materials Article X-ray Determination of Compressive Residual Stresses in Spring Steel Generated by High-Speed Water Quenching Diego E. Lozano 1 , George E. Totten 2 , Yaneth Bedolla-Gil 1 , Martha Guerrero-Mata 3 , Marcel Carpio 4 and Gabriela M. Martinez-Cazares 1, * 1 Departamento de Ingeniería, Universidad de Monterrey, Av. Morones Prieto 4500, San Pedro Garza García 66238, Mexico; diego.lozanod@udem.edu (D.E.L.); yaneth.bedolla@udem.edu (Y.B.-G.) 2 Department of Mechanical and Materials Engineering, Portland State University, Portland, OR 97201, USA; getotten@gmail.com 3 Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Pedro de Alba s/n, San Nicolás de los Garza 66455, Mexico; mpgmus@yahoo.com.mx 4 Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica, EBBE, Universitat Politecnica de Catalunya, Av. Eduard Maristany 16, 08019 Barcelona, Spain; marcel.francisco.carpio@upc.edu * Correspondence: gabriela.martinezc@udem.edu  Received: 26 March 2019; Accepted: 8 April 2019; Published: 9 April 2019  Abstract: Automotive components manufacturers use the 5160 steel in leaf and coil springs. The industrial heat treatment process consists in austenitizing followed by the oil quenching and tempering process. Typically, compressive residual stresses are induced by shot peening on the surface of automotive springs to bestow compressive residual stresses that improve the fatigue resistance and increase the service life of the parts after heat treatment. In this work, a high-speed quenching was used to achieve compressive residual stresses on the surface of AISI/SAE 5160 steel samples by producing high thermal gradients and interrupting the cooling in order to generate a case-core microstructure. A special laboratory equipment was designed and built, which uses water as the quenching media in a high-speed water chamber. The severity of the cooling was characterized with embedded thermocouples to obtain the cooling curves at different depths from the surface. Samples were cooled for various times to produce different hardened case depths. The microstructure of specimens was observed with a scanning electron microscope (SEM). X-ray diffraction (XRD) was used to estimate the magnitude of residual stresses on the surface of the specimens. Compressive residual stresses at the surface and sub-surface of about −700 MPa were obtained. Keywords: residual stresses; X-ray; quenching; steel 1. Introduction Medium carbon alloyed steels are used in the manufacture of springs, which are typically hardened by oil quenching and tempering, and subsequently subjected to a warm shot peening process [1,2]. The quenching process generates tensile residual stresses on the surface and the shot peening is carried out to induce compressive residual stresses through plastic deformation [3]. The residual stresses influence the net stress applied in service, while tensile residual stresses increase the net stress range during fatigue conditions; the presence of compressive residual stresses reduce the stress range. Therefore, components with compressive residual stresses have shown considerably better performance under cyclic loading [4,5]. Materials 2019, 12, 1154; doi:10.3390/ma12071154 www.mdpi.com/journal/materials Materials 2019, 12, 1154 2 of 11 Aronov et al. [6] demonstrated that the use of conventional water at high agitation can reduce the film boiling stage during quenching that causes cracking and distortion. Additionally, Kobasko et al. [7] showed that with the use of highly-agitated water it is possible to generate compressive residual stresses on the surface of steel components if the quenching is interrupted before the sub-surface layers reach the martensite transformation. Further studies have revealed that severe cooling of steel parts produced compressive residual stresses at the surface when the quenching is properly interrupted at a specific time for a particular quenching system [8,9]. The temperature difference between the surface and the core needs to be high enough that a martensite case is produced at the surface, while martensitic transformation is avoided at the center. The component experiences thermal contraction due to the cooling before the martensite transformation temperature is reached; at the surface, an expansion caused by the austenite to martensite takes place while sub-surface layers are still hot. The volumetric expansion of the surface case generates compressive residual stresses, which continue increasing up to the maximum value possible. Then, as the core starts to transform, the compressive residual stresses at the surface decrease in magnitude and eventually switch to tensile stresses due to the bulk’s expansion [10]. In the present study, an interrupted high-speed quenching (or intensive-quenching) followed by isotherm heat treatment at the Martensite start temperature (Ms ) was applied to 5160 steel samples with the goal of generating a tempered martensite case with a bainite core. This heat treatment was carried out with the scope of generating compressive residual stresses at the surface. 