The Influence of the Post-Weld Heat Treatment on the Microstructure of Inconel 625/Carbon Steel Bimetal Joint Obtained by Explos

metals

Article The Inﬂuence of the Post-Weld Heat Treatment on the Microstructure of Inconel 625/Carbon Steel Bimetal Joint Obtained by Explosive Welding

Robert Kosturek 1 , Marcin Wachowski 1,* , Lucjan Snie´ zek˙ 1 and Michał Gloc 2

1 Military University of Technology, Faculty of Mechanical Engineering, 2 gen. W. Urbanowicza str., 00-908 Warsaw, Poland; [email protected] (R.K.); [email protected] (L.S.)´ 2 Warsaw University of Technology, Faculty of Materials Science and Engineering, 141 Woloska str., 02-507 Warsaw, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-261-839-245

Received: 11 January 2019; Accepted: 12 February 2019; Published: 19 February 2019

Abstract: Inconel 625 and steel P355NH were bonded by explosive welding in this study. Explosively welded bimetal clad-plate was subjected to the two separated post-weld heat treatment processes: stress relief annealing (at 620 ◦C for 90 min) and normalizing (at 910 ◦C for 30 min). Effect of heat treatments on the microstructure of the joint has been evaluated using light and scanning electron microscopy, EDS analysis techniques, and microhardness tests, respectively. It has been stated that stress relief annealing leads to partial recrystallization of steel P355NH microstructure in the joint zone. At the same time, normalizing caused not only the recrystallization of both materials, but also the formation of a diffusion zone and precipitates in Inconel 625. The precipitates in Inconel 625 have been identiﬁed as two types of carbides: chromium-rich M23C6 and molybdenum-rich M6C. It has been reported that diffusion of alloying elements into steel P355NH takes place along grain boundaries with additional formation of voids. Scanning transmission electron microscope observation of the grain microstructure in the diffusion zone shows that this area consists of equiaxed grains (at the side of Inconel 625 alloy) and columnar grains (at the side of steel P355NH).

Keywords: explosive welding; heat treatment; Inconel; steel; microstructure

1. Introduction Corrosive wear is a significant problem for the utilization of components of equipment operating in an aggressive environment, such as reactors, tanks, heat exchangers, and pipelines in the chemical industry and geothermal power plants [1,2]. Basically, the alloys which can provide a specified resistant against corrosion in the aggressive environment are expensive. However, there is no need for making the entire component from corrosion resistant material—only its surface, which has direct contact with an aggressive medium. An approach worth considering is the use of relatively inexpensive material (e.g., non-alloy steel) and cladding it with layer of corrosion resistant alloy, such as stainless steel or nickel alloy providing potentiality of operating in the aggressive environment. This solution allows to reduce the material cost of industry equipment significantly depending on used manufacturing technique [3–5]. As an example of such bimetal clad plate Inconel/steel system can be given, in which steel is a load-bearing component while Inconel provides resistance to an aggressive environment. This paper investigates steel P355NH cladded with Inconel 625 as bimetallic material of the above type. A pressure vessel steel grade P355NH, weldable constructional steel with fine grain microstructure, is used as material for the manufacture of pressure equipment operating under high temperature (up to 450 ◦C) [6]. The poor corrosion resistance of this steel limits its applications as a construction

