Retained Austenite and Pitting

8/10/2019 Retained Austenite and Pitting

1/6


2/6

of the results. More important is the fact that this comparative

assessment is highly relevant [57]. Thus, the study of welding

metallurgy requires a consideration of the following:

(a) a welded joint should normally resist corrosion the same as

the metal being joined;

(b) variations in composition or metallurgical or mechanical

changes brought about by welding can introduce corrosion

problems because corrosion thrives on microstructural dif-

ferences adjacent to the base metal;

(c) when subjected to a corrosive environment the weld metal

may corrode more, or less as compared to the base metal.

Consequently, using appropriate heat treatments to simulate

the thermal cycles in this work we performed a comparative

evaluation of localized corrosion susceptibility of low carbon

13CrNiMo stainless steel weld and parent metals.

2. Experimental

2.1. Materials, specimen preparation and characterization

Specimens were cut from 13CrNiMo plate metals and they were

subjected to different heat treatments (HTs). The composition of

the plate metal is shown in Table 1, and a summary of the heat

treatments and nomenclature used along the text are shown in

Table 2.

For each condition, volume fractions of the austenite were mea-

sured by X-ray diffraction from a Rietveld analysis [8]. X-ray dif-

fraction patterns were obtained at room temperature with a

Philips PW 1710 diffractometer, furnished with diffracted beam

graphite monochromator. Data were collected by using Cu-Karadiation in the range 10 6 2h 6 120 at a step interval of 0.02.

Rietveld analysis was performed by using the Fullprof program

[9]. According to this analysis, double tempered conditions re-

sulted in higher amounts of retained austenite content, Table 2.

2.2. Pitting examination

Cyclic potentiodynamic measurements (sweeping rate 0.5

mVs1) were performed in a conventional three-electrode cell.

Specimens cut from plate metal and subjected to different HTs as

indicated inTable 2were used as working electrodes. These elec-

trodes exhibiting 1 cm2 apparent area were ground to a mirror-like

finish before the application of the potential perturbation. A mir-

ror-like finish was obtained after initial grinding with grit paper

and diamond paste and subsequent electropolishing in a solution

of HClO4 (62 ml), methanol (700 ml), butyl cellosolve (100 ml)

and H2

O (137 ml). The counter-electrode was a large Pt sheet area.

All electrode potentials were measured and referred to in the text,

tables and figures against a Hg/Hg2SO4, K2SO4 (satd) reference

electrode (0.65 V in the NHE scale).

Experiments were carried out at room temperature (25 2 C)

in 0.05 M K2SO4+ 0.04 M NaCl (pH = 4.1) solution made from ana-

lytical grade reagents and triple-distilled water. Test solution com-

position was originally proposed in the literature[10]for this kind

of stainless steels and specified by the real environment.

A nitrogen stream was purged through the electrolyte solutions

to eliminate dissolved oxygen before each measurement. Inert gas

circulation was maintained above the solution level during the

measurements.

Optic and scanning electron microscopy was applied to observe

relatively large, already stable pits (diameter > 10 lm).

3. Results and discussion

The microstructures of all specimens consist of a typical lath

martensite with different content of retained austenite particles

precipitating either at grain boundaries or inside the martensite

grains (both in lath borders and packet borders). The morphology

of retained austenite corresponds to particle-platelet precipitates

with thickness smaller than 1 lm(Fig. 1).Carrying out subcritical tempering below 600 C (E condition)

renders a ferritic structure that does not contain a relevant amount

of retained austenite (Fig. 2).

The intercritical tempering at 600 C (B and G conditions)

produces martensite decomposition together with precipitation

of a very thin austenite dispersion, since it is known that the pre-

cipitation of austenite takes place at tempering temperatures

slightly higher than Ac1.In the microstructures with double tempering a structural

refinement can be produced. This corresponds to higher austenite

content together with a more uniform distribution of this phase

(H, M and P conditions). Fig. 3 shows this result for M

condition.

X-ray diffraction patterns for M and E conditions are shown

inFigs. 4 and 5, respectively and can be comparatively analyzed.

Fig. 4 corresponds to a sample with retained austenite resulting

from double tempering at 670 + 600 C, while Fig. 5 corresponds

Table 1

Nominal composition (wt.%).

Grade C Mn Si Cr Ni Mo S P

13CrNiMo 0.03 0.6 0.3 12.74 3.71 0.53 0.01 0.008

Table 2

Description of the applied heat treatments.

