Effect of Graphite Nodule Diameter on Water Embrittlement of Austempered Ductile Iron

Vol.12 No. 3J. Iron &. Steel Res.，Int.May 2005Effect of Graphite Nodule Diameter on Water Embrittlementof Austempered Ductile IronCAI Qi-zhou' ,WEI Bo-kang' ，TANAKA Yuichi2(1. Huazhong University of Science and Technology, W uhan 430074，China;2. Muroran Institute of Technology, Muroran 050- 8585, Japan)Abstract: Effects of graphite nodule diameter on the water embrittlement of austempered ductile iron (ADI) isstudied. The water embrittlement mechanism is discussed. Due to water adhesion, local embrittlement occurs onthe surface of ADI specimen，resulting in early fracture and significant reduction in tensile strength and elonga-tion. The water embrittlement is the cracking of stress induced martensite formed during tensile deformationcaused by hydrogen diffusion decomposed from water and as a result tensile strength and elongationof ADI are re-markably reduced. The segregation of alloying elements in ductile iron is weakened with decreasing nodule diame-ter，reducing the residual austenite in grain boundaries，then decreasing the amount of stress induced martensiteduring tensile plastic deformation and finally restraining ADI water embrittlement.Key words: austempered ductile iron; water embrittlement; graphite nodule diameter; tensile propertyDuctile iron was developed by MorroghI] in1 Experimental Procedure1940s. The appearance of austempered ductile iron(ADI) in 1970s esentially affected the metallurgicalThe pig iron was melted in a medium frequencyresearch of ductile iron[2.3]ADI has very goodinduction furnace, and then treated with FeSiMg6properties[4-15]It is produced by austempering con-for sphoidization and FeSi75 for inoculation. The Y-ventional ductile iron， and the microstructure is ablock with 25 mm in thickness was cast with localmixture of ferrite and stable high carbon austenitechilling casting 1，while 25 mm thick casting 2 and(ausferrite) which usually refers to reacted austen-50 mm thick Y-block casting 3 were poured in resin-ite. However, if the alloying element segregation inbonded sand mold. The chemical composition ( massthe intercellular region occurs during solidification,percent，%) is: C 3.80，Si 2. 64，Mn 0. 32，Pthe austempering in these areas is delayed，and leads0.033，S0.012 and Mg 0. 030. The microstructuralto the transformation of low carbon austenite inparameters including nodularity， nodule amountsADI. Then, this austenite often refers to unreactedand nodule diameter were measured by image analy-austenite that could transform to martensite duringzing system, as shown in Table 1.cooling from the austempering temperature or inThe specimens for tensile test were cut from Y-loading condition[].blocks. After rough machining, the specimens wereThe recent researches indicate that ADI tensileaustenitized at 1 173 K for 2 h and then quenched instrength and elongation are considerably reduced un-a molten salt bath at 648 K for 1.5 h, after whichder water adhesion condition and remarkable embrit-they were cooled in air. The microstructure of ADItlement occurs, named“ADI water embrittlement"" ,consisted of acicular ferrite and austenite with dis-which depends on chemical composition, austemper-persed graphite nodules， and it is shown that theing parameters and environment16- 18] .small nodule size (d=17.7 μm) is accompanied withADI production is complicated and its propertiesfine acicvlar forite. whinh. ic markedly coarsened中国煤化工，can be greatly affected by nodule amount and nodulewith idiameter[19.20]. Effects of nodule diameter, however ，YHCNMHGsoftensilesampleason water embrittlement are still not well known.well as the test method of water adhesion are shownFoundation item: Item Sponsored by China Postdoctoral Science FoundationBiography: CAI Qi-zhou(1964-)，Male, Post doctor, Associate professor; E-mail: caiqizh@ public. wh. hb. cn;Revised date: August 18，2003No.3Effect of Graphite Nodule Diameter on Water Embrttlement of Austempered Ductile Ironin Fig. l. The tensile test was performed on 49 kNand yield strength are similar both in wet and dryINSTRON test machine with a loading rate of 0. 