SPECIAL IRONS FOR WEAR, HEAT AND CORROSION RESISTANCE
In addition to the conventional Meehanite flake and nodular graphite irons for general engineering applications, certain ranges of material have been specially developed to suit environmental conditions involving wear, heat and corrosion. These are essentially types of flake and nodular graphite iron where, by suitable changes in composition, such as by alloy additions or by heat treatment, the irons have been modified to meet specific difficult conditions.
Wear is essentially a surface phenomenon. It involves many properties such as hardness, impact, fatigue under compression, friction, corrosion, thermal shock, grain structure, solidity and lubrication. If follows, therefore, that every wear problem must be carefully considered and selection of material must be made according to the service conditions.
Normal flake and nodular graphite irons show different reactions to changes in temperature. For example, up to 500² C, all irons show a rapid drop in tensile strength and hardness. Above 500² C, structural breakdown and the effect of atmospheric conditions affect service life.
No single type of iron can successfully withstand all possible thermal environments. Special types of iron are therefore required. Selection of these must be based on various conditions, such as type of service, temperature cycle and rate of heating, condition of stress, both mechanical and thermal, resistance of the material to oxidation and thermal cycling, and machinability requirements
Meehanite Metal castings possess superior resistance to corrosion as compared with ordinary cast iron, due to the close grained structure, uniform dispersal of graphite and freedom from porosity. In addition, Meehanite Metal castings have the advantage of close control in their manufacture.
Where the service conditions are so severe that it is necessary to use special material such as stainless steel, none of the standard Meehanite types, possessing a pearlitic matrix, can be recommended, and one of the austenitic types should be used.
Three low alloy pearlitic type of Meehanite are available for corrosion resistance, their main characteristic being extremely low sulfur contents. Type CC is intended for resistance to acids and general chemicals. Type CB3 for resistance against concentrated sulfuric acid and Type KC for resistance to alkali.
The austenitic range of irons are high alloy materials which provide a range of specific corrosion resisting properties, while certain of the higher alloy types possess particularly valuable heat-resisting properties. They are austenitic at ambient temperature due to the presence of such alloys. The carbon can be present either as flake graphite (the CR series) or as nodular graphite (the CRS series).