Metals 2019, 9, 246; doi:10.3390/met9020246 www.mdpi.com/journal/metals Metals 2019, 9, 246 2 of 16 material for the equipment working in the aggressive environment significantly. As it was mentioned previously, the potential solution of this problem is to clad steel P355NH with a layer of corrosion resistant material, e.g., Inconel 625, high-temperature creep resistant nickel alloy which is characterized by high resistance to oxidizing and reducing environment as well as pitting and crevice corrosion and it also displays tolerance to a wide range of the operating temperature (from −150 ◦C to 982 ◦C) [7]. Materials cladding or modifying their surface layer are widely used technologies in the production of industry equipment and machinery, but not all of this process can provide sufficient properties in terms of the formability of the cladded material [8–10]. The technology which allows to produce such bimetal clad-plate and provide both high quality of joint and formability is the explosive welding technique [11–14]. In this process, the energy released during detonation of the high explosive is used to accelerate one metal plate into another, and as a result, the high velocity collision of metal plates occurs [15,16]. The high energy collision results in bringing the surfaces of the colliding metals close enough to each other to obtain interaction between their atoms and make the formation of a metallic bond between them possible [14,17–20]. The severe plastic deformation of the materials significantly influences their microstructure and causes strain hardening in the joint zone. Clad-plates manufactured by this method are subjected to further technological processes to form specific equipment components for the industry, e.g. pipes, pressure vessels, tube plates for heat exchangers. For this reason, the high strain hardening of bimetal is a phenomenon which makes difficulties during plastic forming of clad-plate in the as-welded state. In order to decrease the degree of strain hardening of both materials, as well as, to reduce the residual stresses, the clad-plates are subjected to heat treatment [21–23]. However, the annealing of bimetallic materials in many cases leads to microstructural changes within the joint zone, which may decrease the mechanical properties of bond between welded metals [24]. According to previous investigations performed by the authors of this paper, the normalizing of Inconel 625—steel P355NH joint obtained by explosive welding decreases its shear strength by 33% (decrease from 572 MPa to 383 MPa, determined according to PN-EN13445:2014) [22]. The heat treatment of the explosively welded clad-plate may result in such changes in the joint zone as grainy microstructure evolutions (recrystallization, grain growth), diffusion processes, as well as, the formation of new phases [16,21,25–28]. The character of the diffusion zone depends on the mutual solubility of the alloy chemical components. As a result of the diffusion changes within joint zone it is possible of brittle intermetallic compounds to be formed, new solid solutions or precipitates [29,30]. The important phenomenon, which can take place during heat treatment of the bimetal system is the Kirkendall effect, which results in the formation of the voids in the joint area as a consequence of the differences in diffusion rates of specific alloying elements of the welded materials [25,27,31–33]. The second important aspect that has to be taken into consideration is the fact that exposing of Inconel alloys to the long-term annealing process may have consequences in the formation of precipitates in their microstructure (e.g. carbides, γ” and δ phases) [34–37]. Although, the classic heat treatments of the clad-plates are not long enough to cause the precipitation processes in Inconel alloys, the severe plastic deformation which affects the material in the joint area influence the kinetic of precipitating significantly promotes quicker formation of precipitates [38–40]. Beside plastic deformation, another important factor which promotes formation of carbide precipitates during heat treatment is the carbon diffusion from steel into Inconel alloy which contains chemical elements having a high affinity for carbon (Cr, Mo, Nb) [41]. For this reason, Inconel 625 layer close to joint line has a high potential to form the precipitates during heat treatment of the investigated explosively welded clad-plate. These precipitates can not only decrease the mechanical properties of Inconel alloy but also cause the reduction of the joint strength between welded materials, what can result in high risk of delamination of clad plate during its utilization. Although Inconel/steel explosively welded clad-plate has been a subject of some studies, the literature does not contain sufficient research on the influence of the heat treatment on the changes in the joint zone microstructure [11–14]. Heat-activated phenomena may significantly decrease strength of the joint and cause the risk of failure during heat treatment and forming at the manufacturing stage of specific component. The present work is aimed to investigate Metals 2019, 9, 246 3 of 16 the influence of stress relief annealing and normalizing on the microstructure of Inconel 625/steel P355NH bimetallic joint. Metals 2018, 8, x FOR PEER REVIEW 3 of 17 2. Materials and Methods 2. Materials and Methods In this study, the materials used for the manufacturing of bimetal clad-plate were a 10 mm thick plateIn of this steel study, P355NH the materials and a 3 mm used thick for the sheet manufa of Inconelcturing 625 of bimetal alloy. The clad-plate dimensions were ofa 10 plates mm werethick equalplate of to steel 860 ×P355NH1000 mm. and Thea 3 mm surfaces thick to sheet be joined of Inconel have 625 been alloy. polished The dime and cleanednsions of with plates acetone were beforeequal to welding. 860 × 1000 The mm. chemical The surfaces compositions to be joined of the have materials been polished are presented and cleaned in Table with1. acetone The samples before werewelding. cut outThe of chemical the workpieces compositions in order of the to perform materials a microstructureare presented in investigation Table 1. The of samples materials were in thecut as-received.out of the workpieces The process in of order explosive to perform welding a ofmicrostructure steel P355NH andinvestigation Inconel 625 of alloymaterials was performedin the as- byreceived. EXPLOMET The process High-Energy of explosive Techniques welding Works of steel company P355NH (Opole, and Inconel Poland). 625 The alloy explosive was performed used in theby processEXPLOMET was modified High-Energy ammonium Techniques nitrate Works fuel oil company (ANFO), (Opole, with detonation Poland).velocity The explosive of 2700 m/s,used whichin the hasprocess been was determined modified using ammonium optical fibernitrate sensors. fuel oil The (ANFO), stand-off with distance detonation between velocity plates wasof 2700 equal m/s, to 3which mm. Thehas been explosive determined has been using placed optical directly fiber on sens Inconelors. 625The platestand-off and nodistance buffer between plate has plates been used. was Threeequal samplesto 3 mm. were The explosive cut out of has the bimetallicbeen placed clad-plate directly producedon Inconel by 625 explosive plate and welding, no buffer as shownplate has in Figurebeen used.1. Three samples were cut out of the bimetallic clad-plate produced by explosive welding, as shown in Figure 1. Table 1. Chemical composition of the joined alloys [% mass]. Table 1. Chemical composition of the joined alloys [% mass]. Al Cr Fe Mo Nb Ti Ni Inconel 625 Al Cr Fe Mo Nb Ti Ni Inconel 625 0.16 21.5 4.6 8.7 3.32 0.18 Base 0.16 21.5 4.6 8.7 3.32 0.18 Base C Cr Si Mn Ni Cu Fe St. P355NH C Cr Si Mn Ni Cu Fe St. P355NH 0.18 0.02 0.35 1.19 0.22 0.2 Base 0.18 0.02 0.35 1.19 0.22 0.2 Base

Figure 1. Area from which samples of bimetallic-clad plates were collected for microstructure investigation.Figure 1. Area from which samples of bimetallic-clad plates were collected for microstructure investigation. In order to analyze the inﬂuence of the post-weld heat treatment on the microstructure of the obtainedIn order joint, to theanalyze ﬁrst samplethe influence has been of the investigated post-weld heat in the treatment as-welded on state,the microstructure the second sample of the hasobtained been joint, subjected the first to heat sample treatment has been of stressinvestigated relief annealing in the as-welded (at 620 ◦ state,C for the 90 min),second and sample the third has samplebeen subjected was subjected to heat treatment to the normalizing of stress relief (at 910 annealing◦C for 30(at min).620 °C The for 90 parameters min), and ofthe the third post-weld sample heatwas treatmentsubjected haveto the been normalizing selected for (at base 910 material°C for 30 (P355NH), min). The which parameters plays a load-bearingof the post-weld role in heat the investigatedtreatment have clad-plate. been selected As a result,for base for material the further (P355NH), investigation which the plays three a load-bearing samples were role obtained: in the ainvestigated sample after clad-plate. explosive As welding a result, (InSt for EXW), the further sample investigation after post-weld thestress three reliefsamples annealing were obtained: (InSt HTR) a andsample sample after after explosive post-weld welding normalizing (InSt EXW), (InSt sample HTN). after In order post-weld to perform stress therelief microstructure annealing (InSt analysis, HTR) and sample after post-weld normalizing (InSt HTN). In order to perform the microstructure analysis, the samples have been subjected to the metallographic preparation. The cut samples were mounted in hot-mounting resin, ground with abrasive paper of 80, 320, 600, 1200 and 2400 gradations and polished using diamond paste of 1 µm gradation. In order to reveal the microstructure of steel P355NH, 2% nital etchant with etching time of 5–10 seconds was used and in case of Inconel 625 alloy,

