Three necessary properties in material used at elevated temperatures:

• Retention of strength at the working temperature
• Resistance to oxidation and to scaling.
• Structural stability as regards carbide precipitation, spheroidisation, sigma formation and temper embitterment.

In particular applications other properties may also be important, such as coefficient of expansion for constructional units and resistance to penetration by products of combustion in many furnace applications, specific resistance and temperature coefficient for electrical purposes.

It has to be considered in gas turbine steels additional characteristics especially as large rotors may have to be built of small sections welded together. To internal damping capacity and fatigue strength, notch sensitivity and impact strength (hot and cold), machining and welding characteristics.

The main scale which forms on iron is porous and loosely adherent but in the addition of certain elements to the steel it's protective a rendered adherent. The elements are chromium, silicon and aluminum their great affinity for combining with oxygen make them characterized, but the formation of inert oxide films make the reaction rapidly stifled.

By heating at 1000°C in contact with powdered aluminum, or by metal-Spraying the steel surface with aluminum, coating with bit mastic paint to prevent oxidation and heating to 780°C, Which make the resistance of mild steel to oxidation vastly enhanced.

Enhanced creep strength attained by alloying elements to raise the softening temperature. (Hardening):

At the working temperature to prevent willingly over-ageing we need to wisely Use precipitation hardening.

Second phase dependent on the degree and uniformity of the dispersion achieved.

Creep rate related to a critical range of particle spacing.

To reduce the extent of the primary creep stage we Control the degree of work-hardening in the appropriate temperature ranges.

A marked effect on creep properties is done by the alteration in the process of manufacture, deoxidizers and particles in the crystal boundary.

Vacuum melting permits the use of advantageous compositions which cannot be melted by conventional methods. It also ameliorates ductility in the transverse direction.

Mechanical properties are ameliorates by the supplementation of various elements:

Cobalt, tungsten and molybdenum cause the steels to withstand the action of tempering.

High alloy nonmagnetic solid solution of ferric carbide steels (austenitic) have no change points so it doesn't harden by air cooling, but their resistance to wear resistance isn't major.

Silicon and chromium in sufficiency elements raise the Ac, point to temperatures above the ones reached in service, and prevent the steel from air hardening on cooling.

Steels with high nickel content can't be used at high temperatures because intercrystalline films of nickel sulphide will be formed in contact with gases containing sulphur dioxide or other sulphur compounds.

In high chromium steels the carbides coalesce into large particles which have less obstructive action on grain growth of the ferrite at temperatures above 700°C.

The extreme grain growth still reduces the toughness which these steels possess. Also occurs in the austenitic steels above 1000°C, since they remain tough and ductile even in the coarse grained condition there is no problem.

When heated in the range 500-900°C austenitic steels precipitate carbides along the boundaries of the austenitic steels, and as a result intercrystalline cracks are liable to develop, if the steel is stressed continuously in this temperature range.

With certain compositions both ferritic and austenitic steels are embitter by the formation of sigma phase.