rus
Issue #6/2024
D. V. Zhurba, V. M. Zhurba, V. P. Veiko, A. E. Puisha
Investigation of the Process of Laser Descaling of Rolled Metal
Investigation of the Process of Laser Descaling of Rolled Metal
DOI: 10.22184/1993-7296.FRos.2024.18.6.436.449
The article investigates the process of laser cleaning of mill metal in exposure modes that do not lead to heating of the scale above its melting point. The possibility of destruction of mill scale due to thermochemical reactions in the scale and subsequent thermomechanical destruction has been identified and substantiated. The search and optimization of laser exposure modes has been carried out to increase the cleaning efficiency. For a more complete description of the laser cleaning process, attention is paid to the structure of mill scale; the features of its formation and probable phase transformations under the action of laser heating are described. An area of laser treatment modes in the range of exposure durations from 30 to 400 µs and power densities of from 50 to 750 kW/cm2 leading to thermomechanical scale destruction has been experimentally discovered. The process of laser descaling due to thermomechanical destruction is implemented using a continuous-wave fiber ytterbium laser with a maximum power of 2 kW.
The article investigates the process of laser cleaning of mill metal in exposure modes that do not lead to heating of the scale above its melting point. The possibility of destruction of mill scale due to thermochemical reactions in the scale and subsequent thermomechanical destruction has been identified and substantiated. The search and optimization of laser exposure modes has been carried out to increase the cleaning efficiency. For a more complete description of the laser cleaning process, attention is paid to the structure of mill scale; the features of its formation and probable phase transformations under the action of laser heating are described. An area of laser treatment modes in the range of exposure durations from 30 to 400 µs and power densities of from 50 to 750 kW/cm2 leading to thermomechanical scale destruction has been experimentally discovered. The process of laser descaling due to thermomechanical destruction is implemented using a continuous-wave fiber ytterbium laser with a maximum power of 2 kW.
Теги: лазерная очистка микросекундная длительность воздействия непрерывный иттербиевый волоконный лазер прокатная окалина термомеханическое разрушение окалины
Investigation of the Process of Laser Descaling of Mill Metal
D. V. Zhurba 1, 2, V. M. Zhurba 2, V. P. Veiko 1, A. E. Puisha 2
ITMO University
NPP VOLO LLC)
The article investigates the process of laser cleaning of mill metal in exposure modes that do not lead to heating of the scale above its melting point. The possibility of destruction of mill scale due to thermochemical reactions in the scale and subsequent thermomechanical destruction has been identified and substantiated. The search and optimization of laser exposure modes has been carried out to increase the cleaning efficiency. For a more complete description of the laser cleaning process, attention is paid to the structure of mill scale; the features of its formation and probable phase transformations under the action of laser heating are described. An area of laser treatment modes in the range of exposure durations from 30 to 400 µs and power densities of from 50 to 750 kW/cm2 leading to thermomechanical scale destruction has been experimentally discovered. The process of laser descaling due to thermomechanical destruction is implemented using a continuous-wave fiber ytterbium laser with a maximum power of 2 kW.
Key words: Laser cleaning, continuous-wave ytterbium fiber laser, microsecond exposure time, mill scale, thermomechanical scale destruction
Article received: 23.05.2024
Article accepted: 01.07.2024
Introduction
The removal of oxide layers from the surface of metals is a pressing challenge for the metalworking industry. The most demanded and, at the same time, the most difficult task here is the removal of oxide layers from the surface of hot-mill carbon steels (mill scale). Mill scale does not protect steel from corrosion, and in some cases leads to an acceleration of corrosion processes, which reduces the service life of mill metal products [1–4], therefore, the surface of steel in all cases needs to be cleaned from it.
By now, various methods of mechanical or chemical descaling of metal surfaces are most common. Mechanical cleaning methods include shot blasting and sandblasting, grinding with abrasive brushes and others. Chemical methods include pickling in hydrochloric acid, sulfuric acid and nitric acid baths or in alkalis, including using electrolysis [5].
The significant disadvantages of these methods are: insufficient degree of cleaning, damage to the metal surface, as well as a negative impact on personnel and the environment associated with the use of hazardous consumables, reagents, the formation of by-products (sand dust and scale dust, acid vapors, waste pickle liquor, contaminated sludge, etc.), which must be disposed of. Due to the need to improve working conditions and environmental safety of production facilities, serious restrictions are imposed on these methods.
Laser sources are increasingly being used to remove various contaminants, in particular oxide layers from the surface of metals. Laser cleaning has already established itself as an environmentally safe and easy-to-use method for scale removals, characterized by its being contactless, no need for consumables and tools, low operating costs, the fundamental ability to remove any types of impurities and a number of other advantages associated with the use of laser radiation with high power density [6–12].
In the last decade, laser cleaning technology based on evaporation of impurity material by nanosecond pulses with high power density (>107 W / cm2) has become the most widespread. The use of ytterbium pulsed 100–200 ns fiber laser sources is justified by the versatility of their use: an acutely focused laser pulse makes it possible to remove almost any impurities from the surface of most materials [6–9, 13–15]. The short pulse duration also minimizes the impact on the base material; in metals, the zone of thermal exposure and reflow is limited to a few microns. However, evaporation of oxide layers and, especially, scale is a very energy-intensive process. Therefore, pulsed laser removal of mill scale from the surface of hot-mill steel is characterized by low productivity [13, 14].
