistance. The nitrided layer thickness 'depth*
was e-amined by light microscope after grinding and polishing then etching using 89 (ital
agent '8 ml nitric acid ; <= ml ethanol*. >ptimum settings for the time and temperature
Metals and alloys have a diverse application in the medical field, particularly as implantable internal (in vivo)structural, load-bearing materials in devices for partial and total joint replacement, fracture fixation, andinstruments (see Figure (1)). hey have a successful history in the human body because of their appropriatemechanical properties, corrosion resistance, biocompatibility, and manufacturability. Metals have high elasticmoduli and useful yield strengths such that components may be manufactured that !ill sustain significant loads!ithout large elastic deformations or any plastic deformations. hese metals also possess sufficient fatiguestrength, ma"ing them suitable for cyclic load applications for joint replacements or fracture fixation plates. #f properly fabricated, the fatigue strength of metal components is very predictable and can be designed usingappropriate margins of safety. $ne restriction for implant metals is that they cannot have magnetic propertiesdue to the use of magnetic resonance imaging (M%#) systems used as diagnostic tools in the medical field. hefuture development of ne! biomedical materials and the development of ne! manufacturing processes toimprove the performance of traditional biomedical materials !ill increase the need for more materials science
and metallurgical engineering research, including metallography and microstructural characteri&ation. $negroup of most metals and alloys used for orthopaedic medical device are stainless steels '1
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7/23/2019 Fatigue and Corrosion for Nitrided 304 Stainless Steel
he application of austenitic stainless steel in surgical applications began in the 1*+s. he austeniticmicrostructure of these stainless steels is very important due its superior corrosion resistance and nonmagnetic property. hese alloys can be processed by forging, annealing, stress relieving, cold !or"ing, and !elding to avariety of mechanical properties depending on the processing method used and the properties reuired for theapplication '1.
Figure (1) a - he components of a prosthetic total hip system. b - he components for a prosthetic total "neesystem. c - xamples of fracture fixation components such as plates, scre!s, cables, and rods
#n addition, austenitic stainless steels are attractive materials for various industrial sectors to combatenvironmental and corrosive attac". /o!ever, their inherently poor tribological behaviour (in terms of highfriction and lo! !ear resistance) has been the main barrier to !ider application under corrosion and !ear conditions. 0ince then much research and development has aimed to combine improvements in !ear, corrosionand fatigue properties. itriding process can improve fatigue life by producing a plastically deformedcompressive s"in over a relatively undeformed core '*-2. Furthermore, the additional compressive residualstresses, !hich are developed on the surface, decreases the li"elihood of fatigue failure at that surface. %esidual
stresses have been produced by means of the volume changes accompanying the nitriding process '3. For highcycle fatigue ( )"1 ), !here pea" stresses are in the elastic range and the number of cycles reuired to causefailure is in excess of 1+3, the nucleation of micro-crac" in plain specimen constitutes 4+ - +5 of the totalfatigue life. #n lo! cycle fatigue ( "1 ) !here the stresses are high enough to cause macroscopic plasticdeformation, fatigue life is correspondingly reduced (usually belo! 1+ 3), and the initiation and nucleation of micro-crac" in plain specimen may represents only 6+ - 2+5 of the total life '7-4. #n case of nitrided material,the crac" initiation usually tends to shift from surface to sub-surface in high cycle fatigue ( )"1 ). his may bedue to the increased hardness of the surface layer, resulting in better resistance to cyclic slip '7-4. 0everalstudies !ere performed on gas nitriding of stainless steel and other steel types as !ell. 8 brief revie! is provided hereafter, !hich !ould be treated as a reference for designing the experimental !or" and for comparing results obtained./ussein et.al.' investigation sho!ed that nitriding process played the principal role in the improvement of fatigue strength and sub-surface crac" nucleation of the maraging steel.
