The Influence of Heat Treatment and Laser Alternative Surface Treatment Methods of Non-Alloy Steels: Review

Abstract

This paper focuses on the microstructural characteristics of non-alloy structural steels with carbon contents below 0.3% (further—Low-Carbon Steel—LCS), as well as the possible structural transformations and the resultant mechanical properties attainable through conventional heat treatment or alternative surface treatment methods. The principal microstructural constituents that govern the properties of these steels include both equilibrium and non-equilibrium phases, such as martensite, retained austenite, sorbite, and troostite. Conventional methodologies for enhancing rigidity involve the implementation of supplementary stiffening ribs, which augment rigidity while concomitantly contributing to an increase in overall weight or dimensions of the structure. In structures where supplementary stiffening ribs are incorporated within the thin-walled steel shell, this may reduce manufacturing efficiency and simplicity of design. Modern laser treatment technologies for thin-walled steel structures, however, involve modifying the internal microstructure and creating rigidity ribs within the structure itself, thus circumventing the need for additional elements.

1. Introduction

Recent scientific interest in laser processing technology has surged, focusing on its impact on the microstructure and properties of laser-treated surfaces, particularly in producing metal structures. This article addresses methods to improve the mechanical performance of such structures. The article investigates conventional and contemporary techniques, shedding light on the advantages and challenges associated with each. It examines in more detail the microstructures that are formed in steel during heat treatment. The review explains how important factors such as the alloy composition and the cooling medium are. It provides a comprehensive overview of various heat treatment methods, such as annealing, quenching, tempering, and normalization. The review looks at chemical thermal surface treatment methods such as cementation, nitrocementation, and microalloying, in addition to traditional heat treatment. It discusses laser transformation hardening (LTH) and its applications in surface hardening, emphasizing the importance of controlling treatment depth and area. This review thoroughly analyzes heat treatment and laser processing methods, giving useful information on their applications, challenges, and potential to improve the mechanical properties of steel structures with thin walls.

Traditional processing methods will be considered as a first focus. Rolled steel is employed in the fabrication of diverse shell-type structural components, which are classified based on their intended use, design features, and production processes. This classification typically encompasses large sizes and capacity containers, oversized cylindrical vessels, high-pressure tanks, pipelines, and hollow bodies. Nevertheless, it is important to acknowledge that such classifications are not absolute, as various industries adopt distinct schemes for categorizing shell-type equipment. In addition, numerous steel structures incorporate elements that are segments of different shell-like forms—such as end caps, pup joints, hatches, and fittings.

Even though thin-walled structures can be used for many purposes, they usually have low rigidity, which means that their whole range of strengths cannot be used because of unacceptably large deformations (Figure 1). Because these structures are susceptible to stability loss, the modulus of elasticity, not the strength, determines the critical loads.

In addition, some types of shell structures can be loaded and unloaded slowly while being exposed to high pressure from an aggressive medium. Changes in pressure occur at the same time as the loading cycle. Given that stability loss signifies a near complete loss of the load-bearing capacity of the structural element, it is imperative to consider this phenomenon in the design of new constructions. Thin-walled steel structures can be strengthened with flanging, reinforcing linings, stiffeners, cross-links to stop movements, and a smart arrangement of stiffness nodes that raise local stiffness, lower stress concentration, and increase cyclic strength.

Stability loss is particularly pronounced in lightweight, thin-walled shells and walls subjected to compressive forces (Figure 2) [1,2]. Consequently, specialized approaches have been developed to analyze these structures. The “soft shell” theory is commonly used when assessing thin-walled shells with minimal bending stiffness. Within this framework, the shell is treated as having negligible moment capacity and thus is assumed incapable of bearing compressive loads; it primarily resists tension. However, unforeseen bending effects can arise from external impacts, localized forces, or support reactions, potentially leading to significant damage or complete failure [3].

In modern engineering applications, certain problems necessitate design considerations that factor in the plastic behavior of metals, such as steel. Accounting for plastic deformation—through methods like ultimate load or limit state design—often enables more efficient material usage than traditional elastic-only (working stress) approaches. Consequently, design approaches for heavily loaded components (e.g., building shells, rocket structures, chemical reactors, thin-walled pipelines) frequently incorporate elastoplastic analyses. In these situations, the calculations extend beyond the elastic limit, allowing local plastic flow and thus deviating from Hooke’s law.

When plastic deformation is taken into account in calculations, it is common to find extra structural strength in steel structures. This can lead to higher loads (up to 45% for profile bending) or ultimate bending moments (up to 1.5 times for beams) compared to results from traditional working stress methods that only consider elastic conditions [4,5].

Increasing compressive, tensile, and bending strengths is crucial for thin-walled steel structures. In many cases, heat treatment effectively boosts the yield strength and ultimate tensile strength of structural steels. Another way to enhance strength, especially bending strength, involves modifying the dimensions, altering the shape, or adding specialized reinforcing elements [6]. A variety of treatments are typically used to improve the surface or mechanical properties of steel products, including heat treatments or thermochemical processes, integration of reinforcing elements, adoption of intricate geometrical profiles, and incorporation of structural thickening [3,7,8]. However, these measures tend to elevate production costs, add weight, and demand specialized equipment and precise designs.

