KR100781335B1 - Method for increasing the hardness and erosion resistance of vitreous enamel coatings applied to carbon steels - Google Patents

Abstract

A method for improving hardness and erosion resistance of a vitreous enamel coating layer applied to carbon steel is provided to stably and effectively remove air-bubbles generated from an enamel coating of carbon steel manufactured by a conventional method, and a carbon steel thermal element with improved hardness and erosion resistance treated by the method is provided. A method for improving hardness and erosion resistance of a vitreous enamel coating layer applied to carbon steel comprises the following steps of: forming a vitreous enamel coating layer on a surface of the carbon steel by a conventional method comprising the steps of preparing an enamel coating solution by mixing silica, borax, sodium oxide, potassium oxide, aluminum oxide, nickel oxide, copper oxide and manganese oxide to improve corrosion resistance and erosion resistance(or wear resistance) of the carbon steel, dipping the carbon steel into the enamel coating solution, drying the carbon steel coated with the enamel coating solution, and firing the enamel coating solution-dried carbon steel; and treating the vitreous enamel coating layer under an argon atmosphere having a high temperature of 500 to 727 deg.C and a high pressure of 1000 to 1500 atmospheric pressures.

Description

Method for increasing the hardness and erosion resistance of vitreous enamel coatings applied to carbon steels}

1 is a schematic diagram of a GGH (Gas-Gas Heater) that is a heat exchanger of a flue gas desulfurization facility,

2 is a severely damaged thermal element of GGH,

3 is a cross-sectional shape of the enamel coating layer of Example 1,

4 is a distribution map of the main component measured by the interface shape between the enamel coating layer and the base material of Example 1 and EDS,

5 is a cross-sectional shape of the enamel coating layer of Example 2,

6 is a distribution map of the main component measured by the interface shape between the enamel coating layer and the base material of Example 2 and EDS,

7 is a cross-sectional shape of the enamel coating layer of Example 3,

8 is a distribution map of the main component measured by the interface shape between the enamel coating layer-base metal and EDS of Example 3,

9 is a thermal element coating layer erosion tester used in Example 4,

10 is a shape of the coating layer after (a) the used SiC particles and (b) the test of the erosion test according to Example 4,

11 is a graph showing the erosion rate for the specimen of Example 1 and Example 2 after the erosion test according to Example 4.

The present invention is a method of improving the physical properties, ie erosion resistance of the glass enamel coating layer is applied to improve the corrosion resistance, erosion resistance (or wear resistance) of carbon steel (mainly SPP grade ultra-low carbon steel, SPP = special purpose for porcelain), After forming a glass enamel coating layer on a carbon steel base material by a conventional method, and processing under conditions of high temperature and high pressure to densify the glass coating layer to improve its hardness and erosion resistance.

By vitreous enamel coating is meant a thin layer of glass on the surface of the metal and is applied primarily to protect the underlying metal from corrosive environments. For this reason, glass enamel coatings have long been used in tableware. In this case, because the strength of the product may be low, the base metal used is cold rolled ultra low carbon steel having a very low carbon content (<0.005 wt.%). On the other hand, some strength is required to apply to structural materials such as water or fuel storage, and the base metal is hot rolled high carbon steel.

Flue gas desulfurization facilities in domestic thermal power plants use gas-gas heaters (GGH) to exchange heat between flue gas containing sulfur dioxide and desulfurized flue gas. The heat elements of the heat exchanger are made of plate-like ultra-low carbon steel, and a glass enamel coating layer is formed on both sides in order to protect against the poor corrosive environment of the flue gas desulfurization plant. Flue gas desulfurization equipment removes sulfur dioxide contained in flue gas in the form of gypsum using limestone. In this process, the gypsum produced in the absorber of the flue gas desulfurization facility is partially introduced into the GGH together with the flue gas as indicated by the arrow in FIG. 1. Damage of the enamel coating layer due to the inflow of gypsum has been reported (see FIG. 2).

Therefore, in order to cope with poor corrosive environment such as flue gas desulfurization equipment and erosion environment by gypsum, it is required to have a coating property having higher hardness and better wear resistance and erosion resistance than conventional enamel coating.