Menthe et. al.'1+ conducted a series of experiments to study the influence of gas nitriding on the mechanical properties of austenitic stainless steel. /is experiments !ere on the effects of nitriding process on stainless steeltype 8#0# 6+29 in a temperature range of 6:3 - 2:3 o; using pulsed-<; plasma !ith different (* and /*) gasmixtures and treatment times. /e concluded that the treatment influenced the fatigue life, !hich can be raised bymore than 1+5 at a lo! stress level (*6+ M=a). he obtained results sho!ed that plasma nitriding of austeniticstainless steel is a suitable process for improving the mechanical and the tribological properties (especiallyfatigue strength) !ithout significantly effecting the corrosion resistance of this material.>ell '* overvie!ed the development of lo! temperature thermo-chemical surface alloying processes. /ereported that the fatigue properties of the austenitic stainless steels can be substantially improved by lo!temperature nitriding. his is mainly due to the formation of a hardened layer !hich delays the fatigue crac" initiation, and the introduction of compressive residual stress !hich reduces the fatigue crac" propagation rate.%ahman '11 states that surface treatments, such as nitriding, cold rolling or shot peening, are useful to improvefatigue performance. his is due to producing a compressive residual surface stresses, and hence cause the
maximum tensile stress to occur belo! the surface. herefore, these treatments increase the fatigue strength(endurance limit). /e concluded that, fatigue life after nitriding surface treatments is much longer than that dueto other surface treatment processes.
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7/23/2019 Fatigue and Corrosion for Nitrided 304 Stainless Steel
>iela!s"i '1* conducted a nitriding process on chromium steel at a temperature range of 2++-3++ o; inammonia gas atmosphere. he microstructure of the resulted layers !as investigated using scanning electronmicroscopy (0M) and light microscopy (9M) techniues. #ts phase build-up !as chec"ed by ?%< methods,and the thic"ness and microhardness of the layers !ere also measured. /e found that, by applying gas nitridingon chromium steel, it is possible to obtain layers !ith good mechanical properties (microhardness) and goodcorrosion resistance. Moreover, as a result of gas nitriding process, it !as possible to obtain uniform layers
during lo! temperature process. Moreover, he found that for nitriding in temperature belo! 3++o
;, the obtainedlayers remained !hite after etching, !hich could reflect their good corrosion resistance. 8ll the layers sho!edvery good mechanical properties (high hardness) corresponding to a high nitrogen content in the layers.;onseuently, the previous !or" on gas nitriding process of stainless steel sho!ed that the fatigue properties of the austenitic stainless steels can be significantly improved after nitriding depending upon the treatmentconditions. his is mainly due to the formation of a hardened layer !hich delays the fatigue crac" initiation, andthe introduction of compressive residual stress !hich reduces the fatigue crac" propagation rate. 8lso, there is acommon conclusion that lo!-temperature nitriding is preferable to high-temperature nitriding. here is not anoverall agreement on the effect of other process conditions such as ammonia flo! rate or time of nitriding.>arano!s"a '16 presented the results of investigations on the influence of gas nitriding conditions (atmospherecomposition and temperature) on the corrosion resistance of the layers produced on austenitic stainless steel.he treatment !as made in gas atmosphere in the temperature range of 213@2:3 o;. he microstructure and phase composition of the layers !ere investigated using scanning and light microscopy and ?-ray diffraction,
the elements composition !as evaluated by electron probe microanalysis, and the corrosion resistance !asdetermined on the basis of the anodic polarisation corrosion tests in 65 a;l solution. he results revealed thatcorrosion resistance for layers containing expended austenite is better even than that of austenite. he presenceof nitrides in the nitrided layers reduces their protective properties.
herefore, the aim of the present !or" is to subject an austenitic stainless steel material type 8#0# 6+2 to aconventional gas nitriding process throughout a temperature range of 2++@7++ o;, a nitriding time of 1+-3+ hrsand ammonia (/6) flo! rate of 1++-7++ literAhrB and fatigue behaviour of nitrided specimens !ere thenexamined (studied or investigated). he corrosion rates (in mmAyear) !ere also detected (noticed) in order toidentify the corrosion resistance reduction due to nitriding process uantitatively. he ranges of processing parameters (emperature, time, and flo! rate) !ere estimated based on the previous literature revie!.;onventional gas nitriding is adopted in this research !or", because its cheap and can be used for mass production of industrial parts !ith all si&es such as gears, bearings etc.