Metals 2019, 9, 246 4 of 16 the samples have been subjected to the metallographic preparation. The cut samples were mounted in hot-mounting resin, ground with abrasive paper of 80, 320, 600, 1200 and 2400 gradations and polishedMetals 2018 using, 8, x FOR diamond PEER REVIEW paste of 1 µm gradation. In order to reveal the microstructure of steel P355NH,4 of 17 2% nital etchant with etching time of 5–10 seconds was used and in case of Inconel 625 alloy, acetic acetic glyceregia (15 mL HCl 38%, 10 mL of acetic acid 99%, 5 mL HNO3 65%, 1–2 drops of glycerol) glyceregia (15 mL HCl 38%, 10 mL of acetic acid 99%, 5 mL HNO3 65%, 1–2 drops of glycerol) with withetching etching time oftime 15 min.of 15 Grain min. sizeGrain of size the materialsof the materials has been has measured been measured with Mountains with Mountains Map 7 software. Map 7 Thesoftware. microstructure The microstructure of the samples of the was samples investigated was usinginvestigated light microscope using light OLYMPUS microscope LEXT OLYMPUS OLS 4100 (MilitaryLEXT OLS University 4100 (Military of Technology, University Warsaw, of Technolo Poland)gy, and Warsaw, scanning Poland) electron and microscope scanning (SEM) electron Jeol JSMmicroscope 6610 (Military (SEM) Jeol University JSM 6610 of (Military Technology, University Warsaw, of Poland) Technology, equipped Warsaw, with energy-dispersivePoland) equipped x-raywith energy-dispersivespectroscopy (EDS) x-ray and spectroscopy back-scattered (EDS) electron and back (BSE)-scattered detector. electron The diffusion (BSE) detector. zone, which The diffusion has been zone,formed which in the has post-weld been formed normalizing in the post-weld (InSt HTN) norm samplealizing was (InSt investigated HTN) sample using was scanning investigated transmission using scanningelectron microscope transmission (STEM) electron Hitachi microscope S-5500N (Military(STEM) UniversityHitachi S-5500N of Technology, (Military Warsaw, University Poland). of TheTechnology, sample forWarsaw, STEM Poland).. was prepared The sample using dualfor STEM beam was system prepared Hitachi using NB-5000 dual beam (Military system University Hitachi NB-5000of Technology, (Military Warsaw, University Poland). of InTechnology, order to establish Warsaw, the Poland). strain hardening In order of to analyzed establish samples the strain the Vickershardening microhardness of analyzed samples test was the performed Vickers microhardness with loading of test 100 was g. Microhardness performed with distributions loading of 100 were g. preparedMicrohardness for each distributions sample. The were ﬁrst prepared two measurements for each sample. were performed The first 200twoµ mmeasurements from the joint were line, performedin the layer 200 of Inconel µm from 625 the alloy joint and line, in steelin the P355NH layer of as Inconel shown 625 in Figurealloy and2. Subsequently, in steel P355NH measurement as shown inimprints Figure were2. Subsequently, guided towards measurement the edge of imprints the samples, were atguided the distance towards of the 2000 edgeµm. of the samples, at the distance of 2000 µm.

Figure 2. The scheme of microhardness testing. 3. Results 3. Results 3.1. Microstructure of the Raw Materials 3.1. Microstructure of the Raw Materials In the ﬁrst part of the investigation the light microscopy observations of the base materials after etchingIn the have first been part performed. of the investigation The microstructures the light mi ofcroscopy steel P355NH observations (Figure 3ofA) the and base Inconel materials 625alloy after (Figureetching3 haveB) in been the as-received performed. state The aremicrostructures presented in Figureof steel3 .P355NH As it can (Figure be observed, 3A) and the Inconel steel P355NH 625 alloy (Figurehas a ferrite-pearlite 3B) in the as-received microstructure state are with presented noticeable in Figure pearlite 3. As bands, it can characteristic be observed, for the the steel plates P355NH after hasrolling a ferrite-pearlite process. The microstructuremicrostructure ofwith steel noticeable has ﬁne equiaxialpearlite bands, grains characteristic with their measured for the plates size equal after rollingto 15.5 process.± 4.1 µm. TheAt microstructure the same time, of Inconel steel has 625 fine alloy equiaxial has far grains more with heterogenous their measured microstructure size equal tocharacterized 15.5 ± 4.1 µm. by presenceAt the same of twinstime, Inconel and measured 625 alloy grain has sizefar more of 49.4 heterogenous± 15.6 µm. microstructure Additionally, characterizedthe microhardness by presence of base materials of twins has and been measured measured grain with size registered of 49.4 values± 15.6 ofµm. 150.6 Additionally,± 4.9 HV0.1 the for microhardnesssteel P355NH and of base 249.6 materials± 16.3 HV0.1 has been for Inconel measured 625 with alloy, registered respectively. values of 150.6 ± 4.9 HV0.1 for steel P355NH and 249.6 ± 16.3 HV0.1 for Inconel 625 alloy, respectively.

Metals 2019, 9, 246 5 of 16 Metals 2018, 8, x FOR PEER REVIEW 5 of 17

FigureFigure 3. 3. MicrostructureMicrostructure of of materials materials in in the as-received state: ( AA)) steel steel P355NH; P355NH; ( (B)) Inconel Inconel 625. 625.

3.2.3.2. Microstructure Microstructure of a Sample After Explosive Welding (InSt EXW) Joint TheThe obtained obtained joint has a a wavy wavy structure, structure, typical for explosively welded bond. The grains grains of of both both materialsmaterials in the jointjoint zonezone areare deformeddeformed due due to to severe severe plastic plastic deformation deformation during during collision, collision, as as it canit can be beobserved observed after after etching etching of steelof steel P355NH P355NH (Figure (Figure4A) 4A) and and Inconel Inconel 625 625 alloy alloy (Figure (Figure4B). 4B).

FigureFigure 4. Microstructure ofof thethe jointjoint inin samplesample after after explosive explosive welding welding (InSt (InSt EXW): EXW): (A ()A after) after etching etching of ofsteel steel P355NH; P355NH; (B )(B after) after etching etching of of Inconel Inconel 625 625 alloy. alloy.

