Influence of heat treatment on the microstructure of steel coils of a heating tube furnace

Introduction

Despite the search for new alternative energy sources and carbon-free technologies, raw hydrocarbons remain the key component of the mineral resources sector [25]. Extraction, transportation, refining, export and efficient use of hydrocarbon materials constitute one of the main pillars in the development of the country [24, 26]. A steadily increasing demand for high-quality oil products depends on the operating conditions of certain assemblies and steel parts of heating furnaces and oil refinery units. In addition to the development of new types of alloys for improving performance characteristics of coils and other steel parts, it is necessary to provide effective protection against corrosion and to develop modes for stable and trouble-free operation at high temperatures in chemically aggressive media. In oil refineries, it occurs on a very limited scale. The use of steel coils of various types depends primarily on the prices of high-alloy steel. This is explained by the fact that during oil heat treatment in tube furnaces, destruction of the steel coil surface is predominantly caused by the processes of electrochemical corrosion; therefore, modifying and alloying elements need to be added to the steel to reduce the overall level of corrosion [20]. There is a tendency and statistics of emergency shutdowns of oil refineries due to corrosion processes, which prematurely damage steel coils and consequently lead to environmental non-compliance, direct economic losses and industrial accidents [7, 17].

Preliminary heat treatment of steel coils before their use can be considered as a promising method of limiting premature surface damage, caused by corrosion. The process of heat treatment creates conditions for structure transformation, which is associated with reduction of the corrosion rate, since interval heating is accompanied by phase transitions that add to homogeneity of the microstructure. Thus, the operating life of steel coils can be extended due to the rate-limiting step of the corrosion mechanism [31].

Preliminary heat treatment can be classified as a standard technological solution, but it is also possible to use a number of innovative approaches in this area [6, 8, 11]. Another significant option is the search for alternative methods of heat conservation and its recycling during heating [5, 29]. In these circumstances, there is methodological relevance both in mathematical modeling and in experimental studies that determine the parameters of oil and gas transportation systems [14, 15, 22].

The aim of this research is to determine the best mode of heat treatment for a particular type of steel alloy, which is most commonly used in tube furnace coils, in order to apply preliminary heat treatment to the coils as a precautionary measure. The use of heat treatment can serve as a protective procedure against the wear of steel coils. The effect of obtained microstructures on the corrosion resistance was determined using heat-treated coil samples (15KH5M), immersed in a stagnant aggressive (corrosive) medium, composed of 50 % NaCl solution (3.5 %) acidified with HCl (pH = 5) and 50 % of all-season electrolyte at 25 °C. The study is dedicated to improving performance characteristics of steel coils of tube furnaces and increasing their corrosion resistance by means of preliminary heat treatment.

Methodology

Methodology. The program of experimental research involved studying the effect of heat treatment temperature (in the interval of 200-700 °C), duration of heat treatment and the number of stages for the mass loss of steel samples in a standard corrosion test. The main method for structural study of the samples was metallographic analysis, performed on a ZEISS Axio Lab.A1 optical microscope (Germany) at 20x and 100x magnification. Microstructures were obtained using a ZEISS Axiocam ERc 5s digital camera, mounted on a microscope, and computer image processing by means of specialized software. Corrosion rate was estimated by the mass loss of coil samples made of 15KH5M steel.

Results

It is well known that preliminary thermomechanical treatment of certain steel types leads to a 10-fold increase in wear resistance without changing mechanical properties of the workpiece [1, 12]. As a rule, various methods of workpiece heat treatment (tempering, quenching, annealing, homogenization) extend the operating life of tube thermal furnaces and their elements due to short-range structural changes, associated with phase transitions and distribution of the carbon phase, material hardening and reducing the effect of localized corrosion [27, 30].

The parts of tube furnaces are made of heat-resistant low-alloy 15KH5M steel (martensitic class) able to withstand 600-650 °С. Chemical composition of 15KH5M steel is presented in Table 1, with mass percent of the elements (in wt.%) according to the technical standards GOST 550-75, TU 14-3R-62-2002 and GOST R 53932-2010. Critical temperatures of 15KH5M steel are the following: Ас1 – 815, Ас3 – 848, Ar3 – 775, Ar1 – 818 °С.

