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Abstract
This study introduces a convergence approach named laser oxyfuel and oxy-flame hybrid (LOXAFH) cutting, integrating laser cutting and oxyfuel cutting techniques for cutting clad steel plates. The LOXAFH method combines laser, oxyflame, and oxygen jet in a coaxial manner, harnessing 1 kW power concentration. The LOXAFH cutting head was meticulously developed on the basis of a ray transfer matrix and estimated refractive index by incompressible flow with a computational fluid dynamics program and an optical design program. The LOXAFH cutting is validated through three experiments on 15 mm clad steel plates: oxyfuel cutting, laser-assisted oxygen cutting (LASOX), and LOXAFH cutting.
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1. Introduction
The clad steel plate offers numerous advantages such as increased strength, cost savings, and corrosion resistance compared to a single steel plate, making them a popular choice for use in reactor pressure vessels of power plants and exterior materials in building and ship. However, due to the design life, dismantling clad steel plates with a thickness of 300 mm or more in a confined space poses a significant challenge [1,2]. Conventional dismantling method, such as mechanical cutting, electric discharge machining (EDM), and abrasive waterjet cutting, has proven to be dependable for cutting thick materials. However, it has limitations when it comes to tight spaces, generating secondary waste, and remote operation [2]. Consequently, thermal cutting has been extensively researched due to its faster cutting speed, remote operation, less cutting waste, and improved cut quality [3,4].
Thermal cutting methods for cutting clad steel plates have their own advantages and disadvantages. Oxyfuel cutting is one of the thermal cutting methods that involves preheating a material with a mixture of oxygen and fuel gas, then melting it with an oxygen jet through exothermic reactions. Oxyfuel cutting is capable of cutting materials up to 1200 mm in thickness, such as structural steel, but can be only applied to low-alloy steels due to exothermic reactions, and perform poor cutting quality compared to the other thermal cutting methods [5–7].
The other thermal cutting method is laser cutting, which rapidly heats a localized area with a focused laser beam and removes the molten material with an inert gas. Laser cutting yields excellent cutting quality that is not influenced by the type of material. Also, the cutting thickness of the steel structure is mainly dependent on the laser power. The thicknesses that can be cut with a 500 W laser are known to be around 2 mm for stainless steel and 6 mm for carbon steel [8–10]. Also, One of the thickest reported cuts was 300 mm of stainless steel with a 30 kW laser [11–13]. The use of oxygen for exothermal reactions instead of inert gas is called laser-assisted oxygen (LASOX) cutting method [14,15]. LASOX cutting does not require preheating process like oxyfuel cutting, but instead, heats a larger area with a laser beam spot to induce exothermic reactions and melt the material [16]. However, LASOX cutting is only capable of cutting ferrous metals, mainly low-alloy mild carbon steels. Furthermore, LASOX has a maximum cut thickness of 50 mm in reported cases. [17].
Therefore, we propose a convergence technology named laser-oxyfuel and oxyflame hybrid (LOXAFH) cutting to overcome the limitations of conventional cutting methods. Through theoretical analysis and experimental results of the cutting method, we validate the effectiveness of LOXAFH cutting for dismantling clad steel plates. The LOXAFH cutting method can overcome the limitations of oxyfuel cutting by melting high-alloy metals using laser, preheating flames, and exothermic heat from oxidation from mild steel. Furthermore, the cost-effectiveness of oxyfuel cutting and the ability to cut thick materials has the potential to overcome the thickness limitations of laser cutting which has only 2-3 mm high alloy steel cut report by 0.5 kW laser [9,10].
2. Experimental setup
The system for the new hybrid cutting method was developed using a 1 kW continuous (CW) laser, oxygen jet (4-6 bar), low-pressure(0.6-0.8 bar) oxygen, and propane gas. The 1 kW laser is mounted to the head and coaxially irradiated with oxygen jet and preheated flame, shown in Fig. 1(a). The specimen used in the cutting test is a metal composite with a 3 mm non-oxidizable metal layer (stainless steel; SUS 304) on a 12 mm oxidizable base metal (carbon steel; SS400). To effectively cut the 15 mm clad steel plate, a numerical simulation of fluid was carried out with an oxygen jet supplied to the head using the computational fluid dynamics program (COMSOL Multiphysics). The simulation model was two-dimensional axisymmetric model with steady-state analysis and k-$\epsilon$ turbulence model [18,19]. The inlet boundary condition applied 4-6 bar (gauge pressure) of oxygen, and atmospheric pressure was applied to the buffer at the outlet to visualize the flow. Fig. 1(c) shows that the distance where the flow of the oxygen jet exceeds Mach 1 from the nozzle is 57.0 mm, which is a sufficient velocity to oxidize SS400 with a thickness of 13 mm. The flow velocity in the region from 20 mm to 40 mm from the nozzle was an average of 401.9 $\pm$ 6.9 m/s, which appears to be the most stable flow of oxygen jet. Standoff distance (SOD) is deeply related to the flow of flame and oxygen jet, thus was selected as 20 mm based on the most consistent oxygen jet delivered to the base material through computational flow analysis, shown in Fig. 1(c).
