The impurities sulphur, phosphorus, lead, bismuth, tin and antimony affect the hot workability of stainless steels. In this study we show that lead segregate to phase and solidification-grain boundaries, where cracks appear during hot deformation. The workability was tested by the hot bending of laboratory steels.Nečistoće poput sumpora, fosfora, olova, bizmuta i antimona negativno utječu na toplo preoblikovanje nehrđajućih čelika. U ovaj je studiji pokazano, da olovo segregira na granicama zrna skrućivanja gdje se mogu pojaviti pukotine za vrijeme tople deformacije. Toplo preoblikovanje laboratorijski dobivenih čelika ispitano je metodom toplog savijanja

B. Arzenšek

F. Tehovnik

F. Vodopivec

R. Celin

English

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F. TEHOVNIK, F. VODOPIVEC, B. ARZEN[EK, R. CELINTHE EFFECT OF LEAD ON THE HOT WORKABILITY OF AUSTENITICSTAINLESS STEEL WITH A SOLIDIFICATION STRUCTUREReceived – Prispjelo: 2008-10-12Accepted – Prihva}eno: 2008-12-20Preliminary paper – Prethodno priop}enjeINTRODUCTIONThe impurities sulphur, phosphorus and oxygen, etc.are introduced into steels with the charge, metallic andnon-metallic additions during melting and during thesecondary treatment of the molten steel. With the in-creasing use of steel scrap, the content of some impuri-ties has increased. Their removal requires additionalprocessing, or, as in the case for lead, it cannot beachieved. It is less expensive to remove the sulphur,since a content of 0,001 % can be achieved by industrialprocessing, while, for example, reducing the content oflead, phosphorus, tin and antimony requires a cleancharge and additives are more suitable. Some elements,e.g., copper and tin, are not oxidised during the soakingof the steel and are enriched in the metal at the surfaceand at the grain boundaries, emerging on the surface andinducing hot shortness in structural steels 1.This kind of brittleness is not meant for stainlesssteels, because the solubility of copper and tin is in-creased in austenite with a sufficient content of nickel2.The melt-processing temperature is sufficient for thepartial volatilization of lead and bismuth, especiallyduring the secondary vacuum treatment, since at highprocessing temperatures both elements have a highvapour pressure  The non-evaporated lead is foundmostly as a liquid residue on the bottom of the furnaceand, as shown later, it is found in the solid steel as verysmall spherical inclusions accumulated at the solidifica-tion boundaries. These inclusions, which in reality aredroplets of liquid lead, form when the limit of the lead’ssolubility in liquid steel is exceeded. This formationmust take place below the monotectic temperature.The effect of lead on the hot workabilityof stainless steelsAll the impurities that segregate at the grain and so-lidification boundaries decrease the hot workability to adifferent extent for different elements, since, accordingto ref. 4, the enrichment of an element at a grainboundary is inversely proportional to its solid solubilityin the matrix. For the effect of the most harmful ele-ments, termed as w.(Pbeq.), the following empirical rela-tion was proposed 5-7.(Pbeq.) = (Pb) + 4(Bi) + 0,025(Sb) ++0,01(Sn) + 0,007(As) (1)with the content of the element in wt %.The effect of impurities on the workability is muchstronger in the case of solidification to austenite and thehot elongation decreases strongly with an increasedvalue for Pbeq. 6. It was found that with hot tensile teststhe elongation is sufficient with Pbeq. lower than 0,005 %for steels with a solidification to -ferrite, while forsteels with a solidification to austenite the value of Pbeq.should be lower than 0,003 % 6.According to the iron side of the binary equilibriumdiagram Fe-Pb 8, the solid