Carburized and quenched member and method for production thereof

Abstract

A carburizing and hardening method enhances strength while sufficiently reducing hardening strain, without increasing the production cost, and a carburized and hardened member produced thereby. The raw material is an alloy steel which contains Fe as a main component, 0.10 to 0.50 wt. % C and 0.50 to 1.50 wt. % Si and having a hardenability J, based on an end quenching test, in a range of 35 to 50 (at 12.5 mm). After the raw material is formed into the desired shape, a carburized layer is formed by carburizing in an oxidation inhibiting atmosphere. After the carburizing, quenching is performed with cooling, uninterrupted by temperature rise, from a pearlite transformation point (A1 point) to a martensite transformation start point (Ms point), and with a severity of quenching H in a range of 0.01 to 0.08 (cm −1 ).

1 - 15 . (canceled)  
     
     
         16 . A steel carburizing and hardening method comprising: 
 providing an alloy steel, as a raw material, which contains Fe as a main component, 0.10 to 0.50 wt. % C and 0.50 to 1.50 wt. % Si, said alloy steel having a hardenability J, based on an end quenching test, in a range of 35 to 50 at 12.5 mm;    forming the alloy steel into a desired shape;    carburizing the shaped alloy steel in an oxidation inhibiting atmosphere; and    quenching the carburized alloy steel by cooling from a pearlite transformation point (A1 point) to a martensite transformation start point (Ms point), with severity of quenching H in a range of 0.01 to 0.08 (cm −1 ), and without interruption by any rise in temperature.    
     
     
         17 . A method according to  claim 16  wherein said carburizing is performed in an atmosphere having a reduced pressure of 1 to 30 hPa.  
     
     
         18 . A method according to  claim 16  wherein said carburizing is performed in an atmosphere containing an inert gas as a main component.  
     
     
         19 . A method according to  claim 16  wherein said carburizing produces 0.6 to 1.5 wt. % carbon in a carburized layer.  
     
     
         20 . A method according to  claim 16  wherein intergranular oxidation progresses from a surface of the raw material to a depth which is at most 3 μm.  
     
     
         21 . A method according to  claim 16  wherein the raw material has a surface compression residual stress of 300 to 800 Mpa.  
     
     
         22 . A method according to  claim 16  wherein said quenching is performed with the severity of quenching H being in said range during transition from a temperature in an austenite region to 300° C.  
     
     
         23 . A method according to  claim 16  wherein said quenching is accomplished by gas cooling.  
     
     
         24 . A method according to  claim 23  wherein said quenching accomplished by the gas cooling uses an inert gas.  
     
     
         25 . A method according to  claim 24  wherein the inert gas is nitrogen.  
     
     
         26 . A carburized and hardened steel alloy member produced by the method of  claim 16  having a carburized layer with a surface hardness in a range of 700 to 900 Hv, and an internal non-carburized portion, located inward of the carburized layer, having a hardness in a range of 250 to 450 Hv.  
     
     
         27 . The carburized and hardened steel member according to  claim 26  wherein the area of retained austenite in section of the carburized layer is at most 25%.  
     
     
         28 . The carburized and hardened steel alloy member according to  claim 26  wherein the area of troostite structure in section at the surface of the carburized layer is at most 10%.  
     
     
         29 . The carburized and hardened steel alloy member according to  claim 26  having an internal structure of bainite.  
     
     
         30 . The carburized and hardened member according to  claim 26  in the form of a toothed gear.