Qualification of Temper Bead Welding by an Alternative Hardness Testing Approach

Abstract

In 2004, ASME Section IX added maximum hardness essential variables for temper bead procedure qualification when impact testing is not specified or required by the applicable book section. The assumption with specifying a maximum hardness criterion is that high hardness after temper bead welding indicates inadequate tempering. As discussed in PVP2013-97793 [2], imposing a maximum hardness criterion for temper bead qualification can actually lead to acceptance of a heat affected zone (HAZ) microstructure with less than optimum impact properties. In fact, depending on the weld HAZ microstructure the impact properties can vary widely from very low to very high at the same hardness. This paper describes an alternative hardness test protocol for temper bead procedure qualification. Rather than using a single maximum hardness acceptance threshold, this new test protocol characterizes the base material response to temper bead welding by determining the maximum achievable hardness with a bead-on-plate test and with a hardness calculation. Research shows that a high hardness in the HAZ prior to depositing the tempering weld layers provides the optimum microstructure for achieving desired HAZ impact properties. With proper tempering the HAZ hardness is reduced below the maximum achievable hardness. Temper bead procedure acceptance is thus determined by the drop in HAZ hardness after depositing the temper bead weld layers. Application of this new hardness test protocol for temper bead qualification is proposed as an alternative to a single maximum hardness acceptance criterion.