1964 Volume 33 Issue 3 Pages 182-190
A series of root cracking tests were conducted with y-groove restraint cracking specimen of high strength steels with thickness from 30 to 50 mm. Effects of size of specimen, including bead length and plate thickness, and of multi-layer weld on root cracking were mainly studied in the report.
The conclusions obtained in the study are as follows:
(1) The root cracking tendencies of six high strength steels with their strength levels from 60 to 79 kg/mm2, that is, HT 60 and HT 70 steels, were evaluated and preheating temperatures necessary for the prevention of root cracking were decided.
The maximum hardnesses in HAZ of these steels, which were safe to root cracking, ranged from 350 to 400 VHN under the load of 1 kg.
Microstructures of HAZ safe to root cracking were decided with an, aid of of CCT diagrams, that is, 5-10% ferrite, 20-30% intermediate structure and 60-70% martensite for HT 60 and 10-25% intermediate structure and 75-90% martensite for HT 70 in their area percentages of constituents.
(2) Neither the root crack, which was initiated at the first layer weld, was propagated nor a new crack was initiated with the progress of a multi-layer weld.
Cracking tendencies rather decreased in multi-layer welds with a choice of proper interpass temperature.
(3) An increase of root cracking was not detected with an increase of bead length and a root crack rather increased with a decrease of bead length at a bead length below 150 mm, because of a rapid cooling of a short bead.
Therefore it was suggested that the transverse tensile residual stress in a weld would directly affect the initiation of a root crack and the longitudinal tensile residual stress would indirectly affect the initiation of a root crack.
(4) The tendencies of root cracking did not increase with an increase of plate thickness if the welds were cooled by the same cooling process.
(5) It was confirmed that the slotted cracking specimen with a size of 200 mm×150 mm was equivalent to one of 300×200 mm. The smaller size is therefore satisfactory for a root cracking test of high strength steel welds.