TheThe observations observations using scanning scanning electron electron micros microscopecope (BSE) show the occurrence occurrence of melted melted zones zones inin the the joint, joint, where where the the two welded materials have have been been stirred stirred together together (Figure (Figure 55A).A). InIn thethe meltedmelted zones,zones, the the investigation revealed revealed the the presence presence of of joint joint imperfections imperfections in in the the form form of of both cracks and fragmentsfragments of of steel steel P355NH surface layer, which underwent partial fragmentationfragmentation during explosive welding process, as evidenced by linear analysis of the the chemical chemical composition composition (Figure (Figure 55B).B). TheThe resultsresults ofof the chemical composition analysis indicate indicate on on the highest participation of of Inconel 625 alloying elementselements in this area with small fluctuat ﬂuctuationsions near to the steelsteel P355NH fragment.

Metals 2019, 9, 246 6 of 16 Metals 2018, 8, x FOR PEER REVIEW 6 of 17

Figure 5. Melted zone zone in InSt EXW sample: ( (AA)) microstructure microstructure of of the the melted zone; ( (B)) with linear analysis of the chemical composition (yellow marker). 3.3. Microstructure of Sample After Post-Weld Stress Relief Annealing (InSt HTR) Joint 3.3. Microstructure of Sample After Post-Weld Stress Relief Annealing (InSt HTR) Joint Post-weld stress relief annealing of Inconel 625/steel P355NH bimetallic clad-plate, performed at Post-weld stress relief annealing of Inconel 625/steel P355NH bimetallic clad-plate, performed 620 ◦C for 90 min slightly inﬂuences the microstructure of the joint zone. The partial recrystallization of at 620 °C for 90 min slightly influences the microstructure of the joint zone. The partial steel P355NH microstructure has been reported in the area of 20–30 µm from the joint line (Figure6A). recrystallization of steel P355NH microstructure has been reported in the area of 20–30 μm from the New, equiaxial grains formed on the joint line do not have deformation texture of previous compressed, joint line (Figure 6A). New, equiaxial grains formed on the joint line do not have deformation texture elongated grains. The microstructure of steel P355NH farther from the joint line (about 30 µm) of previous compressed, elongated grains. The microstructure of steel P355NH farther from the joint maintains the deformation texture and no recrystallized grains have been observed. On the other hand, line (about 30 μm) maintains the deformation texture and no recrystallized grains have been the microstructure of Inconel 625 alloy did not reveal any visible changes in the grain morphology observed. On the other hand, the microstructure of Inconel 625 alloy did not reveal any visible after stress relief annealing compared with its microstructure in the as-welded state (Figure6B). changes in the grain morphology after stress relief annealing compared with its microstructure in the as-welded state (Figure 6B).

Metals 2019, 9, 246 7 of 16 Metals 20182018,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1717

Figure 6. Microstructure of the joint in sample after post-weld stress relief annealing (InSt HTR) sample: Figure(A) after 6. etchingMicrostructure of steel P355NH; of the joint (B) afterin sample etching after of Inconel post-weld 625 alloy.stress relief annealing (InSt HTR) sample:sample: ((A)) afterafter etchingetching ofof steelsteel P355NH;P355NH; ((B)) afterafter etchingetching ofof InconelInconel 625625 alloy.alloy. The scanning electron microscope observations did not show any visible changes in the concentrationThe scanning of the electron chemical microscope elements in observatio the joint zonens did (Figure not 7show). Both any the visible bound betweenchanges joinedin the concentrationconcentrationmaterials and meltedofof thethe chemicalchemical zone are elements notelements affected inin the bythe stress jointjoint zone reliefzone (Figure annealing(Figure 7).7). in BothBoth terms thethe of boundbound chemical betweenbetween composition. joinedjoined materialsThe small and imperfections melted zone in are form not of affected voids are by possiblestress relief to observe annealing in melted in terms zone, of chemical which is composition. localized on Theone small side of imperfections the intersurface in form wave of (Figure voids7 are). possiblele toto observeobserve inin meltedmelted zone,zone, whichwhich isis localizedlocalized on one side of the intersurface wave (Figure 7).

FigureFigure 7. 7. ImageImageImage ofof of thethe the jointjoint joint inin in InStInSt InSt HTRHTR HTR samplesample sample fromfrom from thethe the scanningscanning scanning electronelectron microscope.microscope.

3.4.3.4. Microstructure Microstructure of of InSt InSt HTN HTN Joint Joint ◦ TheThe microstructuremicrostructure of of the the joint joint subjected subjected to normalizing to normalizingnormalizing at 910 atCat for910910 30 °C°C min forfor changed 3030 minmin significantly. changedchanged significantly.significantly.It has been reported ItIt hashas beenbeen that duereportedreported to this thatthat post-weld duedue toto heat thisthis treatment post-weldpost-weld the heatheat complete treatmenttreatment recrystallization thethe completecomplete of recrystallizationrecrystallizationmicrostructure of ofof both microstructuremicrostructure joined materials—steel ofof bothboth joinedjoined P355NH materials—steelmaterials—steel (Figure8A) and P355NHP355NH Inconel (Figure(Figure 625 (Figure 8A)8A) andand8B) Inconel occurs.Inconel 625The (Figure welded 8B) materials occurs. The have welded microstructures materials have consisting microstructures of fine, equiaxial consisting grains of fine, and equiaxial no deformation grains andtexture no deformation is noticeable. texture The size is ofnoticeable. steel grains The is size about of 20 steelµm, grains which is is about a typical 20 μ valuem, which for this is a material typical valueafter normalizing. for this material Additionally, after normalizing. it has been Additionally, observed the it has presence been observed of ultrafine the grains presence with of size ultrafine about grains5 µm ofwith steel size P355NH about 5 µm on theof steel joint P355NH line (Figure on the8A). joint In line case (Figure of Inconel 8a). In 625 case grains of Inconel have 625 size grains also have size also about 20 µm with low participation of twins and significant amount of precipitates localizedlocalized onon graingrain boundaries.boundaries.