solubility of lead in themolten steel is limited. It is reported 8 according toMETALURGIJA 49 (2010) 1, 49-52 49The impurities sulphur, phosphorus, lead, bismuth, tin and antimony affect the hot workability of stainless ste-els. In this study we show that lead segregate to phase and solidification-grain boundaries, where cracks appe-ar during hot deformation. The workability was tested by the hot bending of laboratory steels.Key words: austenitic stainless steel, hot workability, impurities, leadUtjecaj olova na toplo preoblikovanje austenitnog nehr|aju}eg ~elika s ljevanom strukturom.Ne~isto}e poput sumpora, fosfora, olova, bizmuta i antimona negativno utje~u na toplo preoblikovanjenehr|aju}ih ~elika. U ovaj je studiji pokazano, da olovo segregira na granicama zrna skru}ivanja gdje se mogupojaviti pukotine za vrijeme tople deformacije. Toplo preoblikovanje laboratorijski dobivenih ~elika ispitano jemetodom toplog savijanja.Klju~ne rije~i: austenitni nehr|aju}i ~elik, toplo preoblikovanje, ne~isto}e, olovoISSN 0543-5846METABK 49(1) 49-52 (2010)UDC – UDK 669.15:539.381:669.4=111F. Tehovnik, F. Vodopivec, B. Arzen{ek, R. Celin, Institute of Metals andTechnology, Ljubljana, SloveniaAraki 9 and Kubaschewsky 10 that the solubility oflead is 0,054 % Pb in the temperature range 1650 °C to1600 °C. The solid solubility at the monotectic tempera-ture ( 1530 °C) is, according to the same authors, of2,7 × 10-4 % Pb. Two equations describing the solubilityof lead in molten iron have been reported 10, 11.lg(w.%Pb) = - 11100/T + 5,57 (2)with the content of the element in wt %.lg(x.%Pb) = -13400/T + 6,16 (3)with the content of the element in atomic %.Experience shows that lead, with a greater densitythan iron and a high vapour pressure 3, is partially vo-latilised from the melt, partially deposited on the bottomof the furnace and, as in lead-free machining steels 12partially found in small droplets in the solidified steel.Depending on the content of the main alloying ele-ments and impurities the solidification of austeniticstainless steels can proceed according to two mecha-nisms 13, 14.– melt  austenite with increased content of nickel -ferrite from the residual interdendritic meltenriched in chromium, or– melt  -ferrite enriched in chromium  austen-ite with solidification of the residual interdendriticmelt and/or a solid transformation -ferrite austenite during cooling.Some elements, e.g., molybdenum, silicon and tita-nium, enhance the solidification via -ferrite, others,e.g., carbon, manganese and copper, enhance the solidi-fication via austenite.EXAMINATIONAll the tests were carried out with specimens thathave a solidification structure. The specimens for thehot-bending tests were prepared by casting laboratorysteel in a heated grey-iron mould of section 20 × 40 mm.The chemical composition of the investigated steel isshown in Table 1. The content of lead in the 316L(Pb)steel was approximately 2,5 times higher than that re-ported for the inducing of hot shortness 15, and ac-cording to our industrial experience, five times higherthan that critical for the appearance of edge cracks dur-ing the hot rolling of 200 mm continuously cast slabs.The aim of the bending tests was to establish the ef-fect of lead on the formation of cracks duringhot-stretching deformation. The specimens were heatedto temperatures of 1050 °C, 1150 °C and 1250 °C, heldfor 5 min or 40 min, bent by 90° on a round mandrel andquenched in water. The time from the taking out of thefurnace to the end of the bending was only a few sec-onds. It is assumed, therefore, that the deformation tem-perature was equal to the heating temperature. Thecracks were found by visual inspection and confirmedwith liquid penetrant tests. The hot crack surface wasthen opened by bending the cut-out specimens cooled inliquid nitrogen and investigated with scanning electronmicroscopy (SEM) and EDS analysis.The maximum stretching tensile deformation wasfound for bend specimens of 29 %. The initial shape ofthe cross-section of the specimen preserved after thebending confirms that the deformation at the surface ofthe specimen was almost exclusively uniaxial stretch-ing.Table 1. Chemical composition of the investigatedsteel AISI 316L(Pb)Elements/wt.