Condition Heat treatments Temperature (C) Time (h) Austenite content (vol %)

As-received Annealing 6

B Single tempering 600 2 16

E Solution annealing + tempering 950 + 550 1 2

2

G Solution annealing + tempering 950 + 600 1 8

2

H Solution annealing + double tempering 950 + 670 + 600 1 18

2

2

M Solution annealing + double tempering 950 + 670 + 600 1 22

2

8

P Double tempering 670 + 600 2 20

2

P.D. Bilmes et al./ Corrosion Science 51 (2009) 876881 877


3/6

to a sample without retained austenite resulting from single tem-

pering at 500 C.

Pitting occurs on thin films on stainless steel formed by sputter-

ing deposition, i.e. free from non-metallic inclusions etc. [11,12].

Accordingly, it can be stated that breakdown of the passive film

is one of the origins of pit initiation [1315]. In turn, oxide layer

composition varies according to the underlying microstructure.

For stainless steels, heat treatment, grinding, and abrasive blast-

ing are detrimental to pitting resistance, whereas passive films

formed after pickling in HNO3+ HF or passivation in HNO3are ben-

eficial [16]. Heat treatments in air generate a chromium oxide scale

and a chromium-depleted region under the scale. The scale is typ-

ically removed mechanically, and the chromium-depleted region is

removed by pickling[16]. Seminal work in the literature related to

pitting corrosion deals with such aspects as the origins of pitting

corrosion on stainless steels as well as the effects of electrolyte

composition[17,18].

Electrochemical studies of pitting corrosion have shown that

characteristic potentials exist. Stable pits form at potentials noble

to the pitting potential EP, and will grow at potentials noble to

the repassivation potential,ER, which is lower than EP [19].

A typical cyclic potentiodynamic anodic polarization curve is

shown inFig. 6, for a sample according to preparation condition B.

Since EP values exhibit the typical scatter for a pitting process, at

least 20 identical sweeps were performed for each condition, i.e.

for each different heat treatment, as indicated above. The large

set ofEPvalues, obtained for each condition, was analyzed accord-

ing to the usual stochastic approach considering the probability for

pitting P(E) [20,21]

PE n=1 N 1

where Nis the total number of samples studied, and n is the number

of samples that were pitted at a potential ofEor lower. The poten-

tial atP= 0.5 was considered as theEPvalue, representative for the

material and its preparation condition.Fig. 7 shows the derived pit-

ting probability as a function ofEfor a sample according to prepa-

ration condition B. The potential atP= 0.5 is indicated in this figure.With regard toER, these values vary linearly with the logarithm

of the charge density q passed under the hysteresis during the

sweep [22]. The potential for an arbitrarily selected amount of

pit propagation (q= 0.1 C cm2) was considered here as a normal-

ized ERvalue to be used for comparison of the different sample

preparations.

Figs. 8 and 9 show pitting potentials EP and repassivation poten-

tialsER, respectively, plotted against the amount of retained aus-

tenite for the different preparation conditions.

The linear dependence found for both EPand ER, confirms previ-

ous results obtained with weld metal samples[4]. The shift in the

noble direction according to the retained austenite content, deter-

mines an enhanced pitting resistance.

Enhanced pitting resistance corresponds to double temperingPWHT conditions. These results indicate that, under present test

conditions, pitting resistance can be strongly affected by two

microstructural factors, namely, the amount and size of carbides

and the amount of retained austenite[4].

These results can be understood in terms of a structural refine-

ment resulting from two-stage tempering according to the model

previously advanced [23,24]. Single tempering at 600 C renders

a microstructure composed of tempered martensite and some re-

tained austenite. Double tempering, initially at 670 C and then

at 600 C, generates the precipitation of new stable austenite par-

ticles-platelets through newly formed interfaces (fresh martensite/

austenite), while fresh martensite is decomposed into tempered

martensite. Most notably, longer tempering times at 600 C (8 h

for condition M) promotes a higher content of retained austenitethan for the other double-stage tempered conditions (2 h for

Fig. 2. Ferrite matrix for 13CrNiMo plate metals with applied heat treatment

according to the E condition.

Fig. 3. Austenite particles in a ferrite matrix for 13CrNiMo plate metals withapplied heat treatment according to the M condition.

Fig. 1. Austenite particles (white particle-platelets) in a ferrite matrix (gray/black

areas) for 13CrNiMo plate metals.

878 P.D. Bilmes et al./ Corrosion Science 51 (2009) 876881


4/6

conditions H and P). Hence, as a result of this double tempering,

the number of austenite platelets increase and they present a more

uniform distribution. Moreover, because the transformation isdiffusional the austenite is enriched in elements such as nickel,

carbon and nitrogen, what in turn determines dissolution or refine-

ment of carbide or carbonitride particles. The whole process gener-

ates a lower susceptibility to pitting corrosion [4 and referencestherein]. As in the case of surface-melted martensitic stainless steel

Fig. 4. X-ray diffraction patterns of a sample with applied heat treatment according to the M condition.

