A high strength low alloyed (HSLA) V-Nb steel was heat treated to martensite and lower bainite with different grain size, reheated for 3 seconds at 750 °C and air cooled. Charpy notch tests were performed from -100 °C to 60 °C and the hardness assessed at room temperature. For as delivered steel and lower bainite, the upper shelf toughness was above 200 J and the transition temperature low, while, for martensite the upper shelf toughness threshold was aproximateky at 0 °C. After reheating, notch toughness was decreased moderatly for martensite and strongly for lower bainite. Independently on grain size, lower bainite was more propensive than martensite to embritlement after short reheating in the (α + β) range. For martensite, the change of notch toughness was not related to change of hardness, as by lower hardness lower, also toughness was lower.Nisko legirani V-Nb čelik visoke čvrstoće (HSLA) je toplinsko obrađen na martenzit i bainit sa različitom veličinom zrna, odžaren 3 sekunde na 750 °C i ohlađen na zraku. Chapy žilavost je određena u intervalu od -100 °C do 60 °C, a tvrdoća kod sobne temperature. Kod čelika sa početnom mikrostrukturom čelika i donjeg bainita, je bila veoma visoka žilavost iznad 200 J i tranzicijska temperatura niska, dok je bio kod martenzita prag približno kod 0 °C. Poslije odžarivanja se je žilavost smanjila malo kod martenzita i jako kod bainita. Nezavisno od veličine zrna, donji bainit je bio mnogo osetljiviji na pojavu krtosti poslije kratkog odžarivanja u području (α + β). Kod martenzita, promjena žilavosti nije u vezi za tvrdoćom, jer kod niže žilavosti je niža i tvrdoća

B. Šuštaršič

F. Vodopivec

G. Kosec

J. Vojvodič-Tuma

Nisko legirani V-Nb čelik visoke čvrstoće (HSLA) je toplinsko obrađen na martenzit i bainit sa različitom veličinom zrna, odžaren 3 sekunde na 750 °C i ohlađen na zraku. Chapy žilavost je određena u intervalu od -100 °C do 60 °C, a tvrdoća kod sobne temperature. Kod čelika sa početnom mikrostrukturom čelika i donjeg bainita, je bila veoma visoka žilavost iznad 200 J i tranzicijska temperatura niska, dok je bio kod martenzita prag približno kod 0 °C. Poslije odžarivanja se je žilavost smanjila malo kod martenzita i jako kod bainita. Nezavisno od veličine zrna, donji bainit je bio mnogo osetljiviji na pojavu krtosti poslije kratkog odžarivanja u području (α + β). Kod martenzita, promjena žilavosti nije u vezi za tvrdoćom, jer kod niže žilavosti je niža i tvrdoća.A high strength low alloyed (HSLA) V-Nb steel was heat treated to martensite and lower bainite with different grain size, reheated for 3 seconds at 750 °C and air cooled. Charpy notch tests were performed from -100 °C to 60 °C and the hardness assessed at room temperature. For as delivered steel and lower bainite, the upper shelf toughness was above 200 J and the transition temperature low, while, for martensite the upper shelf toughness threshold was aproximateky at 0 °C. After reheating, notch toughness was decreased moderatly for martensite and strongly for lower bainite. Independently on grain size, lower bainite was more propensive than martensite to embritlement after short reheating in the (α + β) range. For martensite, the change of notch toughness was not related to change of hardness, as by lower hardness lower, also toughness was lower

Vodopivec, F.

Šuštaršič, B.

Vojvodič-Tuma, J.

Kosec, G.

