Fatigue crack growth analysis in porous ductile cast iron microstructure

In this paper, we explore the effect of inoculants and solidification time on the mechanical properties of an EN-GJS-400-type ferritic ductile cast iron (DCI). For this purpose, static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests are carried out on three different material conditions. They are produced under fast cooling (solidification time 2h45min), representative of thin walled castings, and very slow cooling (solidification time 10—13h), representative of thick walled castings, this latter with and without the addition of Sb. It has been found that the long solidification time leads to an overgrowth and degeneration of the spheroidal graphite nodules. The addition of Sb avoids the formation of chunky graphite observed in the slowly cooled condition but results in large exploded graphite nodules. These effects impact negatively on tensile strength, total elongation and fatigue strength. Conversely, the resistance to fatigue crack growth is even superior and the fracture toughness comparable to that of the fast cooled condition. Metallurgical and fractographic analyses are carried out to explain this behaviour.

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Effect of Test Frequency on Hydrogen-Enhanced Fatigue Crack Growth in Type 304 Stainless Steel and Ductile Cast Iron

Volume 6B: Materials and Fabrication ◽

10.1115/pvp2016-63536 ◽

2016 ◽

Cited By ~ 2

Author(s):

Hisao Matsunaga ◽

Takuya Nakashima ◽

Kosei Yamada ◽

Takashi Matsuo ◽

Junichiro Yamabe ◽

...

Keyword(s):

Stainless Steel ◽

Fatigue Crack ◽

Cast Iron ◽

Fatigue Crack Growth ◽

Crack Growth ◽

Ductile Cast Iron ◽

Hydrogen Gas ◽

Test Frequency ◽

Internal Hydrogen ◽

Type 304

The effect of hydrogen gas environment (external hydrogen) and hydrogen-charging (internal hydrogen) on the fatigue crack growth (FCG) in two materials, austenitic stainless steel Type 304 and ductile cast iron, was investigated at various test frequencies. The pressure of hydrogen gas was 0.7 MPa. Both in the tests of external hydrogen and internal hydrogen, ratio of hydrogen-induced FCG acceleration was gradually increased with a decrease in test frequency in the range of 10 ∼ 0.1 Hz, and then peaked out at 0.1 ∼ 0.01 Hz. The frequency at the maximum acceleration was dependent on materials and test types (i.e. external hydrogen or internal hydrogen). It has been pointed out that, in the test of external hydrogen, a small amount of oxygen impurity contained in hydrogen gas, if any, adsorbs on newly-created crack surface, which inhibits hydrogen penetration into the material near crack tip. Lower frequency allows longer time for oxygen adsorption, and consequently, hydrogen-induced acceleration cannot be prominent at very low frequencies (e.g. 0.001 Hz). However, in this study, similar frequency dependence of hydrogen-induced FCG acceleration was also observed in the case of internal hydrogen. The results inferred the presence of another mechanism producing the frequency dependence of hydrogen-induced FCG acceleration, i.e. hydrogen-induced slip localization dominated by the gradient of hydrogen concentration ahead of crack tip.

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