The properties of structural steel during processing are influenced by various structural components, including equilibrium structures, which include austenite and pearlite, as well as non-equilibrium structures like martensite, residual austenite, sorbite, and troostite. The formation of these structures is determined by factors that include temperature, alloy composition, and cooling medium. For example, pearlite has a lamellar structure, while sorbite, troostite, and pearlite differ in their degree of cementite dispersion and hardness [4]. The mechanical properties of the ferrite–cementite structure are affected by the degree of dispersion; finer structures exhibit increased hardness, strength, ductility, and toughness. Among the different structures, sorbite exhibits the highest plasticity characteristics (δ and ψ), while troostite exhibits the lowest.

The transformation of supercooled austenite into bainite occurs within a temperature range below the pearlitic range but above the martensitic range [5,6,7]. When maintained isothermally at temperatures above 350 °C, upper bainite (~HB 450) is formed, characterised by a laminar structure similar to pearlite. In contrast, when maintained isothermally at temperatures below 350 °C, lower bainite (~HB 550) forms, exhibiting a needle-like structure similar to martensite. However, the formation of such a structure requires prolonged isothermal treatment, typically several hours in a furnace, within a temperature range near 350 °C [8].

At extremely high cooling rates exceeding 1200 °C/s, the diffusion decay of austenite is impeded, leading to supercooling and martensitic transformation. Unlike pearlitic transformation, martensitic transformation exhibits polymorphism, which is a non-diffusional or displacive process. The transformation of austenite into martensite is incomplete, resulting in the presence of residual austenite alongside martensite in quenched steel. The wide temperature range of martensitic transformation leads to various types of martensite in steels in the form of plates or laths, which form at different temperatures. Each type of martensite possesses unique properties. The formation temperature of the structural type of martensite is influenced by alloy composition, cooling medium, and other factors. Plate (twin) martensite structure, for example, develops below 200 °C and is a characteristic feature of high-carbon steels (above 0.6% C) and alloy steels [9].

The properties exhibited by the specific martensite type, which appears in the metal structure in the form of plates, are influenced by the presence of a midrib on these plates. This specific martensite type is referred to as twin martensite due to the midrib of each plate being composed of numerous twins. These twins, located on the planes of the martensite plates, have a thickness of 5–30 nm; the hardness of this martensite is 60–75 HRC. Ribbon or packet martensite, on the other hand, is primarily characteristic of the structure of low- and medium-carbon steels, with a carbon content below 0.5% [10]. The temperature threshold above which the martensitic structure is formed in such steels is 300 °C. As its name implies, this martensite type assumes the form of elongated ribbons in one direction, with each ribbon having a thickness of 0.2–2 μm (their length is approximately 5 times greater than their width). The resulting metal structure consists of parallel crystal ribbons, referred to as groups or packets. Between the martensite ribbons, layers of residual austenite can be observed with a thicknesses of 10–20 nm, depending on the alloy type. This particular martensite type exhibits lower levels of hardness (40–60 HRC) but demonstrates increased wear resistance, dynamic viscosity, and ductility compared to plate martensite. Considering the characteristics of the microstructures discussed above, for thin-walled elements made of non-alloy steel with a carbon content below 0.3%, lower bainite or sorbite can be considered to be the preferred options.

In the metallurgical and metalworking industries, various techniques, such as heat treatment, chemical–thermal processes, and mechanical processing, are employed to manufacture and prepare metals for subsequent processing, as well as to enhance the operational properties of metal products. Among these techniques, heat treatment is the most widely used. When austenite, the high-temperature phase of steel, undergoes cooling, it transforms into different structures depending on the cooling rate; these structures include martensite, bainite, sorbite, and pearlite. Sorbite and bainite are formed through the slow cooling of austenite, while martensite is formed through rapid cooling.

The primary heat treatment methods employed for steels are annealing, quenching, tempering, and normalisation. Although it is not categorised as a fundamental heat treatment, normalisation is frequently employed and is considered a variant of either annealing or quenching, depending on the specific steel grade and workpiece dimensions. The objective of annealing is to attain a steel structure close to equilibrium. To achieve this, carbon steel is subjected to controlled cooling at a rate of 0.05–0.1 °C/s. Annealing leads to a reduction in internal stresses, the enhancement of machinability, the improvement of ductility, and the refinement of the steel microstructure. The cooling rate during annealing is carefully controlled to manipulate the transformation of the steel’s microstructure in accordance with the desired properties [11,12]. Following the annealing process, the hypoeutectoid steel exhibits a microstructure composed of ferrite and pearlite. For low-carbon steels with carbon content of less than 0.3%, normalisation can be employed as an alternative to annealing to achieve a homogeneous fine-grained structure. Normalisation offers advantages in terms of cost-effectiveness due to the reduced cooling time required; the cooling rate during normalisation is typically 1–10 °C/s [13,14]. When steel is cooled from the austenitic state at cooling rates less than the critical cooling rate but higher than the rate necessary for the equilibrium transition of austenite to pearlite, the supercooled austenite undergoes a transformation, resulting in the formation of pearlitic structures [5].

