ASM Metals HandBook Volume 12 - Fractography
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ASM Metals HandBook Volume 12 - Fractography


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or intergranular decohesion fracture 
modes. When a fatigue crack advances by one of these other fracture modes, the fracture segments technically are not 
fatigue fractures. However, because the occurrence of these fracture modes is a natural consequence of a fatigue crack 
propagating under the influence of a specific environment, these mixed fracture modes can be considered as valid a part 
of a fatigue fracture as fatigue striations. 
The effect of environment of fatigue fracture is divided into five principal categories: 
· Effect of gaseous environments 
· Effect of liquid environments 
· Effect of vacuum 
· Effect of temperature 
· Effect of loading 
The effect of these environments on fatigue fracture will be discussed, and where applicable, the effects will be illustrated 
with fractographs. 
Effect of Gaseous Environments. If an environment such as air or hydrogen gas affects a material under static load 
conditions, it will generally affect the rate at which fatigue cracks propagate in that material, especially at low fatigue 
crack growth rates when the environment has the greatest opportunity to exert its influence. The effect of hydrogen on 
steels of various compositions and strength levels is to increase the fatigue crack growth rate (Ref 216, 217, 218, 219, 
220, 221, 222). When compared to dry air or an inert gas atmosphere at equivalent cyclic load conditions, hydrogen in 
steels accelerates the Stage II crack growth rate, often by a factor of ten or more (Ref 216, 217, 218, 221) and promotes 
the onset of Stage III and premature fracture (Ref 218, 223, 224). Depending on the degree of embrittlement, the effect of 
hydrogen on the fracture appearance can range from one that is barely perceptible to one that exhibits brittle striations; 
however, the more common effect is the addition of quasi-cleavage, cleavage, or intergranular decohesion to the fracture 
modes visible on the fatigue fracture surfaces. The basic embrittlement mechanisms responsible for these changes are 
essentially the same as those discussed in the section "Effect of Environment on Dimple Rupture" in this article. 
Effect of Gases on Steels. Fatigue testing of an API-5LX, grade X42 (0.26C-0.82Mn-0.014 Si-0.02Cu), 511-MPa 
(74-ksi) ultimate strength pipeline steel in hydrogen resulted in up to a 300-fold increase in the fatigue crack growth rate 
as compared to the crack growth rate in nitrogen gas (Ref 218). The fatigue tests were conducted at room temperature in 
dry nitrogen and dry hydrogen atmospheres (both at a pressure of 6.9 MPa, or 1 ksi), using a stress intensity range of \u2206K 
= 6 to 20 MPa m (5.5 to 18 ksi in ), a load ratio of R = 0.1 to 0.8, and a cyclic frequency of 1 to 5 Hz. An example of 
the effect of hydrogen on the fracture appearance is illustrated in Fig. 80. The fatigue fracture in hydrogen showed more 
bands of intergranular decohesion, which was associated with the ferrite in the microstructure, and fewer regions of a 
serrated fracture than the specimens tested in nitrogen. The mechanism responsible for the serrated fracture was not 
established; however, more than one mechanism may be involved. Compared to nitrogen, the serrated fracture in 
hydrogen exhibited little deformation, and at high values of \u2206K, the serrated areas resembled cleavage or quasi-cleavage 
(Ref 218). 
 