The use of a continuous laser makes it possible to vary processing parameters such as exposure time, power density and energy in wide ranges. Therefore, it seems promising to investigate and implement various descaling mechanisms, including those not related to evaporation, using continuous lasers. Cleaning of mill metal by continuous laser radiation due to evaporation is difficult to implement in practice. Heating a thin layer of scale to the boiling point also leads to significant heating of the steel. As a result, the surface of the steel is melted and a new layer of scale is formed on it [16]. Such strong heating will lead not only to damage to the surface, but also to significant thermal deformations of the entire article. Therefore, for productive laser descaling of mill metal (and structures made of it), it is necessary to look for more effective mechanisms of destruction and removal of scale that do not require high-temperature heating. In this direction, the most promising is the study of the mechanisms of scale destruction due to thermal stresses. The advantages of a laser heat source are obvious: the high power density allows to heat the entire thickness of the scale layer within several tens of microseconds, and in the absence of radiation, to cool it almost as fast due to thermal conductivity and heat dissipation into the metal base.
This paper proves the possibility of removing mill scale due to thermomechanical destruction when heated by radiation of a continuous-wave fiber ytterbium laser and carries out the search and optimization of exposure modes that cause thermomechanical destruction of mill scale on a thin-sheet St3sp mill carbon steel. The heating temperature of the scale in this process does not exceed its melting temperature, and the destruction and removal of scale fragments occurs in the solid phase state due to tensile thermal stresses and a decrease in adhesion during the formation of wustite. Heating to the temperature of scale destruction occurs only in its own layer. The surface of the steel does not undergo significant thermal effects. Laser scale destruction due to thermal stresses is a less energy-consuming process than laser evaporation.
Structure and process
of mill scale formation
Scale on the surface of steels is an oxide layer (black, bluish or dark red) formed as a result of heat treatment of steels in an oxidizing atmosphere, for example during mill. Normally, at a steel heating temperature above 570 °С, the scale consists of wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3) [17, 18]. Mill (or air) scale is formed on the surface of steels at the final stage of mill, when cooled after the last mill mill stand. The final form and composition of the mill scale depends on the temperature at the end of mill and the temperature of winding into rolls, the cooling rate before winding and the roll cooling rate, the composition of the atmosphere, the presence of water vapor and many other factors [1, 18–21].
Oxides are formed on iron, the specific volume of which oxides is greater than the volume of the metal, so the scale is porous and brittle. Scale can consist of various oxide layers, which differ in thermophysical properties. During high-temperature heating in furnaces, a three-layer scale is formed on steel, the lower layer of which is wustite [22]. At high temperatures, the wustite becomes plastic. At temperatures above 1 250 °С, the shear resistance in the wustite layer becomes close to zero. Therefore, at high temperatures, a reduced amount of scale adhesion to the metal surface is characteristic [19]. Differences in the coefficients of thermal expansion of the scale and metal layers, as well as a decrease in adhesion during the formation of a wustite layer, are prerequisites for the destruction of scale during heating and rapid cooling.
This effect is successfully used in mill production in the hydraulic removal of furnace scale, which has a classic three-layer structure. However, the hydraulic descaling works well with scale thicknesses of more than 1 mm and with high-temperature heating of the entire workpiece. In this case, the jet of water effectively cools the scale layer, while the base metal remains hot. Due to the shear stresses arising in the scale layer, as well as the additional kinetic energy of the water jet, the scale crust is destroyed and detached [23].
Unlike furnace scale, which is formed at a constant high temperature and consists mainly of wustite and a thin upper layer of magnetite, mill scale is formed on a finished roll at a constantly decreasing temperature. The temperature at the end of the mill process (at the exit from the finishing stand of the mill mill) is usually below 900 °C. The sheets are quickly cooled to the winding temperature and winded into rolls, which are left to cool in the air. Air access to the surface of the mill sheet is limited, and the oxidation of steel slows down. Due to the shorter time of high-temperature oxidation, the thickness of the formed mill scale does not exceed several tens of microns. Prolonged cooling of mill products with mill scale at temperatures above 570 °C leads to the formation of a scale consisting mainly of magnetite, at temperatures below 570 °C – in the scale, the initial wustite decomposes by eutectoid reaction to magnetite and metallic iron. If the winding of the sheets takes place at a lower temperature, and the cooling of the rolls occurs quickly enough, then a large amount of wustite is stored in the mill scale and such scale is easier to remove. However, as a rule, the cooling of the rolls is slow and the main component in the scale is magnetite or the product of the eutectoid decomposition reaction of wustite [20, 21, 24–28]. The scheme of formation of various types of mill scale is shown in Fig. 1. A thin film of mill scale turns out to be more homogeneous in its properties and more plastic than classical furnace scale. Such scale has no pronounced layering, has high adhesion to steel and resists well mechanical action [23].
