Temper Brittleness (Embrittlement)

Embrittlement is a loss of ductility of a material, making it brittle. Various materials have different mechanisms of embrittlement. Hydrogen embrittlement is the effect of hydrogen absorption on some metals and alloys. It is more susceptible to BCC and HCP structured metals as compare to FCC structured metals. As little as 0.0001 weight percent of hydrogen can cause cracking in steel. 

1. Alloy steels containing nickel, manganese, and chromium when cooled slowly from tempering temperature of about 350°C to 550°C becomes brittle in impact. However, they, usually show normal ductility in the standard tension test. If these steels are quenched in oil or water from the above temperature, they remain tough in impact. The embrittlement, produced during slow cooling may be due to the separation of some brittle phase. This phase may be soluble above 350°C and hence its separation suppresses during rapid cooling, eliminating the embrittling effect.
2. Addition of about 0.5% molybdenum also eliminates temper embrittlement.
3. Steels produced at about 350°C appear blue in color and hence the brittleness observed at 350°C is called Blue brittleness.

     

Quench cracks

          Surface of component, during quenching, cools rapidly and centre cools slowly; therefore phase appearing at the surface and centre are likely to be different. This results in non-uniform volume changes. The overall effect of non-uniform cooling and non-uniform volume changes is to cause heavy distortion and cracking of the components. The cracking may result during quenching or sometimes after quenching, if tempering is delayed or in the easily stages of tempering. 

Quenching cracks are liable to occur due to following reasons:

1. Over heating: During the austenizing portion of heat treatment cycle can coarsen normally fine grained steels. Coarse grained steels increase hardening depth and are more prone to quench cracking than fine grain steels. Avoid over heating and overly long dwell times while austenizing.
2. Improper selection of steel.
3. Time delay between hardening and tempering operations.
4. Improper design: Sharp changes of section, lack of radii, design of keyways, holes, mass-distribution, and non-uniform sections.
5. Improper quenchant or medium: Water, brine or caustic will get the steel "harder". If the steel is an oil hardening steel, the use of these overly aggressive quenchants will lead to cracking.
6. Excessive amount of non-metallic inclusions in steel.

Age Hardening

          Age hardening is a type of heat treatment used in metallurgy to strengthen metal alloys. It is also called precipitation hardening, as it strengthens metal by creating solid impurities, or precipitates, in the alloy that prevents dislocations in the alloy's crystalline structure. Its name comes from the point in the hardening process in which the metal is aged, either by heating it for an extended of time or keeping it stored at a lower temperature for an extended period before use so that these precipitates can form. This treatment is used on malleable alloys, such as those made from nickel, magnesium, and titanium, as well as some types of steel.

The process consists of two stages:

          In first stage an unstable condition is produced by the formation of a supersaturated solid solution. In this state, there is no appreciable change in physical properties and the alloy remains soft and ductile. Metal undergoing age hardening is heated to a high temperature, which varies according to the materials being used and the desired properties of the final result. For example, maraging steel is heat treated at about around 1510 (about 820). Alloying materials are added and allowed to diffuse through the metal until the heated metal is supersaturated with them, meaning that the amount of these materials dissolved in the metal is higher than would be possible for a solid solution at room temperature.

          Next the metal is aged. In some alloys, this is done by keeping the metal heated for several hours at a temperature lower than that of the initial phase but still much hotter than room temperature. Other alloys are stored for days or weeks at room temperature. At lower temperatures, it is no longer possible for all of the alloying materials to remain dissolved in the supersaturated metal, and so some of it under goes precipitation and separates from the solid solution, becoming impurities spread throughout the metal. The temperature at which the aging process occurs affects how this precipitation occurs, and so influences the mechanical properties of the resulting alloy.

          These impurities created by the hardening process strengthen the metal by interfering with the movement of crystallographic defects called dislocations, which result from misalignment in the atoms that from the metal's crystalline structure. Dis locations make metal more vulnerable to being irreversibly  bent by outside forces. Their resistance to dislocation gives age-hardened alloys high yield strength and the ability to resist permanent deformation when under heavy strain.

          Alloys created by the age hardening process have many uses, especially in applications where high strength and good performance at high temperatures are needed. Maraging steel is used in engine parts and in the construction of missiles and rockets. Age-hardened aluminium alloys made with metals such as nickel, copper, and zinc have frequently been used in the construction of aircraft. The alloy Rene 41, made from nickel alloyed with molybdenum, titanium, chromium, and cobalt, is used in applications involving extreme strain and temperature, such as jet engines.