Fig. 80 Effect of hydrogen on the fatigue fracture appearance of a grade X42 pipeline steel. (a) Tested in dry 
nitrogen gas at a stress intensity range of \u2206K = 20 MPa m (18 ksi in ), a load ratio of R = 0.1, and a cyclic 
frequency of 5 Hz. (b) Tested in dry hydrogen gas at a stress intensity range of \u2206K = 10 MPa m (9 ksi in ), a 
load ratio of R = 0.1, and a cyclic frequency of 1 Hz. Both room-temperature fatigue tests resulted in a fatigue 
crack growth rate of approximately 2 × 10-4 mm/cycle. The fatigue fracture in nitrogen exhibited principally a 
serrated transgranular fracture, along with occasional bands of intergranular decohesion (not shown); however 
the fracture in hydrogen exhibited fewer regions of serrated fracture and more bands of intergranular 
decohesion. Source: Ref 218 
Although testing of the grade X42 pipeline steel in hydrogen increased the fatigue crack growth rate by a factor of nearly 
300 over that in nitrogen gas, precharged ASTM A533B class 2 (0.22C-1.27Mn-0.46Mo-0.68Ni-0.15Cr-0.18Si) 790-MPa 
(115-ksi) ultimate tensile strength commercial pressure vessel steel showed only a maximum fivefold increase in the 
fatigue crack growth rate as compared to uncharged specimens (Ref 223). Lightly charged (240 h at 550 °C, or 1020 °F, 
in 17.2 MPa, or 2.5 ksi, hydrogen gas) and severely charged (1000 h at 550 °C, or 1020 °F, in 13.8 MPA, or 2 ksi, 
hydrogen gas) specimens were both fatigue tested in air at room temperature with a stress intensity range of \u2206K = 7 to 50 
MPa m (65.5 ksi in ), a load ratio of R = 0.05 to 0.75, and a cyclic frequency of 50 Hz. 
Compared to uncharged material, the lightly charged specimens were found to show only a moderate increase in the 
fatigue crack growth rate and only at crack propagation rates of less than 10-6 mm/cycle. There was no significant change 
in fracture appearance. The severely charged material showed a large decrease in mechanical properties: the 0.2% yield 
strength decreased from 660 to 242 MPa (96 to 35 ksi), the ultimate tensile strength from 790 to 315 MPa (115 to 46 ksi), 
the percent elongation from 22.4 to 9%, and the percent reduction of area from 73 to 5%. However, there was only a 
slight increase in the fatigue crack growth rate at growth rates of less than 10-6 and greater than 10-5 mm/cycle, although 
the fatigue fracture surfaces showed evidence of substantial hydrogen attack in the form of cavitated intergranular fracture 
(Fig. 81). The cavities on the intergranular facets were due to the formation of methane gas bubbles or cavities at the grain 
boundaries. 
 
Fig. 81 Effect of severe hydrogen attack on the fatigue fracture appearance of an ASTM 533B pressure vessel 
steel. The severely charged material was tested at room temperature at a stress intensity range of \u2206K = 20 
MPa m (18 ksi in ), a load ratio of R = 0.1, and a cyclic frequency of 50 Hz. The fracture exhibited fatigue 
striations (upper left) as well as cavitated intergranular facets resulting from methane gas bubble formulation 
at grain boundaries. Arrow indicates crack growth direction. Source: Ref 223 
The reaction of hydrogen with the carbon in the steel results in methane gas production and depletion of carbon from the 
matrix (decarburization). The surprisingly small effect that the severe hydrogen damage had on the fatigue crack growth 
rate was due to the competing effects of two opposing processes. The accelerating effect of the hydrogen damage was 
nearly offset by the decarburization softening-induced enhancement of crack closure (the premature contact between 
crack walls during the declining-load portion of the fatigue cycle), which lowers the crack tip stress intensity (Ref 223). 
Another factor that contributed to a lowering of the crack propagation rates was the tortuous crack path resulting from 
grain-boundary cavitation. 
A unique effect was observed in an iron-nickel-chromium-molybdenum-vanadium (0.24C-3.51Ni-1.64Cr-O.39Mo-
0.11V-0.28Mn-0.01Si) 882-MPa (128-ksi) ultimate tensile strength rotor steel that was fatigue tested in air and hydrogen. 
The fatigue tests were conducted at 93 °C (200 °F) in 30 to 40% relative humidity air and in 448-kPa (65-psi) dry 
hydrogen gas at a stress intensity range of \u2206K = 3 to 30 MPa m (2.5 to 27 ksi in ), a load ration of R = 0.1 to 0.8, and 
cyclic frequency of 120 Hz. From near threshold (\u2206K 3 MPa m , or 2.5 ksi in ) fatigue conditions, the fatigue crack 
growth rate in hydrogen was found to be lower than in air (Ref 220). As the \u2206K increased, the crack growth rates in 
hydrogen increased