Flow forming method

Abstract

A method of forming a shaped article. The method comprises providing a pre-form 40 comprising a chromium molybdenum vanadium steel alloy, heat treating the pre-form to obtain a surface hardness of between 420 and 480 according to the Vickers hardness test, and, subsequent to the heat treating step, flow forming the pre-form 40 to shape the pre-form.

1 . A method of forming a shaped article, the method comprising:
 providing a pre-form comprising a steel alloy comprising chromium, molybdenum and vanadium;   heat treating the pre-form to obtain a surface hardness of between 420 and 480 according to the Vickers hardness test; and   subsequent to the heat treating step, flow forming the pre-form to form a shaped article.   
     
     
         2 . A method according to  claim 1 , wherein the step of heat treating the pre-form is carried out to obtain a surface hardness of between 450 and 480 according to the Vickers hardness test. 
     
     
         3 . A method according to  claim 2 , wherein the step of heat treating the pre-form is carried out to obtain a surface hardness of between 460 and 480 according to the Vickers hardness test 
     
     
         4 . A method according to  claim 1 , wherein the step of heat treating the pre-form to obtain a surface hardness of between 420 and 480 according to the Vickers hardness test may comprise one or both of a hardening process to increase the hardness of the pre-form, and a tempering process to reduce the hardness of the pre-form. 
     
     
         5 . A method according to  claim 4 , wherein the step of heat treating the pre-form comprises hardening the pre-form followed by tempering the pre-form. 
     
     
         6 . A method according to  claim 4 , wherein the hardening process comprises heating the pre-form to a temperature of between approximately 935 and 945° C. followed by quenching. 
     
     
         7 . A method according to  claim 4 , wherein the tempering process comprises one or more tempering cycles, each tempering cycle comprising heating the pre-form to a dwell temperature of between 500 and 600° C. 
     
     
         8 . A method according to  claim 7 , wherein the dwell temperature is between 560 and 580° C. 
     
     
         9 . A method according to  claim 7 , wherein each tempering cycle comprises maintaining the pre-form at the dwell temperature for approximately 2.5 hours. 
     
     
         10 . A method according to  claim 1 , wherein the method comprises a stress relief heat treatment step subsequent to the flow forming step. 
     
     
         11 . A method according to  claim 10 , wherein the stress relief heat treatment step comprises heating the shaped article to a stress relief temperature. 
     
     
         12 . A method according to  claim 11 , wherein the stress relief temperature is between 500° C. and 575° C. 
     
     
         13 . A method according to  claim 11 , wherein the stress relief heat treatment step comprises heating the shaped article to the stress relief temperature for a duration of approximately 5 hours. 
     
     
         14 . A method according to  claim 1 , wherein the steel alloy comprises, in weight percent, 0.35 to 0.43% carbon, 0.1 to 0.35% Silicon, 3.0 to 3.5% chromium, 0.8 to 1.1% molybdenum, 0.15 to 0.25% vanadium, and 0.4 to 0.7% magnesium, the balance comprising iron. 
     
     
         15 . A method according to  claim 14 , wherein the steel alloy comprises, in weight percent, less than 0.007% phosphorus, less than 0.002% sulphur, less than 0.3% nickel, less than 0.002 arsenic, and less than 0.008% antimony.