Surface hardening

Surface hardening, treatment of steel by heat or mechanical means to increase the hardness of the outer surface while the core remains relatively soft. The combination of a hard surface and a soft interior is greatly valued in modern engineering because it can withstand very high stress and fatigue, a property that is required in such items as gears and anti-friction bearings. Surface-hardened steel is also valued for its low cost and superior flexibility in manufacturing.

The various methods of surface hardening are:

1. Case hardening
• Pack hardening
• Liquid carburizing
• gas carburizing
2. Nitriding
3. Cyaniding
4. Flame hardening
5. Induction hardening.

          Mechanical means of hardening the surface of steel parts include peening, which is the hammering of the heated surface, as by iron pellets shot onto the surface or by air blasting, and cold-working, which consists of rolling, hammering, or drawing at temperatures that do not affect the composition of the steel.

Case hardening

Case hardening :

          As mentioned previously, only those carbon steels can be hardened whose carbon content is about 0.25% or more.

How do we harden dead mild steel? The answer is by case hardening. In this process, the work piece is packed in charcoal and heated as in annealing. It is kept at that high temperature for a few hours. The result is that carbon enters into the surface of the work piece to the depth of a mm or two depending upon the heating time. The work piece now has a case where carbon percentage is as per requirement for hardening. It is then heated and quenched in the usual manner. The result is a component whose surface acquires hardness, but core remains soft and tough.

The Objects of  case hardening are:

1. To obtain a hard and wear resistance surface on machine parts with enrichment of the surface layer with carbon to concentration of 0.75 to 1.2%.
2. To obtain a tough core.
3. To obtain close tolerance in machine parts.
4. To obtain a higher fatigue limit and high mechanical properties in the core.

          Case hardening consists in heating a steel in the presence of a solid, liquid or gas, rich in carbon in order to enable the surface to be hardened, while retaining a tough ductile core.

          There are three methods of adding carbon to the surface of the metal:

1. Pack hardening               2. Liquid Carburizing               3. Gas carburizing.

Pack Carburizing

          This method is the oldest. The steel to be carburized are packed in metal boxes, or pots completely surrounded by the compound, boxes that are usually made of a heat resistant alloy. The boxes are sealed with clay to exclude air and are placed in an oven, or furnace, where they are heated to temperature of between 900°C and 950°C , depending on the composition of steel. The carbon from the carburizing compound soaks into the surface of the hot steel to depth which depends on the time that the box is left in the furnace, so that the low-carbon steel is converted into high-carbon steel in the form of thin case. The internal section of the steel, and any parts, which have been protected by tinning, however, remain unaffected, the result being a piece of steel with a dual-structure. The steel is allowed to cool slowly in the box. 

          The steel is then removed from the box and reheated to a temperature just above its critical point, or appropriately 915-925°C for fine grain steel, followed by quenching in water, brine or oil. This hardens the skin and at the same time refines the core. Smaller articles and thin sections are heated to a lower temperature in order to avoid distortion. The steel is usually given a second heat treatment at about 760-780°C, in order to improve the ductility and impact resistance of the core and case.

          Small parts and single jobs are often carburized by heating them in forge, and covering them with a carburizing powder when the metal has reached a bright red heat. The carburizing compound melts and flows over the surface of the metal, which is then returned to the forge and maintained at a bright red heat for sufficient time to allow the carbon to penetrate the surface, quenching then follows as usual.

          Many commercial 'carburizing' compounds are available in suitable mixed form. Among the ingredients, combined in different percentages, are powdered charred leather, wood charcoal and horn. Wood charcoal is very largely used, although its value varies with the type of wood. Hickory gives the best results and a normal rate of penetration gradually decreases and ceases after eight hours. Wood charcoal gives the slowest rate of penetration of any of the carburizing materials. 

