WO2008150127A2

WO2008150127A2 - Method of metal surface treatment and solution of metal surface treatment

Info

Publication number: WO2008150127A2
Application number: PCT/KR2008/003176
Authority: WO
Inventors: Cheoul-Kyu Song; Young-Woo Kim
Original assignee: Cheoul-Kyu Song; Young-Woo Kim
Priority date: 2007-06-07
Filing date: 2008-06-05
Publication date: 2008-12-11
Also published as: WO2008150127A3; KR100920212B1; KR20080107620A

Abstract

Provided are a method of metal surface treatment by which material deformation does not occur after surface treatment and there is no limitation in material of an object to be treated, and a solution of metal surface treatment used in the method. The method includes: electrolyzing an object to be treated in a solution obtained by using a composition comprising 30 to 89 parts by weight of NaCN, 1 to 60 parts by weight of KCN, 10 to 69 parts by weight of Na2CO3 or a composition comprising 30 to 60 parts by weight of NaCN and 40 to 70 parts by weight of Na2CO3; maintaining the solution to have temperature of 400°C-800°C and immersing the electrolyzed object to be treated into the solution for 1 to 4 hours; and slowly cooling the immersed object to be treated at the room temperature.

Description

METHOD OF METAL SURFACE TREATMENT AND SOLUTION OF METAL SURFACE TREATMENT

Technical Field

The present invention relates to a method of metal surface treatment and a solution of metal surface treatment, and more particularly, to a method of metal surface treatment by which material deformation does not occur after surface treatment and there is no limitation in material of an object to be treated, and a solution of metal surface treatment used in the method.

Background Art

In surface hardening by which the surface of material is made solid, a change in chemical components occurs only in steel, which is a solid material, and the change occurs only on a surface layer of material. Surface hardening includes carburization, nitrification, induction hardening, flame hardening, discharge hardening, and metallic cementation etc. Carburization is a hardening method by which carbon is infiltrated into the surface of soft steel, soft steel is hardened, the surface of soft steel is made as hard steel and an inside thereof is left as soft steel. Carburization steel should be low carbon steel, and crystalline particles of carburization steel should not be grown even when carburization steel is heated for a long time and defects should not occur on the surface of carburization steel. Carburization includes solid, liquid, and gaseous carburization according to the type of a carburizer. In liquid carburization among them, material to be carburized is dipped into a solution made of a carburizer in which NaCN is used as a main component and neutral salt or carbonate salt is added, and material is carburized. In actuality, carburization and nitrification are simultaneously performed and thus, liquid carburization is referred to as liquid carburization nitrification. More specifically, liquid carburization nitrification is referred to as carbonitriding (is also referred to as cyaniding), when, due to a liquid carburizer, steel material is dipped into a molten dyebath in which NaCN is used as a main component, NaCN is decomposed and carbon and nitrogen are simultaneously infiltrated and diffused into steel material. The method is suitable for various small production. However, recently, since a CN gas causes environmental pollution, an exhaust device is necessary. Therefore, research on a method of pollution-free carburization has been made. In a carburization nitrification mechanism by using a dyebath of molten salt, such as NaCN, NaCN is decomposed, reacts with oxygen, moisture, and carbon dioxide in the air on the surface of the dyebath of molten salt and is changed into cyanide. Such cyanide is decomposed at high temperature, and CO and N are generated so that carburization and nitrification are performed. Temperature for carburization is about

700⁰C . However, acupunctu ration is mainly performed at 700⁰C or lower, and carburization is performed at 700⁰C or higher. Conventional temperature for liquid carburization is high temperature of 750^°C-950^°C . However, in such liquid carburization, a large carburization depth cannot be obtained, and hardness after surface treatment is not relatively high. In addition, since liquid carburization is performed at high temperature, carbon is infiltrated and hardened into an outer boundary of material so that material deformation occurs. Heat treatment, such as hardening and tempering, should be performed again after carburization. Thus, processes are complicated, and treatment costs are relatively high, and it is not easy to manage a carburization dyebath, and a harmful gas is generated, and a pollution countermeasure is necessary.

