Properties and formation mechanism of cladding layer on high-strength low-alloy steel subjected to ultrasonic impact treatment with titanium alloy pin

Abstract

The EQ70 high-strength low-alloy steel plate was treated using ultrasonic impact treatment (UIT) with titanium alloy (Ti6Al4V) pins. Scanning electron microscopy with energy dispersive X-ray microanalysis and X-ray photoelectron spectroscopy were employed to study the structure, morphology, surface chemical state, and composition of the treated product on the surface of the EQ70 steel. The microhardness was measured on the surface and the cross-section of the specimens. And the electrochemical corrosion test in the 3.5 wt% NaCl medium was carried out to evaluate the corrosion resistance of the UIT-treated specimens with different impact intensities and that without UIT. It is found that the UIT with Ti6Al4V pins can result in a mixed oxide cladding layer on the surface of the steel plate, the thickness of which increased with the impact intensity (treatment time). The elements in the cladding layer mainly come from the impact pin (Ti, Al, V), base metal (Fe), and the atmosphere (O), and the contents of these elements do not change significantly with the impact intensity. TiO, TiO2, FeO and Fe2O3 are found to be the main oxides in the cladding layer. The mutual effect of the material transfer between the pin and the treated base metal is found. The surface hardness of the cladding layer increases with the impact intensity, reaching an almost constant value (approximately twice the hardness of the parent material) when the impact intensity is over 1.33 s/mm2. The cladding layer can improve the corrosion-resistant properties of the EQ70 steel and get the best corrosion-resistant properties with an impact intensity ranging from 1.00 to 1.33 s/mm2. The possible formation procedure of the cladding layer after UIT with titanium alloy pins is suggested. The chip from the pin (material transfer from the pin) and the mechanochemical oxidation is the main cause to form the cladding layer.