Fatigue Crack Growth Behavior of Austempered AISI 4140 Steel with Dissolved Hydrogen

Transcription

1 metals Article Fatigue Crack Growth Behavior Austempered AISI Steel with Dissolved Hydrogen Varun Ramasagara Nagarajan 1, Susil K. Putatunda 1, * James Boileau 2 1 Department Chemical Engeerg Materials Science, Wayne State University, Detroit, MI 48202, USA; varunblr83@yahoo.com 2 Research Innovation Center, Ford Motor Company, Dearborn, MI 48121, USA; jboileau@ford.com * Correspondence: sputa@eng.wayne.edu, Tel.: Received: 16 August 2017; Accepted: 24 October 2017; Published: 1 November 2017 Abstract: The focus this vestigation to exame fluence dissolved on fatigue behavior an low-alloy AISI steel. The vestigation also examed fluence dissolved on fatigue threshold this material. The material tested two conditions, as-received (cold rolled ) (austenitized at 882 C for 1 h at 332 C for 1 h). The microstructure specimens consisted a mix ferrite fe pearlite; microstructure specimens lower baite. Tensile Compact Tension specimens were prepared. To exame fluence dissolved, two subsets CT specimens were charged with for three different time periods between h. All CT samples were n subjected to fatigue tests threshold lear regions at room temperature. The test results dicate that austemperg resulted significant improvement yield tensile strength as well as fracture toughness material. The test results also show that, absence dissolved, rate threshold lear regions lower samples compared to as-received () samples. The fatigue threshold also slightly greater samples. In presence dissolved, rate dependent upon K value. In low K region (<30 MPa m), presence dissolved caused rate to be higher samples as compared to samples. Above this value, rate creasgly greater specimens when compared to specimens presence dissolved. It is concluded that austemperg steel appears to provide a processg route by which strength, hardness, fracture toughness material can be creased with little or no degradation ductility fatigue behavior. Keywords: fatigue rate; dissolved ; fracture toughness; austemperg 1. Introduction In recent years, re has been significant terest austemperg [1 5] as an alternative heat treatment process relative to traditional quenchg temperg processes. Austemperg volves austentizg steel fully austenitic region (above A 1 temperature) followed by a rapid coolg to baitic temperature region. The steel is n held this region for sufficient time to allow completion baitic phase transformation reaction; fally, air coolg to room temperature is conducted. The absence a sudden quench to form martensite (as case traditional quench temperg processes) significantly reduces rmal gradients arisg material. This results reduced distortion mimization appearance quench s. This is especially case for small parts (like gears, bolts, clips) used automotive naval structural applications that are exposed to alternatg stresses. In addition, austemperg can yield Metals 2017, 7, 466; doi: /met

2 Metals 2017, 7, strengths like those created traditional quench temperg processes, especially medium Metals 2017, 7, high carbon steels. Failure yield strengths under cyclic like those loadg created (fatigue) traditional is a very serious quench problem temperg for structural processes, components. especially Under cyclicmedium loadg, s high can carbon arise steels. grow; this case, if grows from a sub-critical dimension to a critical Failure flaw under size, itcyclic can lead loadg ultimately (fatigue) tois failure a very serious service. problem The critical for structural flaw size components. under a given loadg Under condition cyclic loadg, is givens by can fracture arise toughness grow; this case, material if [6]. The grows fatigue from a sub-critical rate, (da/dn), dimension has been to a related critical to flaw size, stress it can tensity lead ultimately factor range, to failure ( K), through service. The Paris critical equation flaw size [7]: under a given loadg condition is given by fracture toughness material [6]. The fatigue rate, (da/dn), has been related da dn = to C( K)m stress tensity factor range, ( K), through (1) Paris equation [7]: where C m are material constants, K = K max K m, which is difference between (1) maximum (K max ) mimum (K m ) stress tensity factor a fatigue cycle. This equation has where C m are material constants, K = Kmax Km, which is difference between been maximum found to be (Kmax) very useful mimum characterizg (Km) stress tensity fatiguefactor a fatigue behavior cycle. This steels. equation has As been Figure found 1to illustrates, be very useful when characterizg experimentally-measured fatigue behavior rate data steels. is plotted agast K on a log As scale, Figure 1 illustrates, graph shows when three experimentally-measured distct regions. In Region I (rate threshold data is plotted region), agast K rate on a log is low scale, deviates graph shows fromthree distct Paris equation. regions. In Region In Region I ( II threshold ( lear region), region), Paris equation models rate is low deviates rate well. from In Region Paris equation. III ( fast In Region fracture II ( region), lear region), Paris rate accelerates equation models aga deviates from rate well. Paris In Region equation. III ( In addition fast fracture to se region), regions, re is a threshold rate accelerates aga deviates from Paris equation. In addition to se regions, re is a stress tensity factor ( K th ), below which rate approaches a zero value. Fatigue threshold stress tensity factor ( Kth), below which rate approaches a zero value. threshold is a very important parameter for structural design; structural components designed on Fatigue threshold is a very important parameter for structural design; structural components basis designed fatigue on threshold basis are fatigue expected threshold to survive are expected service to survive under cyclic service loadg under conditions cyclic loadg without undergog conditions anywithout catastrophic undergog failure. any catastrophic failure. Figure 1. Fatigue Crack Growth Curve Schematic. Figure 1. Fatigue Crack Growth Curve Schematic. AISI is an extensively used commercial-grade medium-carbon, low alloy steel. It contas AISI chromium, ismolybdenum, an extensively used manganese commercial-grade as prcipal medium-carbon, alloyg elements low alloy steel. has Ithigh contas chromium, hardenability molybdenum, good fatigue manganese resistance as [1,2]. prcipal This steel alloyg has many elements applications has mission-critical high hardenability good structural fatigue components resistance [8] because [1,2]. This its steel high strength has many applications toughness. This steel mission-critical can be hardened structural by components a variety [8] because common heat-treatment its high strength processes toughness. yield a This wide steel variety can bemechanical hardenedproperties. by a variety While service critical applications, steel can be exposed to -bearg common heat-treatment processes to yield a wide variety mechanical properties. While service environments. Therefore, -duced fatigue data for this material is needed critical applications, steel can be exposed to -bearg environments. Therefore, for safe life prediction failure-safe design components. Although a significant number -duced fatigue data for this material is needed for safe life prediction studies have been carried out on fatigue corrosion fatigue behavior high strength steels, most failure-safe se design studies were components. carried out Although 3.5% NaCl a significant solutions [9 14]. number studies have been carried out on fatigue corrosion fatigue behavior high strength steels, most se studies were carried out 3.5% NaCl solutions [9 14].

