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

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

Varun Ramasagara Nagarajan 1, Susil K. Putatunda 1,* and James Boileau 2 1 Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA; [email protected] 2 Research and Innovation Center, Ford Motor Company, Dearborn, MI 48121, USA; [email protected] * Correspondence: [email protected], Tel.: +1-313-577-3808

Received: 16 August 2017; Accepted: 24 October 2017; Published: 1 November 2017

Abstract: The focus of this investigation was to examine the influence of dissolved hydrogen on the fatigue crack growth behavior of an austempered low-alloy AISI 4140 steel. The investigation also examined the influence of dissolved hydrogen on the fatigue threshold in this material. The material was tested in two conditions, as-received (cold rolled and annealed) and austempered (austenitized at 882 ◦C for 1 h and austempered at 332 ◦C for 1 h). The microstructure of the annealed specimens consisted of a mix of ferrite and fine pearlite; the microstructure of the austempered specimens was lower bainite. Tensile and Compact Tension specimens were prepared. To examine the influence of dissolved hydrogen, two subsets of the CT specimens were charged with hydrogen for three different time periods between 150 and 250 h. All of the CT samples were then subjected to fatigue crack growth tests in the threshold and linear regions at room temperature. The test results indicate that austempering resulted in significant improvement in the yield and tensile strength as well as the fracture toughness of the material. The test results also show that, in the absence of dissolved hydrogen, the crack growth rate in the threshold and linear regions was lower in austempered samples compared to the as-received (annealed) samples. The fatigue threshold was also slightly greater in the austempered samples. In presence of dissolved hydrogen, the crack growth rate was dependent √ upon the ∆K value. In the low ∆K region (<30 MPa m), the presence of dissolved hydrogen caused the crack growth rate to be higher in the austempered samples as compared to annealed samples. Above this value, the crack growth rate was increasingly greater in the annealed specimens when compared to the austempered specimens in presence of dissolved hydrogen. It is concluded that austempering of 4140 steel appears to provide a processing route by which the strength, hardness, and fracture toughness of the material can be increased with little or no degradation in the ductility and fatigue crack growth behavior.

Keywords: fatigue crack growth rate; dissolved hydrogen; fracture toughness; austempering

1. Introduction In recent years, there has been significant interest in austempering [1–5] as an alternative heat treatment process relative to traditional quenching and tempering processes. Austempering involves austentizing the steel in the fully austenitic region (above the A1 temperature) followed by a rapid cooling into the bainitic temperature region. The steel is then held in this region for sufficient time to allow the completion of the bainitic phase transformation reaction; finally, air cooling to room temperature is conducted. The absence of a sudden quench to form martensite (as in the case of traditional quench and tempering processes) significantly reduces the thermal gradients arising in the material. This results in reduced distortion and minimization of the appearance of quench cracks. This is especially the case for small parts (like gears, bolts, and clips) used in automotive and naval structural applications that are exposed to alternating stresses. In addition, austempering can yield

Metals 2017, 7, 466; doi:10.3390/met7110466 www.mdpi.com/journal/metals Metals 2017, 7, 466 2 of 18 strengths like those created in traditional quench and tempering processes, especially in medium and high carbonMetals 2017 steels., 7, 466 2 of 18 Failureyield strengths under cyclic like those loading created (fatigue) in traditional is a very seriousquench problemand tempering for structural processes, components. especially in Under cyclicmedium loading, and cracks high cancarbon arise steels. and grow; in this case, if the crack grows from a sub-critical dimension to a criticalFailure ﬂaw under size, it cyclic can leadloading ultimately (fatigue) to is failure a very inserious service. problem The critical for structural ﬂaw size components. under a given loadingUnder condition cyclic loading, is given cracks by the can fracture arise and toughness grow; in ofthis the case, material if the [crack6]. The grows fatigue from crack a sub-critical growth rate, (da/dN),dimension has been to a relatedcritical toflaw the size, stress it can intensity lead ultimately factor range, to failure (∆K), in through service. theThe Pariscritical equation flaw size [7 ]: under a given loading condition is given by the fracture toughness of the material [6]. The fatigue crack growth rate, (da/dN), has been relatedad to the stress intensity factor range, (∆K), through the = C(∆K)m− (1) Paris equation [7]: dN ∆− where C and m are material constants, and ∆K = Kmax − Kmin, which is the difference between(1) the maximum (Kmax) and the minimum (K ) stress intensity factor in a fatigue cycle. This equation has where C and m are material constants,min and ∆K = Kmax − Kmin, which is the difference between the beenmaximum found to be(Kmax very) and useful the minimum in characterizing (Kmin) stress the intensity fatigue factor crack in growth a fatigue behavior cycle. This of steels.equation has Asbeen Figure found1 toillustrates, be very useful when in experimentally-measuredcharacterizing the fatigue crack crack growth growth behavior rate dataof steels. is plotted against ∆K on a logAs scale,Figure the1 illustrates, graph shows when threeexperimentally-measu distinct regions.red In crack Region growth I (the rate threshold data is plotted region), against the crack growth∆K rateon a log is low scale, and the deviatesgraph shows from three the distinct Paris equation.regions. In Region In Region I (the II threshold (the linear region), region), the crack theParis equationgrowth models rate is the low growth and deviates rate well. from Inthe Region Paris equation. III (the fastIn Region fracture II (the region), linear the region), crack the growth Paris rate acceleratesequation and models again the deviates growth from rate thewell Paris. In Region equation. III (the In additionfast fracture to theseregion), regions, the crack there growth is a threshold rate accelerates and again deviates from the Paris equation. In addition to these regions, there is a stress intensity factor (∆Kth), below which the crack growth rate approaches a zero value. Fatigue threshold stress intensity factor (∆Kth), below which the crack growth rate approaches a zero value. threshold is a very important parameter for structural design; structural components designed on the Fatigue threshold is a very important parameter for structural design; structural components basisdesigned of fatigue on threshold the basis of are fatigue expected threshold to survive are expected in service to survive under cyclicin service loading under conditions cyclic loading without undergoingconditions any without catastrophic undergoing failure. any catastrophic failure.

Figure 1. Fatigue Crack Growth Curve Schematic. Figure 1. Fatigue Crack Growth Curve Schematic. AISI 4140 is an extensively used commercial-grade medium-carbon, low alloy steel. It contains AISIchromium, 4140 ismolybdenum, an extensively and used manganese commercial-grade as the principal medium-carbon, alloying elements low alloy and steel. has Ithigh contains chromium,hardenability molybdenum, and good and fatigue manganese resistance as [1,2]. the principal This steel alloying has many elements applications and hasin mission-critical high hardenability and goodstructural fatigue components resistance [8] because [1,2]. Thisof its steelhigh strength has many and applicationstoughness. This in steel mission-critical can be hardened structural by componentsa variety [ 8of] becausecommon of heat-treatment its high strength processes and toughness. to yield a Thiswide steelvariety can of be mechanical hardened properties. by a variety of commonWhile heat-treatment in service in processescritical applications, to yield a widethe 4140 variety steel of can mechanical be expos properties.ed to hydrogen-bearing While in service environments. Therefore, hydrogen-induced fatigue crack growth data for this material is needed in critical applications, the 4140 steel can be exposed to hydrogen-bearing environments. Therefore, for safe life prediction and failure-safe design of components. Although a significant number of hydrogen-induced fatigue crack growth data for this material is needed for safe life prediction and studies have been carried out on fatigue and corrosion fatigue behavior of high strength steels, most failure-safeof these design studies of were components. carried out Althoughin 3.5% NaCl a signiﬁcant solutions [9–14]. number of studies have been carried out on fatigue and corrosion fatigue behavior of high strength steels, most of these studies were carried out in 3.5% NaCl solutions [9–14]. Metals 2017, 7, 466 3 of 18

