The Influence of Pearlite Volume Fraction on Rayleigh Wave Propagation in A572 Grade 50 Steel

The acoustoelastic effect is the interaction between ultrasonic wave velocity and stress. To estimate the stress a perturbation signal is introduced and the shift in time of flight is measured at the receiving location. In addition to the stress, the wave velocity can be affected by the amount of phases in the material’s microstructure. This study investigates the changes in Rayleigh wave velocity for A572 grade 50 steel as a function of stress and pearlite phase volume fraction. In order to obtain different amounts of pearlite the samples are heat treated at 970 °C for time durations of 30 min, 1 hour, 2 hours and 4 hours and then furnace cooled. The acoustoelastic coefficient for 0.5 and 1 MHz perturbation frequency is calculated by uniaxial loading of each heat treated plate while measuring ultrasonic wave velocity. The results are compared for pearlite phase volume fraction obtained from optical microscopy and hardness measurements

Microstructural Characterization and Damage Detection in Steels with Linear and Nonlinear Ultrasonic Test

The objective of this research is to study the potential of using nonlinear ultrasonic testing (NLUT) to detect damage early due to mechanical deformation or creep, as well as using it as a mean to assess microstructure changes. A comparison is also undertaken between the sensitivity of the NLUT (using nonlinearity parameter and acoustoelastic constant) and linear ultrasonic testing (LUT) technique (using wave velocity), for each damage type and microstructure assessment. This investigation consists of three different parts. In the first part of the investigation, assessment of the mechanical damage in a single-phase metal, Al-1100, was conducted to associate the strain directly with the NLUT and LUT parameters. In the second part, both NDE techniques were correlated with changes in the microstructure of the alloy after heat treatments; an A572 steel was intercritically annealed at different temperatures to generate different volume fractions of ferrite and martensite. In the third part of this study AISI 410 stainless steel specimens were submitted to different levels of creep; such damage includes both mechanical straining and microstructure changes due to exposures to different creep times and total strains. The results showed that the NLUT has the potential to detect the most minute changes in the microstructure with a sensitivity about 150 times more effective than LUT. However, using the LUT methods can still be beneficial in mapping the localized damage especially with the immersion techniques such as Scanning Acoustic Microscope. In the case of mechanical damage, the NLUT showed a continuous increase or decrease depending on the damage. For the case of creep damage assessment more work is needed to interpret the results due to the complexity of this type of damage due to mechanical and microstructural changes that occur simultaneously. There needs to be caution when interpreting the results since several factors are influential, particularly the initial condition of the component before service

The Detection of Burn-Through Weld Defects Using Noncontact Ultrasonics

Nearly all manufactured products in the metal industry involve welding. The detection and correction of defects during welding improve the product reliability and quality, and prevent unexpected failures. Nonintrusive process control is critical for avoiding these defects. This paper investigates the detection of burn-through damage using noncontact, air-coupled ultrasonics, which can be adapted to the immediate and in-situ inspection of welded samples. The burn-through leads to a larger volume of degraded weld zone, providing a resistance path for the wave to travel which results in lower velocity, energy ratio, and amplitude. Wave energy dispersion occurs due to the increase of weld burn-through resulting in higher wave attenuation. Weld sample micrographs are used to validate the ultrasonic results