3 Metals 2017, 7, Little formation is available literature on fluence dissolved on fatigue behavior AISI steel, especially condition. One study [15] compared a quenched tempered structure to an structure a gaseous environment. This study found that samples had lower fatigue rates higher impact toughness environment. A second study [14] conducted on a -type steel that charged with approximately 0.58 ppm. At low K values, presence creased fatigue rates up to 30. When K values exceeded 30 MPa m, fatigue rates were comparable regardless wher present or not. Similar results were also observed [16]. Previous research has shown that baitic microstructures improve fracture toughness ferrous alloys [4,5,17]. This dicates that baitic microstructures can possibly improve fatigue resistance both ambient corrosive environments. Therefore, an vestigation undertaken to exame fluence an austemperg process on microstructure, mechanical properties, fatigue rate, fatigue threshold AISI steel with dissolved. In a previous vestigation [18], fatigue behavior AISI steel upper baitic region examed presence dissolved. This vestigation is contuation that study; this paper, fatigue AISI steel, lower baitic temperature region, examed presence dissolved. The testg designed to simulate what steel might see followg a -contag manufacturg operation an ambient-air operatg environment. 2. Experimental Procedure 2.1. Material The material used this vestigation is AISI steel alloy. The material available form cold-rolled plate with an identifiable rollg direction. The chemical composition material is reported Table 1. Table 1. Chemical Composition Alloy Steel. Element Composition (wt %) C 0.40 Cr 1.12 Mo 0.18 Mn 0.93 Si 0.33 S P Cu 0.02 Ni 0.17 Fe Balance From this steel plate, tensile specimens were prepared as per ASTM E-8 [19]. Compact Tension (CT) specimens were also manufactured with a TL orientation as per ASTM stard E-647 [20]. In CT specimens, width (W) 50.8 mm, thickness ranged between 3 4 mm, notch length (as measured from centerle loadg holes to tip mached notch) mm. Additional compact tension samples were prepared for fracture toughness testg as per ASTM stard E-399 [6]; for se samples, width 40 mm thickness 20 mm Heat Treatment Tensile Testg To underst effect austemperg, all test samples (tensile CT) were divided to two batches. The first batch samples left (as-received) condition. The second

4 Metals 2017, 7, batch samples were heat-treated to produce an condition. This accomplished by itially austenitizg se samples at 882 C for 1 h. The samples were n immediately transferred to a molten salt bath at 332 C, where y were held for 1 h. The austemperg this 332 C temperature region produced desired lower baitic structure. The specimens were n removed from salt bath were allowed to air cool to room temperature. All tensile samples ( as well as heat-treated) were tested as per ASTM E-8 [19]. The tests were performed at a constant engeerg stra rate s 1 on a servo-hydraulic Material Test System (MTS) 810 test mache. All samples were tested at room temperature ambient atmosphere. Load displacement plots were obtaed on an X Y recorder; from se load-displacement diagrams, yield strength, ultimate tensile strength, % elongation values were calculated. Four samples were tested each condition Pre-Crackg CT Specimens To mimize amount time between chargg testg, all CT specimens were pre-ed before chargg occurred. Pre-g performed MTS 810 servo-hydraulic test mache load control mode at room temperature ambient atmosphere. For fatigue threshold specimens, a cyclic loadg frequency five cycles per second (5 Hz) used to produce a 2-mm sharp front accordance with ASTM stard E-647 [20]. The CT samples for fracture toughness tests were pre-ed fatigue at a K level 10 MPa m with a load ratio R = This produced a 2-mm sharp accordance with ASTM stard E-399 [6] External Hydrogen Chargg To underst effect austemperg, all pre-ed Compact Tension (CT) specimens were divided to one four sets listed Table 2. Table 2. Matrix Material Conditions for Alloy CT Specimens. Set Material Externally Charged with Hydrogen? A Annealed No B Annealed Yes C Austempered (332 C/1 h) No D Austempered (332 C/1 h) Yes As detailed Table 2, two sets were set aside; se are referred to as Uncharged specimens. The or two sets were externally charged with ( Charged specimens). Chargg accomplished usg two DC power supplies (Agilent E3616A Sorensen XPL-30-2D) that were capable providg a steady source electric current about A a 4-mm staless steel bar as anode; staless steel bar chosen as it an effective, low-cost alternative to platum. The samples to be charged with were used as cathode (Figure 2). Deionized water, obtaed from Barnstead Nanopure Diamond D11911, havg a resistivity about 16.5 ± 0.5 MΩ-cm, used to dissolve sodium hydroxide so that a 1 N solution obtaed. Dissolution done to avoid uptake deleterious ions durg external chargg [21]. This 1 N solution used as electrolyte for chargg to samples. The current density mataed steady at 300 A/m 2. The samples were charged with for 150, 200, 250 h at room temperature. A mimum three samples were charged from each set for each given exposure times; each case, sample tested with 15 m after chargg complete. In addition, six special CT samples were created for purpose measurg concentration conditions. These samples were prepared manner detailed previously.

