metals Article Effect of Post Weld Heat Treatment on the Microstructure and Mechanical Properties of a Submerged-Arc-Welded 304 Stainless Steel Tae-Hoon Nam 1,2, Eunsol An 1, Byung Jun Kim 1, Sunmi Shin 1, Won-Seok Ko 3, Nokeun Park 4, Namhyun Kang 2,* ID and Jong Bae Jeon 1,* 1 Functional components and Materials R&D Group, Korea Institute of Industrial Technology, Yangsan 50635, Korea; [email protected] (T.-H.N.); [email protected] (E.A.); [email protected] (B.J.K.); [email protected] (S.S.) 2 School of Materials Science and Engineering, Pusan National University, Busan 46241, Korea 3 School of Materials Science and Engineering, University of Ulsan, Ulsan 44610, Korea; [email protected] 4 School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Korea; [email protected] * Correspondence: [email protected] (N.K.); [email protected] (J.B.J.); Tel.: +82-55-367-9407 (N.K.); +82-51-510-3274 (J.B.J.) Received: 8 November 2017; Accepted: 22 December 2017; Published: 2 January 2018 Abstract: The present study is to investigate the effect of post heat treatment on the microstructures and mechanical properties of a submerged-arc-welded 304 stainless steel. The base material consisted of austenite and long strips of delta-ferrite surrounded by Cr-carbide, and the welds consisted of delta ferrite and austenite matrix. For the heat treatment at 850 ◦C or lower, Cr-carbides were precipitated in the weld metal resulting in the reduction of elongation. The strength, however, was slightly reduced despite the presence of Cr-carbides and this could possibly be explained by the relaxation of internal stress and the weakening of particle hardening. In the heat treatment at 1050 ◦C, the dissolution of Cr-carbide and disappearance of delta ferrite resulted in the lower yield strength and higher elongation partially assisted from deformation-induced martensitic transformation. Consequently, superior property in terms of fracture toughness was achieved by the heat treatment at 1050 ◦C, suggesting that the mechanical properties of the as-weld metal can be enhanced by controlling the post weld heat treatment. Keywords: austenitic 304 stainless steels; sub-merged arc welding; post-weld heat treatment 1. Introduction Austenitic stainless steels are widely used in important parts of chemical plants such as pressure vessels, chemical machineries, and pipes because they have excellent corrosion resistance, acid resistance, formability, and weldability. They are also extensively applied in gas turbines, jet propellants, and nuclear power plants because of their excellent toughness in high-temperature and high-pressure environments [1–4]. Submerged Arc Welding (SAW) is widely used for joining ship structures and piping thick-wall stainless steels [5,6]. In SAW, a solid filler wire is automatically inserted to pre-dispersed granular flux and an arc generates between the base metal and the filler wire while being covered with flux. The flux in contact with the arc melts and then the molten slag covers and protects the molten metal. Generally high-efficiency welding is achieved by flowing a large current through a large-diameter wire. It has the advantage of improving welding productivity through high speed solidification during welding and improving weld quality through welding heat control [7]. Metals 2018, 8, 26; doi:10.3390/met8010026 www.mdpi.com/journal/metals Metals 20182018, ,8,8 ,26 26 2 of 13 Due to the heat input from the SAW welding process, there is a degradation problem of microstructuralDue to the and heat mechanical input from properties the SAW such welding as inhomogeneity process, there of the is microstructure, a degradation concentration problem of ofmicrostructural residual stress, and brittleness, mechanical and properties deterioration such of as toughness inhomogeneity in the ofwe thelded microstructure, material [8]. Post-weld concentration heat treatmentof residual (PWHT) stress, brittleness, is usually and introduced deterioration to solve of toughness such degradation. in the welded However, material if the [8]. delta Post-weld ferrite heat in austenitictreatment stainless (PWHT) steel is usually is exce introducedssively present to solve in the such weld degradation. metal during However, SAW welding, if the deltaembrittlement ferrite in ofaustenitic the welded stainless part may steel occur is excessively due to transformation present in the weldinto another metal during brittle SAWphase welding, such as a embrittlement sigma phase duringof the welded post heat part treatment. may occur In dueorder to transformationto solve these problems, into another it is brittle important phase to such investigate as a sigma post-heat phase treatmentduring post process heat treatment.to control the In order content