2. Materials and Methods 2.1. Materials Square bars of 5160 steel were received in the hot rolled condition. The chemical composition was obtained using an optical emission spectrometer, see Table 1. Table 1. Chemical composition of the studied steel. Element C Mn P S Si Ni Cr Cu Nb Ti Al wt% 0.58 0.885 0.017 0.016 0.26 0.01 0.78 0.013 0.003 0.0038 0.0264 The samples used for the quenching process consisted in square bars with a cross section of 20 × 20 mm and 100 mm in length. In order to measure the cooling curve and to analyze the severity of the cooling at different depths, 3 thermocouples were used. Blind holes of Ø 1 mm were made with electrical discharge machining (EDM) up to a middle length at 1/2 and 1/4 of the thickness and 1 mm below the surface. The holes were pre-filled with graphite and K-Type thermocouples were placed in. The graphite power ensured the tightening and good contact between the thermocouple and the sample. A ceramic sealant was employed to prevent water entering the holes during quenching. Only 2D heat transfer takes place at the center since the ratio of the thickness/length is greater than 4, thus the area of interest is at the center of the sample and quenched-ends were discarded. 2.2. Quenching System A schematic representation of the quenching equipment built for this research is shown in Figure 1. The system consists of a 15 HP (horse power) centrifugal water pump, 500 liters reservoir, a Ø3 in. piping line, 2 servo hydraulic valves, air compressor, air line, 2 pneumatic cylinders, manual stopcocks, a manometer gauge, a temperature sensor, and a programmable logic controller (PLC). The water is initially recirculating into the tank with one servo hydraulic valve open and the valve of the chamber is closed until the quenching is manually started. Then, the valves switch and the water flow is redirected to the high-speed chamber during the programmed time. The system operates with a flow rate of 1325 lpm and a pressure of 4 bars. The PLC controls the dwell time for the opening and closing of the valves and the pneumatic cylinders. A sample holder is fixed in the cylinder and is operated Materials 2019, 12, 1154 3 of 11 with a button. The hot sample is placed on the holder and the start button is pressed. The cylinder is raised into the quenching chamber, and at the same time the valves switch the flow to the chamber. The cylinder takes less than one second to reach the upper position and the water flow requires 1.5 seconds to reach the outlet. In the upper position, the sample holder partially blocks the water Materials 2019, 11, x FOR PEER REVIEW 3 of 11 flow exit, thus the chamber is filled and the sample remains fully immersed while the water passes at high-speed through the chamber. After the programed cooling-time is reached, the servo hydraulic passes at high-speed through the chamber. After the programed cooling-time is reached, the servo valves switch Materials again 2019, and 11, x switch FOR the PEER cylinder lowers. The PLC can control the valves with a precision REVIEW of 0.1 3 of 11with hydraulic valves again and the cylinder lowers. The PLC can control the valves a seconds. Figure 2 shows the high-speed chamber operation. precision of 0.1 seconds. Figure 2 shows the high-speed chamber operation. passes at high-speed through the chamber. After the programed cooling-time is reached, the servo hydraulic valves switch again and the cylinder lowers. The PLC can control the valves with a precision of 0.1 seconds. Figure 2 shows the high-speed chamber operation. Figure 1. Figure Figure 1. High-speed 1. High-speed High-speedquenching quenching quenchingsystem. system. system. Figure2.2.High-speed Figure High-speed quenching quenchingchamber in operation. chamber in operation. 2.3. Heat Treatments Figure 2. High-speed quenching chamber in operation. Samples of 5160 steel were pre-heated below Ac1 at 650 °C prior to be austenitized at 850 °C for 2.3. 