Liquid Carburizing

          Where a fairly thin case is required a more economical process is to carburize the parts in a liquid bath. A molten salt bath which will give up carbon to the steel, thus producing a carburized surface, has certain advantages, in that heating is rapid and distortion is minimized. The development of such salt baths is comparatively recent. Briefly, the salt baths usually contains sodium cyanide, barium chloride, sodium carbonate, and other salts, which is heated by electrical immersion elements or by a gas burner. For a constant time and temperature of carbonization, the depth of case depends on the cyanide content. The sodium cyanide content will not exceed 12% of the weight of the bath if a deep case is desired, and may go up to 23% for lighter cases. The temperature range is 1550°F-1750°F. With the lower temperatures more nitrogen will enter the case, which is benificial, and at higher temperatures more carbon will be taken in. This general process may be thought of as an extension of the cyanide process, wherein different compositions of the salt bath and higher operating temperatures are used. The parts leave the bath with a clean, bright finish, the scaling experienced during pack hardening being avoided.

Gas Carburizing

          It is another method of extra carbon into the surface of the steel; in this case by heating the metal in a furnace into which a gas which is rich such as methane, propane, butane is introduced. In gas carburizing, the gas may be generated from suitable liquids, as done in certain processes, or the gas such as methane or propane may be led directly into the container in which the work is placed. It is necessary to maintain a continuous flow of carburizing gas into the furnace, and to extract the spent gas. Some city gasses are quite suitable. Some source of oxygen is necessary for best results, either as air or CO2, but usually it is not necessary to add any oxygen.

          There are explosive ranges of gas composition, which should be avoided. The work is placed in a gas-tight container, which can be heated, in a suitable furnace, or the furnace itself may be the container. The carburizing gas is admitted to the container, and the exit is vented. It should be apparent that there are certain advantages in the gas carburizing process. The steel does not have to be packed, and there is no handling and storage problem of the solid carburizer with which to contend. The time required to bring the work to the carburizing temperature is less than is necessary with solid carburizers. On the other hand, the pack method requires less expensive equipment, and control of the composition of the gasses is simple. Continuous furnace can be used in either process. Continuous gas-carburizing furnace have been developed only recently. In continues gas carburizing, the quenching and tempering cycles take place in the same unit, that is the carburizing, quenching, and tempering processes are carried out in sequence in the same closed furnace as the work progresses on a conveyor from one operation to the next. There are several variations of gas carburizing;

          The horizontal, rotary type of gas carburizing furnace has a retort of muffle which revolves slowly so that the parts are rotated in the steam of gas; this is suitable for smaller parts such as ball and roller bearings, chain links, pins, axels and so on. Carburizing gas led into the retort, and the exit gas is burned. Case depths in relating to time and temperature. Larger parts are usually carburized in a vertically rotary furnace, in which gas is given a swirling rotary motion so that it circulates around the parts.

Nitriding

          The nature of the nitriding process used to obtain a case hardened is very different from that of the carburising process. Nitrogen, instead of carbon, is added to the surface of the steel. Carbon does not play any part in the nitriding operation but influence the machinability of steel. The temperature used in nitriding are much lower than those used in carburising and below the critical temperature of the steel.

          Simple carbon steels, which are often used for carburising are not used for nitriding. Steels used in the process are specially alloy steels. With the nitriding developing rather thin cases, a high core hardness is required to withstand any crushing loads. High tempering temperature call for a steel with a higher carbon content in order to develop this increase in core hardness. In addition to higher carbon content various alloying elements are called for in the steel to bring about an increase in the formation of these nitrides. Aluminium seems to display the strongest tendency in the formation of these nitrides. Chromium, molybdenum, vanadium and tungsten., all being nitrides former, are also used in nitriding steels. Nickel in nitriding steels hardens and strengthens the core and toughens the case but slight loss in its hardness.

Nitriding operation:

Advantages:

1. Better retention of hardness at elevated temperature.
2. Greater fatigue strength under corrosive conditions.
3. Less warping or distortion of pans treated.
4. High endurance limit under bending stresses.
5. Greater resistance to wear and corrosion.
6. Greater surface hardness.

Disadvantages:

1. Necessity of using high alloy containers to resist the nitriding.
2. High furnace costs due to the long lime of treatment.
3. Necessity of using special alloy steels.
4. Medium used is expensive.

