Study on the Fatigue Property of HT60 Weld Metal Containing Microcrackings

Abstract

Microcrackings could be simulated in HT60 weld metal by Restrained T type Tensile (RTT) test. They had two types of cracking. One initiated in the weld metal, called type I cracking, and another initiated from the surface of weld metal, called type II cracking. Type II cracking could be detected by M.T., but type I cracking could not be detected by M.T.. Therefore, if type I cracking exists in the fillet weld between the lowest course shell plate and the annular plate in oil storage tank, no inspection method can be applied to detect them. Fatigue test specimen was sampled from RTT test specimen including type I cracking. Various moistening condition of D6216 electrode were applied to initiate microcrackings in the weld metal. Experimental results were as follows.
1) At 350°C, 3.6 ksec drying of D6216, the hydrogen content in HT60 weld metal was 7 cc/100 gFe, and at 80°C, 86.4 ksec moistening of D6216, it was 37 cc/100 gFe.
2) At 350°C, 3.6 ksec drying of D6216, no microcracking was detected in HT60 weld metal.
3) At 350°C, 3.6 ksec drying of D6216, tensile strength of weld metal did not change before and after fatigue test.
4) At 80°C, 86.4 ksec moistening of D6216, number of cycles to fracture increased with decreasing cyclic load.
5) Even if type I cracking existed in the fillet weld between the lowest course shell plate and the annular plate, these might not make a oil storage tank fracture in operation for 50 years.