For example, the automotive industry is one of the largest markets for steel industry. Therefore, it must fulfill some specific requirements like enhancing the fuel efficiency and reducing the vehicle mass without sacrificing current strength and stiffness of the steels. Young's modulus and internal friction data are key elements to investigate the defect interaction during the different manufacturing processes and their influence on the mechanical properties.
Choi et al. used the impulse internal friction technique to study the aging behavior of the 22MnB5 PHS grade (containing 0.24 wt% C, 1.22 wt% Mn, and 20 ppm B) during the paint-baking process with a RFDA LTVP800 measurement setup. Specimens of different lengths were used to obtain a resonant frequency of 0.7 kHz ,1.2 kHz ,and 2.1 kHz. The peak parameters obtained at these three frequencies were used to calculate the activation energy corresponding to each damping peak.
The IF spectrum of the 22MnB5 PHS was obtained (a) in the as- quenched state, (b) after the paint bake-hardening simulation.
In fig. 1 the internal friction spectrum and the temperature dependence of Young’s modulus of die-quenched 22MnB5 PHS is shown. A thermally activated relaxation process gives rise to a peak in the IF spectrum. Four distinct Debye peaks, related to specific dislocation processes, can be observed with their specific activation energies and relaxation times. For example, the P3 peak (dislocation-enhanced Snoek (DES) peak) represent the interaction between interstitial C atoms and already existing kinks on non-screw dislocation segments.
In fig. 2 is clearly shown that the paint bake hardening after press hardening affected the DES type P3 peak significantly, as a result of the C diffusion to edge locations and their pinning effect on the dislocations by the suppression of kink formation, and the formation of transition carbide precipitates.
A tensile deformation increased again the height of the P3 peak. This may due to the introduction of new dislocations both edge and screw type, or the strain-induced transformation of a small volume fraction of retained austenite to fresh martensite.
- W.S.Choi et al., Internal-friction analysis of dislocation – interstitial carbon interactions in press-hardened 22MnB5 steel, Materials Science & Engineering, A639 (2015) 439–447.