Types of steel and cast iron

Alloy Steel

Steel in itself is an alloy of iron with main carbon along with elements like chromium, manganese, nickel, silicon, vanadium, molybdenum, etc. Alloying of steel is done in order to acquire desired characteristics.

Types of Alloy Steels

1. HSLA(High Strength Low Alloy Steels) Steel

Also called micro-alloyed steels are designed to provide better mechanical properties, better resistance to environmental corrosion than conventional carbon steels. They are designed to meet the required mechanical properties than acquire a certain chemical composition. Hence, the composition of the alloy depends on the mechanical property to be acquired. HSLA in plate or sheet form has low carbon content (0.050 to .25%) to increase the formability and weldability of steel with manganese content up to 2%. Chromium, nickel, copper, nitrogen, vanadium, molybdenum, niobium, titanium are also added in small amounts for various combinations.

1. Stainless Steels

All true stainless steels contain a minimum of 11% Chromium which makes them excellent to corrosion. The secret lies in the protective surface layer of chromium oxide that forms when the steel is exposed to oxygen. Chromium causes the austenitic region to shrink and hence the ferrite region increases in size making it a ferrite stabilising agent.

Types of Stainless Steels

1. Ferritic Stainless Steels

These contain 12% to 25% Cr and less than 0.1% carbon. Because of B.C.C structure, the ferritic stainless steels have good strengths and moderate ductility obtained from strain hardening and solid solution strengthening . They are magnetic and are not heat treatable. They have excellent corrosion resistance,

2. Martensitic Stainless Steels

These combination contain 12% to 25% Cr and 0.1% to 1.0%C. A 17%Cr, 0.5%C alloy heated to 1200ºC forms complete 100% austenite, which transforms to martensite on quenching in oil. The martensite is then tempered to produce high strengths and hardness.

3. Austenitic Stainless Steels

Nickel, which is an austenite stabilizing element, increases the size of austenite field while nearly eliminating ferrite from the iron chromium-carbon alloys. If the carbon content is below, about 0.3%, the carbides do not form and the steel is virtually all austenite at room temperature. The austenite stainless steels have very good properties like ductility, formability and corrosion resistance with F.C.C structure. These are non-magnetic, which is an advantage for many applications.

Tool Steels

Tools steels are high carbon steels that obtain high hardness by quench and temper heat treatment. Hardenability and stability at high temperatures are the results of alloying elements. Tools steels may be Water hardenable steels or oil hardenable steels. Tool steels are mainly used in making cutting tools for machining, die casting as high strength, hardness, toughness and temperature resistance is needed of them. With a carbon content between 0.7% and 1.5%, tool steels are produced carefully under controlled conditions to produce the required quality. The manganese (Mn) percentage is often kept low to minimize the possibility of cracking.

Weldability of Steels

Weldability of steels refers to the ability of steels to be joined by welding. As steels are used as important structural components along with the strength of steels, joints must also be strong for the integrity of structures. Many low carbon sheets of steel weld easily. Welding of medium and high carbon steel is comparatively more difficult since martensite can form in the heat affected zone rather easily, thereby causing a weld with poor toughness. Several strategies such as preheating the material or minimizing incorporation of hydrogen have been developed to counter these problems. In low carbon steels, the strength of the welded zones is higher than the base material itself. This occurs as a result of the formation of pearlite microstructure during cooling.

Embrittlement of Steel

Embrittlement is defined as the complete loss or partial loss of ductility in a material where an embrittled product characteristically fails by fracture without considerable deformation. The embrittlement in steel is usually encountered in galvanized steel related to aging phenomena, cold working, and absorption of hydrogen. Hydrogen Embrittlement may also be caused due to the of atomic hydrogen that gets absorbed by the steel. The susceptibility of a steel to hydrogen Embrittlement is influenced by the type of steel, previous heat treatment process, and degree of previous cold work. Loss of ductility of cold-worked steels is in inter-relation with many factors like the type of steel (aging characteristics, strength level), thickness of steel specimen, and degree of previous cold work, and is increased considerably by areas of stress concentration such as caused by sharp bends , holes, notches, fillets of small radii, bores, etc.

