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Effect of Post-Weld Heat Treatment on the Fatigue Behavior ofMedium-Strength Carbon Steel Weldments

Byeong-Choon Goo

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Citation: Goo, B.-C. Effect of

Post-Weld Heat Treatment on the

Fatigue Behavior of Medium-Strength

Carbon Steel Weldments. Metals 2021,

11, 1700. https://doi.org/10.3390/

met11111700

Academic Editor: Giovanni

Meneghetti

Received: 29 September 2021

Accepted: 20 October 2021

Published: 25 October 2021

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4.0/).

Korea Railroad Research Institute, Uiwang-si 16105, Korea; bcgoo@krri.re.kr; Tel.: +82-31-460-5243

Abstract: Railway vehicle makers manufacture the bogie frame by welding medium-strength carbonsteel sheets. It has been a long-standing practice to perform post-weld heat treatment (PWHT) toremove welding-residual stress, but rail car manufacturers are moving toward producing bogieframes without PWHT. Since securing the fatigue strength of the bogie frame is essential for ve-hicle operation safety, it is necessary to systematically evaluate the effects of PWHT on hardness,microstructure, mechanical properties, corrosion, fatigue strength, etc. In this study, small-scalewelding specimens and full-size components were produced using S355JR used in general structures,automobiles, shipbuilding, railroad vehicles, etc. The effect of PWHT on material properties-thehardness of the base material, heat-affected zone and weld metal, microstructure, shock absorptionenergy, yield strength, tensile strength, and fatigue were investigated. When the weld specimenwas annealed at 590 ◦C and 800 ◦C for 1 h, the yield strength and tensile strength of the specimendecreased, but the elongation increased. For specimens not heat-treated, the parent material’s yieldstrength, the yield strength in HAZ, and the yield strength of the weld metal were 350 MPa, 345 MPa,and 340 MPa. For specimens heat-treated at 590 ◦C, they were 350 MPa, 345 MPa, and 340 MPa.For specimens heat-treated at 800 ◦C, they were 350 MPa, 345 MPa, and 340 MPa. Annealing heattreatment of the specimen at 800 ◦C homogenized the structure of the weldments similar to that ofthe base material and slightly improved the shock absorption energy. For specimens not heat-treated,the Charpy impact absorption energies at 20 ◦C of the parent material and weld metal were 291.5 Jand 187 J. For specimens heat-treated at 590 ◦C, they were 276 J and 166 J. For specimens heat-treatedat 800 ◦C, the Charpy impact absorption energy at 20 ◦C of the parent material was 299 J. PWHT at590 ◦C had the effect of slightly improving the fatigue limit of the specimen but lowered the fatiguelimit by 10.8% for the component specimen.

Keywords: post-weld heat treatment (PWHT); fatigue; residual stress; tensile property; butt-weld

1. Introduction

Welding is a convenient and economical technology for joining steel, alloy, and non -ferrous metals, and is used in various fields such as automobiles, railways, aviation, bridges,pressure vessels, construction, etc. [1]. Welding has the advantages of simple equipmentand work process, easy automation, high bonding efficiency, and few restrictions on thethickness of the welded object. On the other hand, welding also has shortcomings suchas thermal deformation, residual stress, microstructural change, material embrittlement,welding defects, and bad working conditions [1]. Among them, the welding-residual stressis closely related to the fatigue behavior of the welded structure and has been studied for along time. The magnitude of the welding-residual stress depends on several parameterssuch as the welding method, welding procedure, welding parameters, and constraintconditions of the welded objects. It can reach a magnitude similar to the yield stress ofthe base material [2–5]. If there is no constraint and external force in the structure, theresidual stress is always at self-equilibrium. The sum of each stress component is zero.When an external force acts on an object in which residual stress exists, the stress causedby the external force overlaps on the residual stress, so the residual stress has the effect of

Metals 2021, 11, 1700. https://doi.org/10.3390/met11111700 https://www.mdpi.com/journal/metals