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F. VODOPIVEC, B. [U[TAR[I^, J. VOJVODI^-TUMA, G. KOSECCHARPY NOTCH TOUGHNESS AND HARDNESSOF REHEATED MARTENSITE AND LOWER BAINITEReceived – Prispjelo: 2009-09-18Accepted – Prihva}eno: 2010-02-08Original scientific paper- Izvorni znanstveni radINTRODUCTIONThe resistance to hydrogen embrittlement is of es-sential importance for steels for vessels for storage ofhydrocarbons and depends on the effect of absorbedhydrogen on steel ductility.API tests have shown for the 490 MPa yield stressHSLA steels with the microstructure of ferrite andcementite particles a much greater reduction of area thanfor the 350 MPa steel with the microstructure of polygo-nal ferrite and pearlite 1,2. By routine tests during theconstruction of a 60.000 m3 vessel, it was found that bythe equal welding procedure, Charpy toughness waslower for 15 mm plates than for 25 mm plates of the sameHSLA steel. It was assumed that the differences were re-lated to the propensity of constituents of heat affectedzone (HAZ) to embrittlemenst by short reheating in thetwo phase (ferrite+austenite) range for the steel that pro-duces local brittle zones in the HAZ of welds 3–15.Earlier simulation tests have shown 3 that the sensi-bility to embrittlement after reheating was different for amicroalloyed than a conventional steel. As shown in Fig-ures 1 and 2, after cooling from 1300 0C Charpy notchtoughness was slightly lower for the steel ^.0562 withyield stress of 350 MPa than for the microalloyed steelNiomol 490 K with yield stress of 490 MPa, while, afterreheating toughness was for this steel sligthly lover. Thedifferent behaviour of both steels was confirmed withCharpy tests on HAZ of welds of 20 mm plates 3. Theaim of this work was to check these findinds with moresystematical tests and to verify if the embrittlement pro-pensity was related to the difference of steels in chemicalcomposition or to the constituents of microstructure ob-tained with cooling from high temperature. Martensiteand lower bainite in the microalloyed steel Niomol 490 Kwere investigated because assumed to be most sensible toreheat embrittlement than other constituents of HAZ ofwelds. For comparison, the steel with the as deliveredmicrostructure was used.EXPERIMENTAL WORKThe tests and examinations were performed withspecimens cut out from a plate of microalloyed struc-tural steel Niomol 490K (0,1%C-0,5%Mn-0,7%Cr-0,27%Mo- 0,032%Nb-0,025%Al) with the initialMETALURGIJA 49 (2010) 3, 149-154 149A high strength low alloyed (HSLA) V-Nb steel was heat treated to martensite and lower bainite with differentgrain size, reheated for 3 seconds at 750 °C and air cooled. Charpy notch tests were performed from -100 °C to60 °C and the hardness assessed at room temperature. For as delivered steel and lower bainite, the upper shelftoughness was above 200 J and the transition temperature low, while, for martensite the upper shelf toug-hness threshold was aproximateky at 0 °C. After reheating, notch toughness was decreased moderatly for mar-tensite and strongly for lower bainite. Independently on grain size, lower bainite was more propensive thanmartensite to embritlement after short reheating in the (+) range. For martensite, the change of notch toug-hness was not related to change of hardness, as by lower hardness lower, also toughness was lower.Key words: HSLA steel, martensite, lower bainite, reheating, Charpy notch toughness, hardnessCharpy udarna `ilavost i tvrdo}a od`arenog materijala i donjeg bainita. Nisko legirani V-Nb ~elik vi-soke ~vrsto}e (HSLA) je toplinsko obra|en na martenzit i bainit sa razli~itom veli~inom zrna, od`aren 3 sekundena 750 °C i ohla|en na zraku. Chapy `ilavost je odre|ena u intervalu od -100 °C do 60 °C, a tvrdo}a kod sobnetemperature. Kod ~elika sa po~etnom mikrostrukturom ~elika i donjeg bainita, je bila veoma visoka `ilavost iz-nad 200 J i tranzicijska temperatura niska, dok je bio kod martenzita prag pribli`no kod 0 °C. Poslije od`arivanjase je `ilavost smanjila malo kod martenzita i jako kod bainita. Nezavisno od veli~ine zrna, donji bainit je biomnogo osetljiviji na pojavu krtosti poslije kratkog od`arivanja u podru~ju ( + ). Kod martenzita, promjena`ilavosti nije u vezi za tvrdo}om, jer kod ni`e `ilavosti je ni`a i tvrdo}a.Klju~ne rije~i: HSLA ~elik, martenzit, donji bainit, od`arivanje, Charpy udarna `ilavost, tvrdo}aISSN 0543-5846METABK 49(3) 149-154 (2010)UDC – UDK 669.14/15:620.171.70/178:620.18 = 111F. Vodopivec, B. [u{tar{i~, J. Vojvodi~-Tuma, Institute