Metals 2019, 9, 246 8 of 16

Metalsabout 2018 20, µ8, mx FOR with PEER low REVIEW participation of twins and signiﬁcant amount of precipitates localized8 of on17 Metalsgrain 2018 boundaries., 8, x FOR PEER REVIEW 8 of 17

Figure 8. Microstructure of the joint in post-weld normalizing (InSt HTN) sample: (A) after etching of Figure 8. A steelFigure P355NH; 8. MicrostructureMicrostructure (B) after etching of of the the joint jointof Inconel in in post post-weld 625-weld alloy. normalizing normalizing (InSt HTN) sample: ( (A)) after after etching etching of of steel P355NH; (B) after etching of Inconel 625 alloy. steel P355NH; (B) after etching of Inconel 625 alloy. The observations using scanning electron microscope show significant changes in the The observations using scanning electron microscope show signiﬁcant changes in the concentrationThe observations of alloying using elements scanning in the joint electron area. Itmi hascroscope been stated show that significant alloying elements changes of Inconelin the concentration of alloying elements in the joint area. It has been stated that alloying elements of concentration625 were found of alloyingto diffuse elements into steel in the P355NH joint area. along It has grain been boundaries stated that alloying(Figure elements9A). Additionally, of Inconel Inconel 625 were found to diffuse into steel P355NH along grain boundaries (Figure9A). Additionally, 625diffusion were zonefound contains to diffuse voids, into which steel areP355NH localized along mainly grain on boundaries grain boundaries (Figure and 9A). the Additionally, joint line at diffusion zone contains voids, which are localized mainly on grain boundaries and the joint line at the diffusionthe side of zone steel contains P355NH voids, (Figure which 9B). areAnother localized noti ceablemainly change on grain compared boundaries with and the the joint joint in theline as- at side of steel P355NH (Figure9B). Another noticeable change compared with the joint in the as-welded thewelded side stateof steel is formationP355NH (Figure of precipitates 9B). Another in the noti jointceable zone, change which compared are localized with in the Inconel joint in625 the alloy as- state is formation of precipitates in the joint zone, which are localized in Inconel 625 alloy (Figure9A,B). welded(Figure 9A,B).state is formation of precipitates in the joint zone, which are localized in Inconel 625 alloy (Figure 9A,B).

FigureFigure 9. 9. ImageImage of of the the joint joint in in InSt InSt HTN HTN sample sample fr fromom the the scanning scanning electron electron microscope: microscope: ( (AA)) diffusion diffusion ofFigureof alloying 9. Image elementselements of the throughthrough joint in the InStthe joint jointHTN line line sample and and precipitates frprecipitatesom the scanning in Inconelin Inconel electron 625 625 alloy; microscope: alloy; (B) diffusion(B) diffusion (A) zonediffusion zone with ofwithvisible alloying visible voids elements voids along along the through grain the grai boundaries then boundariesjoint line and and theand precipitates joint the line.joint line. in Inconel 625 alloy; (B) diffusion zone with visible voids along the grain boundaries and the joint line. ScanningScanning electron electron microscope microscope observations revealed the presence of two types of precipitates precipitates in in InconelInconelScanning 625625 alloy—lightalloy—light electron microscope precipitates precipitates observations (suggesting (suggesting a revealed higha high concentration concentration the presence of alloyingofof two alloying types elements elementsof precipitates heavier heavier than in Inconelthannickel) nickel) and625 darkalloy—lightand precipitatesdark precipitates precipitates (high (high concentration (suggesting concentratio ofa alloyinghighn of concentration alloying elements elements lighter of alloying lighter than nickel). elementsthan nickel). The heavier size The of thansizeprecipitates ofnickel) precipitates isand about dark is 0.5–1 aboutprecipitatesµm 0.5–1 (Figure µm(high 10 (FigureA). concentratio In order10A).to Inn establish oforder alloying to a establish chemical elements a composition chemicallighter than composition of nickel). precipitates The of sizeprecipitatesEDX of area precipitates analysis EDX area hasis about beenanalysis performed.0.5–1 has µm been (Figure Dark performed. precipitates10A). In Darkorder are precipitatesto characterized establish area chemical by characterized high concentration composition by high of precipitatesconcentrationchromium andEDX of increased chromium area analysis participation and hasincreased been of performed.molybdenumparticipation Dark of (Figure molybdenum precipitates 10B). At are the(Figure characterized same 10B). time, At it the hasby same beenhigh concentrationtime, it has been of chromiumfound that andchemical increased compositio participationn of light of molybdenumprecipitates has (Figure high molybdenum10B). At the same and time,niobium it has content been (Figurefound that10C). chemical Additionally, compositio the presencen of light of precipitatescarbon in both has types high ofmolybdenum precipitates andhas niobiumbeen reported. content (Figure 10C). Additionally, the presence of carbon in both types of precipitates has been reported.

Metals 2019, 9, 246 9 of 16 found that chemical composition of light precipitates has high molybdenum and niobium content

(FigureMetals 2018 10, C).8, x FOR Additionally, PEER REVIEW the presence of carbon in both types of precipitates has been reported.9 of 17

Figure 10. A Figure 10. Precipitates inin InconelInconel 625625 ( (A)) together together with with the the results results of of chemical chemical composition composition analysis analysis of dark precipitate (B) and light precipitate (C). of dark precipitate (B) and light precipitate (C). The width of the diffusion zone in InSt HTN sample (Figure 11A) has been estimated using linear The width of the diffusion zone in InSt HTN sample (Figure 11A) has been estimated using analysis of chemical composition (Figure 11B). The diffusion zone has about 15 µm width and can be linear analysis of chemical composition (Figure 11B). The diffusion zone has about 15 μm width and divided into two sections: iron-rich and chromium and nickel-rich. (Figure 11B). can be divided into two sections: iron-rich and chromium and nickel-rich. (Figure 11B).