Moreover, heat and thermomechanical treatments are expensive, complex, and time-intensive, which can render them impractical for large structures or heavy sections made from LCS. Nonetheless, thinner plates and sheets of such steels can be effectively hardened to deliver a range of strength and toughness combinations after heat treatment. In this context, modern laser processing technologies present a viable alternative for localized surface modifications. Simple and cost-effective Nd:YAG lasers [9,10,11], in particular, can be easily employed to enhance stiffness and other beneficial properties in shell structures composed of LCS. Figure 3 illustrates the simplest example of increasing rigidity in a thin-walled steel shell (pipe).

Laser processing involving melting produces a thicker modified layer than other techniques, making it especially suitable for localized strengthening of thin-walled steel products [12,13]. Heat treatment, in general, is described as a sequence of controlled heating and cooling steps applied to a metal in the solid state to achieve specific property goals. Annealing is commonly performed to improve machinability and cold formability, restore ductility, eliminate or reduce structural non-uniformities, refine the grain size, relieve internal stresses, and condition the steel structure for subsequent heat treatments. By altering the surface characteristics of steel, heat treatment significantly enhances mechanical, tribological, and corrosion properties. Hence, economical LCS treated appropriately can sometimes serve as substitutes for more expensive alloy steels. In essence, heat treatment modifies both the mechanical and metallurgical properties of metal alloys by employing systematic heating and cooling processes [14].

In recent years, there has been growing scientific interest in laser processing methods. Multiple studies have investigated the impact of laser-based treatments on the microstructure and properties of treated surfaces, particularly for the fabrication of various metal structures [15,16,17]. Driven by global demand, the need for thin-walled structural components made from LCS has notably increased. Therefore, examining and gaining valuable knowledge to enhance mechanical and other performance characteristics in these types of thin-walled steel structures remains highly significant.

Contemporary mechanical engineering practices frequently employ thin-walled structures to achieve high resistance and strength while maintaining relatively low weights [18]. The primary objective of metal processing in this context is to reduce the weight and volume of metals required for steel structures by enhancing their strength, hardness, and rigidity [19].

Standard methods of improving rigidity involve the installation of additional stiffening ribs, which increase rigidity but also significantly add to the overall weight or dimensions of the structure, or even reduce the throughput/capacity within the thin-walled steel shell in cases where additional stiffening ribs are installed inside the structure. Conversely, modern laser-treated techniques for thin-walled steel structures imply altering the internal microstructure and creating internal rigidity ribs directly within the structure, without the addition of any extra elements. This maintains the overall dimensions and weight of the structure while significantly increasing its strength against external loads. This paper investigates both conventional and contemporary methods, including laser processing, for the treatment of thin-sheet steels with a carbon content of less than 0.3%.

2. Overview Effect of Heat Treatment on the Properties of LCS

LCS are extensively employed in the fabrication of engineering components and fabricated metal objects for relatively uncritical infrastructural applications, including tube and pipe components, pressure vessel components, structural sections, thin-walled sections, panels, and various forms utilized in industrial metalworking [20]. The performance characteristics of high-quality, LCS and its semi-finished products are contingent on factors such as chemical composition, the selected heat treatment technique, and the condition of supply [21].

During the processing of LCS, the formation of both equilibrium phases (e.g., austenite and pearlite) and non-equilibrium phases (e.g., martensite, residual austenite, sorbite, or troostite, which are quasi-eutectoid mixtures of ferrite and cementite with carbon contents above or below 0.8%) can be observed. The process of formation of a determined microstructure is influenced by a number of factors, including temperature, phase composition, the medium through which cooling occurs, and other relevant environmental factors. The transformation of pearlite gives rise to a lamellar structure, whilst the presence of sorbite, troostite, and pearlite is characterized by the extent of cementite dispersion and the resulting degree of hardness, as illustrated in

Table 1. Characteristics of microstructural phases in steel [22,23].

Parameters	Perlite	Sorbite	Troostite	Bainite	Martensite
Sheet thickness L, µm	0.6–2.0	0.2–0.3	0.1–0.2	0.08–0.12	0.15–0.25
Hardness, HRC	10–25	25–32	32–42	47–54	42–66

[22]. The dispersion level within the ferrite-cementite network exerts a significant influence on the mechanical properties of the material, with higher dispersion leading to enhanced hardness, tensile strength, yield strength, and fatigue resistance. It is noteworthy that sorbite exhibits the greatest plasticity, while troostite displays the least.

The bainitic (intermediate) transformation of supercooled austenite occurs within a temperature range situated below pearlite formation and above the martensitic region [24,25]. When subjected to isothermal heating at temperatures in excess of 350 °C, upper bainite (approximately HRC 47) is formed, exhibiting a feather-like structure similar to pearlite. Conversely, isothermal treatment at temperatures below 350 °C produces lower bainite (around HRC 54), which features a needle-like structure resembling martensite. It is noteworthy that the successful establishment of this lower bainite phase is contingent upon the prolonged isothermal holding, extending over several hours within a furnace, at temperatures in close proximity to 350 °C [26].