Disclosure of the Invention The present invention provides a method of metal surface treatment, by which there is no limitation in the kind of steel to be treated, there is no material deformation after surface treatment, costs are low, the depth of carburization cannot be increased and a manufacturing process is simple, and a solution of metal surface treatment used in the method. According to an embodiment of the present invention, there is a method of metal surface treatment, the method including: electrolyzing an object to be treated in a solution obtained by using a composition comprising 30 to 89 parts by weight of NaCN, 1 to 60 parts by weight of KCN, 10 to 69 parts by weight of Na₂CU₃ or a composition comprising 30 to 60 parts by weight of NaCN and 40 to 70 parts by weight of Na₂CO₃; maintaining the solution to have temperature of 400^°C-800^°C and immersing the electrolyzed object to be treated into the solution for 1 to 4 hours; and slowly cooling the immersed object to be treated at the room temperature. The method may further include, before the electrolyzing, pre-heating the object to be treated at 400 ^°C or lower.

The method may further include, before the electrolyzing or the pre-heating, pretreatment in which at least one or two or more cleaning selected from the group consisting of cleaning the object to be treated by using an at least volatile cleanser, cleaning the object to be treated by using an alkaline cleanser, and cleaning the object to be treated by applying ultrasonic waves to the object to be treated in water, are sequentially performed.

The method may further include, after the cooling, cleaning the object to be treated by using water and drying the object to be treated. The method may further include treating the cleaned and dried object to be treated, by using a rust-preventing solution.

The electrolyzing may include maintaining the object to be treated to have temperature of 500⁰C -700 ^°C in the solution and electrolyzing the object to be treated, by using current of 10-20 V and 0.05-0.25 A/cm² for 0.5 to 2 hours. The cooling may include first slow-cooling in which the object to be treated is pre-heated at 650 ^°C and is air-cooled for 2 hours, and second slow-cooling in which the object to be treated is pre-heated again at 575 ^°C and is air-cooled for 3 hours. According to another aspect of the present invention, there is provided a solution of metal surface treatment obtained by using a composition comprising 30 to 89 parts by weight of NaCN or NaCNO, 1 to 60 parts by weight of KCN or KCNO, and 10 to 69 parts by weight of Na₂CO₃. According to another aspect of the present invention, there is provided a solution of metal surface treatment obtained by using a composition comprising 30 to 60 parts by weight of NaCN or NaCNO and 40 to 70 parts by weight of Na2CO₃.

The solution may further include 5 or less parts by weight of KCI. The solution may further include 10 or less parts by weight of

K₄[Fe(CN)₆].

Brief Description of the Drawings

FIG. 1 is a photo showing the tissue of a sample #1 according to an embodiment of the present invention.

FIG. 2 is a photo showing the tissue of a sample #2 according to an embodiment of the present invention.

FIG. 3 is a graph showing the result of measuring hardness of the samples #1 and #2 of FIGS. 1 and 2. FIG. 4 is a photo showing the sample #1 of FIG. 1 photographed by a scanning electron microscope (SEM) with magnifications of X300, X500, X1.0K, X2.0K, X5.0K, and X10.0K, respectively.

FIG. 5 is a photo showing the sample #2 of FIG. 1 photographed by a scanning electron microscope (SEM) with magnifications of X300, X500, X1.0K, X2.0K, X5.0K, and X10.0K₁ respectively.

FIG. 6 is a photo showing the tissue of a surface hardening layer of the sample #1 of FIG. 1 photographed with a magnification of X10.0K and is a graph showing the result of energy dispersive spectroscopy (EDS) analysis of the surface hardening layer.

FIG. 7 is a photo showing the tissue of a surface hardening layer of the sample #2 of FIG. 2 photographed with a magnification of X10.0K and is a graph showing the result of energy dispersive spectroscopy (EDS) analysis of the surface hardening layer.

FIG. 8 shows the tissues of an RD surface, a TD surface, and an ND surface edge after etching a sample, according to an embodiment of the invention.

Best mode for carrying out the Invention

Exemplary embodiments of the present invention will now be described with reference to the attached drawings. A method of metal surface treatment according to the present invention comprises operations of pretreatment, pre-heating, electrolyzing, immersing, cooling, cleaning and drying, and rust preventing.