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Figure 11. Image of the joint in InSt HTN sample: (A) microstructure of the joint; (B) linear analysis FigureFigure 11.11. ImageImage ofof the the joint joint in in InSt InSt HTN HTN sample: sample: (A (A) microstructure) microstructure of of the the joint; joint; (B )(B linear) linear analysis analysis of of the chemical composition (yellow marker). theof the chemical chemical composition composition (yellow (yellow marker). marker). 3.5. Scanning Transmission Electron Microscope Observations of the Diffusion Zone 3.5.3.5. Scanning Scanning Transmission Transmission Electron Electron Micr Microscopeoscope Observations Observations of of the the Diffusion Diffusion Zone Zone Observations of the diffusion zone performed on scanning transmission electron microscope allow ObservationsObservations ofof thethe diffusiondiffusion zonezone performedperformed onon scanningscanning transmissiontransmission electronelectron microscopemicroscope to investigate the grainy microstructure of this area. It has been stated that in terms of grain structure allowallow to to investigate investigate the the grainy grainy microstructure microstructure of of this this area. area. It It has has been been stated stated that that in in terms terms of of grain grain the diffusion zone consists of two subzones: area of equiaxed grains (at side of Inconel 625 alloy) structurestructure the the diffusion diffusion zone zone consists consists of of two two subzones: subzones: area area of of equiaxed equiaxed grains grains (at (at side side of of Inconel Inconel 625 625 and area of columnar grains (at side of steel P355NH) (Figure 12A). In both cases, the microstructure alloy)alloy) andand areaarea ofof columnarcolumnar grainsgrains (at(at sideside ofof steelsteel P355NH)P355NH) (Figure(Figure 12A).12A). InIn bothboth cases,cases, thethe consists of ultraﬁne grains with their size within the range of 400 nm–1 µm. Predominantly, the ﬁnerμ microstructuremicrostructure consistsconsists ofof ultrultrafineafine grainsgrains withwith theirtheir sizesize withinwithin thethe rangerange ofof 400400 nm–1nm–1 μm.m. grains close to Inconel 625 are equiaxed and columnar grains close to P355NH have slightly larger size. Predominantly,Predominantly, thethe finerfiner grainsgrains closeclose toto InconelInconel 625625 areare equiaxedequiaxed andand columnarcolumnar grainsgrains closeclose toto Additionally, it has been reported the occurrence of precipitates with their width about 50 nm in the P355NHP355NH have have slightly slightly larger larger size. size. Additionally, Additionally, it it has has been been reported reported the the occurrence occurrence of of precipitates precipitates diffusion zone (Figure 12B). withwith their their width width about about 50 50 nm nm in in the the diffusion diffusion zone zone (Figure (Figure 12B). 12B).

FigureFigureFigure 12. 12.12. Image ImageImage of ofof the thethe diffusion diffusiondiffusion zone zonezone grainy grainygrainy microstructure microstructuremicrostructure in inin InSt InStInSt HTN HTNHTN sample samplesample ( (A(AA););); Image ImageImage of ofof the thethe precipitateprecipitateprecipitate in inin the thethe diffusion diffusiondiffusion zone zone ( B(B).).).

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The linear analysis of chemical composition has been performed in order to investigate subzone The linear analysis of chemical composition has been performed in order to investigate subzone of of columnar grains (Figure 13). Results indicate on similar concentration of chromium and niobium columnarin the analyzed grains (Figure area. 13On). the Results other indicate hand, the on similarelements concentration such as iron of and chromium nickel, andwhich niobium are main in the analyzedcomponents area. of On the the diffusion other hand, zone, the show elements significant such fl asuctuations iron and nickel,in their whichconcen aretration main in components this zone. It of thehas diffusion been reported zone, show that concentration significant fluctuations of nickel decreases in their concentration drastically on in the this border zone. between It has been columnar reported thatgrains concentration area of the of diffusion nickel decreases zone and drastically steel P355 onNH. the At border the same between time columnarthe concentration grains area of iron of the diffusionincreases, zone also and rapidly. steel P355NH.The results At of the linear same analysis time the together concentration with scanning of iron transmission increases, also electron rapidly. Themicroscopy results of linearobservations analysis of together this area with suggest scanning the predominate transmission role electron of nickel microscopy and iron concentration observations of thisratio area in suggest the forming the predominate of diffusion zone. role of The nickel precipitat and irones founded concentration in this ratioarea inwere the a forming subject of of further diffusion zone.investigation The precipitates and observations founded performed in this area on were scanning a subject transmission of further electron investigation microscope and reveal observations their performedspecific structure on scanning consisted transmission of core and electron shell (Fig microscopeure 12B). In reveal order their to examine specific chemical structure composition consisted of coreof andprecipitate shell (Figure in the 12diffusionB). In order zone tothe examine linear analysis chemical was composition performed of (Figure precipitate 14). The in the shell-core diffusion zonestructure the linear has been analysis confirmed wasperformed in terms of chemical (Figure 14 composition,). The shell-core since it structurehas been reported has been that confirmed in the precipitate area there are significant differences in distribution of alloying elements. The shell has inprecipitate terms of chemical area there composition, are significant since differences it has been in distribution reported thatof alloying in the precipitateelements. The area shell there has are high concentration of niobium and molybdenum, while core consists of chromium and also of significanthigh concentration differences of in distributionniobium and of molybdenum, alloying elements. while The core shell consists has high of chromium concentration and of also niobium of niobium and molybdenum (compared to the average value of concentration of these elements in the andniobium molybdenum, and molybdenum while core (compared consists of to chromium the average and va alsolue of of concentration niobium and of molybdenum these elements (compared in the diffusion zone). todiffusion the average zone). value of concentration of these elements in the diffusion zone).

(a) (b)

FigureFigure 13. 13.The The results results of linearof linear analysis analysis of the of chemical the chemic compositional composition of columnar of columnar grains grains in the diffusionin the zone.diffusion Lines zone. designation: Lines designation: Fe (green), Fe Ni (gr (blue),een), Ni Cr (blue), (red), NbCr (red), (purple). Nb (purple).