Cast iron , composition and use

Cast iron is essentially an iron-carbon alloy along with the presence of manganese, sulphur ,phosphorus, and silicon in varying amounts. The cast iron contains a much higher proportion of carbon than is found in steel. Cast iron may have 2.5 percent to 5.5 percent carbon content. Whilst the cast iron can be cast, it can be forged. Cast iron is brittle, its brittleness is a factor of carbon composition. Sometimes CI. may easily be broken if a heavy hammer blow is done upon it

Classification of cast iron

The cast iron may be classified as:

1. Gray cast iron

It contains 2.3-3.8%C, 1.2-2.8% Si, 0.4-1.0% Mn, 0.15% and 0.1% S. when the carbon is not chemically combined with iron, it is known as free carbon or graphite carbon and the resulting cast iron is known as gray cast iron. Two conditions are responsible for formation of grey CI

1. a slow rate of cooling
2. the presence of silicon will cause the carbon to change from the combined form of the graphitic form to produce gray cast iron.

It is marked by in presence of flakes of graphite in a matrix of ferrite, pearlite or austenite

Uses :

1. Automatic parts such as cylinder block and cylinder heads
2. Machine cool structure example bed, frame etc
3. Ductile or nodular cast iron

Nodular or ductile cast iron contains 3.2-4.2% C, 1.1- 3.5% Si, 0.3-0.8% Mn, 0.08 % P and 0.2% S. Graphite appears as rounded particles or nodules or spheroids form. The spheroidizing elements, when added to the melt, eliminate sulphur and oxygen (from the melt), which changes solidification characteristics and possibly account for the nodulization. Ductile cast iron possesses very good machinability.

Uses:

1. Power transmission equipment
2. Farm implements and tractors
3. White cast iron

. The white iron contains 1.8-3.6% C, 0.5- 2.0% Si, 0.2-0.8% Mn, 0.18 % P and 0.10% S. when iron and carbon are chemically combined in the form of Cementite, the iron is known as the white cast iron. The presence of Cementite or iron-carbon Fe3C either free or lamellar pearlite makes the metal hard and brittle White cast iron under normal circumstances is brittle and not machineable.

Uses:

1. For producing malleable cast iron.
2. For manufacturing those component parts which required a hard and abrasion resistance material.
3. Malleable cast iron

Malleable cast iron contains 2-3%C, 0.6-1.3% Si, 0.2-0.6% Mn, 0.15 % P and 0.10% S. malleable cast iron is nothing but an alloy in which all the combined carbon is a special type of white cast iron that has been changed to free or temper carbon by predesigned heat treatment. Malleable cast iron is one which can be subjected hammered and even rolled to obtained different shapes(cold work).

Uses:

1. Railroad
2. Rear axle housing

iii. Truck axle assembly parts

5. Alloy cast iron

The main alloying elements used for producing alloy cast iron are nickel, chromium, and molybdenum. Because the cast iron is supposed to be very hard, brittle lacking in tensile strength, it is alloyed with other metal to improve its properties. Two recent example of alloy case iron is acicular and spheroidal cast iron.

Applications of Cast Iron

 Cookware, such as pots and pans, is one of the common uses of cast iron. Once the cookware is sealed, it protects in the seasoning of the foods cooked in them and even adds this cookware iron to the diet.

 For producing Cooking utensils for stove-top and outdoor cooking. Many of these cast-iron utensils include spoons, grill presses and tongs can be made .

 Carpenters use cast iron to build furniture, particularly outdoor patio pieces. Cast-iron furniture includes chairs and glass-top tables.

 Welded barbecue grills use heavy cast-iron materials and stainless steel. Larger models of these grills are often so heavy that two people have to carry them.

Construction of Industrial and residential places use cast iron on buildings, especially for beams and posts that hold up architectural structures. Cast iron is also used in pipe plumbing.

 In the late 1700s, bridges constructed in Europe and the United States used cast iron as their primary building material.