The operation of pretreatment is the operation of cleaning metal to be treated. In the pretreatment operation, an object to be treated is immersed into a TCE solution, which is a general volatile cleanser, at 150⁰C for 10 seconds, thereby removing oil that exists in the object to be treated. Next, in order to remove smudged contaminants of the firstly-cleaned object to be treated, the object to be treated is immersed into a cleanser for practical use, such as lye, at 80⁰C-IOO⁰C for 1 hour. In this case, when many contaminants exist in the object to be treated, ultrasonic waves having the frequency of 28 KHz are applied to the object to be treated for 20 minutes, thereby increasing the effect of cleaning. Next, the object to be treated is immersed into water at 50 ^°C for 5 minutes, thereby removing an alkaline cleanser. Even in this case, if necessary, ultrasonic waves are applied to the object to be treated, thereby improving the effect of cleaning. The pretreatment operation may be properly, sequentially performed or may be performed alone according to the contamination of the object to be treated.

The object to be treated, in which cleaning is finished, is dried in the air and its humidity is removed.

Subsequently, the object to be treated is pre-heated in a pre-heating furnace at 400⁰C or lower. The pre-heating operation is performed to minimize thermal shock that may occur in a next operation. Next, chemicals at a predetermined rate according to target temperature are put in a surface treatment solution according to a type of a pre-heated object to be treated, and temperature increases, and the chemicals are made in a liquefied state, and when it reaches target temperature, electrodes are put, and electricity is applied to the object to be treated, for a predetermined amount of time. Condition for electrolyzing in this case is determined according to an object to be treated and the amount of the object to be treated. Electrolyzing is performed by using the current of 10-20 V and 0.05-0.25 A/cm² at 500⁰C -700⁰C for 0.5-2 hours.

The solution of surface treatment is prepared to have the content of 30 to 89 parts by weight of NaCN, 1 to 60 parts by weight of KCN, 10 to 69 parts by weight of Na₂CO₃ or the content of 30 to 60 parts by weight of NaCN and 40 to 70 parts by weight of Na₂CO₃. When a user wants to increase the convection speed of the solution of surface treatment, KCN is added to a larger content than Na₂CO₃, and when the user wants to decrease the convection speed of the solution of surface treatment, Na₂CO₃ is added to a larger content than KCN. In addition, in order to correct pH of the solution of surface treatment after treatment, KCI of a predetermined amount is added, and potassium hexacyanoferrate(ll) (K₄[Fe(CN)₆] having the content of 10 or less parts by weight may be added to the solution of surface treatment according to an object to be treated. The ratio of mixing of a composition of the solution of surface treatment varies according to the treatment temperature of the object to be treated and may be largely classified into four categories as below. a) 400 ^°C < treatment temperature < 500 ^°C - NaCN 55%

- KCN 40%

- Na₂CO₃ 5%

b) 500^°C < treatment temperature < 610⁰C - NaCN 60%

- KCN 20%

- Na₂CO₃ 20%

c) 610⁰C < treatment temperature < 700⁰C - NaCN 50%

- KCN 10%

- Na₂CO₃ 40%

d) 700 ^°C < treatment temperature < 800 ^°C - NaCN 50%

- KCN 0%

- Na₂CO₃ 50%

When electrolyzing is finished, the above-described solution is maintained to have temperature of 400 °C -800⁰C , and the electrolyzed object to be treated is immersed in the solution for 1-4 hours, thereby performing hardening by using surface diffusion. The convention of the solution occurs during the immersing operation, thereby facilitating surface diffusion. If necessary, stirring may be performed by using a stirrer, so as to improve the effect of immersion.

When immersing is finished, the object to be treated is slowly cooled up to 25⁰C in first and second slow cooling operations. In the first slow cooling operation, the object to be treated is pre-heated to 650⁰C , thereby air-cooling (slowly cooling) the object to be treated for 2 hours. Subsequently, the object to be treated is again pre-heated to 575⁰C and then is air-cooled (slowly-cooled) for 3 hours.

Next, the slowly-cooled object to be treated is immersed into water of about 4O⁰C for about 1 hour, thereby cleaning a water-soluble ingredient, such as NaCN, that exists on the surface of the object to be treated, and the object to be treated is again dried in the air and humidity of the object to be treated is removed.

Last, a rust-preventing operation is performed by using a conventional rust-preventing solution, so as to prevent the surface of the object to be treated from rusting.

Hereinafter, exemplary embodiments of a method of metal surface treatment and a solution of metal surface treatment according to the present invention will be described.

