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CAVITATION EROSION OF DUPLEX AND SUPER DUPLEX STAINLESS STEELS C.T. Kwok, H.C. Man* and F.T. Cheng Department of Applied Physics and *Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong (Received April 7, 1997) (Accepted June 29, 1998) Introduction Owing to their excellent corrosion resistance, stainless steels are widely used both in the marine, urban water, chemical and food industries. In addition to the corrosive environment, high fluid flow speeds are always encountered for components used in these industries. Examples of these components are pump impellers, valves, ultrasonic mixer and stirrers, etc. High fluid speed could induce cavitation erosion on the metallic surface due to the collapse of cavities which are caused by the sudden change of the local pressure within the liquid. A failure investigation was carried out recently by the authors on a large impeller of a high speed urban water pump. The S31603 austenitic stainless steel impeller was seriously damaged due to cavitation erosion after it was put into service for only three months. This investigation has prompted the search for a better material to replace the S31603 stainless steel used for the high speed impeller. A number of stainless steels were considered including the newly developed super duplex stainless steel which has recently stimulated a lot of research interest. Super duplex stainless steel (SDSS) is a newly developed stainless steel and possesses application potential in the marine, chemical, food and pharmaceutical industries. It has excellent pitting resistance (PREN . 40), high yield strength, good casting and machining characteristics and lower price because of the low nickel content. The cavitation characteristics of S30400 and S31600 austenitic stainless steels and duplex stainless steels were studied in detail by a number of authors [1–7]. It was generally agreed that S30400 has higher cavitation erosion resistance than that of S31600 due to higher tendency of strain induced martensitic transformation under high impulse of stress [2]. A considerable number of results on stress corrosion cracking characteristics of SDSS and duplex stainless steels have been published but data concerning their cavitation erosion property are extremely rare. Experimental Procedures The alloys studied include super duplex stainless steels UNS S32760 (Zeron 100), duplex stainless steel UNS S31803, austenitic stainless steels UNS S30400 and UNS S31603 respectively. The chemical composition and the mechanical properties of the four stainless steels are summarised in Table 1 and 2 respectively. The austenite and ferrite ratio in S32760 and S31803 is approximately 1:1. Pergamon Scripta Materialia, Vol. 39, No. 9, pp. 1229 –1236, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00 PII S1359-6462(98)00308-X 1229

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Page 1: Cavitation erosion of duplex and super duplex stainless steels

CAVITATION EROSION OF DUPLEX AND SUPER DUPLEXSTAINLESS STEELS

C.T. Kwok, H.C. Man* and F.T. ChengDepartment of Applied Physics and *Department of Manufacturing Engineering, The Hong Kong

Polytechnic University, Hung Hom, Kowloon, Hong Kong

(Received April 7, 1997)(Accepted June 29, 1998)

Introduction

Owing to their excellent corrosion resistance, stainless steels are widely used both in the marine, urbanwater, chemical and food industries. In addition to the corrosive environment, high fluid flow speedsare always encountered for components used in these industries. Examples of these components arepump impellers, valves, ultrasonic mixer and stirrers, etc. High fluid speed could induce cavitationerosion on the metallic surface due to the collapse of cavities which are caused by the sudden changeof the local pressure within the liquid.

A failure investigation was carried out recently by the authors on a large impeller of a high speedurban water pump. The S31603 austenitic stainless steel impeller was seriously damaged due tocavitation erosion after it was put into service for only three months. This investigation has promptedthe search for a better material to replace the S31603 stainless steel used for the high speed impeller.A number of stainless steels were considered including the newly developed super duplex stainless steelwhich has recently stimulated a lot of research interest.

Super duplex stainless steel (SDSS) is a newly developed stainless steel and possesses applicationpotential in the marine, chemical, food and pharmaceutical industries. It has excellent pitting resistance(PREN. 40), high yield strength, good casting and machining characteristics and lower price becauseof the low nickel content.