According to Korean Patent Publication No. 1999-0071044, the enamel coating used in the flue gas desulfurization facility is made of borax (B ₂ O ₃ ) in order to increase the fluidity at the time of high temperature firing as silica (SiO ₂ ) is the main component. , Sodium oxide (Na ₂ O), potassium oxide (CaO) and the like, aluminum oxide (Al ₂ O ₃ ) to increase the heat resistance, nickel oxide (NiO), copper oxide (CuO) to increase the interfacial adhesion with the base material And manganese oxide (MnO) are mixed. The enamel coating having such a composition is applied to the base material through a immersion, drying and firing process, and the coating layer after the final process contains a large amount of bubbles as shown in FIG. 3. These bubbles reduce the heat transfer characteristics of the GGH and the properties of the coating layer, which ultimately degrades the acid corrosion resistance, thermal shock resistance and abrasion resistance of the thermal element.

Table 1. Composition Analysis of Enamel Coating Layer (Unit: Weight%)

SiO ₂ LiO K ₂ O TiO ₂ Na ₂ O Al ₂ O ₃ B ₂ O ₃ NiO CaO MnO BaO CuO 50-55 2.5-5 8-10 5-10 6-10 3-5 7.5-10 1.5-5 4.5-5 1.5-5 3.8-4 1.5-5

Therefore, the present invention is to solve the conventional technical problem, the first object of the present invention is to provide a stable and effective removal of the bubbles generated in the enamel coating of carbon steel produced by a conventional method, the hardness of the glass enamel coating layer of carbon steel And to provide a method for improving the corrosion resistance characteristics.

The second object of the present invention is to provide a carbon steel heating element which is effectively removed from the air bubbles in the coating layer by improving the enamel coating layer of a conventionally manufactured carbon steel at high temperature and high pressure.

In order to achieve the above object, a method for stably and effectively removing bubbles generated in an enamel coating of carbon steel prepared by the conventional method of the present invention and improving the hardness and corrosion resistance of the coating layer is poor, such as GGH of flue gas desulfurization equipment. An enamel coated carbon steel thermal element operated in a corrosive and eroded environment is manufactured by a conventional method, and then the parts are treated at a high temperature and a high pressure using a high temperature isostatic compression device.

It is preferable that the temperature at the time of the high temperature high pressure process is 500 degreeC-727 degreeC, and the pressure is 1,000 atmospheres-1.500 atmospheres. If the temperature is too low, the material to be treated does not have sufficient ductility, and if the pressure is too low, it will not secure enough stress to bond the pores of the coating layer. In addition, if the temperature exceeds 727 ℃, volume change (volume reduction) occurs due to the phase transformation of carbon steel, and when it cools, it expands again at this temperature, so that there is a high possibility of cracking or detachment of the coating layer, and the effect is small when the pressure is too high. .

The base material is a cold rolled carbon steel, C <0.008%, Mn <0.5%, P <0.03%, S <0.03% and is a very low carbon steel consisting of a residual amount of iron.

Hereinafter, the present invention will be described in more detail.

The present invention is a method for removing bubbles generated during the fabrication process of glass enamel coated carbon steel thermal elements used in harsh corrosive environments such as flue gas desulfurization equipment and densifying the coating layer structure. After the enamel coating layer is manufactured by the firing process, the enamel coating layer is treated at a high temperature and high pressure using a high temperature isostatic compression device to remove bubbles generated during the firing process and to densify the glassy structure.

Formation of pores when the enamel coating layer is coated on the carbon steel base material by dipping, drying and firing an enamel coating solution mainly containing silica, borax, sodium oxide, potassium oxide, aluminum oxide, nickel oxide, copper oxide, and manganese oxide by the conventional manufacturing method. In order to suppress the increase of fluidity of the coating layer at high temperature firing. For this purpose, an oxide such as borax or sodium oxide is added as shown in Table 1. However, the addition amount is limited because the fluidity of the coating layer is increased by the addition of the oxide, but the physical properties of the coating, that is, the hardness decreases. Therefore, in order to secure a certain level of physical properties, the creation of pores is inevitable. Since the pores literally contain gas, the thermal conductivity is very low compared to the solid and, as mentioned above, deteriorates the physical properties of the coating layer itself.