[Embodiment 1 - surface treatment of SUS 304]

In the present embodiment of the present invention, surface hardness before and after treatment of a sample having a surface treated by using SUS 304 according to the present invention was measured and compared.

1 ) Surface hardness of SUS 304 raw material before treatment

3) Hardness depth

According to the above measured values, the surface hardness of SUS 304 treated by using the method of metal surface treatment according to the present invention was between 1500 and 1600.

Considering that the surface hardness of SUS 304 raw material before treatment was average 200, surface hardness was improved by 7-8 times after treatment. In addition, hardness according to depth decreased gradually due to the characteristic of diffusion. The result of metal surface treatment was applied to all kinds of steel, and about HV 1200 was measured as the result of treating Titan.

[Embodiment 2]

In the present embodiment of the present invention, in order to check the cause for surface hardening of the method of metal surface treatment according to the present invention, two samples #1 and #2 of

SUS-304 were surface-treated by using the method of metal surface treatment according to the present invention and then were enlarged by using an optical microscope OLYMPUS-BX51 M with magnifications of X50, X100, X200, and X500, respectively.

FIG. 1 is a photo showing the tissue of a sample #1 according to an embodiment of the present invention, and FIG. 2 is a photo showing the tissue of a sample #2 according to an embodiment of the present invention. FIG. 3 is a graph showing the result of measuring hardness of the samples #1 and #2 of FIGS. 1 and 2 by using Mitutoyo MVK-HVL, which is a hardness testing machine, on the condition of measuring time 10 second, load 10g, and equal intervals 0.1 mm.

FIG. 4 is a photo showing the sample #1 of FIG. 1 photographed by a scanning electron microscope (SEM) with magnifications of X300, X500, X1.0K, X2.0K, X5.0K, and X10.0K, respectively.

FIG. 5 is a photo showing the sample #2 of FIG. 1 photographed by a scanning electron microscope (SEM) with magnifications of X300, X500, X1.0K, X2.0K, X5.0K, and X10.0K, respectively.

As illustrated in FIGS. 4 and 5, a uniform surface hardening layer having the thickness of about 5 to 10 μm was formed on the samples #1 and #2 treated by using the method of metal surface treatment according to the present invention, and a base material inside the samples #1 and

#2 was not affected by heat treatment.

As illustrated in FIGS. 6 and 7, as the result of EDS analysis of the surface hardening layer, the fraction of Cr/Fe was 0.19 - 0.21 , and there was no great change in comparison with 0.16 of a base material. Thus, there was no change in tissues based on a composition caused by substitutional elements. On the other hand, the fraction of C/Fe of a surface layer increased by 0.05 or more, and simultaneously, there was no remarkable carbide on the surface layer. Thus, in the method of metal surface treatment according to the present invention, the main cause for hardening of the surface hardening layer of metal was the effect of solid hardening caused by carbon infiltration. [Embodiment 3]

In the present embodiment of the present invention, in order to check a change in the tissue of material treated by using the method of metal surface treatment according to the present invention, the internal tissue of a sample after SUS304 was surface-treated according to the present invention, was observed by using an optical microscope. FIG. 8 shows the tissues of an RD surface after etching a sample in an etching solution comprising 1g Picric acid + 5 ml HCL + 100 ml ethanol for 1 minute, a TD surface and an ND surface edge after etching a sample in an etching solution comprising 1 g Picric acid + 5 ml HCL + 100 ml ethanol for 30 seconds, according to an embodiment of the invention. As illustrated in FIG. 8, due to the characteristic of a rolling sample, different textures appeared according to observation positions, and the typical tissue of SUS304 including an Austenite phase containing twin and finely-distributed carbide was shown. Thus, in the method of metal surface treatment according to the present invention, grain boundary precipitation of carbide due to heat treatment and appearance of other new phases were not observed. Thus, there was no change in base material due to surface treatment according to the present invention. While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Industrial Applicability

The method of metal surface treatment according to the present invention as described above has the following effects.

(1 ) Material deformation after surface treatment does not occur and it is deemed as zero tolerance. Thus, an object to be treated can be treated even in a completely-processed state, and post-treatment is not needed, and assembling is possible immediately after treatment. Therefore, the present invention can be used in heat treatment of precise parts and treatment of a mold for die casting having accurate dimensional tolerance. (2) Even when the surface hardness of the object to be treated increases, there is no change in internal tissues of the object to be treated so that there is no brittleness and a ductile characteristic increases.