The cavitation characteristics of S30400 and S31600 austenitic stainless steels and duplex stainlesssteels were studied in detail by a number of authors [1–7]. It was generally agreed that S30400 hashigher cavitation erosion resistance than that of S31600 due to higher tendency of strain inducedmartensitic transformation under high impulse of stress [2]. A considerable number of results on stresscorrosion cracking characteristics of SDSS and duplex stainless steels have been published but dataconcerning their cavitation erosion property are extremely rare.

Experimental Procedures

The alloys studied include super duplex stainless steels UNS S32760 (Zeron 100), duplex stainless steelUNS S31803, austenitic stainless steels UNS S30400 and UNS S31603 respectively. The chemicalcomposition and the mechanical properties of the four stainless steels are summarised in Table 1 and2 respectively. The austenite and ferrite ratio in S32760 and S31803 is approximately 1:1.

Pergamon

Scripta Materialia, Vol. 39, No. 9, pp. 1229–1236, 1998Elsevier Science Ltd

Copyright © 1998 Acta Metallurgica Inc.Printed in the USA. All rights reserved.

1359-6462/98 $19.001 .00PII S1359-6462(98)00308-X

1229

Page 2: Cavitation erosion of duplex and super duplex stainless steels

Cavitation erosion experiments were carried out using an ultrasonic induced cavitation facility with550W ultrasonic probe conforming to ASTM Standard G32–92 [8]. The vibratory frequency was 20kHzand a peak-to-peak vibratory amplitude of 50mm was used. The specimens were subjected to cavitationerosion test in 3.5% NaCl solution at 23°C. Each cavitation erosion test was completed after 4 hours,which included eight intermittent periods each of half an hour. After each test period, the weight lossof the specimens was measured to an accuracy of60.1mg. The erosion loss of materials was expressedin terms of mean depth of penetration (MDP) and mean depth of penetration rate (MDPR) as calculatedby the following equations:

MDP (mm) 5DW

10rAand MDPR (mm/h)5

DW

10rADt

whereDW is the weight loss at each time interval in mg,Dt is the time interval in hour, A is the exposedsurface area of the specimen in cm2 andr is the density of the specimen in g cm23.

Before and after the cavitation erosion test, the structure of the specimens was studied by x-raydiffractometry (XRD, Philips, Model MPD1880). As austenite would undergo strain induced marten-sitic phase transformation, the respective volume fractions of transformed martensite and retainedferrite present in austenitic and duplex stainless steels were therefore determined. The volume fractionsof martensite and ferrite were evaluated by the direct comparison method [9]. The integrated intensitiesof (111) diffraction peak for austenite (Ig) and (110) for martensite or ferrite (Ia) were measured. Thevolume fraction of martensite or ferrite (Ca) was calculated using the following equation [10]:

Ca 5 S1 1 1.25Ig

IaD21

TABLE 1Nominal Chemical Compositions of Various Stainless Steels and their Phases.

Stainless steels Fe Cr Ni Mo Mn Cu Zu Cc Nc P S Si W PRENd

S31603 (g)a bal. 17.6 11.2 2.5 1.4 1.4 0.4 0.03 2 2 2 0.4 2 25S30400 (g)a bal. 18.4 8.7 2 1.6 2.1 1.7 0.08 2 0.1 0.1 0.3 2 19S31803 (bulk) bal. 22.3 5.6 2.9 1.5 1.6 2 0.03 2 2 2 0.4 0.2 35

(g)a bal. 21.8 7.4 2.1 1.3 0.4 0.4 0.03 2 2 0.3 0.5 2 2(a)b bal. 24.6 5.2 4.9 1 0.6 0.1 0.03 2 2 2 0.5 1.3 2

S32760 (bulk) bal. 25.6 7.2 4 0.6 0.7 0.5 0.03 0.22 0.1 0.3 0.8 40(g)a bal. 24.3 9.4 3.5 0.1 0.7 0.2 0.03 0.22 2 0.4 2.2 2(a)b bal. 25.9 7.0 4.5 0.1 0.7 0.1 0.03 0.22 2 0.4 1.4 2

ag 5 austenite,ba 5 ferrite, cmaximum values,dPREN5 pitting resistance equivlaent number for stainless steels.