/o!ever, previous research !or" !as performed using the classical methods of changing one factor at a time!hile holding the other factors constant. his methodology reuires a lot of specimens and extensiveexperimental !or" !hich is both costly and time consuming. Furthermore, the classical method is not capable of investigating the interaction effects bet!een the factors and also can not be used to perform experimentoptimi&ation. 8ll these dra!bac"s are tac"led by using the response surface methodology (%0M) in the designof experiment statistical methods. %eason for not using this method in the past is its complex mathematicalformulation, !hich needs a lot of effort and time. hese are no! facilitated by the recent computer technologicaldevelopment and the generation of po!erful statistical pac"ages such as M#8> program '12.
" #aterials and #ethods
he material used for this investigation !as austenitic stainless steel type 8#0# 6+2 !ith a chemical compositionsho!n in able (1). he material !as stress relieved for 6 hrs at 11++ o; in nitrogen atmosphere, then oil
uenched to avoid oxidation. 8ll specimens !ere subjected to pic"ling pre-treatment using a hot hydrochloricacid (:+ o; C 3+5) to brea" the oxide film, !hich is an essential step for gas nitriding process of stainlesssteels. he response surface methodology (%0M) !as employed to determine the reuired points of experiments(<esign of experiment) !ithin considered ranges of nitriding temperature, nitriding time and ammonia flo! rate.8nhydrous ammonia gas !as used to accomplish the gas nitriding processes. he nitriding processes !ereconducted using a =it Furnace type :3* (0#> company) sho!n in Figure (*). he fatigue specimens !erefabricated in accordance !ith 80M standard 277 @ 4*, specimen shape and dimensions is as sho!n in Figure(6). he specimens !ere subjected to gas nitriding process according to experiment matrix obtained from %0M(see able *). Fatigue tests !ere performed at a unity stress ratio (% D E min A Emax D 1) using 8very-<enisontesting machine sho!n in Figure (2). he pure bending loading condition of the smooth samples is sho!n inFigure (3). 0even samples !ere tested for each nitriding process design point and tests !ere executed up tocomplete failure of the specimens. he nitrided layer thic"ness (depth) !as examined by light microscope after grinding and polishing then etching using *5 ital agent (* ml nitric acid 4 ml ethanol). he corrosion tests
!ere performed (carried out) on 8;M instrument machine (Gill 1*) sho!n in Figure (7), the test specimen and
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7/23/2019 Fatigue and Corrosion for Nitrided 304 Stainless Steel
0even samples for each nitriding process !ere fatigue tested, and their corresponding fatigue strength(ndurance limit) is obtained after fitting results. Figure (4) sho!s one of the obtained S%( fatigue limit curvescorresponding to complete failure and able (*) summari&es the total experimental results. 8ll 0- curvesobtained for nitriding processes matrix are given in appendix@8. hese results !ere then analy&ed usingresponse surface methodology (%0M), and the interactions of nitriding processing parameters (nitridingtemperature, nitriding time and ammonia flo! rate) !ere identified. he effects of nitriding processing parameters on fatigue limit are sho!n in Figure () as contours and three dimensional graphs !hile Figure (1+)sho!s (illustrates) one of 8F9 curve and corresponding =otential vs current (# corr in m8Acm*) curve that theobtained from cyclic s!eeps corrosion test. From Figure ()-a, the optimum temperature setting, !hich is veryclose to about 32+ o;, can easily be estimated from both the 6< surface plot and the contour plot. his setting!ill precisely be determined from the optimi&ation chart, !hich should agree !ith these plots. Figure ()-bsho!s that an optimum time setting can also be figured out as compared !ith flo! rate. Figure ()-c also sho!sthat optimum temperature is clear !hen plotted against the flo! rate. he optimum setting of the flo! rate isvery close to about 2++ literAhrs and can easily be monitored (observed or vie!ed) from both the 6< surface plot and the contour plot. his needs further confirmation from the optimi&ation chart. herefore, a third andcomprehensive !ay of presenting these effects is by developing the optimi&ation chart of the fatigue limit !iththe nitriding conditions !hich is sho!n in Figure (11). Figure (11) sho!s the optimi&ation chart for the performed fatigue tests, and corrosion tests on the gas nitrided specimens. he optimi&ation result is sho!n inthe left column, !hile the optimum setting of each parameter is sho!n at the top ro!. he behaviour curve of each factor is sho!n underneath. 