Optimization Of Steel Metallurgy To Improve Broach Tool Life

Abstract

A steel for use as a broachable workpiece having a substantially pearlite-free microstructure whereby the service life of the broach tool is maximized. Formation of pearlite is suppressed by alloy modification; on-line processing; off-line processing; or combinations of these techniques which control the microstructure and hardness levels. The workpiece is broached into the form of a powertrain component such as, for example, a gear or race which can be surface hardened after broaching by one of carburizing, nitriding, or induction hardening depending upon the carbon content of the steel. Disclosed are steel compositions and processing methods for making the steel, the broachable workpiece and the broached and surface hardened powertrain article.

1 . A steel suitable for making a broachable workpiece, said steel having a substantially pearlite-free microstructure and a hardness of between 150 to 250 BHN prior to broaching and subsequent surface hardening by one of carburizing or induction hardening.  
     
     
         2 . The steel of  claim 1  wherein the microstructure comprises one or more of bainite, martensite and ferrite phases and a hardness of between 160 to 240 BHN.  
     
     
         3 . The steel of  claim 1  wherein said steel has a carbon content between 0.15-0.35 wt. % and is suitable for carburizing after broaching.  
     
     
         4 . The steel of  claim 1  wherein said steel has a carbon content between 0.32-0.80 wt. % and is suitable for induction hardening after broaching.  
     
     
         5 . A steel workpiece for broaching, said steel workpiece having a substantially pearlite-free microstructure and a hardness of between 150 to 250 BHN, wherein said workpiece is suitable for surface hardening subsequent to broaching by one of carburizing or induction hardening.  
     
     
         6 . The steel workpiece of  claim 5  wherein the microstructure comprises one or more of bainite, martensite and ferrite phases and a hardness of between about 160 to 240 BHN.  
     
     
         7 . The steel workpiece of  claim 5  wherein said steel is one of a carburizing type having a carbon content of between 0.15-0.35 wt. %.  
     
     
         8 . The steel workpiece of  claim 5  wherein said steel is an induction hardening type having a carbon content of between about 0.35 to 0.80 wt. %.  
     
     
         9 . A broached steel article suitable for use as a powertrain component, said article having a substantially pearlite-free microstructure and a broached surface profile wherein said surface profile has a hardened surface applied by one of carburizing or induction hardening.  
     
     
         10 . The article of  claim 9  in the form of one of a gear or race.  
     
     
         11 . The article of  claim 9  wherein the microstructure comprises one or more of bainite, martensite and ferrite phases.  
     
     
         12 . The article of  claim 11  in the form of one of a gear or race.  
     
     
         13 . The article of  claim 9  wherein said steel has a carbon content of between 0.20-0.32 wt. % and is surface hardened by carburizing.  
     
     
         14 . The article of  claim 13  in the form of one of a gear or race.  
     
     
         15 . The article of  claim 9  wherein said steel has a carbon content of between about 0.33 to 0.70 wt. % and is surface hardened by induction hardening.  
     
     
         16 . The article of  claim 15  in the form of one of a gear or race.  
     
     
         17 . A steel composition suitable for broaching comprising, in % by weight: 0.15-0.65% C; 0.5-1.75% Mn; 0.010-0.10% S; 0.10-0.70% Si; 0.1-0.50% Cr; 0.01-1.0% Ni; 0.10-0.50% Mo; 0.001-0.07% Al; 0.001-0.20% V; 0.005-0.03% B; and balance Fe plus incidental additions and impurities, less than 0.05% each.  
     
     
         18 . The steel of  claim 17  comprising 0.20-0.65% C; 0.7-1.5% Mn; 0.015-0.07% S; 0.15-0.35% Si; 0.03-0.35% Cr; 0.03-0.25% Ni; 0.15-0.40% Mo; 0.01-0.05% Al; 0.01-0.10% V; and 0.007-0.025% B.  
     
     
         19 . The steel of  claim 17  comprising 0.3-0.6% C; 0.80-1.30% Mn; 0.018-0.030% S; 0.20-0.30% Si; 0.05-0.25% Cr; 0.05-0.20% Ni; 0.20-0.30% Mo; 0.015-0.04% Al; 0.01-0.03% V; and 0.010-0.02% B.  
     
     
         20 . The steel of  claim 17  comprising 0.3-0.6% C and nominally: 1.0% Mn; 0.025% S; 0.25% Si; 0.1% Cr; 0.1% Ni; 0.25% Mo; 0.03% Al; 0.001% V; and 0.015 B.  
     
