Method of carburizing and quenching a steel member

Abstract

A method of carburizing and quenching a steel member includes: a reduced pressure carburization step in which a steel member is contacted with carburization gas under reduced pressure, a slow cooling step in which the steel member is then slowly cooled in a cooling gas, and a quenching step of heating a selected portion of the cooled steel member using high-density energy and subsequently subjecting the selected portion to rapid cooling. The steel member subjected to the low-pressure carburization step includes a first portion in which a diffusion rate of carbon taken therein during carburization is high because of its shape and a second portion in which the diffusion rate of carbon is lower than that of the first portion. The reduced-pressure carburization step is controlled to give a carbon concentration at the surface of the first portion in a range of 0.65±0.1 weight % after diffusion.

1 . A method of carburizing-quenching a steel member, comprising:
 (a) a reduced-pressure carburization step of carburizing a steel member in a carburization gas at a pressure below atmospheric pressure giving a first portion of the steel member a carbon concentration at 0.65±0.1 weight % after diffusion at the exterior surface of the first portion;   (b) a slow cooling step of subjecting the steel member, treated in step (a) to have a carbon concentration of 0.65±0.1 weight % after diffusion at the exterior surface, to a first rate of cooling in a cooling gas; and   (c) subsequent to step (b), a quenching step of selectively heating a selected portion of the cooled steel member using high-density energy and subsequently subjecting the selected portion to a second rate of cooling, more rapid than the first rate of cooling, wherein   the first portion of the steel member subjected to the reduced-pressure carburization step (a) has an exterior surface contoured in a manner facilitating the introduction of carbon by diffusion during the reduced-pressure carburization step and wherein the steel member subjected to the reduced-pressure carburization step (a) also has a second portion with an exterior surface shaped to have a carbon diffusion rate less than the carbon diffusion rate of the first portion.   
   
   
       2 . The method according to  claim 1 , wherein:
 the pressure in step (a) is 1 hPa to 100 hPa;   step (a) includes heating the steel member to at least its austenitizing temperature while the steel member is in contact with the carburization gas;   the slow rate of cooling in step (b) is from 0.1 degrees C./second to 3.0 degrees C./second and at a pressure of 100 hPa to 650 hPa;   the heating of the selected portion in step (c) by impinging an electron beam on the selected portion;   the rapid cooling in step (c) is 200 degrees C./second to 2000 degrees C./second;   the first portion has an external surface contour with surface portions meeting at an angle of 130 degrees to 180 degrees.   
   
   
       3 . The method according to  claim 2  wherein the steel member is a gear having a circumferential surface with gear teeth arranged spaced thereon, each gear tooth having a flat top surface and side surfaces meeting the top surface to form corners and wherein the exterior surface of the first portion includes a portion of the top surface between the corners and the exterior surface of the second portion includes the corner. 
   
   
       4 . The method according to  claim 1 , further comprising, subjecting other steel members of the same material to step (a) wherein at least one process parameter is varied between different tests to determine a value for the at least one process parameter resulting in a carbon concentration of 0.65±0.01 weight % after diffusion at the surface of the first portion and subsequently using the at least one process parameter having the determined value in step (a). 
   
   
       5 . The method according to  claim 4  wherein the at least one process parameter is time of contact of the steel member with the carburization gas in step (a). 
   
   
       6 . The method according to  claim 4  wherein the at least one process parameter is selected from temperature, type of carburization gas, pressure, processing time and combinations thereof. 
   
   
       7 . The method according to  claim 1 , wherein the reduced-pressure carburization step (a) is performed under a condition which gives a carbon concentration at the surface of the first portion in a range of 0.65±0.05 weight % after diffusion. 
   
   
       8 . The method according to  claim 1 , wherein the reduced-pressure carburization step (a) is performed under a condition which gives a carbon concentration at the surface of the second portion of 0.85 weight % after diffusion or lower. 
   
   
       9 . The method according to  claim 1 , wherein the reduced-pressure carburization step (a) has a carburization period during which the steel member is in contact with a hydrocarbon carburization gas and carbon is introduced at a surface of the steel member, and a diffusion period during which carbon is diffused inside the steel member no longer in contact with a carburization gas. 
   
   
       10 . The method according to  claim 1 , wherein the first portion of the steel member is any portion having a surface with an angle in cross-section of 130 degrees to 180 degrees. 
   
   
       11 . The method according to  claim 1 , wherein the steel member is a gear having a tooth section, and the first portion is at least one of a tooth face and a tooth bottom of the tooth section. 
   
   
       12 . The method according to  claim 1 , wherein the reduced-pressure carburization step (a) is at a temperature at least as high as the austenitizing temperature of the steel member and at a pressure of 1 to 100 hPa. 
   
   
       13 . The method according to  claim 1 , wherein the slow cooling step is performed at a cooling rate at which the steel member does not transform to martensite during cooling. 
   
   
       14 . The method according to  claim 1 , wherein the slow cooling step is performed at a cooling rate from 0.1 degree C./second to 3.0 degrees C./second while the temperature of the steel member is equal to or higher than an A1 transformation point temperature. 
   
   
       15 . The method according to  claim 1 , wherein the cooling gas used in the slow cooling step (b) is selected from the group consisting of nitrogen, helium, argon, and combinations thereof. 
   
   
       16 . The method of manufacturing a steel member according to  claim 1 , wherein the slow cooling step is performed by contacting the steel member with cooling gas having a reduced pressure lower than atmospheric pressure. 
   
   
       17 . The method according to  claim 16 , wherein the reduced pressure of the cooling gas used in the slow cooling step is in a range from 100 hPa to 650 hPa. 
   
   
       18 . The method according to  claim 16 , wherein the reduced pressure of the cooling gas used in the slow cooling step is in a range from 100 hPa to 300 hPa. 
   
   
       19 . The method according to  claim 16 , wherein the reduced pressure of the cooling gas used in the slow cooling step is raised after the temperature of the steel member becomes equal to or lower than the A1 transformation point. 
   
   
       20 . The method according to  claim 1 , wherein the quenching step is performed by heating a selected portion of the steel member, using high-density energy, to a temperature equal to or higher than the austenitizing temperature of the steel member and subsequently rapidly cooling the steel member at a cooling rate equal to or higher than a critical cooling rate, for rapid cooling at which martensitic transformation occurs in a carburization layer. 
   
   
       21 . The method according  claim 1 , wherein the quenching step is performed by induction heating by high frequency a selected portion of the steel member to heat the selected portion and then rapidly cooling the steel member by water quenching. 
   
   
       22 . The method according to  claim 21 , wherein the high-density energy is applied to a succession of steel members, one by one, and subsequently cooling the heated steel members by spraying cooling water onto each steel member while rotating the steel member. 
   
   
       23 . The method according to  claim 1 , wherein the quenching step is performed by emitting a high-density energy beam onto a selected portion of the steel member to heat the selected portion and then rapidly cooling the steel member by self-cooling. 
   
   
       24 . The method according to  claim 23 , wherein the high-density energy beam is an electron beam. 
   
   
       25 . The method according to  claim 13 , wherein the second rate of cooling is performed at a cooling rate at which the steel member transforms to martensite during cooling. 
   
   
       26 . The method according to  claim 1 , wherein the second rate of cooling is performed at a cooling rate at which the steel member transforms to martensite during cooling.