JPH02207974A - Surface hardening method for titanium or titanium alloy - Google Patents

Abstract

PURPOSE:To provide a hardened layer having a sufficient thickness with a high adhesive property by melting a build up material consisting of a mixture composed of Cr3C2 and Ti or Ti alloy in such a manner as to generate mixing with a base metal, thereby build-up hardening the base metal. CONSTITUTION:The build up material 2 in which the Cr3C2 and the Ti (or Ti alloy) coexist is welded to the base metal 1 consisting of the Ti or Ti alloy. Since the m. p. of the Cr3C2 is approximate to the m. p. of the Ti, the entire part thereof melts once and is nearly homogenized. Carbide TiC4 of the titanium precipitates in the process when the molten metal is cooled and solidified. The build-up layer is hardened as this carbide is finely distributed in the molten metal. Since the Ti component exists in the molten metal, the molten metal fits more easily to the base metal than in the case of merely welding the Co alloy. In addition, this melting is so executed as to generate mixing of the base metal and the build up material and, therefore, the content of the Ti component is higher in the part of the molten metal nearer the base metal.

Description

[Detailed description of the invention]

FIELD OF INDUSTRIAL APPLICATION The present invention relates to a method for hardening the surface of mechanical parts made of titanium or titanium alloys. 00 alloy, typified by Stellite J, is overlaid with gold and then hardened.On the other hand, recently, with the improvement of engine performance, valves are also made of Ti alloy. However, even when a commonly used Co alloy is overlaid on Ti or Ti-gold alloy, it tends to crack and peel from the interface between the base material and the overlay metal layer. Nitriding is a surface hardening technique for Ti and Ti alloys. This involves heating Ti or Ti alloy parts to high temperatures in an N2 gas atmosphere. , hardening is achieved by forming a TiN phase on the surface layer. However, the thickness of the hardened layer that can be formed is as thin as a few μm, and when it wears away or peels off, the wear resistance is lost all at once. Another method is the carbide build-up method. This method uses WC powder in combination with pure Ti powder to harden the build-up, but WC segregates due to gravity in the molten pool. As a result, the distribution of carbides becomes uneven, and it is difficult to expect much hardening in the critical surface layer.

[Problem to be solved by the invention]

The purpose of the present invention is to form a hardened build-up layer with sufficient thickness and high adhesion on the surface of mechanical parts made of Ti or Ti alloy, thereby obtaining a durable and wear-resistant surface. The purpose is to provide a method.

[Means to solve the problem]

The method for surface hardening titanium or titanium alloy according to the present invention includes T
On the surface of the base material made of i or Ti alloy, Cr3
The overlay material made of a mixture of C2 and Ti or a Ti alloy is melted so as to be mixed with the base material, and then overlay hardened. The Ti alloy to which the present invention can be applied has Ti as a base material.
-6AI -4Vk: Representative t'L, in addition to those containing Ti as the main component and a relatively small amount of alloy components added thereto, as well as those containing intermetallic compounds TiA, I) as the main component, This includes those to which relatively large amounts of alloy components are added. The Ti or Ti alloy used for overlay is preferably the same type as the base material, but even if the base material is T: alloy, NTj
Alternatively, a Ti alloy different from that of the base material may be used. Cr3G2 and Ti (or Ti alloy) used for overlay
The composition of the mixture may be selected such that Cr3C2 is in the range of 10 to 70% (by weight), typically 30 to 60%. The powders may be used as a mixture, or the powder mixture may be formed into a rod or the like using an appropriate binder. Melting must be accomplished by supplying sufficient heat so that mixing of the overlay material and base material occurs. For this purpose, means such as plasma arc welding and TIG welding are suitable. For the melting atmosphere, it is easy to work if an inert gas such as Ar is used, but it is also possible to use a CO2,0
It is also advantageous to use an atmosphere containing controlled amounts of 2, N2. It is preferable to melt the weld metal again after overlaying once, because the hardness distribution becomes uniform. [Function] When overlaying a Co alloy on the surface of Ti or a Ti-based alloy, the adhesion of the overlay metal layer is not good because a layer of intermetallic compound TiCo or Ti 2 Co is formed at the interface. It turned out that it was for the purpose of Although such intermetallic compounds are hard in themselves, they are brittle, so if they form in a layer at the interface between the built-up part and the base material, they are likely to peel off from there. According to the invention, as seen in FIG. 1, Ti or T:
When welding the overlay material (2) containing a mixture of Cr3C2 and Ti (or Ti alloy) to the alloy base material (1), Cr3C
Since the melting point of No. 2 (1895° C.) is close to the melting point of Ti (1670° C.), the entire material once melts and becomes almost homogeneous, and in the process of cooling and solidifying, titanium carbide TiC (4) is precipitated. When this is finely distributed in the molten metal, the built-up layer is hardened. Since the molten metal (3) contains a Ti component, it is more compatible with the base metal than when simply welding a CO alloy. Moreover, since this melting is carried out so that the base material and the overlay material are mixed, the content of the Ti component is high in the portion of the molten metal that is close to the base material. If this is shown schematically, it will look like the graph in Figure 2. In other words, when using conventional build-up welding using Stellite as the build-up material, the hardness distribution from the surface to the inside is as shown by the thin line, where the hardness of Stellite itself is maintained from the surface to a certain depth; There, the hardness increases rapidly and then suddenly decreases to the hardness of the base material. This rapid increase in hardness is due to the above-mentioned intermetallic compound JiCo or T.
This is due to the generation of i 2 Go. On the other hand, when a mixture of Cr3C2 and Ti (or Ti alloy) is used according to the present invention, the hardness is maintained over a considerable thickness as shown by the thick line in FIG. The distribution shows a continuous decrease from the molten part toward the base metal. Since a brittle layer is not formed between the built-up metal layer and the base metal, there is no need to worry about peeling. As is easily understood, if the usage rate of Cr3C2 is increased, the amount of carbide produced will increase to a certain extent,
The wear resistance of the built-up hardened layer becomes high, and on the other hand, if the proportion of Ti or Ti alloy is increased, the adhesion with the base material becomes even better. The thickness of the molten metal layer (3) can be varied within a fairly wide range by adjusting the amount of filler material (2) used and the amount of heat given to the welded area, and can be made considerably thicker if desired. Therefore, even if the surface is polished after overlay welding and scraped off to the position shown by the broken line in FIG. 1, a sufficient hardened layer can still remain.