It is not possible for austenite to break down through diffusion when the cooling rates are higher than 1200 °C/s. This causes austenite to supercool and turn into martensite. Unlike the eutectoid decomposition observed in the pearlite transformation, martensitic transformation is polymorphic, characterized by a diffusionless or shear-type mechanism. The change from austenite to martensite does not happen all the way, so the quenched steel has some austenite left over along with the martensite. There is a large temperature range during the martensitic transformation. At different temperatures, different types of martensite (plate or lath) form. Each type exhibits distinct characteristics and properties, such as grain size and chemical composition.

The temperature at which martensite formation occurs, determined by variables such as chemical composition of the alloy, the type of cooling environment, and other conditions, is a critical factor in the resulting martensite phase [27] (Figure 1). Typically, in high-carbon steels (with carbon content above 0.6%) and various alloy steels, martensite appears in a plate-like or lenticular morphology at less than 200 °C. These plate structures are characterized by a distinct midrib and frequently have thicknesses ranging from approximately 5 to 30 nm. Their hardness generally falls in the 60–75 HRC interval. By comparison, low- and medium-carbon steels (under 0.5% C) predominantly undergo lath martensite [28]. This phase emerges at temperatures in excess of 300 °C, producing laths aligned in one direction, with a thickness ranging from approximately 0.2 to 2 µm (whose length is roughly five times more than their width) [28]. Multiple laths possessing similar crystallographic orientations cluster together to form a block, which is visible via light optical microscopy. Adjacent blocks on the same habit plane unite to create a packet. Austenitic interlayers, typically 10 to 20 nm thick, can be found between neighboring laths. Although lath martensite is softer than plate martensite [29,30], it exhibits superior wear resistance along with enhanced dynamic viscous and ductile properties.

Based on the unique properties of each microstructure, it can be concluded that the lower bainite or sorbite is the best structure for thin-walled elements made from LCS (Figure 4).

3. Basics of Lasers and Laser-Material Interaction

It is acknowledged that gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves are all forms of light, and that such phenomena are collectively defined as electromagnetic radiation. The electromagnetic spectrum is the term given to the range of frequencies of electromagnetic radiation. Modern lasers exist within certain ranges of this spectrum.

The etymology of the term ‘laser’ is explained by the abbreviation ‘Light Amplification by Stimulated Emission of Radiation’, which accurately characterizes the physical phenomenon of light emission. A laser is a unique type of light that demonstrates the properties of light while exhibiting a higher degree of energy than that of conventional light [31].

According to the scientific definition, laser beams are characterized by their wavelengths, which range from the infrared to the ultraviolet spectrum, thereby classifying them as a type of electromagnetic radiation. The color and visibility of a laser beam is determined by its specific wavelength. The wavelength of laser light, from shortest to longest, is violet, blue (445 nm, 457 nm, 473 nm), green (515 nm, 532 nm, 535 nm), yellow, red, and near-infrared (1064 nm) [31].

The laser emission is characterized by a high degree of collimation, resulting in a light beam with minimal chromatic aberration and directionality. These properties, when combined, result in a highly structured light field, with the laser light exhibiting coherent properties over spatial and temporal domains. The common characteristics of laser radiation include monochromaticity, directionality, coherence, and high brightness [32].

Monochromaticity is defined as the property of a laser that emits radiation within an extremely limited wavelength range when compared with other electromagnetic radiations.

Directionality is characterized as the property of a laser beam that is both collimated and capable of long-range propagation with minimal spread. The focal point of a laser beam is constrained solely by diffraction effects and not by the dimensions of the source.

Coherence can be defined as the property of all the constituent waves of light moving precisely in tandem through time and space, i.e., they are in phase with each other. The length over which the phase in a beam of light is correlated is termed the coherence length.

The brightness of laser radiation is the result of the characteristics outlined above, in conjunction with high energy or power, and this in turn gives rise to a high level of brightness that surpasses that of any other light source.

Such characteristics of lasers offer considerable potential for the production of high-power densities, rendering it suitable for a wide range of metallurgical applications, including welding, cutting, drilling, and heat treatment [33,34,35]. In order to comprehend the diversity exhibited by laser systems, it is imperative to employ a categorization approach that is specifically defined for each distinct decision. The characterization of laser systems is undertaken on the basis of various criteria, including the active material utilized, the pumping mechanism, the wavelength generated, the operational regime, the temporal operation, the type of quantum transitions, and the safety precautions employed [32]:

• Types of active medium of lasers: solid state, semiconductor, gas, liquid, and fiber lasers;
• Laser radiation wavelengths: infrared, near-infrared, visible, ultraviolet, extreme ultraviolet;
• the energy states of lasers: a ground (lower laser level) state, a metastable state, and an excited state;
• pumping mechanism: optical pumping, electric discharge pumping, beam pumping, gas dynamic, chemical reaction, thermal, injection current and others;
• time and energy parameters of laser radiation: lasers can be divided into continuous (CW), pulsed and quasi-continuous lasers;
• operating modes: free-running, Q-switched, and mode-locked;
• the safety requirements for lasers based on their potential for causing injury to humans’ eyes and skin: lasers are classified into four classes, covering eye protection, flame retardancy, reflectivity and administrative precautions.

