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Influence of Sulfur and Ferrite on SCC and Corrosion Fatigue Behavior of

Model Heats of Stainless Steel

Elaine West, Carl Tackes, George Newsome, Nathan Lewis Acknowledgements: Terry Nolan, Don Redmond, John Mullen, Robb Morris, Kyle Smith

17th International Conference on Environmental Degradation of Materials in Nuclear Power

Systems-Water Reactors

August 9-13, 2015

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Background • Despite controls on material composition invoked with ASTM standards,

variability exists in the stress corrosion cracking (SCC) and corrosion fatigue (CF) resistance of stainless steels.

• Improved understanding of the influence of specific impurities and minor alloying elements could be used to develop more precise SCC and CF crack growth rate (CFCGR) models.

• Isolating the influence of specific elements in commercial heats is difficult due to variability in other elements and material microstructure.

• This study isolates the effect of sulfur and ferrite on crack growth in hydrogenated deaerated water through the generation and testing of model heats of 304L stainless steel with systematically controlled compositions.

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Material Characterization • Materials used for hydrogenated deaerated water SCC experiments underwent single pass

cold rolling to impart 19-20% cold work (CW). • Post-processing microstructural analyses of the model heats (E5174-M0,M2,M4) revealed

they had acceptable and consistent microstructures. • MnS inclusions were detected in area fractions <0.1% in heats with sulfur additions • Average ASTM grain size of 4 • Average Knoop microhardness of 157± 2 HK (exception: E5174-M3 was 167 HK)

Stainless Steel Composition (wt%) Heat ID E5174 - As Received E5174-M0 E5174-M1 E5174-M2 E5174-M3 E5174-M4C 0.024 0.021 0.024 0.022 0.021 0.020Mn 1.49 1.54 1.52 1.54 1.32 1.54P 0.016 0.017 0.016 0.010 0.014 0.016S 0.001 <0.001 0.014 0.012 0.002 0.006Si 0.49 0.43 0.42 0.42 0.38 0.43Cr 18.41 18.28 18.37 18.15 19.52 18.22Ni 9.55 9.53 9.55 9.56 8.29 9.65Co 0.03 0.02 0.03 0.02 0.02 0.02Cu 0.29 0.30 0.31 0.30 0.27 0.30Mo 0.02 0.025 0.02 0.03 0.03 0.03N 0.09 0.08 0.08 0.08 0.06 0.07Ti <0.01 0.005 0.01 0.01 0.01 <0.01Al 0.03 0.01 0.01 0.025 0.01 0.01B 11ppm 15ppm 10 ppm 20ppm 20 ppm 20ppmV 0.04 0.04 0.04 0.04 0.04 0.04Nb (Cb) 0.03 0.04 0.04 0.04 0.03 0.03O --- 0.005 0.003 0.002 0.004 0.005Fe Bal. Bal. Bal. Bal. Bal. Bal.

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SCC Experiments Corrosion Fatigue – High R Experiments Corrosion Fatigue – Low R Experiments

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SCC Experiments • Environment: 338°C deaerated water, 50 cc/kg hydrogen

• Material Condition: 19-20% CW • Test Mode: Active Load • Loading Conditions (1) nominal K value of 38 MPa√m (2) nominal K value of 27 MPa√m with 8.6 periodic partial unloads (PPUs) per day with R=0.7 • Specimen type: 0.6T CTs • Precrack final Kmax value of ≤ 20 MPa√m

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Results of SCC Testing with no PPUs

The SCC growth rates of the heats with sulfur additions were 0.2-14% of the rates measured for the low sulfur heat E5174-M0 (<0.001 wt% S).

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Results of SCC Testing with PPUs

The SCC growth rates of the as-received heat and remelted + reprocessed heats were consistent with one another and with expectations based on testing other heats of 304 and 304L stainless steel.