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increasing or reducing the stress caused by the external force. Even if the magnitude ofthe stress caused by the external load at a point in the structure is smaller than the yieldstress when the sum of the stress caused by the external load and the residual stress isgreater than the yield stress, yielding occurs. As a result, the state of the residual stresswill change after the applied load is removed. It has been found that the residual stress isrelieved to some extent when a cyclic load is applied. In some cases, the tensile residualstress changed to the compressive residual stress [2,4]. Therefore, the residual stress caninfluence crack initiation in the structure. When cracks develop and propagate, the residualstress changes depending on the size and direction of the cracks. So the residual stressmay have a significant effect on the crack life [6–11]. According to the simulation resultsof Wang et al. [6], the residual stress significantly influenced the fatigue life of a T-jointwhen the stress ratio was less than 0.5. But the residual stress effect was negligible whenthe stress ratio was more than 0.5. Liljedahl et al. [7] showed that the residual stresses in2024-T351 aluminum alloy accelerated the fatigue crack growth rate in the middle tensionspecimen. But they retarded the growth rate in the compact tension sample. Kang andLuo [8] reviewed existing fatigue life prediction models considering residual stresses andratcheting. Wang et al. [9] applied Lemaitre’s plastic damage model considering weld-induced residual stresses and weld-induced plastic damage. The effect of residual stresson the fatigue life of butt-weld joints was significant. Baumgartner and Bruder [10] foundthat at higher load amplitudes due to plastic yielding in the sharp notches at the weld toe,welding residual stresses relaxed much, so residual stress effect on the fatigue life wasnot significant. McClung [11] surveyed extensive literature and provided experimentaldata on the redistribution and relaxation of welding residual stresses during fatigue tests.Therefore, to accurately predict the fatigue life of structures, it is necessary to know thestate of stress caused by external load and residual stresses to predict the fatigue life ofstructures. However, in general, it is not easy to accurately measure or predict residualstress at all points of a material. Therefore, methods to reduce or remove the residual stressin the structure are applied. In welded structures, irregularities occur in the toe area wherethe weld metal and the base material meet, acting as a notch, causing stress concentrationand structurally weak points. In most cases on the weld tow surface, tensile residual stressoccurs and hardness and microstructures change. There are methods of applying vibrationto the structure [12–14], methods of mechanical loading [3,15,16], and PWHT [5,17–21]to reduce the residual stress. Tomków and Janeczek [22] conducted an in-situ local heattreatment in underwater conditions. Additional welding stitches tempered the brittlestructures in HAZ and gave a similar effect to PWHT. A more active way is to extendthe fatigue life by striking the toe, the weak part of the welded structure, with a metalpin using a 20–30 kHz ultrasonic exciter (impact pin peening) to generate compressiveresidual stress from the toe surface to a certain depth. The method seems to be effective.Trufyakov et al. [23] showed that the ultrasonic impact peening improved fatigue life byfour to five times for low-carbon steel welded specimens with a yield strength of about200 MPa. Galtier and Stanikov [24,25] compared the fatigue life of samples treated withsand-blast, low transformation temperature welding, and ultrasonic peening on high-tensile steel welded specimens and showed that ultrasonic peening was the most effectivein improving fatigue life.

To manufacture the bogie frame of a rail car (Figure 1), cut and formed steel plates arewelded to produce side beams, and transoms made from steel plates or commercial pipesare joined with the side beams by welding. To the side beam and transom, various bracketsrequired to install devices of railway vehicles: motor, reduction gear, brake, etc. are joinedby welding. Therefore, welding is the core of the bogie frame manufacturing process, andit determines the quality of the bogie frame. After the bogie frame is manufactured withgas metal arc welding (MGAW), it is a long-standing practice to perform post-weld heattreatment (PWHT, or annealing) at about 600 ◦C to remove the residual welding stress.PWHT is specified in international standards for railroads [26,27]. Recently, based on theaccumulated research results on the fatigue characteristics of welded parts, there is a trend

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toward manufacturing the bogie frame without PWHT [28]. Although many studies havebeen conducted to find out the effect of PWHT on the microstructure, hardness, etc. ofthe weldment of carbon steels, there are few studies on the effect of PWHT on the fatiguestrength of the weldment. That is due to the fact that fatigue tests take a lot of time andcost. In general, it has been thought that when tensile residual stress occurs in the toeof a welded structure, the fatigue strength decreases. According to the results of fatiguetests on specimens from which residual stress was removed using PWHT, PWHT didnot improve the fatigue strength of weldments. Leitner et al. [17] made a cross-shapedspecimen with mild steel S355 (yield strength 355 MPa, tensile strength 470 MPa) andconducted a tensile fatigue test. After welding, annealing heat treatment was performed at550 ◦C for 30 min. The test results showed that the fatigue life of the annealed specimenswas shorter than that of the as-welded specimens. The authors argued that this was aresult of the fact that the welding compressive residual stress was relieved by annealing.We also investigated the PWHT effect of the mechanical properties and fatigue behaviorusing butt-weld specimens and T-shape fillet weld specimens made from similar carbonsteel [18]. For T-type specimens, PWHT increased the fatigue strength. But it was difficultto quantify the effect of PWHT on the fatigue strength in the case of the base specimen andthe butt-welded specimen due to the lack of experimental data. Udo and Numakura [19]performed a fatigue test after arc-welding a V-groove of stainless steel (UNS S32750) andsolution treatment at 1050 ◦C for 60 min. PWHT shortened the fatigue life. The causewas the S-phase that occurred during the heat treatment cooling period after welding.Zhang et al. [20] investigated the effect of PWHT on the crack propagation of high-tensilesteel (tensile strength 790 MPa, yield strength 690 MPa) weldments. Arc welding was used,and annealing was carried out at 930 ◦C for one hour. According to their test results, thefatigue life of specimens heat-treated was 9.1% shorter than that of specimens not heat-treated. Jiang et al. [29] investigated the effect of PWHT on the strength of box columnsmade from high-strength steel plates. According to their test results under the compressiveload, PWHT increased the vertical direction strength by 3~7% compared to as-weldedcolumns. They explained that it was due to the fact that tensile residual stresses wererelieved due to PWHT. PWHT is a necessary process in welded pipes and vessels usedat high temperatures and pressures. P91 is one of the creep-resistant steel for pipes andvessels in the power generation industry. Lojen and Vuherer [30] found out that the highPWHT temperature could shorten the holding time. At 800 ◦C, 0.5 h was sufficient in termsof hardness and impact toughness. Silva et al. [31] also searched optimal PWHT conditionsfor P91 pipe welding concerning mechanical strengths, hardness, and microstructures.They applied five different heat treatment cycles. Pandey et al. [32,33] showed that creeprupture life, crack initiation, and propagation characteristics of P91 weld joints dependedvery much on PWHT conditions. A review paper on heat treatment by Kalyankar andChudassama [21], a review paper by Vöhringer [34] on the relaxation of residual stress byheat treatment and mechanical loading, and a review paper on the effect of residual stresson fatigue by McClung [11] are of great help to the overall understanding of the effect ofPWHT and residual stress.