8s sho!n, an optimum nitriding time is 2+ hrs, optimum nitriding temperatureis 32+ o; and optimum ammonia flo! rate setting is 6+ literAhr !hich resulted in fatigue limit of 61:.442M=a. his achievement represents *: 5 increase of the fatigue limit by gas nitriding as compared !ith the un-nitrided value of *3+ M=a. his obtained value of fatigue limit is accompanied !ith relative corrosion rate of 1715 compared !ith un-nitrided value of +.:*: mmAyear corrosion rate !hich does not represent a largedegradation (affecting) of its corrosion resistance. 0o, the nitriding process is preferred process that can be used(applied) on austenitic stainless steel type 8#0# 6+2 to improve (enhance) its fatigue properties. ;omparing these
results !ith literature finding, the 3*3
o
; optimum temperature setting of the gas nitriding process agrees very!ell !ith other researchers findings '1+-1*. Furthermore, the optimum 21 hrs time setting have not beenmentioned in the previous !or", !hich is considered as a further contribution.
Finely (Finally), it is sho!n that the response surface methodology (%0M) is a po!erful tool for studying thenitriding process parameters effects on fatigue limit, and also to find the optimum nitriding process conditions.8lso, the conventional gas nitriding process if properly applied !ould produce excellent surface properties. his process is suitable for mass production of small and even large mechanical components such as gears, and bearings.
3
8uxiliaryelectrode
%eferenceelectrode
est specimenlectrode
7/23/2019 Fatigue and Corrosion for Nitrided 304 Stainless Steel
Figure (1+) ;yclic s!eeps corrosion test curves of nitriding process o :( D 33 o;, t D 2* hrs , Flo! rate D 2 literAhr)
i
Lo0.50(53
ptimal
)ur
d & 0.,522(
i*imum
%el. )or
d & 0.2+156
aimum
Fatigue
& 161.,,56
& 31+.((,4
100.0
600.0
400.0
600.0
10.0
50.0000Temperat Flo$ %atTime
740.08 7540.08 73,0.08
Figure (11) $ptimi&ation chart of gas nitriding process for maximum fatigue limit accompanying!ith the relative corrosion rate obtained by M##8> program
- 0onclusions
From the previous study the follo!ing points are concluded
1. 8 conventional gas nitriding process !as applied on a stress relieved austenitic stainless steel type 8#0#6+2 using anhydrous ammonia. he considered nitriding processing parameters !ere nitridingtemperature, nitriding time and ammonia flo! rate. %esults sho!ed that fatigue limit (endurance limit)has improved by *: 5 as compared !ith the un-nitrided case.
*. he optimum setting for the nitriding temperature, the nitriding time and ammonia flo! rate !ereobtained.
6. he obtained fatigue limit (endurance limit) of nitrided specimens !ere analysed using the responsesurface methodology, !hich proved to be a suitable method for comprehensive gas nitriding parametricstudies and optimi&ation.
herefore, in this study, optimum fatigue limit of 61:.442 M=a is obtained by applying a gas nitriding process at32+ o; for 2+ hrs using 6+ literAhr ammonia flo! rate, !hich represents an increase of *:5 as compared !ith the
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7/23/2019 Fatigue and Corrosion for Nitrided 304 Stainless Steel
*3+ M=a fatigue strength of the un-nitrided materials. ven though, this percentage represents a good achievementespecially !here it is accompanied !ith a little decrease of corrosion resistance not more than 715 . Ac7noledgements
he authors are than"ful to all staff of he 9ibyan echnical %esearch ;anter - Mechanical %esearch >ranch,
ripoli, 9ibya (G0=98I) and (( JKLN OPQR )) for their help and co-operation through out this research program.
%eferences
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itrocarbori&ed Materials V , in Metrological <esign /andboo", ;hap.12, edited by George . otten, Xiyoshi Funtani,and 9in ?ie, Marccl <e"er #nc, (U08), (*++2).
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