As an alloying element in steel, chromium increases the hardenability and in association with high carbon gives resistance to abrasion and wear. 4%is present in high speed steel and up to 5% is present in hot die steels. In Structural steels it may be present in amounts up to about 3 %. Simple chromium-carbon steels are used for ball bearings having high elastic limit and high uniform hardness due to the uniform distribution of the hard carbide particles, but for most structural purposes chromium is used in conjunction with up to 4 % nickel and small amounts of molybdenum or vanadium. Chromium is unique in its effect on resistance to corrosion and scaling and is an essential constituent in all stainless steels, e.g., stainless cutlery steels contain 12% to 14% chromium, whilst in steels of the austenitic corrosion-resisting type, 18% chromium is associated with 8% nickel, and small amounts of other elements. In heat-resisting steels, chromium is present in amounts up to 30%, and it is an important element in many of the highly alloyed heat-resisting materials, whose iron contents are so low that they cannot be regarded as steel. Chromium is also used as an alloying addition to high duty cast irons. Silicon is a powerful deoxidizer, and as such is used in steel making processes in amounts up to about 0•8%. When used as an alloying element, silicon in small percentages will increase the tensile strength and yield point of structural steels. It is used in amounts of 1•5% to 2% in silicon-manganese spring steels and ultra-high tensile steels due to its effect in raising the limit of proportionality and resistance to tempering. Up to 4% in heat resisting steels improves scale resistance owing to the formation of a protective layer (see also Ihrigizing). The higher the silicon, the higher the temperature at which protection against further atmospheric oxidation is given. Water vapour and carbon dioxide, however, attack the layer. Alloys of iron and silicon, containing 15% of the element, are used as acid-resisting materials, but have the properties of cast irons rather than of steels. Carbon-free alloys with up to 4% silicon have a high electrical resistance and low hysteresis loss, and are used as transformer steels. In cast iron, silicon not only serves as a deoxidizer but also has a marked graphitizing effect, thus improving machinability.