The initiation and propagation of root cracks in steel weldments have been investigated by H-slit restraint cracking tests. The experimental variables examined in the tests are chemical compositions in steel in terms of PCM (0.20 to 0.27), diffusible hydrogen in weld metal in terms of H
JIS (0.4 to 30ml/100gr), tensile restraint in terms of R
F (300 to 3, 500 kgf/mm⋅mm), ambient temperature (0, 20 and 30°C) and groove shape (V, y and _??_).
It has been found that a root cracking tendency in root-pass welds may be given by the following parameter P
HM:
P
HM=P
CM+0.075log H
JIS+0.15log (0.017 K
t⋅σ
w)
where, K
t⋅σ
w is a local stress at the weld root, K
t and σ
w being a notch stress concentration factor and an average stress of the weld metal, respectively. σ
w has been given by a function of plate thickness and R
F.
Accoustic emission measurements have revealed that an incubation time for root cracking becomes shorter and that a velocity in crack propagation becomes higher as the P
HM value increases. With a decrease in an ambient temperature, a root cracking susceptibility has increased while a crack velocity decreasing unlike the case of increasing P
HM.
The preheating temperature necessary for preventing root cracking in root-pass welds can be obtained by looking into t
100, the cooling time from the welding completion to 100°C, longer than (t
100)cr given as:
(t
100)cr=10.5×10
4(P
HM-0.276)
2A relationship between t
100 measured at test plates and pheheating temperatures has been prepared for various plate thicknesses and heat-inputs.
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