     
         21 . The steel of  claim 17  in one of a hot worked or normalized condition containing bainite, martensite and/or ferrite and having a substantially pearlite-free microstructure and suitable for surface hardening by one of carburizing or induction hardening.  
     
     
         22 . A method for making a workpiece material to be subjected to broaching and subsequent surface hardening by one of carburizing or induction hardening, comprising the steps of: 
 (a) providing a steel; and    (b) subjecting the steel to one or more treatments comprising: alloy modification, on-line thermal treatment, and off-line thermal treatment to thereby provide a steel having a substantially pearlite-free microstructure and a hardness between 150 to 250 BHN prior to broaching.    
     
     
         23 . The method of  claim 22  wherein the alloy modification treatment comprises adding one or more alloying elements to the steel for suppressing the formation of pearlite in the steel.  
     
     
         24 . The method of  claim 23  wherein Mo is added to the steel in the amount of 0.15 to 0.40 wt. % to suppress the formation of pearlite.  
     
     
         25 . The method of  claim 22  wherein the on-line thermal treatment includes the steps of austenitizing and interrupt quenching to produce a microstructure containing bainite martensite and/or ferrite.  
     
     
         26 . The method of  claim 25  including the step of tempering after the quenching step.  
     
     
         27 . The method of  claim 26  wherein the tempering step is conducted at a temperature of 1100° F. to 1340° F. for 1 to 4 hours.  
     
     
         28 . The method of  claim 22  wherein the off-line thermal treatment includes one of normalizing, normalizing and tempering, and austenitizing, quenching and tempering.  
     
     
         29 . The method of  claim 28  wherein the steel is one of normalized or normalized and tempered to produce a microstructure containing bainite and ferrite.  
     
     
         30 . The method of  claim 28  wherein the steel is austenitized, quenched and tempered to produce a microstructure containing tempered martensite.  
     
     
         31 . The method of  claim 28  wherein the tempering in the off-line thermal treatment is conducted at 1200° to 1340° F. for 1 to 4 hours.  
     
     
         32 . The method of  claim 22  including the step of forming the steel into a hot rolled tubular shape to provide a workpiece for broaching powertrain gears and races.  
     
     
         33 . The method of  claim 22  wherein said steel is subjected to at least two of said treatments.  
     
     
         34 . A method for making a powertrain component comprising the steps of: 
 (a) providing a molten steel having a carbon content between about 0.15 to 0.80 wt. % C and optionally subjecting the molten steel to an alloy modification treatment to suppress the formation of a pearlite phase;    (b) subjecting the steel to one or more treatments including on-line thermal treatment and off-line thermal treatment to suppress the formation of the pearlite phase;    (c) providing a steel workpiece for broaching having a substantially pearlite-free microstructure and a hardness between 150 to 250 BHN;    (d) broaching the steel workpiece to form a broached powertrain component; and    (e) surface hardening the broached powertrain component by selecting one of carburizing or induction hardening.    
     
     
         35 . The method of  claim 34  wherein the steel contains between 0.2 to about 0.32 wt. % C and the surface hardening step selected is carburizing.  
     
     
         36 . The method of  claim 35  wherein the steel contains greater than 0.32 wt. % C up to 0.65 wt. % C and the surface hardening step selected is induction hardening.  
     
     
         37 . The method of  claim 34  wherein the optional alloy modification treatment in step (a) comprises adding 0.10 to 0.50 wt. % Mo to the molten steel.  
     
     
         38 . A method for making a powertrain component comprising the steps of: 
 (a) providing a molten steel having a carbon content between about 0.15 to 0.80 wt. % C;    (b) subjecting the molten steel to an alloy modification treatment to suppress the formation of a pearlite phase;    (c) subjecting the steel to one or more of on-line thermal treatment and off-line thermal treatment to suppress the formation of a pearlite phase and provide a workpiece for broaching having a substantially pearlite-free microstructure;    (d) broaching the workpiece to form a broached powertrain component; and    (e) surface hardening the broached powertrain component by one of carburizing, nitriding or induction hardening.    
     
     
         39 . The method of  claim 38  wherein the steel contains between 0.2 to about 0.32 wt. % C and the surface hardening step is one of carburizing or nitriding.  
     
     
         40 . The method of  claim 38  wherein the steel contains between 0.35 to 0.65 wt. % C and the surface hardening step selected is induction hardening.