As outlined above, it can be determined that the primary benefit of the laser as a material processing instrument is its ability to regulate the precise location and rate at which energy is deposited within the material. This control is accomplished through the selection of distinct laser treatment parameters (e.g., laser medium, wavelengths, pumping methods, time variation, etc.) to achieve the desired treatment of the material [36]. The laser-material interactive process is subject to the influence of various factors, including the laser wavelength (photon energy), amplitude, repetition rate, and the degree of linear polarization. Additionally, the processing parameters such as scanning speed, scanning interval, and degree of defocusing also play a significant role in determining the outcome of the interaction [37,38,39].

Consequently, the determination of the optical absorption and thermal melting point, and thermal diffusion coefficient characteristics of materials is of significant importance in understanding the physical processes involved and the subsequent state of the materials post-manufacturing [40].

The phenomenon of laser-material interaction typically encompasses a series of steps, the initial phase of which may commence with the excitation of the materials involved. This is followed by the thermalization of the atoms within the lattice, a process facilitated by the transfer of photon energy. Subsequent to this, the material undergoes a series of physical changes, including bond breaking, the ablation of the bulk material, and the propagation of a laser-induced shockwave pressure into deeper materials [41].

The interaction between lasers and materials can be categorized into two primary categories [42,43,44]:

• the photothermal process, where laser-induced thermal heating is the predominant phenomenon and structural alterations occur, including laser ablation and sintering.
• photochemical processes, where bond breaking is the dominant mechanism and there is an absence of significant thermal heating effects, such as laser reduction, doping, and graphitization.

Photothermal transformation is a predominant feature in laser processing, especially in cases involving low incident energy. Single-photon absorption processes are found to be instrumental in the transfer of photonic energy to thermal heating [45,46,47]. This process, known as single-photon absorption, occurs in metallic materials and leads to electron excitation within the conduction bands. Subsequent energy transfer to the lattice results in substantial thermal effects. It has been established that the photonic exciton phenomenon is responsible for the generation of surface plasmons, a process which occurs as the movement of the electron gas becomes confined to the ion lattice within particular structures. In this configuration, surface plasmon amplification through resonance is achieved, leading to the focalized delivery of light energy to specific locations. Such a method facilitates the precise and targeted modification of metallic materials with an unprecedented degree of spatial precision [48,49,50,51,52].

4. Characteristics of LCS Processing

The key steelmaking and heat treatment techniques for different steel microstructures are well-established. In metallurgy and metalworking, there are various heat treatments used to improve the characteristics of semi-finished and finished metal products, including thermochemical, thermomechanical [53], and mechanical processing. Importantly, heat treatment is one of the most frequently used processes for steel and iron. As shown in

Table 2. Impact of the cooling rate on the transformation of austenite and steel microstructure [54].

Structure	Perlite	Sorbite	Troostite	Bainite	Martensite
Temperature range of transformation during cooling	650–700 °C	600–650 °C	600–500 °C	250–500 °C	Below 250 °C
Degree of austenite supercooling	Cooling in the oven at a rate of several degrees per minute	Air cooling at a rate of several tens of degrees per minute	Cooling in oil at a rate of several tens of degrees per second	Isothermal exposure in the range of 500–250 °C	Cooling in water at a rate of several hundred degrees per second

, depending on how quickly the steel is cooled during treatment, the structure of the steel can change to different types, such as martensite, troostite, sorbite, and pearlite. The main methods used to treat steel are annealing, quenching, and tempering. Normalization is also often used with those techniques to reduce any remaining stress and make the steel softer and more workable. Normalization is not one of the main heat treatment techniques, but it can be similar to annealing or quenching, depending on the type of steel and the size of the final product. Normalization, despite not being a primary heat treatment method, proves to be valuable for achieving a fine-grained structure in LCS. While annealing seeks equilibrium microstructures, normalization, characterized by shorter cooling times, offers a cost-effective alternative.

Annealing is a process that aims to create a steel structure that is close to being in equilibrium. This is achieved by cooling carbon steels at a rate of approximately 0.05–0.1 °C/s. Once the steel has been annealed, it contains ferrite and pearlite. In hypoeutectoid steels, normalizing promotes the development of a uniform, fine-grained structure. Normalizing can replace annealing in LCS. Normalizing is also a more economical way to treat the steel, as it involves less time during cooling [55] (see

Table 3. Impact of the cooling rate on the different mediums [54].