Little information is available in literature on the inﬂuence of dissolved hydrogen on the fatigue crack growth behavior of AISI 4140 steel, especially in the austempered condition. One study [15] compared a quenched and tempered structure to an austempered structure in a gaseous hydrogen environment. This study found that the austempered samples had lower fatigue crack growth rates and higher impact toughness in the hydrogen environment. A second study [14] was conducted on a 4140-type steel that was charged with approximately 0.58 ppm hydrogen. At low ∆K values, the presence of hydrogen increased the fatigue crack growth rates up to 30×. When the ∆K values √ exceeded 30 MPa m, the fatigue crack growth rates were comparable regardless of whether hydrogen was present or not. Similar results were also observed in [16]. Previous research has shown that bainitic microstructures improve the fracture toughness in ferrous alloys [4,5,17]. This indicates that bainitic microstructures can possibly improve the fatigue crack growth resistance both in ambient and corrosive environments. Therefore, an investigation was undertaken to examine the inﬂuence of an austempering process on the microstructure, mechanical properties, fatigue crack growth rate, and fatigue threshold of AISI 4140 steel with dissolved hydrogen. In a previous investigation [18], the fatigue crack growth behavior of AISI 4140 steel austempered in the upper bainitic region was examined in the presence of dissolved hydrogen. This investigation is the continuation of that study; in this paper, the fatigue crack growth of AISI 4140 steel, austempered in the lower bainitic temperature region, was examined in presence of dissolved hydrogen. The testing was designed to simulate what the 4140 steel might see following a hydrogen-containing manufacturing operation and an ambient-air operating environment.

2. Experimental Procedure

2.1. Material The material used in this investigation is the AISI 4140 steel alloy. The material was available in the form of cold-rolled and annealed plate with an identiﬁable rolling direction. The chemical composition of the material is reported in Table1.

Table 1. Chemical Composition of the 4140 Alloy Steel.

Element Composition (wt %) C 0.40 Cr 1.12 Mo 0.18 Mn 0.93 Si 0.33 S 0.031 P 0.025 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 standard E-647 [20]. In the CT specimens, the width (W) was 50.8 mm, the thickness ranged between 3 and 4 mm, and the notch length (as measured from the centerline of the loading holes to the tip of the machined notch) was 15.24 mm. Additional compact tension samples were prepared for fracture toughness testing as per ASTM standard E-399 [6]; for these samples, the width was 40 mm and the thickness was 20 mm.

2.2. Heat Treatment and Tensile Testing To understand the effect of austempering, all of the test samples (tensile and CT) were divided into two batches. The ﬁrst batch of samples was left in the annealed (as-received) condition. The second Metals 2017, 7, 466 4 of 18 batch of samples were heat-treated to produce an austempered condition. This was accomplished by initially austenitizing these samples at 882 ◦C for 1 h. The samples were then immediately transferred to a molten salt bath at 332 ◦C, where they were held for 1 h. The austempering in this 332 ◦C temperature region produced the desired lower bainitic structure. The specimens were then removed from the salt bath and were allowed to air cool to room temperature. All of the tensile samples (annealed as well as heat-treated) were tested as per ASTM E-8 [19]. The tests were performed at a constant engineering strain rate of 4 × 10−4 s−1 on a servo-hydraulic Material Test System (MTS) 810 test machine. All of the samples were tested at room temperature and ambient atmosphere. Load and displacement plots were obtained on an X–Y recorder; from these load-displacement diagrams, the yield strength, ultimate tensile strength, and % elongation values were calculated. Four samples were tested in each condition.

2.3. Pre-Cracking of CT Specimens To minimize the amount of time between hydrogen charging and testing, all of the CT specimens were pre-cracked before charging occurred. Pre-cracking was performed in the MTS 810 servo-hydraulic test machine in the load control mode at room temperature and ambient atmosphere. For the fatigue threshold specimens, a cyclic loading frequency of ﬁve cycles per second (5 Hz) was used to produce a 2-mm sharp crack front in accordance with ASTM standard E-647 [20]. The CT samples for the fracture √ toughness tests were pre-cracked in fatigue at a ∆K level of 10 MPa m with a load ratio of R = 0.10. This produced a 2-mm sharp crack in accordance with the ASTM standard E-399 [6].

2.4. External Hydrogen Charging To understand the effect of hydrogen and austempering, all of the pre-cracked Compact Tension (CT) specimens were divided into one of the four sets listed in Table2.

Table 2. Matrix of Material Conditions for the 4140 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 in Table2, two of the sets were set aside; these are referred to as the “Uncharged” specimens. The other two sets were externally charged with hydrogen (“Charged” specimens). Charging was accomplished using two DC power supplies (Agilent E3616A and Sorensen XPL-30-2D) that were capable of providing a steady source of electric current of about 1–1.5 A and a 4-mm stainless steel bar as the anode; the stainless steel bar was chosen as it was an effective, low-cost alternative to platinum. The samples to be charged with hydrogen were used as the cathode (Figure2). Deionized water, obtained from Barnstead Nanopure Diamond D11911, having a resistivity of about 16.5 ± 0.5 MΩ-cm, was used to dissolve sodium hydroxide so that a 1 N solution was obtained. Dissolution was done to avoid the uptake of deleterious ions during the external hydrogen charging [21]. This 1 N solution was used as the electrolyte for charging hydrogen into the samples. The current density was maintained steady at 300 A/m2. The samples were charged with hydrogen for 150, 200, and 250 h at room temperature. A minimum of three samples were charged from each set for each of the given exposure times; in each case, the sample was tested within 15 min after charging was complete. In addition, six special CT samples were created for the purpose of measuring the hydrogen concentration in the annealed and austempered conditions. These samples were prepared in the manner detailed previously. Metals 2017, 7, 466 5 of 18

In addition, six special CT samples were created for the purpose of measuring the hydrogen Metalsconcentration2017, 7, 466 in the annealed and austempered conditions. These samples were prepared in5 ofthe 18 manner detailed previously.