Figure 1. A bogie frame made by welding for a rail vehicle.

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In this study, the research material was medium-strength carbon steel (KS D 3515,SM355A) used in railway vehicles, automobiles, shipbuilding, etc. [4]. PWHT can influencemicrostructure, hardness, mechanical properties, fatigue life, etc. If PWHT softened thematerial, its mechanical strength would be lowered, which would decrease the fatiguestrength. On the other hand, relaxation of the tensile residual stress in the structure canimprove fatigue strength. Therefore, to accurately evaluate the PWHT effect on fatiguelife, it is necessary to pinpoint these two effects separately. A literature review reveals thatresearchers have conducted little research in this area. In this study, the PWHT effects onmaterial properties: hardness, microstructure, impact strength, and mechanical strength,were studied. In addition, fatigue tests of welded specimens were carried out, and thePWHT effect on the fatigue of welded specimens was analyzed in detail.

2. Materials and Methods2.1. Effect of PWHT on Mechanical Properties

The SM355A (KS D 3515) welded structural steel used in the test is similar to S355JR(EN 10025-2) steel. It is used a lot for the bogie frame and body structure of railwayvehicles [4]. The chemical components specified in the standard are C (<0.20 wt.%), Si(<0.55%), Mn (<1.6%), P (<0.035%), S (<0.035%). Its minimum yield strength, tensilestrength, and elongation rate are 355 MPa, 490 MPa, and 17%. Welding conditions usedin the railway industry were applied to fabricate the welding specimen, as in Figure 1.Two 10 mm thick steel plates were welded by GMAW (Gas Metal Arc Welding) under theconditions- welding current: 300 A, voltage: 30 V, movement speed: 25 cm/min, shield gas:Ar 85% + CO2 15%, annealing temperature: 590 ± 20 ◦C and 800 ± 20 ◦C, holding time: 1 h;heating and cooling rate: 120 ◦C/h. welding wire: AWS ER 70S-6 Φ1.2 was used, whosechemical compositions are C (0.06–0.15 Wt.%), Ni (0.15% max.), Mn (1.40–1.85%), Cr (0.15%max.), Si (0.80–1.15%), P(0.25% max.), and V (0.03% max). Specimens were produced bycutting with a wire saw so that the rolling direction of the steel plate coincided with thelongitudinal direction of the specimen. Figure 2b shows the shape of a tensile specimenwith a thickness of 5 mm, and Figure 2c shows the shape of the V-notch Charpy impactspecimen. The longitudinal direction of the Charpy impact specimen was the same as thewelding line. Vickers hardness was measured at 0.5 mm intervals under a load of 1.961 N.The tensile test was performed with a gauge length of 50 mm and a speed of 2 mm/min.The Charpy impact specimen test was performed according to ASTM A370. In both tests,three specimens were tested under the same conditions. The microstructure was observedat 500 magnification using an optical microscope.

Figure 2. Plate welding and production of specimens. (a) Plate welding; (b) tensile specimen, thickness 5 mm; (c) Charpyimpact specimen.

2.2. Effect of PWHT on Fatigue Behavior

The material used in this section was the SM355A steel plate used in Section 2.1, butthe production batch was different.

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2.2.1. Hardness Measurement

For butt welded specimens, the hardness of the specimen without PWHT (AAN)and with PWHT (AAY) was measured and compared. The longitudinal direction of thespecimen coincided with the rolling direction of the steel sheet, and the weld line wasperpendicular to the rolling direction. Welding conditions were as follows: current 300 A;voltage 30 V; movement speed 30 cm/min; shield gas Ar 85% + CO2 15%; welding wireAWS ER 70S-6, Φ1.2. PWHT conditions were holding temperature: 590 ± 20 ◦C; holdingtime: 3 h; heating and cooling rate at 120 ◦C/h. The Vickers hardness was measured at0.5 mm intervals in the length direction, and at +1 mm, −1 mm, −3 mm, −5 mm, and−7 mm, −9 mm in depth direction using the AVK-C1 model of AKASHI of Japan with aload of 5 kg (Figure 3c). A welded joint is composed of a base material, a heat-affectedzone, and a weld metal, and each part is different in microstructure and hardness, so thewelded joint is weaker than the base material. In addition, heat treatment after weldingmay cause local defects by changing the size of grains or creating inclusions. Changesin microstructure due to welding and heat treatment methods were analyzed using anoptical microscope.