5conditions, the tensile strength and elongation aremm●min 1. The stress-strain curve was recorded.obviously reduced in wet condition. Furthermore, .The tensile strength and elongation were determinedthe tensile strength is decreased with nodule en-in dry condition (denoted by dry) and water adhe-largement. For sample with a diameter of 49. 1 μm,sion condition (denoted by wet). The water adhe-almost no plastic deformation is observed beyondsion testing condition was built up by wrapping theyield point, resulting in extremely low elongation.degreased specimen with a wet tissue paper.Based on the stress-strain curves in Fig. 2, thecalculated strain hardening index are shown inTable 1 Characteristics of nodulationFig. 3. It is seen that the index is higher in wet thanCasting 1 Casting 2 Casting 3in dry condition. The difference of strain hardeningNodularity/ %929389index between them is enlarged with increasing nod-Nodule amount/ mm'46211559ule diameter. This implies that the increase in nod-Mean graphite noduleule diameter can promote water embrittlement.17.735. 449.1diameter/um2.2 Effect of nodule diameter on tensile propertiesR20中6.0Wet lissue paperThe effects of nodule diameter on tensile prop-erties in dry and wet are illustrated in Fig. 4. Thetensile strength and elongation of ADI are reduced25with nodule enlargement, and the tensile properties in350.25Fig. 1 Tensile specimen and water adhesion method0.202 Experimental Result0.152.1 ADI stress-strain curveDryThe ADI stress-strain curves in dry and wetconditions are illustrated in Fig. 2. ADI has higher0.1002030405(i0tensile strength and elongation with significant plas-Nodule diarneter/lumtic deformation before fracture in dry condition, andthe tensile properties are reduced with increasingFig. 3 Effect of nodule diameter on strain hardeningnodule diameter. Though the modulus of elasticityrate of ADI in dry and wet conditions1 3001 400d=17.7 umTensile strength, dry1 200d-49.1 umWet.11001 000Tensile strength, weld=35.4 μum800 H900d=17.7 μm600骂d=49.1 umElongation,dry10400.(-35.4um中国煤化工ngation, wet200JYHCNMHG_， 5%50050Strain/%Nodule diameter/μmFig. 2 Efct of nodule diameter on stress-strainFig.4 Effect of nodule diameter on tensile properties ofcurves of ADI in dry and wet conditionsADI tested in dry and wet conditionsJ. Iron &. Steel Res.，Int.Vol. 12wet are more decreased. Less tensile strength and e-the boundary between white spots and silver- graylongation reduction appears for ADI with smallerzone is unclear.nodule diameter (d= 17.7 μm). This means thatFig. 7 shows the SEM fractographs of fracturedwater embrittlement is restrained to some extent.tensile specimen in wet condition. The fracture inThough the water embrittlement decreases bothwhite spots was even and lots of graphite nodule losttensile strength and elongation， the reduction of e-[Fig. 7 (a)]. The typical brittle fracture can be ob-longation significantly exceeds tensile strength. Inserved with high magnification [Fig.7 (b)]. Theorder to identify the effect of nodule diameter on wa-ductile fracture characteristic is observed in centralter embrittlement， the tensile fracture energy waszone[Fig. 7 (c)]. While for dry fractured specimen,determined by integrating stress-strain curves shownthe ductile fracture is found on whole fracture sur-in Fig. 2. The results in Fig. 5 indicate that due toface. It is suggested that the white spots formed inwater adhesion, the fracture energy is reduced to awet fractured specimen is the starting point, resul-range from 60% to 80%，resulting in reduction ofting an early fracture due to reduced tensile strengthplastic deformability and elongation. When the nod-and elongation. The embrittlement zone is enlargedule diameter is 17. 7 μm，fracture energy reductionand ADI elongation is dramatically reduced with in-was considerably decreased as the water embrittle-creasing nodule diameter. .ment is restrained. As a result, the water embrittle-ment of ADI could be effectively restrained by de-)0厂creasing nodule diameter to some extent.80 t2.3 Fracture of ADI sampleThe macrostructure of fractures in dry and wet0conditions is shown in Fig. 6. The whole fracture of0tdry sample is of uniform silver grey zone[ Fig. 6(a)]. White spots exist in the periphery of wet frac-203(4(506tured sample [Fig.6 (b)- (d)]. The area of whiteNodule diarmer/umspots is increased with increasing nodule diameter,Fig. 5 Reduction of tensile fracture energy dueand for specimen with a nodule diameter of 49. 