In spot welding, if ordinary welding conditons are adopted, the nugget would begin to cool regardless of flowing the current after a certain elapse of time. Peripheral part of nugget will be columnar and inside of nugget will be cellular or cellualr dendrite structure. The area of columnar structure becomes larger as weld time elapses and at last the whole nugget will assume the columnar structure. Oval-ring-like figure appears in this columnar structure. We corroborated with the help of special control timer made by us that this oval-ring-like figure occurred as the result of cooling with repetition of heating and cooling by the alternating current.
The sensitivity to hydrogen embrittlement of every kind of microstructure was examined by means of Constant Load Rupture Test. Hydrogen was charged into a specimen with each structure cathodically under given constant load. In this report, the microstructures examined were ferrite+pearlite, sorbite, troostite, bainite+martensite and martensite. The martensite was the most embrittled by hydrogen. This depended upon the presence of carbon in the crystal lattice (in the presence of relatively high carbon the martensite was composed of twinned martensite in the microstructure.). Ferrite+pearlite structure appeared to be more insensitive to hydrogen embrittlement than other microstructures and it would be considered that this depended upon the carbide (cementite). The relationship between the sensitivity to hydrogen embrittlement and the hardness (Hv) corresponding to each structure was linear. Therefore the sensitivity to hydrogen of any materials could be roughly estimated by their hardness.