MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF DUPLEX STEEL MIKROSTRUKTURA IN MEHANSKE LASTNOSTI DUPLEKS JEKEL MIRKO GOJIČ1, L. KOSEC2, L. VEHOVAR3 'Željezara Sisak, Sektor za razvoj, B. Adžije 2, 44000 Sisak, Croatia 2OMM, Naravoslovnotehniška fakulteta, Ljubljana 3Inštitut za kovinske materiale in tehnologijo, Ljubljana Prejem rokopisa - received: 1997-05-26; sprejem za objavo - accepted for publication: 1997-10-21 In this work mechanical properties and microstructure of duplex steel after heat treatment are shown. Heat treatment of the steel consisted of water quenching from 1050°C. A ferrite-austenite microstructure was obtained and the brittle a-phase was avoided. The results show that the impact energy depends on the direction of rolling. In rolling direction the share of ferrite and austenite was approximately equal. Key words: duplex steel, mechanical properties, ferrite-austenite microstructure, impact energy V članku so opisani rezultati raziskav mikrostrukture in mehanskih lastnosti dupleks jekel po gašenju v vodi s 1050°C. Na ta način je jeklo dobilo mikrostrukturo iz ferita in austenita in brez krhke 0-faze. Rezultati kažejo, da je udarna energija odvisna od smeri valjanja. Na vzdolžnem prerezu cevi sta deleža ferita in austenita približno enaka. Ključne besede: dupleks jeklo, mehanske lastnosti, feritno-austenitna mikrostruktura, udarna energija 1 INTRODUCTION Because of excellent strength and toughness as well as high resistance to corrosion the high alloy stainless steels are used in car, electronic and petrochemical in-dustnes. Investigations of the development of high alloy steels vvhich have simultaneously a high strength and other physical-chemical characteristics are important1. Depending on chemical composition, especially the content of chromium, nickel, and carbon as vvell as heat treatment high alloy steels can have a ferrite, austenite, martensite or duplex microstructure. Betvveen the high alloy steels duplex stainless steel (DSS) vvith austenite-ferrite microstructure has an important role26. It is a relative^ nevv class of engineering material for different applications because of the excellent combination of mechanical and corrosion characteristics. DSS offer benefits over austenite stainless steels and carbon steels because of their higher strength, good toughness and ductility in combination vvith equivalent resistance to general corrosion, as vvell as better resistance to local-ized corrosion and stress corrosion cracking. Today DSS have lovv carbon content (<0.002 wt.%) and contain opti-mal contents of chromium, nickel, molibdenum, copper and nitrogen for obtaining the required properties. In process industries materials vvith a favorable microstructure are used because it determines their mechanical and corrosion behavior. DSS vvith a austenite-ferrite microstructure are desirable, if free of brittle a-phase7. In the present vvork the investigation of mechanical and microstructural properties of DSS after heat treatment are presented vvith the accent on impact energy and the share of phases. 2 EXPERIMENTAL 2.1 Material A section of commercially produced duplex stainless tubing vvas used for this investigation. The thermal treatment consisted of solution annealing at 1050°C and vvater quenching. The chemical composition the steel is given in Table 1. Table 1: Chemical composition of duplex stainless steel, wt. % C Si Mn P S Cr Mo Al Ni N 0.02 0.45 0.88 0.024 0.018 24.97 3.19 0.04 8.20 0.12 2.2 Mechanical and microstructural testing The mechanical properties vvere assessed on an In-stron 1196 tensile testing machine in accordance vvith standard ASTM procedures8. The average hardness vvas determined by the Brinell method (HB), vvhile the micro-hardness of austenite and ferrite grains vvas determined using the indentor on a Leitz-Wetzlar optical microscope 8196. Impact testing vvas performed at room temperature in both directions of rolling. Microstructural tests vvere carried out vvith optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), image analysis system and X-rays diffraction (XRD). Specimens for metallographic analysis vvere mechanically polished and M. GOJIČ ET AL.: MECHANICAL AND MICROSTRUCTURAL PROPERTIES ... Table 2: Results of mechanical properties of duplex steel Re MPa Rm MPa A %_Z%_Hardness HB Microhardness HV»,io_Impact