30 Heat Treatments min and then water quenched in the high-speed system. The experimental matrix included different quenching times in order to interrupt the transformation at certain depths and to obtain a Samples of 5160 steel were pre-heated below Ac1 at 650 °C prior to be austenitized at 850 °C for case-core (also called shell-core) type microstructure, which is suitable to produce compressive 30 min andstresses. residual then water quenched Quenching in the high-speed was interrupted after 2, 3, 4,system. The experimental and 5 seconds matrix and different case included depths different quenching times in order to interrupt the transformation at certain depths and to obtain a Materials 2019, 12, 1154 4 of 11 2.3. Heat Treatments Samples of 5160 steel were pre-heated below Ac1 at 650 ◦ C prior to be austenitized at 850 ◦ C for 30 min and then water quenched in the high-speed system. The experimental matrix included different quenching times in order to interrupt the transformation at certain depths and to obtain a case-core (also called Materials shell-core) 2019, typeREVIEW 11, x FOR PEER microstructure, which is suitable to produce compressive residual4 stresses. of 11 Quenching was interrupted after 2, 3, 4, and 5 seconds and different case depths were obtained. were obtained. Samples were placedSamples were placed in a furnace in a furnace immediately afterimmediately quenching for after quenching 1 hour ◦ C,1which at 250 for hour at 250 °C, corresponds towhich the Mscorresponds temperature. to Since the Mthe s temperature. cooling wasSince the cooling interrupted, the was interrupted, second cycle wasthe second carried outcycle was to promote carried the bainite outisotherm to promote the bainite in the isotherm center and at theinsame the center time and at the same to temper time to temper the martensite the surface on the martensite of the on the surface of the specimen. Figure 3a shows the heat treatment cycle specimen. Figure 3a shows the heat treatment cycle followed. The surface temperature experiencesfollowed. The surface temperaturewhen recalescence experiences recalescence the cooling when theand was interrupted cooling was interrupted the sample was placed andat the 250sample wasthe ◦ C, while placed cooling at 250 °C, while the cooling path of the core enters directly into the bainite zone in the CCT diagram, path of the core enters directly into the bainite zone in the CCT diagram, see Figure 3b. see Figure 3b. Figure3.3.Schematic Figure Schematicofofthe the heat treatment cycle heat treatment cycleshowing showingthe theinterruption interruption ofof quenching quenching (a),(a), andand cooling cooling curves curvesatatdifferent differentlocations locations(b). (b). 2.4. 2.4.Characterization CharacterizationofofSamples samples One Onesample sample of of each condition each waswas condition characterized characterizedafterafter heatheat treatment by means treatment of scanning by means electron of scanning microscopy (SEM, in a JEOL JSM-6510LV Tokyo, Japan) and X-ray diffraction electron microscopy (SEM, in a JEOL JSM-6510LV Tokyo, Japan) and X-ray diffraction (XRD). Two (XRD). Two different X-ray diffraction different X-ray equipment diffraction were used; awere equipment Bruker D8 aAdvance used; Bruker D8 (Billerica, Advance MA,(Billerica, USA) equipment MA, USA) with copper (Cu) radiation and the powder method was used for the determination equipment with copper (Cu) radiation and the powder method was used for the determination of of phases of the samples; a X’pert phases model of the of the Philips samples; (Eindhoven, a X'pert model of ThetheNetherlands) brand with Philips (Eindhoven, Thechromium Netherlands) (Cr)brand radiation withwas used for the (Cr) chromium XRDradiation evaluation wasofused superficial residual for the XRD stresses. evaluation of superficial residual stresses. TheXRD The XRDmeasurement measurement was was performed performed on on the the surface surfaceofofthethequenched quenched samples samples andandthethe preparation prior to the test consisted of removing the remaining oxide in the preparation prior to the test consisted of removing the remaining oxide in the sample. For thesample. For the samples evaluated samples by the semi-destructive evaluated techniquetechnique by the semi-destructive (residual (residual stresses with respect stresses withto respect the sample depth), to the sample electropolishing was employed between each step. In both cases, the crystal lattice depth), electropolishing was employed between each step. In both cases, the crystal lattice was initially was initially measuredand measured andanalyzed analyzedusing using the the standard standard SenSen2ψψmethod 2 methodagainst againstinterplanar interplanar spacing, d [11]. spacing, Once d [11]. Once the network spacing measurement was obtained, a modulus of