Structure	Cooling Rate, °C/s
	650–550	300–200
Water at a temperature of 18 °C	600	270
Water at a temperature of 25 °C	500	270
Water at a temperature of 50 °C	100	270
Water at a temperature of 75 °C	30	200
Soapy water at 18 °C	30	200
Emulsion oil in water	70	200
Water saturated with carbon dioxide	150	200
Distilled water	250	250
10% aqueous solution of caustic soda at 18 °C	1200	300
10% aqueous solution of table salt at 18 °C	1100	300
10% aqueous solution of sulfuric acid at 18 °C	750	200
10% aqueous soda solution	800	270
5% solution of potassium permanganate	450	100
Kerosene	160–180	40–60
Industrial oil	100–150	20–50
Transformer oil	120	25
Copper cooling plate	60	20
Iron cooling plate	30	15
Air	12	3
Compressed air	30	10

for cooling rates during normalization). When steel is cooled from the austenitic region at speeds below the critical speed but above the threshold for equilibrium transformation to pearlite, the supercooled austenite transforms into structure resembling pearlite (Equations (1) and (2)):

Fe_γ (C) −> Fe_α + Fe₃C (Troostite),

(1)

Fe_γ (C) −> Fe_α(C)_per + Fe_α + Fe₃C (Sorbite)

(2)

Diffusion is the process that causes this transformation. In carbon steels, a dispersed sorbite structure is typically achieved through a process called ’normalization’, which basically involves cooling in air at a rate of about 10 °C/s and higher. By contrast, a troostite microstructure emerges under more rapid cooling in oil, at rates of about 100–150 °C/s [29,30].

Bainite formation during continuous cooling does not usually occur in carbon steels. This intermediate transformation (leading to bainite) requires isothermal holding within the 500–250 °C temperature range, which is essentially unachievable during processes such as welding or laser treatment [56,57]. A crucial parameter is the critical quenching rate, i.e., the minimum cooling speed needed to ensure that supercooled austenite fully converts to martensite. LCS have very high critical quenching rates, which are more than 1200 °C/s [58]. This means that normal quenching methods cannot reach the required cooling rate, so these steels cannot be quenched using standard methods. On the other hand, carbon steels with more than 0.3% carbon are usually quenched in water.

The most effective way to improve the mechanical behaviour of products made from steels with less than 0.3% carbon is to make a type of structure called lower bainite. This offers higher hardness and greater strength than pearlitic structures, while still being ductile and tough. On the other hand, upper bainite becomes brittle due to the formation of coarse carbides along the boundaries of ferritic grains. Steels with an upper bainitic microstructure do not simultaneously enhance hardness and strength.

Other widely adopted thermochemical surface treatment techniques, such as carburizing, nitrocarburizing, microalloying, and related methods, are also used to enhance the performance of steel parts [59].

Carburizing is typically applied to steels containing up to 0.2% carbon. However, it is most suitable for relatively small components and parts of lesser criticality. Under these conditions, austenite transforms into a ferrite–pearlite mixture beneath the carburized surface layer. The workpieces are heated in a furnace, generally between 850 and 950 °C, to introduce carbon into the outer surface. In practice, steel carburizing is carried out using solid, liquid, or gaseous media—commonly termed carburizers. Because carbon diffuses at a rate of only about 0.1 mm per hour, achieving a hardened layer thickness of 0.2–0.5 mm can require approximately five hours of treatment [60].

Nitrocarburizing of steel proceeds similarly, simultaneously introducing both carbon and nitrogen into the steel surface at temperatures of 700–950 °C using a gas mixture composed of carbon-rich and ammonia-based media [61]. Typically performed at 850–870 °C, nitrocarburizing is widely implemented in mechanical engineering for components expected to have a hardened layer that is no thicker than 0.5 mm [62].

Microalloying constitutes another recognized strategy for enhancing surface hardness. It entails adding extremely small quantities—no more than 0.1% of the total mass—of specific alloying elements to a metal or alloy. Common microalloying elements include vanadium, titanium, boron, niobium, zirconium, rare earth metals (cerium, yttrium, lanthanum, etc.), aluminum, nitrogen, barium, calcium, and magnesium. The fundamental principles of microalloying align closely with standard alloying practices in metallurgy [63].

The prevalent boration technologies, although offering consistent outcomes, are associated with limited productivity and high expenses. In particular, processes like electrolysis and liquid boration of parts require the cumbersome tasks of washing and neutralizing the drains. The utilization of powder in boration, while hindering full mechanization and automation, incurs substantial costs for saturating the mixtures. A common drawback across boration methods is the extended duration of the procedure, typically lasting several hours at minimum. Several methods have been suggested to speed up the boration process. These include using an electron beam, electric spark, micro-arc, induction heating, plasma spraying, stimulating glow discharge during saturation from powders, ultrasound, vibrational and fluidization techniques on the saturating mixture under electrical action, and chemical and physical vapor deposition. Despite potential reductions in saturation time, these methods have not gained widespread adoption due to their technical complexity, lack of versatility, and high energy consumption. In some cases, multicomponent boration means that the part surface is saturated with elements other than boron, such as chromium, aluminum, silicon, and others. This supplementary saturation aims to enhance the corrosion resistance and wear resistance of the metal part surface layer. However, the improvements in resistance achieved through these microalloying processes are not extensive enough to make them universally prevalent. After microalloying, the diffusion layer’s thickness, which includes alloying elements, is usually between 20 and 40–50 µm. This is enough for parts used in environments that are rough or corrosive environments [64].

