The welding is on essential process for construction of structure such as fossil power plant due to the manufacture restriction of the tube length and the complicated shape of boiler tube when it is made by forging process. And most of the damage is concentrated on HAZ(Heat Affected Zone) and weldment rather than base metal. So it is necessary to investigate the fatigue properties of the boiler tube material.
The effect of tensile hold time on the low cycle fatigue properties of P122 steel weldment produced by the SMAW(Shield Metal Arc Welding) process was investigated with a strain rate of $4×10^{-3}sec^{-1}$ in air atmosphere. Low cycle fatigue tests were conducted at temperatures of 823K, 873K and 923K. And creep-fatigue tests were also conducted by appling 10 min tensile hold time at the maximum strain at the same test temperatures as the continuous fatigue tests.
Under low cycle fatigue test, the fatigue life of P122 steel weldment is slightly shorter than that of P122 steel base metal. From the observation of tensile peak stress change with cycles, both base metal and weldment show the fatigue softening behavior. The fatigue softening behavior is increased with increasing test temperature and increasing the total strain range. And the fatigue softening of weldment is higher than that of base metal under continuous fatigue test. This fatigue softening behavior is found to be related with the recovery and the rearrangement of dislocations and the growth of carbides. The microstructural observation of P122 steel base metal and weldment after continuous fatigue test shows that the growth of lath width and carbide size of weldment is higher than that of base metal, so the movement of dislocations in weldment may be easier than that in base metal. The lower fatigue life of weldment under continuous fatigue test is caused by the higher fatigue softening of weldment.
When the creep deformation was introduced by imposing tensile hold time, the fatigue life of the weldment is considerably reduced. The fatigue softening of weldment under creep-fatigue test is increased because of the easier dislocation movement by the aids of the accelerated growth of lath width and carbide size. So the fatigue life reduction of weldment when the tensile hold time is imposed is by the higher fatigue softening behavior of weldment under creep-fatigue test.
From the results of observation of metallographic cross section, the internal crack is initiated by the formation of voids near to the inclusion which is formed during welding process. The fatigue life reduction of the weldment under creep-fatigue test is related with the combination of internal crack and surface crack by the further fatigue cycle.