energv J_ 645 774 33 53 225 a=140y= 292 dir.of rolling: 41 ________transverse to dir, of rolling: 33 Figure 1: Microfractography of fracture surface of duplex steel after tensile testing (a) and impact testing in longitudinal (b) and transverse direction (c) Slika 1: Mikrofraktografije prelomnih površin dupleks jekla po raztržnem preiskusu (a) in preiskusu udarne žilavosti v vzdolžni (b) in v prečni smeri valjanja (c) Figure 2: Scanning electron microscopy image of duplex steel in longitudinal direction (a) and the scanning picture for nickel (b) Slika 2: Raster elektronski posnetek dupleks jekla v vzdolžni smeri valjanja (a) in x slika niklja (b) Figure 3: Scanning electron microscopy image of duplex steel in transverse direction (a) and the scanning picture for nickel (b) Slika 3: Raster elektronski posnetek dupleks jekla prečno na smer valjanja (a) in x slika niklja (b) Figure 4: Optical (a) and transmission electron bright-field micrographs (b) of duplex steel. D - twins Slika 4: Optični (a) in transmisijski elektronski posnetek (b) dupleks jekla. D - dvojčki • * WPLEX STEEL 011671 280.8 1671 288.8KV 188ci Figure S: Transmission electron microscopy bright-field micrograph of ferrite phase (a) with indexed area diffraction pattern (b) Slika 5: Transmisijski elektronski posnetek feritne faze (a) z indeksirano difrakcijo za ferit (b) etched in the Kallings reagent, an acid chloride solution (1.5 g of CuCl2, 33 ml of HC1, 33 ml alcohol and 33 ml of distilled vvater)9. The microstructure was examined in optical and SE microscopes, which was equipped for wave dispersive X-ray (WDX) analysis. The quantitative shares of ferrite and austenite were determined using an image analysis system. Thin foil samples for transmission electron microscope were prepared electrochemi-cally and examined TEM operated at 200 kV and equipped for diffraction analysis. The phase identifica-tion was obtained by X-rays diffraction using CuKa ra-diation. 3 RESULTS Mechanical properties (Re -yield strength, Rm - tensile strength, A - elongation and Z - reduction of area), impact energy, hardness and microhardness of DSS vvere measured at room temperature on three specimens. The average values of the properties are given in Table 2. When compared to usual stainless steels DSS have a sig-nificantly higher yield strength (tvvice that of austenite steels) and a good impact energy. Hovvever, the steel shovved also a significant anisotropy in impact energy. The fracture process and valuable evidence concerning the cause of failure can be obtained through microfrac- $ «s M £ <9 -C co Phases Figure 6: Quantitative shares of the ferrite-austenite microstructure in roiling direction Slika 6: Kvantitativni delež ferita in austenita v mikrostrukturi v vzdolžni smeri valjanja 60 70 Angle 20 / degree Figure 7: X-ray diffraction spectrum of duplex stainless steel Slika 7: X-ray difrakcijski spekter dupleks jekel tography10. Figure 1 shovvs the fracture surfaces of DSS after tensile and impact testing. The difference in fracture surfaces is only in size of dimples. At tensile testing DSS fracture in the ductile mode by microvoid coales-cence mechanism (Figure la). The fracture surfaces after impact testing are also ductile but vvith elongated dimples (Figures lb and lc). Figures 2 and 3 show the microstructure of DSS in both rolling directions. In both directions the microstructure is similar and consists of austenite grains embeded p ff £ a a. E |2 1400 1200 1000 800 . L L+5+y L+y —^^^ LTO / S / 8+y / r 1 10 15 Ni _L 30 25 20 15 Cr Content of chromium and nickel, wt. % Figure 8: Quasi binary diagram of FeCrNi alloy with located duplex steel Slika 8: Kvazi binarni diagram Fe-Cr-Ni zlitin z lokacijo dupleks jekel in ferrite. In rolling direction austenite grains are more elongated. Austenite grains contain more and ferrite grains less nickel (Figures 2b and 3b). No o-phase vvas found also by TEM observation, vvhile frequent tvvins vvere observed in ferrite grains (arrovv D, Figure 4a), vvhich vvas identified by selected area diffraction (SAD) pattern (Figure 5). Figure 6 shovvs the results of the quantitative phase analysis in rolling direction. An aver-age of the ten continuous fields vvas used for estimation of the phase share. In the longitudinal direction the shares of ferrite and austenite vvere approximately equal. Figure 7 shovvs the X-ray spectrum the steel. The phases vvere identified using of JCPDS data". Only the presence of ferrite and austenite vvas confirmed by X rays diffrac-tometry. 