High Silicon Cast Iron

High silicon iron-based alloys are excellent corrosion resistant materials with advantages of superior corrosion resistant properties, low cost and saving on noble metals such as nickel and chromium. When containing 14.5% or more silicon, the corrosion resistance of these iron-based alloys is better than that of some cast irons containing a significant amount of chromium, nickel and/or molybdenum, and even than that of certain nickel based alloys in some severe corrosion conditions.

However, because of its intrinsic properties, the high silicon iron-based alloys have some weaknesses such as low strength and impact toughness, high hardness and brittleness, poor weld ability, low thermal conductivity and high thermal expansion (coefficient), resulting in high tendency for shrinkage/porosity and other casting defects in production, unable to handle thermal shock and high pressure, and poor machine ability. Thus, to some extent, the application and promotion of the high silicon cast iron alloys has been limited only to castings.

At present the main products of the material in domestic market are pump parts and some small anode castings, while the study about using the material for large castings is very limited due to the poor properties. With the operating requirements on large repair parts of batch descaling in our steelworks (these sorts of products are entirely depended on import, the over size is 2 500 mm_470 mm_370 mm), the research on the stain resistance of high silicon iron-based alloys in nitric acid was referred, properties in the electricity added condition. The rare earth high silicon iron-based alloy studied in this article can be used to make the auxiliary anode which worked in the added electricity anode protecting condition, showing a promising application future.

High silicon cast irons represent the fifth class of corrosion-resistant cast irons. The main alloying element is 12 to 18% Si, with more than 14.2% Si needed to develop excellent corrosion resistance. Chromium and molybdenum are also used in combination with silicon to develop corrosion resistance to specific environments. High silicon cast irons represent the most universally corrosion-resistant alloys available at moderate cost. When silicon levels exceed 14.2% high silicon cast irons exhibit excellent resistance to H₂SO₄, HNO₃, HCl, CH₃COOH, and most other mineral and organic acids and corrosives. These materials display good resistance in oxidizing and reducing environments and are not appreciably affected by concentration or temperature. Exceptions to universal resistance are hydrofluoric acid (HF), fluoride salts, sulfurous acid (H₂SO₃), sulfite compounds, strong alkali’s, and alternating acid-alkali conditions.

ASTM Standard Specification according to : ASTM A 518/ 518 M Grade: 1, 2 & 3 and on the other hand corrosion resistant high silicon cast iron is standardized to BS 1591; 1949/1995 which contains data about chemical analysis and heat treatment. Table 1 shows the chemical composition according to BS 1591.

Characteristics and application

Table 1: Chemical analysis of corrosion resistant high silicon cast iron

With a higher content of Si, corrosion resistance is greater with a simultaneous loss of relatively good mechanical properties and machinability. Because of this, for an application it's necessary to specifically decide on chemical analysis of this material. The Si Content has to be between 14 and 16%, the reason being is a presence of a continual protection film of SiO₂. This film is easy formed on the casting surface because of its great affinity Si to oxygen.

Structure

Ferrum with strong energy in combination with Si makes solid solution, lowering the meting point of Fe. The temperature interval between liquidus and solidus is extremely small, so it's difficult to form Si segregation. Silicon is an element which makes a smaller y area of ferrum. Because of that, modification points are at higher temperatures, so 17% Si moves these for 50°C and explains the reason for the forming of rough gain during annealing, recrystallization and quenching.

There is influence of carbon on the Fe/Si system diagram but that doesn't change the present phases configuration, but does make a smaller solidification interval of system and also Si content at the eutecticum. Carbon present in high silicon cast microstructures exists like lamellar graplut in the form of fine lamellas, rough lamellas and in the form of small bars. Sometimes in the microstructure carbide is present, especially in case of rapid cooling, immediately after solidification.

Figure 1: Microstructure of high silicon cast iron

Mechanical properties and corrosion resistance are lower with an increasing grain size, and especially with regards to inter-crystalline corrosion resistance lowering.

Technological characteristics

High silicon cast iron has good castability. This characteristic is connected with higher Si content. Pry the rule, for casting, the most usable alloy is with eutectic content which is best because of low melting point and small solidification temperature interval.

Corrosion resistance characteristics

Corrosion resistance characteristics of high silicon cast depends on the formed film silicon dioxide (SiO₂). Ferrum atoms excrete from the silicon-ferritic lattice and residual Si atoms are oxidized and so forms the protective film of silicon oxide which stops further corrosion. High silicon cast iron is resistant to sulphuric and nitrogen acid action though a very large range of concentrations and temperatures, and also to mixes of sulphuric and nitrogen acids. High silicon cast iron is resistant to phosphoric acid action at room temperature, organic acids at all temperatures and concentrations of nitrogen oxides at higher temperatures, carbon disulfide solution at higher temperatures, solutions of aluminum chloride, aldehyde and other aggressive mediums.