KR100791210B1 - Nitriding treatment method about the gray cast iron - Google Patents

Abstract

Optimal ion nitriding conditions are provided to be able to minimize influence of graphite exposed onto a surface of gray cast iron. An ion nitriding method of gray cast iron comprises the steps of: grinding a surface of gray cast iron and removing organic matters from the gray cast iron; vacuuming a pressure of a reaction furnace to 3.0 torr; supplying a gas mixture in which argon is mixed with hydrogen at a volume ratio of 1:2 into the reaction furnace; maintaining temperature in the furnace to 550 deg.C and generating DC plasma in the furnace to perform a pre-sputtering process on the grinded surface of the gray cast iron for 1 hour; vacuuming pressure inside the furnace to 3.0 torr after finishing the pre-sputtering process; supplying a gas mixture in which nitrogen is mixed with hydrogen at a volume ratio of 3:1 into the furnace; increasing an internal temperature of the furnace to 595 deg.C and performing a nitriding process on the gray cast iron for 14 hours; and furnace-cooling the gray cast iron in a nitrogen atmosphere for 1 hour after finishing the nitriding process.

Description

Nitriding treatment method about gray cast iron

1 is a schematic diagram of an ion nitriding device applied to the present invention

Figure 2 is the pre-sputtering hardness curve of the mixed gas ratio of the present invention

Figure 3 is the pre-sputter hardness curve for each vacuum negative pressure of the present invention

4 is a pre-sputtering hardness curve according to the present invention temperature

5 is an ion nitride treatment hardness curve according to the gas mixture ratio of the present invention

6 is an ion nitride treatment hardness curve according to the present invention temperature

7 is an ion nitride treatment hardness curve according to the vacuum negative pressure of the present invention

8 is an ion nitride treatment hardness curve according to the treatment time of the present invention

9 is an ion nitride treatment hardness curve according to the present invention temperature

10 is a time-dependent ion nitride treatment XRD pattern of the present invention

11 is a graph of the graphite thickness curve according to the present invention temperature and time

12 is a polarization curve of the present invention 2% sulfuric acid treatment

13 is an XRD pattern of the high temperature oxidation test of the present invention

14 is EDS line curve of the high temperature oxidation test of the present invention

The present invention relates to a method of ion nitriding the surface of gray cast iron used as a mold.

Cast iron is a main alloy containing 2 to 5% of carbon and has excellent castability, dust absorption and mechanical properties compared to the price. Especially, gray cast iron containing 2.5 to 5% of carbon and 0.8 to 3% of silicon is α ferrite. B. A structure in which black and light graphite is present in the form of a thin plate in a pearlite structure, and the fracture surface is gray, and is used for various applications.

Disadvantages of using gray cast iron as a mold include low conductivity, durability, and abrasion resistance to stress and thermal environments, and microcracks generated due to interfacial separation due to thermal expansion coefficient difference between matrix and graphite propagate along graphite. This affects not only the loss of the mold itself but also the surface of the product.

The conventional method for surface modification was ion nitridation, which can be processed at low temperatures, does not easily crack, has very few dimensional changes after treatment, and enables uniform surface cure over complex shaped products. Unlike steel, cast iron, unlike steel, had a problem that graphite exposed on the surface acts as an obstacle to penetration and diffusion of active species nitrogen ions, thus forming a discontinuous compound layer having a sawtooth shape on the surface after nitriding. .

The present invention was developed to solve the above problems, and an object of the present invention is to provide an optimal ion nitriding treatment condition that can minimize the effect of graphite exposed on the surface of gray cast iron.

In the present invention, experiments were conducted under various process conditions and the chemical tendencies and mechanical properties of nitrides formed on the surface were investigated in order to derive the optimal ionization treatment conditions for gray cast iron.

The test specimens used were FC-25 gray cast iron, and the chemical composition is shown in Table 1.

After the specimen was processed to a size of 20 x 10 x 5mm, the surface was polished using diamond steel paper # 400, # 600, # 800, # 1200, and polished (diamond) to # 3㎛, # 1㎛.

And before nitriding, each specimen was ultrasonically cleaned in acetone solution for 5 minutes to remove organic matter from the surface, washed with alcohol and dried by hot air.

