Method and device for hardening the inner surface of holes, in mechanical pieces of cast iron of predominantly ferritic matrix

Abstract

A method and a device for hardening at least one portion of the inner surface ( 12 ) of a hole ( 11 ) made in a mechanical piece ( 10 ) of cast iron of predominantly ferritic matrix, comprising the step of subjecting said portion of the inner surface ( 12 ) to the action of a laser beam ( 100 ), until it causes the melting of its surface layer. In particular, said method comprises the steps of: projecting the laser beam ( 100 ) in axial direction inside the hole of the piece ( 10 ); deviating the laser beam ( 100 ) through a deviator means ( 4 ), so that the same is projected onto the inner surface of the hole itself, causing the melting of a surface layer; rotating the deviator means ( 4 ) or the piece ( 10 ) with respect to each other, and sliding in axial direction the deviator means ( 4 ) or the piece ( 10 ) with respect to each other, until the desired portion of the inner surface ( 12 ) is subjected to the laser beam ( 100 ).

1 . Method for hardening at least one portion of the inner surface ( 12 ) of a hole ( 11 ) made in a mechanical piece ( 10 ) of cast iron of predominantly ferritic matrix, characterised in that it comprises the step of subjecting said portion of the inner surface ( 12 ) to the action of a laser beam ( 100 ), until it causes the melting of its surface layer.  
   
   
       2 . Method according to  claim 1 , characterised in that it comprises the steps of: 
 a) projecting a laser beam ( 100 ) in axial direction inside the hole of the piece ( 10 );    b) deviating the laser beam through a deviator means ( 4 ) so that the same is projected onto the inner surface ( 12 ) of the hole itself, causing the melting of a surface layer;    c) rotating the deviator means ( 4 ) or the piece ( 10 ) with respect to each other; and    d) making the deviator means ( 4 ) or the piece ( 10 ) slide in axial direction with respect to each other, until said portion to be treated of the inner surface ( 12 ) is entirely subjected to the laser beam ( 100 ).    
   
   
       3 . Method according to  claim 1 , characterised in that the thickness of the melted layer of cast iron of predominantly ferritic matrix is less than 0.5 mm.  
   
   
       4 . Method according to  claim 1 , characterised in that the wavelength of the laser beam ( 100 ) is comprised between 800 nm and 950 nm.  
   
   
       5 . Method according to  claim 1 , characterised in that the power of the laser beam ( 100 ) is comprised between 1500 W and 2200 W.  
   
   
       6 . Method according to  claim 1 , characterised in that the focal distance of the laser beam ( 100 ) is comprised between 66 mm and 500 mm.  
   
   
       7 . Method according to  claim 1 , characterised in that the laser beam ( 100 ) generates on the inner surface ( 12 ) a rectangular luminous spot, with sides comprised between 4 mm and 9 mm.  
   
   
       8 . Method according to  claim 1 , characterised in that the laser beam ( 100 ) generates on the inner surface ( 12 ) a luminous spot which moves, relative to said surface ( 12 ), at a speed comprised between 20 mm/s and 40 mm/s.  
   
   
       9 . Device for hardening, by means of laser, at least one portion of the inner surface ( 12 ) of a hole ( 11 ) made in a mechanical piece ( 10 ) of cast iron of predominantly ferritic matrix, characterised in that it comprises: 
 a laser source ( 2 ) adapted to generate a laser beam ( 100 ), and project it in axial direction inside the hole ( 11 ) of the piece ( 10 );    a deviator means ( 4 ) of the laser beam ( 100 ) which, associated with said laser source ( 2 ), is adapted to be inserted in the hole ( 11 ) of the piece ( 10 ), and to deviate the beam ( 100 ), projecting it onto the inner surface ( 12 ); and    a support device ( 5 ) of the cast iron piece ( 10 ); said support device ( 5 ) and deviator means ( 4 ) being adapted to rotate with respect to each other, and to slide in axial direction with respect to each other, until said portion to be treated of the inner surface ( 12 ) is entirely subjected to the laser beam ( 100 ).    
   
   
       10 . Device according to  claim 9 , characterised in that the support element ( 5 ) comprises a rotating mandrel ( 52 ) adapted to hold the piece ( 10 ) so that the axis of the hole ( 11 ) coincides with the rotation axis of the mandrel ( 52 ) itself, and to set it in rotation at a predetermined speed.  
   
   
       11 . Device according to  claim 9 , characterised in that the support element ( 5 ) comprises a motorised carrier ( 50 ) adapted to slide with reciprocating motion along a rectilinear direction, so to engage the piece ( 10 ) to slide in the direction defined by the axis of the hole ( 11 ).  
   
   
       12 . Device according to  claim 9 , characterised in that the deviator means comprises a faceted body ( 4 ), provided with a reflecting surface ( 40 ) which is sloped with respect to the direction of the laser beam ( 100 ) leaving the laser source ( 2 ).  
   
   
       13 . Device according to  claim 12 , characterised in that the faceted body ( 4 ) is realised in copper.  
   
   
       14 . Device according to  claim 12 , characterised in that the reflecting surface ( 40 ) is made specular by means of polishing.  
   
   
       15 . Device according to  claim 9 , characterised in that it comprises means adapted to cool the deviator means ( 4 ).  
   
   
       16 . Device according to  claim 9 , characterised in that it comprises means adapted to convey a flow of protective gas which, leaving a supply nozzle ( 6 ), is adapted to hit the deviator means ( 4 ) and the surface ( 12 ).