elasticity of the network spacing measurement was obtained, a modulus of elasticity of 205 GPa and a Poisson 205 GPa and a Poisson ratio of 0.29 were used to determine the stresses in the material using the biaxial stress method. ratio of 0.29 were used to determine the stresses in the material using the biaxial stress method. The non-destructive technique was used in the samples to evaluate the surface and a semi- The non-destructive technique was used in the samples to evaluate the surface and a destructive technique was carried out to assess the profile of residual stresses with respect to the semi-destructive technique was carried out to assess the profile of residual stresses with respect depth of the sample [12]. to the depth of the sample [12]. Figure 4 shows a schematic representation of the angles, directions of stresses, and unit strains Figure 4 shows a schematic representation of the angles, directions of stresses, and unit strains used in the uniaxial analysis and the stress tensor with the principal stresses in three dimensions. used in the uniaxial analysis and the stress tensor with the principal stresses in three dimensions. Materials 2019, 11, x FOR PEER REVIEW 5 of 11 Materials 2019, Materials 12, x1154 2019, 11, FOR PEER REVIEW 5 5ofof11 11 Figure 4. Figure 4. Angle Angle and and directions directions of of stresses stresses and and unit unit strains strains in in aa flat flat sample sample (a), (a), and and stress stress tensor tensor with with the principal the principal stresses stresses schema schema (b) (b) [13]. [13]. Figure 4. Angle and directions of stresses and unit strains in a flat sample (a), and stress tensor with 3. the principalDiscussion stresses schema (b) [13]. 3. Results and Results and Discussion 3.1. 3. Thermal Results andAnalysis Discussion 3.1. Thermal Analysis Figure 5a shows the cooling curves and cooling curve rates obtained with embedded Figure Analysis 3.1. Thermal 5a shows the cooling curves and cooling curve rates obtained with embedded thermocouples in the high-speed chamber from the austenitizing temperature to room temperature. thermocouples in the high-speed chamber from the austenitizing temperature to room temperature. Figure temperature The surface 5a shows the wascooling curves calculated from and cooling the inner curve rates temperatures andobtained use of thewith embedded 2D heat transfer The surface temperature was calculated from the inner temperatures and use of the 2D heat transfer thermocouples problem [14]. The in the high-speedof temperature chamber fromhomogenized the sample the austenitizing temperature at room to room temperature temperature. after 25 seconds, problem [14]. The temperature of the sample homogenized at room temperature after 25 seconds, The while surface temperature the surface was calculated cooled down from the The in only 3 seconds. innerhighest temperatures thermaland use ofobserved gradient the 2D heat transfer between the while the surface cooled down◦ in only 3 seconds. The highest thermal gradient observed between the problem [14].core surface and Thewas temperature 778 C afterof3the sample homogenized at room temperature after 25 seconds, seconds. surface and core was 778 °C after 3 seconds. while the surface cooled down in only 3 seconds. The highest thermal gradient observed between the surface and core was 778 °C after 3 seconds. Figure 5. Cooling curves (a) and cooling rate curves (b); T1- 12 thickness, T2- 14 thickness, and T3-1 mm Figure 5. Cooling curves (a) and cooling rate curves (b); T1-½ thickness, T2-¼ thickness, and T3- below the surface. 1 mm below the surface. Figure 5. Cooling curves (a) and cooling rate curves (b); T1-½ thickness, T2-¼ thickness, and T3- 1The mmcooling below theratesurface calculated . for the system, shown in Figure 5b, referred to as the maximum The cooling rate calculated for the system, shown in Figure 5b, referred to as the maximum cooling rate at the surface, was 497 ◦ C/s. According to Kobasko [15], cooling rates above 470 ◦ C/s cooling rate at the surface, was 497 °C/s. According to Kobasko [15], cooling rates above 470 °C/s result Thein cooling a reduction rate of the cracking calculated probability, for the and due system, shown in to the severity Figure of cooling, 5b, referred themaximum to as the process is result in a reduction of the cracking probability, and due to the severity of cooling, the process is referredrate cooling to by atsome authorswas the surface, as intensive 497 °C/s.quenching. According to Kobasko [15], cooling rates above 470 °C/s referred to by some authors as intensive quenching. resultIninorder to