Figure 3. (a) welding specimen geometry; (b) welded specimen; and (c) Vickers hardness measurement locations.

2.2.2. Specimen Tensile Test

Figure 4 shows the shape of the tensile specimen. The longitudinal direction ofthe specimen coincided with the rolling direction of the steel sheet, and the weld linewas perpendicular to the rolling direction. The values of welding parameters and heattreatment conditions were the same as the hardness specimen in Figure 3. A tensile testwas performed to obtain the basic properties of the material, such as yield strength, tensilestrength, and modulus of elasticity. The tensile test was performed following ASTM E8in an Instron 25-ton hydraulic universal testing machine (model 8802) at a test speed of1 mm/min.

Figure 4. Configuration of tensile specimen, thickness 10 mm.

2.2.3. Specimen Fatigue Test

The tensile fatigue test was carried out according to ASTM E 466 at 20 Hz, a stress ratioof 0.1 using Instron model 8802 (25 ton). Figure 5 shows the butt fatigue specimen with athickness of 10 mm. The longitudinal direction of the specimen coincided with the rollingdirection of the steel sheet, and the weld line was perpendicular to the rolling direction.The same welding conditions as those applied to the tensile specimen were applied.

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Figure 5. Butt-weld fatigue specimen, thickness 10 mm.

2.3. Fatigue Test of Members

A three-point bending fatigue test was performed using three types of beams ofa size similar to the side frame of an actual railroad car bogie. The first type of beamwas a box-shaped beam without reinforcement. The second was a beam with a gussetwelded perpendicular to the center of the lower surface. The third was a beam with agusset welded parallel to the bottom surface. The box-shaped beams were welded using aGMAW semi-auto robot. Welding conditions were shield gas: Ar 85% + CO2 15%; weldingwire: AWS ER 70S-6 Φ1.2; weld current 280 A; voltage 29 V; moving speed 30 cm/min.A skilled welder welded the vertical and horizontal gussets. PWHT conditions were asfollows: temperature: 590 ± 20 ◦C; holding time: 3 h; heating and cooling rate: 120 ◦C/h.The fatigue test was performed using Instron’s Model 8503 with a capacity of 50 tons. Aconstant amplitude load with a stress ratio R = 0.1 was applied. A line force was applied tothe center of the upper surface of the beam. The failure occurred on the lower surface ofthe beam, where the maximum tensile stress was estimated. The failure life was defined asthe time when cracks first developed on the bottom surface of the specimen and advancedby 20 mm. Figure 6 shows the cross-sectional shape and fatigue test setup of a 1500 mmlong box-type beam.

Figure 6. Configuration of the real-size beam and fatigue test setup. (a) Side view of the box-shaped beam; (b) detail ‘J’; (c)setup for three-point bending fatigue test [4].

3. Results and Discussion3.1. Effect of PWHT on the Mechanical Properties3.1.1. Tensile Test

Figure 7 shows the fractured specimens after the tensile tests for the base metal, HAZ,and weld metal. There were three similar specimens in every nine groups: parent aswelded, parent PWHT at 590 ◦C, parent PWHT at 800 ◦C, HAZ as-welded, HAZ PWHT at590 ◦C, HAZ PWHT at 800 ◦C, weld metal as-welded, weld metal PWHT at 590 ◦C, weldmetal PWHT at 800 ◦C. In the case of the base material, first, shear deformation occurredin the 45-degree direction from the surface of the specimen, and then the cross-sectionalarea decreased to some extent. Finally, the specimen was separated perpendicular to thetensile load. In the case of weld metal, shear deformation occurred locally on the surface, or

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cracks were first generated and developed in the weak part inside, and the fracture surfaceshowed a torn shape. The fracture surface of the HAZ specimen was similar to that of theparent material, but irregularities of the fracture surface were more severe than that of theparent material. Tables 1–3 show measured tensile properties. Figure 8a is the stress-straincurve of the base specimen. The yield strength and tensile strength of the specimen thatwas not heat-treated were the largest, and the specimen heat-treated at 800 ◦C for 1 h hadthe lowest strength. The elongation was the highest in the case of the specimen heat-treatedat 800 ◦C. In general, the upper and lower yield strengths observed in low carbon steelappeared, and a Lüders band having almost constant stress was observed past the upperyield strength. It is well known that the upper yield point is due to the fact that the C orN atom at the Fe atom site interferes with the displacement of the dislocation [35]. Thestrain in the Lüders band section was 0.18 to 2.17% for not heat-heated specimens, 0.18 to2.28% for 590 ◦C heat-treated specimens, and 0.18 to 2.24% for 800 ◦C annealed specimens.Figure 8b is the tensile curve of the HAZ specimen. The yield strength and tensile strengthof the not annealed specimens were the largest. The strength of the specimen annealed at800 ◦C was the lowest, and the elongation was smaller than the base material. Figure 8c isthe tensile curve of the weld metal. The decrease in strength and increase in elongationdue to heat treatment is evident. It showed the characteristic of sudden fracture after thestress reached the maximum. The tensile stress of the not heat-treated specimens andthe specimens heat-treated at 590 ◦C continued to increase after the yield point. Whenheat-treated at 800 ◦C, deformation characteristics near the yield point were similar tothose of the base material. Figure 9 compares the tensile curves of the base metal, HAZ,and weld metal under various conditions. In the absence of PWHT, the tensile curves of thebase metal and the HAZ specimen were different in elongation, but the tensile strength andyield strength were almost the same. When heat-treated at 590 and 800 ◦C, the yield stressof HAZ was lower than that of the base material. That was due to the fact that restraineddislocations by inclusions between particles moved at high temperatures to soften thematerial. This phenomenon was more pronounced in HAZ and weld metal, where thegrain size was smaller than that of the base metal, and there were many interfaces betweengrains. In the case of heat treatment at 800 ◦C, it was inferred from the tensile curve thatthe structure of the base metal, HAZ, and weld metal was almost homogenized. Figure 10shows yield strengths, tensile strengths, and elongations under various conditions. Theyield strength and tensile strength of the welded specimen without heat treatment werethe greatest. The base metal, HAZ, and weld metal heat-treated at 800 ◦C had almost thesame value. In the case of elongation, HAZ and welded specimens without heat treatmenthad the smallest value. In the case of elongation, HAZ and welded specimens without heattreatment had the smallest value. When heat-treated at 800 ◦C, their elongations were closeto the level of the base material.

