US20070193659A1

US20070193659A1 - Method for nitriding metal in salt bath and metal manufactured using the same

Info

Publication number: US20070193659A1
Application number: US11/588,370
Authority: US
Inventors: Hee Won Jung; Dong Nyung Lee; Young June Park; Dong Sam Kim; Kyu Hwan Oh; Yinzhong Shen
Original assignee: ILJIN LIGHT METAL Co Ltd
Current assignee: ILJIN LIGHT METAL Co Ltd
Priority date: 2006-02-23
Filing date: 2006-10-27
Publication date: 2007-08-23
Also published as: KR20070087475A; KR100812971B1; CA2565769A1

Abstract

Provided is a method for nitriding a metal in a salt bath by using a non-cyanide salt and a nitrided metal manufactured using the same. The method includes the steps of: immerging at least one salt selected from the group consisting of KNO₃, KNO₂, Ca(NO₃)₂, NaNO₃and NaNO₂into the salt bath; melting the salt by heating and maintaining the molten salt at a predetermined temperature; and submerging the metal in the salt bath. Nitriding in non-cyanide salts, such as potassium nitrate (KNO₃), potassium nitrite (KNO₂), sodium nitrate (NaNO₃), sodium nitrite (NaNO₂), calcium nitrate (Ca(NO₃)₂) and their mixtures, is capable of solving an environmental pollution problem and reducing a cost. Also, the method is capable of increasing nitrided depth of the metal two to six times compared to conventional nitriding methods. As a result, the method can be carried out in various application fields.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method for nitriding metal in a salt bath and nitrided metal manufactured using the same; and, more particularly, to a method for nitriding iron or steels by using non-cyanide salt bath, and nitrided iron or steels manufactured using the same.
2. Background of the Related Art
Steels have been widely used for machine parts because of their inherent properties. To be used for machine parts, steels are usually first heat-treated to impart thereto strength, toughness and durability, all of which are the qualities machine parts require. In addition, for machine parts that are often exposed to corrosive environment, surfaces thereof are further heat-treated to impart thereto corrosion resistance.
Nitriding is one of the methods for processing the metal surface to impart thereto a corrosion resistance thereof. The nitriding methods include gas nitriding using NH₃gas, salt bath nitriding using KCN, KCNO etc., gas nitrocarburizing (carbo-nitriding) using a mixture of NH₃gas and RX gas, i.e., endothermic gas, and ion nitriding involving an insertion of a mixture of N₂and H₂gas into plasma.
Generally, although nitriding is applied to steels to improve their abrasion(wear) resistance and fatigue resistance, it can also be carried out to improve the corrosion resistance thereof.
Of the nitriding methods mentioned hereinabove, the salt bath nitriding is most widely used for a variety of machine parts including automobile components, because properties of chemicals for the salt bath and their melting points can be freely controlled to provide stability through a wide range of process temperatures without eroding the surface of the object being treated. To be more specific, in addition to its excellent thermal conductivity, soaking properties and easily controllable processing conditions, it is cheaper to design and maintain, compared with other nitriding methods. For example, it is easy to operate the salt bath, and the heating rate is 4 times faster in the salt bath than in the atmosphere. The salt bath is especially suitable to heat-treatment of high speed steel which is sensitive to crystal(grain) growth. When a material treated in a salt bath comes into a contact with the atmosphere, a film including the salt bath constituents is formed on the surface thereof, the film preventing oxidation by preventing the material from a direct contact with the atmosphere. Furthermore, the surface of the material thus treated is rather clean, making the salt bath an ideal heat-treatment for both mass production and small-lot-sized production.
Cyanide-containing salt is generally used for a salt bath nitriding method, producing cyanide ions inside the bath. Since the cyanide ion is classified as a toxic chemical, it must be carefully and tightly controlled, and this can be an expensive proposition. Also, there is a problem of a cost involved for processing waste water and gas.
Further, the nitriding treatment in a molten salt including cynides is a nitrocarburizing (carbo-nitriding) method involving a simultaneous penetration of carbon and nitrogen. It has a shortcoming in that although the surface hardness of the material thus treated improves significantly, the tensile strength gets only slightly enhanced. The conventional salt bath nitriding method using a cyanide salt also has a problem that its applications are limited to molds or gears since the depth to which the material can be nitrided is limited.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a method for nitriding a metal using non-cyanide salts, and a nitrided metal manufactured using the same.
It is another object of the present invention to provide a salt-bath nitriding method for nitriding a metal, in which nitrogen penetrates into the metal, and a nitrided metal manufactured using the same.