(a) (b) Figure 14. The results of linear analysis of the precipitate in the diffusion zone. Lines designation: Fe (blue), Ni (purple), Cr (green), Nb (black), Mo (burgundy), C (red). Metals 2018, 8, x FOR PEER REVIEW 12 of 17

Metals 2019, 9, 246 12 of 16 Figure 14. The results of linear analysis of the precipitate in the diffusion zone. Lines designation: Fe (blue), Ni (purple), Cr (green), Nb (black), Mo (burgundy), C (red). 3.6. Microhardness Analysis 3.6. Microhardness Analysis The inﬂuence of the explosive welding process on the joined materials in terms of strain hardening wasThe established influence by of microhardness the explosive analysiswelding (Figureprocess 15 on). Thethe joined highest materials degree of in strain terms hardening of strain hardeninghas been revealedwas established close to theby microhardness joint line, where analysis welded (Figure materials 15). were The subjectedhighest degree to the of severest strain hardeningplastic deformation has been revealed due to high close velocity to the collisionjoint line, during where explosivewelded material weldings were process. subjected In this to area, the severestthe microhardness plastic deformation of Inconel due 625 to alloy high increased velocity bycollision about 200during HV0.1 explosive and in welding case of steel process. P355NH In this by area,about the 100 microhardness HV0.1. Stress relief of Inconel annealing 625 reduced alloy increased the microhardness by about of 200 steel HV0.1 P355NH and by in about case 40 of HV0.1. steel P355NHIt is mostly by relatedabout 100 to recrystallizationHV0.1. Stress relief of steel anneal P355NHing reduced grains the and microhardness presumably the of decreasingsteel P355NH of theby aboutresidual 40 stressHV0.1. of It the is mostly welded related materials to recrystallization in the joint zone. of Thesteel post-weld P355NH grains heat treatment and presumably in the form the decreasingof normalizing of the reduced residual the stress microhardness of the welded of steel materials P355NH in and the Inconel joint zone. 625 to The their post-weld baseline value, heat treatmentmeasured in in the the form as-received of normalizing state. reduced the microhardness of steel P355NH and Inconel 625 to their baseline value, measured in the as-received state.

FigureFigure 15. 15. TheThe results results of of microhardness microhardness analysis. 4. Discussion 4. Discussion Both heat treatments cause changes in the microstructure of investigated joint. Stress relief Both heat treatments cause changes in the microstructure of investigated joint. Stress relief annealing has a lowest impact on grainy microstructure and leads only to partial recrystallization of annealing has a lowest impact on grainy microstructure and leads only to partial recrystallization of steel P355NH grains in the joint zone. On the other hand normalizing the complete restructure of steel P355NH grains in the joint zone. On the other hand normalizing the complete restructure of steel grainy microstructure has been noticed. It is a well-known fact that the higher degree of plastic steel grainy microstructure has been noticed. It is a well-known fact that the higher degree of plastic deformation, the lower energy is necessary to initiate and complete heat-activated phenomena and deformation, the lower energy is necessary to initiate and complete heat-activated phenomena and therefore the recrystallization temperature is lower, which explains the incomplete restructure of the therefore the recrystallization temperature is lower, which explains the incomplete restructure of the deformed steel structure after stress relief annealing. Additionally, it can be observed a disappear of deformed steel structure after stress relief annealing. Additionally, it can be observed a disappear of pearlite bands close to the joint line, what can suggest the diffusion process of carbon into Inconel pearlite bands close to the joint line, what can suggest the diffusion process of carbon into Inconel 625 alloy. Inconel contains chemical elements having a high afﬁnity for carbon, such as chromium, 625 alloy. Inconel contains chemical elements having a high affinity for carbon, such as chromium, molybdenum and niobium and for this reason carbon diffusion can result in formation of brittle molybdenum and niobium and for this reason carbon diffusion can result in formation of brittle carbides in the joint zone [15,41–43]. Inconel 625 isothermal transformation diagram shows that the carbides in the joint zone [15,41–43]. Inconel 625 isothermal transformation diagram shows that the parameters of the normalizing are close to the area of carbides formation, especially MC and M6C parameters of the normalizing are close to the area of carbides formation, especially MC and M6C types (Figure 16). types (Figure 16).

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Figure 16. Inconel 625 isothermal transformation diagram [ 35].35]. Parameters of stress relief annealing and normalizing have been marked.

As it was mentioned in the introduction part, the plastic deformation of Inconel 625 alloy, alloy, which took placeplace in in the the joint joint results results in reducing in reducing of the energyof the ofenergy heat-activated of heat-activated processes, processes, including precipitateincluding processes.precipitate Theprocesses. diffusion The of carbondiffusion from of steelcarbon P355NH from intosteel InconelP355NH 625 into alloy Inconel is an additional625 alloy factor,is an whichadditional has afactor, significant which impact has a onsignificant the carbides impact precipitation on the carbides processes precipitation in the layer processes of Inconel in 625 the near layer to ofthe Inconel joint line. 625 near Scanning to the electron joint line. microscopy Scanning electron observations microscopy of sample observations subjected of to sample the normalizing subjected torevealed the normalizing presence revealed of precipitates presence in of Inconel precipitates 625 close in Inconel to the 625 joint close line. to the The joint results line. of The chemical results ofcomposition chemical analysiscomposition of precipitates analysis indicateof precipitates to two types indicate of compounds: to two types light, of molybdenum-rich compounds: light, and molybdenum-richdark, chromium-rich. and The dark, presence chromium-rich. of carbon The in both presence precipitates of carbon has in been both reported. precipitates The literaturehas been reported.on the diffusion The literature bonding on of the Inconel diffusion 625 and bonding low-alloy of Inconel steel describes 625 and both low-alloy morphology steel describes and chemical both morphologycomposition and of carbides chemical formed composition in that of bimetallic carbides fo system,rmed in identifying that bimetallic the precipitates system, identifying as carbides the precipitatesof M6C type as in carbides the case of of M high6C type molybdenum in the case of concentration high molybdenum and of concentration M23C6 type in and the of case M23 ofC6 hightype inchromium the case concentration of high chromium [41,43]. Additionally,concentration M 6[41,43].C carbides Additionally, have been characterizedM6C carbides by have increased been characterizedconcentration by of niobiumincreased and concentration iron what hasof niobium been confirmed and iron in what this investigationhas been confirmed [41]. Scanning in this investigationelectron microscopy [41]. Scanning observation electron of themicroscopy diffusion observation zone formed of duethe diffusion to normalizing, zone formed allow due one to normalizing,draw a conclusion allow thatone chemicalto draw a elements conclusion of Inconelthat chemical 625 alloy elements diffuse of into Inconel steel 625 P355NH alloy diffuse along grain into steelboundaries. P355NH Diffusion along grain zone boundaries. has about Diffusion 15 µm width zone and has can about be divided15 μm width into twoand sectionscan be divided in terms into of twoiron, sections nickel and in terms chromium of iron, concentrations. nickel and chromium The first, concentrations. iron-rich section The consists first, iron-rich of about 90%section of ironconsists and ofbelow about 10% 90% of nickelof iron and and chromium. below 10% An of analysis nickel an ofd the chromium. Fe-Ni-Cr phaseAn analysis equilibrium of the diagram Fe-Ni-Cr suggests phase equilibriumthat this section diagram is composed suggests predominantly that this section of isγ phasecomposed and verypredominantly small amount of γ ofphaseα phase and (Figurevery small 17). Atamount the same of α time, phase the (Figure second 17). section At isthe characterized same time, bythe high second concentration section is ofcharacterized nickel (ca. 45%) by high and concentrationchromium (ca. of 30%). nickel Phase (ca. 45%) diagram and analysischromium also (ca. indicates 30%). Phase on γ phase, diagram as theanalysis main also component indicates of on the γ phase,section as with the themain small component addition of of theα’ phasesection (Figure with the 17). small addition of α’ phase (Figure 17). Scanning transmission electron microscopy observations, allowed to investigate the diffusion zone microstructure, which consists of equiaxed, columnar grains with presence of 50 nm chromium-rich precipitates having core-shell structure. It can be stated that both observed diffusion-based phenomena in sample after normalizing, including diffusion zone with presence of voids and carbides in Inconel 625 alloy have their detrimental effect on the joint strength. The carbides localized in Inconel 625 alloy layer, also result in decreasing of material coherence at the most crucial region—the joint. These factors might cause the deterioration of the joint quality established by the shear test in the previous investigation performed by the authors of this paper [22].