of Metals andtechnology, Ljubljana, SloveniaG. Kosec, SIJ, Acroni d.d. Jesenice, Sloveniamicrostructure of a dispersion of cementite precipitatesin mostly acicular ferrite grains with linear intercept sizeof about 2,5 µm. The specimens were annealed at 920°C or at 1250 °C, than half quenched in lead bath at 4000C and half in water at 70 0C. In this way, two types ofmicrostructure and two austenite grain sizes (ASTMgrades 1-3 and 8-9) were obtained. Half of specimenswas than reheated individually for 3 seconds at 750 °Cwith direct conduction heating and air cooled witht800-500°C = 17 seconds. On all specimens the Charpynotch was cut after heat treatment.For heat treated specimens and the mother steel, theCharpy tests were carried out in temperature range -200°C to 60 °C. The microstructure and the fracture surfacewas investigated with scanning microscopy.MICROSTRUCTURE, NOTCH TOUGHNESSAND HARDNESSThe constituents formed at cooling from auste-nitising temperatures are termed as primary and as sec-ondary those formed at cooling after reheating. Themicrostructure of the as delivered steel consisted of fineferrite grains with a dispersion of cementite precipitates.After reheating, it was changed to platelets of martensiteand ferrite in the interior of grains and inserts of second-ary martensite, mostly at triple points. After waterquenching from 1250 °C, platelets of primary marte-nsite and ferrite were found in coarse grains (Figure 3a).After reheating, this microstructure changed to partiallydecomposed martensite and stringers of cementite parti-cles in ferrite grains and inserts of secondary martensiteat ferrite grain boundaries (Figure 3b). The lead bathcooling from 1250 °C produced a coarse grained micro-structure of stringers of cementite particles and ferriteplatelets (Figure 4a) termed lower bainite. After reheat,it changed to a microstructure of platelets of secondarymartensite in interior of ferrite grains and inserts of sec-ondary martensite at boundaries of coarce grains (Figure4b). After cooling from 920 °C and reheating, themicrostructure was similar than after cooling from thehigher temperature, while, the grain size was smaller forabout 6 ASTM grades.These observations indicate that at short reheating at750 °C, three different processes occur in the investi-gated microstructures:a) dissolution of cementite with formation of sec-ondary austenite around cementite particles in theinterior of ferrite grains and its transformation tosecondary martensite at cooling;150 METALURGIJA 49 (2010) 3, 149-154F. VODOPIVEC et al: CHARPY NOTCH TOUGHNESS AND HARDNESS OF REHEATED MARTENSITE AND LOWER...Figure 1 Thermal cycle (a) and dependence Charpy toughness dependes of test temperature (b) for the classical struc-tural steel ^.0562 with yield stress of 350 MPa and the microalloyed structural steel Niomol 490 K(a) (b)Figure 2 Thermal cycle (a) and dependence Charpy toughness (b) vs. test temperature for the classical structural steel^.0562 with yield stress of 350 MPa and the microalloyed structural steel Niomol 490 K(a) (b)b) decomposition of primary martensite and even-tual formation of some secondary austenite andmartensite;c) formation od secondary martensite inserts (smallgrains) at triple points and boundaries of ferritegrains.In Figures 5a and 5b the dependence Charpy notchtoughness vs. test temperature is shown for coarsemartensite and lower bainite after cooling from 1250 °Cand after reheating. For other heat treated specimens thedependences had a similar shape, while for the as deliv-ered microstructure the effect of reheating was muchlower. The upper shelf notch toughness is high and thecleavage threshold and Charpy transition temperaturesare low for the as delivered steel and fine and coarsegrained bainite. After reheating, the cleavage thresholdtemperature is for aprox. 80 °C higher for bainite, whilethe transition temperature was above the highest testtemperatrure of 60 °C. The effect of reheating on Charpytoughness was much lower for the as delivered steel as,the cleavage threhshold was higher for aprox. 