In this article we will discuss about:- 1. Effects of Carbon on Iron 2. Effect of Carbon on Mechanical Properties of Annealed Steels 3. Effect of Carbon Other Metals.
Effects of Carbon on Iron:
Of all the alloying elements, carbon exerts the most profound and significant effect on the allotropy of iron, as is illustrated in Fe-Fe3C diagram. The crystal structures of BCC- α-Fe and the FCC-y-Fe are modified by adding carbon atoms in the interstices of iron atoms. (Fig. 1.19)
Carbon addition decreases the freezing temperature of iron to become 1147°C at 4.3 % C in cast iron. A lowering of temperature of about 400°C, helps to melt and cast the cast irons, easily as compared to steels.
Carbon is an austenite stabliser, i.e. as the carbon content increases in iron, it increases the range of austenite formation (i.e. raises A4 temperature and lowers A3 temperature), and expands greatly the austenite field and also lowers the fields of ferrite (BCC). The much larger phase-field of austenite compared to α-iron reflects the much greater solubility of carbon in γ-iron than in α-iron. As the solid solubility limit of carbon in γ-iron is exceeded, a new phase cementite forms in Fe-C alloys.
The great difference of solid solubilities of carbon between γ-iron and α-iron leads normally to the rejection of carbon as cementite at the boundaries of γ-phase field. Thus, the difference in the solid solubilities of carbon in γ-iron and in α-iron, introduces another transformation called the eutectoid transformation in the system. This reaction plays a dominant role in the heat treatment of steels, because when high carbon alloys, after solution treatment in γ-region (austenitisation), are rapidly quenched to room temperature, a supersaturated solid solution of carbon in iron, called martensite is formed.
Effect of Carbon on Mechanical Properties of Annealed Steels:
As the carbon content of slowly cooled plain carbon steels increases, the amount of pearlite increases from 0% at 0% C to 100% at 0.77 % C, the remaining other phase being ferrite. Pearlite, the mixture of two phases, i.e., hard, and brittle cementite, embedded in soft and ductile ferrite, makes the steel stronger and stronger as its amount increases to 100 %.
Pearlite can, in a simple way, be compared with the road surfacing mixture of asphalt and pebbles. Road surfaced with either asphalt, or, pebbles is not strong, but mixture of these two (one of which is soft and the other is hard and brittle) makes the road strong and lasting. The cementite plate in pearlite acts as a barrier to the motion of dislocations in ferrite (see Fig. 1.33), thereby increases the resistance to deformation and thus, the strength but, reduces the ductility and toughness.
As the carbon content increases, amount of cementite (hard and brittle) increases, thus, the hardness increases with decrease of % elongation, % reduction in area and impact strength. Increase of carbon increases the pearlite content, thus increases the yield strength and tensile strength of hypoeutectoid and eutectoid steels.
As the carbon content of the steel becomes 1.0 to 1.2 % C, proeutectoid cementite, precipitates at the grain boundaries of austenite (at room temperature-pearlite), and forms as thick network there. Cementite is brittle with no ductility. In tensile testing, under the external stress, high stresses are developed in the network, and thus, cementite fails.
The fracture nucleates and propagates along the grain boundaries. The tensile strength, yield strength decrease and the ductility becomes very small. Fig. 1.35 illustrates this. Fig. 1.36 illustrates the effect of increase of carbon on the transition temperature, which increases with the increase of carbon content of steel. High carbon steels show brittle fracture under impact loading even at room temperature.
Effect of Carbon Other Metals:
Manganese of the added deoxidiser, ferro-manganese, reacts with oxygen of the ferrous oxide and goes over to slag as manganese oxide. Manganese can also remove harmful FeS. Manganese, that remains after the deoxidation, forms solid solution in ferrite and cementite, which appreciably raises the strength of the steel without practically reducing its ductility. It greatly reduces the red-shortness i.e., brittleness at high temperature due to its effects on sulphur. Manganese content normally ranges from 0.5 to 0.8%. Manganese induces depth of hardening but also, the liability to crack in quenching. Thus, water hardening high carbon steels have less than 0.5% Mn.