4 DISCUSSION The microstructure and the depending mechanical and corrosion properties are explained the best through the solidification process according to the quasi binary FeCrNi phase diagram, vvhich is shovvn shematically in Figure 812 vvith dashed lines locating the DSS. By equi-librium solidification, 8-ferrite is the primary solidification phase. 8-ferrite then undergoes solid-state transformation in a tvvo-phase region consisting of austenite (y) and ferrite (5) as the temperature is lovvered. The nuclea-tion of austenite occurs at grain boundaries enriched in carbon and nitrogen because of their limited solubility in ferrite. Given sufficient time, soluble carbon and nitrogen in solid solution diffuse uniformly throughout the austenite phase. It is knovvn that a slovv cooling under 815°C or aging at about 850°C can result in the formation of o-phase13, vvhich is prevented by a final solution annealing at 1050°C and vvater quenching, as shovvn al-ready. In this čase a microstructure consisting of ap-proximately equal shares of austenite and ferrite in the rolling direction is obtained. Taking into account the chromium and nickel equivalents14 the properties of both phases (5 and y) and their respective compositions can be approximately calculated for a given alloy and annealing temperature. The DSS tested here vvas represented in Figure 8 by the dashed lines. It can be seen that at 1050°C for the composition given in Table 1, the steel is located in the tvvo phases 5+y region having an approxi-mate 5/y fraction of 55:45 in accordance to the share of phases shovvn in Figure 6 for the rolling direction. The high tensile strength (Table 2) is the result of several simultaneous mechanisms15: interstitial solid solution hardening (carbon and nitrogen); substitutional solid solution hardening (chromium, molibdenum and nickel) and strengthening by grain refinement (the presence of tvvo phases prevents their mutual grovvth during heat treatment). The values of impact energy are higher in the longitudinal than in the transverse direction of rolling. Thus, the properties of DSS depend on the shape and arrangement of both phases as vvell as on the direc- tion of rolling. The difference in micromorphology frac-ture is explained through the fracture stresses and shape of both phases. The effect of deformation mode is usu-ally explained in terms of localized shear deformation16. Probably, also the interface betvveen the austenite and ferrite in DSS plays a considerable role in the fracture process. Crack propagation may take plače through austenite and ferrite grains. The propagation is associated with the interface it depends greatly on the orienta-tion of ferrite and austenite stringers17. There is a ten-dency of the propagation cracks to deflect along the interface to produce delamination when stringers lie par-allel to the applied stress. Taking into consideration the microstructural texture (Figure 2a), as well as their microhardness lovver impact energy in transverse to the direction vvas expected. A higher ductility of DSS in the longitudinal direction of rolling could indicate to the crack propagation along the 5/y boundaries because of axial stress, in contrast to the transverse direction of rolling vvhere the fracture is produced by orthogonal stresses. This is in agreement vvith the results by Odelstam18 shovving that elongation is by DSS lovver in the transverse than in longitudinal direction. 5 CONCLUSION In this vvork the results of investigation of mechanical and microstructural properties of the duplex stainless steel (DSS) are shovvn the heat treating consisted of an-nealing at 1050°C and vvater quenching, vvhich produced a ferrite-austenite microstructure free of brittle c-phase. Compared vvith usual stainless steels the DSS has a sig-nificantly higher mechanical strength (tvvice higher yield strength than that of austenite steels) vvith a good impact energy. It vvas found that the impact energy depends on the direction of rolling. In the longitudinal direction of rolling the fractions of the ferrite and austenite were ap-proximately equal. 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