20 °C andthe transition temperature for about 60 °C. Above thecleavage temperature, the increase of toughness for the asdelivered steels and as cooled lower bainite is fast and theupper shelf is achieved by a temperature higher for about40 °C. For fine and coarse martensite, as cooled and afterreheating, the cleavage threshold temperature is similarand it is for 40-50 oC higher than for the as delivered steeland bainite, the increase of toughness is slower and theupper shelf temperature is above the highest test tempera-ture of 60 oC. At this temperature notch toughness islower for about 50 J after reheating.Three fracturing mechanisms were identified. Forhigh toughness, the fracture surface had an irregular to-pography of areas of dimples with different size formedwith normal (Figure 6b) and shear decohesion. By lownotch toughness in the range of temperature of growth oftoughness above the cleavage threshold, the fracturesurface consisted of cleavage and ductile areas with in-creasing ductile decohesion for higher toughness mostlyin areas inclined toward the clevage facets (Figure 7).On the ductile to clevage boundary of mixed fracturesclear chacteristics of change of mechanism of crackpropagation were not identified and it is assumed thatMETALURGIJA 49 (2010) 3, 149-154 151F. VODOPIVEC et al: CHARPY NOTCH TOUGHNESS AND HARDNESS OF REHEATED MARTENSITE AND LOWER...Figure 3 Microstructure after water quenching from 1250 oC (a) and after reheating (b).(b)(a)Figure 4 Microstructure after lead bath cooling from 1250 oC (a) and after reheating (b).(b)(a)the fracturing transition occurred with plane slip15-19. In cleavage range the fracture consists ofclevage facets (Figure 6a) by coarser grain size and nodifference in micromorphology was found for facetsmicromorphology for tested specimens. In transitionrange the share of facets decreased with increasingtougness and the share of ductile dimples increased.Details related to the presence of inserts of secondarymartensite at grain boundaries were not found on cleav-age and ductile areas.In Table 1 hardness and room temperature Charpynotch toughness are shown for the different microstruc-tures. After reheating, hardness is little or moderately in-creased for the as delivered steel and lower bainite and itis decreased stronger for martensite. For the as deliveredsteel and martensite, after reheating hardness is lowerand notch toughness at room temperature is lower,while, for bainite hardness and noth toughness are bothlower after reheating. It is evident that the effect of pro-cesses occurring at short reheating in (+) region is dif-ferent for hardness and notch toughnessThe results of this investigation demonstrate that afteridentical short reheating in the two phases range hardnessis changed less than notch toughness and that independ-ently of grain size, toughness is diminished much more152 METALURGIJA 49 (2010) 3, 149-154F. VODOPIVEC et al: CHARPY NOTCH TOUGHNESS AND HARDNESS OF REHEATED MARTENSITE AND LOWER...Figure 5a Charpy toughness vs testing temperature forthe steel quenched in water from 1250 °C andreheated.Figure 5b Charpy toughness vs testing temperature forthe steel cooled from 1250 °C in lead bath andreheated.Figures 6 a) Clevage fracture; specimene cooled in lead bath from 920 °C and reheated at 750 °C. b) Ductile fracture;specimen cooled in lead bath from 920 °C; both cases test temperature -20 °C(b)(a)Figure 7 Fracture surface with mixed ductile and cleavagedecohesion at 40oCfor lower bainite than for austenite. Considering the find-ings in Figures 1 and 2 and earlier investigations, it is alsoconcluded that the propensity for embrittlement is a prop-erty of constituent of microstructure and is not related tothe steel chemical composition. Not only the type of ear-lier stated reheating processes in microstructure is differ-ent for martensite and for lower bainite, also the extent ofthe process is different for both constituents. At this stepof the investigation, it is clear that the dissolution ofcementite particles in lower bainite is faster than the tem-pering of martensite and that, in their initial phase, bothprocesses decrease the notch toughness and have a differ-ent effect on hardness and notch toughness for lowerbainite and for martensite. No details related to the inter-granular inserts of martensite was found on fracturesurfacesand it seems justified to conclude that thechanges in properties are related to the change ofmicrostructure in the interior of grains. These changes arebeing investigated and will be reported later.Table 1 Room temperature hardness and Charpynotch toughness after different thermal tre-atmentThermal treatment Hardness HV5Charpy notchtoughness, JAs delivered 205 243As delivered + 7500C 248 196920 °C  water 282 122920 °C  water + 750 0C 244 68920 °C  lead bath 214 60920 °C  lead bath + 750 0C 222 211250 °C  water 383 2531250 °C  water + 750 0C 320 26,51250 °C  