In order to implement the mechanisms of destruction of mill scale due to internal stresses, it is necessary to increase the intensity of heating and cooling. It is known that when surface heating of steel with mill scale by high frequency currents and additional mechanical action (blows of piezoceramic micro-vibrators), its destruction occurs and a cleaning effect is achieved. An increase in the plasticity of the wustite during heating contributes to the destruction of scale with additional mechanical action [29]. However, the intensity of heating and cooling when using induction heating is not sufficient for effective cleaning. Also, this method is not suitable for cleaning thin-sheet mill products due to significant overheating of the steel sheet and its warping.
Taking into account the structure of the scale and the process of its formation, it was concluded that it is necessary to study the mechanisms of destruction and removal of scale due to intensive local laser heating of it to the full depth and subsequent cooling due to thermal conductivity. As a result of heating the scale above 570 °C, wustite is formed from a mechanical mixture of magnetite and metallic iron. During cooling at temperatures below 570 °C, wustite decomposes again into magnetite and metallic iron. High temperature gradients during cyclic heating and cooling and various linear expansion coefficients of the scale and steel phases form stresses in the scale layer [29, 30]. As a result, the combination of physico-chemical and thermomechanical mechanisms during laser heating makes it possible to destroy and remove the initial layer of scale. Since the heating temperature of the steel surface is significantly lower than its melting point, it does not damage and oxidize [31].
Description of samples
The study of the mill scale cleaning process was carried out on samples of hot-mill products with a thickness of 5 mm of the St3sp steel grade containing from 0.14% to 0.22% carbon. Steel of this grade is widely used in industry. The surface of the sheets in the delivery state is covered with a uniform thin scale of dark blue color without defects visible to the unaided eye. Before laser cleaning, the surface of the samples was not subjected to preliminary preparation. The thickness of the scale was measured on the transverse section of the sheet using an optical microscope. As seen on the micrograph (Fig. 2), the thickness of the scale is about 15–20 microns.
The micrograph also shows oxidized areas of steel under the main dark layer of scale. The formation of these areas is probably associated with defects of mill [32] or with the internal oxidation of steel and the formation of an intra-oxidized transition layer at the steel-scale boundary [33].
Continuous-wave laser scale treatment schemes
The study used a continuous-wave fiber ytterbium laser YLR‑2000-MM-WC with a wavelength of λ = 1.06 µm with a power of 2 kW. Two different optical scanning systems were used: circular and linear (Figure 3). The circle or line obtained with the help of appropriate optical scanning systems was moved rectilinearly to form a continuous processing area. The considered method of scanning the metal surface with a linearly moving circle to perform cleaning was proposed in the patent [34].
The main variable parameters of the exposure were the power density, duration of exposure and the overlap coefficient of the scan figures. The power density and duration of exposure depend on the parameters of the scanning systems: the diameter of the fiber, the optical magnification, and the scanning speed of the laser beam. The overlap coefficient also depends on the speed of linear movement of the scanning figure.
For a circular scanning system, the laser processing area is formed by superimposing rings formed during circular scanning, the center of which shifts with the linear movement of the scanning head. The resulting scan strip consists of rings superimposed on each other with some overlap. The overlap coefficient between adjacent rings depends on the size of the spot, the speed of rotation of the spot and the linear speed of movement of the scanning head. With this scanning method, the overlap coefficient and the total radiation dose increase in the transverse direction, from the center to the edge of the scanning strip. Therefore, the threshold specific energy doses of radiation E necessary to obtain the desired effect of exposure (below – radiation doses) are determined by the values of E and the result of treatment in the central area of the exposure strip. The study of cleaning modes was performed at a 50% ring overlap coefficient in the center of the scan strip.
The linear scanning system forms the processing area in the form of a continuous zigzag strip. The overlap coefficient of the links in the center is maximum and decreases by half to the tops of the links. The threshold values of the radiation dose E (J / cm2) required for the destruction and removal of scale were determined for the central treatment area. The study of cleaning modes was performed with an overlap coefficient in the center of the strip of 50%.
Investigation of thermomechanical scale destruction under the action of laser radiation
The following is an assessment the parameters for heating the scale to a depth of 15 microns by radiation from a continuous-wave ytterbium fiber laser. The thickness of the magnetite layer in which laser radiation with a wavelength of 1.06 microns is absorbed is only 230 nm (absorption index 4.4 ∙ 104 cm‑1) [35]. The thermal diffusivity of magnetite is 1.8 ∙ 10–6 m2/s (determined by formula 1). The reflection coefficient of magnetite at a wavelength of 1.06 microns is 0.14 [35].
α = == 1.8 · 10−6 m2 / s, (1)
where k is thermal conductivity, ρ is density, c is heat capacity, (the values of thermophysical quantities are taken from [36]).
The required heating duration can be estimated as:
t = == 125 µs, (2)
where h is the thickness of the scale, a is the thermal diffusivity of the scale.
