residual stresses resulting from rapid cooling on quenching. During and after machining, these stresses may produce dimensionaldistortions in the casting, which can be avoided or at least reduced with stress relief heat treatments at intermediary temperatures,

Duplex stainless steels, thus called because theirstructure is composed of ferrite and austenite in approx-

Materials Characterization 58 (2taking care to prevent the loss of mechanical properties, mainly impact toughness.The purpose of this study was to investigate the behavior of CD4MCu and CD4MCuN duplex stainless steels in impact tests

under 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 microstructure 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 steel's fracture surface shows cleavagefacets typical of low toughness 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-ray

1. IntroductionMarcelo Martins a,b,, Sergio Mazzer Rossitti c, Marcio Ritoni a, Luiz Carlos Casteletti d

a SULZER BRASIL-DIVISO FUNDINOX, Brazilb Centro Universitrio Salesiano de So Paulo-Diviso de Americana, Brazil

c Grupo MetalStainless Steel and Special Alloys, Brazild Department of Materials, Aeronautics and Automotive Engineering,

So Carlos School of Engineering University of So Paulo at So Carlos, SP, Brazil

Received 15 May 2006; received in revised form 1 September 2006; accepted 13 September 2006

Abstract

The production of 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 during 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 mechanicalproperties 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 containEffect of stress relief at 350 C and 550 C on the impactproperties of duplex stainless steelsCorresponding author. SULZER BRASIL-DIVISO FUNDI-NOX, Brazil. Tel.: +55 11 45892020.

E-mail address: marcelo.martins@sulzer.com (M. Martins).

1044-5803/$ - see front matter 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2006.09.006007) 909916imately equal parts, have been known since the early 20thcentury. Their use on an industrial scale, however, only

Table 2Brinell hardness values for the duplex stainless steels under threedifferent heat treatment conditions

Material Condition Hardness (Brinell)

CD4MCu SA+WQ 232

910 M. Martins et al. / Materials Characterization 58 (2007) 909916began 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 microstructural alterations.

Intermetallic precipitates that can occur in thesematerials when subjected to certain conditions of timeand temperature, are easily found in castings over 125mmthick, Fig. 1. The sigma phase () is the most problematicof all precipitates because it causes a considerablereduction in toughness and corrosion resistance [35].The precipitation conditions of this phase have beenexhaustively studied and described in the literature [68].

Considering the production of massive castings that

Fig. 1. Possible phase precipitations in duplex stainless steel andinfluence of the alloy's elements on the TTT curve [2].require strict dimensional control, it may be necessary toapply a stress relief heat treatment in order to ensure thedimensional stability. However, few studies have focusedon the microstructural transformations that occur at lowertemperatures and with longer times characteristic of stressrelief heat treatments. After solution annealing and waterquenching, thematerial is in principle, free of intermetallicprecipitates, but 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 (normally below

Table 1Chemical composition of the steels studied (% in weight)

Alloy C (%) Cr (%) Ni (%) Mn (%) Si (%)

CD4MCuN 0.027 25.01 5.70 0.88 0.49CD4MCu 0.032 24.67 5.43 0.77 0.87600 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 content 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 composition. This chemical modificationincreases the austenite content improving the material'sweldability [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-tion may degrade the material's tensile strength, impactresistance and increase hardness [10].

2. Experimental procedure

Duplex stainless steels cylindrical test specimens260 mm long and 25 mm diameter were cast fromCD4MCuandCD4MCuN. The casting process employedwas silica sand with phenolurethane type organic resinbinder (PepSet). The casting design was made usingAutocad 2002 software and the solidification of thecastings was simulated with another software package,

SA+WQ+350 C for 4 h 235SA+WQ+550 C for 2 h 241

CD4MCuN SA+WQ 235SA+WQ+350 C for 4 h 231SA+WQ+550 C for 2 h 239

SA+WQ=Solution annealing at 1160 C/2H+Water quench.called SOLSTAR, whose physical principle is based onthe determination of heat conduction through finiteelements. The metal was melted in a vacuum inductionfurnace with network frequency (60 Hz) and a maximumpower of 400 KW, lined with predominantly magnesium

Mo (%) S (%) P (%) Cu (%) N (%) Cr/Ni equiv

1.88 0.004 0.024 2.89 0.172 1.721.86 0.011 0.028 3.03 0.077 2.14

refractory cement (magnesium oxide base). 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-sequent stress relief at 350 C for 4 h and at 550 C for 2 h.

A scanning electron microscope with an EDSdetector was used and the secondary electron imageswere 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 diffraction patterns wasperformed using copper K1 radiation and a scanning

Table 3Results of the Charpy impact tests for diferent heat treatmentconditions

Material Condition Energy (J)

CD4MCu SA+WQ 66SA+WQ+350 C for 4 h 55SA+WQ+550 C for 2 h 26

CD4MCuN SA+WQ 119SA+WQ+350 C for 4 h 140SA+WQ+550 C for 2 h 116

SA+WQ=Solution annealing at 1160 C/2H+Water quench.

