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Corrosion Fatigue Crack Initiation in Duplex Stainless Steel Paper Making Components

Published

Author(s)

Mark R. Stoudt

Abstract

The pulp and paper industry is frequently impacted by the enormous cost of replacing prematurely failed suction roll shells and other metallic components. The results of a great deal of research has lead to numerous improvements in the models used for life prediction and to guide alloy development, but a disparity still exists between laboratory scale life prediction methods and the actual service performance and this disparity is an inherent problem hindering the development of new materials. Sandusky International produced two duplex stainless steel alloys with very similar compositions and the performance predictions based on current life predictions methods indicate that the behaviors of these alloys should be similar in service. However, in the ten year period since the development of these alloys, Sandusky has not experienced a single service failure with one alloy while the other continues to fail prematurely. An evaluation was undertaken to determine the fundamental factors in this difference in performance.Two hypotheses were adopted for this study. The first assumed that a problem existed in the measurement method used to generate the data used in the life prediction models or that this data was deficient in some fashion. The results of that evaluation revealed no reasonable expectation that a modification of the experimental parameters used to predict crack propagation would yield a cause for the reported discrepancy in performance between the two Sandusky alloys. The second hypothesis evaluated whether a process could promote cracking in one alloy and not the other. The results of this evaluation showed that overall, Alloy 75 and Alloy 86 have quite similar electrochemical and mechanical behaviors. The only significant difference observed between the two alloys in an aerated, simulated deinked white water environment was a fairly wide separation between the free corrosion and pitting potentials in the alloy that has not exhibited premature failure (Alloy 86). In general, a small separation between these two potentials establishes a high probability for pitting during service and a wide separation denotes a low probability for pitting under the same corrosion conditions.Slow strain rate tensile tests were used to further evaluate whether the resistance to pitting is the primary factor that determines the performance of these alloys in a paper making environment. The results of that analysis revealed that whenever pitting did occur, the failure mode was identical regardless of the alloy examined. Failure was always due to a reduction in cross sectional area caused by the formation and propagation of pitting which dissolved the more ductile phase (austenite). This action resulted in a simultaneous decrease in the fracture toughness of the alloy and an increase in the applied stress intensity and rapid failure occurred when the stress intensity reached the value of KIscc for the alloy. An analysis of an actual service failure supported this finding. The main conclusion drawn from this study was: the discrepancy in the performance between the two Sandusky duplex stainless alloys was a direct result of the predictive models not properly addressing the role of pitting corrosion in the fatigue crack initiation process.
Citation
NIST Interagency/Internal Report (NISTIR) - 6309
Report Number
6309

Keywords

corrosion, corrosion fatigue, duplex stainless steel, fatigue crack propagation, papermaking, stress corrosion cracking, white water

Citation

Stoudt, M. (1999), Corrosion Fatigue Crack Initiation in Duplex Stainless Steel Paper Making Components, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD (Accessed April 21, 2024)
Created March 1, 1999, Updated October 16, 2008