produce a reduction of thecompressive stresses, the cracking probability, andquenching due to themust be stopped severity when of cooling, thethe thermal process is In order to produce compressive stresses, the quenching must be stopped when the thermal gradients referred toare high enough by some authors that martensite as intensive and austenite are present within the sample. From the quenching. gradients are high enough that martensite and austenite are present within the sample. From the cooling curvestoofproduce In order Figure 5,compressive it can be noted that thethe stresses, thermal gradients quenching mustbetween the surface be stopped when the and thermal the core cooling curves of Figure 5, it can be noted that the thermal gradients between the surface and the core are maximum in the first 6 seconds. Thus, the cooling times selected were 1, gradients are high enough that martensite and austenite are present within the sample. From the 2, 3, 4, 5, and 6 seconds, are maximum in the first 6 seconds. Thus, the cooling ◦ C.times selected were 1, 2, 3, 4, 5, and 6 seconds, followed cooling by immediate curves of Figure isotherm 5, it can beinnoted a furnace at 250 that the thermal gradients between the surface and the core followed by immediate isotherm in a furnace at 250 °C. The quenching are maximum system in the first was programmed 6 seconds. with times Thus, the cooling 1 second steps selected starting were 1, 2, 3,with 4, 5, one and second up 6 seconds, The quenching system was programmed with 1 second steps starting with one second up to 6 to 6 seconds. For each cooling time, the followed by immediate isotherm in a furnace at 250 °C.cooling curve was obtained to evaluate the temperature seconds. For each cooling time, the cooling curve was obtained to evaluate the temperature distribution and the stabilization The quenching temperature ofwith system was programmed the sample 1 second after thestarting steps cooling was withinterrupted. one second Figure up to 66 distribution and the stabilization temperature of the sample after the cooling was interrupted. Figure seconds. For each cooling time, the cooling curve was obtained to evaluate the temperature distribution and the stabilization temperature of the sample after the cooling was interrupted. Figure Materials 2019, 12, 1154 6 of 11 Materials 2019, 11, x FOR PEER REVIEW 6 of 11 6 shows the cooling curve for quenching times of 2, 3, 4, and 5 seconds. After one second of shows the cooling curve for quenching times of 2, 3, 4, and 5 seconds. After one second of quenching, quenching, the surface temperature did not reach the Ms temperature, therefore, no transformation the surface temperature did not reach the Ms temperature, therefore, no transformation occurred. occurred. Figure6.6.Cooling Figure Cooling curves curves obtained obtainedin inthe thehigh-speed high-speedchamber chamberforfor (a)(a) 2, 2, (b)(b) 3, (c) 4, and 3, (c) (d) (d) 4, and 5 s.5 s. Fromthe From thethermal thermal data data displayed displayed in in Figure Figure6,6,ititcancanbebeexpected expected that thatthethe optimal optimal conditions to to conditions achieve the case-core microstructure are between 3 s and 4 s, since the 2 s achieve the case-core microstructure are between 3 s and 4 s, since the 2 s condition was not enough condition was not enough timetotocool time coolthe thesurface surface below below thethe MMss temperature temperatureand andtotopromote promote the the martensite martensite transformation; transformation; while in the 5 s condition, the temperature of the core dropped below while in the 5 s condition, the temperature of the core dropped below the Ms , which would be expectedthe M s, which would be toexpected reduce theto reduce the compressive compressive stresses due stresses to thedue to the transformation transformation expansion expansion of the of the core. In core. In the the conditions conditions corresponding to 3 and 4 s, a sufficient thermal gradient was observed corresponding to 3 and 4 s, a sufficient thermal gradient was observed between the surface and the core, between the surface and the core, such that martensite and austenite are present at the same time; thus, compressive such that martensite and austenite are present at the same time; thus, compressive residual stresses on residual stresses on the surface of the sample are expected to some extent since the hardened case the surface of the sample are expected to some extent since the hardened case experiences expansion experiences expansion when the martensite transformation takes place, while the core that remains when the martensite