Fig. 1. System overview and simulation results. (a) Schematic diagram of laser-oxyfuel and oxyflame hybrid (LOXAFH) cutting head. (b) Lens parameter description. (c) COMSOL multiphysics simulation results; configuration of the head and output buffer(left), flow velocity(right). (d) Ray transfer matrix simulation based on refractive index in atmospheric conditions and estimated refractive index by Eq. (5). (e) Optimization results by optical design software(Optics Studio)
The laser used in this study is a continuous wave (CW) single mode, 1 kW, 1080 nm, $M^2$<1.3 fiber laser (RECI, FC1000). In order to effectively deliver the laser beam to both the top and bottom of the clad steel plate, the depth of focus should be maximized. Also, there should be no interference between the mechanical shape of the nozzle and the laser beam. Since, oxygen jet and the laser beam are irradiated coaxially inside the head, it is necessary to estimate the refractive index inside the head and incorporate the estimated refractive index into the optical system design. The initial design of the optics system was performed using through the ray transfer matrix of Eq. (1)–(4). The schematic of optical system and parameter descriptions are shown in Fig. 1(a) and (b). The numerically computed results through the ray transfer matrix are presented in Fig. 1(d).
Initial design utilizing the estimated refractive index can be done using the ray transfer method with the paraxial approximation, Eq. (1)–(4). In this equation, $r_1$ and $r_n$ represent the radius of incidence and exit beams on the specimen, respectively, and $\theta _1$ and $\theta _n$ are the incidence and exit beam angles between the optical axis and the beams. $t_{l}$ represents the thickness of the lens, and $R_{k1}$ and $R_{k2}$ are the radius of curvatures of the front and back surfaces of the lens respectively. $n_{l}$ and $n_{h}$ are the refractive indices of the lens and inside the head based on gas state, respectively. $D_{k}$ is either the gap between lenses or between the lens and the specimen. As shown in Eq. (1)–(4), the output ray vector $[\begin {smallmatrix} r_n\\ \theta _n \end {smallmatrix}]$ is derived utilizing the matrix for thick lens $M^{(lens)}_k$, free space propagation matrix $M^{(D)}_{k}$, and initial ray vector $[\begin {smallmatrix} r_1\\ \theta _1 \end {smallmatrix}]$. The refractive index of gas inside the head $n_{h}$ can be estimated through Lorentz-Lorenz equation, shown in Eq. (5) [20].
The Lorentz-Lorenz equation is approximately valid in gas applications. From Eq. (5), $N$ is the number of molecules per unit volume, $N_{A}$ is Avogadro constant, $\alpha _{m}$ is the mean polarizability, $A$ is molar refractivity, $P$ is pressure, $R$ is gas constant for oxygen, and $T$ is temperature. The density of the gas, $\rho$, which is the mass of the gas per unit volume at steady state, can be estimated by Eq. (6)–(8) when 5 bar of oxygen gas at room temperature is introduced into the head. The maximum value of refractive index $n_{h}$ due to oxygen estimated from this is $n_{h} = 1.0017$. The numerical calculation of ray tracing is performed using MATLAB, in the condition of the atmospheric refractive index inside the head $n_{h} = 1.0003$(atmospheric environment) and $n_{h} = 1.0017$ (5 bar oxygen flow), shown in Fig. 1(d). The difference in the diameter of the laser beam delivered to the top surface of the specimen in each condition is 0.3 mm.
Based on the results of the initial design, the distance between the lenses was optimized using the optical design software (OpticStudio) real ray tracing analysis, and the results are shown in Fig. 1(e). The root-mean-square (RMS) radius of the beam incident on the upper surface of the sample is 0.54 mm with an intensity of 60 $kW/cm^2$, and the RMS radius at the back surface of the clad steel plate is 0.67 mm. Based on these specifications, a LOXAFH cutting nozzle was designed and utilized for the experiments.
