Page 51 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Chapter 4

Materials and Their Weldability
There are many different definitions concerning the weld-
ability of steel because it often means different things to
different people. Simplistically it can be defined as the ability
of a material to be welded and still retain its specified
properties. This ability to be welded successfully depends on
many factors including the type and composition of the
material, the welding process used and the mechanical
properties required. Poor weldability generally involves
some type of cracking problem and this is dependent upon
factors such as:
. residual stress level (from unequal expansion and con-
traction due to welding);
. restraint stress level (from local restraint such as clamps,
jigs or fixtures);
. presence of a microstructure susceptible to cracking (the
base material may have a susceptible microstructure or the
HAZ and/or weld metal may form a microstructure
susceptible to cracking owing to the welding).
Carbon equivalency
The susceptibility of a microstructure to cracking is heavily
influenced by the amount of carbon and the type and amount
of other alloying elements present in the steel. The carbon
and other alloying elements can be put into a formula that
determines the carbon equivalency value (Cev) of the
material. This Cev is a measure of the hardenability of the
steel. The higher the Cev, the more susceptible the material
will be to cracking by brittle fracture.
Other factors affecting the likelihood of cracking include
the base metal thickness and the combined joint thickness
(i.e. a butt weld has two thicknesses whereas a fillet weld has
three). The combined joint thickness is important because

36

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200904QG Welding chap4.3d Page 36 of 48