Figure 2. Experimental setup of external hydrogen charging. Figure 2. Experimental setup of external hydrogen charging. 2.5. Hydrogen Concentration Analysis 2.5. Hydrogen Concentration Analysis The six special analysis samples were analyzed for the hydrogen concentration via a Vacuum The six special analysis samples were analyzed for the hydrogen concentration via a Vacuum Hot Hot Extraction method using an NRC Model 917 unit. The testing was conducted at Luvak Extraction method using an NRC Model 917 unit. The testing was conducted at Luvak Laboratories Laboratories (Boylston, MA, USA) within 24 h of charging. Each charged sample was first weighed (Boylston, MA, USA) within 24 h of charging. Each charged sample was ﬁrst weighed and cleaned and cleaned with ether to remove surface contaminants. Each sample was then placed into an with ether to remove surface contaminants. Each sample was then placed into an evacuated chamber evacuated chamber surrounded by induction heating coil. Each sample was heated to a point below surrounded by induction heating coil. Each sample was heated to a point below the melting point of the melting point of the sample (approximately 1150 °C) to degas hydrogen from the sample. The the sample (approximately 1150 ◦C) to degas hydrogen from the sample. The hydrogen was collected hydrogen was collected in a separate chamber in the system, and was measured by a McLeod gauge. in a separate chamber in the system, and was measured by a McLeod gauge. All of the measurements All of the measurements were accurate to ±0.1 ppm. were accurate to ±0.1 ppm. 2.6. Fatigue Testing 2.6. Fatigue Testing Within 15 min after completion of charging, each fatigue sample was tested on a MTS 810 Within 15 min after completion of charging, each fatigue sample was tested on a MTS 810 servo-hydraulic test machine in the load control mode at room temperature and in ambient servo-hydraulic test machine in the load control mode at room temperature and in ambient atmosphere. atmosphere. All of the fatigue testing was carried out at a cyclic loading frequency of five cycles per All of the fatigue testing was carried out at a cyclic loading frequency of ﬁve cycles per second second (5 Hz). A constant amplitude sinusoidal waveform was applied and the tests were carried (5 Hz). A constant amplitude sinusoidal waveform was applied and the tests were carried out at a out at a load ratio R = Kmin/Kmax = 0.10. Crack lengths were measured on the specimen surface using load ratio R = K /Kmax = 0.10. Crack lengths were measured on the specimen surface using an an optical microscopemin without interrupting the test; at the same time, the number of fatigue cycles optical microscope without interrupting the test; at the same time, the number of fatigue cycles was was also recorded. The crack growth rate, da/dN, was determined as per ASTM standard E-647 [20]. also recorded. The crack growth rate, da/dN, was determined as per ASTM standard E-647 [20]. Four samples from each material condition, as listed in Table 2, were tested and averaged for the Four samples from each material condition, as listed in Table2, were tested and averaged for the values reported in this paper. values reported in this paper. The threshold was obtained using the load-shedding, decreasing ∆K procedure detailed in The threshold was obtained using the load-shedding, decreasing ∆K procedure detailed in ASTM E-647 [20]; the load was periodically decreased until the crack growth rate reached a level of ASTM E-647 [20]; the load was periodically decreased until the crack growth rate reached a level of 10−10 m/cycle. The reduction of load at a given ∆K level was done in such a way that no more than 10−10 m/cycle. The reduction of load at a given ∆K level was done in such a way that no more than 10% of the load was reduced for that ∆K level. This load shedding was also done after the crack has 10% of the load was reduced for that ∆K level. This load shedding was also done after the crack has grown at least 0.5 mm at the previous ∆K level. In this way, any retardation effect (due to a previous grown at least 0.5 mm at the previous ∆K level. In this way, any retardation effect (due to a previous higher ∆K level) was avoided. higher ∆K level) was avoided.

2.7. Fracture Toughness TestTest Within 15 minmin afterafter completioncompletion of charging,charging, eacheach fracturefracture toughnesstoughness specimen was loaded in tension inin aa servo-hydraulicservo-hydraulic MTSMTS testtest machine;machine; the the load-displacement load-displacement diagrams diagrams were were obtained obtained using using a clipa clip gauge gauge in in the the knife knife edge edge attachment attachment on on the the specimen. specimen. FromFrom thesethese load-displacementload-displacement diagrams, PQ values were calculated using the 5% secant deviation technique. From these PQ values, the KQ PQ values were calculated using the 5% secant deviation technique. From these PQ values, the KQ values were determined using the standard stress intensity factor calibration function for the CT Metals 2017, 7, 466 6 of 18 values were determined using the standard stress intensity factor calibration function for the CT specimens. Since Since these these K KQQ valuesvalues satisfied satisﬁed all all of of the the requirements for a valid KICIC as per ASTM standard E-399E-399 [ 6[6],], they they were were judged judged to be to valid be valid KIC values. KIC values. Four samples Four samples from each from material each conditionmaterial listedcondition in Table listed2 were in Table tested 2 were and averagedtested and for aver theaged values for reportedthe values in reported this paper. in this paper.

3. Results and Discussion

3.1. Microstructure The microstructure microstructure of of the the 4140 4140 steel steel alloy alloy in the in annealed the annealed condition condition showed showed a mixed aferrite mixed + ferritepearlite + pearlitestructure; structure; very fine very needles ﬁne needles of pearlite of pearlite were were observed observed in inthe the matrix matrix (Figure (Figure 3)3) X-rayX-ray diffraction [18] [18] as well as transmission electron microscopy (TEM) examinations were were carried carried out. out. Both techniquestechniques detected detected only only body-centered body-centered cubic cu (BCC)bic (BCC) phases phases in the material.in the material. No retained No austeniteretained wasaustenite detected was (viadetected XRD or(via TEM) XRD or or observed TEM) or inobserved prepared in metallographicprepared metallographic specimens. specimens.

Figure 3. Fine Pearlite Observed in the Annealed AI AISISI 4140 Steel Alloy (Sodium Bisulfate etch).

The microstructure of the austempered 4140 samples is shown in the Figure 4. It shows the The microstructure of the austempered 4140 samples is shown in the Figure4. It shows the presence of lower bainite (blue-colored phase) and a limited number of isolated pockets of presence of lower bainite (blue-colored phase) and a limited number of isolated pockets of martensite martensite (brown-colored phase) in the microstructure. These arose even though the samples were (brown-colored phase) in the microstructure. These arose even though the samples were austempered austempered at a temperature above the Ms (Martensite Start) temperature of the material. This was at a temperature above the Ms (Martensite Start) temperature of the material. This was attributed to attributed to segregation effects from the alloying elements present in the steel. The carbides of segregation effects from the alloying elements present in the steel. The carbides of alloying elements alloying elements like Cr, Mo, and Mn tend to segregate to the intercellular regions. In these like Cr, Mo, and Mn tend to segregate to the intercellular regions. In these segregated regions, the segregated regions, the bainitic reaction associated with the austenite decomposing into ferrite and bainitic reaction associated with the austenite decomposing into ferrite and carbide becomes sluggish; carbide becomes sluggish; therefore, complete transformation does not take place during the therefore, complete transformation does not take place during the processing time. Upon cooling, processing time. Upon cooling, these untransformed regions can form martensitic structures. these untransformed regions can form martensitic structures. Optically, a very small amount (<1%) of Optically, a very small amount (<1%) of retained austenite (white phase) was also observed in the retained austenite (white phase) was also observed in the material; however, no retained austenite material; however, no retained austenite was detected using either XRD or TEM. Therefore, it is was detected using either XRD or TEM. Therefore, it is concluded that the amount of austenite concluded that the amount of austenite present is so small as to have no effect upon the resultant present is so small as to have no effect upon the resultant hydrogen concentration or properties in the hydrogen concentration or properties in the austempered material. austempered material.

Metals 2017, 7, 466 7 of 18 Metals 2017, 7, 466 7 of 18

Figure 4. The Microstructure of the Austempered AISI 4140 Steel AlloyAlloy (Sodium(Sodium BisulfateBisulfate etch).etch).