TABLE 2Measured and Calculated Mechanical Properties of Various Alloys.

Stainless steels

Hardness(HRB)

Tensilestrength(N/mm2)

0.2% Yieldstrength(N/mm2)

Engineeringstrain energyd

(N/mm2)

Ultimateresiliencee

(N/mm2)Elongation(%)

Strainhardness rate(N/mm2)

S31603 70 619 550 239 12 45 147S30400 94 619 587 258 17 46 151S31803 96 754 547 276 13 44 340S32760 103 768 645 255 14 39 358

dIntegrated area under stress-strain curve,earea under elastic portion of stress-strain curve.

CAVITATION EROSION1230 Vol. 39, No. 9

Page 3: Cavitation erosion of duplex and super duplex stainless steels

Results and Discussions

Ranking of Stainless Steels

Figs. 1 and 2 show the graphs of cumulative MDP and MDPR as a function of time for various stainlesssteels eroded in 3.5% NaCl solution at 23°C respectively. Among the stainless steels tested, S32760 isthe most resistant to cavitation erosion, whereas S31603 is the least. The MDP of S31603 is almost fourtimes higher than that of S32760 after 4 hours of testing. The ranking of these steels in terms ofcavitation erosion resistance (Re, defined as the reciprocal of MDPR after 4 hours) in the conditionsconsidered is as follows: S32760. S30400. S31803. S31603.

Mechanical Strength and Strain Hardenability of Stainless Steels

Of the two duplex stainless steels S32706 and S31803, S32706 has higher cavitation erosion resistance.S32760 has the highest 0.2% yield strength, ultimate tensile strength and strain or work hardening rate

Figure 1. Cumulative MDP as a function of time for various stainless steels eroded in 3.5% NaCl solution at 23°C.

Figure 2. MDPR as a function of time for various stainless steels eroded in 3.5% NaCl solution at 23°C.

CAVITATION EROSION 1231Vol. 39, No. 9

Page 4: Cavitation erosion of duplex and super duplex stainless steels

in the plastic region as defined in [11] among the four stainless steels studied. The high cavitationerosion resistance of S32760 is probably due to its high 0.2% yield strength, ultimate tensile strengthand strain hardening rate (see Table 2).

In comparing S30400 with S31803, although S31803 has higher tensile strength and strain hardeningrate than S30400, the cavitation erosion resistance of S30400 is superior. In the work carried out byHeathcock and Protheroe [2], the ferritic structure was found to have poor resistance to cavitationerosion because of the high strain sensitivity of the bcc structure that suffers from brittle fracture alongthe transgranular and intergranular structures. Also, Karimi [6] studied the cavitation erosion behaviourof duplex stainless steel and found that the material loss is initiated from the austenite-ferrite boundariesand propagates more rapidly into the ferrite grains than into the austenite grains. Al-Hashem [7]observed a similar mechanism in duplex stainless steel, in which the cracks appeared to be initiated inferrite and were impeded by the austenitic phase during their propagation. Furthermore, from theelectrochemical corrosion point of view, the galvanic effect between the ferrite and austenite of duplexstainless steel leads to a significant increase in active dissolution of the ferritic phase [12]. Super duplexstainless steel is damaged in a similar mode. Fig. 3 shows the cross sectional view of S32760 afterexposure to cavitation for 4 hours. The figure shows that at the eroded front, most of the white austeniticgrains remained whereas the dark ferritic grains have been eroded away. Ferrite is more easily erodedthan austenite. The smaller amount of ferrite remaining at the surface of both duplex stainless steels isan indication of the susceptibility of the ferrite to cavitation erosion. The result obtained is consistentwith the XRD spectrum of S32760 and S31803 which will be discussed later.

