Metallographic structure of heat-resistant cast iron

The metallographic structure of heat-resistant cast iron is mainly composed of metal matrix, graphite phase and special phase formed by alloy elements. The following is a detailed introduction:

Metal matrix

Ferrite matrix: It has good toughness and anti-growth performance, can remain stable at high temperature, and reduce the volume change and microcracks of castings. For example, the matrix of medium silicon cast iron is mainly ferrite.

Pearlite matrix: It has high strength and hardness, but relatively poor heat resistance. In some heat-resistant cast irons, the pearlite content needs to be controlled within a certain range to ensure the heat resistance of the material.

Martensitic matrix: Heat treatments like quenching can produce a martensitic matrix, enhancing the strength and hardness of cast iron. However, due to its relatively low toughness, additional processes such as tempering are typically necessary to improve its ductility.

Graphite phase

Flake graphite: In gray cast iron, flake graphite weakens the matrix and reduces strength and toughness, but provides good damping and lubricity. In heat-resistant gray cast iron, the size, quantity and distribution of flake graphite will affect its heat resistance.

Spheroidal graphite: Spherical graphite in ductile iron minimizes matrix disruption, significantly enhancing strength, toughness, and thermal fatigue resistance. Medium silicon-molybdenum ductile iron belongs to this category, and the spheroidization rate is required to be ≥85%.

Vermicular graphite: Vermicular cast iron contains worm-shaped graphite, with properties between flake and spheroidal graphite. It offers good heat resistance and mechanical strength.

Special phases formed by alloying elements

Carbides: Chromium and molybdenum form stable carbides like Cr₇C₃ and Mo₂C, enhancing hardness, wear resistance, and thermal stability of cast iron.

Silicides: Silicon forms silicides like FeSi and Fe₂Si with iron, enhancing oxidation resistance, growth resistance, and strengthening the matrix.

Borides: Boron in heat-resistant cast iron forms borides like Fe₂B and Fe₃(C,B), improving hardness, wear resistance, and heat resistance.