3.2. Mechanical Properties The mechanicalmechanical propertiesproperties of of the the 4140 4140 steel steel are are reported reported in Tablein Table3. Statistical 3. Statistical analysis analysis of this of data this showeddata showed that thethat austempering the austempering process process had signiﬁcantlyhad significantly increased increased the hardness,the hardness, as well as well as both as both the yieldthe yield and tensileand tensile strengths, strengths, without without any signiﬁcant any significant reduction reduction in the ductility.in the ductility. In addition, In addition, the fracture the toughnessfracture toughness shows a statistically shows a slightstatistically improvement slight improvement as well. Thus, theas austemperedwell. Thus, the microstructure austempered of lowermicrostructure bainite with of lower a limited bainite amount with of a martensite limited am hasount increased of martensite the strength, has increased hardness, the and strength, fracture toughnesshardness, and without fracture any losstoughness of ductility without in the any AISI loss 4140 of ductility steel. Furthermore, in the AISI the 4140 strength steel. andFurthermore, hardness ofthe the strength material and was hardness comparable of tothe that material obtained was by acomparable traditional quenchingto that obtained and tempering by a traditional process in AISIquenching 4140 steel and [tempering22]. process in AISI 4140 steel [22].

Table 3. Mechanical Properties of AISI 4140 Alloy.Alloy.

Material Yield Strength Ultimate Tensile Fracture Toughness Material Yield Strength Ultimate Tensile % Hardness Fracture Toughness % Elongation Hardness (HRc) √ Condition (MPa)(MPa) StrengthStrength (MPa) (MPa) Elongation (HRc) (MPa(MPa√m) Annealed 757 757 ±± 2020 1031 1031 ± 5 ± 5 5.6 5.6 28 28 ±± 1 1 65 65± ± 2 Austempered Austempered◦ 1481 ± 12 1646 ± 9 5.5 45 ± 1 72 ± 6 (332 C/1 h) 1481 ± 12 1646 ± 9 5.5 45 ± 1 72 ± 6 (332 °C/1 h)

3.3. Hydrogen Concentration 3.3. Hydrogen Concentration Table4 shows the effect of charging times on the concentration of dissolved monoatomic hydrogen Table 4 shows the effect of charging times on the concentration of dissolved monoatomic in the 4140 steel alloy. As this table shows, the amount of dissolved hydrogen increases as the charging hydrogen in the 4140 steel alloy. As this table shows, the amount of dissolved hydrogen increases as time increases for both the annealed and austempered conditions. the charging time increases for both the annealed and austempered conditions. Table 4. The Effect of Charging Time on the Dissolved Hydrogen Content of the 4140 Steel. Table 4. The Effect of Charging Time on the Dissolved Hydrogen Content of the 4140 Steel.

MaterialMaterial Condition Condition Hydrogen Hydrogen Charging TimeTime (Hours) (Hours) Measured Measured Hydrogen Hydrogen Content (ppm)(ppm) 150 0.7 150 0.7 Annealed 200 1.5 Annealed 250200 1.5 2.9 150250 2.9 1.6 ◦ Austempered (332 C/1 h) 200150 1.6 3.6 250 8.1 Austempered (332 °C/1 h) 200 3.6 250 8.1

Metals 2017, 7, 466 8 of 18 Metals 2017, 7, 466 8 of 18

Additionally, thethe tabletable showsshows thatthat thethe austemperedaustempered samplessamples werewere foundfound to have over a two-fold increase inin the the amount amount of dissolvedof dissolved hydrogen hydrogen compared compared to the to annealed the annealed samples samples for a given for charginga given time.charging This time. can This be attributed can be attributed to the small to the amount small am ofount martensite of martensite present present in the in microstructure the microstructure after austempering.after austempering. Martensitic Martensitic structures structures are known are kn to haveown ato higher have dislocationa higher di densityslocation when density compared when tocompared other phases to other like phases ferrite, like pearlite, ferrite, orpearlite, bainite or [ 1bainite,2]. These [1,2]. dislocations These dislocat canions act can as hydrogenact as hydrogen traps intraps the in material the material [16,23 [16,23,24].,24]. The presence The presence of these of th trapsese traps also reducesalso reduces the diffusivity the diffusivity of hydrogen of hydrogen to a lowerto a lower value value than whatthan waswhat estimated was estimated by equations by equations based upon based the upon concentration the concentration [25]. One [25]. previous One investigationprevious investigation [16] also concluded [16] also thatconcluded that the that diffusivity that the of diffusivity hydrogen inof steelhydrogen cannot in be steel determined cannot bybe applicationdetermined ofby Fick’s application law in of the Fick’s presence law in of the traps presence in the material. of traps in the material. In addition to dislocationdislocation density effects, the diffusion coefficientcoefficient of hydrogen appears to be affected by the presence of martensite as well. An earlier investigation on Cr-Mo steels [[26]26] showedshowed that the diffusion coefficientcoefficient of hydrogen in martensite was marginally lower than than that that in in bainite. bainite. Further, aa recent recent study study on hydrogen-inducedon hydrogen-induced cracking crac inking steels in [15steels] showed [15] showed that the diffusionthat the coefficientdiffusion decreasescoefficient as decreases the strength as the of strength the alloy of increased. the alloy increased. Thus, it can be concluded that the presence of martensite in the austemperedaustempered alloy resulted in a larger number of dislocations, which created a largelarge number of hydrogen traps in the material. This, in turn, resulted in higherhigher concentrationconcentration of dissolved hydrogen content in the austempered samples compared to the annealed sample.sample.

3.4. Influence Inﬂuence of Austempering on the CrackCrack Growth Behavior ofof UnchargedUncharged 41404140 SteelSteel Figure5 5 comparescompares the fatigue crack crack growth growth behavi behavioror in in the the threshold threshold region region (Region (Region I) for I) forthe theannealed annealed and andaustempered austempered conditions; conditions; this data this datais for is the for samples the samples without without dissolved dissolved hydrogen hydrogen (Sets (Sets“A” and “A” “C” and in “C” Table in Table 2). Table2). Table 5 details5 details the thethreshold threshold stress stress intensity intensity values values for forthese these materials. materials. As Asthisthis figure ﬁgure and and table table illustrate, illustrate, the the fatigue fatigue crack crack growth growth rate rate of of the the annealed annealed samples samples in the near-threshold region was higher than the austempered samples; additionally, the ∆K of the annealed near-threshold region was higher than the austempered samples; additionally,th the ∆Kth of the samplesannealed was samples lower was than lower that ofthan the that austempered of the austempered samples in samples this region in this as well.region as well.

Figure 5. The Effect of Austempering on the Fatigue Crack Growth Rate of Uncharged 4140 Steel in Figure 5. The Effect of Austempering on the Fatigue Crack Growth Rate of Uncharged 4140 Steel in the the Threshold Region. Threshold Region.

