Method of treating the surface of TI or TI alloy valve element

Abstract

 A covering layer made of heat-resistant steel is formed on the outer circumferential surface of a stem of a Ti or Ti alloy valve element in an engine valve. Then, nitriding is applied to the covering layer to achieve deeper diffusion of nitrogen into the layer, thereby forming a relatively thick nitrided layer.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of treating the surface of a valve element in an engine valve moulded from Ti or Ti alloy, and especially a method of treating the outer circumferential surface of a stem of the valve element to improve wear resistance thereof.

[0002] To increase allowable rotation speed of an engine, the largest obstacle is increase in inertia mass owing to weight of a valve operating mechanism. When the total weight of parts of the valve operating mechanism becomes larger, the higher rotation speed becomes, the lower followability of a cam to a valve element becomes, so that whirling motion occurs in the valve element, thereby involving decrease in output power. Thus, instead of a conventional heat-resistant steel, the valve element is moulded from Ti or Ti alloy having low specific gravity and high heat resistance, thereby lightening the valve element. However, in an engine valve in which the valve element is moulded from Ti or Ti alloy, Ti or Ti alloy has Rockwell hardness (HRC) of about 30 to 40, thereby decreasing wear resistance of a stem which is always guided to a valve guide and slidably reciprocates at high speed, so that it is liable to cause baking or galling.

[0003] To solve the disadvantage, Japanese Utility Model Laid-Open Pub. No.4-103211 describes an engine valve in which salt bath soft nitriding is applied to the outer circumferential surface of a stem to increase wear resistance thereof. But, in the engine valve, when soft nitriding is applied directly to the surface of Ti valve element for normal treating time such as 1.5 to 2 hours, a nitrided layer (hardened layer) is relatively thin, which does not satisfy suitable wear resistance and fatigue strength. Therefore, it required very long time to form a preferable nitrided layer, thereby decreasing productivity remarkably.

SUMMARY OF THE INVENTION

[0004] To solve the disadvantage, it is an object of the present invention to provide a method of treating the surface of Ti or Ti alloy valve element in an engine valve to improve wear resistance and durability of the valve.

[0005] According to the present invention, there is provided a method of treating the surface of Ti or Ti alloy valve element in an engine valve, the valve element comprising a head and a stem, the method comprising the steps of forming an Fe covering layer on the outer circumferential surface of the stem which slidably contacts a valve guide; and applying nitriding to said covering layer to form a nitrided layer.

[0006] After Fe covering layer is formed on the outer circumferential surface of the stem, nitriding is applied to the covering layer, so that nitrogen deeply diffuses compared with a conventional engine valve in which nitriding is applied directly onto Ti. Thus, relatively thick nitrided layer is formed. Accordingly, wear resistance is remarkably increased at the valve stem which slidably contacts a valve guide, thereby providing a durable engine valve.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The feature and advantage of the present invention will be described with respect to the following embodiment based on drawings wherein:

Fig. 1 is a central longitudinal sectional front view of an embodiment of an engine valve according to the present invention; and

Fig. 2 is an enlarged view of portion "A" in Fig. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0008] Fig. 1 illustrates an engine valve according to the present invention, in which a valve element 3 which comprises a head 1 and a stem 2 is integrally moulded from Ti or Ti alloy (Ti-Al-V). On the whole surface of the valve element 3 except the head, there is formed a covering layer 4 from thermal spraying material of a heat-resisting steel or alloy steel such as Fe, Ni or Co as base. The covering layer 4 may be made of the thermal spraying material like powder, rod or wire with thermal spraying means such as plasma and gas flame to provide thickness of 50 to 200µm.

[0009] Prior to formation of the covering layer 4, the outer circumferential surface of the valve element 3 may be a rough surface 5 by blasting treatment as shown in Fig. 2, thereby increasing bond strength of the spraying material to the surface of the element to increase peel resistance of the covering layer 4.

[0010] On the surface of the covering layer 4, there is formed a nitrided layer 6 with thickness of 10 to 30µm. The nitrided layer 6 is easily formed by general salt bath soft nitriding (Tufftriding). For example, a treating bath which contains KCN, KOCN and Na₄ [Fe(CN)₆] as main ingredient is heated to about 600°C, and the valve element 3 is immersed in the bath for 1 to 2 hours. Owing to diffusion in the surface layer of the steel covering layer 4 with cementation and nitriding, there is formed a compound (nitrided) layer which contains very hard carbide and nitride. The compound layer provides excellent wear-, galling- and baking-resistance, thereby increasing wear resistance not only of a portion of the stem which slidably contacts a valve guide 7, but also of a stem face 8 which contacts a locker arm and a tappet (not shown), a valve face 9 which contacts a valve seat (not shown) and a cotter groove 10.

[0011] As mentioned above, in the foregoing embodiment, after the covering layer 4 made of steel is formed on the surface of the Ti valve element, soft nitriding is applied on the surface of the covering layer 4 to form the nitrided layer 6, so that diffusion of nitrogen becomes deeper than a conventional direct nitriding on the surface of Ti valve element, thereby forming a thick nitrided layer 6 for usual treating time.

[0012] To form the covering layer 4, padding may be applied instead of thermal spraying, and material for thermal spraying may comprise cheaper carbon- or stainless-steel instead of the foregoing heat-resisting steel. Nitriding may be gas-, liquid-, ion- or gas-carburizing nitriding instead of the salt bath soft nitriding.

[0013] The foregoing merely relates to an embodiment of the present invention. Any changes and modifications may be made by person skilled in the art without departing from the scope of claims wherein:

Claims

1. A method of treating a surface of Ti or Ti alloy valve element in an engine valve, the valve element comprising a head and a stem, the method comprising the steps of:

forming an Fe covering layer on an outer circumferential surface of the stem which slidably contacts a valve guide; and

applying nitriding to said covering layer to form a nitrided layer.

2. A method as defined in claim 1 wherein the step of forming the covering layer comprises thermal spraying.

3. A method as defined in claim 1 wherein the step of nitriding comprises salt bath soft nitriding.

Drawing