[Example 11] A flat plate of Ti-6Aρ-4v alloy was used as the base material, and C
A mixture of r3C2 powder and the same Ti-6A, I)-4 alloy powder as above in the following proportions was overlay welded by the plasma arc method (current 100 A) in the respective atmospheres. In some cases, the weld metal was remelted by heating using a plasma arc alone. No Cr3C2 Excess atmosphere Remelting Ar Yes Ar No Ar Yes Ar Yes Ar Yes 3%02-Ar Yes 3%02-Ar No 5%Co2A r Yes 5%N2-Ar Yes Thickness of built-up part was approximately 1.5 m. We investigated the hardness distribution near the interface between the base material and the built-up layer up to the surface of the hardened build-up layer. The results are shown in Figure 3 for N (11~5, NQ6~
9 are shown in FIG. 4, respectively. Comparison of Nα2 and Nα1, 3 to 5 in Figure 3, and the fourth
A comparison between Nα6 and Nα7 in the figure clearly shows the effect of remelting to make the hardness uniform, and a comparison between Figures 1 and 2 shows that controlling the atmosphere during backfill welding is effective. [Example 21] A pure Ti flat plate was used as the base material, and a build-up material in which Cr3 c2 powder and pure Ti powder were mixed so that the former was 50% by weight, and under the same conditions as Example 1. Overlay hardening was performed. In addition, a portion was subjected to remelting treatment. Examine the hardness distribution of each and get the results shown in Figure 5)
It's q. [Example 3] A fill-up material containing 35% by weight of Cr3C2 powder and the remainder being the same Ti-6AfJ-4V alloy powder was applied to the face of an engine valve made of Ti-6AI-4V alloy.
Overlay welding was performed by the plasma arc method (current 98 A, in an Ar atmosphere). After finishing the face by grinding, we examined the hardness of the part that spans the hardened layer and the base metal. The results are shown in FIG. Effects of the Invention By performing overlay hardening by the method of the present invention, a sufficiently thick hardened layer with high adhesion can be provided on the surface of the Ti or Ti-gold alloy base material. Therefore, the surface hardening method of the present invention is useful for manufacturing parts that are required to be lightweight and wear resistant, such as internal combustion engine valves, rocker arms, connecting rods, or piston pins.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a base material and build-up weld metal showing surface hardening performed by the method of the present invention. FIG. 2 is a graph schematically showing differences in hardness depending on surface hardening materials. FIG. 3, FIG. 4, FIG. 5, and FIG. 6 are all graphs showing the hardness distribution near the built-up hardened layer obtained in the examples of the present invention. 1...Base metal 2...Welding material 3...Weld metal Patent applicant Daido Steel Co., Ltd. Agent Patent attorney Suga So Otsutsu 311 □Wa□ No, 1 1.0 0.5 0 etc. I or 3=-Cicada 0.5 (mm) 1.0 Rei→ Internal wholesale Figure 4 1--@--- No, 6] Ω 0.5 0 Cicada i6-Exclusive (mm)

Claims (4)

[Claims] (1) On the surface of the base material made of Ti or Ti alloy, melt the overlay material made of a mixture of Cr_3C_2 and Ti or Ti alloy so that it mixes with the base material,
A method for surface hardening Ti or a Ti alloy, which comprises hardening overlay. (2) The surface hardening method according to claim 1, wherein the method is carried out in an atmosphere containing oxygen and/or nitrogen, and hardening is also performed by solid solution of these gas components. (3) The surface hardening method according to claim 1 or 2, further comprising a step of remelting the built-up portion. (4) The surface hardening method according to any one of claims 1 to 2, which is applied to a Ti alloy valve face.