Special Alloy Steels

Quenched and Tempered Steels

The composition of this steel is

The steel is water quenched from 1,650 °F and tempered at 1,150 - 1,200 °F. The resulting structure is tempered Martensite. In addition, the yield strength is above 90,000 psi, tensile strength - 105,000 psi, elongation is 2 inches at least 16 %. It is used for welded structure such as pressure vessels, bridges, heavy machinery. It has excellent impact resistance at low temperatures.

Corrosion-resistant steels:

They may be divided into 2 categories

a) those which are only corrosion resistance

b) those which combine good appearance with corrosion resistance.

In both groups, chromium is the principle alloying element.

4 to 6% chromium is added to low carbon steel, a product result which has improved resistance to corrosive media. These steels are used for chemical industries. It has usually 0.4 - 0.8 % silicon and 0.5 % molybdenum. Carbon content is kept as low as possible usually under 0.15%. For severe corrosive condition 11-18% chromium may be used.

Stainless Steel is commonly used to indicate any or all rustless steels or iron alloys designed to resist atmospheric corrosion, the attack of hot or cold acids, and scaling at elevated temperatures.

Ultra-high Strength Steels:

By adding some alloying elements and by cold working high strength stainless steels can be formed. They are called superalloy.

Managed steel starts with a low carbon steel with a composition of 18 - 25% nickel along with 7% cobalt, 5% molybdenum, 0.4% titanium and traces of zirconium, aluminum, boron, silicon and manganese. The steels are then heated to 1500° F for 1 hour per inch of thickness, air cooled to room temperature, then reheated to 900° F for about 3 hours. The steels can be cold worked to achieve tensile strengths of close to 300000 lb/in². They can be welded, but must be carefully heat treated after the weld.

If a chromium -molybdenum -vanadium steel is heated to the austenite range, quenched or cooled very rapidly to a temperature of about 900° F, rolled or forged at this temperature, then rapidly quenched to room temperature, an ausformed steel is the product. They have tensile strength upto 370,000 lb/in².

Heat Resisting Steels:

These steels are suitable for working at high temperatures. It is used for rockets, jet aircraft, and ballistic missiles.

• These steels have good strength characteristics, corrosion resistance, and creep resistance of high temperature.
• Offer high resistance to oxidation and scaling
• Possess structural stability at high temperature. Micro structural changes reducing their strength do not occur;
• Have high creep resistance.

The ferrous alloys are used below 1,400° F and are of 2 types with carbon less than 0.1% carbon. These are

• 18-8 stainless steel stabilized with either columbium or titanium. Such alloys have 1,000 hour rupture stress at 1,400° F of 6,000 to 7000 psi with higher strengths of low temperature.
• Other uses 13 - 16% chromium, 25 - 26% nickel, 1.5 - 3% molybdenum and 2.4% titanium and gives a 1,000 hour rupture strength of 14,000 - 18,000 psi at 1400° F.

Alloy steels containing 23 to 30% chromium with carbon less than 0.35 % are used for service at temperatures between 815° C and 1150° C. Furnace parts, annealing boxes and other equipment requiring high temperatures are often made of these steels

Shock Resisting Steels:

These steel resist shock and severe fatigue stresses. One grade of steel contains 0.50 %  carbon, 2.25 % tungsten, 1.50% chromium and 0.25% vanadium. Another grade of shock resisting steel, known as silicon manganese steel contains 0.55% carbon, 2.00% silicon, 0.80 % manganese, and 0.30 percent molybdenum.

Magnet Steel:

High cobalt steels are used for making of permanent magnets for magnetos, loud speakers and other electrical machines and instruments. Steels having 15 to 40% cobalt, 0.4 to 1% carbon, 1.5 to 9% chromium and upto 10% tungsten possess improved magnetic properties.

Maraging Steels

These steels are used in aluminum die casting dies, inserts, cores, extrusion dies, forging dies, punches, cold heading dies, die holders etc.

Maraging steels are low carbon, high -nickel alloy steels capable of attaining yield strengths in excess of 185 MN/m² in combination with good fracture toughness.

Those alloys contain carbon about 0.02%, Ni 18%, Co 8% and molybdenum 5% along with small amounts of titanium and aluminum as hardening agents.

This alloys is martensitic in the annealed condition and acquires ultra high strength by an aging treatment.

Hadfield's Manganese Steel

It is an austenitic steel possessing excellent strength and toughness with high wear resistance under impacts loads as experienced in material handling and earth moving machinery such as crushers, shovels, bulldozers, coal grinding mill rings etc.

Hadfield's manganese steel contains C 1.2-1.4%, Mn 12-14%.  Hadfield's manganese steel is austenitized by heating it to 1050°C -100°C, kept at that temperature for sufficient time and then quenched in water. Before using this steel, it is plastically deformed on its surface by hammering. Thus, a layer of austenite at the surface, where impact takes place, gets converted into martensite. This layer of Martensite is extremely hard and gives excellent wear resistance backed by tough austenite.