Martensitic Transformability of Stainless Steels

Martensitic transformation and stacking fault energy (SFE) are important aspects for austenitic stainlesssteels and the austenitic phase in duplex stainless steels because they control the work hardening rateand the type and mechanism of martensitic transformation [13]. Their significance for explainingcavitation erosion resistance of the stainless steels is discussed below.

S30400 has a higher cavitation erosion resistance than S31803 and S31603. S30400 has mechanicalproperties and strain hardening rate very similar to S31603, as indicated in Table 2. The abnormallyhigh cavitation erosion resistance of the S30400 austenitic stainless steel is attributed to the straininduced austenite to martensite transformation (g3a’) [3]. The transformation is dependent on thecomposition of alloys, temperature and strain rate. This phase transformation absorbs the energyproduced by cavitation and thus reduces cavitation damage of the austenitic stainless steels. Fig. 4shows the X-ray diffractometry spectra of various stainless steels S30400, S31603, S31803 and S32760before and after 4 hours of exposure to cavitation erosion. The spectra of S30400 and S31603 provide

Figure 3. Cross-sectional view of super duplex stainless steel S32760 after being eroded for 4 hours.

CAVITATION EROSION1232 Vol. 39, No. 9

Page 5: Cavitation erosion of duplex and super duplex stainless steels

evidence of the phase transformation. The spectra and volume fraction of martensite (peak of bct (110))show that the degree of phase transformation in S30400 is more significant than that in S31603, asshown in Fig. 4(a) and (b) and Table 3. This result indicates that the transformability of S30400 ishigher than that of S31603.

For the spectra of S32760 and S31803, as shown in Fig. 4(c) and (d), it is difficult to distinguish thepeak of the martensitic phase (bct (110)) after cavitation erosion since the existence of the originalferritic phase (bcc [110]) may overlap with the peak of the martensite. As Md30 is predicted to be lowand negative for the duplex stainless steels by equation (1), martensitic transformation in both S32760and S31803 after cavitation is unlikely. The volume fractions of ferrite of S32760 and S31803 aftercavitation are found to be much less than before cavitation, as shown in Table 4. This is because the

Figure 4. X-ray diffractometry spectra of (a) S30400, (b) S31603, (c) S32760 and (d) S31803 before and after cavitation.

TABLE 3Volume Fraction of Transformed Martensite of Austenitic Stainless Steels before and after Cavitation.

Stainless steels volume fraction of martensite before cavitation volume fraction of martensite after cavitation

S30400 0% 35%S31603 0% 20%

CAVITATION EROSION 1233Vol. 39, No. 9

Page 6: Cavitation erosion of duplex and super duplex stainless steels

strain sensitive bcc ferrite was eroded away more easily than fcc austenite. Also, the removal of ferritein S31803 is more severe than that of S32760 after cavitation.

Martensitic transformability of austenite of stainless steels under repetitive cyclic strain by cavitationdepends on the temperature at which 50% martensite is transformed under the action of a true strain of0.3. This transformation temperature is an index of stability and is denoted by Md30. A higher value ofMd30 means that the austenite of the steels is more susceptible to martensitic transformation. Md30 ofvarious stainless steels are estimated by substituting the typical values of their chemical compositions(in weight %) of the austenite into the following formula [13,14]:

Md30(°C) 5 4972 462([C]1 [N]) 29.2 [Si] 28.1 [Mn] 2 20 [Ni] 2 13.7 [Cr]2 18.5 [Mo] (1)