According to formula (3), the power density of laser radiation is estimated at which the surface temperature of the scale in the center of the spot reaches a predetermined temperature, in this case the melting point of magnetite Tmelt = 1597 °С [30]. The following estimate uses the formula for pulsed heating during time t = h2 / a, which is realized when scanning a continuous beam at a velocity of Vsc. In this case, an approximation of a uniform distribution of power density over a spot with a characteristic size d is taken, so that: t = d / Vsc, from where the required scanning speed can be found [37]. The initial temperature of the sample is assumed to be 20 °С.
q ==≈
≈ 57,4 кВт / с2, (3)
where k is the thermal conductivity of magnetite, R is the reflection coefficient of magnetite at a wavelength of 1.06 microns.
The modes of laser action leading to the destruction and removal of mill scale in the solid phase state have been experimentally found. The variable parameters of the modes were: power density, duration of exposure and number of impacts.
If the scale removal effect is achieved in the studied mode, then the number of impacts (in practice, the scanning speed) and the specific energy dose E of all impacts (J / cm2) (threshold energy dose of cleaning) necessary for scale removal were determined for it. The threshold energy density of cleaning is a criterion for the effectiveness of the studied mode. The lower it is, the more energy efficient this mode is and the higher the cleaning performance can be achieved.
The criterion for the destruction and removal of scale was considered to be the destruction and detachment of the upper component. At the same time, the appearance of the surface changed from glossy to matte. Figure 4 shows a micrograph of the sample surface (a), a micro profile of the surface (b) and a micrograph of separated scale particles (c). Laser processing parameters: power density 100 kW / cm2, exposure duration 150 µs, total energy dose of treatment 90 J / cm2. In this sample, about half of the surface is free of the original scale. Scale removal occurs in the form of fragments whose longitudinal dimensions significantly exceed the transverse one.
Qualitatively, the process of scale removal was described as a result of the following experiment. For a duration of exposure of 150 µs, processing was performed with power densities in the range from 35 to 350 kW / cm2. At a power density of 35 kW / cm2 (a single dose of 5.3 J / cm2), no scale removal effect was obtained. At a power density of 350 kW / cm2 (a single dose of 52.8 J / cm2), the scale is melted, and the efficiency of thermomechanical destruction decreases. For treatment modes leading to scale removal, the dependence of the threshold energy dose of cleaning on the energy dose of a single exposure was constructed (Fig. 5). The results show that the dose of a single exposure of 13.2 J / cm2 is near the optimal value.
On circular (Fig. 6) and linear (Fig. 7) scanning systems, dependences of the threshold energy dose of cleaning on the duration of exposure for certain levels of power density were obtained. If the required duration of exposure is exceeded at a given power density, scale melts and its removal efficiency decreases. If the duration of exposure is insufficient, the scale may crack, but it is not removed.
It can be seen from the obtained graphs that with an increase in power density, the optimal duration of exposure decreases. The optimal power density levels are in the range of 70–250 kW/cm2, while the duration of exposure must be selected from the range of 280–80 µs. At power densities above 300 kW / cm2, scale melting is observed; the higher the power density is, the less effective the thermomechanical destruction is and the greater the heating of the steel surface.
With a duration of exposure of about 100 µs, the scale is heated to its entire thickness. During cyclic heating and cooling, phase transformations occur in the scale layer and residual tensile stresses are formed. As a result, the scale cracks into individual flakes, which lose their adhesion to the surface during laser treatment and are removed. Scale destruction occurs in the scale layer, and not along the steel-scale boundary, so a thin loosened layer of scale may remain on the surface of the steel. The residual layer can be effectively removed in the evaporative mode by a pulsed nanosecond fiber laser. The energy dose of the final cleaning by pulses with a duration of 200 ns and a pulse energy of 0.5–1 mJ is about 20 J/cm2.
Conclusion
The paper experimentally demonstrates the possibility of removing mill scale with a thickness of about 15 microns, consisting mainly of magnetite, in a solid phase state due to thermomechanical destruction by radiation from a continuous-wave fiber laser.
The destruction of scale during continuous laser treatment at power densities and exposure durations below the evaporation thresholds is associated with tensile stresses and thermochemical transformations in the scale structure. The conversion of a mixture of magnetite and metallic iron into wustite reduces the adhesion of the modified scale and probably contributes to the formation of cracks along the scale plane. Intensive cooling of the heated scale after the end of laser exposure leads to the appearance of tensile stresses that form cracks across the scale plane and, with sufficient softening of the scale, lead, as a result, to the detachment of scale particles. A softened loose thin layer of scale remains on the metal surface, which can be easily removed using a laser in evaporative mode.
When examining samples of hot-mill steel, a transition layer was found at the steel-scale boundary, which was formed by the introduction of scale particles into the metal matrix during mill or due to internal oxidation of steel. This layer contains conditions for the occurrence of corrosion processes when interacting with air. In some cases, the removal of the scale layer may not be sufficient to ensure the required level of corrosion resistance of metal surfaces. Therefore, in the future it is necessary to investigate the problem of removing this transition layer.
AUTHORS
Zhurba Danila Vladimirovich, postgraduate student, Federal State Autonomous Educational Institution of Higher Education “ITMO National Research University”, St. Petersburg; Junior Researcher, LLC “Scientific and Manufacturing Enterprise of Fiber-optic and Laser Equipment”, St. Petersburg, Russia.