Table 1. Yield strength, tensile strength, and elongation rate of the parent material.

Item Parent Material as Welded Parent Material Heat Treated at 590 ◦C Parent Material Heat Treated at 800 ◦C

Specimen No. 1 No. 2 No. 3 No. 1 No. 2 No. 3 No. 1 No. 2 No. 3σy (MPa) 364 363 366 356 355 354 310 323 322σu (MPa) 522 524 524 509 506 511 475 479 482

ε f (%) 24.0 23.9 23.4 26.8 24.3 28.7 26.2 26.6 25.2

Table 2. Yield strength, tensile strength, and elongation rate of the HAZ material.

Item HAZas Welded

HAZHeat Treated at 590 ◦C

HAZHeat Treated at 800 ◦C

Specimen No. 1 No. 2 No. 3 No. 1 No. 2 No. 3 No. 1 No. 2 No. 3σy (MPa) 366 366 359 327 335 332 310 314 305σu (MPa) 529 521 522 506 511 510 478 480 478

ε f (%) 25.1 16.8 13.3 24.8 24.8 26.3 19.8 24.8 19.9

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Table 3. Yield strength, tensile strength, and elongation rate of the weld metal.

Item Weld Metalas Welded

Weld MetalHeat Treated at 590 ◦C

Weld MetalHeat Treated at 800 ◦C

Specimen No. 1 No. 2 No. 3 No. 1 No. 2 No. 3 No. 1 No. 2 No. 3σy (MPa) 442 449 448 385 413 393 327 329 327σu (MPa) 549 557 568 520 549 535 483 486 481

ε f (%) 13.1 11.3 19.0 15.9 21.3 21.9 28.9 26.4 19.7

Figure 7. Specimens after tensile tests.

Figure 8. Strain-stress curve. (a) Parent material; (b) HAZ; and (c) weld metal.

Figure 9. Comparison of tensile curves of parent, HAZ and weld metal. (a) Non PWHT (b) PWHT at 590 ◦C; (b) PWHT at590 ◦C; (c) PWHT at 800 ◦C.

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Figure 10. Comparison of yield stresses. P, parent material non heat treated; P5, parent heat treated at 590 ◦C; P8, parentheat treated at 800 ◦C; H, HAZ non heat treated; H5, HAZ heat treated at 590 ◦C; H8, HAZ heat treated at 800 ◦C; W, weldmetal non heat treated; W5, weld metal heat treated at 590 ◦C; W8, weld metal heat treated at 800 ◦C. (a) Yield stress;(b) tensile stress; and (c) elongation rates.

3.1.2. Microstructure and Hardness

Figure 11 is the Vickers hardness distribution of the base metal, HAZ, and weldmetal measured on the surface of the weld specimen. Maximal Vickers hardness valueswere summarized in Table 4. Hardness values of the weld metal and HAZ were higherthan the base metal. When the specimens were not heat-treated, or when specimenswere heat-treated at 590 ◦C, hardness values of the weld metal and HAZ were 40–60 Hvhigher than the base metal. When PWHT was carried out at 800 ◦C, the base metal, HAZ,and weld metal had almost the same hardness value, and the microstructures were morehomogenized. At the boundary between HAZ and the weld metal, there was a pointwith higher hardness. That seems to be due to cementite inclusions existing at the grainboundary. The hardness distribution is consistent with the effect of heat treatment ontensile strength, yield strength, and elongation observed in the tensile curve. Figures 12–14are optical micrographs of the base metal, HAZ, and weld metal. The base material hada commonly observed carbon steel microstructure composed of the perlite (black) andthe ferrite (gray) matrix. Almost all of the as-welded base material was ferrite. Someelongated pearlite grains were distributed at ferrite grain boundaries. The HAZ and weldmetal that received welding heat were composed of bainite, ferrite, and pearlite. In thebase material of the specimen heat-treated at 590 ◦C, the pearlite particles were larger andmore numerous than those of the as-welded specimen. Although the HAZ structure wasdifferent from the as-welded, there was no significant difference in the microstructure ofthe weld metal. In the base material of the specimen heat-treated at 800 ◦C, pearlite wasspheroidized. The microstructures of HAZ and weld metal were similar to those of the basematerial. Their microstructures became more homogenized. This characteristic explainsthat the hardness distribution of the specimen heat-treated at 800 ◦C was almost constant,and the elongation increased. The microstructure heat-treated at 800 ◦C was significantlydifferent from the as-welded and heat-treated structures at 590 ◦C.