Ni, Mn and N are the stabilisers of austenite (g) and Cr, Mo and Si are stabilisers of ferrite (a). Thesolubilities of the alloying elements in austenite and ferrite in the duplex stainless steels are different.The compositions ofg (see Table 1) in duplex stainless steels were used for calculating Md30. Fromequation (1), Md30will decrease with the addition of alloying elements such as Ni, Cr, Mo and N whichare used for enhancing the pitting resistance of the stainless steels. Table 5 shows the Md30 values ofthe stainless steels studied. For S30400, Md30 is high and close to room temperature so it has the highesttransformability among the four stainless steels examined. For S31603, the higher Ni and Mo contentsresults in low Md30. Similarly, for both duplex stainless steels S31803 and S32760, the high Cr, Mo andN content of their austenitic phases results in low Md30. So S31603, S31803 and S32760 are predictedto be less susceptible to strain induced martensitic transformation. The Md30 or the martensitictransformability for both austenitic stainless steels is consistent with the data of x-ray diffractometry andagrees with the findings of Heathcock and Protheroe [3]. In addition, Pohl and Gocke [15] have foundthat uniformly distributed dislocations and a large number of stacking faults were observed due to thelow SFE for S31803. However, the transformed martensite was not observed by them. This observationagrees with the fact that the Md30 of S31803 is low.

TABLE 4Volume Fraction of Retained Ferrite and Transformed Martensite of Duplex Stainless Steels before and after

Cavitation.

Stainless steels volume fraction of ferrite before cavitation volume fraction of ferrite1 martensite after cavitation

S31803 45% 29%S32760 45% 39%

TABLE 5Calculated Values of Re, Md30 Temperatures and SFE of Various Stainless Steels.

Stainless steels

Cavitation erosionresistance Re(h/mm)f Md30 (°C)

SFE (mJ/m2)Eqt. (2)[13]

SFE (mJ/m2)Eqt. (3a & 3b)[15]

SFE (mJ/m2)Eqt. (4)[15]

SFE (mJ/m2)Eqt. (5)[16]

S31603 2.93 243.3 42.9 26.8 34.3 56.5S30400 7.52 18.2 53.5 21.1 39.3 18.9S31803 (g phase) 4.30 217.5 25.1 32.8 21.9 55.2S32760 (g phase) 9.74 2199.4 14.2 28.1 25.3 31.4

f Re 5 (MDPR)21 where MDPR is the average mean depth of penetration rate after 4 hours.

CAVITATION EROSION1234 Vol. 39, No. 9

Page 7: Cavitation erosion of duplex and super duplex stainless steels

Stacking Fault Energy (SFE) of Stainless Steels

In addition to the martensitic transformability, SFE was also considered to be responsible for thecavitation erosion resistance of the stainless steels [3]. Low SFE may delay the development oflocalised stresses required to initiate fracture and hence result in high cavitation erosion resistance. Anumber of empirical equations that correlate the SFE and the compositions of major alloying elements(in weight %) for commercial grade austenitic stainless steels have been published [13,16,17]. Forexample,

SFE (mJ/m2) 5 25.71 2 [Ni] 1 410[C]2 0.9 [Cr] 2 77[N] 2 13 [Si] 2 1.2 [Mn] (2) [13]SFE (mJ/m2) 5 17.01 2.29 [Ni] 2 0.9 [Cr] for [Cr]#20% (3a) [16]SFE (mJ/m2) 5 226.61 0.73 [Ni] 1 2.26 [Cr] for [Cr]$20% (3b) [16]SFE (mJ/m2) 5 1.21 1.4 [Ni] 1 0.6 [Cr] 1 17.7 [Mn] 2 44.7 [Si] (4) [16]SFE (mJ/m2) 5 2531 6.2 [Ni] 1 0.7 [Cr] 1 3.2 [Mn] 1 9.3 [Mo] (5) [17]

The SFEs for theg phases in the steels concerned are estimated using these equations and the valuesare shown in Table 5. It is obvious that different equations result in different SFEs for the same steel.This creates problems and uncertainty in the use of SFE to explain the ranking of cavitation erosionresistance.

