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1、Effect of stress relief at 350 C and 550 C on the impactproperties of duplex stainless steelsMarcelo Martinsa,b, Sergio Mazzer Rossittic, Marcio Ritonia, Luiz Carlos CastelettidaSULZER BRASIL-DIVISO FUNDINOX, BrazilbCentro Universitrio Salesiano de So Paulo-Diviso de Americana, BrazilcGrupo MetalSta
2、inless Steel and Special Alloys, BrazildDepartment of Materials, Aeronautics and Automotive Engineering,So Carlos School of Engineering University of So Paulo at So Carlos, SP, BrazilReceived 15 May 2006; received in revised form 1 September 2006; accepted 13 September 2006AbstractThe production of
3、massive duplex stainless steel castings weighting over 2 t, with thicknesses exceeding 5 in. represents a majorchallenge for the foundry industry. The difficulty in manufacturing such castings lies in the fact that thick sections experiment lowcooling rates during the solidification process and duri
4、ng the solution annealing and water quenching heat treatment.As a result, intermetallic phases such as sigma phase (), Chi phase (), G phase, R phase, and complex carbides mayprecipitate, causing the material to be extremely brittle Martins M, Casteletti LC. Effect of heat treatment on the mechanica
5、lproperties of ASTM A890 grade 6A super duplex stainless steel. J ASTM Int 2005;2(1) January.1.After solution annealing and water quenching, the steel is, in principle, free of intermetallic precipitates, but will containresidual stresses resulting from rapid cooling on quenching. During and after m
6、achining, these stresses may produce dimensionaldistortions in the casting, which can be avoided or at least reduced with stress relief heat treatments at intermediary temperatures,taking care to prevent the loss of mechanical properties, mainly impact toughness.The purpose of this study was to inve
7、stigate the behavior of CD4MCu and CD4MCuN duplex stainless steels in impact testsunder the conditions of solution annealing and water quenching and stress relief at 350 C for 4 h and at 550 C for 2 h. Comparedto CD4MCu the high nitrogen content of CD4MCuN stainless steel has a more balanced microst
8、ructure with similar ferrite andaustenite contents, providing it with higher energy-absorbing capacity in impact tests. CD4MCuN fracture surfaces havepredominantly fibrous structures typical of high toughness materials, while the CD4MCu steels fracture surface shows cleavagefacets typical of low tou
9、ghness materials. The stress relief heat treatments reduced the impact toughness of the CD4MCu alloy butdid not affect the CD4MCuN alloy. 2006 Elsevier Inc. All rights reserved.Keywords: Duplex; Impact; Fractography; X-ray1. IntroductionDuplex stainless steels, thus called because theirstructure is
10、composed of ferrite and austenite in approx-imately equal parts, have been known since the early 20thcentury. Their use on an industrial scale, however, onlyMaterials Characterization 58 (2007) 909916Corresponding author. SULZER BRASIL-DIVISO FUNDI-NOX, Brazil. Tel.: +55 11 45892020.E-mail address:
11、(M. Martins).1044-5803/$ - see front matter 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2006.09.006began in the second half of the century. Even today, largecastings of these materials are difficult to manufacturebecause their metallurgical structure is metastable and,prone to micro
12、structural alterations.Intermetallic precipitates that can occur in thesematerials when subjected to certain conditions of timeandtemperature,areeasilyfoundincastingsover125mmthick,Fig.1.Thesigma phase ()isthe mostproblematicof all precipitates because it causes a considerablereduction in toughness
13、and corrosion resistance 35.The precipitation conditions of this phase have beenexhaustivelystudied and described inthe literature 68.Considering the production of massive castings thatrequire strict dimensional control, it may be necessary toapply a stress relief heat treatment in order to ensure t
14、hedimensional stability. However, few studies have focusedon the microstructural transformations that occur at lowertemperaturesandwithlongertimes characteristic ofstressrelief heat treatments. After solution annealing and waterquenching,thematerialisinprinciple,freeofintermetallicprecipitates, but
15、still contains residual stresses resultingfrom its abrupt cooling on water quenching. During andafter machining, these stresses can cause dimensionaldistortions in the components. However, this can beavoided, or at least reduced, by applying stress relief heattreatments at intermediary temperatures
16、(normally below600 C), while taking care to prevent the loss ofmechanical properties, specifically impact toughness.Even today, one of the duplex steels most widelyused for valve and centrifugal pump components is theCD4MCu, whose microstructure contains up to 70% offerrite. This high ferrite conten
17、t renders the materialmore prone to the formation of solidification and heattreatment cracks and favors the formation of sigmaphase. With a view to reducing these adverse effects, theCD4MCuN material was developed from the throughthe addition of 0.10% to 0.25% in weight of Nitrogen toits chemical co