Table 4. Vickers hardness maximum.

Hardness Value (Hv) Parent Material HAZ Weld Metal

As-welded 170.0 193.3 192.0Heat treated at 590 ◦C 143.1 180.7 196.9Heat treated at 800 ◦C 139.8 150.1 175.2

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Figure 11. Micro hardness distribution.

Figure 12. Microstructure of the as-welded specimen. (a) Parent material; (b) HAZ; and (c) weld metal.

Figure 13. Microstructure of the heat-treated specimen at 590 ◦C. (a) Parent material; (b) HAZ; and (c) weld metal.

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Figure 14. Microstructure of the heat-treated specimen at 800 ◦C. (a) Parent material; (b) HAZ; and (c) weld metal.

3.1.3. Charpy Impact Characteristics

Figure 15 shows the results of the Charpy impact test results of various specimens.In the case of the base material, the shock absorption energy value of the specimens heat-treated at 800 ◦C was higher than those of the specimens not heat-treated and heat-treatedat 500 ◦C at temperatures below 20 ◦C. In the case of the base material, the shock absorptionenergy of the heat-treated specimen at 500 ◦C was slightly smaller than that of the basematerial not heat-treated at temperatures above −20 ◦C. In terms of shock absorptionenergy, heat treatment at 800 ◦C can improve the impact resistance of the bogie frame usedin extreme climatic conditions. Another interesting result was that the shock absorptionenergy was maximum at 20 ◦C regardless of the presence or absence of heat treatment.In the case of weld metal, due to micro-defects existing inside the specimens, a fracturesometimes started not in the notch of the test pieces. So sufficient data on shock absorptionenergy could not be obtained. The shock absorption energy of the weld metal showed asmaller value than that of the base metal in all temperature ranges.

Figure 15. Charpy impact absorption energy.

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3.2. Fatigue Characteristics of Specimens3.2.1. Microstructure and Hardness

Figure 16 is a microstructure photograph taken with an optical microscope [4]. It is atypical carbon steel structure composed of ferrite and pearlite. The perlite was elongated,and the perlite had a higher fraction in HAZ than in Figure 12. The microstructure wasquite different from the results in Figures 12 and 13 obtained from the different batches ofthe same steel manufacturer. Figure 17 shows the hardness distribution of the butt-weldedspecimen with and without PWHT. The hardness decreased in the order of weld metal,HAZ, and base material. The distribution of hardness was slightly different depending onthe depth from the surface of the specimen. As shown in Figure 3, it was due to the shapeof the weld bead. HAZ is a region that has different microstructures in the solid phase andexhibits unique mechanical properties due to grain growth, solid solution, precipitation ofparticles, and residual stress. In the hardness measurement of AAN and AAY specimens,there was no unusual tendency to distinguish HAZ. The hardness increased from the basemetal to the weld zone. Metallographic observations can explain this kind of phenomenon.In HAZ, re-crystallization and coarsening of grains often occur, so HAZ has coarse-grainedand fine-grained structures together. For the grains to become coarse, a lot of welding heatmust be input, but in multi-layer welding, enough weld heat was not input at once. Thus,re-crystallization took place without coarsening of the grains. As a result, the base materialwith coarse grains had the lowest hardness value, and the hardness increased in HAZ withfine grains, and the weld metal had the highest hardness value.

Figure 16. Microstructures (×200). (a) Parent material, non PWHT; (b) HAZ, non PWHT; (c) parent material, PWHT at590 ◦C; (d) HAZ, PWHT at 590 ◦C.

Figure 17. Micro hardness distribution, AAN: non PWHT, AAY: PWHT at 590 ◦C. (a) −1 mm deep; (b) −5 mm; and(c) −9 mm.

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3.2.2. Tensile Test Results

Figure 18 shows tensile stress-strain curves at room temperature. The base metal andthe welded specimen presented distinctly different tensile behaviors. In the case of weldedspecimens, PWHT decreased strength and increased elongation to a significant extent. Inthe case of the base material specimen, after heat treatment (BMY), the tensile strengthdecreased by 20 MPa (7.2%), and the 0.2% offset yield strength decreased by 25 MPa (3.8%).In the case of butt-welded specimens, after heat treatment (AAY), the tensile strengthdecreased by 54 MPa (16.6%), and the yield strength decreased by 59 MPa (10.4%), but theelongation was increased by 73%. In the case of the specimen that was heat-treated aftergrinding the toe (GAY), the tensile strength became lower by 49 MPa (20.1%), and the yieldstrength became lower by 72 MPa (9.4%), but the elongation was increased by 238%.

