In the case of alloy steels, the zone adjacent to weld metal is profoundly affected by the heat of arc welding. The hardness of the heat-affected zone is very high, and the so-called hard cracks are apt to occur in this zone.
In this research, first, it was revealed that under some definite conditions for welding hard cracks did not occur when austenitic steel electrodes were used, but appeared always in the case of mild steel electrodes.
Next, using four types of electrodes, which were made of mild steel (very low carbon) and austenitic steel core rods shielded with a thin or thick coating, single beads were deposited on the surfaces of rectangular specimens of ducol and Mn-Cr-Mo steel. The welding was carried out taking into consideration the characteristics of each electrode. Then the hardness and microstructure of the heat-affected zones were surveyed.
Under an approximately identical condition for the uses of thick or thin coated mild steel electrodes and thick or thin coated austenitic steel electrodes, the heat quantities supplied to base metals appear to be arranged from large to small in due order of the above-mentioned. And the maximum hardness of the haet-affected zone of alloy steel having an air hardening property like ducol steel is arranged from low to high in good order of the above description.
Order differences among the microstructures were found in accordance with the tendency for the hardness.
In the case of alloy steel having a severe air hardening-property like Mn-Cr-Mo steel, the differences among the hardness values and microstructures of the heat-affected zones were not found, but cracks were always induced when mild steel type electrodes were used as previously described.
Finally, the grooves of mild steel specimens under restraint (θ-type specimens) were welded using the four types of electrodes, and the amount of welding contraction perpendicular to the weld line was measured.
The amount of contraction is arranged from large to small in order of the before mentioned arrangement for four types of electrodes.
Judging from these results alone, it is unable to explain the reason why the hard cracks do not occur in the case of austenitic steel type electrodes.
The explanation of this reason shall be done in another research conducted by the author.
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