High-Concentration Carburized/Low-Strain Quenched Member and Process for Producing the Same

Abstract

A super carburized, low-distortion quenched member with higher performance and minimized heat-treatment distortion is provided. A process for the production thereof includes a primary treatment and a secondary treatment. The primary treatment includes heating a steel member for a machine structure to a temperature within an austenite region by vacuum carburizing (low-pressure carburizing) to have carbon dissolved at least at a eutectoid carbon concentration of a surface layer portion of the member and then quenching the member to have at least one of ultrafine carbide and nuclei of the carbide formed in the surface layer portion of the member. The secondary treatment includes subsequently heating and soaking the member to a temperature within the austenite region and then conducting rapid quenching to have ultrafine carbide precipitated in an outermost surface layer portion.

1 . A process for producing a super carburized, low-distortion quenched member, which comprises a primary treatment of heating a steel member for a machine structure to a temperature within an austenite region by vacuum carburizing (low-pressure carburizing) to have carbon dissolved at least at a eutectoid carbon concentration of a surface layer portion of said member and then quenching said member at a cooling rate of from 3 to 15° C./sec from said temperature within said austenite region to a temperature not higher than an A 1  transformation point to have at least one of ultrafine carbide and nuclei of said carbide formed in said surface layer portion of the said member; and a secondary treatment of subsequently heating and soaking said member to a temperature within said austenite region and then conducting rapid quenching to have ultrafine carbide precipitated in a range of from 10 to 30% in terms of effective case depth percentage in an outermost surface layer portion. 
     
     
         2 . A process according to  claim 1 , wherein in said secondary treatment, additional carburizing treatment is applied to said surface layer portion of said member. 
     
     
         3 . A process according to  claim 2 , wherein in said secondary treatment, said ultrafine carbide is caused to precipitate in said surface layer portion of said member to form a structure composed primarily of martensite and containing a mixed structure of troostite, retained austenite and the like in parts thereof such that said outermost layer portion (a portion A) of said layer, a layer portion (a portion B) inner than said portion A and a layer portion (a portion C) inner than said portion B are in an order of A≧C≧B in terms of the fineness of austenite grain size. 
     
     
         4 . A super carburized, low-distortion quenched member comprising a surface layer portion of a structure composed primarily of martensite and containing a mixed structure of troostite and retained austenite or the like in parts thereof, wherein in said surface layer, an outermost surface layer (a portion A), a layer (a portion B) inner than said portion A and a layer (a portion C) inner than said portion B are in an order of A≧C≧B in terms of the fineness of austenite grain size.