Table 1

Chemical composition of 15KH5M steel, wt.%

Steel coils are not prone to embrittlement when quenched. Removal of the formed scale (organic and inorganic deposits) from the inner surface of the coil is carried out by means of mechanical treatment using blowing or sandblasting devices during short-term equipment shutdowns. However, steel parts of this type have to withstand high operating temperatures and maintain their performance characteristics. Due to high corrosion rate on the surface of the metal, it is impossible to avoid internal cracking of the coils at critical temperatures, pressure and motion of the aggressive medium, which leads to aggressive localized corrosion. Further overheating and burning through the walls of steel coils, caused by overall or localized corrosion, lead to emergency shutdown of the equipment, to the extent of its destruction by the explosion of oil products. Such cases are typical for oil refineries, and therefore on a scheduled periodic basis the coils are replaced [4, 33].

In order to perform heat treatment of steel coil samples, we assembled an integrated laboratory installation, the diagram of which is presented in Fig.1. The temperature in the furnace was measured and maintained with the help of a Cr/Al thermocouple connected to the controller. To protect the surface of steel templates obtained from the coil from oxidation, their heat treatment was performed in the atmosphere of technical nitrogen.

The studies performed earlier [10, 16] estimate that thermal and/or mechanical treatment increase corrosion resistance of steel workpieces. At the same time, it is noted that reduced corrosion rate is associated with a more favorable microstructure of the studied steel samples [9]. The studies also indicate elevated rates of corrosion on the inner surface of steel parts of the tube furnace, which is primarily attributed to the presence of metals in the hydrocarbon medium (oil) [19, 32]. Taking into account high viscosity of metal-bearing oil, its transportation and refining are naturally associated with the development of electrochemical corrosion of the coil surface [18, 28, 34], the rate of which significantly increases with the heating in the tube furnace and in the presence of sediments of various origin on the heat-transmitting surface [2, 23]. Notably, elevated corrosion rates result from localized temperature increase due to low thermal conductivity of the sediments, which ultimately leads to premature failure of steel coils and shutdown of the entire technological chain.

The influence of the temperature of steel coil heat treatment was studied in the interval of 200-700 °C with a step of 100 °C (Table 2). Before examination with an electron microscope, all the samples of the tube furnace steel coils were prepared as templates and then processed and etched with a 5 % solution of nitric acid in ethanol (nital), according to the conventional method [13]. To estimate sample corrosion rate according to the accepted conventional method, we used an electrolyte composed of 50 % NaCl solution (3.5 %) mixed with HCl (pH = 5) and 50 % sulfuric acid based solution with thermostatic control of the samples for 5 days at 25 °C. The results obtained are presented in Table 2 and Fig.2.

Table 2

Influence of heat treatment temperature on mass loss after 1 h of hold time and air cooling

In Table 2, the sample without heat treatment is designated as I₀ in the graph it is shown as a brown dot (Fig.2). Samples subjected to heat treatment under 200, 300, 400, 500, 600 and 700 °C are designated as I₁, I₂, I₃, I₄, I₅ and I₆, respectively; М₀ is the mass loss of the sample at the end of treatment at a given temperature of heating; ∆$\overline{M}$ – величина потери массы образца в конце обработки при заданной температуре нагрева; ∆$\overline{M}$ – средняя величина потери массы образца; (∆$\overline{M}$/М0) ∙100 – относительная потеря массы образца при коррозионном испытании.

In Table 2, the sample without heat treatment is designated as I₀ in the graph it is shown as a brown dot (Fig.2). Samples subjected to heat treatment under 200, 300, 400, 500, 600 and 700 °C are designated as I₁, I₂, I₃, I₄, I₅ and I₆, respectively; М₀ is the mass loss of the sample at the end of treatment at a given temperature of heating; ∆$\overline{M}$ is the average mass loss of the sample; (∆$\overline{M}$/М₀) ∙100 is the relative mass loss of the sample in a corrosion test

According to Fig.2, it can be seen that in case of thermal heating the mass loss decreases by factor of 0.65/0.07 = 9.3 after 1 h of hold time at 500 °C compared to the samples without heat treatment. It was assumed that these conditions corresponded to the rate-limiting step of the process, when the corrosion rate tends to zero, there are no phase transitions and pearlite does not form under temperatures below 500 °C, which is significantly lower than the critical point (Ac1 = 815 °C) for the coil sample made of 15KH5M steel.