In a purely corrosive environment, these pores are not a big problem, but if the erosion by gypsum, such as GGH in flue gas desulfurization facilities, is concerned, the presence of pores greatly affects the lifetime of the GGH thermal element. However, when the enamel coating layer prepared according to the present invention is treated at a high temperature and high pressure, fine pores are removed and a densified enamel coating layer can be obtained.

The present invention will be described in more detail with reference to the following examples. However, an Example is for illustrating this invention and this invention is not limited to these Examples.

Example  One

The heat element used in the present example is enamel coated carbon steel which is currently supplied for the GGH flue gas desulfurization plant in Korea. Since the components for the base material and the coating were not provided by the manufacturer, the coating layer components were roughly measured using energy dispersive x-ray spectroscopy (EDS) attached to the scanning electron microscopy (SEM) of JEOL, Japan. Is shown in Table 2. In the case of borax (B ₂ O ₃ ), EDS analysis is not possible, so that borax is dried at 110 ° C, takes a certain amount, dissolved in aqua regia (mixture of hydrochloric acid and nitric acid) to a certain amount, and an inductively coupled plasma emission spectrometer ( Perkin Elmer Optima 4300DV). Since only part of the sample was dissolved because the oxide of the enamel coating layer was dissolved, the quantitative analysis of borax was impossible and only the presence was confirmed by qualitative analysis. The base material was quantitatively analyzed by this method to obtain the results shown in Table 3, and the used base material was identified as SPP steel.

Table 2. Composition of enamel coating layer used in Example 1: weight ratio (%)

SiO ₂ LiO K ₂ O TiO ₂ Na ₂ O Al ₂ O ₃ B ₂ O ₃ NiO CaO MnO BaO CuO 69.4 - - 3.3 10.4 2.2 ? 3.3 1.5 - - -

Table 3. Composition of carbon steel base material used in Example 1: weight ratio (%)

C B Al Mn Ti Fe 0.01 0.08 0.08 0.16 0.11 Remainder

As a result of cutting the enamel coated carbon steel and observing the cross section with an optical microscope and SEM, the base material thickness was 0.8 mm, the thickness of the coating layer was about 200 μm, and as shown in FIG. Including a large amount of bubbles were observed in the coating layer. In addition, as a result of component map analysis of the interface area with the base material by EDS as shown in Figure 4 it can be seen that the iron protrudes from the surface of the base material to the coating layer.

Table 4

Item Contents producer ABB (Sweden) Container size 1124 (O.D) × 700 (I.D) × 3,285 (O.H) × 2,171 (I.H) Work size (maximum) 75 (outer diameter) * 890 (height) Working temperature 2,000 ℃ Pressure (max) 200 Mpa Working fluid Ar Heating element SiC Heater 21ea (3 Zone) Load type Top Loading and Closure

Example  2

After preparing the same specimen as in Example 1, it was treated in an argon atmosphere of high temperature and high pressure. The high temperature and high pressure treatment was performed at 700 ° C. and 120 MPa for 2 hours using equipment having the specifications as shown in Table 4 below. Minutes).

Specimens subjected to high temperature and high pressure were observed in a cross section using an optical microscope and SEM as in Example 1. 5 shows optical and SEM photographs. Compared with the cross-sectional photograph of the enamel coating layer before the high temperature and high pressure treatment of Example 1, it can be seen that the thickness was first reduced to about 15% to 170 μm and the internal pores were almost disappeared to densify the coating layer. Moreover, the acicular phase which was not observed before the process was observed at the interface with a base material. 6 shows an enlarged SEM photograph of the interface and distribution of main components using X-ray mapping. In the SEM photograph of FIG. 6, the grains appearing darker than the surroundings can be seen that silicon oxide is concentrated and sodium deficient. The results of the EDS analysis on the grains revealed that the grains were oxygen-deficient silicon dioxide (SiO _2-x , x = 0.38). In addition, needle-like particles near the interface appear to grow on the surface of the carbon steel base material, and the results of the EDS component analysis are shown in Table 5. This particle was not observed in the result of Example 1, and it can be seen that it was produced by the treatment of Example 2. Based on the elemental component ratios in Table 5, this phase was estimated to be a kind of complex oxide in which silicon oxide contains a considerable amount of iron oxide. Therefore, iron oxide is not independently present on the surface of the base material, and it is expected to have a good effect on the adhesion with the base material by forming a composite with silicon oxide.