From the study by Richman (equations 3a & b) [18], the linear SFE dependence of some austeniticstainless steels (e.g., S30400, S30300 and S30100) that form martensite under cavitation attack isdifferent from the SFE dependence of those (e.g. S31603 and S31000) that do not or are less susceptibleto form martensite. A high susceptibility to form martensite together with low SFE results in greaterresistance to cavitation erosion. However, the SFE and martensitic transformation dependence of thestainless steels found by Richman cannot be used to explain satisfactorily the ranking of cavitationresistance of the four stainless steels studied here. This is mainly due to the confusing results of SFEas estimated by different empirical equations. Determination of the SFE for each steel requires lengthyexperimental work and is not feasible for the ranking of a large number of materials.

According to the foregoing investigation, as SFE cannot provide a satisfactory explanation of thecavitation erosion resistance of the stainless steels studied, the strain hardening rate seems to be a morepractical factor since it measures the bulk properties of stainless steels. On the other hand, martensitictransformability has been proved to be a factor which plays an important role in cavitation erosionresistance. Furthermore, bulk properties such as the yield strength and hardness are also prime factorsthat influence cavitation erosion resistance.

For the purpose of comparison, the relevant data of S30400 are taken as the reference value andrespective data of the other three stainless steels are normalised. Table 6 shows the normalised values

TABLE 6Normalised Values of Re, 0.2% Yield Strength, Strain Hardening Rate, Md30 Temperatures and SFE of Various

Stainless Steels.

Stainlesssteels Re

0.2% yieldstrength

Strainhardeningrate Md30

SFEEqt. (2)[13]

SFEEqt. (3a & 3b)[15]

SFEEqt. (4)[15]

SFEEqt. (5)[16]

S31603 0.39 0.93 0.97 23.37 0.80 1.27 0.87 2.90S30400 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00S31803 0.57 0.93 2.25 21.96 0.47 1.55 0.56 2.91S32760 1.29 1.10 2.37 211.96 0.26 1.33 0.64 1.68

CAVITATION EROSION 1235Vol. 39, No. 9

Page 8: Cavitation erosion of duplex and super duplex stainless steels

of the cavitation erosion resistance Re, Md30, 0.2% yield strength, strain hardening rate and SFEs. Aclear conclusion can be drawn from Table 6. It can be seen that 0.2% yield strength is the primary factorthat influences Re. The higher the yield strength, the higher the Re. If the yield strength values of thestainless steels are very close to each other (e.g., S30400 and S31603), then Md30 becomes thesecondary factor for comparison. The higher the Md30, the higher the Re. The strain hardening ratewhich is associated with SFE, however, is of the least importance among the three factors considered.

Conclusions

(1) The ranking of stainless steels in terms of cavitation erosion resistance is S32760. S30400.S31803. S31603.

(2) The above ranking can be primarily explained in terms of 0.2% yield strength supplemented by themartensitic transformability.

(3) SFE cannot be used to explain the Re of various stainless steels, especially those of duplex structure.

Acknowledgments

The authors would like to acknowledge the Research Committee of the Hong Kong PolytechnicUniversity for the provision of a research grant (No. 350/654) and Weir Metals (U.K) for the supply ofZeron 100.

References

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10. J. L. Gonalez, R. Aranda, and M. Jonapa, Application of Stainless Steel ’92, vol. 2, p. 1009, Stockholm, Sweden (1992).11. J. R. Newby et al., Metal Handbook, Mechanical Testing, vol. 8, p. 24, ASM International, Metals Park, OH (1995).12. J. H. Potgieter and S. C. Tjong, S. Afr. J. Chem. 43, 59 (1990).13. F. B. Pickering, Stainless Steels ’84, p. 9 (1984).14. F. B. Pickering, Physical Metallurgy and Design of Steel, p. 229, Harwood Academic, New York (1978).15. M. Pohl and A. Gocke, Pract. Metall. 26, 428 (1989).16. C. G. Rhodes and A. W. Tompson, Metall. Trans. A. 8A, 1901 (1977).17. R. E. Schramm and R. P. Reed, Metall. Trans. 6A, 1345 (1975).18. R. H. Richman, International Conference on Martensitic Transformation, ICOMAT 95 II, France, 1995, 1155 (1995).

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