After the prepared specimen was charged in the reactor, the exhaust port was opened and exhausted to 3.0 torr.

Next, the mixed gas was supplied at the selected mixed gas ratio, and a DC plasma was generated to perform pre-sputtering under the selected process conditions.

After the pre-sputtering, H 2 and N 2 gas, which is a reactor body, were supplied at a selected mixing ratio, and the temperature was raised to each nitriding temperature while maintaining a constant pressure.

After nitriding, the DC power supply was turned off and the furnace was cooled in a nitrogen gas atmosphere for 1 hour.

A schematic diagram of the ion nitriding device is shown in FIG. 1, and the nitriding treatment conditions are shown in Tables 2 and 3.

The treated specimens were cut and microstructure and component analysis were observed through an optical microscope (Nikon-EPI photo), SEM and EDS (JOEL: JXA-860). While moving at intervals, the load was measured at 25gf and a load time of 10 seconds.

Using CuKα (λ = 1.504) target XRD analysis to a range of 20 ° ~ 80 ° at a scanning rate of 0.02 ° / sec to analyze the resulting phase after the nitriding treatment.

In order to evaluate the oxidation resistance at high temperature, the weight change was measured by treating nitrided specimens and untreated FC-25-based gray cast iron at 800 ° C for 24 hours and 60 hours, and the formed phases were analyzed by XRD analysis. The thickness and structure were confirmed by EDS analysis.

In addition, a corrosion test was performed with a high density carbon electrode as a counter electrode on the Ag / AgCl reference electrode.

The electrolyte used in the experiment was bubbled with high-purity argon gas for 30 minutes to remove dissolved oxygen and fix the exposed area to 1 cm².

V/sec이었다.Potential Scanning Rate is 1x

V / sec.

)의 질량은 수소이온(

)의 질량보다 아주 크기 때문에 소재표면의 산화피막을 큰 가속에너지로 두드려 효과적으로 제거시키는 기계적 작용을 한다.Argon ion (

) Mass is hydrogen ion (

Because it is much larger than the mass of), it has a mechanical action to effectively remove the oxide film on the surface of the material with a large acceleration energy.

이온은 소재의 산화를 방지하고, 스퍼터되어 분해된 산소 포텐셜을 낮추는 화학적 작용을 한다.

The ions act as a chemical to prevent oxidation of the material and to lower the sputtered and decomposed oxygen potential.

This interaction was utilized to investigate the effect of the argon / hydrogen ratio on the ion nitridation during pre-sputtering prior to ion nitridation (FIG. 2).

At this time, the pre-sputtering conditions except for the gas mixture ratio were made constant (T = 550 ° C., P = 3 Torr).

Fig. 2 shows the cross-sectional hardness at the time of processing while varying the gas ratio of argon / hydrogen.

Here, argon / hydrogen = 1/2 shows the most effective nitride layer in terms of thickness and hardness.

The results show that the results may vary depending on the degree of oxides or other organics on the surface of the specimen when subjected to nitriding under the same conditions. The ratio of argon and hydrogen with different functions during pre-sputtering does not show a tendency in one direction. It can be seen that the nitride is best formed at argon / hydrogen = 1/2, which is expected to be optimal.

As the sputtering pressure decreases, the mean free path of the particles increases, and thus the distance of electron acceleration increases, so that the average energy of the electrons increases.

Increasing the average energy of the electrons increases the amount of ions excited by the electrons with high energy, which increases the sputtering rate up to a certain pressure.

In sputtering, the reduction in furnace pressure can have the same effect as an increase in DC power density, which will double the decarburization and thus activate the specimen surface further, as it sputters with higher energy at relatively high pressures.

3 is a result of examining the effect of the pressure in the furnace during the sputtering of the pre-sputtering factor on the subsequent ion nitriding behavior.

At this time, pre-sputtering conditions except vacuum were argon / nitrogen = 1/2, temperature = 550 ° C., and vacuum = 3.0torr.

From 3 to 20㎛ has a high hardness value in the case of 2torr but when it is 30㎛ shows a tendency to decrease rapidly.

3torr shows that the diffusion layer depth is kept constant at a higher value up to 40 µm than at other vacuum levels.