to solveof delta these ferrite problems, and the itharmfu is importantl carbides to investigate[9]. Proper post-heat treatment can process improve to control the mechanical the content properties of delta ferrite by homogenizing and the harmful the carbidesmicrostructure [9]. Proper of the post-heat welded materialtreatment and can controlling improve the the mechanical formation of properties beneficial by and homogenizing harmful phases the [10–13]. microstructure of the welded materialAlthough and controlling extensive theresearches formation have of conducted beneficial and on the harmful effectphases of PWHT [10 –on13 ].the welded austenitic stainlessAlthough steels, extensiveindustrial researches field still haverequires conducted the comprehensive on the effect ofresearch PWHT on on the the resultant welded austenitic effect of PWHT,stainless especially steels, industrial for the field practical still requires and the the cost comprehensive effective process research condition on the resultantof PWHT. effect The of present PWHT, studyespecially thus for investigated the practical the and effect the cost of effectivePWHT on process the change condition of ofmicrostructural PWHT. The present and mechanical study thus propertiesinvestigated of thethe effectwelded of part PWHT of AISI on the STS change 304 st ofeel microstructural produced by SAW. and mechanicalThe microstructural properties changes of the duringwelded the part heat of AISI treatment STS 304 were steel analyzed produced and by the SAW. resultant The microstructural effect on mechanical changes properties during the were heat discussed.treatment were analyzed and the resultant effect on mechanical properties were discussed. 2. Materials Materials and Methods In this study, submerged-arcsubmerged-arc welding was performedperformed in austenitic stainlessstainless steel STS304 using STS308L as as a a welding welding electrode. electrode. The The chemical chemical compos compositionsitions of the of materials the materials used usedare shown are shown in Table in 1.Table A plate1. A with plate the with width the of width 100 mm, of 100 the mm, length the of length 1000 mm of 1000 and mmthe thickness and the thickness of 25 mm ofwas 25 fabricated mm was byfabricated the SAW. by the In SAW.order In to order evaluate to evaluate the micros the microstructuretructure and and mechanical mechanical properties, properties, PWHT PWHT was was ◦ ◦ performed from 650–1050 °CC with an interval of 200 °CC as shown in Figure1 1a.a. AfterAfter maintainingmaintaining atat these temperatures for 0.5–4 h, the specimensspecimens were thenthen quenchedquenched inin water.water. In order order to to observe observe the the microstructure, microstructure, the the test test pieces pieces were were grinded grinded using using sandpapers sandpapers and then and werethen micro-polished were micro-polished using a using diamond a diamond suspension suspension with a 1 μ withm diameter. a 1 µm According diameter. to AccordingASTM E-340, to theASTM polished E-340, surface the polished was etched surface using was a etchedmixed usingsolution a mixed of distilled solution water, of distilledhydrochloric water, acid hydrochloric and nitric acid with and nitrica ratio acid of 4:3:3. with Through a ratio ofthe 4:3:3. optical Through microscopy, the optical the microstructures microscopy, the of the microstructures base metal, the of weldthe base part metal,and the the heat weld affected part zone and (HAZ) the heat were affected analyzed. zone According (HAZ) were to ASTM analyzed. E-8, subsized According plate- to typeASTM tensile E-8, subsized specimens plate-type were taken tensile from specimens the middle were of taken the fromwelded the plate middle with of theperpendicular welded plate to with the weldingperpendicular direction to the as weldingshown in direction Figure 1b. as shown in Figure1b. Figure 1. Schematic diagrams showing (a) post-weld heat treatment schedules and (b) ASTM-E8 Figure 1. Schematic diagrams showing (a) post-weld heat treatment schedules and (b) ASTM-E8 subsized tensile specimen. subsized tensile specimen. The tensile tests were conducted to measure yield strength, tensile strength and elongation with −3 the strain rate of 1010−3/s/s according according to to ASTM ASTM A370 A370 using using universal universal tensile tensile machine machine (MTS, (MTS, Eden Eden Praire, Praire, MN, USA). The yield strength was determined by the 0.2% offset method and the elongation was measured by the travel distance of crosshead. Vickers hardness test was conducted with the load of Metals 2018, 8, 26 3 of 13 MN, USA). The yield strength was determined by the 0.2% offset method and the elongation was measured by the travel distance of crosshead. Vickers