Ultrasonic Attenuation and Velocity in Three Transformation Products in Steel

Abstract

Ultrasonic attenuation measurements were made from 2 to 100 Mc/sec in the pearlitic‐plus‐ferritic, bainitic, and martensitic transformation products in SAE 4150 steel, a low‐alloy, 0.5% carbon variety. Measurements were also made in the martensitic specimen after tempering. Ultrasonic velocity measurements were made at 10 Mc/sec in each case. To eliminate the question of grain size and grain size distribution, three specimens were treated identically through the austenitizing operation. Then they were cooled differently to produce the three transformation products. The attenuation can be expressed as Af⁴+Cf², where f is frequency. The first term is Rayleigh scattering and the second may be from dislocation damping, atomic relaxations, or magnetic domain boundary effects. Both A and C are strong functions of microstructure. Both coefficients decrease in the order pearlite‐plus‐ferrite, bainite, martensite, tempered martensite. On tempering, A decreased in the ratio 3:2 while C decreased 3:1. In pearlite‐plus‐ferrite, A is larger by a factor of 225 than it is in tempered martensite, and C is larger by a factor of 10. The ratio C_T/C_L (T = transverse waves, L=longitudinal waves) was a constant independent of microstructure and equal to 2.4. This suggests either dislocation damping, an atomic relaxation, or a magnetic effect. The large change in C on tempering (with C_T/C_L = constant) indicates that the interstitial carbon is involved. The ultrasonic velocity measurements showed an increase in velocity on tempering the martensite. Pearlite‐plus‐ferrite has the highest velocities and density, while raw martensite has the lowest. The differences in velocities arise primarily from differences in the elastic moduli of the transformation products, not from the density differences.