In pre-sputtering, the result of processing by varying the temperature is shown in FIG. 4.

At this time, pre-sputtering conditions except for temperature were argon / hydrogen = 1/2 and vacuum = 3torr.

Although the hardness is high at 525 ℃ and 550 ℃, the depth is kept constant up to 30㎛ in case of 550 ℃ and the microstructure is considered. I think it is the best.

The results are shown in FIG. 5 when the ion nitriding treatment is performed while varying the ratio of hydrogen gas to nitrogen gas.

At this time, the ion nitriding conditions except gas mixing ratio were constant at temperature = 550 ° C., vacuum = 3.0torr, and 6hr.

5 shows the cross-sectional hardness and the nitride layer thickness.

When nitrogen / hydrogen = 3/1, it can be seen that a high hardness and a thick nitride layer were formed.

As the amount of nitrogen for the nitriding reaction increases, the nitride layer becomes deeper, but when the nitrogen gas is reacted, the nitride layer is thinner and the hardness is lower than that of nitrogen / hydrogen = 3/1.

In general, in the ion nitriding, when hydrogen is mixed in the atmosphere, plasma generation is easy, and thus plasma generation of nitrogen is also easy, thus increasing the formation rate of the nitride layer.

As the amount of nitrogen increases, the compounding ratio with Fe increases, and the hardness and thickness are deep, but the plasma generation of nitrogen and the stability of the nitride layer formation should be considered together.

The results of the treatment temperature change in ion nitriding are shown in FIG. 6.

At this time, ion nitridation conditions except for temperature were carried out in the same manner as nitrogen / hydrogen = 3/1, temperature = 550 ° C., and time = 6hr.

It can be seen from FIG. 6 that the surface hardness increases with temperature and the thickness of the nitride layer increases from the surface.

Referring to FIG. 9, which shows the XRD results, Fe₄N is generated in the nitride layer as Fe, FeN, and Fe 3 N are produced and the temperature is increased.

Moreover, the peak of Fe was not confirmed at 595 degreeC.

In the nitriding treatment, it is thought that as the temperature rises, the diffusion of nitrogen is accelerated and thus a very strong Fe₄N phase is formed to improve the overall hardness value.

The result when the vacuum degree was changed is shown in FIG.

At this time, the nitriding conditions except vacuum were set to nitrogen / hydrogen = 3/1, temperature = 575 ° C., and time = 6hr.

As a result of the treatment in 3torr, the hardness value is higher than that in other vacuum degree up to 50㎛.

In 4torr, the plasma was not well formed during the operation, which made the operation difficult.

If the degree of vacuum is too high or too low to cause a reaction, it is difficult to reach nitrogen ions or particles with energy for reacting on the surface.

8 and 10 show the results of the treatment with varying time in the ion nitriding treatment.

At this time, other ion nitriding conditions were made constant by nitrogen / hydrogen = 3/1, temperature = 575 ° C., and vacuum = 3torr.

8 shows the cross-sectional hardness and nitride layer thickness with time, and FIG. 10 shows the phase of nitride.

As the time increases, the thickness of the nitride layer increases because the reaction with nitrogen increases, and as shown in the XRD results (FIG. 10), Fe gradually decreases and Fe FeN is formed.

As a result, in a short time, the diffusion layer appears to form a deep layer over time, compared to that strengthened by the compound layer on the outermost surface.

In this experiment, the trend was confirmed, and the hardness value showed similar tendency over 14 hours.

11 shows the results of measuring the thickness of the graphite after nitriding in order to investigate the effect of the nitriding treatment on the shape of the graphite.

It was confirmed that graphite expanded as temperature and time increased.

This is because even though the structure of the FC-25 gray cast iron is made from graphite and cementite and nitriding occurs under the transformation point, cementite decomposes to form FeN-based nitrides, thereby promoting the growth of graphite.

This reaction causes the growth of graphite as the temperature rises, and the growth of graphite causes the brittleness to check the growth degree and then to determine the characteristics of the material according to the growth.

The corrosion test results when the anode polarization characteristics of the nitrided specimens were observed by the coin-top method are shown in FIG. 12.

