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


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at a rate faster than those in air, and at about \u2206K = 12 MPa m (11 ksi in ), the fatigue crack growth 
rate in hydrogen began to exceed the crack growth rate in air and kept exceeding it at the higher levels of \u2206K. Although 
the precise mechanism for this unusual behavior is not known, it was postulated that at the lower value of \u2206K water from 
the moist air could be adsorbed and dissociated at the crack tip to supply atomic hydrogen to the steel faster than could be 
supplied by the dissociation of the hydrogen gas. However, at \u2206K levels exceeding 12 MPa m (11 ksi in ) the 
dissociation of gaseous hydrogen was faster and more efficient in embrittling the steel than the dissociation of water. 
Another interesting observation in this material was the appearance of the fracture surface. Although the fatigue fractures 
in air were transgranular at all levels of \u2206K, the fractures in hydrogen exhibited transgranular and intergranular fracture, 
with the intergranular fracture being a function of \u2206K. The fatigue fractures in hydrogen were transgranular in the near-
threshold region, the amount of intergranular fracture increased until it peaked at 30% at \u2206K 11 MPa m (10 ksi in ) 
at R = 0.1, then gradually decreased again to near zero at the highest levels of \u2206K (Ref 220). There was no correlation 
between the amount of intergranular fracture and the rate at which the fatigue cracks propagated. 
The general effect of hydrogen on austenitic grades of stainless steels follows the trends observed in most heat-treatable 
steels. For example, when compared to air, the fatigue crack growth rate in hydrogen gas for an annealed austenitic type 
302 stainless steel showed a threefold increase, and a type 301 stainless steel exhibited as much as a tenfold increased in 
the fatigue crack growth rate when tested at mean stress of 66 MPa (9.6 ksi) and a stress intensity factor range of \u2206K = 60 
MPa m (54.5 ksi in ) (Ref 217). The fatigue tests were conducted at 25 °C (75 °F) in laboratory air and in dry hydrogen 
gas, which was at slightly above atmospheric pressure, with a stress intensity range of about \u2206K = 50 to 70 MPa m (45.5 
to 64 ksi in ), a mean stress of 66 and 72 MPa (9.6 and 10.4 ksi), a load ratio of R = 0.05, and a cyclic frequency of 0.6 
Hz. Although the actual test conditions in this investigation covered a much broader range of \u2206K and mean stresses, 
narrower testing parameters were selected for this discussion in order to overlap the air and hydrogen data. 
The fracture surfaces of annealed type 301 and 302 austenitic stainless steels are shown in Fig. 82. The type 302 stainless 
steel exhibited fatigue striations in all atmospheres, but the type 301 stainless steel formed few striations under any 
fatigue condition. In the presence of hydrogen gas, however, the type 301 stainless steel fatigue fractures exhibited a 
significant number of relatively flat, cleavagelike facets and smaller, parallel plateaus of facets, each at a different 
elevation, forming a stepped structure. The difference in the response between the two stainless steels was due to the 
presence of unstable austenite in the type 301 stainless steel, which could transform to martensite as a result of strain at 
the crack tip. Also, cold-worked (19 to 40% reduction in thickness) material of both stainless steels was more susceptible 
to the effects of hydrogen than the annealed material. This was also due to the presence of strain-induced martensite in the 
cold-worked stainless steels, which showed no fatigue striations under any testing condition (Ref 217). 
 
