KR100263769B1 - High chrome cast steel with a good acid resisting and pitting corrosion resistance - Google Patents

Abstract

PURPOSE: A Cr-rich cast iron having superior pitting resistance and acid resistance is provided to prevent facility damages usually caused by corrosive materials from all kinds of chemical plants. CONSTITUTION: The Cr-rich cast iron comprises 25-35wt.% of Cr, 1-18wt.% of Ni, 1-6wt.% of Mo, 3wt.% or less of Si, 15wt.% or less of Mn, 3wt.% or less of V, 1-6wt.% of W, 0.5-4.0wt.% of C, 1wt.% or less of N, 3wt.% or less of Cu, one or more elements (3wt.% or less) selected from Ti, Zr, Nb, a balance of Fe and other inevitable impurities. Especially, the Cr/(C+N) ratio is in the range of 8 to 70.

Description

High chrome cast iron with excellent acid and formula resistance

The present invention relates to a method for producing high chromium cast iron having high hardness and excellent acid resistance and formula resistance, and more specifically, formulas, erosion corrosion, abrasion, etc., generated in pumps or pipes for transferring corrosive substances and various contaminants. The present invention relates to high chromium cast iron having excellent abrasion resistance, acid resistance and formula resistance that can withstand the damage of.

In general, since thermal power plants obtain thermal energy by burning fossil fuels, many by-products of the combustion of fossil fuels are generated.

The combustion gas generated by the combustion of the fossil fuel as described above contains a lot of components that cause air pollution, and the representative product is ash.

For this reason, as regulations on environmental pollution are tightened, all thermal power plants are building desulfurization facilities to remove harmful components, particularly sulfuric acid, from the emitted gases.

However, the flue gas desulfurization slurry produced by the operation of the desulfurization facility and the ash are very corrosive and contain a large amount of hard solid particles, and are contaminated with various materials in sewage treatment plants. Various damages are caused to the facilities for transporting them by solid materials such as water and sand.

That is, many damages such as formula, erosion, corrosion, and the like are generated in pumps or pipes used in various facilities as described above, and when the corrosion resistant steel such as STS 304 is used as a component, the hardness of the steel is low. Therefore, it is very likely that equipment failure will be caused by erosion or wear in a short time.

Until now, high chromium (> 20% Cr) cast iron is the main material used for wear-resistant equipment parts, which is comparable to stainless steel because chromium in the cast iron improves high temperature oxidation and corrosion resistance of cast iron. This is because it passes the heat resistance corrosion resistance.

In addition, since the high chromium cast iron has a high carbon content, it is limited in use as a material having a high carbide content and a mechanical strength, especially because of low ductility, but it is widely used as a wear-resistant material due to the presence of a high hardness carbide in the base. It is used.

However, the 27% Cr-based white cast iron for wear-resistant parts, which is typically used among the high chromium cast irons, is more than twice as good as AlSl 316 stainless steel in wear resistance, but is about three times larger in corrosion rate. Corrosion forms and erosion is mainly generated as the corrosion form, the corrosion resistance to various acids also had a weak problem.

In order to solve the above problems, the present invention maintains the hardness at a high value while greatly improving corrosion resistance and acid resistance to various acids, and thus has high corrosion resistance and corrosion resistance. It is an object of the present invention to provide a method for producing the same.

1 is a graph showing the relationship between the hardness and the Cr / (C + N) ratio of the alloy according to the present invention.

In order to achieve the above object, the present invention provides a weight percent of Cr: 25 to 35. Ni: 1-18, Mo: 1-6, Si: 3 or less, Mn: 15 or less, V: 3 or less, W: 1-6, C: 0.5-4.0, and the rest is made of inevitable impurities and iron, It provides a high chromium cast iron having excellent acid and formula resistance, characterized in that the ratio of Cr / (C + N) is between 8 ~ 70.

In addition, the present invention is characterized in that the acid resistance and characterized in that the addition of each of the high chromium cast iron or more than 3.0% by weight of Cu or less to 3.0% in order to always greatly increase the acid resistance to improve the resistance to the formula and Provides high chrome cast iron with good formula resistance.

In addition, the present invention is characterized in that one or two or more components of Ti, Zr, Nb is added to 3% by weight or less in order to increase the hardness by forming a precipitation mole by combining with carbon or nitrogen in the high chromium cast iron It provides a high chrome cast iron excellent in acid resistance and formula resistance.

Hereinafter, the reason for numerical limitation of this invention is demonstrated.

(1) Chromium (Cr): 250-35.0%

It is the most important and basic element for maintaining the hardness and corrosion resistance of cast iron. It should contain more than 25.0% of chromium for proper corrosion resistance. It is limited to less than 35.0% in consideration of price and ease of production.

Chromium added in the alloy contributes to the maintenance of hardness in the form of chromium carbide and nitride, and elemental chromium plays a very large role in maintaining corrosion resistance.

