극후판 고장력강(EH36-TMCP)의 용접부 파괴인성에 대한 연구

Abstract

For the promotion of structural integrity and pursuit of economical interest on large welding construction and operation of the construction, the thorough and rigorous evaluation and working control are an essential precondition to prevent the safety concern during the manufacturing and operating the construction. Considering the benefit from economics of scale is important, many large welding constructions such as offshore structures, VLCCs (Very Large Oil Carriers), VLOCs (Very Large Ore Carriers) and even container ships have been constructed and this trend may be maintained continuously in future.

In case of large container ships, this trend has been applied actively, and actually the size of the ship is being continuously increased. Along with this trend, the applications of thick steel plates with high strength (EH36-TMCP) that exceed more than 70mm and 355N/mm2 in thickness and yield strength respectively are increased rapidly to satisfy the deduction of lightweight and increase of longitudinal strength of the ship.

The application of these thick steel plates, however, cause an advance in workload and man-hours to be invested for welding processes, accordingly the necessity of more efficient welding process by using the large heat input welding i.e. EGW (Electro Gas Welding) are raised to the cost-reduction, instead of using the conventional semi-automatic FCAW (Flux Cored Arc Welding).

But, the application of the EGW as an efficient welding process causes to increase the brittleness on the welding region because of large heat input, and resultantly the fracture toughness of welding materials may be affected, comparing with an application of ordinary multi-layer FCAW welding. Considered the assurance of fracture toughness on thick steel plates with high strength is indispensible to the large welding construction, the characteristics of EGW need to be investigated and furthermore the effectiveness of the using EGW as a substitution of FCAW also needs to be verified for applying the thick steel plates with high strength (EH36-TMCP).

For the purpose of this, the analyses for welding residual stress are conducted to the FE models which have been welded by EGW and FCAW respectively. And, for consideration of actual working condition, the redistributed welding residual stresses, when the external bending load and initial crack are induced, are re-analyzed on the basis of the conducted FE analyses for welding residual stress.

Acknowledging the fracture toughness is the most important factor among the large welding constructions to which the thick steel plates with high strength are used, the analyses of fracture toughness for each welding process i.e. EGW and FCAW are conducted in this study and the result shows that the fracture toughness (KIC) for the EGW, which is considered as a terms of crack initiation, is lower than FCAW. But, it is considered to negligible as the difference of the toughness values between two welding processes is very small compared with mechanical properties of welding materials.

For the confirmation of mechanical characteristics and integrity after welding, the mechanical experiments have been carried out and it is confirmed that the properties are complied with the classification rules for EH36-TMCP. As a verification of fracture toughness in way of welding regions, CTOD test in accordance with British standard BS 7448 and the review of fractured surfaces by SEM(Scanning Electron Microscope) have been carried out. The result shows that the FCAW have higher value than EGW in CTOD value and the fractured surfaces show that ductile fracture in FCAW and cleavage fracture in EGW have been occurred.

Conclusively, the analyses and experimental result shows that characteristics of EGW have been slightly deteriorated compared with FCAW but the result can be acceptable to the concerned thick plates with high strength, EH36-TMCP as an substitution of conventional multi-layer welding, FCAW.