It is yet another object of the present invention to provide a salt bath nitriding method for nitriding a metal, capable of increasing hardness and tensile strength of the metal to be treated, and a nitrided metal manufactured using the same.
It is still another object of the present invention to provide a salt bath nitriding method for nitriding a metal, capable of maximizing a nitriding depth, and a nitrided metal manufactured using the same.
In accordance with one aspect of the present invention, there is provided a method for nitriding a metal in a salt bath, the method including the steps of: a) immersing a non-cyanide salt into the salt bath; b) melting the salt by heating and maintaining the molten salt at a predetermined temperature; and c) submerging the metal in the salt bath.
In the present invention, it is preferred that the non-cyanide salt includes at least one selected from a group consisting of NaNO₃, NaNO₂, KNO₃, KNO₂and Ca(NO₃)₂, and the metal is one of iron and steels.
At this time, the predetermined temperature is within a range of 400° C. to 700° C., and the submerging time is within a range of 1 minute to 24 hours.
In the present invention, when iron is nitrided in the salt bath including at least one of the group consisting of KNO₃, KNO₂, Ca(NO₃)₂, NaNO₃, and NaNO₂, the iron can be nitrided into a depth of 0.1 mm to 3.0 mm from its surface.
In the present invention, when a steel is nitrided in the salt bath including at least one of the group consisting of KNO₃, KNO₂, Ca(NO₃)₂, NaNO₃, and NaNO₂, the steel can be nitrided into a depth of 0.1 mm to 3.0 mm from its surface.
The steel includes ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel, alloy steel and IF steel.
The ultra-low carbon steel nitrided by the present invention has the surface hardness ranging from more than 120 Hv to equal to or less than 450 Hv. The low carbon steel has the surface hardness being more than 200 Hv to equal to or less than 410 Hv. The medium carbon steel has the surface hardness being more than 130 Hv to equal to or less than 420 Hv. The high carbon steel has the surface hardness being more than 150 Hv to equal to or less than 400 Hv. The alloy steel has the surface hardness being more than 200 Hv to equal to or less than 410 Hv. IF steel has the surface hardness being more than 165 Hv to equal to or less than 400 Hv. The surface hardness of the steels nitrided by the present invention can be improved to a maximum of 420 Hv. The surface hardness of the iron nitrided by the present invention is also improved.
The ultra-low carbon steel nitrided by the present invention has the tensile strength ranging from more than 35 kgf/mm²to equal to or less than 110 kgf/mm². The low carbon steel has the tensile strength ranging from more than 45 kgf/mm²to equal to or less than 110 kgf/mm². The medium carbon steel has the tensile strength ranging from more than 45 kgf/mm²to equal to or less than 100 kgf/mm². The high carbon steel has the tensile strength ranging from more than 60 kgf/mm²to equal to or less than 95 kgf/mm². The alloy steel has the tensile strength ranging from more than 55 kgf/mm²to equal to or less than 110 kgf/mm². The tensile strength of IF steel and iron can be improved by the nitriding method of the present invention.
The salt-bath nitriding method of the present invention can be applied to the iron, the IF steel, the carbon steel including the ultra-low carbon steel having a carbon content of at least 0.0001 wt % to less than 0.13 wt %, the low carbon steel having a carbon content of at least 0.13 wt % to less than 0.2 wt %, the medium carbon steel having a carbon content of at least 0.21 wt % to less than 0.51 wt %, and the high carbon steel having a carbon content of at least 0.51 wt % to less than 2.0 wt %, the steel having a chrome content of 0.1 wt % to 1.5 wt %, the steel having a molybdenum content of 0.05 wt % to 0.5 wt %, the steel having a nickel content of 0.1 wt % to 10 wt %, the steel having a manganese content of 0.1 wt % to 2.0 wt %, the steel having a boron content of 0.001 wt % to 0.1 wt %, the steel having a titanium content of 0.01 wt % to 0.1 wt %, the steel having a vanadium content of 0.05 wt % to 0.15 wt %, the steel having a niobium content of 0.005 wt % to 0.1 wt %, and the steel having an aluminum content of 0.005 wt % to 0.1 wt %. Also, the salt-bath nitriding method of the present invention can be applied to the alloy steel including at least two kinds of the steels suggested above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph illustrating relationship between a nitriding time and a hardness profile in a steel nitrided in accordance with a first embodiment of the present invention;

FIG. 2 is a graph illustrating relationship between the nitriding time and the hardness profile in the steel nitrided in accordance with the first embodiment of the present invention;

FIG. 3 is a graph illustrating relationship between a nitriding temperature and the hardness profile in the steel nitrided in accordance with the first embodiment of the present invention;

FIG. 4 is a graph illustrating relationship between the nitriding time and the surface hardness of the steel nitrided in accordance with the fourth embodiment of the present invention;