lead bath 257 2171250 °C  lead bath + 750 0C 298 14CONCLUSIONS– Charpy notch toughness is higher and the transi-tion temperature is lower for lower bainite than formartensite, independently on grain size;– after reheating, the microstructure of steel inchanged differently for different initial constitu-ents in the interior of grains, while independentlyon initial microstructure and grain size after re-heating inserts of secondary martensite are formedat triple points of initial grains;– after reheating at 750 °C and air cooling, Charpynotch toughness is greatly diminished for lowerbainite independently on grain size. The cleavagetemperature is increased after reheating for about80 °C for lower bainite and it is virtually un-changed for martensite;– particularly harmfull after reheating for notchtoughness and transition temperature is the pres-ence of secondary martensite platelets in the inte-rior of ferrite grains;– the change of the initial microstructure at reheat-ing affects very differently notch toughness andhardness. By smaller changes of hardness, verysignificant changes of notch toughness were ob-tained and lower notch toughness was found forlower hardness, also;– the results of this investigation and in quoted ref-erences suggest that the reheating embrittlement isa propety of the constituent of the microstructureand not of the chamical composition of the struc-tural steel.The authors aknowledge the support of the Ministryof High Education, Science and Technology ofSlovenia, Ljubljana and of the company SIJ, Acronid.o.o., Jesenice, Slovenia and are indebted to Mr. BorisArzen{ek for the experimental tests.REFERENCES1 S. A`man, L. Vehovar, J. Vojvodi~-Tuma: Development ofa Steel microalloyed with niobium molybdenum; Intern.Symposium Welding, 1996, Belgrade, Proceed. 58-60.2 Vojvodi~-Tuma J., Jenko M.: Strain ageing of a 490 MPaMicroalloyed Structural Steel; Welding in South-East Asia:A Challenge for the Future – Proceedings, 2006, 758-766.3 F. Vodopivec, G. Kosec, S. Grbi}, D. Kmeti~: Materiali inTehnologije 38 (2004), 149-153.4 H.G. Pisarski: HAZ Toughness Evaluation in: Proceed.Intern. Conf. “The Metallurgy, Welding and Qualificationof Microalloyed (HSLA) Steels Weldments”, AmericanWelding Society, 1990, 351-382.5 T.M. Sconover: Evaluation of Local Brittle Zones inHSLA-80 Weldments: Proceed. Intern. Conf. “The Metal-lurgy, Welding and Qualification of Microalloyed (HSLA)Steels Weldments”, American Welding Society, 1990,276-305.6 S. Aihara, K. Okamoto: Influence of Local Brittle Zones onHAZ Toughness of TMCP Steels: Proceed. Intern. Conf.“The Metallurgy, Welding and Qualification of Microallo-yed (HSLA) Steels Weldments”, American Welding So-ciety, 1990, 401-425.7 Q. Liu, X Diao, Y.W Mai: Engineering Fracture Mechanics49 (1994), 741-750.8 Q. Liu, T. Varga: Engineering Fracture Mechanics 49(1994), 435-444.9 T. Moltubakk, C. Thaulow, Z.L. Zhang: Engineering Frac-ture Mechanics 62 (1999), 445-462.10 J. Vojvodi~-Tuma, A. Sedmak: Engineering Fracture Me-chanics 71 (2004), 1435-1451.11 B. Bezensek, J.W. Hancok: Engineering Fracture Mecha-nics 74 (2007), 2395-2419.12 Q. Liu, T. Varga: Engineering Fracture Mechanics 49(1994, 435-444.13 T. Lee, Y.T. Pan: Materials Science and Engineering A 136(1991), 109-119.14 J. Il Jang, Y.-S. Lee, J.-B. Ju, B.W.Lee, D. Kwon, W.S.Kim: Materials Science and Engineering 351(2003),183-189.METALURGIJA 49 (2010) 3, 149-154 153F. VODOPIVEC et al: CHARPY NOTCH TOUGHNESS AND HARDNESS OF REHEATED MARTENSITE AND LOWER...15 J.Il Jang, B.W. Lee, J.B.Ju, D. Kwon, W.S. Kim: Enginee-rimng Fracture Mechanics 70 (2003), 1245-1257.16 G. Kosec, B. Arzen{ek, J. Vojvodi~-Tuma, F. Vodopivec:Materiali in Tehnologije 43 (2009), 31-37.17 F. Vodopivec, B. Arzen{ek, J. Vojvodi~-Tuma, R. Celin:Metalurgija 47 (2008), 173-179.18 F. Vodopivec, B. Breskvar, B. Arzen{ek, D. Kmeti~, J. Voj-vodi~-Tuma: J Mater. Sci. Techn. 17 (2001), 1-719 F. Vodopivec, J. Vojvodi~-Tuma: The Charpy behaviour ofstructural steels with different microstructure; Proceedingsof the Intern. Conf. Mechanical Properties of AdvancedEngineering Materials, ed. M. Tokuda nd B. Xu, Mie Un.Japan and Tsingua Un. China, 2001, 187-200.Note: The responsible translator for English language is Andrej Paulin,Ljubljana, Slovenia154 METALURGIJA 49 (2010) 3, 149-154F. VODOPIVEC et al: CHARPY NOTCH TOUGHNESS AND HARDNESS OF REHEATED MARTENSITE AND LOWER...

Charpy udarna žilavost i tvrdoća odžarenog materijala i donjeg bainita

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