transformation takes place, while the core that remains as austenite experiences as austenite experiences thermal shrinkage. By observing the thermocouple at the core location for thermal shrinkage. By observing the thermocouple at the core location for the different conditions, the different conditions, it can be observed that 4 s is the maximum time before the core reaches the it transformation can be observed that 4 s is the temperature. Themaximum thermal pathtimeofbefore the core reaches the conditions of 2 andthe3 s transformation at the core enterstemperature. directly The thermal path into the bainite nose.of the conditions of 2 and 3 s at the core enters directly into the bainite nose. The ◦ C and showed a bainitic microstructure in the Thesample sampleof ofthe the 33 ss condition stabilizedatat289 condition stabilized 289°C and showed a bainitic microstructure in the core coredue duetotoaahigh highthermal thermal gradient duringthe gradient during thequenching quenchingtreatment treatment creating creating favorable favorable conditions conditions for for thetheformation formationof ofaacore-shell core-shell microstructure, microstructure, as asdepicted depictedininFigure Figure7.7. Canale Canale [16] [16] described described an increase an increase ofofcompressive compressiveresidual residual stress stress due to to the the expansion expansionofofmartensite martensite ononthethe surface surface andand contraction contraction of of thethecore. core. Materials 2019, 11, x FOR PEER REVIEW 7 of 11 Materials 2019, 12, 1154 7 of 11 Materials 2019, 11, x FOR PEER REVIEW 7 of 11 Figure 7. Metallography observed by SEM of 3 s showing tempered martensite at the surface (a) and columnar Figure 7.and nodular bainite Metallography at the by observed core (b).of 3 s showing tempered martensite at the surface (a) and SEM Figure 7. Metallography observed by SEM of 3 s showing tempered martensite at the surface (a) and columnar and nodular bainite at the core (b). 3.2. X-Raycolumnar and nodular bainite at the core (b). Diffraction 3.2. X-ray Diffraction Firstly, 3.2. X-Rayx-ray diffraction was performed to evaluate the presence or absence of retained austenite. Diffraction FigureFirstly, x-ray the 8a displays diffraction was performed X-ray diffraction pattern to corresponding evaluate the presence or absence to cooling times ofof1,retained austenite. 2, 3, 4, and 5 s, Firstly, x-ray diffraction was performed to evaluate the presence or absence of retained austenite. Figure 8a followed bydisplays the isotherm the X-ray diffraction treatment at the pattern corresponding Ms temperature (250 °C).to cooling times of 1, 2, 3, 4, and 5 s, Figure 8a displays the X-ray diffraction pattern corresponding to◦ cooling times of 1, 2, 3, 4, and 5 s, followed The bcc byofthe isotherm retained treatment austenite peak at corresponding the Ms temperature to the (250 planeC). followed by the isotherm treatment at the Ms temperature (250 °C). (2 0 0 γ) was not observed in any The bcc condition, ofitretained austenitethatpeak corresponding to the plane (2 0 0 γ) wasthe notuntransformed observed in any Thethus bcc of can be stated retained austenite the tempering peak corresponding caused transformation to the plane (2 0 0 γ) wasofnot observed in any condition, austenite to thus bainite it can [17].beUpon stated that the tempering evaluation of angle caused 2θ, the the transformation samples corresponding of the to 2,untransformed 3, 4, and 5 s condition, thus it can be stated that the tempering caused the transformation of the untransformed austenite showed to bainite affinity austenite [17]. with bainite to bainite Upon [17]. Upon evaluation indexed cards. of angle 2θ, the samples corresponding to 2, 3, 4,3,and evaluation of angle 2θ, the samples corresponding to 2, 4, and 5s5s showed affinity affinitywith The presence showed bainite bainiteindexed of residual with stressescards. indexed is reflected in the width of the x-ray diffraction peak and cards. The consequently presence of residual stresses The presence of residual stressesanalysis in its intensity. A detailed is reflected is reflected of theininprincipal thewidth the width ofofthe peak the1x-ray (1 0x-rayis diffraction α) diffraction displayed inpeak peak andand Figure consequently 8b. consequently in The peak widens its intensity. A detailed and a reduction in its intensity. A detailed analysis is analysis of observedofin the principal thethe peak intensity principal peak (1 axis. 1 0 (1 1The α) is 0 α) peakdisplayed suffered is displayed in a width8b. Figure in Figure The 8b.peak increase as widens The the cooling