%C Si Mn P S Cr Ni Cu Mo Pb0,03 0,6 1,43 0,026 0,001 16,57 10,36 0,37 2,14 0,0083EXAMINATION RESULTSHot-bending testsDuring a visual examination of the tensile surface ofthe hot-bend specimens of the leaded AISI 316L(Pb)steel fine transverse cracks were found, more frequentlynear the specimens’ borders. This was confirmed withliquid penetrant tests (Figure 1). A microscopic exami-nation of the cracked section showed the cracks to beintergranular and to have a surface covered with a thinlayer of oxide. In the metal near the crack lip, smallspherical inclusions (Figure 2) were found and con-firmed to be pure lead (Figure 3, Table 2). No lead inclu-sions were found at a greater distance from the bound-ary. The solubility of the lead is greater for the liquidthan for the solid steel and, with the advance of the solid-ification front, the content of lead in the remnant liquidwas increased. The end product of the solidification is ananotectic consisting of solid steel with a very low con-tent of lead in solid solution and droplets of lead that so-lidify at a much lower temperature. The results of theEDS analysis of the lead inclusions in Table 2 are ap-proximate, because the size of the inclusions was sma-ller than the depth and the lateral penetration of the elec-tron beam, which meant that elements from the matrixwere detected. The analysis showed a high lead contentin the inclusions and, considering that iron, nickel andchromium are virtually insoluble in lead, the conclusionthat the inclusions are virtually pure lead is justified.Thin platelets were cut out from the cracked area ofthe bend surface, cooled in liquid nitrogen, fractured bybending around a thin mandrel and the opened, crackedsurface was examined with SEM. The macrophotographyfracture surface of the specimen is shown in Figure 4.In Figure 5 part of the cracked surface is shown,where two mechanisms of crack propagation are visible:the area of dimpled ductile fracturing of the cooled steeland the area of propagation during the solidification ofthe grain boundaries (Figure 6). This area of the surfacein Figure 5 was covered with a thin layer of oxide, oblit-50 METALURGIJA 49 (2010) 1, 49-52F. TEHOVNIK et al: POSSIBILITIES OF REMOVING LEAD FROM INDUSTRIAL Cu-Pb-Fe ALLOY WITH OF ARGON...erating the fracture micromorphology and confirmingthat the surface was formed by cracking during the hotbending. Only rare and small lead inclusions were foundon the interdendritic, cracked surface (Figure 7). Theirsize did not allow a reliable chemical analysis; however,the line scanning and the content of 4,4 % Pb, estab-lished by point analysis, confirm the presence of lead in-clusions on the hot-fracture surface and the presence ofliquid droplets of lead at the solidification grain bound-aries during hot bending. No other impurities werefound in the interdendritic surface in Figure 6. We canconclude that grain-boundary fracturing was induced bythe presence of droplets of lead and that lead droplets diddrop out mostly from the cracked surface when it wasopened because the size of the solid lead inclusion wassmaller than the corresponding microscopic hollowformed around the liquid droplet during the steel’s solid-ification.METALURGIJA 49 (2010) 1, 49-52 51F. TEHOVNIK et al: POSSIBILITIES OF REMOVING LEAD FROM INDUSTRIAL Cu-Pb-Fe ALLOY WITH OF ARGON...Figure 1. Hot bending specimen, view from convex sides(penetrant test)Figure 2. SEM micrograph of the grain-boundary regionwith lead inclusionsFigure 3. Points of EDS analysis in Figure 2Figure 4. Macrophotography fracture surface of the speci-men broken after cooling in liquid nitrogenTable 2. Results of the EDS analysis at points markedin Figure 3Element/wt.