## 1 Introduction  
Steels with yield strength greater than 1300 MPa are becoming more widely used in different applications. An increase in strength provides the possibility to use less material, i.e., to design and use lighter structures. The result is increased energy efficiency and fuel savings that not only translate into financial gains but also may be required by different norms and regulations. Also, increased steel strength leads to higher hardness and abrasive wear resistance. Generally, the strongest steels used in structural, wear, and protection applications have martensitic microstructures. The hardness of quenched martensite increases with the increasing carbon content (up to 0.6-0.8 wt.%) up to more than 800 HV with tensile strength levels exceeding 2500 MPa for quenched plain carbon and low-alloy compositions [1]. Martensite may be lath or plate-like or a mixture of both. Below the 0.6 wt.% carbon content, the fine lath martensite is dominant. The substructure of prior austenite grains in martensite can be divided into packets which contain blocks and sub-blocks of individual laths [2; 3]. These features comprise the fine and complex structure of the martensite phase that can be altered by chemical composition and processing. Carbon atoms segregated in the structure, prior austenite grain size, dislocation density, different precipitates, and retained austenite together contribute to the superior strength and hardness of martensite, with carbon responsible for the main contribution [1; 2; 3; 4; 5; 6; 7; 8]. Furthermore, the highest hardness and strength levels are often achieved by expensive, heavy alloying, or by the utilization of time-consuming heat treatments in order to meet requirements related to usability and toughness. However, thermomechanical controlled processing (TMCP) is nowadays utilized to refine the grain structure for improved impact toughness, especially in the case of ultrahigh-strength steels. The rolling schedule is adjusted to include passes in the non-recrystallization regime (NRX), which results in an elongated austenite grain structure with an increased dislocation density and improved impact toughness [9; 10; 11]. However, the formability of ultrahigh-strength steels with relatively high carbon content is limited and might turn out be challenging when producing complex shapes or small components, i.e., blades for furrow ploughs or disc narrows. Therefore, these types of high-hardness steels are provided in the hot-rolled condition to the parts manufacturer for further processing: the final shape is given to the part in the reheating process while in the soft and easily formable condition, after which suitable quenching can be applied. Different press and hardening methods are utilized for processing high-hardness steels. Oil, water, or different aqueous solutions are used for the quenching, which requires careful planning to avoid quench-cracking. Often the process includes tempering to improve the impact toughness of the material. When abrasive wear resistance is required, high hardness needs to be retained after tempering. AISI 4340, 300M, and 51 CrV4 with a carbon content of around 0.4-0.5 wt.% are commonly used in the aforementioned applications. Tempering is a common method to improve the ductility and impact toughness of steels. It is widely applied for ultrahigh-strength martensitic steels due to the limited toughness properties of quenched medium- and high-carbon steels. Often, the increasing carbon content and the subsequent increase in hardness and strength impairs the toughness properties of carbon steels in their as-quenched state. Therefore, different temperatures and tempering times can be used depending on the desired mechanical properties, such as suitable hardness and impact toughness levels [1; 12; 13]. Tempering temperatures up to 650 \({}^{\circ}\)C can be utilized to adjust the mechanical properties of steels for different applications: low-alloy, low-carbon steels benefit from improved low-temperature impact toughness and bendability properties.