18、mposition. This chemical modificationincreases the austenite content improving the materialsweldability 9. These materials contain copper in theirchemical composition and their stress relief heattreatment may cause precipitation of the phase,which is basically a copper precipitate. This precipita-ti
19、on may degrade the materials tensile strength, impactresistance and increase hardness 10.2. Experimental procedureDuplex stainless steels cylindrical test specimens260 mm long and 25 mm diameter were cast fromCD4MCuandCD4MCuN.Thecastingprocessemployedwas silica sand with phenolurethane type organic
20、resinbinder (PepSet). The casting design was made usingAutocad 2002 software and the solidification of thecastings was simulated with another software package,called SOLSTAR, whose physical principle is based onthe determination of heat conduction through finiteelements. The metal was melted in a va
21、cuum inductionfurnace with network frequency (60 Hz) and a maximumpower of 400 KW, lined with predominantly magnesiumFig. 1. Possible phase precipitations in duplex stainless steel andinfluence of the alloys elements on the TTT curve 2.Table 1Chemical composition of the steels studied (% in weight)A
22、lloyC (%)Cr (%)Ni (%)Mn (%)Si (%)Mo (%)S (%)P (%)Cu (%)N (%)Cr/Ni equivCD4MCuN0.02725.015.700.880.491.880.0040.0242.890.1721.72CD4MCu0.03224.675.430.770.871.860.0110.0283.030.0772.14Table 2Brinell hardness values for the duplex stainless steels under threedifferent heat treatment conditionsMaterialC
23、onditionHardness (Brinell)CD4MCuSA+WQ232SA+WQ+350 C for 4 h235SA+WQ+550 C for 2 h241CD4MCuNSA+WQ235SA+WQ+350 C for 4 h231SA+WQ+550 C for 2 h239SA+WQ=Solution annealing at 1160 C/2H+Water quench.910M. Martins et al. / Materials Characterization 58 (2007) 909916refractory cement (magnesium oxide base)
24、. Chemicalanalysis was done by optical emission spectroscopy onsolid samples. The material was solution annealed at1160 C for 2 h, followed by water quenching and sub-sequentstressreliefat350Cfor4handat550Cfor2h.A scanning electron microscope with an EDSdetector was used and the secondary electron i
25、mageswere obtained digitally.Brinell hardness was measured using a UniversalTesting Machine with a capacity of 300,000 N and a10 mm diameter sphere with a load of 3000 Kgf and theimpact tests were done using a OTTO WOLPERT-WERKE-GMBH, type PW 30K machine, with maxi-mum capacity of 294J. X-ray diffra
26、ction patterns wasperformed using copper K1 radiation and a scanningvelocity of 1/min to characterize the structures for eachalloy.Table 3Results of the Charpy impact tests for diferent heat treatmentconditionsMaterialConditionEnergy (J)CD4MCuSA+WQ66SA+WQ+350 C for 4 h55SA+WQ+550 C for 2 h26CD4MCuNS
27、A+WQ119SA+WQ+350 C for 4 h140SA+WQ+550 C for 2 h116SA+WQ=Solution annealing at 1160 C/2H+Water quench.Fig. 2. Aspects of the fracture of CD4MCu in the condition of solutionannealing at 1160 C and water quenching.Fig. 3. Aspects of the fracture of CD4MCuN in the condition ofsolution annealing at 1160
28、 C and water quenching.Table 4Electron microprobe analysis of the particles contained in the dimplesMaterialO (%)Al (%)Si (%)Zr (%)S (%)Ca (%)CD4MCu9.770.8662.72ND4.77ZeroCD4MCuN5.510.3510.256.522.901.17MaterialCr (%)Mn (%)Fe (%)Ni (%)Cu (%)CD4MCu6.336.518.470.350.23CD4MCuN31.0414.6325.291.530.81911
29、M. Martins et al. / Materials Characterization 58 (2007) 9099163. Results and discussionThe chemical compositions of the materials studiedare shown in Table 1. The Crequivalent/Niequivalentratiosobtained through Eqs. (1) and (2) were: 2.14 for theCD4MCu steel and 1.72 for the CD4MCuN. Accordingto AS
30、TM Standard A800/800M 11 for theseCrequivalent/NiequivalentCD4MCu has 69% of ferriteand CD4MCuN has 49% of ferrite.Creq Cr% 1:5%Si? 1:4%Mo? %Nb4:99111.Nieq Ni% 30%C? 0:5%Mn? 26N0:02? 2:77211.Results from quantitative metallography was 63%ferrite for the CD4MCu and 46% ferrite for theCD4MCuN in a goo
31、d agreement with A800/800M.The presence of nitrogen strongly favors the formationof austenite in the microstructure.Table 2 lists the Brinell hardness values found for thematerials when solution heat-treated at 1160 C andwater quenched and stress relieved at 350 C for 4 h and550 C for 2 h. In the ca
32、se of the CD4MCu steel, stressrelief at 350 C for 4 h promoted an insignificantincrease in hardness, on the order of 2.6%, which iswithin the range of accuracy of the test. Although thematerial contains 2.89% copper, the temperature andheat treatment time were insufficient to promote theprecipitatio
33、n of phase particles, which are rich in thiselement 12. However, the same alloy, when subjectedto stress relief at 550 C for 2 h, showed a 5.2% increasein hardness, possibly indicating that submicroscopic phase particles began to precipitate in the ferritic phase.The CD4MCuN steel remained practical