ORCID: 0009-0001-6814-1737
The author’s contribution: planning and conducting experimental studies, literature review, analysis and description of the results, preparation of the article.
Zhurba Vladimir Mikhailovich, General Director, Scientific and Manufacturing Enterprise of Fiber-Optic and Laser Equipment, St. Petersburg, Russia.
The author’s contribution: the idea of the study, the general management of the project.
Veiko Vadim Pavlovich, Doctor of Technical Sciences, Professor, Chief Researcher, Federal State Autonomous Educational Institution of Higher Education “ITMO National Research University”, St. Petersburg, Russia.
The author’s contribution: scientific guidance, assistance in presenting the results, editing the article.
Puisha Alexander Eduardovich, Candidate of Technical Sciences, Head of the Scientific and Technical Department, Scientific and Production Enterprise of Fiber-Optic and Laser Equipment, St. Petersburg, Russia.
The author’s contribution: scientific advice, assistance in editing the article.
Information about
a conflict of interest
The authors state that there is no real or potential conflict of interest between them.
D. V. Zhurba 1, 2, V. M. Zhurba 2, V. P. Veiko 1, A. E. Puisha 2
ITMO University
NPP VOLO LLC)
The article investigates the process of laser cleaning of mill metal in exposure modes that do not lead to heating of the scale above its melting point. The possibility of destruction of mill scale due to thermochemical reactions in the scale and subsequent thermomechanical destruction has been identified and substantiated. The search and optimization of laser exposure modes has been carried out to increase the cleaning efficiency. For a more complete description of the laser cleaning process, attention is paid to the structure of mill scale; the features of its formation and probable phase transformations under the action of laser heating are described. An area of laser treatment modes in the range of exposure durations from 30 to 400 µs and power densities of from 50 to 750 kW/cm2 leading to thermomechanical scale destruction has been experimentally discovered. The process of laser descaling due to thermomechanical destruction is implemented using a continuous-wave fiber ytterbium laser with a maximum power of 2 kW.
Key words: Laser cleaning, continuous-wave ytterbium fiber laser, microsecond exposure time, mill scale, thermomechanical scale destruction
Article received: 23.05.2024
Article accepted: 01.07.2024
Introduction
The removal of oxide layers from the surface of metals is a pressing challenge for the metalworking industry. The most demanded and, at the same time, the most difficult task here is the removal of oxide layers from the surface of hot-mill carbon steels (mill scale). Mill scale does not protect steel from corrosion, and in some cases leads to an acceleration of corrosion processes, which reduces the service life of mill metal products [1–4], therefore, the surface of steel in all cases needs to be cleaned from it.
By now, various methods of mechanical or chemical descaling of metal surfaces are most common. Mechanical cleaning methods include shot blasting and sandblasting, grinding with abrasive brushes and others. Chemical methods include pickling in hydrochloric acid, sulfuric acid and nitric acid baths or in alkalis, including using electrolysis [5].
The significant disadvantages of these methods are: insufficient degree of cleaning, damage to the metal surface, as well as a negative impact on personnel and the environment associated with the use of hazardous consumables, reagents, the formation of by-products (sand dust and scale dust, acid vapors, waste pickle liquor, contaminated sludge, etc.), which must be disposed of. Due to the need to improve working conditions and environmental safety of production facilities, serious restrictions are imposed on these methods.
Laser sources are increasingly being used to remove various contaminants, in particular oxide layers from the surface of metals. Laser cleaning has already established itself as an environmentally safe and easy-to-use method for scale removals, characterized by its being contactless, no need for consumables and tools, low operating costs, the fundamental ability to remove any types of impurities and a number of other advantages associated with the use of laser radiation with high power density [6–12].
In the last decade, laser cleaning technology based on evaporation of impurity material by nanosecond pulses with high power density (>107 W / cm2) has become the most widespread. The use of ytterbium pulsed 100–200 ns fiber laser sources is justified by the versatility of their use: an acutely focused laser pulse makes it possible to remove almost any impurities from the surface of most materials [6–9, 13–15]. The short pulse duration also minimizes the impact on the base material; in metals, the zone of thermal exposure and reflow is limited to a few microns. However, evaporation of oxide layers and, especially, scale is a very energy-intensive process. Therefore, pulsed laser removal of mill scale from the surface of hot-mill steel is characterized by low productivity [13, 14].
The use of a continuous laser makes it possible to vary processing parameters such as exposure time, power density and energy in wide ranges. Therefore, it seems promising to investigate and implement various descaling mechanisms, including those not related to evaporation, using continuous lasers. Cleaning of mill metal by continuous laser radiation due to evaporation is difficult to implement in practice. Heating a thin layer of scale to the boiling point also leads to significant heating of the steel. As a result, the surface of the steel is melted and a new layer of scale is formed on it [16]. Such strong heating will lead not only to damage to the surface, but also to significant thermal deformations of the entire article. Therefore, for productive laser descaling of mill metal (and structures made of it), it is necessary to look for more effective mechanisms of destruction and removal of scale that do not require high-temperature heating. In this direction, the most promising is the study of the mechanisms of scale destruction due to thermal stresses. The advantages of a laser heat source are obvious: the high power density allows to heat the entire thickness of the scale layer within several tens of microseconds, and in the absence of radiation, to cool it almost as fast due to thermal conductivity and heat dissipation into the metal base.