Cyaniding

          Cyaniding is a superficial case-hardening process that utilizes carbon and nitrogen. The part to be cyanide case hardened is heated in a molten sodium cyanide at about 850°C  followed by quenching. The cyanide-casing operation is the fastest type of casing that can be applied to a mild steel core, less distortion due to use of salt bath; however, its disadvantages are that the cyanide salts are extremely toxic and the process is messy. Special protective equipment must be on by personnel operating in the vicinity of the cyanide salts. Parts must also be limited to a size that can be handled by one or two people. Not suitable for components subjected to shock, fatigue and impact because nitrogen has adverse effect on these properties.

Process:

Medium: Parts immersed in liquid bath containing NACN varying between 25% and 90%.

Bath heated in a range of 800 to 950°C.

Measured amount of air passed through the molten bath.

Reactions: 

                   2NaCN + O2 ----> 2NaCNO 

                   2NaCNO + O2 ----> Na2CO3 + CO + 2N

                   2CO  ---->  CO2 + C 

C and N2 so formed diffuse into steel and give thin wear resistant layer of carbonitride  ϵ  phase.

Quenched in oil or water.

Cyaniding time of 1.5 to 6hrs for case depth of 0.13 to 0.35mm @ 850° C 

Higher the temperature, higher the C diffusion (0.8 to 1.2%) on surface as compared to N (0.2 to 0.3% )

Case hardness: 850 VHN

Flame hardening

          Flame hardening is a surface-hardening method that uses an oxyacetylene flame to heat treatment the surface of the metal. Flame hardening can be performed on only medium or high carbon steel or cast iron. When flame hardening is applied to steels with over 70 points of carbon, extra care must be used in order to prevent surface cracking of the high-carbon steel.

          The process is based on the rapid heating of the outer surface of a ferrous metal to or above its transformation temperature.The minimum distance that the oxyacetylene flame must be held from the base metal is approximately 9/16 inches. If the oxyacetylene flame is closer than 9/16 inches., the base metal will be deformed by the flame. The metal is moved rapidly under the flame, allowing the flame to heat the base metal only on the surface. This surface heating creates two heat-affected zones, a primary heat-affected zone, where the transformation of the metal has taken place, and a secondary heat affected zone, where grain growth has been developed or where the grains have been enlarged or decreased by the application of heat to the base metal. Immediately after it is heated, the metal is subjected to a quenching spray, generally water, that hardens the metal area that has undergone transformation.

          The depth of hardness depends entirely on the hardenability of the material being treated since no other elements are being added or diffused, as in case hardening. With proper control, the interior of the metal will not be affected by this process. Often an average application of this process involves heat treating a complete piece to a certain specified softness or toughness. The exterior then may be flame hardened so that the finished piece resembles an item that has been case hardened. It is quick and the hardening is restricted to parts which are affected by wear.

          Fig shows, a flame hardening of gear teeth. A flame from an oxy-acetylene or similar burner is played on to the teeth so as to arise temperature rapidly above the hardening temperature. Hardening results when the austenised surface is quenched by spray that follows spray.

Induction hardening

          Induction hardening is a method similar to flame hardening, with the exception that the heat is generated in the metal by an induced electrical alternating current. The only metals that can be induced hardened are those that are conductors or semiconductors. The ACHF (alternating current high frequency) obtained from a pulsating magnetic field about a wire produces the heat in induction hardening. The heat results from molecular agitation induced by the electricity. The high-frequency, low-voltage, high ampere current produce a great number of eddy currents which are primarily responsible for the heating of the metal, although hysteresis is another source of heating.

          An inductor block, similar to a primary coil in a transformer, is placed around the part to be hardened. This coil does not touch the metal. A high frequency current is passed through the block or the coil and induces a sympathetic current in the surface of the metal, creating heat, a process called hysteresis. As the temperature of the metal reached the transformation range, the power is turned off, the heat source is removed, and the area is quenched, usually by a spray from a water jet built in to the inductor block. The most important aspects of induction heating is its rapid action. For example, it requires only a few seconds to heat steel to a depth of 1/8 inches.

          The heat produced by induction is the result of both current and frequency. Higher currents produce stronger magnetic fields, while higher frequencies produce more pulsation of the field within a given time. A specific degree of heating can be obtained either by using high current at low frequencies or low current at high frequencies. However, high-frequency induction heating of metals requires a device that can convert 60 hertz (Hz) power to a high frequency of several hundreds or more cycles per second. the frequency most used for metal treating applications is 450 kilohertz per second(KHz/S).