Here, the corrosion potential (Ecorr) showed a nearly constant potential (-0.5V) for the ionized and non-ionized specimens.

When the potential rises above the corrosion potential, the nitriding treatment shows that the current density increases and then the current decreases to -0.25V.

In comparison, the untreated specimens did not degrade much in current density.

This is thought to be that when the potential is increased, the nitride slows the movement of electrons and the current decreases.

On the other hand, untreated specimens exhibited higher corrosion rates as their potentials increased.

FC-25 cast iron containing iron as its main component is easy to oxidize.

In this experiment, a high temperature oxidation test was carried out to investigate whether the nitride had an oxidation resistance by forming a nitride layer on the surface of the specimen by ion nitriding.

The results are shown in FIGS. 13 and 14.

FIG. 13 shows the XRD results of the specimens subjected to the high temperature oxidation and the untreated specimens, whereby peaks of Fe, FeO, and Fe₂O₃ were observed on the surface of the specimen when subjected to high temperature oxidation at 800 ° C.

의 결합이 불충분하게 되므로 전체의 산화피막은 얇게 되고 산화를 방지한다.If a nitride layer is present on the surface of the specimen,

Because of insufficient bonding, the entire oxide film becomes thin and prevents oxidation.

FIG. 14 shows that the diffusion of oxygen by the nitride is hindered as a result of the oxidation of the EDS line profile after the high temperature oxidation experiment at 800 ° C. for 60 hours.

In addition, as the oxide thickened, brittleness occurred, and the oxide layer was separated.

The following results were obtained by investigating the characteristics of nitride formation by ion-nitriding the FC-25-based gray cast iron.

1. Prior to ion nitriding, argon / hydrogen gas mixture ratio, working temperature, and vacuum degree were pre-sputtered with variables, and ion nitride treatment under the same conditions showed that the formation degree of nitride was different in each specimen.

This suggests that the results may vary depending on the degree of oxides or other organics on the surface of the specimen even under the same conditions, and at the same time, the effects of argon (mechanical) and hydrogen (chemical) with different functions during pre-sputtering Experimental results show that ratio, temperature, and vacuum degree can be most effectively influenced under proper conditions without showing tendency in one direction.

In this experiment, nitride was best formed at the gas mixing ratio argon / hydrogen = 1/2, temperature 550 ℃ and pressure 3torr.

2. After the pre-sputtering treatment, the ion nitriding treatment was performed according to the conditions of each process variable. As a result, the tendency was observed through the increase of hardness.

Compared to the base metal, the hardness value was about 200 Hv, and the nitrides formed on the specimen surface were FeN, Fe₃N, and Fe₄N.

Nitride / hydrogen = 3/1, temperature = 595 ℃, time = 14hr, and vacuum = 3torr showed the most effective nitride production conditions in this experiment.

3. When ionization of FC-25 gray cast iron was observed, the growth of graphite was almost linearly proportional to temperature and parabolic in time.

Corrosion test results of the nitrided and untreated specimens in 2% sulfuric acid solution showed improved corrosion resistance.

In addition, as a result of the high temperature oxidation experiment at 800 ℃ for 24, 60 hours, mainly Fe₂O₃ oxide was formed and the oxidation rate was decreased by nitride.

As described above, the present invention has an effect of making the surface of gray cast iron modified for use in molds and the like by providing an optimal ion nitriding method while minimizing the influence of graphite contained in a large amount of gray cast iron.

Claims (1)

In the method for ionizing the gray cast iron in the reactor, Polishing the gray cast iron surface and removing organic matter; Vacuuming the pressure of the reactor to 3.0 torr; Mixing and supplying argon and hydrogen in a volume ratio of 1: 2; Generating a DC plasma while maintaining the temperature in the furnace at 550 ° C. and pre-sputtering for 1 hour; Vacuuming the pressure in the furnace to 3.0 torr after the pre-sputtering is finished; Supplying nitrogen and hydrogen in a volume ratio of 3: 1; Raising the temperature in the furnace to 595 ° C. and nitriding for 14 hours; Ion nitriding treatment method for gray cast iron, comprising the step of furnace cooling for 1 hour in a nitrogen atmosphere after nitriding.

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