5 Metals 2017, 7, In addition, six special CT samples were created for purpose measurg concentration conditions. These samples were prepared manner detailed previously. Metals 2017, 7, Figure 2. Experimental setup external chargg. Figure 2. Experimental setup external chargg Hydrogen Concentration Analysis 2.5. Hydrogen Concentration Analysis The six special analysis samples were analyzed for concentration via a Vacuum The six special analysis samples were analyzed for concentration via a Vacuum Hot Hot Extraction method usg an NRC Model 917 unit. The testg conducted at Luvak Extraction method usg an NRC Model 917 unit. The testg conducted at Luvak Laboratories Laboratories (Boylston, MA, USA) with 24 h chargg. Each charged sample first weighed (Boylston, MA, USA) with 24 h chargg. Each charged sample first weighed cleaned cleaned with er to remove surface contamants. Each sample n placed to an with er to remove surface contamants. Each sample n placed to an evacuated chamber evacuated chamber surrounded by duction heatg coil. Each sample heated to a pot below surrounded by duction heatg coil. Each sample heated to a pot below meltg pot meltg pot sample (approximately 1150 C) to degas from sample. The sample (approximately 1150 collected a separate C) to degas from sample. The collected chamber system, measured by a McLeod gauge. a separate chamber system, measured by a McLeod gauge. All measurements All measurements were accurate to ±0.1 ppm. were accurate to ±0.1 ppm Fatigue Testg 2.6. Fatigue Testg With 15 m after completion chargg, each fatigue sample tested on MTS 810 With 15 m after completion chargg, each fatigue sample tested on a MTS 810 servo-hydraulic test mache load control mode at room temperature ambient servo-hydraulic test mache load control mode at room temperature ambient atmosphere. atmosphere. All fatigue testg carried out at a cyclic loadg frequency five cycles per All fatigue testg carried out at a cyclic loadg frequency five cycles per second second (5 Hz). A constant amplitude susoidal waveform applied tests were carried (5 Hz). A constant amplitude susoidal waveform applied tests were carried out at a out at a load ratio R = Km/Kmax = Crack lengths were measured on specimen surface usg load ratio R = K m /K max = Crack lengths were measured on specimen surface usg an an optical microscope without terruptg test; at same time, number fatigue cycles optical microscope without terruptg test; at same time, number fatigue cycles also recorded. The rate, da/dn, determed as per ASTM stard E-647 [20]. also recorded. The rate, da/dn, determed as per ASTM stard E-647 [20]. Four samples from each material condition, as listed Table 2, were tested averaged for Four samples from each material condition, as listed Table 2, were tested averaged for values reported this paper. values reported this paper. The threshold obtaed usg load-sheddg, decreasg K procedure detailed The threshold obtaed usg load-sheddg, decreasg K procedure detailed ASTM E-647 [20]; load periodically decreased until rate reached level ASTM E-647 [20]; load periodically decreased until rate reached a level m/cycle. The reduction load at a given K level done such way that no more than m/cycle. The reduction load at a given K level done such a way that no more than 10% load reduced for that K level. This load sheddg also done after has 10% load reduced for that K level. This load sheddg also done after has grown at least 0.5 mm at previous K level. In this way, any retardation effect (due to previous grown at least 0.5 mm at previous K level. In this way, any retardation effect (due to a previous higher K level) avoided. higher K level) avoided Fracture Fracture Toughness Toughness Test Test With With m m after after completion completion chargg, chargg, each each fracture fracture toughness toughness specimen specimen loaded loaded tension tension a servo-hydraulic servo-hydraulic MTS MTS test test mache; mache; load-displacement load-displacement diagrams diagrams were were obtaed obtaed usg usg a clip a clip gauge gauge knife knife edge edge attachment attachment on on specimen. specimen. From From se se load-displacement load-displacement diagrams, diagrams, PQ values were calculated usg 5% secant deviation technique. From se PQ values, KQ P Q values were calculated usg 5% secant deviation technique. From se P Q values, K Q values were determed usg stard stress tensity factor calibration function for CT