Applications:

          The induction hardening is at present extensively used for producing hard surface on crank shaft, axels, and gears. the principle advantages are as listed below:

1. Increases the labour productivity by reducing the time requirement for this heat treatment operation.
2. Deformation due to heat treatment is considerably reduced.
3. No scale effect.
4. By controlling the current the hardening of the surface can be easily controlled.
5. By changing frequency of supplying voltage the depth of hardness can be controlled easily.

Defects due to Heat treatment of steel

1. Overheating.
2. Burning.
3. Quenching cracks.
4. Warping*.
5. Decarburisation.
6. Corrosion.
7. Excessive hardness of hot worked annealed steel.
8. Insufficient hardness after quenching.
9. Excessive hardness after tempering.
10. Insufficient hardness after tempering.

*warping means become bent or twisted, out of shape

Surface Finish After Heat treatment

          Due to heat treatment certain defects like burning, decarburisation, overheating, quenching cracks, deformation, scaling, soft spots etc., may occur. In order to remove these defects following finishing operations are carried: 

1. Acid pickling

2. Sand blasting

3. Degreasing

4. Straightening.

 1. Acid pickling:

          If the heat treatment is not performed in a controlled gas atmosphere oxide or scale layer is formed on the surface of the heat treated component. Pickling is the most common chemical procedure used to remove oxides and iron contamination. Besides removing the surface layer by controlled corrosion, pickling also selectively removes the least corrosion-resistant areas. Pickling normally involves using an acid mixture containing nitric acid (HNO3), hydrofluoric acid (HF) and, sometimes, also sulphuric acid (H2SO4). Owing to the obvious risk of pitting corrosion, chloride containing agents such as hydrochloric acid (HCL) must be avoided.

Descaling can be performed by acid pickling in the following two ways:

         Direct dissolution of a component in all acid bath:

         Mechanical circulation of acid solution:

- 8 to 12% or 20% HCL is taken for the above purpose.

- The acid bath is used at 50 to 70.

- The components removed from the pickling bath are thoroughly cleaned with steam and then neutralised in a week alkaline bath for 5 to 10 minutes. Finally they are rinsed by warm water.

2. Sand blasting:

          In sand blasting process the surface to be cleaned is exposed to a jet of compressed air carrying sand composed of dry, sharp, quartz grain of 1 to 10 mm of size. This process gives fine surface finish and useful to large areas to be cleaned. 
          In another process surface cleaning is done by centrifugal action of abrasive material like iron shorts instead of sand blowing with pneumatic pressure. This method also produces cold case hardening of the surface, increasing the fatigue resistance of the articles.

3. Degreasing:

          Salts are deposited on the surface of the components when treated in salt baths. These are removed by soaking the component in boiling water.

          Both slats and oil deposits can be cleaned by immersing the components in a tank containing boiling data solution and then washing in water. The operation is known as vat cleaning.

4. Straightening:

      During heat treatment certain tools and machine parts get distorted or warped. In order to bring them original shape, straightening is carried out.

     Straightening of rolled stock is done by straighteners or hydraulic straightening process.

     

INTRODUCTION

          Ferrous material refers to those materials whose main constituents is iron; while non-ferrous materials are those which do not contain iron in any appreciable quantity. Ferrous materials are usually stronger and harder and are used extensively in our daily lives. One very special property of ferrous materials is that, their properties can be significantly altered by heat treatment processes or by addition of small quantities of alloying elements. Ferrous materials are relatively cheap but suffer from great disadvantage. They are subject to corrosion and rusting.

IRON AND STEEL

          Most common engineering materials are ferrous materials such as mild steel and stainless steel which are alloys of iron. It is truly said that gold is metal for kings and iron is king of metals. Otto Von Bismark of Germany once said that "for development of a nation, lectures and meetings are not important, but what is important are blood and steel". Incidentally, what is common in blood and steel is "Iron". Though iron is important, but it is mostly used in the form of its alloy, namely steel.

          To a layman, words iron and steel convey the same meaning. But iron and steel are two different things. Iron is the name given to the metal, whose chemically symbol is Fe and refers to pure (or almost pure iron). Pure iron is relatively soft and less strong. Its melting point is about 1540°C. In industry, wrought iron is the material which is nearest to iron in purity; but is rarely used these days.