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Table 5. Experimentally-Determined Values of ∆Kth for the Uncharged 4140 Steel Alloy. √ Material Condition ∆Kth (MPa m) Annealed 6.68 Austempered (332 ◦C/1 h) 8.12

The lower crack growth rate in the austempered samples was an interesting result and cannot be attributed to the lower crack closure stress intensity factor (Kop) in this material. During cyclic loading, the compressive stress in the crack tip region will cause the crack to remain partially closed during the unloading part of the fatigue cycle. This stress intensity factor (for crack closure) is often deﬁned as Kop. Thus, the effective stress intensity factor, ∆Keff, is a measure of crack driving force and is given by ∆Keff = Kmax − Kop. This crack-opening stress intensity factor depends on the cyclic plastic zone size, which is inversely proportional to the yield strength of the material. Thus, the Kop value decreases as the material strength increases. In this study, the cyclic plastic zone size was calculated using Rice’s formula [27] for each set of specimens at the different charging conditions, based upon where the maximum transgranular (or intergranular) feature was observed. However, these calculations yielded identical results for all of the materials; the cyclic plastic zone sizes were approximately 14–22 µm. Further, the austempered samples had a signiﬁcantly higher yield strength than the annealed samples (Table3). This means that the Kop value will be lower in the austempered samples as compared to the annealed samples. Therefore, it would be expected that a higher crack driving force (∆Keff) would be present in the austempered samples. Consequently, a higher near-threshold fatigue crack growth rate and lower fatigue threshold (∆Kth) would also be expected for the austempered samples. However, the opposite was observed in the samples in this study: a lower crack growth rate and higher ∆Kth were found for the austempered samples. This unique behavior is hypothesized to be due to the following two factors:

• First, the austempering process has increased the fracture toughness of the material due to the presence of a microstructure containing a large amount of lower bainite. The lower bainitic microstructure increases the fracture toughness of the material [3–5]. The higher fracture toughness is indicative of a greater crack growth resistance in this material. This, in turn, causes a lower fatigue crack growth rate and a higher fatigue threshold in the material. • Secondly, the austempered samples had a very fine-scale microstructure consisting of lower bainite with a limited amount of tempered martensite. A lower bainitic structure has a much finer grain size than upper bainite or pearlite. This creates additional resistance to crack growth since the crack tip encounters large number of fine scale grain boundaries. This, in turn, reduces the crack propagation rate because the crack grows along a longer, more torturous path.

Figure6 shows the fatigue crack growth behavior of the uncharged annealed and austempered specimens in the linear region (Region II). This is further shown by the data in Table6. The austempered material had a much lower value of the Paris constant “C” and a slightly higher “m” value. It is evident from the plot that the austempered specimens have a lower crack growth rate than the annealed specimens. Again, the general expected trend is that the fatigue crack growth rate increases with a higher strength and hardness. However, the austempered samples in the present study again have a lower crack growth rate despite the higher hardness and strength; this behavior is similar to what was seen previously in the threshold region. The behavior is also attributed to the increased crack growth resistance provided by the lower bainitic microstructure. Metals 2017, 7, 466 10 of 18

Table 6. Comparison of Paris Constants for the Uncharged 4140 Steel Alloy. Metals 2017, 7, 466 10 of 18 Paris Law Constant Hence, upon comparingMaterial the Condition fatigue crack growth rates in the linear region (Figure 6), as well as C m in the threshold region (Figure 5), it can be concluded that the lower bainitic microstructure provides Annealed 1 × 10−10 2.08 an improved fatigue crack growth resistance in 4140 steels when dissolved hydrogen is not present. Austempered (332 ◦C/1 h) 5 × 10−11 2.14 Similar results have been reported for a series of low alloy steels [28].

Figure 6. The Effect of Austempering on the Fatigue Crack Growth Rate of Uncharged 4140 Steel in Figure 6. The Effect of Austempering on the Fatigue Crack Growth Rate of Uncharged 4140 Steel in the the Linear Region. Linear Region. 3.5. Influence of Hydrogen on the Fatigue Crack Gowth Behavior of 4140 Steel Hence, upon comparing the fatigue crack growth rates in the linear region (Figure6), as well as in the thresholdFigure 7 shows region the (Figure fatigue5), itcrack can begrowth concluded behavior that of the the lower annealed bainitic samples microstructure in the linear provides region anin presence improved of fatigue dissolved crack hydrogen; growth resistance for comparis in 4140on, steels the crack when growth dissolved behavior hydrogen of isthe not annealed present. Similarsamples results without have dissolved been reported hydrogen for ais seriesalso plotted of low alloyin the steels same [ 28figure.]. The data in Figure 7 shows that the presence of dissolved hydrogen causes a large amount of variation in the crack growth rate 3.5.data. Influence In addition, of Hydrogen the presence on the Fatigueof hydrogen Crack appears Gowth Behavior to increase of 4140 the Steel average crack growth rate at ∆K values that are greater than 60 MPa√m. Figure 7 also shows that the hydrogen charging time does Figure7 shows the fatigue crack growth behavior of the annealed samples in the linear region in not have any significant effect on the crack growth rate, given the high degree of variation in the presence of dissolved hydrogen; for comparison, the crack growth behavior of the annealed samples data. without dissolved hydrogen is also plotted in the same figure. The data in Figure7 shows that the presence of dissolved hydrogen causes a large amount of variation in the crack growth rate data. In addition, the presence of hydrogen appears to increase the average crack growth rate at ∆K values √ that are greater than 60 MPa m. Figure7 also shows that the hydrogen charging time does not have any significant effect on the crack growth rate, given the high degree of variation in the data.

Metals 2017, 7, 466 11 of 18 Metals 2017, 7, 466 11 of 18

Figure 7. The Effect of Hydrogen Charging Time on the Fatigue Crack Growth Rate in the Linear Figure 7. The Effect of Hydrogen Charging Time on the Fatigue Crack Growth Rate in the Linear Region for the Annealed 4140 Steel Samples. Region for the Annealed 4140 Steel Samples. Figure 8 shows the fatigue crack growth behavior of the austempered samples in the linear regionFigure in the8 shows presence the fatigueof dissolved crack growthhydrogen; behavior again, of for the comparison austempered purposes, samples in the the crack linear growth region behaviorin the presence of the ofaustempered dissolved hydrogen; samples without again, for dissolv comparisoned hydrogen purposes, is also the plotted crack growthin the same behavior figure. of Similarthe austempered to that seen samples in Figure without 7, Figure dissolved 8 shows hydrogen that the ishydrogen also plotted charging in the time same does figure. not Similarhave any to significantthat seen in effect Figure on7 ,the Figure crack8 shows growth that rate the of hydrogenthe austempered charging samples. time does However, not have in any contrast significant to the dataeffect in on Figure the crack 7, Figure growth 8 shows rate of that the the austempered presence of samples. dissolved However, hydrogen in does contrast not tocause the dataa large in amountFigure7 ,of Figure variation8 shows in the that crack the presencegrowth rate of dissolved data. In addition, hydrogen Figure does not8 shows cause that a large the amountpresence of of variation in the crack growth rate data. In addition, Figure8 shows that the presence of dissolved dissolved hydrogen increases the crack growth rate significantly at ∆K values√ below 30 MPa√m. Thus,hydrogen the increaseseffect of hydrogen the crack growthis relatively rate significantly important when at ∆K valuesthe contribution below 30 MPaof otherm. mechanisms Thus, the effect is small.of hydrogen When is∆K relatively is low, there important is little when stress the built contribution up at the ofcrack other tip; mechanisms the effect of is hydrogen small. When on crack∆K is growthlow, there is increased is little stress since built there up is at little the crack stress tip; to theprovide effect increased of hydrogen energy on crack for the growth propagation is increased of a crack.since there is little stress to provide increased energy for the propagation of a crack. The results in Figures 77 andand8 8agree agree with with those those from from two two published published research research studies studies [ [14,16]14,16] on 4100-series steelsteel alloysalloys charged charged with with hydrogen hydrogen and an testedd tested in laboratoryin laboratory air. Theyair. They found found that thethat crack the −5 −7 crackgrowth growth rates inrates the rangein the of range 10 toof 1010−5 m/cycleto 10−7 m/cycle were increased were increased in the hydrogen-charged in the hydrogen-charged samples whensamples compared when compared to the uncharged to the uncharged samples. samples. Table7 7details details the the fatigue fatigue threshold threshold of both of both the annealed the annealed and austempered and austempered 4140 alloy 4140 as aalloy function as a functionof dissolved of dissolved hydrogen hydrogen content. The content. data inThe this data table in shows this table that thereshows are that only there minor are differences only minor in differencesthe threshold in valuesthe threshold of the annealed values of and the austemperedannealed and samples; austempered in addition, samples; all in these addition, values all are these low valuesregardless are oflow hydrogen regardless content. of hydrogen This is an content. important This result, is an as important it shows thatresult, austempering as it shows can that be austemperingused to provide can improved be used strength to provide properties improved while strength avoiding properties a significant while reduction avoiding in fatiguea significant crack reductionresistance; in this fatigue combination crack resistance; is not usually this combinat observedion in mostis not materials.usually observed in most materials.

