## 2 Materials and Methods  
Commercial 45 steel long bar specimens with the size of 55 mm \(\times\) 10 mm \(\times\) 10 mm and \(\Phi\)15 \(\times\) 170 mm tensile specimens were selected. Their chemical composition is shown in Table 1.  
After austenitizing the specimens in a muffle furnace at 850 \({}^{\circ}\)C for 25 min, some of the specimens were intensively quenched in aqueous NaCl (10%) solution at 25 \({}^{\circ}\)C under high-speed stirring, while others were quenched in quenching oil at 25 \({}^{\circ}\)C. Subsequently, all specimens were held at 580 \({}^{\circ}\)C for 120 min for tempering treatment to prepare for intensive quenching and high-temperature tempering of tempered carbon steel (CS_IQAT) and hardening-tempering carbon steel (CS_QAT) specimens of 45 steel. The heat-treated samples were ground and polished and then corroded with 4% nitric acid alcohol, and the microstructure was characterized using an optical microscope (VS200-500U from Olympus, Tokyo, Japan) and a scanning electron microscope (Zeiss Supra55 VP from Carl Zeiss AG, Oberkochen, Germany). Physical phase analysis was carried out using an X-ray diffractometer (XRD, D8 Advance from Bruker, Berlin, Germany) with a scanning speed of 4\({}^{\circ}\)/min. Tensile testing was carried out using a microcomputer-controlled electronic universal testing machine MTS (E44.304 from MTS, Shenzhen, China). Impact toughness test was carried out using impact testing machine (JB-30B from MTS, Washington, DC, USA). Vickers hardness tester (HXD-1000 TB from caikon, Shanghai, China) was used to measure the hardness with a load of 1000 g and by holding the load for 10 s. The tensile test and impact test involved three specimens, whose average value is taken as the experimental results. The fiber hardness test involved the same selection of three specimens, where five adjacent points of each specimen were selected for hardness measurement, and finally the average value is taken as the hardness measurement results. The reciprocating motion is a common form of movement for moving parts, such as linear sliding guides. We used the MFT-5000 friction from Rice Silicon Valley, San Jose, CA, USA and wear testing machine for reciprocating wear experiments; its working principle is shown in Figure 1. Because the intensive quenching and high-temperature tempering of 45 steel is a new process, we conducted friction experiments of 45 steel that had undergone tempering treatment and compared it with a selection of commonly used parameters: a hardness of 770 HV and a commercial GCr15 ball with a diameter of 9 mm as a pair of wear vice. We used 45 steel tempering specimens measuring 25 mm \(\times\) 10 mm \(\times\) 10 mm in size and intensive-quenching-and-high-temperature-tempering specimens as the friction and wear specimens. In the test, the fixed load was 30 N, and the frequencies were 2 Hz, 3 Hz, 4 Hz, and 5 Hz, which were designed to simulate the different working conditions of linear sliding guides. The wear time was 40 min and the reciprocating distance was 20 mm. In the second set of tests, the sliding speed and total distance were (0.04 m/s and 96 m), (0.06 m/s and 144 m), (0.08 m/s and 192 m), and (0.10 m/s and 240 m), respectively. The wear surfaces of 45 steel-tempered specimens and intensely quenched-high-temperature tempered specimens were characterized using scanning electron microscopy (SEM). The wear track profiles were determined using a surface 3D profiler (SuperView W1 from CHOTEST, Shenzhen, China) and the relative wear rate \(k\) was calculated using Equation (1):  
\[K=\frac{AL}{FS} \tag{1}\]  
where \(F\) is the normal load (N), \(S\) is the total sliding distance (m), \(A\) is the average cross-sectional area of the trajectory (mm\({}^{2}\)), and L is the stroke length (mm).