Silicon, of the added deoxidiser, deoxidises the ferrous oxide in molten steel and its resulting oxide joins the slag. The remaining silicon, after the deoxidation, forms solid solution with ferrite, which greatly increases the yield point, but reduces the deep drawing and cold heading properties of the steels. Steels for cold press work and cold-heading, have always less amounts of silicon. Silicon content in normal steels ranges from 0.35 to 0.5%.
Sulphur joins the steel (as well as cast irons) from raw materials and furnace gases, while making steel. Sulphur is harmful impurity in steels. Sulphur forms FeS with iron. This compound forms a eutectic with iron, which is low melting (988°C). This brittle and fusible, yellowish-brown looking eutectic, is located mainly along the grain boundaries as films [Fig. 1.37 (a)]. This form of sulphur inclusions is very detrimental.
When steel is heated to the rolling, or forging temperature (1000° to 1200°C), the eutectic film melts. When the steel is worked, it fractures and fissures develop along the grain boundaries, and the object gets fragmented. This phenomenon is called hot shortness or red shortness.
This harmful effect of sulphur can be practically removed, that is, red-shortness can be avoided by adding manganese. After deoxidation, the amount of manganese should be about five times than theoretically required to combine with the sulphur present. Manganese has greater affinity for sulphur than iron has and thus, forms manganese sulphide, which melts at 1620°C, i.e., at a temperature well above the common temperature of hot working of steels.
As manganese sulphide is almost insoluble in steel (liquid as well as solid), it forms dove-gray coloured large globules, irregularly distributed through the steel [Fig. 1.37 (A)]. At the temperature of hot working, manganese sulphide is plastic and gets elongated into threads by rolling, without seriously impairing the properties of the steels [Fig. 1.37 (c)].
Sulphide inclusions adversely effect the mechanical properties, especially the impact strength and ductility, particularly in the direction transverse to the rolling and forging, and also, decreases the endurance limit as sulphide inclusions act as stress concentrators. Sulphide inclusions lower the weldability and corrosion resistance.
Thus, sulphur content is, always, kept low in steels in range from 0.035 to 0.05 %, or even less. Sulphur improves machinability of steels and that is why, free-cutting steels have 0.2% sulphur and 1.5% manganese.
It joins the steels and cast irons from the raw materials. The content of phosphorus is kept low 0.02-0.05%. Phosphorus dissolves up to 1.2% in α-iron, but the solubility decreases with the increase of carbon in the steels and cast irons, and then, phosphorus forms a compound, Fe3P.
Phosphorus, when present as solid solution in ferrite, severely distorts its crystal lattice, resulting in increase of tensile strength and yield point, but seriously reduces the ductility and toughness. It also raises sharply the ductile to brittle transition temperature as illustrated in Fig. 1.38, thus causing cold brittleness of steels, i.e., cold shortness in steel. Each 0.01 % P raises the cold brittleness temperature by 20° to 25°C. This harmful effect of phosphorus becomes more cause of worry because of the marked tendency of phosphorus to segregate that is, at places the content of phosphorus may become alarmingly high.
As the solubility of phosphorus decreases with the increase of carbon content, particularly in cast iron having 3.5% C, the solubility is 0.3%. The segregated phosphorus forms Fe3 P, which forms ternary eutectic called steadite, of ferrite, cementite and iron phosphide (Steadite: Fe = 91.19 % , C = 1.92 %, P = 6.89%) having a melting point of 960°C and is brittle.
Phosphorus presence is taken to an advantage in some cases:
(i) 0.07% P with 0.3% Cu in steel causes increased resistance to atmospheric corrosion;
(ii) with 0.08% P, the tendency of tin-plates to stick together is prevented and the fire-welding of spades is improved.