FIG. 5 is a graph illustrating relationship between the nitriding temperature and time and the hardness profile in the steel nitrided in accordance with the fourth embodiment of the present invention;

FIG. 6 is a graph illustrating relationship between the nitriding time and the hardness profile in the steel nitrided in accordance with the fourth embodiment of the present invention;

FIG. 7 is a graph illustrating the hardness profile in the steel nitrided in accordance with the fifth embodiment of the present invention;

FIG. 8 is a graph illustrating the hardness profile in the steel nitrided in accordance with the sixth embodiment of the present invention; and

FIG. 9 is a graph illustrating relationship between a mixture ratio of a mixed salt and the hardness profile in the steel nitrided in accordance with the sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in more detail.
In nitriding of a metal, the present invention incorporates therein the nitrogen dissolution principle involving a non-cyanide molten salt, more particularly, NaNO₃, NaNO₂, KNO₃, KNO₂, Ca(NO₃)₂and mixtures thereof as a molten salt, as opposed to a conventional nitriding method such as a nitrocarburizing (carbo-nitriding) method involving the use of cyanides, e.g., KCN and NaCN, as the molten salt wherein carbon and nitrogen are simultaneously diffused into the metal.
The method for nitriding the metal in accordance with the present invention involves immersing at least one salt from a group consisting of NaNO₃, NaNO₂KNO₃, KNO₂and Ca(NO₃)₂into a salt bath, melting the salt and maintaining of the molten salt at a predetermined temperature ranging from 400° C. to 700° C.
Subsequently, the metal to be nitrided is submerged in the bath for 1 minute to 24 hours.
During this time, nitrogen, oxygen and nitrogen oxides are generated from the non-cyanide molten salts of the present invention, NaNO₃, NaNO₂KNO₃, KNO₂, Ca(NO₃)₂and mixtures of thereof, by the following reaction formulae 1 to 3.
The following reaction formula 1 represents nitrogen formation reaction in the molten salt bath of NaNO₃and NaNO₂.
NaNO₃→NaNO₂+½O₂
2 NaNO₂→Na₂O+NO₂+NO
2NaNO₂+2NO→2NaNO₃+N₂ [Reaction formula 1]
The following reaction formula 2 represents nitrogen formation reaction in the molten salt bath of KNO₃and KNO₂.
KNO₃→KNO₂+½O₂
2KNO₂→K₂O+NO₂+NO
2KNO₂+2NO→2KNO₃+N₂ [Reaction formula 2]
The following formula 3 shows nitrogen formation reaction in the molten salt bath of Ca(NO₃)₂.
Ca(NO₃)₂→CaO+2NO₂+½O₂
2NO₂→2O₂+N₂ [Reaction Formula 3]
As shown in Table 1, those metals nitrided, including carbon steel (including ultra-low carbon steel, low carbon steel, medium carbon steel and high carbon steel), alloy steel, IF steel and iron using the salt-bath nitriding method in accordance with the present invention are nitrided to a depth of 0.1 mm to 3.0 mm from the surface. The range of nitrided depth/diffusion layer thickness obtained through the present invention is 2 to 6 times larger than that obtained using the conventional nitriding methods, meaning that a nitrided/diffusion layer formed using the nitriding method of the present invention extends from the surface to the metal inner part, and consequently the surface hardness and tensile strength of the metal also improve compared to those of the metal nitrided using the conventional nitriding method. Reference for the table 1 are as follows:
K. Funatani, “Low-Temperature Salt Bath Nitriding of Steels”, Metal Science and Heat Temperature, Vol. 46, No. 7, PP. 277-281 (2004).

TABLE 1

			Thickness of
	Temperature		diffusion
Nitriding method	(K)	Type of Steel	layer (μm)

Nitride process by	953	Low carbon	3000
the present		steel
invention	913	IF steel	1500
Tufftride TFI	853	1015	800
	853	1045	780
	853	34Cr4	480
	853	X210Cr12	160
Tufftride NSI	843	1015	780
	843	SCM435	171
“Soft” Nitriding in	843	SS2250	353
gas medium
“Soft” Nitriding in	793	38CrMoAl	78–97
gas medium	—	40Cr	63–80
Gas Nitriding	773	SAE9254	49
Plasma Nitriding	793	722M24	72
	(Pused)
	793 (DC)	722M24
Plasma Nitriding	833	En40B	100
	813	En19	110
	793	Nitraps	46
	823	36CrMo	100
	793	36CrMo + 0.1Y	200
	823	36CrMo + 0.1Ce	215
Low-temperature	753	SKD61	150
salt bath Nitriding	843	SKD61	106
(palsonite)	753	SCM435	141
	843	SCM435	200

Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

First Embodiment

In accordance with the first embodiment of the present invention, steel is nitrided using the NaNO₃molten salt. The nitrided steel includes ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel.
Each of the ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel is submerged in the NaNO₃molten salt bath for 2 hours at a temperature of 500° C.
Table 2 shows changes in surface hardness and tensile strength of the samples nitrided in the molten salt bath, wherein the hardness was measured using a Vickers hardness tester under a load of 1 kgf.
In case of ultra-low carbon steel, the surface hardness increases by 119% and the tensile strength increases by 47%. In case of low carbon steel, the surface hardness increases by 47% and the tensile strength increases by 19%.
In case of medium carbon steel, the surface hardness increases by 32% and the tensile strength increases by 18%. In case of high carbon steel, the surface hardness increases by 28% and the tensile strength increases by 16%. In case of alloy steel, the surface hardness increases by 24% and the tensile strength increases by 17%.
That is, in case of steel, the surface hardness increases by 20% to 120% and the tensile strength increases by 15% to 50%.
The differences in the amount of increases shown in the surface hardness depending on the steel type can be attributed to the differences in the nitrogen diffusion rate associated with each type of steels determined by the carbon content therein.

	TABLE 2

	Change of Hardness	Change of Tensile
	(Hv)	Strength (kgf/mm²)

			In-		After	In-
Type	Before	After	creasing	Before	nitrid-	creasing
of	nitriding	nitriding	rate	nitriding	ing	rate
steel	process	process	(%)	process	process	(%)

Ultra	128	280	119	34	50	47
low
carbon
steel
Low	194	286	47	62	74	19
carbon
steel
Medium	183	241	32	56	66	18
carbon
steel
High
	230	294	28	73	85	16
carbon
steel
Alloy	226	281	24	71	83	17
steel

FIG. 1 is a graph the showing the hardness distribution in the thickness direction of the ultra-low carbon steel before (As) and after nitriding in the NaNO₃molten salt bath at 500° C. for 30 minutes, 1 hour, 2 hours and 5 hours, respectively.
The nitrided depth or the diffusion depth increases with increasing nitriding time, and the hardness decreases with increasing distance from the surface because the nitrogen concentration decreases with increasing distance from the surface. When the steel is nitrided for 5 hours, it can be seen that the steel is nitrided to a depth of about 0.6 mm from the surface.
FIG. 2 shows the hardness distribution along the thickness direction of low carbon steel nitrided in the NaNO₃molten-salt bath at 680° C. for 3, 6, 12 and 24 hours, respectively, wherein the hardness is measured using a Vickers hardness tester under a load of 3 kgf.
As shown in FIG. 2, the nitrided depth or the diffusion depth of the steel increases with increasing nitriding time. The nitrided depth of the steel after nitriding for 24 hours is about 3 mm, which is 6 times deeper than that obtained from the conventional nitriding method.
Also, the surface hardness after nitriding is 450 Hv, which is more than 4 times higher than that of the non-treated specimen.
Accordingly, the nitriding method of the present invention can increase the nitrided depth of the steel by 2 to 6 times compared to the conventional cyanide-based salt bath nitriding method.
FIG. 3 shows hardness distributions along the thickness direction of the ultra-low carbon steel before and after nitriding in the NaNO₃molten-salt bath at 500° C. and 600° C. for 3 hours. The nitrided depth of the steel nitrided at 600° C. is 3 times deeper than that of the steel nitrided at 500° C. The surface hardness of the steel nitrided at 600° C. is 100 Hv higher than that of the steel nitrided at 500° C. That is, the surface hardness and nitrided depth of steel increase with increasing nitriding temperature.
Table 3 shows changes in tensile strength of ultra low carbon steel depending on the nitriding temperature wherein the samples are nitrided for 3 hours at 450° C., 500° C., 550° C. and 600° C., respectively, using the salt-bath nitriding method of the first embodiment of the present invention.
As shown in FIG. 3, in case of the nitriding temperature of 450° C., the tensile strength increases by 5%. As the temperature increases, the tensile strength of the steel also increases. Accordingly, when the temperature is 600° C., the tensile strength increases by 134%.

TABLE 3

	Nitriding	Nitriding	Tensile	Increasing
	temperature	time	strength	rate
Division	(° C.)	(h)	(kgf/mm²)	(%)

Before	—	—	34.8	0
nitriding
After	450	3	36.6	5
nitriding	500		50.8	46
	550		64.5	85
	600		81.4	134

That is, since it is possible to simultaneously improve the hardness and the tensile strength by nitriding the steel according to the first embodiment, the present invention can be applied to diverse fields including diverse components and structural members.