Metals 2017, 7, 466 12 of 18 Metals 2017, 7, 466 12 of 18

Figure 8. The Effect of Hydrogen Charging Time on the Fatigue Crack Growth Rate in the Linear Figure 8. The Effect of Hydrogen Charging Time on the Fatigue Crack Growth Rate in the Linear Region for the Austempered 4140 Steel Samples. Region for the Austempered 4140 Steel Samples.

Table 7. The Effect of Hydrogen-Charging Time on the ∆Kth for the 4140 Steel Alloy. Table 7. The Effect of Hydrogen-Charging Time on the ∆Kth for the 4140 Steel Alloy.

Material Condition Hydrogen Charging Time (Hours) Hydrogen Concentration (ppm) ∆Kth (MPa√m) Hydrogen Charging Hydrogen ∆K Material Condition √th Time0 (Hours) Concentration--- (ppm) (MPa m) 6.68 150 0 —0.7 6.68 5.58 Annealed 150 0.7 5.58 Annealed 200 1.5 5.49 200 1.5 5.49 250 250 2.92.9 7.85 7.85 0 0 —--- 8.12 8.12 150 1.6 6.59 Austempered (332 ◦C/1 h) 150 1.6 6.59 Austempered (332 °C/1 h) 200 3.6 6.17 200 250 8.13.6 6.01 6.17 250 8.1 6.01 Table7 also illustrates that, as the hydrogen concentration increases, the fatigue threshold decreasesTable slightly7 also illustrates in the austempered that, as the sample. hydrogen This concentration is the expected increases, result, the and fatigue it is believed threshold to bedecreases due to theslightly increased in the amountaustempered of hydrogen sample. that This will is the cause expected lattice result, dilation and in it the is steel.believed This, to inbe turn, due increasesto the increased the strain amount fields and of internalhydrogen energy, that resultingwill cause in alattice greater dilation driving in force the for steel. crack This, propagation; in turn, thisincreases results the in astrain lower fields threshold and stress internal that mustenergy be, achievedresulting toin grow a greater the crack. driving force for crack propagation;Table7 also this shows results that, in a inlower the annealedthreshold samples, stress that the must dissolved be achieved hydrogen to grow content the crack. did not have any significantTable 7 also effect. shows The that, reason in the for annealed this has not sample beens, conclusively the dissolved determined. hydrogen Itcontent could did be a not result have of theany largesignificant variation effect. in theThe experimental reason for this data. has Or,not it b mayeen conclusively be due to a threshold determined. limit, It could above be which a result the presenceof the large of hydrogenvariation couldin the beexperimental beneficial to data. certain Or microstructures., it may be due to Further a threshold investigation limit, above is necessary which tothe understand presence of these hydrogen results andcould is inbe progress.beneficial to certain microstructures. Further investigation is necessaryFigures to9 understand–11 compare these the fatigue results crackand is growth in progress. behavior in the linear region for the annealed and austemperedFigures 9–11 specimens compare as a the function fatigue of crack the charging growth timebehavior (dissolved in the hydrogen linear region content.) for the Interestingly, annealed √ alland of austempered these figures showspecimens that, in as the a low function (<30 MPa of thm)e ∆chargingK region, time the annealed(dissolved specimens hydrogen have content.) a lower crackInterestingly, growth rateall thanof these the austemperedfigures show specimens. that, in the As low the ∆ (<30K values MPa increase√m) ∆K beyondregion, approximatelythe annealed √ 30specimens MPa m, have a transition a lower stresscrack growth intensity rate factor than range the austempered exists where thespecimens. annealed As specimens the ∆K values have anincrease increasingly beyond higherapproximately crack growth 30 MPa rate√m, than a transition austempered stress specimen.intensity factor This range observation exists where indicates the that,annealed in the specimens higher ∆ haveK regions, an increasingly the austempered higher cr specimensack growth arerate more than austempered resistant to crack specimen. growth This in hydrogen-chargedobservation indicates atmospheres. that, in the Thishigher was ∆K unexpected, regions, the as austempered several investigators specimens [28 ,are29] more have reportedresistant to crack growth in hydrogen-charged atmospheres. This was unexpected, as several investigators [28,29] have reported that the embrittlement effects of hydrogen are more severe in the case of high

Metals 2017, 7, 466 13 of 18

Metals 2017, 7, 466 13 of 18 Metals 2017, 7, 466 13 of 18 that the embrittlement effects of hydrogen are more severe in the case of high strength steels. Due to strength steels. Due to the higher strength and hardness of the alloy in austempered conditions, strengththe higher steels. strength Due andto the hardness higher ofstrength the alloy and in hardness austempered of the conditions, alloy in austempered hydrogen embrittlement conditions, hydrogen embrittlement would be expected to have a stronger effect. It is hypothesized that the hydrogenwould be embrittlement expected to have would a stronger be expected effect. Itto is have hypothesized a stronger that effect. the lowerIt is hypothesized bainitic microstructure that the lower bainitic microstructure developed during the austempering process is counteracting this lowerdeveloped bainitic during microstructure the austempering developed process during is counteracting the austempering this embrittling process is effect;counteracting the increased this embrittling effect; the increased tortuosity of the crack path is believed to be key. embrittlingtortuosity ofeffect; the crackthe increased path is believed tortuosity to beof the key. crack path is believed to be key.

Figure 9. The Effect of Austempering on the Fatigue Crack Growth Rate of the 4140 Steel FigureFigure 9. 9. TheThe Effect Effect of ofAustempering Austempering on on the the Fatigue Fatigue Crack Crack Growth Growth Rate Rate of of the the 4140 4140 Steel Steel Hydrogen-Charged for 150 h. Hydrogen-ChargedHydrogen-Charged for for 150 150 h. h.

Figure 10. The Effect of Austempering on the Fatigue Crack Growth Rate of the 4140 Steel Figure 10. The Effect of Austempering on the Fatigue Crack Growth Rate of the 4140 Steel Hydrogen-ChargedFigure 10. The Effect for 200 of h. Austempering on the Fatigue Crack Growth Rate of the 4140 Steel Hydrogen-Charged for 200 h. Hydrogen-Charged for 200 h.