34、ly unaltered,indicating greater stability of Cu in solid solution in theaustenite phase.Fig. 4. Fractures in the CD4MCu (A) and CD4MCuN (B) steelssolution annealed and stress relief at 350 C for 4 h.Fig. 5. Appearance of the fractures in CD4MCu (A) and CD4MCuN(B) solution annealed and stress relieve
35、d at 550 C for 2 h.912M. Martins et al. / Materials Characterization 58 (2007) 909916Although the microstructure of the CD4MCuN steelhas a higher volume of austenite, the hardness of theCD4MCu and CD4MCuN materials in the solutionannealed state was very similar, i.e., 232HB and 235HB,respectively. T
36、his behavior is probably due to the factthat the nitrogen in solid solution in the CD4MCuNpromotes hardening by interstitial solid solution.TheenergyabsorbedintheCharpyimpacttestsatroomtemperature by the two materials is shown in Table 3. Thevalues represent the arithmetic mean of three test speci-m
37、ens. The CD4MCuN steel containing nitrogen hasgreater impact toughness under all the conditions studied.The presence of nitrogen in the materials compositionleads to a greater concentration of the austenitic phase inthe microstructure, increasing the energy absorptioncapacity in the Charpy impact te
38、st. The face-centeredcubic crystalline structure has greater plasticity than thebody-centered cubic crystalline structure.In the solution annealed condition, the energyabsorbed by the CD4MCuN steel was 80% greaterFig. 6. Magnified region of the diffractograms of Fig. 7.Fig. 7. Diffractograms of the
39、CD4MCu and CD4MCuN materials in the solution annealing condition.913M. Martins et al. / Materials Characterization 58 (2007) 909916than that absorbed by the CD4MCu steel. In the case ofthe materials stress relieved at 350 C, the energyabsorbed by CD4MCuN was 154% greater than inCD4MCu, while the imp
40、act test showed that thenitrogenized steel stress relieved at 550 C absorbed346% more energy than the steel without nitrogen.In the case of the CD4MCu, the stress relief at 350 Cfor 4 h reduced the absorbed energy by 16.7% in relationto the sample only solution heat-treated, while stressrelief at 55
41、0 C for 2 h decreased the absorbed energy by60.6% when compared with the same steel in thesolubilized state.This behavior was not repeated by the CD4MCuNsteel, in which the stress relief at 350 C for 4 hincreased the absorbed energy by 17.6% in relation tothe sample only solution annealed at 1160 C.
42、 Thisincrease may be associated with the effect of the relief ofall stresses originated during cooling on quenching.Stress relief at 550 C for 2 h hardly altered the energyabsorbed in the Charpy impact test when compared withthe sample only solution annealed. Nitrogen increasesthe austenite content
43、in the microstructure and austenite,in turn, dissolves higher concentrations of copper insolid solution compared to ferrite. Hence, the stressrelief at both 350 C and 550 C in the samples con-taining nitrogen were probably insufficient to promoteprecipitation of the phase in the microstructure.Figs.
44、 2 and 3 illustrate the aspect of the fracturesurfaces of the CD4MCu and CD4MCuN after solutionannealing and water quenching. CD4MCu has fracturepredominantly faceted, with few areas of ductilefracture, due to the mostly ferritic microstructure. Theparticles visible in the dimples probably originate
45、 fromthe deoxidization process of the metal.CD4MCuN has fracture predominantly ductile, i.e.,withfibrous regions,due tothe largequantityof austenitein the microstructure, which, being a very tough phase,absorbs the impact energy in the test to deform itselfplastically, making difficult for cracks to
46、 propagate.Analysis of Figs. 2 and 3 indicates that themorphologies of the particles found in the two materialsare completely dissimilar, for the CD4MCu particles arebasically spherical while those of the CD4MCuN areangular and smaller. Electron microprobe analyses ofthe particles indicated that the
47、y are probably inclusions,but have different chemical compositions, as shown inTable 4. In the case of the CD4MCu steel, the particlesare basically silica inclusions originating from thedeoxidization process and the Cr and Fe elements thatappear in the microanalysis are due to the small size ofthe a
48、nalyzed particles, which produce an effect on theadjacent matrix. With regard to the CD4MCuN steel, theparticles are complex silicon, zirconium, chromium,manganese and iron oxides. This is because thematerials were deoxidized differently. Only a metallicsilicon was used to deoxidize the CD4MCu. A ca
49、lciumsiliconmanganese, ironsiliconzirconium, and me-tallic silicon was used to deoxidize the CD4MCuN steel.Moreover, since the same electron beam phenomenonreached the matrix of this material, the chromium andiron contents, in particular, must have significantlyinfluenced its effect.Fig. 4(A) and (B) depict the fractures in both steels inthe condition of solution annealed and stress relief atFig. 8. Diffractograms of the CD4MCu and CD4MCuN materials after solution annealing and stress relief at 350 C for 4 h.914M. Martins et al. / Materials Characterization 58 (2007) 909916350 C for 4 h. Simi