Second Embodiment

In accordance with the second embodiment of the present invention, steel is nitrided by using the NaNO₂molten salt.
Steels including ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel are submerged in the salt bath at 450° C. for 2 hours.
Table 4 shows changes in surface hardness and tensile strength of the samples nitrided in the molten salt bath, wherein the surface hardness is measured using a Vickers hardness tester under a load of 1 kgf.
For ultra-low carbon steel, the surface hardness increases by 54% and the tensile strength increases by 21%. For low carbon steel, the surface hardness increases by 32% and the tensile strength increases by 15%.
For medium carbon steel, the surface hardness increases by 19% and the tensile strength increases by 13%. For high carbon steel, the surface hardness increases by 18% and the tensile strength increases by 12%.
For alloy steel, the surface hardness increases by 17% and the tensile strength increases by 14%.
That is, in case that steels are nitrided by the molten salt bath nitriding method of the second embodiment of the present invention, the surface hardness increases by 15% to 60%, and the tensile strength increases by 10% to 25%.
Accordingly, the molten salt bath nitriding method in accordance with the second embodiment of the present invention also increases the surface hardness and tensile strength of the steels.

	TABLE 4

	Change of Hardness	Change of Tensile
	(Hv)	Strength (kgf/mm²)

			In-			In-
Type			creasing		After	creasing
of	Before	After	rate	Before	nitrid-	rate
steel	nitriding	nitriding	(%)	nitriding	ing	(%)

Ultra-	128	197	54	34	41	21
low
carbon
steel
Low	194	257	32	62	71	15
carbon
steel
Medium	183	218	19	56	63	13
carbon
steel
High
	230	271	18	73	82	12
carbon
steel
Alloy	226	265	17	71	81	14
steel

Third Embodiment

In accordance with the third embodiment of the present invention, steels are nitrided using the KNO₂molten salt.
The steels including ultra-low carbon steel, low carbon steel, high carbon steel and alloy steel are submerged in the molten salt bath at 480° C. for 2 hours.
Table 5 shows changes in hardness and tensile strength of the samples submerged in the molten salt bath, wherein the surface hardness is measured using a Vickers hardness tester under a load of 1 kgf.
For ultra-low carbon steel, the surface hardness increases by 45% and the tensile strength is increases by 15%. For low carbon steel, the surface hardness increases by 25% and the tensile strength increases by 11%.
For high carbon steel, the surface hardness increases by 17% and the tensile strength increases by 10%. For alloy steel, the surface hardness increases by 12% and the tensile strength increases by 11%.
That is, when the steels are nitrided using the molten salt bath nitriding method of the third embodiment of the present invention, the surface hardness increases by 10% to 50%, and the tensile strength increases by 10% to 20%.
Accordingly, the molten salt bath nitriding method in accordance with the third embodiment of the present invention also increases the surface hardness and the tensile strength of the steels.

	TABLE 5

	Change of Hardness	Change of Tensile
	(Hv)	Strength (kgf/mm²)

			In-			In-
Type			creasing		After	creasing
of	Before	After	rate	Before	nitrid-	rate
steel	nitriding	nitriding	(%)	nitriding	ing	(%)

Ultra-	128	186	45	34	39	15
low
carbon
steel
Low	194	243	25	62	69	11
carbon
steel
High
	230	268	17	73	80	10
carbon
steel
Alloy	226	252	12	71	97	11
steel

Fourth Embodiment

In the fourth embodiment of the present invention, steel is nitrided using the KNO₃molten salt.
The steel to be nitrided is Interstitial-Free (IF) steel, which includes carbon (C) of 0.003 wt %, manganese (Mn) of 1.23 wt %, aluminum (Al) of 0.037 wt %, titanium (Ti) of 0.027 wt %, phosphorus (P) of 0.050 wt %, nitrogen (N) of 0.002 wt % and sulfur (S) of 0.008 wt %.
The IF steel is nitrided in the KNO₃molten bath at 560° C., 580° C., 600° C., 620° C. and 640° C., respectively.
FIG. 4 shows the surface hardness of the IF steel nitrided in the KNO₃molten bath as functions of time and temperature.
As shown in FIG. 4, as the nitriding time and temperature increase, the surface hardness increases under most temperature conditions. Although the increase of the hardness can be explained as solution strengthening, the present invention is not limited to this theory.
However, when the nitriding time in the KNO₃molten salt at 620° C. exceeds 8 hours, or the nitriding time in the KNO₃molten salt at 640° C. exceeds one hour, the surface hardness decreases. It is understood that this decrease in the surface hardness is caused by the formation of the nitrided layer in the grain boundaries of the IF steel.
In Table 6, the surface hardness values of the IF steel nitrided by the third embodiment of the present invention are given. When the IF steel is nitrided at temperatures of 560° C. to 640° C., the surface hardness increases by 75% to 130%.

