ALLOY STEELS

ALLOY STEELS:

          Just as the properties of cast iron can be improved by adding some alloying elements to its composition, so can the properties of plain carbon steels be improved tremendously by addition of alloying elements. The main object of alloying in steels is :

(i) Alloy steels can be hardened by heat treatment processes to greater depth and with less distortion and less chance of cracking.

(ii) Alloying develops corrosion resisting property as in stainless steels.

(iii) Alloying develops the property of red hardness as in cutting tool.

(iv) Alloying develops the strength and toughness of steels as in high strength low alloy (HSLA) steels.

(v) Some alloy steels show a marked resistance to grain growth and oxidation at high temperatures etc., main alloying elements used are chromium, nickel, tungsten, molybdenum, vanadium, cobalt, manganese and silicon. Alloy steels are available in great variety, each one has been developed for a specific purpose. We shall study them by grouping them in 

1. Stainless steels, 
2. Tool steels, and 
3. Special steels.

Stainless steels

(i) Stainless steels:

What is stainless steel ? Types of stainless steel

          Stainless steel is iron base alloy that has a great resistance to corrosion because of this these steels are called stainless. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust (or) stainless than other types of steel. It is observed that a thin, transparent and very tough film forms on the surface of stainless steel which is inert or passive and does not react with many corrosive materials. In a temperature range of 235 to 980, it exhibits strength, toughness and corrosion resistance superior to other metals. It is thus ideally suited for handling and storage of liquid helium, hydrogen, nitrogen and oxygen that exist at cryogenic temperature. The property of corrosion resistance is obtained by adding chromium only (or) by adding chromium and nickel together. Stainless steel is manufactured in electrical furnaces.

Why stainless steel is corrosive resistance

          Stainless steels are very slightly oxidisable. Slight oxidation forms very thin film of oxide and this oxide film acts as a protective coating and in this way further corrosion is stopped. This protective film of oxide is so thin that the color and beauty of the basic materials is not affected.

Types of stainless steels

Stainless steels are further divided into the following three categories:

(a) Ferrite stainless steel:

          These steels contain a maximum of 0.15% carbon, 6–12% chromium, 0.5% nickel besides iron and usual amounts of manganese and silicon. These steels are stainless and relatively cheap. They are also magnetic. These steels have a similar micro-structure to carbon and low alloy steels. These days, one and two rupee coins are made from such steels. These steels are essentially Iron-chromium alloys and cannot be hardened by heat treatment. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. Main usage for such steel is in manufacture of dairy equipment, food processing plants, chemical industry etc., 

(b) Martensitic stainless steel:

          These stainless steels have 12–18% chromium but contain higher carbon percentage (0.15-1.2%). These are also called chromium steel, and are hardenable and magnetic. These steels can be hardened by heat treatment, but their corrosion resistance is reduced. They are used where high strength and moderate corrosion resistance is required. They have generally low weldability and formability. They are magnetic. These steels are used for making surgical knives, hypodermic needles, bolt, nut screws and blades etc.,

(c) Austenitic stainless steels:

          These are the most important and costliest among all stainless steels. Austenitic steels have austenite as their primary phase. In these steels, besides chromium, nickel is also added. Nickel is a very strong austenite stabilizers and therefore the micro-structure of these steels is austenitic at room temperature. The most common amongst stainless steel is 18/8 steel. Its composition is 24% chromium, 8% nickel, 0.08- 0.2% carbon, manganese 1.25% maximum and silicon 0.75% maximum. Such steels have extremely good corrosion resistance but they cannot be hardened by heat-treatment. However, they are very susceptible to "strain hardening". In fact, due to strain hardening, their machining becomes very difficult. It is generally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel. It is used extensively for household utensils and in chemical plants and other places where high corrosion resistance is required.

Tool steels

(ii)Tool steels:

          The requirements in a tool steel are that it should be capable of becoming very hard and further, that it should be able to retain its hardness at high temperatures commonly developed during cutting of steel and other materials. This property is called "red hardness". Further tool steel should not be brittle and should have good strength. High speed steel (HSS) is the name given to a most common tool steel. Its name implies that it can cut steel at high cutting speeds. At high cutting speed, the temperature rise is higher but high speed steel tools can retain their hardness up to 600-625. The property of red hardness comes from addition of tungsten. A typical composition of H.S.S is tungsten 18%, chromium 4%, vanadium 1%, carbon 0.75 - 1%, rest iron. Tungsten is a costly metal. It has been found that molybdenum can also impart "red hardness" to steel and actually half per cent of molybdenum can replace one percent of tungsten. Molybdenum is far cheaper than tungsten. H.S.S with tungsten are known as T-series and H.S.S with molybdenum are known as M- series steels. A very useful H.S.S has a composition of tungsten 6%, molybdenum 6%, chromium 4% and vanadium 2%, besides iron and carbon. Another version of H.S.S is called super high speed steel. It is meant for heavy duty tools and has about 10-12% cobalt, 20-22% tungsten, 4% chromium, 2% vanadium, 0.8% carbon, rest iron. These days, tools are made of tungsten carbide and other materials, besides H.S.S.

Special Alloy Steels

(iii) Special Alloy Steels:

(a) Manganese steels:

          All steels contain small amounts of manganese to mitigate the bad effects of sulphur. The true manganese alloy steels contain much larger amounts of Mn. More manganese reduces strength and ductility. Manganese steels show high percentage of elongation. Specific gravity = 7.92. Melting point = 1343°C. They have work hardening properties. Manganese steels can be forged but special care is necessary to avoid degrading the steel. After forging the steel should be heat-treated by raising it to temperature of 1010 and quenching in water. They are used for railway points and crossings, and with usage, they become more wear-resistant.

(b) Nickel steels:

         Nickel can be added in steels up to 50%. Nickel makes the steel highly resistant to corrosion, non-mgnetic, and having very low coefficient thermal expansion. Invar (Ni=36%) and super invar (Ni=31%) are the popular materials for least co-efficient of expansion and are used for measuring instruments, surveyor tapes and clock pendulums. Such steels are used for turbine blades, internal combustion engine valves etc.,

(c) Chromium steels:

         Chromium makes steel corrosion resistant, and increases its UTS. And IZOD strength. Very often alloy steels are used with both chromium and nickel being added. Ni-Cr steel wires are often used in furnaces, toasters and heaters.

(d) Silicon steels:

        A steel containing 0.05% carbon, about 0.3% Mn and 3.4% of silicon possesses extremely low magnetic hysteresis and is used widely for making laminations of electrical machines. Silico-manganese steels are also used frequently for making springs.