Method of nondestructive examination of chromium-containing nickel-based alloy for grain boundary corrosion and examination apparatus

Abstract

Discloses is a nondestructive inspection method of grain-boundary attack due to thermal sensitization in a chromium-containing nickel-based alloy, such as Inconel 600 alloy. The method comprises measuring a saturation magnetization M s (T i ) of a test piece at each of a plurality of measuring temperatures defined by equally dividing a given measuring temperature range in the range of a minimum to a maximum of Curie temperatures corresponding to respective chromium concentrations in a chromium impoverished region of the alloy, and then quantitatively determining an average spatial distribution of the chromium impoverished region of the test piece, or the chromium-concentration-specific volume of the chromium impoverished region adjacent to the crystal grain boundaries of the test piece, according to a given calculation formula. The present invention can solve disadvantages in a conventional method of inspecting a chromium impoverished region of a chromium-containing nickel-based alloy, such as destruction of the alloy surface caused by an etching or breaking operation, which is incongruous with the philosophy of a nondestructive inspection, and poor information about chromium impoverished region, which is obtainable only in the alloy surface.

1 . A method for nondestructive inspection of grain-boundary attack due to thermal sensitization in a chromium-containing nickel-based alloy, comprising: 
 measuring a saturation magnetization M s (T i ) of a test piece at each of a plurality of measuring temperatures defined by equally dividing a given measuring temperature range in the range of a minimum to a maximum of Curie temperatures corresponding to respective chromium concentrations in a chromium impoverished region of said alloy; and    calculating vk according to the following formula (1) to quantitatively determine the volume of the chromium impoverished region in a divided manner on the basis of the chromium concentrations:                        M   s     ⁡     (     T   i     )       =       ∑     k   =   1     i     ⁢           ⁢         v   k     ⁢       M   k     ⁡     (     T   i     )         V         ,           (   1   )                 wherein: vk is the volume of the chromium impoverished region having a chromium concentration C k ;    V is the volume of said test piece;    k is a natural number to be determined in conjunction with dividing the range of a minimum measuring temperature T min  to a maximum measuring temperature T max , into n equal parts, in conformity to measurement conditions; and    M k  (T i ) is a saturation magnetization at a measuring temperature T i  in the chromium impoverished region having the chromium concentration C k , said saturation magnetization being obtained in advance based on the following data (a), (b) and (c):    (a) the relationship between saturation magnetization and chromium concentration at an absolute temperature of 0 (zero) K in the chromium impoverished region;    (b) the relationship between Curie temperature and chromium concentration in the chromium impoverished region; and    (c) the relationship between saturation magnetization and measuring temperature in the chromium impoverished region.    
     
     
         2 . An apparatus for detecting magnetic characteristics of the test piece for use in the method as defined in  claim 1 , comprising: 
 a cooling-medium tank for containing a cooling medium;    a test-piece housing disposed at the central region of said cooling-medium tank to receive said test piece therein;    an exciting device mounted on the inner wall of said test-piece housing to excite said test piece;    a support member for supporting said test piece in such a manner that it is located at the center position of said exciting device;    a magnetic flux detector disposed around said test piece;    a cooling device for supplying a cooling medium to said cooling-medium tank to circulatingly cool said test piece while allowing cooling gas generated from said cooling medium to flow into said test-piece housing;    a heating device disposed below said test piece to heat said test piece; and    means for controlling the measuring temperature of said test piece through the use of said cooling medium and said heating device.