| 摘要 |
PURPOSE: To optimize a process of gas nitriding steel that contains 0.15 to 0.42 wt.% of carbon(C), 1.0 to 2.25 wt.% of chromium(Cr), 0.4 to 1.05 wt.% of molybdenum(Mo), 0.0 to 0.65 wt.% of manganese(Mn), 0 to 0.22 wt.% of vanadium(V), 0.0 to 0.5 wt.% of silicon(Si) and 0.0 to 1.08 wt.% of aluminum(Al) and is used for valves for steam turbine. CONSTITUTION: The gas nitriding method of steel comprises a step(a) of charging an ammonia gas into a nitriding furnace until a ratio of ammonia gas becomes 98% in the nitriding furnace; a step(b) of increasing temperature of the furnace to 480 deg.C at a maximum heating rate of 100 deg.C/hr while injecting ammonia gas into the furnace at a flow rate of 4 m¬3/hr; a step(c) of rapidly heating the product to 530 deg.C within 1 hour while increasing an ammonia flow rate to 4.5 m¬3/hr if temperature of a product reaches 480 deg.C; a step(d) of maintaining the product for 1 hour while adjusting an amount of ammonia gas flown in the furnace such that a decomposition ratio of ammonia gas is maintained between 25% and 35% if temperature of the product reaches 530 deg.C; a step(e) of rapidly heating temperature of the product to 555 deg.C within 1 hour, and adjusting an amount of ammonia gas flown in the furnace such that a decomposition ratio of ammonia gas is maintained between 60% and 80% if temperature of the product reaches 550 deg.C; a step(f) of maintaining the product at 555 deg.C for 23 to 30 hours in the state that a decomposition ratio of ammonia gas is maintained in a range of 60 to 80%; a step(g) of stopping supply of ammonia gas, starting supply of nitrogen gas into the furnace, and cooling the product to 500 deg.C by cooling the product at a cooling rate of 75 deg.C/hr; a step(h) of turning off a heater, and cooling the nitriding furnace to 100 deg.C at a cooling rate of 75 deg.C/hr while supplying 4 m¬3/hr of nitrogen gas into the furnace if ammonia gas is not remained in the nitriding furnace as a result of measurement after measuring density of gas contained in the nitriding furnace; and a step(i) of stopping supply of nitrogen gas and opening a cover for the nitriding furnace when temperature of the nitriding furnace is cooled to 100 deg.C.
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