Ultrasonic nondestructive evaluation of matrix structures and nodularity in cast irons
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I.
INTRODUCTION
S O M E twenty years ago, the nondestructive testing (NDT) techniques in the evaluation of material structures and the prediction of material behavior began to evolve. The techniques include X-ray, eddy current, neutron scattering, electromagnetic, and ultrasonic characterization of materials. K i m et al. worked on the microstructure evaluation of a ferrite stainless steel by small-angle neutron scattering, m Houska characterized the composition and structure of near-surface zones by X-ray diffraction, ~21 and Polanschutz estimated the martensite content in steel by an electromagnetic method. ]31 Other nondestructive evaluation methods are often based on the consideration of physical mechanisms which control materials behavior in some specific ways. Among these, the ultrasonic technique is very useful and has been applied rather extensively. In addition to the determination of material parameters such as elastic modulus and densities of P / M p a r t s , 141 it was also applied to the determination of grain size, [5,6'71 degree of porosity and amount of second-phase particles, t81 measurement of texture, ~9~ and residual stress, t~~ Acoustic velocity and ultrasonic attenuation were used as investigative tools for the above purposes. However, the correlation of ultrasonic characteristics with a material's SHEN-CHIH LEE, Professor and Chairman, is with the Department of Materials Engineering, Tatung Institute of Technology, Taipei, Taiwan, 10451, Republic of China. JIA-MING SUEN, formerly Graduate Student, Department of Materials Engineering, Tatung Institute of Technology, is now in the compulsory ROTC military service of the Republic of China. Manuscript submitted December 23, 1988. METALLURGICAL TRANSACTIONS A
intrinsic properties is difficult to obtain if there are too many variables involved. Hence, it is necessary to have a significant parameter which may affect both the material properties and ultrasonic behaviors to render meaningful results, such as the graphite morphology in cast irons. In the production of ductile cast iron, the spheroidal graphites in the structure may degenerate to flake or vermicular graphites due to a number of reasons mJ and result in the deterioration of mechanical properties. It is of vital importance to develop an N D T method which could identify the nodularity, and possibly the matrix structures, of the ductile iron for quality control and product reliability purposes. It is the aim of this research to apply ultrasonic NDT for the evaluation of the nodularity/matrix structures, as well as their correlation to the mechanical properties, of the cast irons under study.
II.
THEORETICAL BACKGROUND
A. The Velocity of Ultrasonic Wave Propagation in Material The velocities of the various kinds of sound waves which propagate in solid material can be calculated from the modulus of elasticity E, density p, and Poisson's ratio /x. tiE1 For longitudinal waves,
VL = X/[E/p] • [(1 - / z ) / ( 1 +/.t) (1 - 2/z)]
[1]
For transverse waves,
Vr=
[E/pl X
2(1+~)
VOLUME 20
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