6 Metals Metals 2017, 2017, 7, 7, values were determed usg stard stress tensity factor calibration function for CT specimens. Sce se KQ K Q values satisfied all requirements for a valid KIC as per ASTM stard E-399 [6], [6], y y were were judged judged to beto valid be valid K IC values. KIC values. Four samples Four samples from each from material each condition material listed condition Table listed 2 were Table tested 2 were tested averaged for averaged values for reported values reported this paper. this paper. 3. Results Discussion Microstructure Microstructure The The microstructure microstructure steel steel alloy alloy condition condition showed showed a mixed a ferrite mixed + ferrite pearlite + pearlite structure; structure; very fe very needles fe needles pearlite pearlite were were observed observed matrix matrix (Figure (Figure 3) 3) X-ray X-ray diffraction diffraction [18] [18] as as well well as as transmission transmission electron electron microscopy microscopy (TEM) (TEM) examations examations were were carried carried out. out. Both Both techniques techniques detected detected only only body-centered body-centered cubic cubic (BCC) (BCC) phases phases material. material. No retaed No austenite retaed austenite detected (via detected XRD or (via TEM) XRD or or observed TEM) or observed prepared metallographic prepared metallographic specimens. specimens. Figure 3. Fe Pearlite Observed Annealed AISI Steel Alloy (Sodium Bisulfate etch). The microstructure samples is shown Figure 4. It shows The microstructure samples is shown Figure 4. It shows presence lower baite (blue-colored phase) a limited number isolated pockets presence lower baite (blue-colored phase) a limited number isolated pockets martensite martensite (brown-colored phase) microstructure. These arose even though samples were (brown-colored phase) microstructure. These arose even though samples were at a temperature above Ms (Martensite Start) temperature material. This at a temperature above M s (Martensite Start) temperature material. This attributed to attributed to segregation effects from alloyg elements present steel. The carbides segregation effects from alloyg elements present steel. The carbides alloyg elements alloyg elements like Cr, Mo, Mn tend to segregate to tercellular regions. In se like Cr, Mo, Mn tend to segregate to tercellular regions. In se segregated regions, segregated regions, baitic reaction associated with austenite decomposg to ferrite baitic reaction associated with austenite decomposg to ferrite carbide becomes sluggish; carbide becomes sluggish; refore, complete transformation does not take place durg refore, complete transformation does not take place durg processg time. Upon coolg, processg time. Upon coolg, se untransformed regions can form martensitic structures. se untransformed regions can form martensitic structures. Optically, a very small amount (<1%) Optically, a very small amount (<1%) retaed austenite (white phase) also observed retaed austenite (white phase) also observed material; however, no retaed austenite material; however, no retaed austenite detected usg eir XRD or TEM. Therefore, it is detected usg eir XRD or TEM. Therefore, it is concluded that amount austenite concluded that amount austenite present is so small as to have no effect upon resultant present is so small as to have no effect upon resultant concentration or properties concentration or properties material. material.

7 Metals 2017, 7, Metals 2017, 7, Figure 4. The Microstructure Austempered AISI Steel Alloy (Sodium Bisulfate etch) Mechanical Mechanical Properties Properties The The mechanical mechanical properties properties steel steel are are reported reported Table Table 3. Statistical 3. Statistical analysis analysis this data this showed data showed that that austemperg austemperg process process had significantly had significantly creased creased hardness, hardness, as well as well as both as both yield yield tensile tensile strengths, strengths, without without any significant any significant reduction reduction ductility. ductility. In addition, In addition, fracture toughness fracture toughness shows a statistically shows a slight statistically improvement slight improvement as well. Thus, as well. Thus, microstructure lower microstructure baite with lower a limited baite amount with a martensite limited amount has creased martensite strength, has creased hardness, strength, fracture toughness hardness, without fracture any loss toughness ductility without any AISI loss ductility steel. Furrmore, AISI strength steel. Furrmore, hardness strength material hardness comparable to that material obtaed by a comparable traditional quenchg to that obtaed temperg by a traditional process AISI quenchg steel [22]. temperg process AISI steel [22]. Table 3. Mechanical Properties AISI Alloy. Material Yield Strength Ultimate Tensile Fracture Toughness Material Yield Strength Ultimate Tensile% Elongation % Hardness Hardness (HRc) Fracture Toughness Condition (MPa) Strength Strength (MPa) (MPa) Elongation (HR c ) (MPa m) Annealed 757 ± ± 5 ± ± ± ± 2 Austempered Austempered ( ± ± ± 1 72 ± 6 C/1 h) 1481 ± ± ± 1 72 ± 6 (332 C/1 h) 3.3. Hydrogen Concentration 3.3. Hydrogen Concentration Table 4 shows effect chargg times on concentration dissolved monoatomic Table 4 shows effect chargg times on concentration dissolved monoatomic steel alloy. As this table shows, amount dissolved creases as chargg steel alloy. As this table shows, amount dissolved creases as time creases for both conditions. chargg time creases for both conditions. Table 4. The Effect Chargg Time on Dissolved Hydrogen Content Steel. Table 4. The Effect Chargg Time on Dissolved Hydrogen Content Steel. Material Condition Hydrogen Chargg Time (Hours) Measured Hydrogen Content (ppm) Material Condition Hydrogen Chargg Time (Hours) Measured Hydrogen Content (ppm) Annealed Annealed Austempered (332 C/1 h) Austempered (332 C/1 h)