	TABLE 6

	Change of Hardness (Hv)		Change of Hardness
	after nitriding for 16 h.		(Hv) after nitriding for 1 h.

			Increasing				Increasing
Nitriding	Before	After	rate	Nitriding	Before	After	rate
Temperature	nitriding	nitriding	(%)	Temperature	nitriding	nitriding	(%)

560° C.	165	289	75	620° C.	165	336	104
580° C.	165	329	99	640° C.	165	355	115
600° C.	165	379	130

FIG. 5 shows the hardness distribution along the thickness direction of the IF steel nitrided by the fourth embodiment of the present invention.
The IF steel is nitrided in the KNO₃molten salt at 560° C. for 16 hours and at temperatures of 560° C., 580° C., 600° C. and 620° C. for 8 hours.
Referring to FIG. 5, the hardness of the IF steel decreases with increasing depth from the surface because the nitrogen concentration decreases with increasing distance from the steel surface. When the nitrided depth is defined as the distance between the surface and the position where the hardness value is equaled to 110% of that of the center of the IF steel before nitriding, the nitrided depth formed in each condition ranges from about 1.38 mm to 1.5 mm, which is 3 to 5 times thicker than the thickness of the nitrided layer formed using the conventional method.
FIG. 6 is a graph showing hardness distribution along the thickness direction of the IF steel nitrided in the KNO₃molten salt at 640° C. for 1 hour, 2 hours, 4 hours, 8 hours and 16 hours.
As shown in the FIG. 6, for IF steel, as the nitriding time increases, the difference in hardness between the surface and the interior decreases, resulting in the IF steel having, as well as an increased surface hardness, an increased bulk hardness, as a consequence of nitrogen diffusing into the interior and the difference in concentration thereof between the surface and the interior decreasing. In other word, the nitriding method in accordance with the present invention will lead to an IF steel having an increased surface and bulk hardness, resulting from a nitrogen diffusing into the interior at a higher diffusion rate.

Fifth Embodiment

In the fifth embodiment of the present invention, steel is nitrided using the Ca(NO₃)₂molten salt.
The steel to be nitrided in the fifth embodiment is low carbon steel.
Since Ca(NO₃)₂is highly hygroscopic at a room temperature, including combined water, it is preferred to use Ca(NO₃)₂after removing moisture by heating for a predetermined time.
The fifth embodiment of the present invention includes the process of removing moisture by heating Ca(NO₃)₂for 4 hours at 100° C. to 150° C., heating Ca(NO₃)₂to 580° C. to form the Ca(NO₃)₂molten-salt bath and submerging the low carbon steel in the bath for 3 hours.
FIG. 7 is a graph showing the surface hardness profile in low carbon steel nitrided by the fifth embodiment of the present invention.
As shown in FIG. 7, the low carbon steel nitrided by the fifth embodiment is nitrided to a depth of 0.5 mm from the surface, and has the surface hardness that is 2 times higher than the surface hardness (As) of the steel before nitriding.

Sixth Embodiment

In the sixth embodiment of the present invention, steel is nitrided using a molten mixture of KNO₃and NaNO₃.
In the sixth embodiment of the present invention, the low carbon steel is nitrided in the molten mixture of KNO₃and NaNO₃whose mixture ratios are 1:1, 8:2 and 2:8.
Table 7 shows the surface hardness values of steels nitrided by the sixth embodiment of the present invention. Various types of steel are submerged in the molten mixture of KNO₃and NaNO₃whose ratio is 1:1 for 12 or 24 hours at 650° C.
At this time, the hardness is measured using a Vickers hardness tester under a load of 3 kg.
The hardness values of the steels nitrided in the mixture of KNO₃and NaNO₃increase by 69% to 251% depending on the steel type.

	TABLE 7

	Change of Hardness (Hv)

	Nitriding			Increasing
	Time	Before	After	rate
Type of steel	(h)	nitriding	nitriding	(%)

Ultra-low	24	128	449	251
carbon steel
low carbon	12	194	406	109
steel
Medium carbon
	12	183	391	114
steel
High carbon	24	230	389	69
steel
Alloy steel	24	226	387	71

Various steels are submerged in the mixture of KNO₃and NaNO₃whose ratio is 1:1 at 580° C., and changes in surface hardness and tensile strength of the nitrided steels depending on nitriding time are measured.
As shown in Table 8, nitriding in accordance with the fifth embodiment of the present invention increases the hardness and the tensile strength of all the steels. The hardness and tensile strength increase with increasing nitriding time.