1 μm,to water adhesiona)_(b(d)(a) d=35.4 pm, dry;(b) d=17. 7 μm, wet; (c) d=35. 4 pm，wet;(d) d=49.1 pm, wetFig. 6 Macrostructure of fracture surface of ADI tested in dry and wet conditions.o..中国煤化工HCNMHG(a) White spots in periphery; (b) White spots in high magnification;(c) Specimen centerFig.7 SEM micrographs of fracture of specimen tested in wet conditionNo.3Effect of Graphite Nodule Diameter on Water Embrittlement of Austempered Ductile Iron3 DiscussionH2O 1Hydrogen embrittlement occurs in highDOHustrength steel[21.22 and most intermetallicsC23.24] irwet or hydrogen environments under the effect ofQtensile stress. The research on hydrogen embrittle-、ADINment has been carried out for a long time and ADIH2O+2M= MOH+M-Ha (H+ +e- + M= M-Ha)water embrittlement was found just recently. The :M Hd=M+Hao M: ADIpreliminary research results indicated that ADI wa-Fig. 8 Schematic of atomic hydrogen generationter embrittlement was caused by the hydrogen em-process from water on ADI surfacebrittlement induced by decomposition of water dur-ing plastic deformation17]. The steps of water dein austenite is higher than that in martensite and fer-composition and hydrogen diffusion process on sur-ritel-27，the most of hydrogen decomposed from wa-face of ADI are shown in Fig. 8; namely,①adsorp-ter is dissolved in austenite. When unreacted austen-tion of water;②decomposition of adsorbed water ;ite is transformed into stress induced martensite,③adsorption of hydrogen (Hd);④transfer of H。ddue to low solubility of hydrogen in martensite, theto the interior of ADI (H。b).latter is supersaturated with hydrogen， and theFig. 9 shows the microstructure of fracture forcrack formation is observed in such martensite underspecimens with nodule diameters of 17. 7 μm andtensile stress, then the cracks propagate into the ad-49.1 μm. It is evident that there were a lot of mar-jacent ferrite with high dislocation density, resultingtensite at the cell boundary when d = 49. 1 μm,in ADI early fracture.while a few martensite were observed for d = 17.7On the other hand， the matching betweenμm.' This is because the element segregation of man-graphite nodule and matrix is lost after austemperingganese and silicon in grain boundaries is enhancedand the gap between graphite nodule and matrix iswith increasing nodule diameter-25.26]， leading to un-formed during tensile plastic deformation. The wa-finished austempering reactions during heat treat-ter or hydrogen decomposed from water can intrudement，and the formation of unreacted austenite ininto specimen along gaps. The contact interface ofthese areas. This unreacted austenite could trans-water or hydrogen with stress induced martensite isform to martensite ( stress- induced martensite) un-increased with the increase of nodule diameter. As ader tensile stress during plastic deformation.result, the brittle fracture zone is widened and waterBased on the above observation and analysis，embrittlement is severely aggravated. .the model of ADI water embrittlement is given inResearch results5 28.29 indicate that grain inter-Fig.10. The ADI matrix is composed of acicular fer-face, inclusion and atomic void could be acted as hy-rite, reacted austenite and unreacted austenite beforedrogen trap. It is considered that the interface be-tensile deformation. .tween graphite and matrix in ADI might serve asDue to the fact that the solubility of hydrogenmain hydrogen trap. The diffusion of hydrogen into中国煤化工MHCNMHG10 pum(a) d=17.7 pum;(b) d=49.1 umFig. 9 Stress-induced martensite at cell boundaries during tensile test●44.J. Iron &. Steel Res.，Int.Vol. 12cell boundary. This could decrease the quantity ofH< -H2O H,OSpecinen surface且__ ___stress-induced martensite during tensile plastic de-formation and restrain ADI water embrittlement.AcicularferriteVoidReferences:仁Grnphite =[1] MorroghJ H, Williams w J. Graphite Formation in Cast Iron .Stressand in Nickel-Carbon and Cobalt Carbon Alloys [J]. J Iron andSteel Inst, 1947, 155: 321-371.Unreacted austenite[2] Johansson M. Austenitic-Bainitic Ductile Iron [J]. AFS Trans,1977，85: 117-122.[3] Tanaka Y, Kage H. Development and Application of Austem-H<- HO H,Opered Spheroidal Graphite Cast Iron [J]. Materials Trans,JIM，1992, 33(6); 543-557.Cracks