Figure 18. Tensile curves. BMY: parent, PWHT at 590 ◦C; BMN: parent non PWHT; AAY: Butt weld,PWHT at 590 ◦C; AAN: butt weld, non PWHT; GAY: butt weld, ground and PWHT at 590 ◦C; GAN:butt weld, ground and non PWHT.

3.2.3. Specimen Fatigue Test Results

Figure 19 is an S-N diagram for a tensile fatigue test performed at a stress ratio R = 0.1.Each point is the test data, and the straight line is a curve fit of the test data to a 50%probability of failure. Specimens that did not break over 2 × 106 cycles were not includedin the test data. Looking at the fatigue test results of the base metal specimen (Figure 19a),PWHT smoothed the slope of the S-N curve, but there was no significant difference infatigue strength. If the magnitude of residual stress in the base material were negligible, thisdifference would be entirely due to heat treatment. In the case of butt-welded specimens(Figure 19b), the effect of PWHT was a bit greater than that of the base metal. In the regionof 2 × 105 cycles or less, the fatigue strength of the as-welded specimen was higher thanthat of the heat-treated specimen and was smaller over 2 × 105 cycles [2]. The difference infatigue strength seemed to be due to the simultaneous action of PWHT and residual stresssince a considerable amount of residual stress existed in the case of welded specimens.In the low cycle region, if the total stress due to external loading and the residual stressexceeded the yield stress, the magnitude and distribution of the residual stress wouldchange [2,4–6]. In the high cycle fatigue test, since the magnitude of the stress appliedby the external force was small, the total stress was less than the yield stress, so the effectof the residual stress was insignificant [2,4–6]. Figure 19c compares the fatigue strengthof the (GAY) and (GAN) specimens that were not heat-treated after grinding the weldtoe of the butt-welded specimen. The scatter of fatigue lives was investigated by testing20 specimens at each stress amplitude. Fatigue strength in the case of heat treatmentwas small in the whole life range. Grinding had the effect of suppressing the occurrence

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of fine cracks occurring in the notch by removing the irregularities existing in the weldtow. Since compressive residual stress was generated near the surface when grinding wasperformed, it was a predictable result that the fatigue strength of the specimen withoutPWHT was greater. Figure 19d compares the S-N curves of the as-welded specimen andthe tow-ground specimen. The fatigue strength of the ground specimen was slightly higher.It was due to the surface compressive residual stress caused by grinding and the effectof surface finishing. Figure 19e compares the S-N diagrams when as-welded specimensand ground specimens were heat treated. Contrary to expectations, the fatigue strengthof the GAY specimen was slightly lower. Inferring from these results, grinding based onthe operator’s experience could lower the fatigue strength of the welded structure withoutimproving it. Table 5 shows fatigue limits at 2 × 106 cycles of the S-N diagram of Figure 18(Log ∆σ = a Log N + b). The fatigue limit decreased in the order of BMY > BMN > AAY >GAN > GAY > AAN.

Figure 19. Comparison of S-N curves. (a) Comparison between BMN and BMY; (b) comparison between AAN and AAY; (c)comparison between GAN and GAY; (d) comparison between AAN and GAN; and (e) comparison between AAY and GAY.

Table 5. Parameter values of the S-N curves (Log ∆σ = a Log N + b).

Specimen a b Fatigue Strength at 2 × 106 Cycles (MPa) R2 (Coefficient of Determination)

BMN −0.10848 2.83143 140.5 0.99BMY −0.0940 2.74645 142.6 0.91AAN −0.23124 3.35092 78.3 0.91AAY −0.1734 3.04473 89.5 0.97GAN −0.22232 3.31668 82.3 0.89GAY −0.21682 3.26416 79.1 0.88

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3.2.4. Residual Stress Measurement by the Contour Method

The contour method [36] was applied to measure the residual stress in the specimennear the weld toe. First, the test piece was amputated using a wire electrical dischargemachine. In the next step, the displacements normal to the cut surfaces, which wereassumed to arise from elastic relaxation of residual stresses after the cut, were measuredat 0.1 mm intervals in the horizontal and vertical directions using a laser displacementmeasurement sensor with a very high resolution of 0.01 µm. The average of displacementsof two cut faces gives the vertical coordinate of the reference plane. After some raw datacleanup and smoothing [37], the reversed measured displacements were the input to thefinite element model for the back-calculation of residual stresses in the specimen. Figure 20shows the cut specimen. Figure 21 represents the measured displacement normal to thecut surface. Wedge elements with 15 nodes were used in the finite element model of theright part of the specimen. Young’s modulus E = 210 GPa and Poisson’s ratio ν = 0.3were used. The right end side was fixed as a displacement boundary condition. Thefinite element analysis was carried out using the Abaqus code. Figure 22 presents thestress corresponding to the displacement in Figure 21. The magnitude of the tensile andcompressive residual stresses was near to the yield stress. According to [36], when weuse a wire electrical discharge machining for cutting, the contour method tends to predictslightly larger stresses at the edges [36,38]. The analysis results showed a similar trend.

