What is Corrosion Fatigue?
Corrosion fatigue is the combined action of fatigue and corrosion. It can produce a failure in fewer cycles and lower loads than if either corrosion or fatigue were acting alone. Of all corrosion failure mechanisms, it is the most difficult to identify especially in the presence of low-frequency loading. At very low frequency loads, such as when there is a conveyor push or shear pass, corrosion fatigue failures may sometimes exhibit the same fractographic features as a stress-corrosion crack, i.e. intergranular fracture mode as opposed to the usual transgranular fracture mode typical for fatigue.
The conditions under which corrosion fatigue occurs can depend on a specific combination of material, cyclic loading (frequency and stress) and environment. It is the synergistic effect of fatigue and stress-corrosion cracking acting together that can lead to greater degradation in material and load-carrying capacity than either acting alone. Corrosion fatigue can reduce the effective fatigue limit by as much as a factor of 10. Similar failures have occurred in unused components, stored outdoors and fabricated of high hardness/strength abrasion resistant steel.
Case Study: AFC Conveyor Pan Top Deck Plate Failure
Cracks were detected in the top deck plate of an armored-faced conveyer (AFC) used in a longwall mining system for coal. It had been in service on five panels and had conveyed approximately 10M raw tons before the cracks were noticed in the top deck plate. Visual examination of the pan line revealed that most of the cracks were coincident with the chain tracks. An investigation into the cause of the cracks included chemical analysis, determination of hardness/strength and Charpy V-notch (CVN) impact properties as well as fractographic and metallographic studies using optical and scanning electron microscopy.
When the conveyer was fabricated, two grades of abrasion-resistant (AR) alloy steel were used for the deck plates. One grade was specified to have hardness of about 450 HB and the other 500 HB. Results of the chemical analysis confirmed that some of the pan decks were fabricated from AR 450 and others from an AR 500 grade. However, the Material Test Certificates (MTC) revealed that carbon content was the only significant difference in the chemical composition of the plates.
When the pans were fabricated, the vendor plasma-cut the deck plates to obtain maximum yield from the plates. This resulted in the longitudinal (L) direction of the hot-rolled plate becoming the transverse (T) direction of the plate. In other words, the T properties of the deck plates were the L properties of the hot-rolled plates.
Tensile tests were conducted to determine if there was any significant difference in longitudinal and transverse properties between the failed deck plates and if they differed significantly from plate material recently supplied by the vendor. Test results revealed that there was no significant difference between the AR 450 and AR 500 exemplars recently supplied, or the RAEX 500 used for the failed deck plates.
Within their respective group (tensile, CVN, deck plates), the fractographic features were found to be similar. Scanning electron microscopy (SEM) revealed that the tensile and CVN fractures exhibited a predominantly ductile mode. However, after the corrosion products were removed, SEM analysis of areas of the opened cracks indicated the presence of intergranular (IG) fracture, characteristic of stress corrosion cracking (SCC).
Metallographic examination of multiple transverse cross sections revealed the presence of a uniform martensitic microstructure and multiple intergranular (IG) cracks that initiated independently at corrosion pits on the chain contact surface of the deck plate. The absence of IG fracture in the tensile and CVN impact test specimens and the IG fracture morphology of all of the service-induced cracks was consistent with both stress corrosion and corrosion fatigue.
To learn more about the effects of mine waters on carbon steel coupons as well as forged splice link failures in longwall mining, read my article in Coal International Magazine “Impact of Corrosion on Mining Equipment,” March/April 2014. This article was originally presented at Aachen 2013 Fourth International Symposium, Mineral Resources and Mine Development.