The transformation process during normalisation occurs through diffusion phenomena. When carbon steel is subjected to normalisation with cooling rates exceeding 10 °C/s in an air stream, a dispersed sorbite structure is typically obtained. Alternatively, under faster cooling conditions, such as immersion in oil to achieve a cooling rate of 100–150 °C/s, a troostite structure is formed [11,12].

The formation of bainite in carbon steel is not typically observed during continuous cooling. The intermediate transformation leading to bainite formation can only occur through isothermal holding within a specific temperature range of 500–250 °C, which is typically unattainable in welding or laser processing applications [5]. Achieving the critical cooling rate for quenching, which represents the minimum rate at which all austenite is supercooled and transformed into martensite, is crucial for achieving the desired microstructure. Low-carbon steels with carbon content below 0.3% exhibit the highest critical quenching rate, exceeding 1200 °C/s [15]. Consequently, these steels are often not subjected to conventional heat treatment because typical cooling media are unable to achieve the necessary quenching rate, thus hindering the transformation of austenite into martensite. Carbon steels containing more than 0.3% carbon content are typically subjected to water quenching. For non-alloy steels, the heat treatment method aimed at obtaining lower bainite represents a preferred approach for enhancing the mechanical properties of steel products, despite its inherent challenges. Lower bainite exhibits superior hardness and strength compared to pearlitic structures, while still retaining adequate levels of plasticity and toughness. In contrast, upper bainite is brittle, primarily due to the formation of coarse carbides along the boundaries of ferrite grains. Consequently, upper bainite structures do not significantly improve the hardness and strength of steel.

In addition to the aforementioned heat treatment techniques, various established chemical–thermal surface treatment methods are employed to enhance the operational characteristics of steel components, including cementation, nitrocarburising, and micro-alloying [16,17]. These techniques involve modifying the surface composition and structure of steel components to achieve desired properties such as improved wear resistance, hardness, corrosion resistance, and fatigue strength. The techniques rely on controlled diffusion processes and the interaction of specific elements or compounds with the steel surface to create a modified layer with enhanced properties.

The cementation process is primarily applicable to low-carbon steels with a carbon content of less than 0.2%. However, cementation is often limited to small-sized components with less critical functional requirements. During the cementation process, the austenite phase of the steel transforms into a mixture of ferrite and pearlite beneath the cemented layer. To achieve carburisation of the outer surface layer, the parts are subjected to elevated temperatures of 850–950 °C within a furnace. Steel cementation can be carried out using various media, including solid, liquid, or gaseous carburisers. The cementation process can be time-consuming, with a carbon penetration rate of approximately 0.1 mm/h. Consequently, the resulting thickness of the hardened layer typically does not exceed 0.2–0.5 mm. Therefore, considering the required operational depth of 0.5 mm, the estimated time required to attain the desired strengthening depth is approximately 5 h [18].

Nitrocarburising is a surface treatment method employed for steels, wherein the steel is exposed to a gaseous environment composed of carburising gas and ammonia. This process involves the simultaneous introduction of carbon and nitrogen into the steel structure at elevated temperatures of 700–950 °C. Generally, nitrocarburising is conducted within the temperature range of 850–870 °C. This technique has garnered considerable recognition in the field of mechanical engineering, particularly for components that require the formation of a hardened layer with a thickness not exceeding approximately 0.5 mm to meet operational requirements [19].

Microalloying is a widely recognised and established technique for surface hardening in which minute quantities of alloying elements are introduced into a metal or alloy, with the total mass of these additives not exceeding 0.1% of the original metal or alloy mass. Typical microalloying additives include vanadium, titanium, boron, niobium, zirconium, and various rare earth elements such as cerium, yttrium, and lanthanum, either individually or in combination. Additionally, aluminium, nitrogen, barium, calcium, and magnesium are also commonly utilised. The methodology employed for microalloying is similar to the alloying techniques employed in the metallurgical industry, which aim to enhance the surface properties of the material [20].

