JPS6143430B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for forming a carbide layer on the surface of a treated material using a molten salt method. Examples of this type of method include the method of using a borate as a salt bath agent as disclosed in Japanese Patent Publication No. 47-19844, and the method of using a neutral salt as disclosed in Japanese Patent Publication No. 53-35637. Additionally, the present inventor has invented that the use of a chloride-borate mixed salt of a group element and a group element of the periodic table makes the process easier to process industrially than the above method, and has has applied. Since then, we have continued to investigate better industrial treatment methods, and when we add 2 to 10 wt% of Group Va element salts such as sodium vanadate to the above salt bath agents, the following excellent properties are exhibited, and it has been found that industrial He invented something that would be advantageous. (1) A carbide layer can be uniformly coated anywhere in the processing furnace. In particular, in the conventional method described above, the carbide layer formed on the surface of the workpiece immersed in the upper part of the furnace may be thinner than the lower part, or in some cases, no carbide layer may be formed at all. However, when group Va element salts are added, a carbide layer can be formed with a uniform thickness up to the bath surface in contact with the atmosphere in the upper part of the furnace. (2) Longer pot life. Of the conventional methods, the neutral salt method of JP-A No. 53-35637 has a relatively good pot life, but when Group Va element salts are added,
Furthermore, the pot life is extended. Why does adding Group Va element salts make the coating layer more uniform and extend the life of the pot?
The reason for this is unknown, but experiments with a large number of component compositions revealed that the effect appears when added in a range of 2 to 10 wt%. If it is less than 2wt%, the effect of addition is small, and if it is added in excess of 10wt%, the life of the salt bath will be shortened (it becomes difficult to coat the carbide layer after 10-odd hours of treatment). Further, as shown in the following examples, even if only a Group Va element salt is added as a carbide-forming metal source, no carbide will be formed, and a Group Va element or a substance containing it must be added at the same time. It is desirable to use alloy powder as the substance containing Group Va elements. Group Va element salts can be left as is if they are anhydrous, but those with crystallization water should be dried at around 200℃ for a long time before being added to the salt bath agent, or added and then dried together with the salt bath agent. It's good to do that. It is preferable that the addition be carried out at room temperature, thoroughly mixed with the salt bath agent and the Group Va element or a substance containing it, and then heated and melted. Even if a Group Va element salt is added within the scope of the present invention, other properties such as bath viscosity, evaporation, and ease of removing deposited salts do not change. Therefore, it is only necessary to newly add 2 to 10 wt% of Group Va element salt to the conventional method, and the treatment steps and treatment conditions may be the same as the conventional method. In particular, the method of using a mixed salt of a group A element of the periodic table, a chloride of a group element, and a borate of 5 to 30 mo% as a salt bath agent, which has been separately filed by the present inventors, involves adding a group Va element acid salt. It is industrially superior to add, and furthermore, when the periodic table group a element and the chloride of the group a element are Bac ₂ and a mixed salt with 5 to 15 mo% borate added, The most industrially advantageous treatment is possible. Further, even if the material to be treated is electrolyzed in the above-mentioned molten bath as a cathode, a good carbide layer can be coated. A more detailed explanation will be given below based on examples. Example 1 Nine types of salt bath compositions shown in the table were prepared. Among these, A to D are the compositions of the present invention, E to G are the compositions used for comparison, and H and I are the compositions of the conventional method.
An anhydrous primary reagent was used as Na ₂ B ₄ O ₇ , and an industrial grade reagent was used as BaC ₂ . NaVO ₃ was calculated using industrial NaVO ₃ 4H ₂ O containing water of crystallization, and assuming that all the water of crystallization was decomposed and evaporated during heating and drying.
Fe-V was used as a fine powder with a purity of 76% and a size of 100 mesh or less. After mixing these at room temperature,
It was heated and melted in the air in a SUS304 pot. The material to be treated was a SKD11 plate, and after its surface was ground and finished, it was degreased and held immersed in the bottom of the pot. The treatment temperature was maintained at 1000°C for 4 hours and then cooled with oil in the air. After washing and removing salt adhering to the surface of the treated material, X-ray diffraction and optical microscopic observation were performed to measure the type and thickness of the coating layer. The results are shown in the table, with symbols E, F,
In all cases except for I, VO carbide with a thickness of about 9 μm was formed on the surface of the treated material. In the composition of symbol E, the carbide layer was somewhat thin, and after using this salt bath for more than 10 hours, the carbide layer was no longer covered. In the composition of symbol F, no carbide layer was formed at all, and on the contrary, the surface was oxidized. In the composition of symbol I, the surface layer
A composite layer of approximately 9 μm of V ₂ O and VO was formed in the inner layer.
These results show that when the material to be treated is placed at the bottom of the pot, the compositions of methods A to D of the present invention have substantially the same carbide coating ability as comparative method G and conventional method H.