(3) There are various materials to be treated, such as special alloy, including SS41 , SPC, SUS 30 group, FCD 20-40, and Ni etc. Even a conventional material in which surface hardening is not well performed, can obtain surface hardness and hardening depth remarkably. In particular, when a sheet material, such as SPC or SUS304 etc. in which surface hardening is not well performed, is treated according to the present invention, higher strength and surface hardness than in thin material can be obtained so that light-weight and cost reduction is possible and a surface hardening layer increases the elasticity of the sheet material and the rigidity of the material can be increased.

(4) There are various temperatures between 400⁰C and 800⁰C , and various temperatures can be selected according to characteristics of a target material.

(5) The object to be treated enters into or goes out freely so that treatment time can be easily adjusted and can be reduced.

(6) A compound layer is not formed on the surface hardening layer so that cracks and exfoliation do not occur in the surface hardening layer.

(7) Facility cost is lower than in other methods of metal surface treatment.

(8) Process control is easy, and error defects caused by process control do not occur. In addition, processes can be easily automated. (9) Convection occurs in a treatment solution so that distortion of the object to be treated due to a temperature difference in solution does not occur.

(10) Thermal resistance, wear resistance, and rust resistance are very higher than in carburization induction and ion nitrification, and the thickness of the surface hardening layer is large. Thus, the mechanical life span of the object to be treated is extended.

(11 ) The metal surface treatment according to the present invention is age-hardening type treatment so that, as time elapses, tissues of the object to be treated can be further stabilized and hardness thereof is improved.

Claims

What is claimed is:

1. A method of metal surface treatment, the method comprising: electrolyzing an object to be treated in a solution obtained by using a composition comprising 30 to 89 parts by weight of NaCN, 1 to 60 parts by weight of KCN, 10 to 69 parts by weight of Na₂CO₃ or a composition comprising 30 to 60 parts by weight of NaCN and 40 to 70 parts by weight of Na₂COa; maintaining the solution to have temperature of 400 ^°C -800⁰C and immersing the electrolyzed object to be treated into the solution for 1 to 4 hours; and slowly cooling the immersed object to be treated at the room temperature.

2. The method of claim 1 , further comprising, before the electrolyzing, pre-heating the object to be treated at 400 ^°C or lower.

3. The method of claim 1 or 2, further comprising, before the electrolyzing or the pre-heating, pretreatment in which at least one or two or more cleaning selected from the group consisting of cleaning the object to be treated by using an at least volatile cleanser, cleaning the object to be treated by using an alkaline cleanser, and cleaning the object to be treated by applying ultrasonic waves to the object to be treated in water, are sequentially performed.

4. The method of claim 1 or 2, further comprising, after the cooling, cleaning the object to be treated by using water and drying the object to be treated.

5. The method of claim 4, further comprising treating the cleaned and dried object to be treated, by using a rust-preventing solution.

6. The method of claim 1 or 2, wherein the electrolyzing comprises maintaining the object to be treated to have temperature of 500^°C-700^°C in the solution and electrolyzing the object to be treated, by using current of 10-20 V and 0.05-0.25 A/cm² for 0.5 to 2 hours.

7. The method of claim 1 or 2, wherein the cooling comprises first slow-cooling in which the object to be treated is pre-heated at 650°C and is air-cooled for 2 hours, and second slow-cooling in which the object to be treated is pre-heated again at 575⁰C and is air-cooled for 3 hours.

8. A solution of metal surface treatment obtained by using a composition comprising 30 to 89 parts by weight of NaCN or NaCNO, 1 to 60 parts by weight of KCN or KCNO, and 10 to 69 parts by weight of Na₂CO₃.

9. A solution of metal surface treatment obtained by using a composition comprising 30 to 60 parts by weight of NaCN or NaCNO and 40 to 70 parts by weight of Na₂CO₃.

10. The solution of claim 8 or 9, further comprising 5 or less parts by weight of KCI.

1 1. The solution of claim 8 or 9, further comprising 10 or less parts by weight of K₄[Fe(CN)₆].