**1. Introduction**

Wear, fracture and corrosion are usually the main failure modes during the service of metal materials. Wear will not directly cause the failure of metal parts, but equipment parts are difficult to repair due to wear. Besides, frequent replacement significantly reduces the working efficiency and service life of equipment, thus leading to a large amount of material and energy loss [1–4]. At present, among the metal wear-resistant materials, austenitic high manganese (Mn) steel, high chromium (Cr) cast iron and low alloy wear-resistant steel are the most widely used. Among them, austenitic high manganese steel has a surface austenitic structure, quickly producing work hardening by a phase transition under strong extrusion or impact. The core of austenitic high manganese steel still retains good toughness and plasticity because of the austenitic structure [5–7]. However, the wear resistance of austenitic high manganese steel is relatively low under low or medium stress conditions, which severely limits their application scope in the wear-resistant materials field [8]. High chromium cast iron, as a second-generation wear-resistant material, is currently recognized as the best wear-resistant material [9–11]. A lot of non-network carbide M7C3 with a hardness of 1600 HV is precipitated in high chromium cast iron and the toughness and plasticity is better than that of white cast iron [12]. Therefore, mechanical equipment made of high chromium cast iron can meet the needs of long-term wear resistance in complex environments. However, due to the large number of valuable elements such as Cr and

**Citation:** Li, Z.; Yuan, Q.; Xu, S.; Zhou, Y.; Liu, S.; Xu, G. In Situ Observation of the Grain Growth Behavior and Martensitic Transformation of Supercooled Austenite in NM500 Wear-Resistant Steel at Different Quenching Temperatures. *Materials* **2023**, *16*, 3840. https://doi.org/10.3390/ ma16103840

Academic Editors: Andrea Di Schino and Claudio Testani

Received: 24 April 2023 Revised: 12 May 2023 Accepted: 17 May 2023 Published: 19 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

nickel (Ni), the production of high chromium cast iron is complex, which also increases the production cost and limits its wide application in industrial production [13]. In view of the many problems regarding the service and production of the above two steels, low-alloy wear-resistant steel has gradually become the research topic of the new generation of wear-resistant metal materials [14–18].

Various wear-resistant steel products are made by Chinese iron and steel manufacturers, among which the production technology of the products below NM400 is relatively mature. Tempered martensite is obtained by off-line re-austenitizing after rolling to improve the strength hardness, and then the toughness is improved by subsequent tempering. Martensite has ultra-high strength and hardness among the different microstructures in steel, and it is usually selected as an important microstructure in the production of ultra-high strength steel. Therefore, among all the kinds of low-alloy wear-resistant steels, martensitic wear-resistant steel is promising. The wear resistance of martensite mainly relies on its high hardness, but the wear resistance under high impact is erratic due to its poor toughness. Therefore, many studies on martensitic wear-resistant steel have focused on improving its toughness and plasticity [19–21]. It has been revealed that martensitic laths and blocks are the organizational units affecting strength and hardness, while martensitic packets affect the plasticity and toughness [22,23]. In addition, Liang et al. found that crack propagation could be effectively inhibited by using smaller sizes and angles of the martensitic packet [24]. The main structure control unit affecting fracture, the size of martensitic block, was identified by Inoue et al. through a study on the cleavage fracture of tempered martensitic steel [25]. Moreover, some scholars improved the fracture toughness and elongation of martensitic steel by optimizing the composition and heat treatment process, so that about 30% residual austenite was obtained at room temperature [26].

In addition, the wear resistance of martensitic steel had been studied extensively. Liang et al. reported that low-alloy martensitic wear-resistant steel exhibited better wear resistance under moderate impact wear, and its comprehensive mechanical property was more than twice that of austenitic high manganese steel [27]. Cao et al. prepared Ti-Cr-B (boron) microalloyed high-strength wear-resistant steel with tempered martensite, in which a high dislocation density and tempered carbide precipitation hardened the matrix [28]. In the work of Ma et al., they found that the solid solution carbon content in the martensitic structure was a direct factor affecting the wear resistance and subsurface hardness [29].

