Mechanical and corrosion resistance properties were investigated of welded joints of Zircaloy-2 sheets, 1 and 2 mm thick, which were welded in a controlled atmosphere welding chamber filled with either pure argon or impure argon mixed with an impurity gas of air, nitrogen, oxygen or hydrogen.
Characteristics of welded joints which welded in pure and impure argon atmospheres, namely, hardness, microstructure, tensile and bending properties and corrosion resistance against high temperature high pressure water were studied and the following conclusions were obtained ;
(1) Weld metal was the most hardened zone in welded joint because its microstructure consisted of α'+α structure. The heat-affected zone was more hardened than base metal.
(2) Tensile strength of weld metal was stronger than that of base metal at each temperature in the range of room temperature up to 500°C. Thus tensile fracture of welded joint occured in base metal.
(3) Rupture strengths at 1000 hr. of weld metal and base metal at 316°C were about 19 kg/mm
2 and 16.5 kg/mm
2, respectively.
(4) Corrosion resistance of welded joint against high temperature high pressure water was better than that of base metal when was pickled after welding.
(5) It is recommended for zircaloy-2 to pickle (HF 5%+HNO
345%+H
2O 50%, at room temperature for 30 sec) after welding for high temprature high pressure corrosion test.
(6) The effects of impurity gases in argon atmosphere on mechanical and corrosive properties of weld metal are as follows (shown in Table 2) :
(a) More than 5×10
2-10
3 ppm (vol.) of air, N
2 gas and 10
3-5×10
3 ppm of O
2 gas embrittled the mechanical properties, and hardness of the weld metal.
However, H
2 did not change the hardness and tensile strength although more than 10
2-5×10
3 ppm it enbrittled the weld metal.
(b) More than 7 × 10
2 ppm of air and 10
2 ppm of N
2, O
2 and H
2 gas if specimens would not be pickled after welding decreased greatly the corrosion resistance against high temperature high pressure water. On the contrary, if specimens would be pickled after welding, up to 10
4 ppm of air, 5 × 10
3 ppm of N
2, 10
4-10
5 ppm of O
2 and H
2 gas did not decrease corrosion resistance.
(7) As far as the controlled atmosphere welding is concerned, it seems to be necessary for reactor purposes to use on initial vacuum less than 10
-1 mmHg and filling with high pure grade (99.99% up) argon to one atmosphere and to pickle (HF 5%+HNO
3 45%+H
2O 50%) the specimens after welding.
View full abstract