8 Metals 2017, 7, Metals 2017, 7, Additionally, table shows that samples were found to have over a two-fold crease amount amount dissolved dissolved compared compared to to samples samples for a given for chargg a given time. chargg This time. canthis be attributed can be attributed to small to amount small amount martensite martensite present present microstructure microstructure after austemperg. after austemperg. Martensitic Martensitic structures structures are known are known to have ato higher have dislocation a higher dislocation density when density compared when to compared or phases to or like phases ferrite, like pearlite, ferrite, or pearlite, baite or [1,2]. baite These [1,2]. dislocations These dislocations can actcan as act as traps traps material material [16,23,24]. [16,23,24]. The presence The presence se se traps traps also reduces also reduces diffusivity diffusivity to a lower to a lower value value than what than what estimated estimated by equations by equations based upon based upon concentration concentration [25]. One[25]. previous One vestigation previous vestigation [16] also concluded [16] also that concluded that that diffusivity that diffusivity steel cannot besteel determed cannot by be application determed by Fick s application law Fick s presence law traps presence material. traps material. In addition to dislocation density effects, diffusion coefficient appears to be affected by presence martensite as well. An earlier vestigation on Cr-Mo steels [26] showed that diffusion coefficient martensite margally lower than that baite. Furr, aa recent recent study study on on -duced g g steels [15] steels showed [15] showed that diffusion that coefficient diffusion decreases coefficient as decreases strength as strength alloy creased. alloy creased. Thus, it can be concluded that presence martensite alloy resulted a larger number dislocations, which created a large number traps material. This, turn, resulted higher concentration dissolved content samples compared to sample Influence Influence Austemperg Austemperg on on Crack Crack Growth Growth Behavior Behavior Uncharged Uncharged Steel Steel Figure Figure 5 compares compares fatigue fatigue behavior behavior threshold threshold region region (Region (Region I) for I) for conditions; conditions; this data this data is for is for samples samples without without dissolved dissolved (Sets (Sets A A C C Table Table 2). Table 2). Table 5 details 5 details threshold threshold stress stress tensity tensity values values for se for se materials. materials. As As this this figure figure table table illustrate, illustrate, fatigue fatigue rate rate samples samples near-threshold near-threshold region region higher higher than than samples; samples; additionally, additionally, K th Kth samples samples lower than lower that than that samples samples this region this as well. region as well. Figure 5. The Effect Austemperg on Fatigue Crack Growth Rate Uncharged Steel Figure 5. The Effect Austemperg on Fatigue Crack Growth Rate Uncharged Steel Threshold Region. Threshold Region.

9 Metals 2017, 7, Table 5. Experimentally-Determed Values K th for Uncharged Steel Alloy. Material Condition K th (MPa m) Annealed 6.68 Austempered (332 C/1 h) 8.12 The lower rate samples an terestg result cannot be attributed to lower closure stress tensity factor (K op ) this material. Durg cyclic loadg, compressive stress tip region will cause to rema partially closed durg unloadg part fatigue cycle. This stress tensity factor (for closure) is ten defed as K op. Thus, effective stress tensity factor, K eff, is a measure drivg force is given by K eff = K max K op. This -openg stress tensity factor depends on cyclic plastic zone size, which is versely proportional to yield strength material. Thus, K op value decreases as material strength creases. In this study, cyclic plastic zone size calculated usg Rice s formula [27] for each set specimens at different chargg conditions, based upon where maximum transgranular (or tergranular) feature observed. However, se calculations yielded identical results for all materials; cyclic plastic zone sizes were approximately µm. Furr, samples had a significantly higher yield strength than samples (Table 3). This means that K op value will be lower samples as compared to samples. Therefore, it would be expected that a higher drivg force ( K eff ) would be present samples. Consequently, a higher near-threshold fatigue rate lower fatigue threshold ( K th ) would also be expected for samples. However, opposite observed samples this study: a lower rate higher K th were found for samples. This unique behavior is hyposized to be due to followg two factors: First, austemperg process has creased fracture toughness material due to presence a microstructure contag a large amount lower baite. The lower baitic microstructure creases fracture toughness material [3 5]. The higher fracture toughness is dicative a greater resistance this material. This, turn, causes a lower fatigue rate a higher fatigue threshold material. Secondly, samples had a very fe-scale microstructure consistg lower baite with a limited amount tempered martensite. A lower baitic structure has a much fer gra size than upper baite or pearlite. This creates additional resistance to sce tip encounters large number fe scale gra boundaries. This, turn, reduces propagation rate because grows along a longer, more torturous path. Figure 6 shows fatigue behavior uncharged specimens lear region (Region II). This is furr shown by data Table 6. The material had a much lower value Paris constant C a slightly higher m value. It is evident from plot that specimens have a lower rate than specimens. Aga, general expected trend is that fatigue rate creases with a higher strength hardness. However, samples present study aga have a lower rate despite higher hardness strength; this behavior is similar to what seen previously threshold region. The behavior is also attributed to creased resistance provided by lower baitic microstructure.

10 Metals 2017, 7, Table 6. Comparison Paris Constants for Uncharged Steel Alloy. Metals 2017, 7, Paris Law Constant Hence, upon comparg Material Condition fatigue rates lear region (Figure 6), as well as C m threshold region (Figure 5), it can be concluded that lower baitic microstructure provides Annealed an improved fatigue resistance Austempered (332 steels C/1 h) when dissolved is not present Similar results have been reported for a series low alloy steels [28]. Figure 6. The Effect Austemperg on Fatigue Crack Growth Rate Uncharged Steel Figure 6. The Effect Austemperg on Fatigue Crack Growth Rate Uncharged Steel Lear Region. Lear Region Influence Hydrogen on Fatigue Crack Gowth Behavior Steel Hence, upon comparg fatigue rates lear region (Figure 6), as well as threshold Figure 7 shows region (Figure fatigue 5), it can be concluded behavior that lower baitic samples microstructure lear provides region an presence improved fatigue dissolved ; resistance for comparison, steels when dissolved behavior is not present. Similar samples results without have dissolved been reported for ais series also plotted low alloy steels same [28]. figure. The data Figure 7 shows that presence dissolved causes a large amount variation rate 3.5. data. Influence addition, Hydrogen presence on Fatigue Crack appears Gowth Behavior to crease Steel average rate at K values that are greater than 60 MPa m. Figure 7 also shows that chargg time does Figure 7 shows fatigue behavior samples lear region not have any significant effect on rate, given high degree variation presence dissolved ; for comparison, behavior samples data. without dissolved is also plotted same figure. The data Figure 7 shows that presence dissolved causes a large amount variation rate data. In addition, presence appears to crease average rate at K values that are greater than 60 MPa m. Figure 7 also shows that chargg time does not have any significant effect on rate, given high degree variation data.