peak and time widens aand reduction increased is observed a reduction when in theinintensity compared is observed axis. The withintensity the a reference peak axis.of thesuffered The a width samesuffered peak material increase a without width as the cooling increase as the time increased cooling time when increased compared when heat treatment; that is, with no residual stresses (solid line). with compared a reference with a of reference the of same the samematerial materialwithout without heat treatment; that is,that heat treatment; withis, no withresidual stresses no residual (solid stresses line).line). (solid Figure 8. X-ray diffraction pattern (a) and close-up of the principal peak (110-α) (b). Figure 8. X-ray Figure diffraction 8. X-ray diffractionpattern pattern(a) (a)and andclose-up close-up of of the the principal peak(110-α) principal peak (110-α)(b). (b). The diffraction peak width at 2w or FWHM (full-width at half-maximum), which is related to TheThe diffraction diffraction peak peak widthatat2w width 2wororFWHM FWHM(full-width (full-width at half-maximum), half-maximum), which whichisisrelated relatedtoto the hardening [18,19], is displayed in Table 2 along with the angle, 2θ’, and d2θ’ . The sample of 5 s the the hardening hardening [18,19], [18,19], is is displayedininTable displayed Table22along alongwith with the the angle, 2θ’, and dd22θθ'.'.The 2θ’, and Thesample sampleofof5 5s s exhibited the greater width increment with a 2w value of 0.49, followed by conditions 4, 3, and 2 in Materials 2019, 11, x FOR PEER REVIEW 8 of 11 Materials exhibited the 2019, 12,greater width increment with a 2w value of 0.49, followed by conditions 4, 3, and8 of 1154 2 11 in order of peak deformation. Using the Bragg's Law, the interplanar d-spacing can be calculated using the stress free distance as a reference to obtain the crystallographic strain. The stress free d-spacing order of peak deformation. Using the Bragg’s Law, the interplanar d-spacing can be calculated using considered was 1.1705 (Å). the stress free distance as a reference to obtain the crystallographic strain. The stress free d-spacing considered was 1.1705 (Å). Table 2. Diffraction peak width at maximum height (FWHM parameter). Table 2. Diffraction peak width at maximum 2w height2θ´ (FWHM parameter). Sample Intensity d2θ' Sample1 s 1562 Intensity 2w 0.25 44.665 2θ’ 2.0274 d2θ’ 1s 2s 1562 1497 0.250.35 44.655 44.665 2.0276 2.0274 2s 3s 1497 1134 0.350.39 44.585 44.655 2.0308 2.0276 3s 4s 1134 1123 0.390.45 44.615 44.585 2.0295 2.0308 4s 5s 1123 1058 0.450.49 44.615 44.635 2.0295 2.0287 5s 1058 0.49 44.635 2.0287 3.2.1. Residual Stresses 3.2.1. Residual Stresses Additionally, from the analysis of dφψ vs sin2 ψ, the sign of the residual stresses can be Additionally, determined. A negativefrom the analysis slope means of the vs sin2 ψ, dϕψpresence of the signstresses tensile of the residual while a stresses positivecan be slope determined. A negative slope means the presence of tensile stresses while a positive corresponds to compressive stresses [12]. Figure 9 shows the dφψ vs sin2 ψ pattern obtained from slope corresponds to compressive X´pert stressessoftware Data Collector [12]. Figure 9 shows for tilt anglesthe ofd0°, vs sin2 ϕψ45°, pattern and ψ90°. It canobtained from X’pert be observed that theData 2 s Collector software for tilt angles of 0 ◦ , 45◦ , and 90◦ . It can be observed that the 2 s condition slope is condition slope is positive, which means that it is under tensile residual stresses, and the conditions positive, which of 3, 4, and means 5 s are underthat it is under ones. compressive tensile residual Table stresses, 3 shows and the stress the uniaxial conditions of 3,of4,the analysis and 5 s are different under compressive conditions. ones. Table 3 shows the uniaxial stress analysis of the different conditions. Figure 9. Analysis of dϕψ vs sin2 ψ. Figure 9. Analysis of dφψ vs sin2 ψ. Table 3. Uniaxial stress analysis. Table 3. Uniaxial stress analysis. Angle 2s 3s 4s 5s Angle σϕ2s τϕ σϕ 3s τϕ σϕ 4s τϕ σϕ 5s τϕ ϕ (◦ ) (MPa) (MPa) (MPa) (MPa) (MPa) (MPa) (Mpa) (MPa) φ (°) 0 σφ (MPa) 358.3 τφ (MPa) 6.6 -601.1 σφ (MPa) 20.1 τφ (MPa) σφ–261.2 (MPa) τφ9.9 (MPa) –490.7 σφ (Mpa) 22.9 τφ (MPa) 45 278 19.9 -621.6 –5.6 –316.3 19.4 –508.5 19.9 0 90 358.3189.7 6.612.6 –551 -601.1 –9.3 20.1 –431.8 –261.2 17.4 9.9 –470.6 –490.7 3122.9 45 278 19.9 -621.6 –5.6 –316.3 19.4 –508.5 19.9 90 A biaxial 189.7analysis was 12.6 calculated –551 –9.3 –431.8 17.4 –470.6 31 4. from the stress tensor to obtain the principal stresses, see Table The 3 s condition was the condition that showed the maximum magnitude