Page 114 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

stainless steel grades such as 321 or 347 are used. These
have stabilisers such as titanium (321) or niobium (347)
added. The titanium and niobium are stronger carbide
formers than chromium and form titanium carbide and
niobium carbide thereby leaving the chromium in the
grain.
. Quench cooling. Austenitic stainless steel is not hardened
by quenching and is not generally susceptible to hydrogen
cracking so cooling rapidly to reduce the time in the
critical heat range can be done without detrimental effects.
. Keeping heat inputs and interpass temperatures low. These
actions reduce the time that the material is held in the
critical temperature range.
. Solution heat treatment after welding. This involves heating
to around 1100 8C and quenching, which will dissolve the
chromium carbides and restore the chromium to the grain.
Do not carry out this procedure without consulting a
metallurgist for advice because other problems can be
induced in the material if you get the temperatures/timing
wrong. Also, be aware that a major disadvantage of this
method is the high level of distortion it causes.
Porosity
Porosity (Fig. 7.8) is the entrapment of gases (H 2 ,O 2 ,N 2 ,
etc.) within the solidifying weld metal. Causes of porosity
include:
. Loss of gas shield. Nitrogen and oxygen contamination
result from poor gas shielding. As little as 1% air
entrainment in the shielding gas will cause porosity.
Draughts, leaks in the gas hose or incorrect gas flow
rates are frequent causes of porosity.
. Damp electrodes or fluxes. Hydrogen can originate from
moisture in insufficiently dried electrodes and fluxes.
. Arc length too large.
. Damaged electrode flux.
. Moisture or contamination on the parent material or

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200907QG Welding chap7.3d Page 98 of 107