3. Results and discussion
In this study, three types of experiments were performed: oxyfuel cutting, LASOX cutting, and laser-oxyfuel and oxyflame hybrid (LOXAFH) cutting. The oxygen jet flow from the cutting head and 1 kW laser are transmitted through a cone-straight nozzle, as shown in Fig. 1(a). The oxygen and propane gas were supplied at gauge pressures of 0.6-0.8 bar for making preheating flame, while the other oxygen inlet for making oxygen jet was maintained at gauge pressures of 4-6 bar. All pressures for gas were kept constant using a pressure regulator. LOXAFH cutting involves creating a flame for preheating the clad steel plate, and then melting and blowing the molten material by coaxially transmitting the laser and oxygen jet. The cutting speed for all three methods (oxyfuel cutting, LASOX cutting, and LOXAFH cutting) is 30 mm/min. For LASOX cutting, the laser power was 1 kW, while the laser power for the LOXAFH cutting experiment was 1 kW and 0.5 kW.
Figure 2 shows the results of the oxyfuel cutting, LASOX cutting, and LOXAFH cutting on a 15 mm clad steel plate. The experimental results show that LOXAFH cutting can cut 15 mm of the clad steel plate with 0.5 kW laser and small beam radius 0.54 mm. It implies that LOXAFH cutting can overcome the limitation of LASOX, which requires a laser beam diameter as large as the gas jet footprint, the limitation of laser cutting, which can only cut 2-3 mm per 0.5 kW, and the limitation of oxyfuel cutting, which cannot cut high alloys. As shown in Fig. 2, the preheating flame and oxygen jet from the oxyfuel cutting method did not reach the temperature high enough to melt the SUS 304 steel layer. This is because the melting point of the high-alloy steel is close to that of the slag produced during cutting, which prevents slag from being expelled and mixed with the molten metal near the cutting edge to hinder oxidation [21]. In the case of LASOX cutting, the non-oxidizable metal on the front surface was melted by laser, but the molten layer was not effectively removed by the oxygen jet. However, under the same conditions, LOXAFH cutting cut the clad steel plate even at a lower laser beam input of 0.5 kW.
Fig. 2. Cutting results for clad steel plate by oxyfuel cutting, laser-assisted oxygen (LASOX) cutting with laser power of 1 kW, LOXAFH cutting with laser power of 1 kW and 0.5 kW.
Laser cutting involves melting the metal using a high-intensity focused laser beam and using an inert gas jet to remove the molten material [17]. In contrast, LASOX cutting is influenced by the flow of the oxygen jet affected by the melting width of the laser beam, thus cannot concentrate the laser beam, because the exothermic reaction is directly related to the melting of mild steel [22]. LOXAFH cutting can cut clad steel plates with a relatively small laser beam because it can improve the influence of the gas jet to the base metal compared to laser cutting and LASOX cutting. The LOXAFH cutting mechanism and is illustrated in Fig. 3(a) and (b). The laser mainly melts high alloy steel, and the flame widens the width of the molten layer. The widened top-kerf helps to deliver the sufficient flow of the oxygen jet for an effective exothermic reaction of the base metal. The mechanism can be inferred from the side view of cut clad steel plate by LASOX cutting and LOXAFH cutting, as shown in Fig. 3(c). Therefore, LOXAFH cutting can locally heat a small area with high energy density by focusing the beam to about 1 mm in diameter. This allows the cutting of thicker 15 mm cladding metals, compared to the ability to cut SUS 304 of 2$-$3 mm thickness per kW of power in conventional laser cutting [8,10]. The side view of the specimen cut by LOXAFH cutting in Fig. 2(a) shows that the melting of SUS 304 was due to the input heat from the laser and preheating flame dominantly which validate that it is sufficient to melt cladding metal containing high-alloy steel (SUS 304) and compensates for the disadvantage that only mild steel can be cut in oxyfuel cutting.
Fig. 3. LOXAFH cutting mechanism with affecting factor analysis for melting (a) LOXAFH cutting mechanism (b) Side view of clad steel plate by LASOX and LOXAFH cutting and its heat inducement (c) Cutting experiment and specification of the clad steel plate.