Fig. 5. X-ray diffraction patterns of a sample with applied heat treatment according to the E condition.



5/6

UNS S42000 the improvement in pitting corrosion resistance re-

sulted from the dissolution or refinement of carbide particles and

the presence of retained austenite, as evidenced by the fact that

pitting and repassivation potentials increased linearly with the

amount of retained austenite[4,25].A comparison between the results in Figs. 8 and 9 and those

previously obtained for weld metal samples [4], shows that both

EP andER values are always more noble for parent (or base) me-

tal samples than for weld metal samples, for each volume frac-

tion of retained austenite. However, the difference in EP values

DEP between the parent metal and the weld metal samples de-

creased with increasing the retained austenite content, while

for contents of P20 wt.% DEP becomes negligible. This finding

supports the viewpoint that the beneficial condition of the par-

ent metal (having a more homogeneous structure, with lower

density of inclusions and other defects acting as pit initiation

sites) becomes less prevailing compared to the beneficial effect

of a high retained austenite content (associated with structural

refinement and an increase in dissolved Cr content [26]). The dif-ference in ER values (DER) decreases with increasing retained

austenite content in a much less marked way. Thus, the relative

influence of austenite content (measured as the ratio of slopes

dE/d[%aust]) is 2.13 for EP while only 1.25 for ER. This reflects

the fact that the beneficial effect of retained austenite on the

repassivation kinetics imposing a stop of growth on stably grow-

ing pits (related to ER) is not as strong as on the presence of pitinitiation sites (related to EP).

Pit size and shape depend on the selected experimental condi-

tions. In the electrochemical tests regular-shaped pits are gener-

ated with hemispherical morphology (Fig. 10). Consequently,

these pits grow without following the crystallographic orientation

of the metal and cross the interface of crystallites. It has been sta-

ted that this is the result of the presence of sulfate in the electro-

lyte[13].

4. Conclusion

The presence and amount of retained austenite as a microstruc-

tural component resulting from the applied heat treatments has a

beneficial effect on the pitting corrosion resistance of both weldand parent metal 13CrNiMo stainless steels. When retained

Fig. 6. Cyclic potentiodynamic polarization curve recorded in de-aerated 0.05 M

K2SO4+ 0.04 M NaCl (pH = 4.1), sweep rate 0.5 mVs1, for preparation condition B.

Pitting and repassivation potentials are indicated in the figure.

Fig. 7. Plot of the cumulative probability P for pitting potential values (Eq. (1)) as a

function of potential, for preparation condition B. The potential at P =0.5 is

representative for the material and its preparation condition.

Fig. 8. Retained austenite (wt.%) vs. pitting potential (EP) for different preparation

conditions as indicated in the text.

Fig. 9. Retained austenite (wt.%) vs. repassivation potential (ER) for different

preparation conditions as indicated in the text.

880 P.D. Bilmes et al./ Corrosion Science 51 (2009) 876881


6/6

austenite is controlled at 2025% both materials exhibit compara-

ble pitting corrosion resistances, given not only by similar critical

potentials for stable pit growth, but also exhibiting analogous

repassivation kinetics.

Acknowledgements

Drs. Gervasi and Llorente are grateful to the Comisin de Inves-

tigaciones Cientficas y Tcnicas Buenos Aires for their positions as

members of the Career of Scientific Researcher.

References

[1] T.G. Gooch, Heat treatment of welded 13%Cr-4%Ni martensitic stainless steelsfor sour service, Welding J. 74 (1995) 213223.

[2] A.W. Marshall, J.C.M. Farrar, Welding of ferritic and martensitic 13%Cr steels,

in: IIW Doc 1998, No. IX-H422-98, International Institute of Welding,

Cambridge, England, 1998, pp. 8.

[3] A. Turnbull, A. Griffiths, Corrosion and cracking of weldable 13 wt-%Cr

martensitic stainless steels for application in the oil and gas industry,

Corrosion Eng. Sci. Technol. 38 (2003) 2150.

[4] P.D. Bilmes, C.L. Llorente, L. Saire Huamn, L.M. Gassa, C.A. Gervasi,

Microstructure and pitting corrosion of 13CrNiMo weld metals, Corrosion

Sci. 48 (2006) 32613270.

[5] P. Sathiya, S. Aravindan, R. Soundararajan, A. Noorul Haq, Effect of shielding

gases on mechanical and metallurgical properties of duplex stainless-steel

welds, J. Mater. Sci. 44 (2009) 114121.