	TABLE 8

	Change of Hardness	Change of Tensile
	(Hv)	strength (kgf/mm²)

	Nitriding			Increasing			Increasing
Type of	Time	Before	After	rate	Before	After	rate
steel	(h)	nitriding	nitriding	(%)	nitriding	nitriding	(%)

Ultra-	3	120	283	136	35	48	37
low	12	120	421	251	35	92	163
carbon
steel
low	3	200	283	42	45	55	22
carbon	12	200	403	102	45	79	76
steel
Medium	3	130	181	39	45	57	27
carbon	12	130	398	206	45	88	84
steel
High	3	150	201	34	60	76	27
carbon	12	150	391	161	60	87	45
steel
Alloy	3	200	274	37	55	75	36
steel	12	200	409	105	55	90	64

FIG. 8 is a graph showing the hardness profiles of steel nitrided at 680° C. for 200 minutes in the KNO₃bath, the NaNO₃bath, the 50% KNO₃-50% NaNO₃mixture bath at 680° C. for 200 minutes.
The hardness was measured using a Vickers hardness tester.
In FIG. 8, the steel nitrided in the mixture bath has a nitrided depth of 1.5 mm and a surface hardness of 160 Hv, which is higher than that of the steel nitrided in the single salt baths and 3 times higher than that of the steel before nitriding.
FIG. 9 is a graph showing the hardness profiles of the low carbon steel nitrided in the 80% KNO₃-20% NaNO₃bath and 20% KNO₃-80% NaNO₃bath at 650° C. for 4 hours, respectively.
As shown in FIG. 9, the surface hardness of the steel nitrided in the mixture baths is about 2 times higher than that of the steel before nitriding.
The present invention can solve an environmental pollution problem and can reduce a cost for nitriding steels by using molten non-cyanide salts, such as sodium nitrate (NaNO₃), sodium nitrite (NaNO₂), calcium nitrate (Ca(NO₃)₂) and their mixtures.
Since the present invention can increase the nitrided depth or nitrogen-diffusion depth of steels two to six times higher than that obtained using conventional nitriding methods, thereby nitriding the inner part as well as the surface of the metal, its applications are extended to various fields.
Since the present invention can be applied to bulk hardening as well as surface hardening of steels by increasing hardness and tensile strength of the metal, it is possible to apply the present invention to many fields including light and highly strong automobile components and diverse structural members which require improved wear resistance, corrosion resistance and fatigue life.
The present application contains subject matter related to Korean patent application No. 2006-0049077, filed in the Korean Intellectual Property Office on May 30, 2006, the entire contents of which are incorporated herein by reference.
The terms and words used in the present specification and claims should not be construed to be limited to the common or dictionary meaning, because an inventor defines the concept of the terms appropriately to describe his/her invention as best he/she can. Therefore, they should be construed as a meaning and concept fit to the technological concept and scope of the present invention.
Therefore, the embodiments and structure described in the present specification are nothing but one preferred embodiment of the present invention, and do not represent all of the technological concept and scope of the present invention. Therefore, it should be understood that many equivalents and modified embodiments that can substitute those described in this specification exist.

Claims

1. A method for nitriding a metal in a salt bath, comprising the steps of:

a) immersing at least one salt selected from a group consisting of KNO₃, KNO₂, Ca(NO₃)₂, NaNO₃and NaNO₂into the salt bath;

b) melting the salt by heating and maintaining the molten salt at a predetermined temperature; and

c) submerging the metal in the salt bath.

2. The method as recited in claim 1, wherein the predetermined temperature is within a range of 400° C. to 700° C.

3. The method as recited in claim 1, wherein, in the step c), a submerging time is within a range of 1 minute to 24 hours.

4. The method as recited in claim 2, wherein the metal is one of iron and steels.

5. The method as recited in claim 3, wherein the metal is one of iron and steels.

6. A metal nitrided in a salt bath including at least one selected from a group consisting of KNO₃, KNO₂, Ca(NO₃)₂, NaNO₃and NaNO₂, wherein the metal is iron and the iron is nitrided to a depth of 0.1 mm to 3.0 mm from the surface.

7. A metal nitrided in a salt bath including at least one selected from a group consisting of KNO₃, KNO₂, Ca(NO₃)₂, NaNO₃and NaNO₂, wherein the metal is steel and the steel is nitrided to a depth of 0.1 mm to 3.0 mm from the surface.

8. The metal as recited in claim 7, wherein the steel is at least one selected from a group consisting of ultra low carbon steel, low carbon steel, medium carbon steel, high carbon steel, alloy steel and IF steel.

9. The metal as recited in claim 8, wherein the ultra low carbon steel has a surface hardness being more than 120 Hv to equal to or less than 450 Hv.