Table 5. Composition Element Composition Ratio (%) of Acicular Particles Present at Coating Layer-Material Interface of Example 2

O Si Fe Na Al Ca Ti 65.3 18.5 7.1 6.6 1.0 0.45 1.0

Example  3

After preparing the same specimen as in Example 1, only the temperature was set to 727 ℃ and was treated as in Example 2. The treated specimens were observed in section using an optical microscope and SEM in accordance with Example 1. 7 shows optical and SEM photographs. As in the result of Example 2, compared to the enamel coating layer before the treatment, the thickness was reduced by about 15% to 170 μm, and the internal pores were almost disappeared to densify the coating layer. In addition, a needle-like phase was observed at the interface with the base material as in the result of Example 2. 8 shows an enlarged SEM photograph of the interface and distribution of main components using X-ray mapping. As in the result of Example 2, the grains darker than the surroundings in the SEM photograph of FIG. 8 can be seen that silicon oxide is concentrated and sodium deficient. In addition, the needle-like particles near the interface were clearly identified.

However, in this case, it can be seen that the crack occurs from the surface to the interface with the base material as shown in the SEM photograph. The temperature of 727 DEG C selected in Example 3 is a temperature at which the crystal structure of the carbon steel base material phase transforms from the body centered cubic (BCC) to the face centered cubic (FCC). . Therefore, when the specimen is treated at a temperature of 727 ° C. or higher, there is a concern that a crack may be generated as shown in FIG. 7 by inducing stress in the coating layer due to volume change due to phase transformation. Therefore, the treatment temperature is preferably set to 727 占 폚 or lower.

Example  4

Erosion tests were performed on the specimens according to Examples 1 and 2. Erosion test was used for the erosion tester as shown in FIG. In the erosion test, SiC powder was used as the erosion medium (see (a) of FIG. 10). The SiC powder was filled in the upper inlet (about 2000 cm ^{3 in} volume) to allow 910 mm free fall through a stainless steel pipe having an internal diameter of 17 mm. The specimen was allowed to collide at an angle of 45 degrees. After one erosion test, the specimens were taken and weighed using an electronic balance. This process was repeated five times or more to measure the weight loss. In each erosion test, new SiC powder was used to maintain the same erosion conditions.

Hardness measurement was measured and compared for the specimens according to Example 1 and Example 2. The hardness was measured 10 times under a load of 50 g and a time of 15 seconds using a Vickers hardness tester (model name: AMT-7FS) of Matsuzawa, and the average value was calculated.

Figure 10 (b) is a photograph showing the surface of the specimen after the erosion test according to Example 4, Figure 11 is a graph showing the weight loss after the erosion test. As can be seen from this graph, in the case of the specimen treated at high temperature and high pressure according to Example 2, it can be seen that the corrosion resistance property is increased by about 2 times compared to the specimen of Example 1 not treated. As a result of the hardness measurement, the specimen of Example 1 was increased by about 37% to 486 Hv and the specimen of Example 2 was 664 Hv.

As can be seen from the above results, the present invention removes air bubbles in the glass enamel coating layer, which is applied to improve corrosion resistance and erosion resistance (or wear resistance) of alloy steel (mainly carbon steel), thereby increasing heat exchange efficiency of the thermal element and simultaneously enamel coating layer. By densifying and effectively improving the physical properties of the coating layer, that is, hardness and erosion resistance can extend the life of the product more than two times.

In addition, the present invention can be expected to improve the adhesiveness compared to the conventional coating layer by changing the characteristics of the coating layer-base material interface can further extend the life of the product.

Claims (2)

In order to improve the corrosion resistance and corrosion resistance (or wear resistance) of carbon steel, a conventional enamel coating liquid mainly containing silica, borax, sodium oxide, potassium oxide, aluminum oxide, nickel oxide, copper oxide, and manganese oxide is immersed, dried and calcined. Method of improving the hardness and corrosion resistance of the glass enamel coating layer of carbon steel, characterized by coating the glass enamel coating layer on the surface of the carbon steel by phosphorus method, and then treating it under a high temperature of 500 ° C to 727 ° C phosphorus and a high pressure argon of 1,000 to 1,500 atm. . Carbon steel thermal element for the flue gas desulfurization facility treated by the method of improving the hardness and corrosion resistance of the glass enamel coating layer according to claim 1.

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