Quenching method

Abstract

A quenching method of cooling an object to be treated placed on a support base, the quenching method includes a lowering control step controlling the support base so that the support base is lowered into a coolant accumulated in a state of the coolant not flowing in a cooling tank, and immersion of the object is completed and the support base is stopped while a vapor film of the coolant is formed around the object by heat of the object, and a state maintaining step of maintaining a state in which the coolant does not flow and a state in which the support base does not move so that a relative flow velocity is not applied between the object and the coolant at least until a surface of the object to be treated undergoes martensite transformation after the lowering of the support base is stopped.

1 . A quenching method of cooling an object to be treated placed on a support base, the quenching method comprising:
 a lowering control step of controlling the support base so that the support base is lowered into a coolant accumulated in a state of the coolant not flowing in a cooling tank, and immersion of the object to be treated is completed and the support base is stopped while a vapor film of the coolant is formed around the object to be treated by heat of the object to be treated; and   a state maintaining step of maintaining a state in which the coolant does not flow and a state in which the support base does not move so that a relative flow velocity is not applied between the object to be treated and the coolant at least until a surface of the object to be treated undergoes martensite transformation after the lowering of the support base is stopped.   
     
     
         2 . The quenching method according to  claim 1 , wherein
 a period during which the vapor film is formed   is a period from when immersion of a portion immersed first of the object to be treated is started to when a vapor film disappears and boiling starts around the portion.   
     
     
         3 . The quenching method according to  claim 1 , wherein
 a period during which the vapor film is formed   is a period from when immersion of the object to be treated is started to when a portion where a surrounding vapor film disappears and boiling starts occurs.   
     
     
         4 . The quenching method according to  claim 1 , wherein
 a period during which the vapor film is formed   is a period in which an amount of change of a surface heat transfer coefficient between the coolant and the object to be treated with respect to an amount of decrease in temperature reaches a predetermined value or less first and then exceeds the predetermined value in a process in which the object to be treated is immersed in the coolant and temperature decreases.   
     
     
         5 . The quenching method according to  claim 1 , wherein
 a lowering speed of the object to be treated in the lowering control step   is a distance from a liquid level of the coolant to a lower end of the object to be treated at a descending stop position/a period during which the vapor film is formed.   
     
     
         6 . The quenching method according to  claim 1 , wherein
 a plurality of the objects to be treated is placed at a plurality of different positions in a vertical direction of the support base, wherein   the lowering control step includes completing immersion of an uppermost portion of an uppermost object to be treated and stopping the support base during a period in which the vapor film of a lowermost object to be treated is formed, and wherein   the state maintaining step includes maintaining a state in which a relative flow velocity is not applied between the object to be treated and the coolant until a surface of all of the plurality of object to be treated undergoes martensite transformation.   
     
     
         7 . The quenching method according to  claim 1 , wherein
 a plurality of the objects to be treated is placed at a plurality of different positions in a vertical direction of the support base, wherein   the lowering control step includes completing immersion of an uppermost portion of an uppermost object to be treated and stopping the support base in a period from when immersion of the plurality of objects to be treated is started to when a portion where a vapor film around a lowermost object to be treated disappears and boiling starts occurs, and wherein   the state maintaining step includes maintaining a state in which a relative flow velocity is not applied between the object to be treated and the coolant until a surface of all of the plurality of object to be treated undergoes martensite transformation.   
     
     
         8 . The quenching method according to  claim 1 , wherein
 the object to be treated is asymmetrical in a vertical direction, and is placed on the support base so that a portion, of the object to be treated, where a surrounding vapor film disappears earliest and boiling starts faces upward in the vertical direction.   
     
     
         9 . The quenching method according to  claim 1 , wherein
 a surface of the object to be treated is regarded to have undergone martensite transformation when a martensite fraction of the surface of the object to be treated reaches a predetermined value.   
     
     
         10 . The quenching method according to  claim 1 , wherein
 the surface of the object to be treated is regarded to have undergone martensite transformation when a predetermined period has elapsed since immersion of the object to be treated was started.   
     
     
         11 . The quenching method according to  claim 1 , wherein
 a state in which a relative flow velocity is not applied between the object to be treated and the coolant   is a state in which the coolant is not moved by an external force, and the support base is not moved.