      Steel on the other hand, is an alloy of iron and carbon,; the percentage of carbon theoretically varies from 0 to 2%. However in actual practice, carbon rarely exceeds 1.25 to 1.3%. Carbon forms anointermetallic compound called Cementite (Fe3C), which is very hard, brittle and strong. The presence of Cementite in steel makes steel much stronger and harder than pure iron.

CLASSIFICATION OF STEELS

Steel can be classified into (1) Plain carbon steel, and (2) Alloy steel.

          Plain carbon steel is that steel in which the only alloying element present is carbon. In alloy steel, apart from carbon, other alloying elements like chromium, nickel, tungsten, molybdenum, and vanadium are also present and they make an appreciable difference in the properties of steel.

        Before we go further, readers must note that in steel, besides iron and carbon, four other elements are always present. These are S, P, Mn ans Si. Removing these elements from steel is not a practical proposition. However, the effect of sulphur and phosphorous on the properties of steel is detrimental and their percentage is generally not allowed to exceed 0.05%. Similarly, the usual percentage of manganese ans silicon in steel is kept below 0.8 and 0.3%, although their effect is not determinantal to the properties of steel. In fact, manganese counters the bad effect of sulphur. The presence of these four elements to the extent indicated does not put plain carbon steel into the category of alloy steel. However, if higher percentages of Mn and Si are intentionally added to steel in order to alter its properties, then the resulting steels come within the category of alloy steels.

Plain Carbon Steels

(1) Plain Carbon Steels:

Since the properties of plain carbon steels are so dependent upon their carbon percentage, these steels are further classified into following categories on the basis of carbon percentage only:

(i) Low carbon or dead mild steel having carbon below 0.15%,

(ii) Mild steel having carbon between 0.15 - 0.3%,

(iii) Medium carbon steel having carbon between 0.3 - 0.7%, and

(iv) High carbon steels having carbon content above 0.7% (the higher practical limit of C% is 1.3%).

As the carbon percentage increases, the strength and hardness of plain carbon steel increases while ductility decreases. Reference is invited to Fig. 2.1. which shoes the effect of increasing carbon percentage on certain mechanical properties of carbon steels.

Applications and Uses of Plain Carbon Steel

Applications and Uses of Plain Carbon Steel

1. Dead mild steel: It has got very good weld ability and ductility. Hence, it is used in welded and solid drawn tubes, thin sheets and wire rods, etc., It is also used for those parts which undergo shock loading but must have good wear-resistance. To increase its wear-resistance, the parts have to undergo casehardening process; which provides a hard surface, while the core remains soft and tough.
2. Mild steel: It is used very extensively for structural work. It retains very good weld ability if carbon percentage is limited to 0.25%. Forgings, stamping's, sheets and plates, bars, rods and tubes are made of mild steel.
3. Medium carbon steel: It has little weld ability but is stronger and has better wearing property than mild steel. It is used for railway axles, rotors and discs, wire ropes, steel spokes, marine shafts, carbon shafts, general agricultural tools etc.,
4. High carbon steels: It is used for hand tools like cold chisels, cold working dies, hammers, boiler maker's tools, wood working tools, hand taps and reamers, filers, razors, shear blades etc., High carbon steels can be hardened by the process of quenching and being hard can be used for cutting tools which are not used in hot condition. If they become hot (above 150°C), they begin to loose their hardness and become blunt.

WROUGHT IRON

          It is the purest form of iron; although it may contain traces of carbon. It is usually made by "puddling process" and besides iron contains a small quantity of slag. It is very costly and its use has been almost totally replaced by cheaper steels. However, for some components like chains-links and chain-hooks wrought iron is still the preferred raw materials. In old havelis/houses, one can still see iron railings and gates made of wrought iron.

Effect of Impurities on Cast Iron

The effects of impurities on the cast iron are as follows:

1. Silicon: It may be present in cast iron up to 4%. It provides the formation of free graphite which makes the iron soft and easily machinable. It also produces sound castings free from blow-holes, because of its high affinity for oxygen. It reduces the melting point but enhances the percentage content of uncombined carbon.
2. Sulphur: It makes the cast iron hard and brittle. It causes red-hardness of metals (Brittleness at high temperatures). Since too much sulphur gives unsound casting, therefore, it should be kept well below 0.1% for most foundry purposes.
3. Manganese: It makes the cast iron white and hard. It is often kept below 0.75%. It helps to exert a controlling influence over the harmful effect of sulphur by forming MnS (which is not injurious in small quantities).
4. Phosphorus: It aids fusibility and fluidity in cast iron, but induces brittleness. It is rarely allowed to exceed 1%. Phosphoric irons are useful for casting of intricate design and for many light engineering castings when cheapness is essential. It gives rise to cold shortness (Brittleness at ordinary temperatures).