Various surface treatment methods, including boration, mechanical treatment, and laser processing, further contribute to enhancing material properties.

Low productivity, high cost, and prolonged procedures hinder boration technologies, despite their stable results. Attempts to accelerate boration using advanced techniques face challenges related to technical complexity and energy intensity. Multicomponent boration shows promise in enhancing corrosion resistance and wear resistance.

In industrial practice, mechanical surface treatment is frequently employed to induce plastic deformation in thin surface layers, thereby strengthening the outer layer. This approach enhances strength characteristics and electrical resistance, accelerates diffusion processes, reduces plasticity, and lowers the corrosion resistance of the treated surface. Such mechanical surface treatment through plastic deformation is especially effective during the final manufacturing stages of machine components. This process is generally categorized into two main types: Dynamic Surface Plastic Deformation (DSPD) and Static Surface Plastic Deformation (SSPD).

DSPD involves surface hardening through methods such as bombarding with a stream of metal shot, balls, or a suspension containing abrasive particles; rolling with rollers, balls, or smoothing with a diamond tool; and chasing. An alternative technique utilizes explosion energy to harden the surface, known as the plasma-pulsed detonation (PPD) method, where surface hardening is achieved rapidly using explosives. Shot blasting, a dynamic form of DSPD, is a widely adopted variant that uses metal or corundum shot with particle sizes ranging from 0.5 to 2.0 mm. The optimal particle velocity upon collision with the treated surface is maintained at 50–70 m/s, with the fraction striking at an angle of 75–90°. The typical processing time does not exceed 2–3 min, resulting in a hardened layer thickness of 0.2–0.4 mm. In the surface layer, the density of lattice defects increases, leading to potential changes in the shape and orientation of grains. Consequently, the microhardness of the near-surface layers can experience an increase of up to 50%. For example, the microhardness of steel 20 may increase from 1600 MPa to 2400 MPa after such treatment. However, a drawback of this method is the substantial capital investment required for chamber construction. Shot blasting is generally most effective on large, thick-walled structures and is less suitable for finished, thin-walled components that are under 1.5–2 mm thick. Additionally, the benefits gained from shot blasting are negated if the metal temperature exceeds 450 °C, meaning that shot-blasted parts cannot be subjected to subsequent heat treatment. It should also be noted that blasted surfaces may experience reduced corrosion resistance [65].

Mechanical surface treatments, such as DSPD and SSPD, effectively harden thin surface layers, providing advantages in industrial applications. However, challenges, such as the need for substantial capital investments and limitations in applicability to thin-walled products, are evident.

Another popular technique used to modify surfaces is laser processing (Figure 5). Laser treatment is recognized as a key surface modification method, enabling precise control over heating and cooling in order to tailor the metal’s microstructure. Technologies such as laser hardening, laser nitriding, and laser carburizing are employed to enhance various properties of metals [66], including surface strength, hardness, roughness, friction behavior, chemical resistance, and corrosion resistance.

During laser treatment, the heating rate surpasses 1000 °C/s. It is crucial to highlight that laser processing not only involves rapid material heating but also requires sufficiently swift cooling. The cooling mechanism in laser processing is facilitated by intense heat dissipation into the metal mass, resulting in a cooling rate typically ranging from (5–10)·10³ °C/s at temperatures below the melting point and around 10⁶ °C/s during crystallization from a liquid phase. This rapid cooling occurs due to the small volume of the welding bath during laser treatment, keeping the surrounding metal practically cold [66,67,68,69].

In industrial practice, laser processing can be applied to selective regions of a large structural surface, facilitated by the adaptability of modern laser technologies. Laser-based methods include both techniques that involve surface melting and those that preserve the solid-state microstructure without melting.

Laser transformation hardening (LTH) is frequently utilized for the surface hardening of materials such as cast iron, medium-carbon steel, and tool steel [70,71,72]. LTH involves the rapid irradiation of the workpiece’s surface with a high-power laser beam, elevating the surface temperature above the austenite transformation temperature but below the melting point (Figure 6). In LTH, melting is generally undesirable. The typical thickness of the hardened layer after LTH using a CO₂ laser without surface melting typically does not exceed 0.3 mm, employing a laser pulse of 0.15 mm. A more concentrated laser beam or a powerful laser (1 kW) can then be applied to increase the hardening depth up to 1.5 mm [68,70,71].

In most mixtures and composite materials fabricated via laser processing, including their structural strength, the properties typically comply with the principle of additivity. Under this principle, such properties vary linearly with the percentage or volumetric proportion of each constituent in the mixture. Consequently, one can estimate the approximate strength of dual-phase steels by applying the rules of mixtures [73].

H_c = H_m F_m + H_r F_r,

(3)

where H_c—hardness of composite phase; H_m—hardness of matrix phase; H_r—hardness of reinforcement phase; F_m, F_r—volume fraction of matrix and reinforcement phase, respectively.

