溶接継手のルート割れにおよぼす外的拘束の影響

抄録

Control of weld cracking is one of the most important problems for welding engineers. Weld cracking is influenced by welding stress during cooling as well as properties of weld metal and heat affected zone. The problems of weld cracking, therefore, should be explored not only from metallurgical strandpoint but also from standpoint of welding stress. In this report research was focused on the influence of restraint on weld cracking.
A new test apparatus as shown in Figs. 3 and 4 was designed for evaluation of sensitivity for root cracking of steel welds. With this apparatus, "Rigid Restraint Weld Cracking test (RRC test)" as shown in Fig. 1 was developed, it is such that the length of a specimen is kept constant during cooling. The RRC test method is similar to the TRC test developed by Dr. Suzuki and his collaborators for the researches on root cracking of high strength steel welds. However the authors' test method differs from the TRC test as follows : In the TRC test a constant load was applied at a few minutes or immediately after welding because emphasis was laid on only delayed root cracking of high strength steel welds. The authors' test method is very significant, on the other hand, for the two reasons which (1) it gives gradual increase of reaction load by keeping the length of a specimen always constant during welding and cooling and also (2) it permits a wide degree for adjustability in the restraint intensity by changing the gauge length of restraining (l), (See Fig. 1). This may be much similar to the actual behavior of restrained welds and will be reasonable as the test method for weld cracking under restraint during cooling as well as after cooled to room temperature.
Experiments were carried out on the weldments of a mild steel and high strength steels of 60 and 80 kg/mm² tensile strength level.
The results obtained are as follows.
(1) In the RRC test, shinkage rate and magnitude of reaction stress were widely changed with changing the gauge length of restraining, and weld crack occurs in wide range of temperature. Reaction load increases gradully with cooling, and higher intensity of restraint usually favours crack initiation or decreases the time required for cracking.
(2) In mild steel weldments, weld crack occurs only during cooling and no crack initiation is observed after cooled to room temperature.
(3) In high strength steel weldments, on the other hand, crack initiation was observed not only during cooling but also after cooled to room temperature.
(4) This delayed cracking was observed even at lower reaction stress level. Minimum reaction stress level for the delayed cracking is 33 kg/mm² for HT-80 steel and 60 kg/mm² for HT-60 steel. The incubation period was measured 1-10 hours for HT-80 steel and 20 hours for HT-60 steel after welding.
(5) Critical gauge length (l_cr) of restraing, over which no crack initiation was observed, was 300-370 mm for mild steel, 450-500 mm for HT-60 steel and 550-600 mm for HT-80 steel.