Fig. 3. Aspects of the fracture of CD4MCuN in the condition ofsolution annealing at 1160 C and water quenching.

911M. Martins et al. / Materials Characterization 58 (2007) 909916Fig. 2. Aspects of the fracture of CD4MCu in the condition of solutionannealing at 1160 C and water quenching.velocity of 1/min to characterize the structures for eachalloy.

Table 4Electron microprobe analysis of the particles contained in the dimples

Material O (%) Al (%) Si (%) Zr (%) S (%) Ca (%)

CD4MCu 9.77 0.86 62.72 ND 4.77 ZeroCD4MCuN 5.51 0.35 10.25 6.52 2.90 1.17

Material Cr (%) Mn (%) Fe (%) Ni (%) Cu (%)

CD4MCu 6.33 6.51 8.47 0.35 0.23 CD4MCuN 31.04 14.63 25.29 1.53 0.81

3. Results and discussion

The chemical compositions of the materials studiedare shown in Table 1. The Crequivalent/Niequivalent ratiosobtained through Eqs. (1) and (2) were: 2.14 for theCD4MCu steel and 1.72 for the CD4MCuN. Accordingto ASTM Standard A800/800M [11] for theseCrequivalent/Niequivalent CD4MCu has 69% of ferriteand CD4MCuN has 49% of ferrite.

Creq Cr% 1:5%Si 1:4%Mo %Nb4:99 1

[11].

Nieq Ni% 30%C 0:5%Mn 26N0:02 2:77 2

[11].Results from quantitative metallography was 63%

ferrite for the CD4MCu and 46% ferrite for the

CD4MCuN in a good 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 case 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 theprecipitation 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 practically unaltered,indicating greater stability of Cu in solid solution in the

Fig. 5. Appearance of the fractures in CD4MCu (A) and CD4MCuN(B) solution annealed and stress relieved at 550 C for 2 h.

912 M. Martins et al. / Materials CharaFig. 4. Fractures in the CD4MCu (A) and CD4MCuN (B) steelssolution annealed and stress relief at 350 C for 4 h.cterization 58 (2007) 909916austenite phase.

Fig. 6. Magnified region of the diffractograms of Fig. 7.

913M. 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. This behavior is probably due to the factthat the nitrogen in solid solution in the CD4MCuNpromotes hardening by interstitial solid solution.

The energy absorbed in the Charpy impact tests at roomtemperature by the two materials is shown in Table 3. The

values represent the arithmetic mean of three test speci-

Fig. 7. Diffractograms of the CD4MCu and CD4MCumens. The CD4MCuN steel containing nitrogen hasgreater impact toughness under all the conditions studied.The presence of nitrogen in the material's compositionleads to a greater concentration of the austenitic phase inthe microstructure, increasing the energy absorptioncapacity in the Charpy impact test. The face-centeredcubic crystalline structure has greater plasticity than thebody-centered cubic crystalline structure.

In the solution annealed condition, the energy

absorbed by the CD4MCuN steel was 80% greater

N materials in the solution annealing condition.

than 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 impact 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 550 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. Thisincrease may be associated with the effect of the relief ofall stresses originated during cooling on quenching.

particles visible in the dimples probably originate fromthe deoxidization process of the metal.

CD4MCuN has fracture predominantly ductile, i.e.,with fibrous regions, due to the large quantity of austenitein the microstructure, which, being a very tough phase,absorbs the impact energy in the test to deform itselfplastically, making difficult for cracks to 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 they 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 of

914 M. Martins et al. / Materials Characterization 58 (2007) 909916Stress 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 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. 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. TheFig. 8. Diffractograms of the CD4MCu and CD4MCuN materialsthe analyzed 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 calciumsiliconmanganese, 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 atafter solution annealing and stress relief at 350 C for 4 h.

terials

harac350 C for 4 h. Similarly, Fig. 5 shows the appearance ofthe fractures that occurred in the two materials solutionannealed and stress relieved at 550 C for 2 h. Ananalysis of Figs. 4 and 5 reveals that the CD4MCustainless steel subjected to stress relief at both 350 Cand 550 C displayed fractures with cleavage facetstypical of brittle materials. These same heat treatmentsdid not affect the impact behavior of CD4MCuN, whichshowed predominantly fibrous fractures characteristic of

Fig. 9. Diffractograms of the CD4MCu and CD4MCuN ma

M. Martins et al. / Materials Cductile materials.The X-ray diffraction results for the two steels indicate

the major phases present are ferrite () and austenite ().This means that after the solution annealing at 1160 C,the microstructure of these steels is composed solely ofthese two phases. Fig. 6 shows only a small region of thediffraction pattern of Fig. 7, but on a magnified scale,clearly revealing the peaks corresponding to the (111),(110) and (200) planes. Similarly, Figs. 8 and 9 showthe diffraction patterns of the CD4MCu and CD4MCuNmaterials after solution annealed followed by stress reliefat 350 C and 550 C, respectively. These figures showonly peaks corresponding to the reflections of ferrite andaustenite crystalline planes. No secondary phase wasidentified by this technique, although there was evidenceof its occurrence, mainly in the Charpy impact tests on theCD4MCu steel.

