Effects Of Nb In High CE Gray Iron And Its Application For Production Of Brake Discs

Niobium is widely used to be a trace alloying element in steel production. Recently, the application of niobium in cast iron is becoming increasingly attractive and it has been thought to be one of important elements in the production of high temperature wear-resisting cast irons.In this thesis, the behavior of niobium element in high carbon equivalent gray cast iron, the effect on structure and mechanical properties and its mechanism was investigated. Some craft technology as the addition of niobium was performed as well. Based on these results, the application of niobium in high carbon equivalent gray cast iron brake discs was conducted.The experimental results indicate that there are two forms of niobium in high CE (CE=3.8-4.6%) gray iron, solid solution and niobium-rich phase. Because of the good affinity of niobium with carbon and nitrogen, it would form some hard phases such as Nb(C,N).There are three kinds of niobium-rich phases in high CE gray cast iron, block-like phase NbC; strips ( including Y-style and V-style ) and carburizing type with an unified whole with the fine cementite of pearlite. When the addition of niobium is lower than 0.3%, the niobium-rich phases tend to form stripes; and when the addition is high enough, block-like niobium-rich phases can be formed.Niobium cannot change the type of flake graphite. When niobium added in gray cast iron, long and coarse graphite becomes short, curl and fine. Primary graphite would disappear when the graphite amount reaches a critical value. Eutectic temperature decreases with the addition of niobium, which enlarges super-degree and having a refine effect on eutectic cell, refining and stabilizing pearlite. At the same time, lamellar spacing of pearlite reduces, but there is no effect on the amount of pearlite. Niobium purifies grain boundary, increases bonding force between cells and improves hardness, strength, and toughness. With increasing of niobium addition, the micro-hardness of pearlite improved due to solute strengthening effect and refined solidification structure.When niobium is added to 0.1% in the high carbon equivalent gray cast iron brake discs, the solution strengthening effect is obvious.With an addition of 0.1% niobium in high carbon equivalent gray iron, ideal graphite of type A is obtained and the obtained materials processing a good properties as regenerative ability, conduction capacity and damping capacity. Hardness and wedge strength increased slightly to make up for the shortage of gray iron. Hot cracking property and thermal fatigue resistance of high carbon equivalent gray iron brake disc also increased. The dispersedly and uniformly distributed Nb(C,N) hard phase improves the wear resistance effectively. The gray cast iron of niobium addition also has good mechanical processing properties and casting performance. All above advantages make niobium gray cast iron ideal material of high CE brake discs in medium and advanced cars.