Abstract
21/4Cr-1 Mo steel are generally used for the huge reactor vessels that operate at high pressure andelevated temperature, because of its good short and long time elevated tensile properties and its goodresistance to hydrogen attack. Requirements for higher operating volume and pressure have accelerateda demand for reactor vessels of extra heavy gauge wall thickness and upgraded mechanical properties throughout the thickness.
To obtain informations needed for effetive uses of 21/4Cr-1 Mo steels, a fundamental and systematicstudy was carried out on the heavy gauge steels of 100-400 mm in thickness. In this study, the effectof cooling rates from austenitizing temperature on microstructures, the effect of cooling rates andtempering cycles on mechanical properties and elevated temperature tensile properties were investigated.
The results obtained are as follows:
(1) Uniform bainitic structure is obtained when the cooling rate is faster than 6°C/min, and theblockish ferrite increases with decrease rate, which deteriorates mechanical properties.
(2) The effect of various heat treatment cycles on room temperature tensile properties is shownas a function of LARSON-MILLER'S tempering parameter, and 21/4Cr-1 Mo steels in the ultimate tensilestrength range from 56kg/mm2 to 90 kg/mm2 can be manufactured by the suitable tempering cycles.
(3) To maintain excellent notch toughness, the cooling rate faster than 6°C/ min is desirableand in the special case of manufacturing high tensile steels which are tempered at low temperature (UTS is over than 70kg/mm2), 10-15°C/min or faster cooling rate is recommendable.
(4) By applying the results mentioned above, 365 mm thickness forged shell was cooled in agitatingwater and the excellent mechanical properties and satisfactory bainitic structure was obtainedthroughout the thickness.