Conventional boronising techniques provide consistent outcomes, but face challenges related to low productivity and high resource expenses. Electrolytic and liquid boronising approaches necessitate wastewater treatment, while powder boronising cannot be fully automated and is relatively costly. A notable limitation of these methods is the time required for the process, which can span several hours. To accelerate the boronising process, several alternative approaches have been proposed, including electron-beam, electro-spark, micro-arc, and induction heating methods, as well as plasma spraying. Other techniques involve generating a glowing discharge during powder saturation, utilising ultrasound, employing vibration or pseudo-liquification induced by electrical influence, or employing chemical and physical deposition from the vapour phase [21]. However, despite the reduced saturation duration offered by these methods, their widespread adoption has been limited due to their technical complexity, limited versatility, and high energy consumption. In certain scenarios, a multi-component boronising approach can be employed, in which the component’s surface is saturated not only with boron but also with other elements, including chromium, aluminium, and silicon. The aim of saturation with other elements is to enhance the corrosion and wear resistance of the component’s surface layer. However, the resulting increase in resistance is insufficient for these processes to be widely adopted. After microalloying, the diffusion layer thickness in the presence of alloying elements is typically 20–50 microns, which may be sufficient for components subjected to abrasive wear or corrosive environments [21].

In industrial applications, mechanical surface treatment techniques are employed to induce plastic deformation in thin surface layers, leading to modification of the properties of these layers while preserving the properties of the underlying metal core. This process, known as surface cladding, enhances strength properties, electrical resistance, and the rate of diffusion processes, while reducing the plasticity and corrosion resistance of the treated layer. Surface hardening treatments via plastic deformation are utilised in the final stages of the manufacturing of machine parts. Two types of treatment are commonly employed: dynamic surface plastic deformation (DSPD) and static surface plastic deformation (SPD). Surface cladding can be achieved by bombarding the component with metallic shot, balls, or abrasive particle suspensions; rolling it with rollers, balls, or diamond tools; or stamping. To increase surface hardness through DSPD, rapid surface hardening can be achieved using an explosive substance. Shot blasting is the most frequently employed form of DSPD in a dynamic regime, utilising metallic or corundum shot with particle sizes of 0.5–2.0 mm. The optimal particle velocity during the impact with the treated surface is 50–70 m/s, with a corresponding angle of incidence of 75–90°. The treatment duration is typically limited to 2–3 min, and the cladding layer thickness should not exceed 0.20–0.4 mm. Within the cladding layer, the density of lattice defects increases, altering the grain orientation and shape. As a result, the microhardness of the near-surface layers can be enhanced by up to 50%. For steel, available data indicate an increase in microhardness of 1600–2400 MPa. However, it should be noted that this technique requires relatively high capital investments in a blasting chamber. In addition, this approach is primarily applicable to bulky and thick-walled structures, as thin-walled finished products and components with wall thicknesses below 1.5–2 mm are unsuitable for this method. Also, when exposed to temperatures exceeding 450 °C, all the properties attained through the shot blasting of steel are compromised. Consequently, treated components are unsuitable for subsequent heat treatment. Furthermore, surfaces subjected to cladding lose their corrosion resistance properties [22].

The different heat treatment or thermochemical treatment processes described above and the addition of strengthening elements, complex geometry profiles, and structural thickenings are mostly used to improve the mechanical properties of steel products and their surfaces. But the application of strengthening elements, complex geometry profiles, and structural thickenings increases the manufacturing costs and the weight of metal parts and requires complex equipment and accurate designs. Heat treatment and thermomechanical treatment technologies are expensive, complicated, and long-running processes. These technologies are not appropriate or effective for large constructions from non-alloy and low-carbon steels containing less than 0.3% carbon.

An additional widely recognised and effectively employed technique for localised surface treatment is laser processing, the heating rate of which exceeds 1000 °C/s during laser processing. It should be noted, however, that laser processing encompasses not only rapid heating of the material, but also swift cooling. The cooling of the heated region occurs through heat transfer into the bulk of the metal. Consequently, for more efficient cooling, the total volume of the component should be significantly larger than the volume of the processed zone or be artificially increased via a substantial substrate with considerable thermal conductivity. The cooling rate at temperatures below the melting temperature is typically (5–10)·10³ °C/s; during crystallization from the liquid phase, the cooling rate reaches 10⁶ °C/s. This disparity is likely due to the fact that the volume of the welding bath in laser processing is extremely small, and the surrounding metal remains cold [23,24,25].

Hence, laser treatment can also be applied to non-alloy steels containing less than 0.3% carbon. Laser treatment techniques exist both with and without surface melting. In the case of laser transformation hardening using a CO₂ laser without surface melting, the typical thickness of the hardened layer does not exceed 0.3 mm when employing a laser pulse of 0.15 mm [26]. The effectiveness of laser transformation hardening and the ultimate strength of the laser-treated structure depend on the overall area treated by the laser and the depth of hardening. Consequently, the use of laser transformation hardening without surface melting for reinforcing non-alloy steel structures with less than 0.3% carbon is limited by the hardened layer thickness, as an extensive surface area must be treated. In contrast, laser treatment with a surface melting process yields greater thickness of the treated layer and is preferred for localised laser treatment.