10. The metal as recited in claim 8, wherein the low carbon steel has a surface hardness being more than 200 Hv to equal to or less than 410 Hv.

11. The metal as recited in claim 8, wherein the medium carbon steel has a surface hardness being more than 130 Hv to equal to or less than 420 Hv.

12. The metal as recited in claim 8, wherein the high carbon steel has a surface hardness being more than 150 Hv to equal to or less than 400 Hv.

13. The metal as recited in claim 8, wherein the alloy steel has a surface hardness being more than 200 Hv to equal to or less than 410 Hv.

14. The metal as recited in claim 8, wherein the IF steel has a surface hardness being more than 165 Hv to equal to or less than 400 Hv.

15. The metal as recited in claim 9, wherein the ultra-low carbon steel has a tensile strength being more than 35 kgf/mm²to equal to or less than 110 kgf/mm².

16. The metal as recited in claim 10, wherein the low carbon steel has a tensile strength being more than 45 kgf/mm²to equal to or less than 110 kgf/mm².

17. The metal as recited in claim 11, wherein the medium carbon steel has a tensile strength being more than 45 kgf/mm²to equal to or less than 100 kgf/mm².

18. The metal as recited in claim 12, wherein the high carbon steel has a tensile strength being more than 60 kgf/mm²to equal to or less than 95 kgf/mm².

19. The metal as recited in claim 13, wherein the alloy steel has a tensile strength being more than 55 kgf/mm²to equal to or less than 110 kgf/mm².

20. The metal as recited in claim 14, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

21. The metal as recited in claim 15, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

22. The metal as recited in claim 16, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

23. The metal as recited in claim 17, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

24. The metal as recited in claim 18, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

25. The metal as recited in claim 19, wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt %.

26. The metal as recited in claim 14, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

27. The metal as recited in claim 15, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

28. The metal as recited in claim 16, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

29. The metal as recited in claim 17, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

30. The metal as recited in claim 18, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

31. The metal as recited in claim 19, wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %.

32. The metal as recited in claim 14, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

33. The metal as recited in claim 15, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

34. The metal as recited in claim 16, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

35. The metal as recited in claim 17, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

36. The metal as recited in claim 18, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

37. The metal as recited in claim 19, wherein a nickel content of the steel ranges from 0.1 wt % to 10 wt %.

38. The metal as recited in claim 14, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

39. The metal as recited in claim 15, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

40. The metal as recited in claim 16, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

41. The metal as recited in claim 17, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

42. The metal as recited in claim 18, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

43. The metal as recited in claim 19, wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt %.

44. The metal as recited in claim 14, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

45. The metal as recited in claim 15, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

46. The metal as recited in claim 16, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

47. The metal as recited in claim 17, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

48. The metal as recited in claim 18, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

49. The metal as recited in claim 19, wherein a boron content of the steel ranges from 0.001 wt % to 0.1 wt %.

50. The metal as recited in claim 14, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

51. The metal as recited in claim 15, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

52. The metal as recited in claim 16, wherein a titanium content of the steel ranges from 0.1 wt % to 0.1 wt %.

53. The metal as recited in claim 17, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

54. The metal as recited in claim 18, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

55. The metal as recited in claim 19, wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt %.

56. The metal as recited in claim 14, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

57. The metal as recited in claim 15, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

58. The metal as recited in claim 16, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

59. The metal as recited in claim 17, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

60. The metal as recited in claim 18, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

61. The metal as recited in claim 19, wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt %.

62. The metal as recited in claim 14, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

63. The metal as recited in claim 15, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

64. The metal as recited in claim 16, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

65. The metal as recited in claim 17, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

66. The metal as recited in claim 18, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

67. The metal as recited in claim 19, wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt %.

68. The metal as recited in claim 14, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

69. The metal as recited in claim 15, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

70. The metal as recited in claim 16, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

71. The metal as recited in claim 17, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

72. The metal as recited in claim 18, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

73. The metal as recited in claim 19, wherein an aluminum content of the steel ranges from 0.005 wt % to 0.1 wt %.

74. The metal as recited in claim 15, wherein the ultra-low carbon steel has a carbon content ranging from at least 0.0001 wt % to less than 0.13 wt %.

75. The metal as recited in claim 16, wherein the low carbon steel has a carbon content ranging from at least 0.13 wt % to less than 0.2 wt %.

76. The metal as recited in claim 17, wherein the medium carbon steel has a carbon content ranging from at least 0.21 wt % to less than 0.51 wt %.

77. The metal as recited in claim 18, wherein the high carbon steel has a carbon content ranging from at least 0.51 wt % to less than 2.0 wt %.