Alloys or metallic compositions – Containing over 50 per cent metal but no base metal – Iron containing
Reexamination Certificate
1999-02-24
2001-06-19
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Containing over 50 per cent metal but no base metal
Iron containing
C420S586100, C420S429000, C420S052000, C420S010000, C420S011000, C420S102000, C420S107000
Reexamination Certificate
active
06248292
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an overlaying alloy advantageously usable for overlaying an engine valve of diesel engines using light oil as a fuel, engines using compressed natural gas (CNG) or liquified petroleum gas (LPG) as a fuel, or other engines in which the oxidizing power of the combustion atmosphere is weaker than that of gasoline engines.
2. Description of the Related Art
FIGS. 1A and 1B
show an engine valve
1
having a valve face
2
, which is required to have good properties including wear resistance, non-damaging property, heat resistance and thermal shock resistance over a wide temperature range from room temperature to elevated temperatures. The term “non-damaging property” means that an overlaid part does not significantly damage a counterpart which is in butting relationship with the overlaid part. The term “damaging property” means the intensity at which an overlaid part damages a counterpart. The heat resisting steels commonly used as a material of engine valves cannot satisfactorily provide these properties, and therefore, alloys having these properties are overlaid on the valve face to impart the required properties.
When a gasoline engine is operated, a combustion product is formed on an overlaid layer of the engine parts such as a piston ring, a valve face, and a valve seat and suppresses wear which would otherwise occur in such parts. The combustion product is substantially composed of oxides of the alloy components of the overlaid layer and acts as a protective film providing solid lubrication.
However, diesel engines using light oil as a fuel and engines using CNG, LPG or other gases as a fuel provide a combustion atmosphere having a weak oxidizing power such that the combustion product substantially composed of oxides hardly forms on the surface of the engine parts. As a result, no effective lubrication occurs and an overlaid layer of a wear resistant material only causes metal-to-metal contact between the parts leading to significant wear.
A known solution to the above problem is adding Cr and/or Mo in an alloy composing the parts of diesel engines and engines using CNG or LPG as a fuel to allow Cr oxides and/or Mo oxides to be formed on the surface of the alloy and to provide solid lubrication.
For example, Japanese Unexamined Patent Publication (Kokai) No. 59-100263 discloses an iron-based alloy as a material for thermal-spraying the pistons of diesel engines, which alloy consists of 1-4 wt % C, 10-30 wt % Cr, 10-30 wt % Mo, 1-5 wt % Nb, 20-40 wt % Co, 2-15 wt % Ni and the balance of Fe and unavoidable impurities.
Stellite alloy is generally used as an alloy for overlaying the valve face of the engine valves of engines using CNG or LPG as a fuel, which alloy consists of 1.2 wt % C, 29.8 wt % Cr, 0.01 wt % Mn, 1.35 wt % Si, 4.7 wt % W and the balance of Co and unavoidable impurities.
However, the above recited conventional overlaying alloys cannot satisfactorily provide non-damaging property and good wear resistance at practically acceptable level, and therefore, an overlaying alloy having an improved non-damaging property and wear resistance is desired.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an overlaying alloy for overlaying the parts of diesel engines and engines using CNG, LPG or other gases as a fuel, which alloy allows formation of the oxide film thereon necessary for providing effective solid lubrication for an improved non-damaging property and wear resistance.
To achieve the object, the present inventors conducted various studies and found the following novel facts.
FIG. 2
shows the relationship between the heating temperature and the percent weight gain due to oxidation for simple Mo, simple Cr, Mo carbide (MO
2
C) and Cr carbide (Cr
3
C
2
) powder samples when oxidized by heating in air. The samples are bonded with oxygen or oxidized to form oxides and are increased in the weight, so that the percent weight gain of the powder sample indicates the degree of oxidation.
It can be seen from
FIG. 2
that the simple Mo and the Mo carbide show percent weight gains both being increased with a rise in the heating temperature, indicating that the degree of oxidation increases. The percent weight gain is increased mainly by an increase in the amount of Mo oxide.
The simple Cr and the Cr carbide show percent weight gains both being only slightly increased with a rise in the heating temperature, indicating that the degree of oxidation hardly increases. The percent weight gain is increased mainly by an increase in the amount of Cr oxide and the result shows that the Cr oxide is not substantially formed.
The relationship between the oxidation speed and the diffusion speed of an alloying component is believed to be essential to the formation of an oxide on the surface of an alloy by oxidation of the alloying component. Cr is relatively easily oxidized to form an oxide film on the surface of an alloy but the diffusion speed of Cr in an alloy is relatively low. The oxidation speed of Cr is significantly higher than the diffusion speed of Cr, so that the Cr present in the vicinity of the alloy surface is selectively oxidized to form a thin Cr oxide coating on the alloy surface. The thus-formed Cr oxide coating has a very dense structure to prevent entrance of oxygen from outside. Therefore, it is believed that oxidation of Cr does not significantly proceed when the heating temperature is raised. The thin Cr coating covers the alloy surface to prevent contact of the Cr present therebelow with oxygen and further formation of Cr oxide does not occur. Cr carbide has a diffusion speed lower than that of Cr and oxidation of Cr carbide is also suppressed for the same reason as Cr.
In contrast, Mo is different from Cr in that Mo is easily oxidized to form an oxide on the alloy surface, and at the same time, the diffusion speed of Mo in an alloy is low. Moreover, an Mo oxide coating formed on the alloy surface is not so dense as the Cr oxide coating and does not prevent the entrance of oxygen from outside. Therefore, even if the Mo present in the vicinity of the alloy surface is oxidized to form a thin Mo oxide coating on the alloy surface, the Mo diffuses from inside of the alloy, reaches the alloy surface, and is oxidized by oxygen entering the alloy through the thin Mo oxide coating, so that the formation of Mo oxide proceeds. The diffusion speed of Mo carbide is lower than that of Mo but the difference from the oxidation speed is not so significant as for Cr and, therefore, the oxidation of Mo carbide also proceeds with a rise in the heating temperature similarly to Mo.
Cr oxide has a standard free energy of formation smaller than that of Mo oxide and is formed more easily than the latter. Therefore, when both Mo and Cr are present, Cr and Cr carbide are oxidized before the oxidation of Mo and Mo carbide and form a thin Cr oxide coating on the alloy surface to prevent further formation of Cr oxide and Mo oxide.
Thus, when an alloy contains Cr, a thin Cr oxide coating is formed on the alloy surface, covers the alloy surface and prevents further formation of oxides, so that the amount of Cr oxide or Mo oxide is insufficient to provide effective solid lubrication for a good non-damaging property and wear resistance.
The present invention is thus based on the novel finding that an overlaying alloy containing either no Cr or a small amount of Cr is advantageously usable for overlaying an engine valve of diesel engines using light oil as a fuel, engines using compressed natural gas (CNG) or liquified petroleum gas (LPG) as a fuel, or other engines in which the oxidizing power of the combustion atmosphere is weaker than that of gasoline engines.
According to the first aspect of the present invention, there is provided an overlaying alloy, containing no Cr, comprising:
20-70 wt % Mo,
0.5-3 wt % C,
5-40 wt % Ni, and
the balance being Fe and unavoidable impurities.
The alloy containing no Cr of the first aspect is particularly advantageous for the uses in which the combustio
Ando Kimihiko
Ito Eiji
Manabe Akira
Yasuda Akio
Burns Doane , Swecker, Mathis LLP
Toyota Jidosha & Kabushiki Kaisha
Yee Deborah
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