448 R K Dayal and N Parvathavarth<strong>in</strong>i Figure 20. Factors affect<strong>in</strong>g C K and C H (Timm<strong>in</strong>s 1997).
<strong>Hydrogen</strong> <strong>embrittlement</strong> <strong>in</strong> <strong>power</strong> <strong>plant</strong> <strong>steels</strong> 449 dazol<strong>in</strong>es, aliphatic am<strong>in</strong>es, quaternary ammonium salts, nitrites and phosphate can be used. In gas-phase hydrogen atmosphere, even a small amount <strong>of</strong> oxygen present may elim<strong>in</strong>ate subcritical crack growth rate. Elements such as As, Se, Te, S, P, Sn, Hg, Pb and Bi that promote hydrogen entry by <strong>in</strong>hibit<strong>in</strong>g hydrogen recomb<strong>in</strong>ation reaction should be strictly avoided <strong>in</strong> hydrogen ion-conta<strong>in</strong><strong>in</strong>g environments. These elements when present <strong>in</strong>side steel segregate to gra<strong>in</strong> boundaries and should be avoided as much as possible. Avoid<strong>in</strong>g cathodic clean<strong>in</strong>g, pickl<strong>in</strong>g or activation treatments, and by us<strong>in</strong>g alkal<strong>in</strong>e soak clean<strong>in</strong>g, anodic etch<strong>in</strong>g or electropolish<strong>in</strong>g, hydrogen <strong>embrittlement</strong> can be elim<strong>in</strong>ated. If pickl<strong>in</strong>g is necessary, <strong>in</strong>hibited acid can be used to decrease metal dissolution. Electroplat<strong>in</strong>g can be replaced by us<strong>in</strong>g vacuum coat<strong>in</strong>g or organic coat<strong>in</strong>g. Bak<strong>in</strong>g <strong>of</strong> pickled or electroplated parts enhances resistance to hydrogen <strong>embrittlement</strong>. To avoid HIC dur<strong>in</strong>g weld<strong>in</strong>g, the amount <strong>of</strong> hydrogen enter<strong>in</strong>g the weld metal can be limited by the use <strong>of</strong> clean, low hydrogen consumables. Electrodes should be stored at an appropriate temperature <strong>in</strong> ovens or used from freshly opened airtight conta<strong>in</strong>ers. Cleanl<strong>in</strong>ess <strong>of</strong> weld preparation, weld<strong>in</strong>g wire and weld<strong>in</strong>g apparatus is also important. Pa<strong>in</strong>t, rust, grease, degass<strong>in</strong>g agents can all be hydrogen sources. Lubricants from wire-draw<strong>in</strong>g operation are another potential source. An additional approach to hydrogen control is to allow hydrogen removal by diffusion so that the levels are reduced to acceptable values by the time the weld has cooled. This is done by pre-heat<strong>in</strong>g which decreases the cool<strong>in</strong>g rate and allows more time for the hydrogen to diffuse. Moisture and contam<strong>in</strong>ation are also burned <strong>of</strong>f by this. Us<strong>in</strong>g higher weld<strong>in</strong>g heat <strong>in</strong>puts also <strong>in</strong>creases the weld thermal cycle time. Residual stresses developed <strong>in</strong> the welded assembly are <strong>of</strong>ten difficult to control, although a certa<strong>in</strong> degree <strong>of</strong> control can be exercised by alter<strong>in</strong>g the design. The residual stress may also be reduced to a degree by heat<strong>in</strong>g the weld preparation and the surround<strong>in</strong>g base metal prior to weld<strong>in</strong>g. This preheat<strong>in</strong>g reduces the non-uniformity <strong>of</strong> cool<strong>in</strong>g and allow more time for relaxation <strong>of</strong> residual stress by reduc<strong>in</strong>g the weld metal cool<strong>in</strong>g rate. Diffusivity <strong>of</strong> hydrogen at ambient temperature can be vastly different for consumables and base plate materials. Hence it is better to use <strong>steels</strong> <strong>of</strong> match<strong>in</strong>g composition for deposit<strong>in</strong>g the consumable rather than those recommended <strong>in</strong> different standards. The higher the strength <strong>of</strong> the steel, the lower the acceptable weld hydrogen content which can be as low as 1 or 2 ml H 2 /100 g deposit metal. With proper selection and use <strong>of</strong> weld<strong>in</strong>g consumable, a m<strong>in</strong>imal hydrogen content can be <strong>in</strong>troduced to the weld pools. 8. Conclusions <strong>Hydrogen</strong> <strong>embrittlement</strong> has caused failures <strong>of</strong> various steel components <strong>in</strong> <strong>power</strong> <strong>plant</strong>s. Some <strong>of</strong> such failures have been described <strong>in</strong> this article. The effect <strong>of</strong> hydrogen <strong>in</strong> <strong>steels</strong>, classification <strong>of</strong> damage types, <strong>in</strong>fluence <strong>of</strong> different factors and related mechanism have been expla<strong>in</strong>ed. Several preventive methods to m<strong>in</strong>imize the hydrogen embrittelment problems have been provided. Based on the discussion it can be concluded that there is no general remedy for hydrogen <strong>embrittlement</strong> problems and specific problems require specific solutions. The best way is to understand the phenomenon thoroughly before provid<strong>in</strong>g any solution to HIC. References Akhurst K N, Baker T 1981 The threshold stress <strong>in</strong>tensity for hydrogen-<strong>in</strong>duced crack growth. J. Metall. Trans. A12: 1059–1070
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