Stress relief above 550 C in duplex stainless steelscontaining copper normally causes the precipitation of phase particles extremely rich in this element [12]. Thefact that the X-ray test did not detect other precipitates inthe material's microstructure does not mean they do notexist, but that they may be present in concentrations ofless than 3%, which is the lowest detection limit, or thatthere are only atom clusters in the initial stages offormation of a precipitate coherent with the matrix.

4. Conclusions

The two materials showed very different values ofabsorbed energy in the Charpy impact tests under the

after solution annealing and stress relief at 550 C for 2 h.

915terization 58 (2007) 909916various conditions of heat treatment. In the solutionannealing condition at 1160 C followed by waterquenching, the higher volumetric concentration ofaustenite in the microstructure of CD4MCuN led tohigh energy absorptionwhen comparedwith the energy ofCD4MCu. The stress relief treatments at both 350 C and550 C significantly reduced the impact energy ofCD4MCubut did not have the same effect onCD4MCuN.

Although the microstructure of the nitrogenizedCD4MCuN presented more austenite, the hardness ofthe CD4MCu and CD4MCuN steels in the solubilizedand quenched condition was practically the same. Thehardening effect caused by N in interstitial solid solutionprobably explains this fact.

The stress relief heat treatment at 350 C for 4 h didnot significantly alter the materials' hardness, but thetreatment of CD4MCu at 550 C for 2 h increased itshardness by 9 HB, while the change in this property wasnot very significant in CD4MCuN.

The fracture surfaces of CD4MCu were predomi-nantly faceted under all the conditions, while those ofCD4MCuN were typically fibrous.

The X-ray diffraction tests indicated the presence ofonly two phases: ferrite and austenite in the materials'microstructures under all the conditions studied.

Although the absorbed energy values of CD4MCudeclined in response to the stress relief heat treatmentthe presence of a secondary crystalline phase responsi-ble for this behavior was not detected in X-ray.

References

[1] Martins M, Casteletti LC. Effect of heat treatment on themechanical properties of ASTM A890 grade 6A super duplexstainless steel. J ASTM Int 2005;2(1) [January].

[2] Charles J. Super duplex stainless steels: structure and properties.In: Charles J, Bernhardsson S, editors. Duplex stainless steels'91, Beaune, Bourgogne, France, October 28th30th of 1991;1991a.

[3] Charles J. Super duplex stainless steels: structure and properties.In: Charles J, Bernhardsson S, editors. Duplex stainless steels91,Beaune, 1991. Proceedings. Les ditions de PhysiqueLes Ulis:France; 1991b. p. 348.

[4] Norstrm L-, Pettersson S, Nordin S. -Phase embrittlement insome ferriticaustenitic stainless steels. ZWerkstofftech 1981;12:22934.

[5] Truman JE, Pirt KR. Properties of a duplex (austeniticferritic)stainless steel and effects of thermal history. In: Lula RA, editor.Duplex stainless steel conference, ASM, St. Louis, 1982.

[6] Josefsson B, Nilsson J-O, Wilson A. Phase transformations induplex steels and the relation between continuous cooling andisothermal heat treatment. In: Charles J, Bernhardsson S, editors.Duplex stainless steels91, Beaune, 1991. Proceedings. Lesditions de PhysiqueLes Ulis: France; 1991. p. 6878.

[7] Wang XG, Dumortier D, Riquier Y. Structural evolution of zeron100 duplex stainless steel between 550 C and 1100 C. In:Charles J, Bernhardsson S, editors. Duplex stainless steels91,Beaune, 1991. Proceedings. Les ditions de PhysiqueLes Ulis:France; 1991. p. 12734.

[8] Maehara Y, Ohmori Y, Fujino N, Kunitake T. Effects of alloyingelements on phase precipitation in duplex phase stainlesssteels. Met Sci 1983;17:5417.

[9] Stephenson N. Welding status of duplex stainless steels foroffshore applications. Weld Met Fabr J May/June and July/August1987;55(4 and 5).

[10] Rossitti SM. Efeito do Nibio na Microestrutura e nas Proprie-dades Mecnicas do Ao Inoxidvel Super duplex Fundido SEW410 W. Nr. 1.4517, Tese de Doutorado, Universidade de SoPaulo, Interunidade EESCIFSCIQSC. 2000.

[11] American Society for Testing and Materials. ASTM A800/A800M-91. Standards Practice for Steel Casting, Austenitic Alloy, EstimatingFerrite Content Thereof. Annual book of ASTM standards. V.01.02.Ferrous Castings; Ferroalloys. 1999. p. 458463.

[12] Banas J, Mazurkiewicz A. The effect of copper on passivity andcorrosion behaviour of ferritic and ferriticaustenitic stainlesssteels. Mater Sci Eng A 2000;277:18391.

916 M. Martins et al. / Materials Characterization 58 (2007) 909916Proceedings. ASMOhio: EUA; 1983. p. 11342.

Effect of stress relief at 350C and 550C on the impact properties of duplex stainless s.....IntroductionExperimental procedureResults and discussionConclusionsReferences