PROCEDE DE CARBURATION EN CONTINU

1503179 Continuous carburizing MONSANTO CO 29 Oct 1975 [30 Oct 1974] 44704/75 Heading C7U The continuous carburizing of low carbon steel stock up to 0À051 cm thick, is carried out by passing the cleared strip S through a pre-heat zone 16 of a carburization furnace 4 in which a neutral gas atmosphere is maintained. The strip S then passes through baffles 20 into the carburizing zone 17 held at 950-1150‹C in which a carburizing atmosphere is maintained. The atmosphere has a high "carbon availability", (defined as the ratio Kg C entering the furnace to Kg of steel passing through the furnace per hour) in the range 0À010-0À080. On leaving the carburization zone 17, the strip passes through baffles 20 into a homogenization zone 18, wherein a neutral atmosphere, similar to that of pre-heat zone 16 is maintained, and held at a temperature above 800‹C. The homogenized strip is then passed, again through baffles 20, into a cooling zone 6, split into two zones 30, 22, in which cooling gas is directed on to the upper and lower surfaces of the strip. Cooling zone 30, in which the coolant is hydrogen, rapidly quenches the strip to approximately 600‹C in about 10 seconds while zone 32, supplied with nitrogen, further cools the strip to ambient temperature prior to recording on reel 51, via guide roll 48 and tension rolls 49. The residence time of the strip in the carburizing zone 17 is less than 10 minutes, the amount of carburization necessary being defined by a sootfree surface and a uniform microstructure of predominantly fine pearlite with a maximum of 5% proeutectoid ferrite present; the control being the "carbon availability" and residence time. Suitable material for treatment is 0À0254 cm thick black plate, and the C content can be increased from 0À08% to 0À6% with residence times of less than 10 minutes, using a high (0À02) carbon availability and a carburization temperature of 950-1150‹C.