Fig. 82 Fatigue fracture surfaces of annealed type 301 and type 302 stainless steels tested at 25 °C (75 °F) in 
1 atm hydrogen gas. The type 302 stainless steel (a) showed well-developed fatigue striations. The type 301 
stainless steel (b) showed a more brittle-appearing fracture surface with few striations but containing flat, 
cleavagelike facets and small, parallel facets at different elevations joined by tear ridges. Source: Ref 217 
Effect of Gases on Nonferrous alloys. Steels are not the only alloys affected by gaseous environments; some 
aluminum alloys are also susceptible. When a 2219-T851 aluminum alloy was fatigue tested at room temperature within a 
stress intensity range of \u2206K = 10 to 24 MPa m (9 to 22 ksi in ), a load ratio of R = 0.05, and a cyclic frequency of the 5 
Hz in 0.2-torr water vapor and 760-torr (1-atm) dry argon gas (at a cyclic test frequency of 20 Hz), the fatigue crack 
growth rate in the water vapor was three times greater than that in the argon gas (Ref 225). The increase in the crack 
propagation rate was attributed to hydrogen embrittlement and was essentially equivalent to the fatigue rates observed 
when the 2219-T851 aluminum alloy is tested in 40 to 60% humidity air, distilled water, and even a 3.5% sodium chloride 
solution (Ref 225, 226). A significant change in the fatigue fracture appearance (Fig. 83) accompanies the increase in the 
fatigue crack growth rate. 
 
Fig. 83 Effect of water vapor on the fatigue fracture appearance of a 2219-T851 aluminum alloy tested at room 
temperature. (a) 760-torr dry argon, \u2206K = 16.5 MPa m (15 ksi in ), R = 0.05, and f = 20 Hz. (b) 0.2-torr 
water vapor, \u2206K = 16.5 MPa m (15 ksi in ), R = 0.05, and f = 5 Hz. The magnifications are too low to resolve 
the fatigue striations clearly, but the general change in the fracture morphology is apparent. The crack 
propagation direction is from left to right. Source: Ref 225 
The nickel-base superalloys are not immune to the effects of reactive atmosphere. Although hydrogen showed no effect, 
gases containing hydrogen sulfide and sulfur dioxide increased the crack propagation rate substantially in an Inconel alloy 
718 that had received a modified heat treatment that improved the notch rupture properties of the material (Ref 227). 
The fatigue tests were conducted at 650 °C (1200 °F) at slightly above atmospheric pressure in dry hydrogen gas, dry 
helium gas, helium with 0.5% hydrogen sulfide, helium with 5% sulfur dioxide, air, and air with 0.5 and 5% sulfur 
dioxide (other gas atmospheres were also tested). The approximate stress intensity range was \u2206K = 40 to 70 MPa m 
(36.5 to 63.5 ksi in ), the load ratio R = 0.1, and the cyclic frequency 0.1 Hz. At a stress intensity range of \u2206K = 40 
MPa m (36.5 ksi in ), the fatigue crack growth rate in air was five times greater than that in dry helium gas, and at \u2206K 
= 60 MPa m (54.5 ksi in ), the crack growth rate in air containing 5% sulfur dixoide was about four times greater than 
that in air. 
The greatest increase, however, was observed when testing at a stress intensity range of \u2206K = 40 MPa m (36.5 ksi in ). 
in atmospheres containing helium with 0.5% hydrogen sulfide and helium with 5% sulfur dioxide, which increased the 
fatigue crack growth rate by almost a factor of 30 over the fatigue fractures produced in air that contained 0.5% sulfur 
dioxide, the fracture surfaces produced in sulfur-containing atmospheres were covered with thick, adherent scales that 
obscured the fracture detail. The fracture surfaces of specimens tested in hydrogen gas exhibited distinct fatigue striations 
whose spacing approximated the fatigue crack growth rate. The fatigue fractures in air consisted of intergranular 
decohesion among the principally transgranular fracture, which contained fatigue striations with spacings approximating 
the fatigue crack growth rate. Although there was no significant difference in the fatigue propagation rates between the air 
and air containing 0.5% sulfur dioxide, the fractures produced in the latter atmosphere showed no fatigue striations and 
consisted of intergranular decohesion and flat, transgranular facets. All atmospheres that increased the fatigue crack 
growth rate tended to promote intergranular fracture, and atmospheres containing hydrogen sulfide were more aggressive