Yangyang Zhao

A custom-designed high-temperature sliding-on-sheet-strip (SOSS) tribo-tester was used to simulate the high-temperature friction process of 10Mn5 medium manganese steel bare plate under actual hot stamping conditions. To reveal its high-temperature friction mechanism in the hot forming process, the high-temperature friction behavior of 10Mn5 steel and 22MnB5 steel was compared. The scanning electron microscope (SEM), energy spectrum analyzer (EDS) and X-ray diffractometer (XRD) were used to investigate the structure of the oxide layer, composition of physical phase, wear surface morphology and elemental composition. The results show that the average coefficient of friction of 10Mn5 steel is 12.7% lower than that of 22MnB5 steel. The cross-section of both steel consists of an oxide layer, an alloying element-rich layer and the matrix. The oxide layer of 10Mn5 steel is mainly composed of Fe₃O₄, approximately 63.7%, while that of 22MnB5 is mainly composed of Fe₂O₃, approximately 66.9%. The complete and less flaking scale of 10Mn5 steel provides good wear protection, and the mechanism is abrasive with slight adhesive wear. Meanwhile, oxide particles and fragments are embedded in the 22MnB5 surface thus increasing the wear, and the mechanism evolves into severe abrasive and adhesive wear. The difference in the mechanism between the two steels is mainly caused by different austenitizing temperatures, which for 10Mn5 is lower than 22MnB5, about 100 °C. This makes the thermal stress of 10Mn5 from the temperature difference between the furnace and the environment not enough to break the scale and decrease abrasion.

The effect of a Cu addition on the corrosion behavior of 304 austenitic stainless steel in a solution was elaborately investigated through salt spray tests, FeCl₃ immersion tests and characterization analyses. The corrosion behavior was further analyzed by optical microscopy, scanning electron microscopy, inductively coupled plasma mass spectrometry and an X-ray photoelectron spectroscopy analysis. 304Cu showed a higher corrosion resistance than 304 after the salt spray tests in a 5 wt.% NaCl solution for 168 h; the beneficial effect of Cu was not obvious after exposure for 480 h. Penetrating (stripped morphology in 304 and fishbone morphology in 304Cu) and non-penetrating dish-shaped pits were both observed on 304 and 304Cu after immersion in a 6 wt.% FeCl₃ solution for 72 h; the corrosion rates of 304Cu were much higher than 304. Cu and Cu²⁺ were the main existing forms in the as-received 304Cu and Cu⁺ formed with the corrosion in chloride solutions. The results indicated a good corrosion resistance of 304Cu in service. The use of steel in an aggressive environment for a long exposure time should be cautiously considered.

Resistance spot welding (RSW) with inserted strips, a recent variant of traditional RSW, was usually adopted in joining thin gage steels to lower the temperature developed at the electrode surface and to extend electrode life. In order to understand the influencing mechanism how the inserted strips affect the heat transfer behavior and to optimize the selection of suitable strips, an approach integrated with FEM and response surface methodology (RSM) was employed. FEM results showed that the inserted strips would not only lead to earlier initiation of weld and bigger weld size in both diameter and thickness but also lower the electrode surface temperature. Based on FEM, uniform design and RSM were further employed to build a regression model between the strip properties (i.e., electrical/thermal conductivity, thickness) and the responses (i.e., electrode tip temperature, weld diameter, and temperature at strip/sheet interface). A graphical optimization was conducted to identify a preferable strip, and a Cu55Ni45 strip with a thickness of 0.12 mm was recommended for a 0.4 mm steel sheet.