Research on high-grade low-alloy wear-resistant steel is insufficient. A high-grade NM500 wear-resistant steel is presented in the present study. Martensite transformation greatly influenced the microstructure and properties of the low-alloy wear-resistant steel, and the quenching temperature and the subsequent cooling also played a decisive role in these properties. However, few studies about NM500 wear-resistant steel have studied the relationship between grain growth and martensite transformation. Moreover, a dynamic investigation into austenite grain growth and martensite transformation in NM500 wearresistant steel has not been conducted. Therefore, the phase transformation behavior of NM500 wear-resistant steel in a continuous cooling process was analyzed by confocal laser scanning high-temperature microscopy (CSLM). The novelty of the present work is summarized in two aspects: (1) the grain growth behavior of high grade NM500 wearresistant steel at two quenching temperatures was first recorded and compared and (2) the martensite transformations in different austenite grains were dynamically analyzed. The results of the present study will provide a reference to understand austenite grain growth and martensite transformation at different quenching temperatures.

#### **2. Experimental Procedures**

Figure 1 shows the VL2000DX-SVF17SP confocal laser scanning high-temperature microscope and the corresponding quenching process. This CSLM equipment consists of a flow control device, a console, a display, high temperature microscope, etc., which can observe and capture all kinds of physical and metallurgical phenomena in real time. It studies the dynamic process at a high resolution in real time through high-speed laser scanning imaging. A higher automation degree was achieved through digital image information storage and processing technology. The light source of CSLM was a blue laser, whose wavelength and resolution were about 410 nm and 0.25 μm, respectively. Various phases emerged under the effect of thermal etching, rather than chemical corrosion.

**Figure 1.** (**a**) The confocal laser scanning high-temperature microscope and (**b**) in situ observation process.

The experimental steel was a developed high-grade NM500 steel with the chemical composition Fe-0.23C-0.20Si-1.49Mn-1.15Cr-0.25Ni-0.37(Nb+V+Ti+Mo)-0.022Cu-0.00174B-0.01P-0.002S (wt.%). Figure 2 illustrates the structure of the confocal laser scanning hightemperature microscope. A small cylindrical sample with dimensions of Φ 6 mm × 5 mm was finish machined, and the two faces of the sample were polished until a mirror surface was obtained. Subsequently, the sample was placed into the Al2O3 crucible for in situ observation. Before the experiment, the sample chamber was vacuumed to 6 × <sup>10</sup>−<sup>3</sup> Pa, and then argon gas was introduced to prevent sample oxidation. Microstructure evolution was recorded throughout the whole process with a recording frequency of 5 photos/s. Figure 1b demonstrates the heating process with two different quenching temperatures. Firstly, the samples were reheated to 860 ◦C and 1160 ◦C, respectively, at a rate of 5 ◦C/s and then held for 1 h. Subsequently, the maximum cooling rate was applied to cool the specimens to room temperature after thermal holding to simulate the quenching process. The heating rate of 5 ◦C/s was determined by an empirical value considering its small size. Two different quenching temperatures of 860 ◦C and 1160 ◦C were chosen according to the minimum and maximum tempering parameters in industrial production. The holding time of 1h was utilized also based on the requirements of industrial production. It should be noted that the average cooling rate during quenching was estimated to be only 8 ◦C/s due to the low cooling ability at a low temperature range. However, the cooling rate before martensite transformation could reach 15 ◦C/s, totally ensuring the martensite transformation. In addition, to facilitate the analysis of grain size evolution, the eyepiece was manually changed during the in situ observation experiment. An appropriate magnification was selected due to the different size of parent austenite grains (PAGs) at 860 ◦C and 1160 ◦C. Meanwhile, the precipitates quenched at 1160 ◦C were identified by transmission electron microscopy (TEM) using a thin film specimen.

**Figure 2.** The structure of a confocal laser scanning high-temperature microscope.