Figure 20. Specimen cut by wire electrical discharge machining.

Figure 21. Measured displacement normal to the cut surface.

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Figure 22. Residual stress distribution in the right part of the specimen shown in Figure 20. (a) Max principal stress;(b) stress component normal to the cut surface.

3.2.5. Fatigue Test Results of Members

Figure 23 shows the S-N curve for the beams similar to the side member of the bogieframe. Table 6 summarizes the parameter values of the S-N curve. In the three-pointbending fatigue test of the box-shaped beam, there was little effect of heat treatment, butrather, the fatigue strength of the member without heat treatment was higher. These resultsare consistent with those reported in the existing literature. According to [2], there wasno effect of improving fatigue strength due to heat treatment on H-type St52, St37, andS355 full-size beams. For high tensile steel StE, the fatigue limit increased by 5% [2]. In thecase of box-type beams, regardless of heat treatment, all cracks started at the lower platewhere the maximum tensile stress took place by the concentrated load and propagated inthe direction perpendicular to the principal stress. The crack initiation location was not inthe center of the beam, but at about 1/3 of the width of the box beam N and the box beamY (Figure 24a,b). That was due to the fact that though the cross-section of the beam wassymmetric about the central axis, the welding-residual stress was asymmetric.

Figure 23. S-N curves of real-size specimens.

Table 6. Parameter values of the S-N curves (Log ∆σ = a Log N + b).

Specimen a b Fatigue Strength at 2 × 106 Cycles (MPa) R2 (Coefficient of Determination)

Box beam, non PWHT 0.29716 3.74015 73.7 0.98Box beam, PWHT 0.31092 3.77661 65.7 0.99

Box beam V, non PWHT 0.28614 3.50966 50.9 0.98Box beam H, non PWHT 0.33162 3.59949 32.4 0.99

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Figure 24. Fatigue crack initiation and propagation of real-size beams. (a) box beam, non PWHT; (b) box beam, PWHT;(c) box beam V with a vertical gusset, non PWHT; (d) box beam H with a horizontal gusset, non PWHT.

The fatigue life of the box beam V and the box beam H without heat treatment, towhich a vertical (V) and a horizontal (H) gusset were welded, decreased significantlycompared to the beam without gusset. The fatigue strength of the box beam H was a lotless than that of the box beam V. The slope values of the S-N curves of all beams werealmost the same. Figure 24c,d show the crack initiation location and propagation pathof the box beam-V and box-beam-H, respectively. The crack occurred in the HAZ andpropagated along with the weld bead.

4. Conclusions

In this study, with medium-strength welded structural steel (SM355A, KS D 3515equivalent to S355JR, EN 10025-2) widely used in industry, PWHT effects on materialproperties such as hardness, microstructure, and mechanical properties of weld specimensand full-size beams were studied. Several results are as follows.

(1) When the base metal, HAZ, and weld metal were annealed at 590 ◦C and 800 ◦Cfor 1 h, the yield and tensile strength decreased but the elongation increased. Whenannealed at 800 ◦C for 1 h, the tensile curves of the base metal, HAZ, and weld metalshowed almost similar behavior. That explained the almost constant hardness valuesand homogenization of the materials. For specimens not heat-treated, the parentmaterial’s yield strength, the yield strength in HAZ, and the yield strength of theweld metal were 350 MPa, 345 MPa, and 340 MPa. For specimens heat-treated at 590◦C, they were 350 MPa, 345 MPa, and 340 MPa. For specimens heat-treated at 800 ◦C,they were 350 MPa, 345 MPa, and 340 MPa.

(2) In the base material, PWHT at 800 ◦C slightly increased the Charpy impact absorptionenergy. But PWHT at 590 ◦C slightly lowered the impact resistance. The shockabsorption energy of the weld metal was significantly lower than that of the basemetal. And it did not increase much by heat treatment. For specimens not heat-treated,the Charpy impact absorption energies at 20 ◦C of the parent material and weld metalwere 291.5 J and 187 J. For specimens heat-treated at 590 ◦C, they were 276 J and 166 J.For specimens heat-treated at 800 ◦C, the Charpy impact absorption energy at 20 ◦Cof the parent material was 299 J.

(3) PWHT had a slight effect on the fatigue strength of the base metal and slightlyimproved the fatigue strength of the butt-welded specimen. In the case of a specimenground in the toe part and not heat-treated, the compressive residual stress occurrednear the surface, and the fatigue strength slightly improved. However, when heattreated, the beneficial effect of the compressive residual stress disappeared, and thelocal damage caused by grinding lowered the fatigue strength.

(4) The fatigue limit of the full-scale beam annealed at 590 ◦C was 10.8% lower than thatof the as-welded beam. These results show the possibility of fabricating a bogie framethat does not require PWHT. Vertical and horizontal gussets welded to as-weldedbeams decreased the fatigue limits by 30.9% and 56%, respectively.

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Funding: This research was funded by Korea Railroad Research Institute.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Not applicable.

Conflicts of Interest: The author declares no conflict of interest.

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