Pure iron is a poor engineering material, often no stronger than most plastics. It is certainly not suitable for use as a structural material and does not respond in any appreciable degree to heat treatment.

Steel, which is an alloy of iron and carbon and/or manganese, is a highly useful engineering material, in part because it can respond to heat treatment (Table 1). Alloying provides the basis for the heat treatability of steel. Even a few percent of other alloying elements allow for a wide range of strength, toughness and ductility properties.

1. Yield Strength (Y.S.) and Ultimate Tensile Strength (UTS) are measures of how strong the steel is after the heat-treatment process shown.
2. Eutectoid composition (0.77%C)

Table 2. Effect of carbon on hardness[2]

Carbon is the most important of these alloying elements in terms of the mechanical properties of steel, and most heat treatments of steel are based primarily on controlling the distribution of carbon. Adding carbon has the following (generally positive) effects:
  • Raising the ultimate strength
  • Enhancing the abrasive (wear) resistance
  • Improving the uniformity of hardness
  • Increasing the depth of hardening
  • Intensifying the fineness of fracture
  • Lowering the hardening and quenching temperature
Of course, other (generally negative) effects also occur:
  • Increasing the resistance to machinability
  • Lowering the hardening and quenching temperature
  • Lessening the heat conductivity
  • Lowering the ductility and toughness
  • Diminishing the electrical or magnetic conductivity
  • Decreased weldability
The influence of carbon additions on the strength of iron should not be underestimated. Small amounts of carbon increasing to as high as 0.80-0.90 wt.% steadily increase its strength and hardness (Table 2).

Fig. 1. Three plain carbons steels (From left to right: 1018, 1045 and 1095[3])

As the carbon content goes above 0.85%, the resulting increase in hardness is correspondingly less than in the lower-carbon ranges. The maximum hardness after quenching also increases with increased carbon, but above 0.60%C the rate of increase is very small.

Tensile strength in the as-rolled condition also increase with carbon content but the effect is less pronounced above 0.85%C. For example, at 0.20%C the incremental increase per point of carbon will be about 0.88 ksi; at 0.40%C about 1.04 ksi and at 0.60%C the increase is 1.20 ksi.

As seen in Figure 1, increasing the carbon content (from 0.18% to 0.95%) causes the amount of ferrite (light) to decrease and the amount of pearlite (dark, lamellar) to increase.

Plain-carbon steels can contain up to 4.5 wt.%C, but the strength and formability deteriorate, and they assume the characteristics of cast irons when the carbon concentration exceeds 2.1%. Finally, carbon has a moderate tendency to segregate within the ingot and is often of greater importance than the segregation of other elements.

Coming up in future installments, the influence of: Manganese (Mn), Phosphorous (P), Sulfur (S), Silicon (Si), Copper (Cu), Nickel (Ni), Chromium (Cr), Molybdenum (Mo) and Vanadium (V).