[Table] Next, after heating the same salt bath as described in the table to 1000℃,
As shown in Figure 1, the material to be treated SKD11 is
The sample was immersed in the salt bath so that mm was exposed from the surface. The shape of the material to be treated is 5 x 10 x 110 mm, and 90 mm is immersed in the molten salt bath. The inner diameter of the SUS304 pot is 46mm, and the depth of the salt bath is approximately 150mm. After thoroughly stirring the molten salt bath, the material to be treated was immersed as shown in FIG. 1, and then left as it was until the treatment was completed. The treatment time was 4 hours, and then oil was applied to wash and remove adhering salts. When the thickness of the coating layer at each position on the treated material was measured using an optical microscope, it was found that the coating ability in the upper part of the salt bath differed depending on the salt bath composition, as shown in FIG. That is, in compositions A to D, which are the methods of the present invention, the VO carbide was uniformly coated to a thickness of about 9 μm up to the very edge of the salt bath surface. However, with other methods, no carbide layer is formed at all in the upper 30 to 70 mm of the salt bath, and the layer deeper than that is gradually coated, but even in this area, the coating layer is thin and the thickness is uneven. Ta. Furthermore, the material to be treated corresponding to the position where the salt bath came into contact with the atmosphere had been eroded and reduced in size, as shown in Figure 3. Therefore, when the amount of dimensional reduction due to erosion for each composition was measured, the results were as shown in Figure 4.
In the compositions of the present invention with symbols A to D in which 2 to 10 wt% of NaVO ₃ is added, almost no dimensional reduction due to erosion is observed. Similarly, no size reduction was observed in the case of symbol E containing 15 wt% NaVO ₃ . Symbols G and I are only slightly eroded, but symbol H shows significant erosion on the steel, and the reduction in size is larger than the others. In conventional molten salt surface treatment methods that have been carried out industrially, heat-resistant steel is usually used as the pot material, and in this case, the same phenomenon as described above occurs when the pot is locally damaged in the area corresponding to the salt bath surface. It has been eroded and is reaching the end of its life relatively early. As shown in FIG. 4, the salt bath composition of the present invention has an extremely small erosive effect on steel, which is advantageous in terms of improving the life of the pot. Example 2 NaC, BaC ₂ and Na ₂ B ₄ O ₇ in molar composition ratio
Fe−V20wt in salt mixed in the ratio of 38.5:50.5:11
% and NaVO ₃ 5wt% were mixed at room temperature and heated and melted in a SUS304 pot. When the same treatment as in Example 1 was carried out, a VO carbide layer of about 9μ was coated. Furthermore, the carbide layer was formed up to approximately 10 mm from the salt bath surface, demonstrating the effect of NaVO ₃ addition. In addition, almost no reduction in size due to erosion was observed. Example 3 The molar composition ratio of KC, NaC, and Na ₂ B ₄ O ₇ is 45:
When 30wt% of Fe-Nb of 100 mesh or less and 10wt% of KNbO ₃ were mixed at room temperature in a salt bath agent mixed at a ratio of 45:13, and the same treatment as in Example 1 was carried out, approx.
A 10μ NbO carbide layer was formed. Furthermore, the carbide layer was formed to a depth of about 15 mm from the salt bath surface, and almost no reduction in size due to erosion was observed. Example 4 BaC ₂ and CaC in a molar composition ratio of 2:4:1
_2. 10wt% Fe-V powder in Na ₂ B ₄ O ₇ mixed salt
When 5wt% of CaVO ₃ was mixed at room temperature and the same treatment as in Example 1 was performed, a VC carbide layer of about 10μ was formed. Carbide was formed up to a depth of about 5 mm from the salt bath surface, and was hardly eroded. Example 5 NaC, LiC, with a molar composition ratio of 3:4:1
Fe-Nb powder of less than 100 mesh in K ₂ B ₄ O ₇ mixed salt
When 10wt% and 5wt% of KNbO ₃ were added, heated and melted, and treated in the same manner as in Example 1, approximately 10μ of
NbC carbide was formed. Carbide was formed to a depth of about 15 mm from the salt bath surface, and almost no erosion was observed.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the method of immersing a sample in order to investigate the carbide coating ability in the upper part of the salt bath, Figure 2 is a diagram showing the regions where no carbide layer is formed for each salt bath composition, and Figure 3 is a diagram showing the area where the carbide layer is not formed in the upper part of the salt bath. FIG. 4 is a schematic diagram showing the situation in which the steel of the treated material is eroded on the surface of the salt bath in contact with the salt bath. FIG. 4 is a diagram showing the amount of dimensional reduction of the treated material due to erosion for each salt bath composition. : Processed material, molten salt bath, : Pot.

Claims (1)

[Scope of Claims] 1. A salt bath agent mainly containing a mixed salt of a group a element of the periodic table and a chloride of a group a element and 5 to 30 mo% of a borate, and further containing 2 to 10 wt% of Va. A group Va element of the periodic table or a substance containing it is added to a mixture containing a group element acid salt and heated and melted to form a carbide layer of the group Va element on the surface of the material to be treated immersed in the bath. Surface treatment method. 2. The surface treatment method according to claim 1, wherein the salt bath agent is a mixed salt of BaC ₂ and 5 to 15 mo% borate. 3. Claim 1 in which the Group Va element is vanadium and the Group Va element salt is a vanadate.
The surface treatment method described in Items 1 and 2.