(2) Nickel (Ni): 1.0-18.0%

As a strong austenite stabilizing element, since corrosion resistance is a useful element in terms of corrosion, it must be at least 1%, and a large amount of nickel must be added to austenite the matrix structure, but it is limited to less than 18.0% in consideration of the price problem.

(3) Molybdenum (Mo): 1.0 ~ 6.0%

Together with chromium, it functions to stabilize the ferrite phase as an important element for maintaining the corrosion resistance of the alloy of the present invention. Mo ₂ C, Fe ₄ Mo ₂ C, Mo ₂ N and the like to form a precipitate to play a major role in maintaining the hardness, elemental molybdenum is important for improving the corrosion resistance.

Therefore, it should contain more than 1.0% minimum, and if added in too much, brittleness is increased, so it is limited to less than 6.0%.

(4) Silicon (Si): 3% or less

As an element to stabilize the ferrite structure, deoxidation effect during dissolution and refining, and an element that increases oxidation resistance or by adding 3% of supernatant decreases the toughness and ductility of the steel.

In terms of corrosion resistance, 1.5% or less is preferable, and a large amount may be added for acid resistance.

(5) Manganese (Mn): 15% or less

It is an element that stabilizes austenite structure and is an element that increases the high capacity of nitrogen. In order to improve the corrosion resistance by increasing the nitrogen content, an appropriate amount of manganese is an essential element. In addition, it forms a manganese sulfide by combining with sulfur in the steel to prevent the occurrence of hot brittleness and deoxidation effect during dissolution refining.

It also plays a large role in increasing hardness when added in large amounts with nitrogen. However, considering the stability of the tissue, it is limited to less than 15%.

(6) Vanadium (V): 3.0% or less

By forming a precipitate by combining with carbon or nitrogen, it contributes greatly to the increase in hardness, and has the effect of increasing the corrosion resistance. However, it is limited to less than 3.0% because a large amount may damage oxidation resistance and formula resistance.

(7) Tungsten (W): 1.0 ~ 6.0%

The effect on corrosion resistance as ferrite stabilizing element is about 50% smaller than that of molybdenum but has a similar tendency. It is important for maintaining the hardness of the alloy of the present invention by forming precipitates such as WC and W ₂ C, and elemental tungsten is important for maintaining corrosion resistance.

Therefore, it should contain more than 1.0% minimum, and if added too much, brittleness is increased, so it is limited to less than 6.0%.

(8) Carbon (C): 0.5-4.0%

It is the most important element for maintaining hardness. That is, the hardness of the alloy is increased by combining with other alloying elements to form carbides. However, if a large amount is added, the corrosion resistance is greatly impaired, so it is limited to 4.0% or less. If it is added too much, the corrosion resistance may be improved, but an appropriate hardness may not be obtained, so it should exceed 0.5%.

(9) The reason that the ratio of Cr / (C + N) is between 8 and 70 is that if the ratio is less than 8, the hardness is increased but the corrosion resistance is weakened. If the ratio is higher than 70, the corrosion resistance is improved but the hardness is large. This is because it is degraded.

(10) Nitrogen (N): 1.0% or less

It is a useful element that improves the resistance to formulas, and its effect is about 30 times more than chromium. As a strong austenite stabilizing element, it is one of the most important elements in terms of corrosion resistance. When present simultaneously with molybdenum, the synergistic effect greatly improves the corrosion resistance.

In addition, by forming a nitride by combining with elements such as chromium and molybdenum, it contributes to the increase in hardness, and when added together with a large amount of manganese, the increase in hardness can be obtained. The solubility of the alloy is limited to a maximum of less than 10%.

(11) Copper (Cu): 3.0% or less

It has the effect of greatly improving acid resistance, and as an austenite stabilizing element, strengthens the matrix structure to improve strength. It should be limited to less than 3.0% as a large amount will weaken formula resistance and impair tissue stability.

(12) Titanium (Ti), zirconium (Zr), niobium (Nb): 3% or less

It forms a precipitate by combining with carbon or nitrogen, which greatly contributes to the increase of hardness. If a large amount is added, the weldability may be impaired, so each addition or sum of three elements is less than 3%.

Hereinafter, an Example is given and this invention is demonstrated in detail.

First, the specimen for use in the embodiment of the present invention using the pure commercial grade electrolytic iron, Cr, Mo, W, Ni, W, Fe-V, Fe-Cr-N, Mn, Fe-Si, etc. Dissolved together.

After dissolving using a magnesia crucible in an air atmosphere using a high frequency induction furnace having a capacity of 80 KW, ingots were prepared by injecting molten metal into a mold.

Thereafter, the surface was processed by cutting and polishing, followed by annealing heat treatment and air cooling.

Table 1 shows the chemical compositions of the inventive alloys and the comparative alloys.

