Page 394 - Failure Analysis Case Studies II

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Fig. 7. Typical detail of gusset welds showing complete absence of “rat hole” at the junction of the three members.

Cracks in the HAZ are usually sited either at the weld toe, the weld root, or in an underbead
position. The interaction between the factors responsible for cracking and the ways in which control
over them may be achieved are discussed below [4].

3.3.1. Hydrogen level. During welding, hydrogen is absorbed by the weld pool from the arc
atmosphere. During cooling, much of this hydrogen escapes from the solidified bead by diffusion
but some also diffuses into the HAZ and the parent metal. The amount which does so depends on
several factors such as the original amount absorbed, the size of the weld, the decreasing solubility,
and the time-temperature conditions of cooling.
In general the more hydrogen present in the metal the greater the risk of cracking. Control over
this hydrogen level may be achieved either by minimising the amount initially absorbed or by
ensuring that sufficient is allowed to escape by diffusion before the weld cools. Frequently a
combination of both measures provides the best practical solution.
3.3.2. Stress level. Stresses are developed by thermal contraction of the cooling weld and these
stresses must be accommodated by strain in the weld metal. The presence of the hydrogen appears
to lower the stress level at which cracking will occur. In rigid structures the natural contraction
stresses are intensified because of the restraint imposed on them by the different parts of the joint.
These stresses will be concentrated at the toe and root of the weld and also at notches constituted
by inclusions and other defects. The higher degrees of strain which result produce higher risks of
cracking for a given microstructural hardness.
The stress acting upon a weld is a function of weld size, joint geometry, fitup, external restraint,
and the yield strengths of the plate and weld metal.

3.3.3. Microstructure. The heat affected zone (HAZ) of the parent metal adjacent to the weld is
raised to a high temperature during welding and subsequent rapid cooling (quenching) by the
surrounding parent metal causes hardening by transformation to martensite. Close to the fusion
boundary the HAZ is raised to a sufficiently high temperature to produce a coarse grain size. This
high temperature region, because of its coarse grain size is not only more hardenable but also less
ductile than regions further from the fusion boundary. It is thus the region in which the greatest
risk of cracking exists. As a general rule, for both carbon-manganese and low alloy steels, the
harder the microstructure the greater is the risk of cracking. Soft microstructures can tolerate more
hydrogen than hard before cracking occurs.

3.3.4. Temperature. Hydrogen embrittlement of ferritic steels occurs only at low temperatures,