11 Metals 2017, 7, Metals 2017, 7, Figure 7. The Effect Hydrogen Chargg Time on Fatigue Crack Growth Rate Lear Figure 7. The Effect Hydrogen Chargg Time on Fatigue Crack Growth Rate Lear Region for Annealed Steel Samples. Region for Annealed Steel Samples. Figure 8 shows fatigue behavior samples lear region Figure 8 shows presence fatigue dissolved ; behavior aga, for comparison purposes, samples lear region behavior presence dissolved ; samples without aga, for dissolved comparison purposes, is also plotted same behavior figure. Similar to that seen samples Figure without 7, Figure dissolved 8 shows that is also plotted chargg time samedoes figure. not Similar have any to significant that seen effect Figure on 7, Figure 8 shows that rate chargg samples. time does However, not have any contrast significant to data effect onfigure 7, Figure 8 shows rate that presence samples. dissolved However, does contrast not to cause data a large amount Figure 7, Figure variation 8 shows that presence rate data. dissolved In addition, Figure does8 not shows cause that a large presence amount dissolved variation creases rate data. In addition, rate Figure significantly 8 shows at that K values presence below 30 dissolved MPa m. Thus, creases effect is relatively rate significantly important when at K values contribution below 30 MPa or m. mechanisms Thus, effect is small. When is K relatively is low, re important is little when stress built contribution up at or tip; mechanisms effect is small. When on K is low, re is creased is little stress sce built re upis atlittle stress tip; to provide effectcreased energy on for propagation is creased a. sce re is little stress to provide creased energy for propagation a. The results Figures 7 8 agree with those from two published research studies [14,16] on 4100-series steel alloys charged with with tested tested laboratory laboratory air. They air. They found found that that rates rates range range 10 5 to m/cycle to 10 7 m/cycle were creased were creased -charged -charged samples when samples compared when compared to uncharged to uncharged samples. samples. Table 7 details details fatigue fatigue threshold threshold both both alloy asalloy a function as a function dissolved dissolved content. The content. data The thisdata table shows this table that re shows arethat onlyre mor are differences only mor differences threshold values threshold values samples; addition, samples; all se addition, valuesall arese low values regardless are low regardless content. This is ancontent. important This result, is an asimportant it shows that result, austemperg as it shows canthat be austemperg used to providecan improved be used strength to provide properties improved while strength avoidg properties a significant while reduction avoidg fatigue a significant reduction resistance; this fatigue combation resistance; not usually this combation observed most is not materials. usually observed most materials.

12 Metals 2017, 7, Metals 2017, 7, Figure 8. The Effect Hydrogen Chargg Time on Fatigue Crack Growth Rate Lear Figure 8. The Effect Hydrogen Chargg Time on Fatigue Crack Growth Rate Lear Region for Austempered Steel Samples. Region for Austempered Steel Samples. Table 7. The Effect Hydrogen-Chargg Time on Kth for Steel Alloy. Table 7. The Effect Hydrogen-Chargg Time on K th for Steel Alloy. Material Condition Hydrogen Chargg Time (Hours) Hydrogen Concentration (ppm) Kth (MPa m) Hydrogen Chargg Hydrogen K Material Condition th Time (Hours) Concentration (ppm) (MPa m) Annealed Annealed Austempered (332 C/1 h) Austempered (332 C/1 h) Table 7 also illustrates that, as concentration creases, fatigue threshold decreases Table slightly 7 also illustrates that, as sample. This concentration is expected creases, result, fatigue it is believed threshold to be decreases due to slightly creased amount sample. that This will is cause expected lattice result, dilation it is steel. believed This, to be turn, due creases to creased stra amount fields ternal energy, that resultg will cause a lattice greater dilation drivg force for steel. This, propagation; turn, this creases results a stra lower fields threshold stress ternal that must energy, be achieved resultg to grow a greater. drivg force for propagation; Table 7 also this shows results that, a lower threshold samples, stress that must dissolved be achieved to grow content. did not have any significant Table 7 also effect. shows The that, reason for this has not samples, been conclusively dissolved determed. It content could did be a not result have any large significant variation effect. The experimental reason for this data. has Or, not it been may conclusively be due to a threshold determed. limit, It could above be which a result presence large variation could be experimental beneficial to data. certa Or, microstructures. it may be due to Furr a threshold vestigation limit, above is necessary which to underst presence se results could is be progress. beneficial to certa microstructures. Furr vestigation is necessary Figures to 9 11 underst compare se fatigue results is progress. behavior lear region for Figures 9 11 specimens compare as a function fatigue chargg time behavior (dissolved lear region content.) for Interestgly, all se figures show specimens that, as a low function (<30 MPa m) K chargg region, time (dissolved specimens have content.) a lower Interestgly, rate all than se figures show specimens. that, As low K (<30 values MPa m) crease K beyond region, approximately 30 specimens MPa m, have a transition a lower stress tensity rate factor than range exists where specimens. As specimens K values have an crease creasgly beyond higher approximately 30 MPa m, rate than a transition stress specimen. tensity factor This range observation exists where dicates that, specimens higher K have regions, an creasgly higher specimens are rate more than resistant to specimen. This -charged observation dicates atmospheres. that, This higher K unexpected, regions, as several vestigators specimens [28,29] are more have reported resistant to -charged atmospheres. This unexpected, as several vestigators [28,29] have reported that embrittlement effects are more severe case high

