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Understanding Valve Seats
Creating a product that performs...

Valve Seat is a critical component in an Engine which undergoes repetitive cycle of hammering by the valves at elevated temperatures during functioning. As a result of this, mechanical deformation of the valve seats is introduced by the thermal stress and strains. The result of expansion and contraction of the material as a function of temperature, the pressure built up during the ignition, varies and the complete ignition of the fuel is not ensured. There arises a greater need for quantitative data information on thermal expansion of the material at the elevated temperatures involved during combustion.

Further the thermal conductivity of the valve seat material is equally important which keeps the valve seat at lower temperature when the engine is running at the optimum efficiency which ensures the heat generated in the combustion chamber is transmitted to the water cooled cylinder head.

Further valve seats properties are influenced by the corrosion due to chemical reaction continuously taking place at elevated temperatures near the valve seat area. The wear resistance property of valve seat material at elevated functional temperature also plays an important role.

Keeping in view the optimum temperature at which the engine has to function and the type of fuel used the valve seat material is designed.
There are two types of valve seat manufacturing methods: Casting Route and Powder Metallurgy Route. In casting route valve seat material is available in the following range:

Iron , Nickel ,Cobalt and Chromium Base.
1. Density and Compactness: The advantage of going in for shell stack molding method is to reduce machining allowance all over without affecting the cooling rate of the molten metal poured into the mold. This ensures closer grain structure which ultimately results in better wear resistant property, compactness of the molecules of the grain structure and leaves no porous structure in the valve seat material. This casting route process either centrifugal or shell stack molding method ensures maximum density for a given chemical composition. In the Powder metallurgy route the compactness of the grain structure is not ensured because the carbide forming elements present in the structure does not completely fuse with the basic matrix and hence leaves the porosity which affects the density of the valve seat material as a whole which is not desirable.

2. Machinability: In the Cast Route method the Homogeneity of the material is ensured if the cooling rates are controlled which results in uniform and smooth machining properties. In Sintered route the homogeneity of the material is not ensured and hence the machinability of the valve seat angle in a valve seat when it is machined is not uniform. This will result in erratic tool consumption pattern at the final stage of seat angle machining.

3. Thermal Expansion: Cast Route valve seats have homogeneous matrix structure and the grain sizes are uniform and compact. Hence the thermal expansion is lower than the sintered route valve seat material as the sintered route material has open grain structure which facilitates thermal expansion which is not desirable in the functioning of the engine.

4. Thermal Conductivity: Cast Route valve seat material has lesser thermal conductivity than the sintered route valve seat. This is the only advantage in sintered route valve seat.

5. Heat Resistant Properties: Valve seats in cast route can have any material composition with Iron, Chromium, Nickel and Cobalt as base material and Tungsten Moly, Vanadium, Manganese as alloying elements. Carbon & Chromium usage is no bar, whereas in Sintered route material Carbon and Chromium used in excess of 1.2% and 3% respectively, the homogeneity of the material is not ensured and hence as detailed above the machinabiltiy will be a problem. 

Cast Route valve seats having material composition Iron, Chromium, Nickel and Cobalt as base material and Tungsten  Moly, Vanadium, Manganese as alloying elements can have very good heat resistant properties at very high elevated temperatures up to 1200"K. the valve seat will not distort and the property known as Hot Hardness is achieved at elevated temperatures.

Since Copper/Lead are used as binding/filling material between the grain boundaries of the sintered route material valve seats the continuity of compactness of the material is not ensured. Hence for heavy fuels having CV high these materials have limitations in their applications.

6. Corrosion and Wear: Cast Route valve seats having material composition Iron, Chromium, Nickel and Cobalt as base material and Tungsten Moly, Vanadium, Manganese as alloying elements the corrosion will be minimal as compared to sintered valve seat, because of very high percentage of free carbides present in cast route material which will provide superior resistance to abrasion, corrosion, oxidation, and sulfidation. In addition, they provide excellent dimensional stability at all temperatures and hot hardness to 1600°F (871°C).


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