Page 105 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Fracture Modes and Welding Defects

hydrogen breaks down at increased temperatures into atomic
hydrogen (which has a small atomic size) and escapes to the
atmosphere through the steel microstructure. When the
temperature reduces to below around 300 8C the hydrogen
starts reforming to the hydrogen element and will no longer
be able to escape from the material. As the H 2 reforms it may
build up an internal pressure stress within the material
structure itself.
A ductile metal structure can absorb this H 2 without
penalty as the metal is able to plastically deform. If, however,
the metal is of a hardened (i.e. martensitic) structure less able
to deform, then the stress level built up in the material may
be enough to cause a fracture to occur. The four critical
factors involved in a hydrogen crack are:
. A hydrogen content of > 15 ml/100 g weld metal. The
hydrogen comes from moisture, paint, oil, grease, damp
electrodes or fluxes, loss of shielding gases or cellulosic
electrodes (H 2 is the shield gas).
. A stress level > 50% yield. The stress comes from residual
welding stress, restraint stress, etc.
. A hardness > 350 Vickers. The hardness refers to a crack-
sensitive microstructure and is related to the Cev of the
steel and the formation of martensite, a hard structure
caused by rapid cooling of steels.
. A temperature < 300 8C. There is nothing that can be done
to stop the temperature eventually falling below the
critical level on completion of the welding and any
required PWHT, so NDT needs to be carried out for up
to 72 hours after welding to check for delayed cracking.
All of the four factors must be present at the same time for a
crack to occur, so if any factor is reduced below its critical
level a crack will be avoided. Each factor can be reduced as
follows:

89

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200907QG Welding chap7.3d Page 89 of 107