20th International Forgemasters Meeting - IFM 2017

Recently, the needs for eco-ships have been increasing from the viewpoint of the strengthened environmental regulations and the demands for improvement of fuel consumption. The engine design places importance on higher power in low revolution range, so that the stroke of engine becomes longer. As a result, the higher torsional vibration is subjected to the intermediate shafts. The torsional vibration stress can be lowered by the increase of shaft diameter, or strengthening of materials. In particular, the higher tensile strength material can lead to reduce shaft weight, and omit the torsional vibration damper. In April, 2015, the special approval of alloy steel which has a minimum specified tensile strength beyond 800 and less than 950 MPa can be used for intermediate shaft material in the Appendix I of IACS UR M68. Because of this special approval, it is expected that the application of high tensile strength alloy steel increases for intermediate shafts. This paper reports the results of the manufacturing and the mechanical properties test of the large-sized intermediate shaft with the length over 10 meters for large container ships. Its specified tensile strength is 930 MPa. This large-sized one has never been manufactured in KOBE STEEL, LTD, due to the size of the conventional polymer quenching tank. At this time, the vertical quenching tower is applied to the quenching after austenitizing. As this is the first heat treatment of intermediate shafts using the vertical quenching tower, we consider the heat treatment condition in advance. In particular, the heat treatment condition to satisfy the product specification is optimized based on the tensile strength estimation equation derived from past productions of low alloy steel, and the cooling rate by thermal analysis. The measured cooling rate during the quenching is corresponded well with the calculated value, and the desired mechanical property is achieved.