Fractography of Cold Crack of Various Carbon Steels and Alloy Steels by Means of the Implant Test : Studies on Fractography of Welded Zone (IV)

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Fractographic investigation is made on weld cold crack of various plain carbon steels and alloy steels, using the implant cracking test in different diffusible hydrogen contents and observing by a scanning electron microscope. Main conclusions obtained are as follows: (1) Fracture mode of hydrogen-induced delayed crack dominates in plain carbon steels whose carbon content is less than about 0.50% and in weldable high strength steels. (2) Fracture mode of quenching crack together with hydrogen-induced delayed crack is obvious in plain carbon steels whose carbon content is more than about 0.50% and in medium carbon low alloy steels. (3) Fracture surface consists of three or four modes among intergranular, quasi-cleavage, cleavage and dimple fractures. Decrease in applied stress increases the area of quasi-cleavage fracture mode in all specimens, where tear ridges and secondary cracks are formed due to hydrogen. Increase in applied stress increases the area of dimple fracture mode in steels described in (1) and intergranular fracture mode in steels described in (2).
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