ABSTRACT:

Three different types of steel, normalized, normalized-z and TMCP-z steels, have been studied for the cracking susceptibility in the thickness direction using the implant tests and the thermal simulator. The results obtained from the implant tests indicated that the TMCP-z steel has the highest critical cold cracking rupture stress even at high diffusible hydrogen content (24.5ml/100gr weld metal) in comparison with the normalized and the normalized-z steels which show a substantial decrease at increased diffusible hydrogen levels. Electron metallography showed that fracture surfaces of the TMCP-z implant specimen are ductile mode, while those of the normalized and the normalized-z are intergranular. Simulated heat-affected zone studies using the induction heating type simulator demonstrated that liquation hot cracking occurs in the normalized steel but the normalized-z and the TMCP-z steels are rather insensitive to liquation hot cracking_

1. INTRODUCTION

TMCP(Thermo-Mechanical Controlled Process) steels provide benefits of low manufacturing costs and good weldability both to steel makers and fabricators. respectively. Cold cracking. hot cracking in the HAZ (Heat Affected Zone) and lamellar tearing still can occur in TMCP steels unless adequate welding parameters are maintained. These defects are related with amounts of alloying elements and the shape of nonmetallic inclusions and microstructures. Both a normalized-z and a TMCP-z quality steels of which ductility in the thickness direction is improved and a normalized steel of T.S.50kg/mm2 have been investigated regarding cracking susceptibilities in the thickness direction using the implant test machine and the thermal simulator.

2. EXPERIMENTAL

Three different types of steel whose chemical compositions are shown in Table 1 were tested. The normalized steel has the highest carbon equivalent and sulfur content while the normalized- z and the TMCP-z have the lowest sulfur contents because of Ca wire treatment in the ladle before pouring.

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