Systematic experimental investigations were conducted to study the microstructures and impact toughness of each heat affected zone (HAZ) formed during rail flash-butt welding. A high-strength carbide-free bainitic rail steel was subjected to different thermal simulation cycles to separately reproduce each HAZ subzone by tailoring the peak temperature (PT) with respect to 700, 850, 920, 1000 and 1350 °C, and hence to generate the corresponding microstructures by using Gleeble-3500 simulator. Results show that the HAZ subzones exhibit complicated microstructures depending on the PTs, and with increasing PT the dominant bainitic microstructure type evolves from polygonal bainitic ferrite (700 °C) to a mixture of fine bainitic ferrite and granular bainite (850–1000 °C), and finally to coarse bainitic ferrite and granular bainite (1350 °C). Impact tests demonstrate that the impact toughness initially increases significantly as the PT reaches 920 °C (i.e., fine-grained HAZ), beyond which the impact toughness starts to decrease. The fine-grained HAZ displays optimal impact toughness in HAZs, yet which is lower than the base metal. Moreover, the morphology and distribution of martensite-austenite (M-A) constituents is strongly dependent on the welding PT, and the high fraction blocky and coarse slender M-A constituents is considered to be detrimental for the impact toughness.@en

A systematic experimental investigation concerning the fretting-induced tribo-chemical state and its effect on the fretting wear behavior of titanium alloys under the vacuum atmospheres (4 × 10–3 Pa and 4 × 10–1 Pa) in different fretting regimes is reported. An in situ XPS analysis tester was developed to capture the real tribo-chemical state of worn surface for all test conditions. Results show that samples subjected to different vacuum atmospheres have varied tribo-chemical states depending on the fretting regime, which play significantly different roles in determining the associated damage mechanisms and the resulting fretting wear resistance. Under both vacuum atmospheres, in the partial slip regime (PSR) the worn scars were mainly covered by TiO2, showing comparable levels of very slight damage, while in the mixed fretting regime (MFR), the tribo-layer is still mainly consisted of TiO2, but with an evident peak of Ti metal for the high vacuum degree (4 × 10–3 Pa) in MFR, showing a mild damage. In contrast, in the gross slip regime (GSR), Ti metal was prone to be oxidized to Ti2O3 and TiO on the worn scar, especially for the low vacuum degree (4 × 10–1 Pa) having a highest content of Ti2O3. It might be inferred that the tribo-layer containing more Ti2O3 formed during fretting wear process is susceptible to be broken, hence showing a highest fretting wear volume in GSR for the low vacuum degree. The results suggest that for the vacuum environments, the Ti6Al4V may be more suitable to be used under the high vacuum atmosphere.@en

An experimental investigation was conducted to study the fretting wear behavior of low alloyed construction steel in the tempered fully martensitic state. The resulting damage mechanism and the resistance to fretting wear of martensitic steels subjected to different tempering temperature was evaluated and compared with the virgin (un-tempered) martensitic steel under the different loading conditions. The results show that the friction coefficient increases with the increase of the tempering temperature for all the applied loads. The fretting wear resistance mainly depends on the tempering temperature. Compared to the virgin (un-tempered) full martensite, most of the tempered martensites have better fretting wear resistance, in which the tempered martensitic (TM) steel of 200° due to a good balance of strength and ductility has a super fretting wear resistance for all loading conditions. In addition, the correlation of fretting wear resistance with the initial hardness was discussed.@en

Systematic experimental observations concerning the response of microstructural features on fretting wear of steel grades were conducted. Five steel grades with different typical microstructural features were selected. The effect of various microstructures on fretting wear behaviour and the resulting wear mechanism were evaluated. Results show that microstructural features play a significant effect on fretting wear resistance depending on the applied loads. The DP and Q&P steel having low hardness show comparable or even better fretting wear resistance than the FM steel of the highest hardness regardless of loads, which are all better than TWIP and IF steel. The result suggests that the combination of soft ferrite and hard martensite with/without retained austenite having low hardness displays superior fretting wear resistance.@en

Systematic experimental observations concerning the response of microstructural features on fretting wear of steel grades were conducted. Five steel grades with different typical microstructural features were selected. The effect of various microstructures on fretting wear behaviour and the resulting wear mechanism were evaluated. Results show that microstructural features play a significant effect on fretting wear resistance depending on the applied loads. The DP and Q&P steel having low hardness show comparable or even better fretting wear resistance than the FM steel of the highest hardness regardless of loads, which are all better than TWIP and IF steel. The result suggests that the combination of soft ferrite and hard martensite with/without retained austenite having low hardness displays superior fretting wear resistance.@en

Systematic experimental observations concerning the response of microstructural features on fretting wear of steel grades were conducted. Five steel grades with different typical microstructural features were selected. The effect of various microstructures on fretting wear behaviour and the resulting wear mechanism were evaluated. Results show that microstructural features play a significant effect on fretting wear resistance depending on the applied loads. The DP and Q&P steel having low hardness show comparable or even better fretting wear resistance than the FM steel of the highest hardness regardless of loads, which are all better than TWIP and IF steel. The result suggests that the combination of soft ferrite and hard martensite with/without retained austenite having low hardness displays superior fretting wear resistance.@en

In this work, an in-situ XPS analysis test combined self-designed high precision fretting wear tester was carried out to study the fretting wear behavior and the resulting tribo-oxidation of thermal-oxidation film on Ti6Al4V titanium alloy under the varied working atmosphere. The fretting-induced tribo-oxidation under the air and vacuum (4 × 10-3 Pa) environment was analyzed and its response on the resulting fretting wear resistance and damage mechanism was discussed. Results show that the working environment plays a significant role in the formation of tribo-oxidation and then determining the fretting wear resistance. Thermal-oxidation film in the vacuum atmosphere shows a better fretting wear resistance than that in the air atmosphere for all fretting regimes, except for partial slip regime (PSR) where there is an equivalent fretting wear resistance. Compared with the substrate Ti6Al4V titanium alloy, the thermal-oxidation film in the vacuum atmosphere performs a good protection for titanium alloy, especially for slip regime (SR), but not applied for air atmosphere.@en