In accordance with this guiding principle, the degree of effectiveness of laser processing, and the ultimate strength of the treated region, are contingent on two factors: the total surface area that is subjected to treatment, and the depth of penetration of the hardening process. Therefore, LTH is inherently constrained by the relatively shallow hardened layer that is required to cover a substantial area without melting the metal. Two primary approaches are employed to mitigate this constraint: the “wide-spot” method, which uses a laser beam with a large spot moving uniformly across the treatment zone, and the scanning method, which employs a smaller laser spot paired with a scanning head capable of quickly shifting the beam. However, it remains challenging to apply LTH to extensive surfaces or large/thick-walled components and to treat an entire metal surface. Consequently, LTH is more frequently employed for other purposes, such as enhancing friction, wear, or corrosion resistance through surface hardening. It is noteworthy that when LTH is conducted without melting the metal, it is less suitable for producing oriented structural reinforcement ribs aimed at preventing outward deformation in metal structures.

As demonstrated in several studies, the efficacy of laser processing is contingent on the attainment of temperatures within the metal that exceed its melting point. While the increases in hardness within both the hardened and melted zones of carbon steel are comparable, disparities emerge due to the presence of a melted zone and the potential modifications in the microstructure of hypereutectoid steel. These alterations influence the extent of the hardened zone. Among these approaches, laser treatment with melting achieves the largest treatment depth [74,75]. By leveraging phase transformations and inducing localized changes in the material’s structure under the laser beam, one can create structural reinforcement ribs composed of a modified, harder microstructure. This strategy offers significant opportunities for enhancing the overall strength of thin-walled metal components. The advent of contemporary hardware and laser processing methodologies has engendered the capacity for precise beam manipulation, thereby facilitating the implementation of selective treatment zones, the fabrication of strengthening ribs within designated locations, and the meticulous calibration of their alignment, depth, and geometric parameters within the treated layer [76].

The spectrum of industrial lasers encompasses fibre, neodymium (yttrium aluminium garnet or Nd:YAG), and semiconductor lasers [77]. Semiconductor (diode) lasers typically operate in a continuous mode, whereas Nd:YAG and fiber lasers can be used in both pulsed and continuous modes (

Table 4. Common type of industrial lasers [32,79,80].

Laser		Wavelength	Oscillation Form	Conversion Efficiency (%)
Gas	CO2 laser	1064 nm	CW	Max. 20
			Pulse
	TEA-CO2	Mainly 1064 nm (varies by gas pressure)	Pulse
Solid-state	YAG laser	1064 nm	CW Pulse	Max. 3
	YVO4	1064 nm	Pulse
Fiber	Yb fiber	915–1040 nm	CW Q-switch Ultra-short pulse	Max. 70
Semiconductor	LD laser (GaAs InGaAsP and another)	Mainly 1064 nm (but exist variety of wavelengths)	CW	Max. 100
			Pulse

). The oscillation characteristics, power output, and conversion efficiency of these lasers differ depending on the mode of operation [78].

Continuous wave (CW) lasers find extensive application in laser cutting, welding, and processing thicker workpieces, particularly metal sheets exceeding 2 mm in thickness. These lasers predominantly employ remelting with deep penetration, featuring a high aspect ratio surpassing 5:1, rapid processing speeds, and minimal thermal deformation.

In the continuous mode, the impact zone adopts a strip shape of a specific width, while in pulse mode, it transforms into a circular shape. This configuration allows for adjustment of the degree of overlap, the introduction of multiple heating zones, and precise control over the metal cooling rate [78].

For the efficient processing of thin metals, softer modes are essential, and these are achieved through pulsed laser processing. This processing method operates on the principle of heat conduction, resulting in minimal overall temperature elevation in the workpiece and, consequently, limited thermal impact and deformation [81].

A key benefit of pulsed operation is the ability to achieve extremely high peak power. While solid-state lasers typically provide 1–3 kW of power in continuous mode, pulsed lasers operated in free-running mode can deliver average power levels of up to 10 kW, with pulse durations in the 10⁻³ to 10⁻⁴ s range.

In pulsed mode operation, a notable drawback of solid-state lasers is their relatively low efficiency, reaching up to 3%, in contrast to the more efficient continuous-wave lasers that boast efficiencies of up to 10%. However, it is essential to acknowledge that laser diodes (LD) for continuous-wave lasers come with a significantly higher cost. Additionally, the processing speed in pulsed mode is significantly slower than that of continuous lasers, which can reach speeds of up to 100 m/h. Despite this, the pulsed nature of processing ensures a minimal depth of metal heating and eliminates the risk of overheating.

Industrial pulsed lasers are offered in a range of pulse durations, encompassing millisecond, microsecond, nanosecond, picosecond, and femtosecond lasers [82].

