Effect of Metal Flaws

8.4.2 Effect of Metal Flaws

Nuclei for hydrogen cracking of steels form at the interface of precipitated phases (e.g., Fe 3 C or intermetallic compounds, such as those occurring in maraging steels) by separation from the plastically deformed matrix. Nuclei of perhaps Fe 3 C in low - carbon 10% Ni – Fe alloys can be oriented by cold rolling, thereby greatly improving resistance to hydrogen cracking of specimens stressed parallel to the rolling direction, but not of specimens stressed at right angles [61] . The role of internal lattice fl aws on hydrogen cracking explains in part the resistance of stressed pure iron to cracking despite its becoming hydrogen - embrittled. On the other hand, pure carbon – iron alloys crack readily.

In steels, nonmetallic inclusions — for example, MnS — provide internal fl aws where hydrogen blisters often nucleate. The blisters contribute to pipeline fail- ures by H.I.C. [53] , which occurs in three steps:

1. Formation of hydrogen atoms at the steel surface and adsorption on the surface.

2. Diffusion of hydrogen atoms into the steel substrate.

3. Accumulation of hydrogen atoms at hydrogen traps, such as voids around inclusions in the steel matrix, leading to increased internal pressure, crack initiation and propagation, and linkage of separate cracks.

Internal cracks are nucleated where stress intensity is highest, such as at elon- gated inclusions oriented parallel to the rolling direction; elliptically and spheri- cally shaped inclusions are less damaging. Eventually, in fi nal failure, cracks appear at right angles, linking the cracks that previously developed at the elon- gated inclusions. H.I.C. requires only a steel with a susceptible microstructure (typically elongated sulfi de inclusions) and suffi cient hydrogen to cause cracking.

H.I.C. can occur in the absence of any applied or residual stress in the steel other than the hydrogen pressure in traps. Under the infl uence of an applied stress, failure can occur by S.O.H.I.C., similar to H.I.C. except that the morphology of cracking is different. Whereas in

H.I.C. the blister cracks form at widely distributed sites and then link in a step- wise pattern, in S.O.H.I.C. the blister cracks tend to form in an array that is per-

HYDROGEN DAMAGE

Figure 8.14. ( a ) Hydrogen - induced cracking (H.I.C.). ( b ) Stress - oriented hydrogen - induced cracking (S.O.H.I.C.). (Courtesy of Malcolm Hay.)

pendicular to the applied stress, and these individual blister cracks link leading to failure. Figure 8.14 presents illustrations of H.I.C. and S.O.H.I.C.

Any factors that reduce hydrogen absorption in the corrosion process are benefi cial; for example, alloying with a small percentage of Pt or Pd, which cata- lyze formation of molecular hydrogen at the steel surface [62] , or with copper, which forms an insoluble sulfi de fi lm that provides some protection at pH above

about 4.5. Similarly, any processing of steel that minimizes sharp internal surfaces at the inclusion interface, for example by avoiding low rolling temperatures, reduces the tendency toward cracking. Increased resistance to H.I.C. can also be obtained by reducing the sulfur content in the steel and by controlling the shape of sulfi des — for example, by adding calcium to the steel composition.

Surface fl aws may infl uence the hydrogen cracking (or sulfi de cracking) of moderate - or high - strength steels exposed to oil - well brines containing H 2 S. Marked susceptibility to cracking in this environment makes it necessary to specify steels below the strength level at which failure occurs. Strength of a steel is proportional to its hardness. The empirically determined maximum hardness

is specifi ed as Rockwell C 22 (R c 22), corresponding to a yield strength of about

90 ksi [63] . It has been suggested [64] , based on threshold values of stress intensity

EFFEC T OF STRESS

factors for steels exposed to aqueous H 2 S solutions, that R c 22 corresponds to a critical surface - fl aw depth of about 0.5 mm (0.02 in). At greater depths, fl aws are likely to develop quickly into major cracks. Small - size fl aws of this order, which are calculated to be still less tolerable for harder steels, are not readily avoided in practice, corresponding with the general experience of the oil industry that, in

H 2 S environments, steels of hardness >R c 22 should be avoided. In general, surface fl aws become more important to cracking the higher the strength of the steel; internal fl aws, on the other hand, affect both low - and high - strength steels.