Effect of Nodular Graphite on the Deterioration of Vibration Fracture Resistance of Ferritic Cast Iron under an Aqueous Environment

Abstract

An intergranular fracture feature could be observed when ferritic spheroidal graphite cast irons were tested under continuously wet conditions. The maximum depth of intergranular fractures was closely related to the microstructural feature and ambient environment that caused the deterioration in vibration fracture resistance. Experimental evidence has confirmed the overall D-N curves can be generalized into three characteristic regions. Intergranular fractures occurred in the vicinity of the surface; the existence of nodular graphite act as porosity should be considered as a dominant microstructural factor on the initiation of intergranular fractures. On the other hand, intergranular fractures can be prevented resulting in better vibration fracture resistance when a specimen of ferritic SG cast iron is covered with oil film, or if an identical test is performed using the Fe-3Si steel specimen containing no nodular graphite.

Tensile properties of ADI material in water and gaseous environments

“…More important, Martinez et al [15] further reported reduction of ductility by 73-83% in water, 29-47% in SAE 30 mineral lubricant oil, and 67% in isopropyl alcohol (99%). Other studies [10][11][12][13][14]16] reported similar results. Furthermore, several authors [10,11] reported that this phenomenon is revertible, that is, after drying, the specimens revert to their previous tensile properties.…”

“…To further distinguish investigated ADI materials in dry condition compared to materials used in literature [10][11][12][13][14][15][16], impact test and hardness were determined. The impact energy was measured for as-cast ductile iron and ADI material in dry condition on unnotched Charpy specimens (10 × 10 × 55 mm) using a Charpy impact testing machine Schenck-Treble RPSW/A.…”

“…Recently, it was noticed that ADI materials in contact with water and other liquids, such as various types of lubricant, exhibit a drop in strength and more critically, ductility [9][10][11][12][13][14][15][16][17]. This behavior may potentially lead to a catastrophic failure in exploitation as strength and elongation of standard ADI grades (ASTM A897M-06: minimum tensile strength from 750 to 1600 MPa, and elongation from 11 to 1%, respectively) could be severely reduced.…”

“…There are several explanations for degradation of the tensile properties. The most common is hydrogen embrittlement [10,12,14,15]. However, in steels, hydrogen embrittlement is most pronounced in the case of ferritic microstructure and martensitic microstructure, and it is slow diffusion process enhanced at slow strain rates [18][19][20].…”

Relation between Surface Tension and Graphite Shape in Cast Iron

“…1,2) To effectively improve the quality of the castings and reduce the amount of rejected materials, rapidly monitoring the graphite shape of cast iron is becoming very important.…”