Development of Vanadium-added Heat Resistant Cast Iron for Exhaust Manifold 2010-32-0073
The exhaust gas is getting to higher temperature in order to adapt higher power and lower fuel consumption of the engines, and the exhaust manifolds tends to use in more severe conditions. The exhaust manifolds of 660cc engines of the turbo specification had been made of the high-Si spheroidal graphite cast iron containing a small amount of Mo so far, but now it cannot help using Ni-resist for higher heat resistance. However, Ni-resist is extremely expensive because it contains much 35wt% nickel of the rare metal. Therefore, the development of the new cast iron having the performance and the cost between the conventional cast iron and Ni-resist is demanded. With such a background, the new spheroidal graphite cast-iron “Vanadium cast iron” for the low-cost exhaust manifolds that greatly improved the heat resistance than the conventional cast iron was developed.
First of all, evaluating the test materials that added various alloy elements to the conventional cast iron, vanadium addition was found to be extremely effective to the high-temperature strength improvement of the conventional cast iron. In addition, optimizing the amount of V and Si addition improved greatly the high-temperature properties compared to the conventional cast iron.
This developed cast iron shows the excellent tensile property compared to the conventional cast iron, on tensile strength and proof stress about 50% or more from 200°C to 800°C region, maintaining the ductility at the room temperature. In addition, this developed cast iron has superior the thermal fatigue life than Ni-resist was showed by the severe durability bench test for the surface temperature of the exhaust manifold to exceed 880°C.
By developing “Vanadium cast iron”, the substitution from Ni-resist used for the exhaust manifolds with the high thermal stress has been achieved, and the cost reduction of about 60% has been attained.