This paper proves the possibility of removing mill scale due to thermomechanical destruction when heated by radiation of a continuous-wave fiber ytterbium laser and carries out the search and optimization of exposure modes that cause thermomechanical destruction of mill scale on a thin-sheet St3sp mill carbon steel. The heating temperature of the scale in this process does not exceed its melting temperature, and the destruction and removal of scale fragments occurs in the solid phase state due to tensile thermal stresses and a decrease in adhesion during the formation of wustite. Heating to the temperature of scale destruction occurs only in its own layer. The surface of the steel does not undergo significant thermal effects. Laser scale destruction due to thermal stresses is a less energy-consuming process than laser evaporation.
Structure and process
of mill scale formation
Scale on the surface of steels is an oxide layer (black, bluish or dark red) formed as a result of heat treatment of steels in an oxidizing atmosphere, for example during mill. Normally, at a steel heating temperature above 570 °С, the scale consists of wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3) [17, 18]. Mill (or air) scale is formed on the surface of steels at the final stage of mill, when cooled after the last mill mill stand. The final form and composition of the mill scale depends on the temperature at the end of mill and the temperature of winding into rolls, the cooling rate before winding and the roll cooling rate, the composition of the atmosphere, the presence of water vapor and many other factors [1, 18–21].
Oxides are formed on iron, the specific volume of which oxides is greater than the volume of the metal, so the scale is porous and brittle. Scale can consist of various oxide layers, which differ in thermophysical properties. During high-temperature heating in furnaces, a three-layer scale is formed on steel, the lower layer of which is wustite [22]. At high temperatures, the wustite becomes plastic. At temperatures above 1 250 °С, the shear resistance in the wustite layer becomes close to zero. Therefore, at high temperatures, a reduced amount of scale adhesion to the metal surface is characteristic [19]. Differences in the coefficients of thermal expansion of the scale and metal layers, as well as a decrease in adhesion during the formation of a wustite layer, are prerequisites for the destruction of scale during heating and rapid cooling.
This effect is successfully used in mill production in the hydraulic removal of furnace scale, which has a classic three-layer structure. However, the hydraulic descaling works well with scale thicknesses of more than 1 mm and with high-temperature heating of the entire workpiece. In this case, the jet of water effectively cools the scale layer, while the base metal remains hot. Due to the shear stresses arising in the scale layer, as well as the additional kinetic energy of the water jet, the scale crust is destroyed and detached [23].
Unlike furnace scale, which is formed at a constant high temperature and consists mainly of wustite and a thin upper layer of magnetite, mill scale is formed on a finished roll at a constantly decreasing temperature. The temperature at the end of the mill process (at the exit from the finishing stand of the mill mill) is usually below 900 °C. The sheets are quickly cooled to the winding temperature and winded into rolls, which are left to cool in the air. Air access to the surface of the mill sheet is limited, and the oxidation of steel slows down. Due to the shorter time of high-temperature oxidation, the thickness of the formed mill scale does not exceed several tens of microns. Prolonged cooling of mill products with mill scale at temperatures above 570 °C leads to the formation of a scale consisting mainly of magnetite, at temperatures below 570 °C – in the scale, the initial wustite decomposes by eutectoid reaction to magnetite and metallic iron. If the winding of the sheets takes place at a lower temperature, and the cooling of the rolls occurs quickly enough, then a large amount of wustite is stored in the mill scale and such scale is easier to remove. However, as a rule, the cooling of the rolls is slow and the main component in the scale is magnetite or the product of the eutectoid decomposition reaction of wustite [20, 21, 24–28]. The scheme of formation of various types of mill scale is shown in Fig. 1. A thin film of mill scale turns out to be more homogeneous in its properties and more plastic than classical furnace scale. Such scale has no pronounced layering, has high adhesion to steel and resists well mechanical action [23].
In order to implement the mechanisms of destruction of mill scale due to internal stresses, it is necessary to increase the intensity of heating and cooling. It is known that when surface heating of steel with mill scale by high frequency currents and additional mechanical action (blows of piezoceramic micro-vibrators), its destruction occurs and a cleaning effect is achieved. An increase in the plasticity of the wustite during heating contributes to the destruction of scale with additional mechanical action [29]. However, the intensity of heating and cooling when using induction heating is not sufficient for effective cleaning. Also, this method is not suitable for cleaning thin-sheet mill products due to significant overheating of the steel sheet and its warping.
Taking into account the structure of the scale and the process of its formation, it was concluded that it is necessary to study the mechanisms of destruction and removal of scale due to intensive local laser heating of it to the full depth and subsequent cooling due to thermal conductivity. As a result of heating the scale above 570 °C, wustite is formed from a mechanical mixture of magnetite and metallic iron. During cooling at temperatures below 570 °C, wustite decomposes again into magnetite and metallic iron. High temperature gradients during cyclic heating and cooling and various linear expansion coefficients of the scale and steel phases form stresses in the scale layer [29, 30]. As a result, the combination of physico-chemical and thermomechanical mechanisms during laser heating makes it possible to destroy and remove the initial layer of scale. Since the heating temperature of the steel surface is significantly lower than its melting point, it does not damage and oxidize [31].
