Aluminium Design and Construction John Dwight

6.4 SEVERITY OF HAZ SOFTENING
6.4.1 Softening factor
The severity of softening in the HAZ is expressed in terms of a softening
factor kz which is intended to represent the ratio of HAZ strength to
parent metal strength. At the current state of the art, it is only possible
to suggest approximate values for this factor. Typically HAZ experiments
employ hardness surveys, and although these give a good indication of
the extent of the softened zone, they say much less about the actual
tensile properties of the softened metal, because the hardness number
correlates only crudely with tensile strength and hardly at all with
proof stress. Also, there tends to be a lot of scatter between specimens.
In fact, for any given material we recognize three different values for
kz (as in BS.8118), and Section 6.6 explains which value to use when.
k z1 This value is used for calculations involving the limiting stress
p a (Table 5.2). Because p a =0.5(f o +f u ), the factor k z1 is notionally
intended to represent the ratio of the HAZ value of this quantity
to that for the parent metal, averaged over the width of region
1 (Figure 6.2).
k z2 This value is employed for resistance calculations that involve the
limiting stress p o . Because p o is normally equal to the proof stress f o ,
we (notionally) take k z2 as the ratio of HAZ proof to parent metal
proof, again averaged over the width of region 1. The fact that the
HAZ material has a lower proof/ultimate ratio than that for the
parent metal (Figure 6.4) means that k z2