Metals 2017, 7, 466 14 of 18 Metals 2017, 7, 466 14 of 18

Figure 11. The Effect of Austempering on the Fatigue Crack Growth Rate of the 4140 Steel Figure 11. The Effect of Austempering on the Fatigue Crack Growth Rate of the 4140 Steel Hydrogen-Charged for 250 h. Hydrogen-Charged for 250 h. 3.6. Fractography 3.6. Fractography The fracture surfaces that are associated with the annealed 4140 alloy samples are shown in The fracture surfaces that are associated with the annealed 4140 alloy samples are shown in Figure 12. As this figure shows, charging with hydrogen only slightly changes the fracture surface Figure 12. As this figure shows, charging with hydrogen only slightly changes the fracture surface morphology. The fracture surfaces of the hydrogen-charged samples (Figure 12b) appear to have morphology. The fracture surfaces of the hydrogen-charged samples (Figure 12b) appear to have limited ductile tearing with a more flat-like appearance as compared to the uncharged samples limited ductile tearing with a more flat-like appearance as compared to the uncharged samples (Figure 12a). This correlates well with experimentally-determined fatigue crack growth rate data (Figure 12a). This correlates well with experimentally-determined fatigue crack growth rate data shown in Figures 6 and 7, which show the relative closeness of the fatigue curves to one another shown in Figures6 and7, which show the relative closeness of the fatigue curves to one another regardless of the charging condition. regardless of the charging condition. Figure 13 shows the fracture surfaces associated with the austempered 4140 alloy samples at Figure 13 shows the fracture surfaces associated with the austempered 4140 alloy samples at low ∆K values. Several interesting characteristics were observed. Similar to that seen in Figure 12a, low ∆K values. Several interesting characteristics were observed. Similar to that seen in Figure 12a, the fracture surface in the uncharged samples (Figure 13a) have little ductile tearing and are the fracture surface in the uncharged samples (Figure 13a) have little ductile tearing and are relatively relatively flat in morphology. However, in contrast to those seen in Figure 12b, Figure 13b shows flat in morphology. However, in contrast to those seen in Figure 12b, Figure 13b shows that the that the hydrogen-charged austempered samples are characterized by both intergranular and hydrogen-charged austempered samples are characterized by both intergranular and transgranular transgranular features. Intergranular features appear increasingly prominently as the ∆K values features. Intergranular features appear increasingly prominently as the ∆K values decrease in the decrease in the charged austempered samples. This also agrees well with the experimental fatigue charged austempered samples. This also agrees well with the experimental fatigue crack growth data, crack growth data, which shows that the austempered samples that are charged with hydrogen which shows that the austempered samples that are charged with hydrogen have fatigue crack growth have fatigue crack growth rates that are faster and fatigue thresholds that are lower when rates that are faster and fatigue thresholds that are lower when compared to those in the annealed compared to those in the annealed material at low ∆K values. material at low ∆K values. Figure 14 shows the fracture surfaces associated with the austempered 4140 alloy samples at Figure 14 shows the fracture surfaces associated with the austempered 4140 alloy samples at high high ∆K values. As this figure details, the fracture surface in the uncharged samples (Figure 14a) are ∆K values. As this figure details, the fracture surface in the uncharged samples (Figure 14a) are very very similar to those in the charged samples (Figure 14b). Both fracture surfaces are characterized similar to those in the charged samples (Figure 14b). Both fracture surfaces are characterized by a large by a large number of transgranular features. Once again, this agrees well with the experimental number of transgranular features. Once again, this agrees well with the experimental fatigue crack fatigue crack growth data for high ∆K values, which show that the fatigue crack growth rates for growth data for high ∆K values, which show that the fatigue crack growth rates for the charged and the charged and uncharged austempered material are similar at high ∆K values. uncharged austempered material are similar at high ∆K values.

Metals 2017, 7, 466 15 of 18 Metals 2017, 7, 466 15 of 18 Metals 2017, 7, 466 15 of 18

(a) (a)

(b) (b) Figure 12. Typical Fatigue Fracture Surface Features Present in the Annealed 4140 Samples. (∆K = 12 Figure 12. 12. TypicalTypical Fatigue Fatigue Fracture Fracture Surface Surface Features Features Present Present in the Annealed in the Annealed 4140 Samples. 4140 Samples.(∆K = 12 MPa√m; crack growth√ direction was from top to bottom of photos). (a) Uncharged (No Hydrogen); MPa(∆K√ =m; 12 crack MPa growthm; crack direction growth was direction from top was to bottom from topof photos). to bottom (a) ofUncharged photos). (No (a) Hydrogen); Uncharged (b) Hydrogen-Charged. ((Nob) Hydrogen-Charged. Hydrogen); (b) Hydrogen-Charged.

(a) (a)

Figure 13. Cont.

Metals 2017, 7, 466 16 of 18 Metals 2017, 7, 466 16 of 18

Metals 2017, 7, 466 16 of 18

(b) (b) Figure 13.13. TypicalTypical FatigueFatigue Fracture Fracture Surface Surface Features Features Present Present in thein the Austempered Austempered 4140 4140 Samples Samples at low at Figure 13. Typical Fatigue√ Fracture Surface Features Present in the Austempered 4140 Samples at low∆K Values.∆K Values. (∆K =(∆ 19K MPa = 19 MPam; crack√m; growthcrack growth direction direction was from was top from to bottom top to of bottom photos). of (a )photos). Uncharged (a) low ∆K Values. (∆K = 19 MPa√m; crack growth direction was from top to bottom of photos). (a) Uncharged(No Hydrogen); (No Hydrogen); (b) Hydrogen-Charged. (b) Hydrogen-Charged. Uncharged (No Hydrogen); (b) Hydrogen-Charged.

(a)

(b) (b) FigureFigure 14. 14.Typical Typical Fatigue Fatigue Fracture Fracture Surface Surface Features Features Present Present in in the the Austempered Austempered 4140 4140 Samples Samples at highat Figure 14. Typical Fatigue√ Fracture Surface Features Present in the Austempered 4140 Samples at ∆Khigh Values. ∆K Values. (∆K = 50 (∆ MPaK = 50m; MPa crack√m; growth crack growth direction direction was from was top from to bottom top to ofbottom photos). of (photos).a) Uncharged (a) highUncharged ∆K Values. (No (Hydrogen);∆K = 50 MPa (b)√ Hydrogen-Charged.m; crack growth direction was from top to bottom of photos). (a) (No Hydrogen); (b) Hydrogen-Charged. Uncharged (No Hydrogen); (b) Hydrogen-Charged. 4. Conclusions and Future Work 4. Conclusions and Future Work To better understand the effect of hydrogen upon the mechanical properties and fatigue crack growthTo better of austempered understand the4140 effect steel, of the hydrogen present uponupresearchon the mechanicalstudy was conducted. properties and The fatiguestudy was crack growthconducted ofof austempered austempered ex-situ to 4140simulate 4140 steel, steel, the the exposure presentthe present research of aresearch 4140 study alloy wasstudy to conducted. hydrogen was conducted. Theduring study manufacturingThe was study conducted was conducted ex-situ to simulate the exposure of a 4140 alloy to hydrogen during manufacturing

Metals 2017, 7, 466 17 of 18 ex-situ to simulate the exposure of a 4140 alloy to hydrogen during manufacturing followed by usage in an ambient environment. From the data obtained in this study, the following conclusions can be drawn:

1. Austempering in the lower bainitic temperature range has signiﬁcantly increased the mechanical properties and the fracture toughness of AISI 4140 steel as compared to the as-received (annealed) condition. 2. In the absence of any charged hydrogen, the austempered samples had a much lower average crack growth rate and higher fatigue threshold than the as-received (annealed) samples. 3. The presence of dissolved hydrogen increased the average crack growth rate in the austempered as well as in the as-received (annealed) samples. √ 4. There is a transition stress intensity factor value of approximately 40–50 MPa m; below this value, the presence of dissolved hydrogen causes the crack growth rate to be higher in the austempered samples when compared to annealed samples. 5. In presence of dissolved hydrogen, above the transition stress intensity factor value, the crack growth rate was increasingly greater in the annealed specimens as compared to the austempered specimens. 6. When compared to the as-received (annealed) condition, austempering of 4140 steel appears to provide a processing route by which the strength, hardness, and fracture toughness of the material can be increased with little or no degradation in the ductility and fatigue crack growth behavior.