of compressive stresses with a value of –628 MPa in the principal stress. Sample Stress Tensor (MPa) Principal Stresses (MPa) 358.3 4 6.6 358.4 0 0 2s σij= 4 189.7 12.6 σij= 0 189.6 0 Materials 2019, 12, 1154 6.6 12.6 0 0 0 0 9 of 11 Table 4. Biaxial analysis showing the stress tensor and principal stresses. −601.1 −45.5 20.1 −628 0 0 Sample 3s σij= Tensor Stress −45.5 (MPa) −551 −9.3 σij= 0Principal −524.1 0 (MPa) Stresses 358.3 20.1 4 −9.3 0 6.6 0 358.40 0 0 0 2s σij = 4 189.7 12.6 σij = 0 189.6 0 6.6 12.6 0 0 0 0 −601.1−261.2−45.530.2 20.1 9.9 –256 −6280 0 0 0 3s σij =4s −ij45.5 σ = 30.2 −551 −431.8 − 9.3 17.4 σijσ=ij = 0 0−437 − 0 524.1 0 20.1 −9.3 0 0 0 0 9.9 17.4 0 0 0 0 −261.2 30.2 9.9 –256 0 0 4s σij = 30.2 −431.8 17.4 σij = 0 −437 0 9.9 17.4 0 0 0 0 −490.7 −27.9 22.9 −510.3 0 0 −490.7 −27.9 22.9 −510.3 0 0 5s σij =5s σ = −27.9−470.6 −ij27.9 −470.6 31 31 σijσ= = ij 0 0 −451 0−451 0 22.9 22.9 31 31 00 0 0 0 0 0 0 3.2.2. Residual Stress Profile 3.2.2. Residual Stress Profile The residual stress profiles as a function of the depth were obtained by performing x-ray The residual measurements stress on the profiles surface and as thena afunction of thewas surface layer depth were by dissolved obtained means by performing of an x-ray electropolishing measurements on the surface and then a surface layer was dissolved by means of an process. The residual stresses were measured at different depths down to a 0.31 mm depth. Figure 10electropolishing process. displaysThethe residual residualstresses were measured stress profiles for 3 andat4 different depths s, showing down tovalue a minimum a 0.31ofmm depth. −698 MPaFigure 10 at a 0.26 displays mm depth theand residual −490 stress MPa atprofiles for 3depth, a 0.26 mm and 4respectively. s, showing aFrom minimum value of the residual −698evaluation stress MPa at a 0.26 and mm depth and −490 MPa at a 0.26 mm depth, respectively. From the residual stress the cooling curves, it can be concluded that the reduction in magnitude of the compressive stressesevaluation and the fromcooling the 3 scurves, to the 4itscan be concluded condition produces that thehigh very reduction in magnitude volumetric expansion of caused the compressive stresses by the martensite from the 3 s to the transformation 4 s condition of deeper layers.produces very high volumetric expansion caused by the martensite transformation of deeper layers. Figure 10. Residual stress profile of the conditions of 3 s and 4 s. 4. Concluding Remarks The quenching severity obtained in high-speed quenching is capable of producing very high thermal gradients at the beginning of cooling greater than 750 ◦ C between the surface and the core, making it possible to produce a shell-core microstructure. The shallow hardening induced compressive residual stresses at the surface. From the slope of the d-spacing vs Sin2 ψ, it was concluded that the conditions of 3, 4, and 5 s resulted in compressive residual stresses and this was corroborated with the uniaxial and biaxial stress analysis. Materials 2019, 12, 1154 10 of 11 The maximum magnitude of the compressive residual stresses was found to take place with 3 s of cooling. The residual stresses at the surface were −621MPa and had a maximum value of −700 MPa at 250 µm below the surface. The shell-core microstructure consisted in tempered martensite at the surface and bainite at the core. The interruption time is critical in the retention of compressive residual stresses, since further cooling of inner layers will reduce compressive stresses when the subsurface layers expand due to martensite transformation. Author Contributions: Research design G.E.T. and M.G.-M.; testing D.E.L., M.C. and G.M.M.-C.; data analysis D.E.L. and G.M.M.-C.; literature research Y.B.-G. and M.C.; manuscript writing Y.B.-G., D.E.L., G.M.M.-C. and M.C. Funding: This research received no external funding. Acknowledgments: The authors want to thank CONACYT and UDEM for their support. Conflicts of Interest: The authors declare no conflict of interest. References 1. Todinov, M.T. Residual stresses at the surface of automotive suspension springs. J. Mater. Sci. 2000, 35, 3313–3320. [CrossRef] 2. Jaramillo S, H.E.; de Sánchez, N.A.; Avila D, J.A. Effect of the shot peening process on the fatigue strength of SAE 5160 steel. Proc. Inst. Mech. Eng. Part C J. Mech. Eng. Sci. 2018, 0954406218816349. [CrossRef] 3. Segurado, E.; Belzunce, F.J.; Pariente, I.F. Effects of low intensity shot peening treatments applied with different types of shots on the fatigue performance of a high-strength steel. Surf. Coat. 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