The results of the top and bottom kerf widths for LASOX and LOXAFH are shown in Fig. 4. The top kerf width for LASOX was 1.5$\pm$0.0 mm, while for LOXAFH cutting, the top kerf width was 8.1$\pm$2.0 mm, and the bottom kerf width was 11.4$\pm$1.1 mm. The top kerf width of LOXAFH cutting in 1 kW increased about four times compared to LASOX. Comparing the LASOX and LOXAFH cutting, it appears that the heat input from the flame aids to melt the SUS 304, which creates an expansion of the top kerf width facilitating the effective propagation of the oxygen jet. Under the condition of 0.5 kW laser in LOXAFH cutting, the top kerf width and bottom kerf width are 15.3$\pm$0.5, 9.3$\pm$0.8 mm, respectively. The reduction in laser incident power increases the thermal influence of the flame, providing a wider top kerf width, which appears to increase the influence of the oxygen jet blowing the molten material. The variability of the oxygen jet with the change in top kerf width can be explained through Fig. 5.
Fig. 4. Top and bottom kerf width comparison of cutting experiment results by oxyfuel cutting, laser-assisted oxygen (LASOX) cutting with laser power of 1 kW, LOXAFH cutting with laser power of 1 kW and 0.5 kW
Fig. 5. Numerical simulation of computational fluid dynamics by COMSOL multiphysics. (a) Simulation of oxyjen jet applying the top kerf width of the SUS 304 steel cut by LASOX as a buffer. (b) Simulation of oxyjen jet applying the top kerf width of the SUS 304 steel cut by LOXAFH as a buffer.
Figures 5(a) and (b) show the numerical analysis of the flow based on the top kerf width of SUS 304 cut by 1 kW LASOX and 1 kW LOXAFH cutting, respectively. The graph at the bottom right of Fig. 5(a) and (b) show the cross-section of the velocity field at the rear end of the SS400. For LASOX cutting, the velocity at the center of the rear end of the SS400 was 233.4 mm/s, with a jet diameter of 3.9 mm. In contrast, in the case of LOXAFH cutting, the velocity and jet diameter were 423.6 mm/s and 9.7 mm, respectively. The jet diameter is the width from the rear end of the SS400 to a point 10${\%}$ above the maximum jet flow velocity. The jet diameter significantly influences the generation of an exothermic reaction of the base metal and blowing molten material. If the oxygen jet is unobstructed, the diameter of the oxygen jet on the front of the top plate and on the back of the back plate would be similar. When the oxygen jet passes through the narrower width compared to the jet diameter, which leads to the diffusion of gas, the bottom kerf width tends to widen due to exothermic reaction. In case of LASOX cutting, the oxygen jet suffers from a narrow top kerf width leading to decreased flow rate and speed, resulting in ineffective removal of the molten material, shown in Fig. 5(a). This result supports the fact that the laser beam diameter must be larger than the gas jet footprint [22]. In LOXAFH cutting, the flame, along with the laser, provides heat to melt the clad metal, increasing the top kerf width and increasing the influence of the oxygen jet. In LOXAFH cutting, if a 0.5 kW laser is applied instead of 1 kW, the influence of the flame on melting the clad metal is increased, which leads to a wider top-kerf width because the heat is applied over a larger area. Since the widened top kerf width is larger than oxygen jet diameter $\phi =9.7mm$ that increases the straightness of the oxygen jet. The fluid straightness reinforces the influence of blowing the molten material rather than the exothermic reaction, which leads to a decrease in the bottom kerf width. This means that the quality of the cut can be improved by controlling the influence of the preheating flame, laser, and oxygen jet.
4. Conclusion
In summary, we demonstrate LOXAFH cutting technique for clad steel plates, addressing the limitations of conventional thermal cutting methods. By combining laser and oxyfuel cutting in a coaxial manner and concentrating power, the LOXAFH method offers a solution for cutting thicker clad steel plates that surpasses the capabilities of traditional laser and oxyfuel cutting. The cutting head design, validated through experiments on 15 mm clad steel plates by LOXAFH cutting with 0.5 kW laser power, demonstrates the effectiveness of the LOXAFH technique in melting high-alloy steel-containing clad steel plate. The heat incident by the laser and flame effectively melts the top plate, which aids in the effective entry of the oxygen jet. Creating a top kerf width larger than the diameter of the oxygen jet was important for effective oxidation of the base metal and blowing of the molten material.
Funding
Korea Institute for Advancement of Technology(KIAT) grant funded by the Korea Government (MOTIE) (P0008763, HRD Program for Industrial Innovation); Ministry of SMEs and 201 Startups (G21S324530901).
Disclosures
The authors declare that they have no conflicts of interest.
Data availability
The data underlying the results presented in this study are not publicly available at this time but may be obtained from the authors upon reasonable request.
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