[6] R.S. Huang, L. Kang, X. Ma, Microstructure and phase composition of a low-

power YAG Laser-MAG welded stainless steel joint, J. Mater. Eng. Perform. 17

(2008) 928935.

[7] P. Ferro, A. Tiziani, F. Bonollo, Influence of induction and furnace postweld heat

treatment on corrosion properties of SAF 2205 (UNS 31803), Welding J. 87

(2008) 298306.

[8] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, MA, 1978.

[9] C. Rodrguez, FULLPROF, a program for Rietveld refinement and patternmatching analysis, in: Proc. Satellite Meeting on Powder Diffraction of

Congress of IUCr, Tolouse, France, 1990, pp.127.

[10] S. Hashizume, K. Masamura, I. Matsushima, Pitting corrosion resistance of 13%

Cr steel in oil field environments, Trans. ISIJ 23 (1983) B351.

[11] D.E. Williams, R.C. Newman, Q. Song, R.G. Kelly, Passivity breakdown and

pitting corrosion of binary alloys, Nature 350 (1991) 216219.

[12] M.P. Ryan, N.J. Laycock, R.C. Newman, H.S. Isaacs, The pitting behavior of iron-

chromium thin film alloys in hydrochloric acid, J. Electrochem. Soc. 145 (1998)

15661571.

[13] H. Strehblow, Pitting corrosion, in: A.J. Bard, M. Stratmann (Eds.), Encyclopedia

of Electrochemistry, in: M. Stratmann, G.S. Frankel (Eds.), Localised Corrosion

Phenomena, vol. 4, Wiley-VCH, Weinheim, Germany, 2003, pp. 308343.

[14] K. Sugimoto, in: Advances in Materials Research, in: Y. Waseda, S. Suzuki

(Eds.), Characterization of Corrosion Products on Steel Surfaces, vol. 7,

Springer, Berlin, Heidelberg, 2006 (Chapter 1).

[15] P. Schmuki, H. Bhni, Illumination effects onthe stabilityof thepassive filmon

iron, Electrochim. Acta 40 (1995) 775783.

[16] A.J. Sedriks, Corrosion of Stainless Steels, second ed., John Wiley, New York,

1996.

[17] P.C. Pistorius, G.T.Burstein, Aspects of the effects of electrolyte composition on

the occurrence of metastable pitting on stainless steel, Corrosion Sci. 36 (1994)

525538.

[18] G.T. Burstein, C. Liu, R.M. Souto, S.P. Vines, Origins of pitting corrosion,

Corrosion Eng. Sci. Technol. 39 (2004) 2530.

[19] G.S. Frankel, Pitting corrosion, in: S.D. Cramer, B.S. Covino Jr. (Eds.), Corrosion:

Fundamentals, Testing, and Protection, ASM Handbook, vol. 13A, ASM

International, 2003, pp. 236241.

[20] G.S. Frankel, Pitting corrosion of metals: A review of the critical factors, J.

Electrochem. Soc. 145 (1998) 21862198.

[21] T. Shibata, T. Takeyama, Stochastic theory of pitting corrosion, Corrosion 33

(1977) 243251.

[22] B.E. Wilde, Critical appraisal of some popular laboratory electrochemical tests

forpredicting thelocalized corrosionresistance of stainless alloys in sea water,

Corrosion 28 (1972) 283291.

[23] C.A. Gervasi, P.D. Bilmes, C.L. Llorente, Metallurgical factors affecting localized

corrosion of low-C 13CrNiMo martensitic stainless steels, in: I.S. Wang (Ed.),

Corrosion Res. Trends, Nova Science Publishers, New York, 2007, pp. 134.Chapter 1.

[24] P.D. Bilmes, M. Solari, C.L. Llorente, Characteristics and effects of austenite

resulting from tempering of 13Cr-NiMo martensitic steel weld metals, Mater.

Characterization 46 (2001) 285296.

[25] C.T. Kwok, H.C. Man, F.T. Cheng, Cavitation erosion and pitting corrosion

behaviour of laser surface-melted martensitic stainless steel UNS S42000, Surf.

Coat. Technol. 126 (2000) 238255.

[26] M. Kimura, Y. Miyata, T. Toyooka, Y. Kitahaba, Effect of retained austenite on

corrosion performance for modified 13% Cr steel pipe, Corrosion 57 (2001)

433439.

Fig. 10. SEM micrograph showing hemispherical pits grown on 13CrNiMo plate

metal in 0.05 M K2SO4+ 0.04 M NaCl (pH= 4.1) solution.


Retained Austenite and Pitting

Documents

Transcript of Retained Austenite and Pitting