Method of treating a workpiece comprising a titanium metal and object

Abstract

Method of treating a workpiece comprising a titanium metal, wherein a titanium metal surface layer of the workpiece is converted to titanium nitrides. The method comprises the following steps; a) heating the workpiece to an initial nitriding temperature (Tn1) and b) subjecting said workpiece to one or more nitriding temperatures (Tn1, Tn2) for predetermined time(s) in a nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions for converting the titanium metal surface layer to a first layer portion consisting of titanium nitrides and a second layer portion comprising a nitrogen gradient in the titanium metal. The method further comprises c) quenching the workpiece in the nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions, in order to strengthen the titanium metal below the in step b) formed first nitride layer portion.

1 . A method of treating a workpiece comprising a titanium metal, wherein a titanium metal surface layer of the workpiece is converted to titanium nitrides, which method comprises the following steps;
 a) heating the workpiece to an initial nitriding temperature (T n1 );   b) subjecting said workpiece to one or more nitriding temperatures (T n1 , T n2 ) for predetermined time(s) in a nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions for converting the titanium metal surface layer to a first layer portion consisting of titanium nitrides and a second layer portion comprising a nitrogen gradient in the titanium metal;   the method being characterized by;   c) quenching the workpiece in the nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions at a quenching rate of at least 150 K/min, in order to strengthen the titanium metal below the, in step b) formed, first nitride layer portion.   
     
     
         2 . The method according to  claim 1 , wherein the work piece, before step c), is subjected to at least one temperature (T n1 ) at or above the β phase transus temperature for the titanium alloy in question for solution treatment of the titanium alloy, to convert prior α phase or α+β phase structure to solely or predominantly β phase structure. 
     
     
         3 . The method according to  claim 1 , wherein step c) comprises quenching the workpiece at a cooling rate which is high enough to, at least partially, transform β phase by a martensitic transformation directly to α′ phase or α″ phase. 
     
     
         4 . The method according to  claim 1 , wherein said quenching rate is chosen for delaying the remaining transformation of β phase to α phase at lower temperatures, resulting in substantially finer microstructures. 
     
     
         5 . The method according to  claim 1 , wherein the workpiece, after step c), is subjected to at least one aging temperature (T q2 ) at a predetermined time to obtain precipitation hardening of the titanium metal. 
     
     
         6 . The method according to  claim 1 , wherein the workpiece is cooled to room temperature after quenching or after subjecting the workpiece to the at least one aging temperature for the predetermined time. 
     
     
         7 . The method according to  claim 1 , wherein the workpiece is positioned in one and the same hot isostatic press chamber during the entire execution of the method. 
     
     
         8 . The method according to  claim 1 , wherein the workpiece is subjected to the nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions during the entire execution of the method. 
     
     
         9 . The method according to  claim 1 , wherein the titanium metal comprises at least one of the following; titanium alloy of Grade 1, Grade 2, Grade 5 or Grade 9. 
     
     
         10 . The method according to  claim 1 , wherein said workpiece, in step b), is quenched at a quenching rate sufficient to prevent the formation of α phase structure. 
     
     
         11 . (canceled) 
     
     
         12 . The method according to  claim 1 , wherein step c) is carried out at hot isostatic pressure conditions where the nitrogen gas pressure is at least initially above 10 MPa. 
     
     
         13 . The method according to  claim 1 , wherein the workpiece comprises titanium alloy of Grade 2, Grade 5 or Grade 9 and wherein step b) comprises quenching the workpiece at a quenching rate of;
 at least 900 K/min and preferably at least 1200 K/min for Grade 2;   at least 210 K/min and preferably at least 420 K/min for Grade 5; or   at least 300 K/min for Grade 9.   
     
     
         14 . An object comprising a titanium metal alloy of Grade 2, Grade 5 or Grade 9, characterized in that the object exhibits a surface layer comprising a first nitride layer portion and second titanium metal portion which exhibits solely or predominantly α′ phase or α″ phase structures and a nitrogen gradient, which surface layer extends to a depth of at least 50 μm when the titanium metal alloy is Grade 2 or Grade 9 and at least 75 μm when the titanium metal alloy is Grade 5. 
     
     
         15 . The object according to  claim 14 , wherein the object exhibits a hardness of at least 265 HV0,1 at 25 μm; at least 325 HV0,1 at 25 μm and at least 420 HV0,1 at 50 μm, when the titanium metal alloy is Grade 2, Grade 9 and Grade 5 respectively. 
     
     
         16 . (canceled) 
     
     
         17 . The object according to  claim 14 , which object constitutes or forms part of a component chosen from a group of components comprising; automotive, aerospace, mining and medical components. 
     
     
         18 . The object according to  claim 14 , which object has been subjected to a method according to  claim 1 .