STAINLESS STEEL -

Stainless steels are essentially those containing chromium, together with other elements such as nickel and are grouped as under:

Austentite stainless steel -

Most important'under this group is that containing 15 to 20% chromium and 7 to 10% nickel. A steel containing 18% chromium and 8% nickel is very widely used and is commonly referred to as 18/8 stainless steels. Additions of molybdenum are made to certain grades to increase their corrosion resistance while other have titanium or niobium added to stabilise the carbon. They cannot be hardened by heat treatment. Austentite in these steels is stable at all service temperatures. Austentite stainless steels are non - magnetic.

Martensite Stainless Steel -

This group often termed plain chromium type stainless steel, which contains 10 to 14% chromium and with the odd exception, have no other major alloying elements. These steels are all hardenable by heat treatment.

Ferrite Stainless steel -

This group contains mainly 14 to 18% or 23 to 30% chromium again with no other major alloying element. They cannot be hardened by heat treatment. Stainless steels have great strength and toughness and extremely good resistance to corrosion and heat. The heat resisting properties are increased by the addition of about 3% silicon. They find wide applications in the brewery, dairy and chemical industries, in making cutlery, all types of surgical and dental instruments, household appliances. Considerable amountsare also used for nuclear power stations.

When the carbon content is reduced to as low as 0.09%, the produce is called stainless iron. Irons of this classification usually have chromium in excess of 22%, but they may range from 20 to 30%. They are highly resistant to corrosion and they may be used in parts for heat exchangers, conveyor chains, furnaces, strokers and other similar high temperature applications.

TITANIUM-

Metallic titanium has a bright silvery lustre, and when polished, resembles steel in appearance. The high purity metal has a relatively low tensile strength (14 tons/sq.in) and high ductility (40%), but commercial grades of the metal may contain impurities which raise the tensile strength to 45 tons/sq. in and reduce ductility to 20%. A titanium alloy containing 4% aluminium and 4% manganese has a tensile strength of 65 tons/sq. in.

Since the specific gravity of titanium is only 4.5, the metal has a high strength/weight ratio. At room temperature, this ratio shows little advantage over some of the aluminium and magnesium alloys. But titanium retains its strength at much higher temperatures than these alloys.

Titanium finds application in* the compressors of jet aero - engines, where the modern tendency is to increase the compression ratio with consequent increase in thermodynamical heating. Moreover, titanium has a high melting point (1725°c) coupled with a good resistance to corrosion.

Molten titanium combines readily with oxygen and nitrogen, and must therefore be melted in a vacuum. It also reacts readily with all known refractories and is melted by means of an electric arc in a water - cooled copper crucible, on to which molten titanium does not weld.

Titanium finds application as a grain refiner in the heat treatment process. Small amounts of titanium are also added which inhibit carbide precipitation and make post - welding heat - treatment uncecessary to the problem of "weld - decay" in fabricated articles of stainless steel.

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