13 Metals 2017, 7, Metals 2017, 7, Metals 2017, 7, that embrittlement effects are more severe case high strength steels. Due to strength higher steels. strength Due to hardness higher strength alloy hardness conditions, alloy embrittlement conditions, strength steels. Due to higher strength hardness alloy conditions, would be expected embrittlement to have would a stronger be expected effect. Itto is hyposized have a stronger that effect. lower It is baitic hyposized microstructure that embrittlement would be expected to have a stronger effect. It is hyposized that lower developed baitic durg microstructure austemperg developed process durg is counteractg austemperg this embrittlg process is effect; counteractg creased this lower baitic microstructure developed durg austemperg process is counteractg this embrittlg tortuosity effect; creased path is believed tortuosity be key. path is believed to be key. embrittlg creased tortuosity path is believed to be key. Figure 9. The Effect Austemperg on Fatigue Crack Growth Rate Steel Figure The TheEffect Austemperg on on Fatigue Crack Growth Rate Steel Hydrogen-Charged for 150 h. Hydrogen-Charged for 150 h. h. Figure 10. The Effect Austemperg on Fatigue Crack Growth Rate Steel Figure 10. The Effect Austemperg on Fatigue Crack Growth Rate Steel Hydrogen-Charged Figure 10. The Effect for 200 h. Austemperg on Fatigue Crack Growth Rate Steel Hydrogen-Charged for 200 h. Hydrogen-Charged for 200 h.

14 Metals 2017, 7, Metals 2017, 7, Figure 11. The Effect Austemperg on Fatigue Crack Growth Rate Steel Figure 11. The Effect Austemperg on Fatigue Crack Growth Rate Steel Hydrogen-Charged for 250 h. Hydrogen-Charged for 250 h Fractography 3.6. Fractography The fracture surfaces that are associated with alloy samples are shown The fracture surfaces that are associated with alloy samples are shown Figure 12. As this figure shows, chargg with only slightly changes fracture surface Figure 12. As this figure shows, chargg with only slightly changes fracture surface morphology. The fracture surfaces -charged samples (Figure 12b) appear to have morphology. The fracture surfaces -charged samples (Figure 12b) appear to have limited ductile tearg with a more flat-like appearance as compared to uncharged samples limited ductile tearg with a more flat-like appearance as compared to uncharged samples (Figure 12a). This correlates well with experimentally-determed fatigue rate data (Figure 12a). This correlates well with experimentally-determed fatigue rate data shown Figures 6 7, which show relative closeness fatigue curves to one anor shown Figures 6 7, which show relative closeness fatigue curves to one anor regardless chargg condition. regardless chargg condition. Figure 13 shows fracture surfaces associated with alloy samples at Figure 13 shows fracture surfaces associated with alloy samples at low K values. Several terestg characteristics were observed. Similar to that seen Figure 12a, low K values. Several terestg characteristics were observed. Similar to that seen Figure 12a, fracture surface uncharged samples (Figure 13a) have little ductile tearg are fracture surface uncharged samples (Figure 13a) have little ductile tearg are relatively relatively flat morphology. However, contrast to those seen Figure 12b, Figure 13b shows flat morphology. However, contrast to those seen Figure 12b, Figure 13b shows that that -charged samples are characterized by both tergranular -charged samples are characterized by both tergranular transgranular transgranular features. Intergranular features appear creasgly promently as K values features. Intergranular features appear creasgly promently as K values decrease decrease charged samples. This also agrees well with experimental fatigue charged samples. This also agrees well with experimental fatigue data, data, which shows that samples that are charged with which shows that samples that are charged with have fatigue have fatigue rates that are faster fatigue thresholds that are lower when rates that are faster fatigue thresholds that are lower when compared to those compared to those material at low K values. material at low K values. Figure 14 shows fracture surfaces associated with alloy samples at Figure 14 shows fracture surfaces associated with alloy samples at high high K values. As this figure details, fracture surface uncharged samples (Figure 14a) are K values. As this figure details, fracture surface uncharged samples (Figure 14a) are very very similar to those charged samples (Figure 14b). Both fracture surfaces are characterized similar to those charged samples (Figure 14b). Both fracture surfaces are characterized by a large by a large number transgranular features. Once aga, this agrees well with experimental number transgranular features. Once aga, this agrees well with experimental fatigue fatigue data for high K values, which show that fatigue rates for data for high K values, which show that fatigue rates for charged charged uncharged material are similar at high K values. uncharged material are similar at high K values.