Description of samples
The study of the mill scale cleaning process was carried out on samples of hot-mill products with a thickness of 5 mm of the St3sp steel grade containing from 0.14% to 0.22% carbon. Steel of this grade is widely used in industry. The surface of the sheets in the delivery state is covered with a uniform thin scale of dark blue color without defects visible to the unaided eye. Before laser cleaning, the surface of the samples was not subjected to preliminary preparation. The thickness of the scale was measured on the transverse section of the sheet using an optical microscope. As seen on the micrograph (Fig. 2), the thickness of the scale is about 15–20 microns.
The micrograph also shows oxidized areas of steel under the main dark layer of scale. The formation of these areas is probably associated with defects of mill [32] or with the internal oxidation of steel and the formation of an intra-oxidized transition layer at the steel-scale boundary [33].
Continuous-wave laser scale treatment schemes
The study used a continuous-wave fiber ytterbium laser YLR‑2000-MM-WC with a wavelength of λ = 1.06 µm with a power of 2 kW. Two different optical scanning systems were used: circular and linear (Figure 3). The circle or line obtained with the help of appropriate optical scanning systems was moved rectilinearly to form a continuous processing area. The considered method of scanning the metal surface with a linearly moving circle to perform cleaning was proposed in the patent [34].
The main variable parameters of the exposure were the power density, duration of exposure and the overlap coefficient of the scan figures. The power density and duration of exposure depend on the parameters of the scanning systems: the diameter of the fiber, the optical magnification, and the scanning speed of the laser beam. The overlap coefficient also depends on the speed of linear movement of the scanning figure.
For a circular scanning system, the laser processing area is formed by superimposing rings formed during circular scanning, the center of which shifts with the linear movement of the scanning head. The resulting scan strip consists of rings superimposed on each other with some overlap. The overlap coefficient between adjacent rings depends on the size of the spot, the speed of rotation of the spot and the linear speed of movement of the scanning head. With this scanning method, the overlap coefficient and the total radiation dose increase in the transverse direction, from the center to the edge of the scanning strip. Therefore, the threshold specific energy doses of radiation E necessary to obtain the desired effect of exposure (below – radiation doses) are determined by the values of E and the result of treatment in the central area of the exposure strip. The study of cleaning modes was performed at a 50% ring overlap coefficient in the center of the scan strip.
The linear scanning system forms the processing area in the form of a continuous zigzag strip. The overlap coefficient of the links in the center is maximum and decreases by half to the tops of the links. The threshold values of the radiation dose E (J / cm2) required for the destruction and removal of scale were determined for the central treatment area. The study of cleaning modes was performed with an overlap coefficient in the center of the strip of 50%.
Investigation of thermomechanical scale destruction under the action of laser radiation
The following is an assessment the parameters for heating the scale to a depth of 15 microns by radiation from a continuous-wave ytterbium fiber laser. The thickness of the magnetite layer in which laser radiation with a wavelength of 1.06 microns is absorbed is only 230 nm (absorption index 4.4 ∙ 104 cm‑1) [35]. The thermal diffusivity of magnetite is 1.8 ∙ 10–6 m2/s (determined by formula 1). The reflection coefficient of magnetite at a wavelength of 1.06 microns is 0.14 [35].
α = == 1.8 · 10−6 m2 / s, (1)
where k is thermal conductivity, ρ is density, c is heat capacity, (the values of thermophysical quantities are taken from [36]).
The required heating duration can be estimated as:
t = == 125 µs, (2)
where h is the thickness of the scale, a is the thermal diffusivity of the scale.
According to formula (3), the power density of laser radiation is estimated at which the surface temperature of the scale in the center of the spot reaches a predetermined temperature, in this case the melting point of magnetite Tmelt = 1597 °С [30]. The following estimate uses the formula for pulsed heating during time t = h2 / a, which is realized when scanning a continuous beam at a velocity of Vsc. In this case, an approximation of a uniform distribution of power density over a spot with a characteristic size d is taken, so that: t = d / Vsc, from where the required scanning speed can be found [37]. The initial temperature of the sample is assumed to be 20 °С.
q ==≈
≈ 57,4 кВт / с2, (3)
where k is the thermal conductivity of magnetite, R is the reflection coefficient of magnetite at a wavelength of 1.06 microns.
The modes of laser action leading to the destruction and removal of mill scale in the solid phase state have been experimentally found. The variable parameters of the modes were: power density, duration of exposure and number of impacts.
If the scale removal effect is achieved in the studied mode, then the number of impacts (in practice, the scanning speed) and the specific energy dose E of all impacts (J / cm2) (threshold energy dose of cleaning) necessary for scale removal were determined for it. The threshold energy density of cleaning is a criterion for the effectiveness of the studied mode. The lower it is, the more energy efficient this mode is and the higher the cleaning performance can be achieved.