% Si Cr Mn Fe Ni Mo PbSpectrum 1 0,40 15,82 1,65 61,66 9,30 2,02 9,15Spectrum 2 0,40 13,78 1,47 50,83 7,82 25,69Spectrum 3 0,54 14,23 0,89 51,14 7,28 25,92Spectrum 4 0,65 16,96 1,46 67,27 10,89 2,77Spectrum 5 0,62 16,45 1,60 68,36 10,90 2,07Spectrum 6 0,60 16,72 1,42 67,97 10,72 2,57Figure 5. Fracture surface of the specimen broken aftercooling in liquid nitrogenCONCLUSIONHot-bending tests were performed on one laboratoryaustenitic stainless steel with up to 0,0083 % Pb. Thetest were performed in the temperature range from 1050oC to 1250 oC. The maximum deformation was 29 %during hot bending. The deformed specimens were in-vestigated with optical and SE microscopy and EDSanalysis. Transverse cracks were found on the surface ofsome specimens deformed with the hot bending.The solubility of lead is greater in liquid than in solidsteel. During solidification the lead is rejected from themelt, and small droplets are formed and accumulated onboundaries of solidification grains at the monotectictemperature. During metallographic examinations, hotcracks and lead inclusions were observed at some of thesolidification boundaries. On the surface of the openedcracks a thin layer of oxide was found, the presence oflead inclusions were confirmed, and the cracking at thedeformation temperature was also confirmed.The following experimental findings of this investi-gations are of significance for the understanding of thehot workability of stainless steels containing of leadwith the as-solidified microstructure consisting of aus-tenite and of -ferrite:– droplets of lead are enriched at the solidificationboundaries;– interphase segregation and the accumulation oflead inclusions decrease the steel’s hot worka-bility.REFERENCES1 F. Vodopivec, M. Torkar, M. Debelak, M. Kmeti~, F. Haler,F. Kau~i~, Mater. Sci. Technol., (1988) 4, 9172 G. L. Fisher: J. Iron Steel Inst., (1969) 207, 10103 J. F. Elliott, M. Gleiser: Thermochemistry for steelmaking,Vol.I, 270; (1960), Reading, Addision-Wesley publishingCompany4 C. L. Briant, Mater. Sci. Technol., (2001) 17, 1317-1323.5 W. Runsheng, Special steel, (2000) 21, 10-13.6 L. Myllykoski, N. Suutala, Metals Technology, (1983) 10,453-460.7 L. Å. Norström, Scandinavian J. of Metallurgy, (1979) 8,95-96.8 B. Burton, J. of Phase Equil., (1991) 12, 200-202.9 T. Araki, Trans. Nat. Res. Inst. Met. Tokyo, (1991) 5.10 O. Kubaschewski, Iron-Binary Phase Diagrams, New York,Springer-Verlag (1982), 87-88.11 A. N. Morozov, Yu. A. Ageyev: Russ. Metall., (Equi Dia-gram; Experimental), (1971) 4, 435.12 A. Razinger: @elezarski zbornik, (1974) 8, 99-109.13 R. Honeycombe, P. Hancook, Steels Microstructure andProperties, 2nd ed; 1995, Cambridge, University of Cam-bridge14 J. R. Davis et all.: ASM Specialty Handbook: Stainless Ste-els; 1996, Ohio, ASM International15 L. G. Ljugström, Scandinavian J. of Metallurgy, (1977) 6,176-184.Note: The responsible translator for English language is FrancVodopivec, IMT - Ljubljana.52 METALURGIJA 49 (2010) 1, 49-52F. TEHOVNIK et al: POSSIBILITIES OF REMOVING LEAD FROM INDUSTRIAL Cu-Pb-Fe ALLOY WITH OF ARGON...Figure 6. Detail of the intergranular surface in Figure 5Figure 7. Image and line scaning on the interdendritic,cracked surface with droplets of lead

Utjecaj olova na toplo preoblikovanje austenitnog nehrđajućeg čelika s lijevanom strukturom

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