15 Metals 2017, 7, Metals 2017, 7, 466 Metals 2017, 7, (a) (a) (b) (b) Figure 12. Typical Fatigue Fracture Surface Features Present Annealed Samples. ( K = 12 Figure 12.Typical Typical Fatigue Fracture Surface Features Present Annealed Samples. Figure 12. Fracture Surface Features Present Annealed Samples. ( K = 12 Fatigue MPa m; direction from top to bottom photos). (a) Uncharged (No Hydrogen); ( K = 12 MPa m; from top to bottom photos). (No (a) Hydrogen); Uncharged MPa m; direction direction from top to bottom photos). (a) Uncharged (b) Hydrogen-Charged. (NoHydrogen-Charged. Hydrogen); (b) Hydrogen-Charged. (b) (a) (a) Figure 13. Cont.

16 Metals 2017, 7, Metals 2017, 7, Metals 2017, 7, (b) (b) Figure 13. Typical TypicalFatigue FatigueFracture FractureSurface SurfaceFeatures Features Present Austempered Samples at Figure 13. Present Austempered Samples at low MPa m; Figure 13. Typical Fatigue Fracture Surface Features Present Austempered Samples at(a) low K Values. ( K = 19 direction from top to bottom photos). K Values. ( K = 19 MPa m; direction from top to bottom photos). (a) Uncharged low K Values. ( K = 19 MPa m; direction from top to bottom photos). (a) Uncharged (No Hydrogen); (b) Hydrogen-Charged. (No Hydrogen); (b) Hydrogen-Charged. Uncharged (No Hydrogen); (b) Hydrogen-Charged. (a) (b) (b) Figure Typical Fatigue Fracture Surface Features Present Austempered Samples at Figure Typical Fatigue Fracture Surface Features Present Austempered Samples at high Figure Fatigue Features Present Austempered Samples high 14. K Typical Values. ( K = 50 Fracture MPa m; Surface direction from top to bottom photos). (a) at K Values. ( K = 50 MPa m; direction from top to bottom photos). (a) Uncharged high K Values. = 50 MPa m; direction from top to bottom photos). (a) Uncharged (No ( K Hydrogen); (b) Hydrogen-Charged. (No Hydrogen); (b) Hydrogen-Charged. Uncharged (No Hydrogen); (b) Hydrogen-Charged. 4. Conclusions Future Work 4. Conclusions Conclusions Future Future Work Work 4. To better underst effect upon mechanical properties fatigue To better underst properties effect steel, present upon research study conducted. The fatigue study To better underst effect upon mechanical mechanical properties fatigue steel, exposure present research study The study study conducted conducted ex-situ to simulate aresearch alloy toconducted. durg manufacturg steel, present study conducted. The conducted ex-situ to simulate exposure a alloy to durg manufacturg

17 Metals 2017, 7, ex-situ to simulate exposure a alloy to durg manufacturg followed by usage an ambient environment. From data obtaed this study, followg conclusions can be drawn: 1. Austemperg lower baitic temperature range has significantly creased mechanical properties fracture toughness AISI steel as compared to as-received () condition. 2. In absence any charged, samples had a much lower average rate higher fatigue threshold than as-received () samples. 3. The presence dissolved creased average rate as well as as-received () samples. 4. There is a transition stress tensity factor value approximately MPa m; below this value, presence dissolved causes rate to be higher samples when compared to samples. 5. In presence dissolved, above transition stress tensity factor value, rate creasgly greater specimens as compared to specimens. 6. When compared to as-received () condition, austemperg steel appears to provide a processg route by which strength, hardness, fracture toughness material can be creased with little or no degradation ductility fatigue behavior. The results from this study also highlight need for three future vestigations. First, it is recommended that a study be conducted to better defe effect austemperg temperature on fatigue resistance. This study would also need to more closely characterize closure effects effects microstructural gras on rate propagation. Second, a study examg effect concentration on fatigue resistance should be conducted. This study should clude content measurements before after fatigue test order to underst effect diffusion, gress, outgassg durg testg process. Third, study needs to be repeated for a quenched tempered steel alloy that exposed to chargg like that done current study. This will allow for an evaluation degree commercial improvement yielded by austemperg compared to current commonly used heat treatment process. Author Contributions: This research study performed fulfillment a PhD Thesis by Varun Ramasagara Nagarajan. Susil K. Putatunda Varun Ramasagara Nagarajan conceived designed experiments; James Boileau provided material for this study; Varun Ramasagara Nagarajan performed experiments; Varun Ramasagara Nagarajan, Susil K. Putatunda James Boileau analyzed data wrote paper. Conflicts Interest: The authors declare no conflict terest. References 1. Reed-Hill, R.E.; Abbaschian, R. Prciples Physical Metallurgy; PWS Kent Publishg Company: Boston, MA, USA, Mangoonan, P.L. The Prciple Material Selection Engeerg Design; Prentice Hall: Salle River, NJ, USA, Putatunda, S.K. Fracture Toughness a high carbon high silicon steel. Mater. Sci. Eng. A 2001, 297, [CrossRef] 4. Putatunda, S.K.; Martis, C.; Boileau, J. Influence austemperg temperature on mechanical properties low carbon low alloy steel. Mater. Sci. Eng. A 2011, 528, [CrossRef] 5. Putatunda, S.K.; Sgar, A.V.; Tackett, R.; Lawes, G. Development a high strength high toughness ausferritic steel. Mater. Sci. Eng. A 2009, , [CrossRef] 6. ASTM E-399. Stard Test Method for determation Pla Stra Fracture Toughness Metallic Materials. In Annual Book ASTM Stards; ASTM: Philadelphia, PA, USA, 2016; pp

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