The criterion for the destruction and removal of scale was considered to be the destruction and detachment of the upper component. At the same time, the appearance of the surface changed from glossy to matte. Figure 4 shows a micrograph of the sample surface (a), a micro profile of the surface (b) and a micrograph of separated scale particles (c). Laser processing parameters: power density 100 kW / cm2, exposure duration 150 µs, total energy dose of treatment 90 J / cm2. In this sample, about half of the surface is free of the original scale. Scale removal occurs in the form of fragments whose longitudinal dimensions significantly exceed the transverse one.
Qualitatively, the process of scale removal was described as a result of the following experiment. For a duration of exposure of 150 µs, processing was performed with power densities in the range from 35 to 350 kW / cm2. At a power density of 35 kW / cm2 (a single dose of 5.3 J / cm2), no scale removal effect was obtained. At a power density of 350 kW / cm2 (a single dose of 52.8 J / cm2), the scale is melted, and the efficiency of thermomechanical destruction decreases. For treatment modes leading to scale removal, the dependence of the threshold energy dose of cleaning on the energy dose of a single exposure was constructed (Fig. 5). The results show that the dose of a single exposure of 13.2 J / cm2 is near the optimal value.
On circular (Fig. 6) and linear (Fig. 7) scanning systems, dependences of the threshold energy dose of cleaning on the duration of exposure for certain levels of power density were obtained. If the required duration of exposure is exceeded at a given power density, scale melts and its removal efficiency decreases. If the duration of exposure is insufficient, the scale may crack, but it is not removed.
It can be seen from the obtained graphs that with an increase in power density, the optimal duration of exposure decreases. The optimal power density levels are in the range of 70–250 kW/cm2, while the duration of exposure must be selected from the range of 280–80 µs. At power densities above 300 kW / cm2, scale melting is observed; the higher the power density is, the less effective the thermomechanical destruction is and the greater the heating of the steel surface.
With a duration of exposure of about 100 µs, the scale is heated to its entire thickness. During cyclic heating and cooling, phase transformations occur in the scale layer and residual tensile stresses are formed. As a result, the scale cracks into individual flakes, which lose their adhesion to the surface during laser treatment and are removed. Scale destruction occurs in the scale layer, and not along the steel-scale boundary, so a thin loosened layer of scale may remain on the surface of the steel. The residual layer can be effectively removed in the evaporative mode by a pulsed nanosecond fiber laser. The energy dose of the final cleaning by pulses with a duration of 200 ns and a pulse energy of 0.5–1 mJ is about 20 J/cm2.
Conclusion
The paper experimentally demonstrates the possibility of removing mill scale with a thickness of about 15 microns, consisting mainly of magnetite, in a solid phase state due to thermomechanical destruction by radiation from a continuous-wave fiber laser.
The destruction of scale during continuous laser treatment at power densities and exposure durations below the evaporation thresholds is associated with tensile stresses and thermochemical transformations in the scale structure. The conversion of a mixture of magnetite and metallic iron into wustite reduces the adhesion of the modified scale and probably contributes to the formation of cracks along the scale plane. Intensive cooling of the heated scale after the end of laser exposure leads to the appearance of tensile stresses that form cracks across the scale plane and, with sufficient softening of the scale, lead, as a result, to the detachment of scale particles. A softened loose thin layer of scale remains on the metal surface, which can be easily removed using a laser in evaporative mode.
When examining samples of hot-mill steel, a transition layer was found at the steel-scale boundary, which was formed by the introduction of scale particles into the metal matrix during mill or due to internal oxidation of steel. This layer contains conditions for the occurrence of corrosion processes when interacting with air. In some cases, the removal of the scale layer may not be sufficient to ensure the required level of corrosion resistance of metal surfaces. Therefore, in the future it is necessary to investigate the problem of removing this transition layer.
AUTHORS
Zhurba Danila Vladimirovich, postgraduate student, Federal State Autonomous Educational Institution of Higher Education “ITMO National Research University”, St. Petersburg; Junior Researcher, LLC “Scientific and Manufacturing Enterprise of Fiber-optic and Laser Equipment”, St. Petersburg, Russia.
ORCID: 0009-0001-6814-1737
The author’s contribution: planning and conducting experimental studies, literature review, analysis and description of the results, preparation of the article.
Zhurba Vladimir Mikhailovich, General Director, Scientific and Manufacturing Enterprise of Fiber-Optic and Laser Equipment, St. Petersburg, Russia.
The author’s contribution: the idea of the study, the general management of the project.
Veiko Vadim Pavlovich, Doctor of Technical Sciences, Professor, Chief Researcher, Federal State Autonomous Educational Institution of Higher Education “ITMO National Research University”, St. Petersburg, Russia.
The author’s contribution: scientific guidance, assistance in presenting the results, editing the article.
Puisha Alexander Eduardovich, Candidate of Technical Sciences, Head of the Scientific and Technical Department, Scientific and Production Enterprise of Fiber-Optic and Laser Equipment, St. Petersburg, Russia.
The author’s contribution: scientific advice, assistance in editing the article.
Information about
a conflict of interest
The authors state that there is no real or potential conflict of interest between them.
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