Example 1

The test vessel used a 1 liter reactor and a condenser was installed at the top to prevent evaporation of the solution. The specimens were tested at an angle of 45 ° using wires insulated with rubber tubing, and replaced with fresh solution at each test. The specimen containing the components shown in Table 1 was cut to a size of 2 × 1 × 1 inch, and the surface of the specimen was wet polished with SiC abrasive up to # 600 and then stored in a desiccator until the test.

Corrosion rate was obtained by immersing in 20% 6% FeCl ₃ = 10% FeCl ₃ 6H ₂ 0) solution for 24 hours in accordance with ASTM G48A standard specification. The weight loss was measured to obtain a temperature at which the cumulative value exceeded 5 mg, which was called Critical Pitting Temperature (CPT).

In addition, the desulfurization solution at 20 ° C. (Green Death Solution, l: 4.2v / o H ₂ SO ₄ + 1.8v / o HCl + 0.6w / o FeCl ₃ + 0.6w / o CuCl ₂ , II: 7.0v / o H ₂ The corrosion rate was obtained in mdd (mg / dm ² / day) by immersion in SO ₄ + 3.0v / o HCl + 1.0w / o FeCl ₃ + 1.0w / o CuCl ₂ for 24 hours.

As shown in Table 2, the formal resistance of the inventive alloy is much better than that of the comparative alloy, and the corrosion resistance of the inventive alloys A11, A100 and A101 is particularly excellent.

Example 2

Surface hardness was measured by using a Rockwell hardness tester (C-scale) for the specimen having a component as shown in Table 1 cut to a size of 1.5 × 1.5 × 1.5cm and subjected to a solution annealing. Hardness was measured on the surface of the specimen polished up to 600 times with SiC abrasive paper.

In general, the hardness of cast iron is an important property that serves as a criterion for judging the wear resistance of a material.

Here, in the case of high chromium cast iron, the hardness value depends on the ratio of Cr / C, and the smaller the value, the higher the hardness.

However, the alloy of the present invention, unlike the comparative alloy, contains a large amount of nitrogen in the material. Therefore, the characteristic of the alloy of the present invention is that the hardness is proportional to the ratio Cr / (C + N).

This is shown in Table 3 and Figure 1, and shows that the hardness increases as the ratio of Cr (C + N) in the alloy decreases.

In particular, the hardness of the inventive alloys A15, A16 and A101 with a low carbon content is greater than that of A11 and A100 with similar carbon content due to the high content of N and Mn.

Example 3

Ingot was prepared from the components shown in Table 1, cut into 2 × 2 × 1 inch, and subjected to annealing, followed by air cooling. The test vessel used a 1 liter glass reactor and a condenser was installed at the top to prevent evaporation of the solution.

The specimens were tested at an angle of 45 ° using wires insulated with rubber tubing, replaced with fresh solution every test, and the surface of the specimens was wet polished with SiC abrasive up to # 600 and then tested. Store in desiccator until

The test solution was immersed in boiling 10% oxalic acid, boiling 100% formic acid, boiling 100% acetic acid for 24 hours, and the weight change was measured. The corrosion rate was measured in mdd (mg / dm ² / day). Obtained.

Table 4 shows that the acid resistance test results of the invention alloy and the comparative alloy is much better than that of the comparative alloy.

In particular, alloys of A11, A14, A15, A100, and A101 showed excellent corrosion resistance in boiling 10% oxalic acid and boiling 100% acetic acid, while corrosion resistance in boiling 100% formic acid was A11, A16. , A17, A100, A101 alloys were excellent.

In addition, the acid resistance of the alloy of the present invention was excellent in acid resistance than the rolled plate material of representative stainless steel STS 304, STS 3041, STS 3161.

According to the present invention as described above has been used as a part of the wear-resistant equipment because it has better corrosion resistance than stainless steel, but added elements to improve the acid resistance and corrosion resistance, etc. to high chromium cast iron exhibiting properties that are vulnerable to the formula, erosion corrosion, etc. This can prevent damage to the equipment due to erosion or wear occurring in a short time.

Claims (5)

It is composed of Cr: 25-35, Ni: 1-18, Mo: 1-6, Si: 3 or less, Mn: 15 or less, V: 3 or less, W: 1-6, C: 0.5-4.0 High chromium cast iron having excellent acid resistance and formula resistance, wherein the rest is made of inevitable impurities and iron. The high chromium cast iron having excellent acid resistance and formula resistance according to claim 1, wherein an amount of 1 wt% or less of N is further added to the high chromium cast iron, and a ratio of Cr / (C + N) is 8 to 70. . The high chromium cast iron of claim 1, wherein 3 wt% or less of Cu is further added to the high chromium cast iron. The method of claim 1, wherein up to 1% by weight of N and 3% by weight of Cu are further added to the high chromium cast iron, and the ratio of Cr / (C + N) is between 8 and 70. High chrome cast iron with excellent acid and formula resistance. The acid-resistant and formula resistant material according to any one of claims 1 to 4, wherein one or more components of Ti, Zr, and Nb are further added to the high chromium cast iron so as to be 3% by weight or less. High chrome cast iron.