Machine part and method for manufacturing same

Abstract

A mechanical component 1 composed of a steel is surface-hardened by nitriding, and specifically has a surficial layer 2 raised in the surface hardness by the nitriding, and an inner portion 3 not affected by the nitriding and showing a nearly constant hardness. Vickers hardness of the inner portion 3 is adjusted to 190 to 260 HV, Vickers hardness of the surficial portion 2 measured at a reference position corresponded to a 50 μm depth from the component surface is adjusted to 340 to 460 HV, and effective depth of hardened layer measured from the component surface 4 , where a Vickers hardness of 270 HV is achieved, is adjusted to 0.3 mm or more. This makes it possible to provide a mechanical component which is composed of a steel and surface-hardened by nitriding, and is successfully improved in the strength and bending straightening property, and also to provide a method of fabricating the same.

1 . A mechanical component composed of a steel and surface-hardened by nitriding, 
 having a Vickers hardness of the surficial portion measured at a reference position corresponded to a 50 μm depth from the surface of said mechanical component of 340 to 460 HV, having a Vickers hardness of the inner portion being not affected by the nitriding and showing a nearly constant hardness of 190 to 260 HV, and having an effective depth of hardened layer measured from the surface of said component, where a Vickers hardness of 270 HV is achieved, of 0.3 mm or more.    
     
     
         2 . The mechanical component as claimed in  claim 1 , 
 having compositions of the individual constituent elements adjusted so as to limit Cr[eq.] to 0.72% or more and 1.0% or less, and C[eq.] to 0.65% or more and 0.86% or less, under definitions of Cr[eq.]=0.475×C+0.164×Si+0.241×Mn+Cr C[eq.]=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr, and    having a hardness distribution profile H(x), which is given by plotting, on H-x plane, Vickers hardness H measured in the depth-wise direction x as viewed from the component surface, fallen in region Z expressed by the equation (1) below:                            H   ′     ⁡     (   x   )       =         H   ′     ⁢   0     +       (         H   ′     ⁢   1     -       H   ′     ⁢   0       )     ×     [     1   -     erf   ⁡     (     x     2   ⁢       α   ⁢           ⁢   Dt           )         ]     ⁢           ⁢   where         ,     
     ⁢         H   ′     ⁢   0     =         C   ⁡     [     eq   .     ]       ×   254     +   33.8         ⁢     
     ⁢         H   ′     ⁢   1     =         Cr   ⁡     [     eq   .     ]       ×   392     +   65.8       ⁢     
     ⁢       Cr   ⁡     [     eq   .     ]       ⁢     :     ⁢           ⁢   chromium   ⁢           ⁢   equivalence   ⁢           ⁢   described   ⁢           ⁢   in   ⁢           ⁢   the   ⁢           ⁢   above     ⁢     
     ⁢         C   ⁡     [     eq   .     ]       :           ⁢     carbon   ⁢           ⁢   equivalence   ⁢           ⁢   described   ⁢           ⁢   in   ⁢           ⁢   the   ⁢           ⁢   above       ;     ⁢     
     ⁢     D   =       D   0     ×     exp   ⁡     (       -   Q       R   ×     (     T   +   273     )         )           ⁢     
     ⁢       D   0     ⁢     :     ⁢           ⁢   1.13   ×     10     -   6         ⁢     
     ⁢     Q   ⁢     :     ⁢           ⁢   83   ×     (     1   -     14.03     T   +   273         )     ×   1000     ⁢     
     ⁢       R   ⁢     :     ⁢           ⁢   8.314     ;     ⁢     
     ⁢     α   =     exp   ⁡     (         -   1.47     ×   Si     -     0.918   ×   Mn     +   0.998     )         ⁢     
     ⁢   Si   ⁢     :     ⁢           ⁢   Si   ⁢           ⁢   content   ⁢           ⁢     (     wt   ⁢           ⁢   %     )       ⁢     
     ⁢       Mn   ⁢     :     ⁢           ⁢   Mn   ⁢           ⁢   content   ⁢           ⁢     (     wt   ⁢           ⁢   %     )       ;   and             (   1   )                 region Z is defined as a region in which H′(x) expressed by the equation (1) can move on the H-x plane while satisfying a condition of H′(0.3×10 −3 )≧270, when t varies from 3.6×10 3  to 18×10 3  and T varies from 500 to 650.    
     
     
         3 . The mechanical component as claimed in  claim 2 , having, in % by weight, an Fe content of 90% or more, and containing constituent elements with the individual contents of C: 0.35 to 0.5%, Si: 0.01 to 0.3%, Mn: 0.6 to 1.8%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr: 0.01 to 0.5%, Al: 0.001 to 0.01% and N: 0.005 to 0.025%.  
     
     
         4 . The mechanical component as claimed in  claim 3 , containing any one species, or two or more species of constituent elements with the individual contents, in % by weight, of Pb: 0.30% or less, S: 0.20% or less, Ca: 0.01% or less, Bi: 0.30% or less, Ti: 0.02 or less, Zr: 0.02% or less and Mg: 0.01% or less.  
     
     
         5 . The mechanical component as claimed in  claim 1 , wherein said mechanical component is a crank shaft.  
     
     
         6 . A method of fabricating a mechanical component composed of a steel and surface-hardened by nitriding, 
 said nitriding being carried out so as to adjust Vickers hardness of the surficial portion measured at a reference position corresponded to a 50 μm depth from the surface of said mechanical component to 340 to 460 HV, so as to adjust Vickers hardness of the inner portion being not affected by the nitriding and showing a nearly constant hardness to 190 to 260 HV, and so as to adjust an effective depth of hardened layer measured from the surface of said component, where a Vickers hardness of 270 HV is achieved, to 0.3 mm or more.    
     
     
         7 . The method of fabricating a mechanical component as claimed in  claim 6 , wherein compositions of the individual constituent elements are adjusted so as to limit Cr[eq.] to 0.72% or more and 1.0% or less, and C[eq.] to 0.65% or more and 0.86% or less, under definitions of  
         Cr[eq.]=0.475×C+0.164×Si+0.241×Mn+Cr  C[eq.]=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr, and said nitriding is  
       carried out by gas soft nitriding or salt-bath nitriding under conditions of a process time of 3.6×10 3  to 18×10 3  seconds, and a process temperature of 500 to 650° C.  
     
     
         8 . The mechanical component as claimed in  claim 2 , wherein said mechanical component is a crank shaft.  
     
     
         9 . The mechanical component as claimed in  claim 3 , wherein said mechanical component is a crank shaft.  
     
     
         10 . The mechanical component as claimed in  claim 4 , wherein said mechanical component is a crank shaft.