The results from this study also highlight the need for three future investigations. First, it is recommended that a study be conducted to better deﬁne the effect of austempering temperature on the fatigue crack growth resistance. This study would also need to more closely characterize crack closure effects and the effects of microstructural grains on the rate of crack propagation. Second, a study examining the effect of hydrogen concentration on the fatigue crack growth resistance in austempered 4140 should be conducted. This study should include hydrogen content measurements before and after the fatigue test in order to understand the effect of hydrogen diffusion, ingress, and outgassing during the testing process. Third, the study needs to be repeated for a quenched and tempered 4140 steel alloy that was exposed to hydrogen charging like that done in the current study. This will allow for an evaluation of the degree of commercial improvement yielded by austempering compared to the current commonly used heat treatment process.

Author Contributions: This research study was performed in fulfillment of a PhD Thesis by Varun Ramasagara Nagarajan. Susil K. Putatunda and Varun Ramasagara Nagarajan conceived and designed the experiments; James Boileau provided the material for this study; Varun Ramasagara Nagarajan performed the experiments; Varun Ramasagara Nagarajan, Susil K. Putatunda and James Boileau analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Reed-Hill, R.E.; Abbaschian, R. Principles of Physical Metallurgy; PWS–Kent Publishing Company: Boston, MA, USA, 2010. 2. Mangoonan, P.L. The Principle of Material Selection in Engineering Design; Prentice Hall: Salle River, NJ, USA, 1999. 3. Putatunda, S.K. Fracture Toughness of a high carbon high silicon steel. Mater. Sci. Eng. A 2001, 297, 31–43. [CrossRef] 4. Putatunda, S.K.; Martis, C.; Boileau, J. Inﬂuence of austempering temperature on the mechanical properties of low carbon low alloy steel. Mater. Sci. Eng. A 2011, 528, 5053–5059. [CrossRef] 5. Putatunda, S.K.; Singar, A.V.; Tackett, R.; Lawes, G. Development of a high strength high toughness ausferritic steel. Mater. Sci. Eng. A 2009, 513–514, 329–339. [CrossRef] 6. ASTM E-399. Standard Test Method for determination of Plain Strain Fracture Toughness of Metallic Materials. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, USA, 2016; pp. 589–611. Metals 2017, 7, 466 18 of 18

7. Paris, P.C.; Erdogan, F. A critical analysis of crack propagation laws. J. Basic Eng. 1963, 85, 528–534. [CrossRef] 8. Cwiek, J. Hydrogen degradation of high-strength steels. J. Achiev. Mater. Manuf. Eng. 2009, 37, 193–212. 9. Lukacs, J. Fatigue crack propagation limit curves for different metallic and non-metallic materials. Mater. Sci. Forum 2003, 31, 414–415. [CrossRef] 10. Philips, T.V.; McCaffrey, T.J. Ultra High-Strength Steels. In Metals Handbook, 10th ed.; ASM: Geauga County, OH, USA, 1990; Volume 1, pp. 430–448. 11. Banerjee, K.; Chatterjee, U.K. Effect of microstructure on hydrogen embrittlement of weld-simulated HSLA-80 and HSLA-100 steels. Metall. Mater. Trans. A 2003, 34, 1297–1309. [CrossRef] 12. Jha, A.K.; Narayanan, P.R.; Sreekumar, K.; Mittal, M.C.; Ninan, K.N. Hydrogen embrittlement of 3.5Ni-1.5Cr-0.5Mo steel fastener. Eng. Fail. Anal. 2008, 15, 431–439. [CrossRef] 13. Lee, H.; Uhlig, H. Corrosion fatigue of type 4140 high strength steel. Metall. Mater. Trans. B 1972, 3, 2949–2957. [CrossRef] 14. Murakami, Y.; Matusuoka, S. Effect of Hydrogen on fatigue crack growth of metals. Eng. Fract. Mech. 2010, 77, 1926–1940. [CrossRef] 15. Chuang, J.H.; Tsay, L.W.; Chen, C. Crack growth behaviour of heat-treated 4140 steel in air and gaseous hydrogen. Int. J. Fatigue 1998, 20, 531–536. [CrossRef] 16. Vergani, L.; Columbo, C.; Gobbi, G.; Bolzoni, F.M.; Fumagalli, G. Hydrogen effect on fatigue behavior of a quenched and tempered steel. Procedia Eng. 2014, 74, 468–471. [CrossRef] 17. Rao, P.P.; Putatunda, S.K. Comparative study of fracture toughness of austempered ductile irons with upper and lower ausferrite microstructures. Mater. Sci. Technol. 1998, 14, 1257–1265. [CrossRef] 18. Ramasagara Nagarajan, V.; Putatunda, S.K. Inﬂuence of dissolved hydrogen on the fatigue crack growth behavior of a low alloy (AISI 4140) steel. Int. J. Fatigue Special Issue Fatigue Damage Struct. Mater. IX 2014, 62, 236–248. 19. ASTM E-8. Standard Test Method for Tensile Testing of Materials. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, USA, 2016. 20. ASTM E-647. Standard Test Method for Fatigue Crack Growth Behavior of Materials. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, USA, 2016. 21. Karim, R.A. Cathodic hydrogen charging of high strength low alloy 4140 steel. Mater. Perform. 2006, 45, 58–63. 22. Hertzberg, R.W. Deformation of Fracture Mechanics of Engineering Materials, 3rd ed.; John Wiley & Sons: Hoboken, NJ, USA, 1989. 23. Parvathavarthini, N.; Saroja, S.; Dayal, R.K.; Khatak, H.S. Studies on hydrogen permeability of 2.25% Cr-1% Mo ferritic steel: Correlation with microstructure. J. Nucl. Mater. 2001, 288, 187–196. 24. Darken, L.S.; Smith, R.P. Behaviour of hydrogen in steel during and after immersion in acid. Corrosion 1949, 5, 1–16. [CrossRef] 25. Dong, C.F.; Li, X.G.; Liu, Z.Y.; Zhang, Y.R. Hydrogen—Induced cracking and healing behavior of X70 steel. J. Alloys Compd. 2009, 484, 966–972. [CrossRef] 26. Kim, C.D.; Longinow, A.W. Techniques for investigating hydrogen—Induced cracking of steels with high yield strength. Corrosion 1968, 24, 313–318. [CrossRef] 27. Rice, J.R. Fatigue Crack Propagation; ASTM STP 415; ASTM: Philadelphia, PA, USA, 1967; p. 247. 28. Nykyforchyn, H. Effect of hydrogen on the kinetics and mechanism of fatigue crack growth in structural steels. Mater. Sci. 1997, 33, 504–515. [CrossRef] 29. Tsay, L.W.; Liu, C.C.; Shieh, Y.H. Fatigue crack propagation in 2.25Cr-1.0Mo steel weldments in air and hydrogen. Mater. Sci. Eng. A 2001, 299, 16–26. [CrossRef]

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