When assessing the effectiveness of short-pulse laser radiation, one must recognize that the depth of the heated layer is governed largely by the pulse duration and the thermal diffusivity of the material. In other words, the shorter the pulse, the shallower the penetration of the laser energy. Hence, for thin steel components or layers up to 0.5 mm thick (Figure 7), millisecond pulsed lasers operating with pulse durations of approximately 5–10 ms are typically the most suitable. By contrast, pulsed laser processing to achieve a penetration depth exceeding 1 mm becomes considerably more challenging [83]. The microstructure located in the laser-processed area of the non-alloy steel sample exhibits a typical sorbite structure [84].

Laser processing emerges as a versatile surface modification technique, offering diverse applications in hardening, nitriding, and carburization. The rapid heating and cooling rates achievable through laser processing enable precise control over material microstructures. LTH proves effective, and the choice between melting and non-melting processes depends on the desired outcomes.

The use of industrial lasers, including fiber, neodymium, and semiconductor lasers, adds another dimension to surface treatment. Pulsed lasers, despite lower efficiency, offer advantages in terms of peak power and minimized thermal impact. Various pulse durations cater to different material thicknesses, allowing flexibility in processing.

The paper [13,84] investigates the development of innovative metal processing technologies focusing on laser treatment. Research addresses a significant challenge in expanding the industrial applications of thin-sheet steel products by developing a laser processing technology capable of creating structural strengthening ribs. This is expected to have a significant impact on the overall strength and stiffness of metal components. The modification in microhardness from 130 HV to 210 HV resulting from laser treatment was substantiated by the alteration in microstructure; specifically, the microstructure present within the laser-processed region of the non-alloy steel specimen reveals a characteristic sorbite structure (Figure 8 and Figure 9).

The data obtained and analyzed in this article [13,84] demonstrate that localized laser heating can be utilized to create internal structural rigidity ribs with changing of mechanical properties of the laser-treated zone compared to the base material (laser-untreated zone) (

Table 5. Mechanical properties of laser-treated zone and base material (laser-untreated zone).

Zone	Modulus of Elasticity E, GPa	Yield Strength σ0.2, MPa	Ultimate Strength σB, MPa
Base material (laser-untreated zone)	200	256	410
Laser-treated zone	210	412	662

). These reinforcing ribs, in turn, are capable of enhancing the overall rigidity of thin sheet LCS by as much as 33% in bending and 26% in tension loads (Figure 10 and Figure 11). This is achieved without the need for complex geometric shapes/forms or an increase in the weight or dimensions of the structure. Consequently, this evidence suggests that the advancement of technology, including laser equipment, allows for a significant improvement in the rigidity of metal structures. This improvement is achieved through entirely new methods of metal processing that deviate markedly from traditional, established thermal treatment methods.

The comprehensive exploration of steelmaking processes and various heat treatment techniques for steel structures underscores their critical role in the metallurgical and metalworking industry (

Table 6. Comparisons of processing techniques.

	Thermochemical Processing	Thermomechanical Processing	Mechanical Processing	Laser Treatment
Advantages	- stable results in boration techniques	- cost-effective technique for annealing - normalization achieving a fine-grained structure in LCS and characterized by shorter cooling times - annealing seeks equilibrium microstructures	DSPD and SSPD effectively harden thin surface layers	- versatile surface modification technique - diverse applications in hardening, nitriding, and carburization - rapid heating and cooling rates - precise control over material microstructures
Disadvantages	- low productivity - high cost - prolonged procedures for boration techniques	- LCS cannot be quenched using standard methods	- substantial capital investments - limitations in applicability to thin-walled products	employing LTH without melting to strengthen LCS faces challenges due to the limited thickness of the hardened layer and the need to treat a large surface area

). Using methods such as thermochemical, thermomechanical, and mechanical processing, the industry aims to optimize the operational properties of metal products. In particular, heat treatment has become a prevalent practice, with annealing, quenching, tempering, and normalization being the key methodologies. Transformations of austenite into structures such as martensite, troostite, sorbite, perlite, and others, on cooling rates, provide a foundation for understanding the effects of heat treatment.

5. Conclusions

The synergy of traditional heat treatment, advanced surface treatments, and laser processing provides a spectrum of tools for optimizing the properties of steel and related materials. Each method brings its own set of advantages and limitations, which require careful consideration based on specific material characteristics and intended applications. Continued research and technological advancements in these areas promise further refinement of these processes, contributing to the ongoing evolution of materials science and industrial practices.

In contrast to other metalworking methods, which may not always be optimal, efficient, or readily applicable, modern laser technologies emerge as highly beneficial for enhancing the rigidity and other operational properties of shell structures constructed from LCS.

Laser processing methods exploit localized heating, steep temperature gradients, and swift cooling rates (on the order of 10⁴ to 10⁶ °C/s), a capability made possible by their high thermal conductivity. Consequently, laser processing enables microstructural modifications that enhance the mechanical properties of LCS. However, employing LTH without melting to strengthen LCS faces challenges due to the limited thickness of the hardened layer and the need to treat a large surface area. On the other hand, laser processing with melting makes it possible to create a thicker treated layer, which makes it a better method for localized laser treatment that aims to strengthen thin-walled steel products.