The effect of the number of reheats at 500 °C and subsequent quenching was studied on four samples. The first sample was used in the control test, where the steel template after heat treatment for a specified time was not subjected to quenching but was cooled in the air to room temperature. The remaining samples were subjected to one, two or three stages of quenching with subsequent tempering at 500 °C for one hour and cooling in the air to room temperature. For uniform cooling of the samples, their quenching was carried out in a low-temperature salt-water mixture based on sodium chloride, which was continuously stirred. The results of the corrosion test are presented in Table 3 and Fig.3. According to the data, corrosion rate estimated by the mass loss of the sample was practically a linear function of the number of reheats, which can be explained by the intensive phase of recrystallization and associated increase in the number of intercrystalline dislocations, which act as corrosive centers and intensify corrosion damage by factor of around 2.2.

Results of the study on the effect of heat treatment duration on the corrosion damage of quenched steel coils are presented in Table 4 and Fig.4. Relative mass loss of the sample decreases from 0.13 to 0.06 %, as heat treatment duration increases from 0.5 to 3 h, which can be explained by stabilization and levelling of sample composition.

Taблица 3

Influence of the number of reheats to 500 °C on the mass loss

Notes. I₀ – sample without quenching; I₁ – sample after the first quenching; I₂ –sample after second quenching; I₃ – sample after third quenching.

Taблица 4

Influence of tempering time at 500 °C on the mass loss

Note. I₀, I₁, I₂, I₃ – samples subjected to heat treatment by tempering at 500 °C for 30 min, 1, 2 and 3 h, respectively, after cooling in the air at room temperature.

Analysis of microstructural characteristics of the samples was carried out depending on the temperature of heat treatment in the interval 200-700 °C and its duration (Fig.5). For this purpose, one can rely on the known patterns of heat treatment for martensitic steels, where excess carbon in the composition of the martensite phase gets liberated due to the release of cementite Fe₃C [3], which in our case holds out the prospect of forming low-carbon phases with the structure of sorbite and/or troostite [21]. Metallographic analysis demonstrated that with an increase in the treatment temperature, one can observe coarsening of the crystalline grains of ferrite, as well as distribution of carbide particles along the grain boundaries (Fig.5, c-e) compared to the initial structure (Fig.5, a). At 300 °C, the structure consists of martensite with troostite colonies and a small fraction of bainite grains (Fig.5, b). At 400 °C, cementite begins to precipitate from the martensite phase. This process results in carbon depletion of the martensite phase and starts a phase transition, which leads to generation of the ferrite phase. At 500 °C, the structure in Fig.5, d indicates the formation of sorbite and bainite grains between the troostite colonies and carbide deposits. According to performed experimental studies, this structure is characterized by increased corrosion resistance due to the formation of small spherical grains of cementite in the ferrite matrix (Fig.5, f ). As the treatment temperature rises, cementite grains grow faster due to lowering carbon concentration in the ferrite structure, which contributes to decreasing corrosion resistance of the coil material.

Conclusions

Performed set of studies allows to draw the following main conclusions:

1. The widespread use of low-alloy steel as a construction material for manufacturing coils of heating tube furnaces requires strict control over their preliminary heat treatment, which is particular-ly important for increasing corrosion resistance and extending operating life of the coils.. 
2. It was identified that preferred conditions of preliminary heat treatment of martensitic steels for in-creasing their corrosion resistance include the temperature of the process at the level of 500 °C and duration of the process around 2-3 h. Such conditions of steel sample treatment provide formation of a uniform steel structure with inclusion of cementite in the form of spherical grains in the ferrite base (matrix), which results in a more than 9-fold increase in corrosion resistance of the material compared to the samples without preliminary heat treatment.

The study was supported by the Government of the Republic of Syria as a part of a dissertation research aimed at improving performance characteristics of metal structures and equipment for oil and gas industries.

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