CAST IRON

CAST IRON:

          Cast irons contain more than 2% carbon, which is the theoretical limit for steels. However, in actual practice, carbon content of most cast irons is between 3 to 4 %. One characteristic of cast irons (except white cast iron) is that much of the carbon content is present in free form as graphite. It is this fact, which determines, largely, the properties of cast iron. Cast iron is generally produced in coke-fired cupola furnaces by melting a mixture of pig iron, scrap cast iron and a small percentage (usually not exceeding 5%) of small sized steel scrap. Melting point of cast iron is much larger that that of steel. Most of the castings produced in a cast iron foundry are of grey cast iron. These are cheap and widely used. There are many varieties of cast iron. These are listed below :

(i) Grey cast iron, 

(ii) White cast iron,

(iii) Malleable cast iron, 

(iv) Nodular cast iron, and

(v) Alloy cast iron.

(1) Grey cast iron:

          Grey cast iron is charecterised by presence of a large portions of its carbon in the form of graphite flakes. Although grey iron is often defined as steel containing graphite, its properties are far different from those of steel. As already mentioned, Grey cast iron is very widely used in the form of castings. In fact, it is so widely used that the term cast iron has come to mean grey cast iron. If a finger is rubbed on a freshly fractured surface of grey cast iron, the finger will get coated with grey color due to the graphite present in the cast iron. grey cast iron has good compressive strength, but is weak in tension. It is relatively soft but brittle. It is very easy to machine and the resulting surface finish is good. It is self lubricating due to the presence of graphite and has good vibration damping characteristics. Compares to steel, it resists corrosion. 

Due to these properties, it is used extensively for making machine beds, slides, gear-housing, steam engine cylinders, manhole covers, drain pipes etc., 

(2) White cast iron:

White cast iron has 2 to 2.5% carbon and most of it is in the form of cementite. If molten cast iron is cooled very quickly and its chemical composition lacks graphite-promoting elements like Si and Ni, then carbon remains in combined form as . However, white cast iron does not have much use as such. It is very hard and shows white colored fracture. Highly resistance to wear. Due to its poor fluidity it does not fill the mould freely.

Only crushing rolls are made of white cast iron. But it is used as raw material for production of malleable cast iron. Used for parts subjected to excessive wear.

(3) Malleable cast iron:

         Malleable cast iron is manufactured by a complex and prolonged heat treatment of white cast-iron castings. Grey cast iron is brittle and has no or very little elongation. Malleable cast iron castings loose some of grey iron's brittleness and become useful even for those applications where some ductility and toughness is required. (Note: 'molten iron' is a name given to cast iron whose structure shows part grey and white cast iron characteristics).

Malleable cast iron is of two types: 1. Black hearth and  2. White hearth. Malleable cast iron can be obtained by keeping temperature and time comparatively of high values.

It is used in differential and steering gear housing, brake pedals, tractor springs, hangers and washing machine parts.

(4) Nodular cast iron:

          This cast iron is also known under the name of spheroidal graphite cast iron. If a little bit of magnesium (0.5%) is added to molten cast iron, the graphite, which is normally present in grey iron in the form of graphite flakes, changes its shape to small balls/ spheres and remain distributed throughout the mass of cast iron. This change in the shape of graphite particles has a very big effect on the properties of resulting castings and their mechanical properties improve considerably. The strength increases, yield point improves and brittleness is reduced. Such castings can even replace some steel-components.

(5) Alloy cast iron:

          The properties of cast iron can be improved by addition of certain alloying elements like nickel, chromium, molybdenum and vanadium, etc. Among the alloying elements, nickel is predominating alloying constituent whose addition to the extent of 0.5 to 1.5% avoid the tendency of chilling and hard spots. Alloying cast irons have higher strength, heat-resistance and grater wear-resistance etc. Such enhanced properties increase the application and uses of cast irons. I.C. Engine cylinders, cylinder liners, piston rings etc, are made of alloy cast irons.