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Figure 17. Fe–Ni–Cr phase diagram [44] together with marked compositions of: iron-rich section Figure 17. Fe–Ni–Cr phase diagram [44] together with marked compositions of: iron-rich section (green(green dot) and dot) nickel-chromium-richand nickel-chromium-rich section section (red (red dot).dot). 5. Conclusions Scanning transmission electron microscopy observations, allowed to investigate the diffusion Analysiszone microstructure, of Inconel which 625/steel consists P355NH of equiaxed, joint microstructurecolumnar grains with in the presence as-welded of 50 nm state chromium- and after two different,rich separatedprecipitates typeshaving of core-shell heat treatment structure. (stress It can reliefbe stated annealing that both and observed normalizing) diffusion-based allowed the phenomena in sample after normalizing, including diffusion zone with presence of voids and following conclusions to be drawn. carbides in Inconel 625 alloy have their detrimental effect on the joint strength. The carbides localized 1. Thein Inconel explosive 625 alloy welding layer, processalso result allowed in decreasing to obtain of material joint betweencoherence steelat the P355NHmost crucial and region— Inconel 625 alloy.the joint. The These wavy-shape factors might joint cause was the found deterioration to include of the melted joint quality zones established having high by the concentration shear test of in the previous investigation performed by the authors of this paper [22]. imperfections such as cracks, voids and fragments of steel P355NH surface layer. ◦ 2. Stress5. Conclusions relief annealing (620 C/90 min) led to partial recrystallization of steel P355NH in the joint area. At the same time no changes in the grainy microstructure of Inconel 625 and chemical Analysis of Inconel 625/steel P355NH joint microstructure in the as-welded state and after two compositiondifferent, separated of the types joint haveof heat been treatment noticed. (stress relief annealing and normalizing) allowed the ◦ 3. Heatfollowing treatment conclusions in the to form be drawn. of normalizing (910 C/30 min) resulted in complete recrystallization of1. grainyThe explosive microstructure welding of process both bondedallowed materials.to obtain joint between steel P355NH and Inconel 625 4. As thealloy. result The of wavy-shape normalizing joint the was diffusion found to ofinclude Inconel melted 625 zones alloying having elements high concentration into steel P355NH of took placeimperfections along the such grain as cracks, boundaries voids and with fragments tendency of steel to formation P355NH surface of voids. layer. 2. Stress relief annealing (620 °C/90 min) led to partial recrystallization of steel P355NH in the joint 5. Additionally, another effect of post-weld normalizing is the formation of M C and M C carbides area. At the same time no changes in the grainy microstructure of Inconel6 625 and chemical23 6 in Inconelcomposition 625 alloy of the in joint the jointhave zone.been noticed. 3. Heat treatment in the form of normalizing (910 °C/30 min) resulted in complete recrystallization Author Contributions:of grainy microstructureFor research of articles both bonded with several materials. authors, a short paragraph specifying their individual contributions4. As must the result be provided. of normalizing The following the diffusion statements of Inconel should 625 be usedalloying “conceptualization, elements into steel R.K.; P355NH methodology, M.W. and M.G.;took place formal along analysis, the grain R.K.; boundaries investigation, with R.K., tendency M.W. to and formation M.G.; writing—original of voids. draft preparation, ´ ´ R.K. and5. M.W.;Additionally, writing—review another effect and editing, of post-weld M.W. andnormalizing L.S.; supervision, is the formation L.S.”, please of M6C turn and to M the23C CRediT6 taxonomy forcarbides the term in Inconel explanation. 625 alloy Authorship in the joint must zone. be limited to those who have contributed substantially to the work reported. Author Contributions: For research articles with several authors, a short paragraph specifying their individual Funding: contributionsThis research must wasbe fundedprovided. by The The following National statements Centre for should Research be andusedDevelopment “conceptualization, of Poland, R.K.; grant number: DZP/M-ERA.NET-2013/2309/2014. methodology, M.W. and M.G.; formal analysis, R.K.; investigation, R.K., M.W. and M.G.; writing—original draft Conﬂictspreparation, of Interest: R.K.The and authorsM.W.; writin declareg—review no conﬂict and editing, of interest. M.W. and L.Ś.; supervision, L.Ś.”, please turn to the CRediT taxonomy for the term explanation. Authorship must be limited to those who have contributed substantially to the work reported. References

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