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SCC Measurement Summary

Influence of ferrite on SCC growth is small or negligible

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Analysis of SCC Tips

The crack tips of elevated sulfur heats were wider and filled

with more oxide relative to the low

sulfur heat

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Detection of Sulfur Enrichment

Sulfur enrichment occurs at metal oxide

interface due to dissolution of MnS

inclusions

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SCC Experiments Corrosion Fatigue – High R Experiments Corrosion Fatigue – Low R Experiments

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High R Corrosion Fatigue Experiments • Environment: 338°C deaerated water, 30 cc/kg hydrogen

• Material Condition: Annealed

• Test Mode: Active Load

• Nominal Loading Conditions •R=0.7 •∆K=7 MPa√m •τR=5.1-5,100 seconds

• Specimen type: 0.6T CTs • Precrack final Kmax value of ≤ 20 MPa√m

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Effect of Sulfur on Corrosion Fatigue

Crack growth rates of heats with sulfur additions show

increasing deviation (slower) from model predictions with

increasing rise time

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SCC Experiments Corrosion Fatigue – High R Experiments Corrosion Fatigue – Low R Experiments

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Low R Corrosion Fatigue Experiments • Environment: 338°C deaerated water, 30 cc/kg hydrogen

• Material Condition: Annealed

• Test Mode: Active Load

• Nominal Loading Conditions (Phase 1, Phase 2, Phase 3) •R=(0.1, 0.1, 0.5) •∆K=(12, 13, 7 MPa√m) •τR= (813, 960, 920 sec)

• Specimen type: 0.6T CTs • Precrack final Kmax value of ≤ 20 MPa√m

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Effect of Sulfur on Corrosion Fatigue

*Crack growth rates were determined from in-situ EPD measurements and have not been DE corrected.

Sulfur dependence is also observed under

lower R loading conditions, but all

rates are slower than model predictions.

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Crack Paths of Low R Test Specimens

• Crack paths of low R fatigue specimens were analyzed to determine if there were signs of crack closure.

• Heavier oxide coverage was observed in the 0.006 wt% sulfur specimens.

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Simple Modeling Work to Investigate Closure

K = stress intensity factor Syc = cyclic yield strength E = elastic modulus v = Poisson’s ratio uclosure = thickness of the feature that is causing closure.

Closure Calculations

Then plugged effective ∆K, τR, and R into Mills stainless steel fatigue crack growth rate model(1)

References: (1) WJ Mills, “Accelerated and Retarded Corrosion Fatigue Crack Growth Behavior of 304 Stainless Steel in an Elevated Temperature Aqueous Environment,” in Proceedings of the 16th

International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, Asheville, NC, 2013.

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EPD Signal Suggests Potential Crack Closure

• EPD active signal evolved during the first test phase conducted under constant K loading conditions.

• Saturation of EPD active signal with increasing load could be indicative of crack closure.

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Calculations from EPD Closure Measurements

Measured corrosion fatigue crack growth rate decreases and saturates with ∆Keff estimated from the

EPD closure measurements.

Future work is required to validate EPD indicated closure measurements by comparing results to compliance measurements of closure.

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Conclusions • Sulfur is effective in reducing stainless steel SCC and corrosion fatigue crack growth

in DW. – SCC experiments on stainless steel model heats with sulfur additions showed that sulfur consistently

reduced the crack growth rates of 20% cold worked material by factors of 3X - 20X in 338°C DW. – Corrosion fatigue crack growth rate testing showed that sulfur additions resulted in improvement

factors that ranged from 1.5X to >50X and were strongly dependent on the loading condition.

• Reduction in crack growth rates in the higher sulfur content materials was associated with thicker crack tip oxide films suggesting that the mechanism of crack retardation may be related to the effect that sulfur has on stainless steel corrosion behavior.

– Environmentally assisted creep or oxide induced crack closure may be influencing crack retardation behavior.

• Crack closure effects appear to be influencing crack growth rates under low stress ratio loading conditions.

– Future work may reveal whether variability in closure effects contributes to the differences in crack growth rates between high and low sulfur content model heats of stainless steel.

• Ferrite additions of approximately 2% did not have a large influence on SCC growth rates of 20% cold worked 304/304L stainless steel in 338°C DW.

References: (1) WJ Mills, “Accelerated and Retarded Corrosion Fatigue Crack Growth Behavior of 304 Stainless Steel in an Elevated Temperature Aqueous Environment,” in Proceedings of the 16th

International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, Asheville, NC, 2013.

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Backup Slides

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Results of SCC Validation Test

The SCC growth rates of the as-received heat and remelted + reprocessed heats were consistent with one another and with expectations based on testing other heats of 304 and 304L stainless steel.

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Results of Corrosion Fatigue Validation Test

Corrosion fatigue crack growth rates of as-received heat and remelted + reprocessed heat were consistent with one another and with expectations based on testing other heats of 304 and 304L stainless steel.

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Summary of Results from Corrosion Fatigue Testing