Unit 002: Engineering Principles and Practice

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2850 Level 3 in EngineeringUnit 302 Handout 2

Unit 302:Engineering principles

Handout 2: The effects of carbon content and heat treatment on the properties of plain carbon steels

One of the most important alloy systems from an engineering standpoint is the one between iron and carbon, which forms the basis of all low alloy steels. The unique position of carbon steel, apart from its cost, is the potential to vary its mechanical properties over a wide range by altering its composition and applying heat treatment.

Although adding carbon to steel affects most of its properties, some of which are intensified by subsequent heat treatment, from a practical point of view the most significant are the mechanical properties. Essentially, the properties that are most important from a mechanical viewpoint are strength, ductility and hardness, as these tend to have most bearing on why a particular steel is selected for a specific application. However, the selection and application of different steels varies according to the carbon content but all alloys fall within the approximate range of 0.05 to 1.15% carbon.

Taking these factors into consideration it is possible to show a how the three mechanical properties vary as the carbon content increases to a maximum of around 1.2%C. The variations are illustrated on the graph in figure 1, as are also the percentages of carbon that provide the optimum strength to ductility and hardness to ductility properties.

Fig 1

The affect of alloying elements on the properties of plain carbon and low alloy steels

Alloy steels are usually classified into two major categories, high and low alloy steels. Generally, low alloy steels possess similar microstructures to plain carbon steels and therefore respond to similar heat treatments. However, the advantage of adding other elements such as chromium, nickel or manganese as opposed to just increasing the carbon content is that the strength increases considerably without the corresponding fall in ductility shown in Figure 1. Moreover, many of the alloying elements above, which are commonly used in low alloy steels produce a marked increase in strength at both ambient and elevated temperatures with very little or virtually no loss to ductility.

The heat treatment of plain carbon and low alloy steels

The concept of solid metals as crystalline substances with grain structures is essential for an understanding of how heat treatment works. It is also necessary to understand that an alloy is a substance consisting of two or more elements of which at least one is a metal. However, in general, commercial alloys consist of several elements.

Taking carbon steel as an example, which is essentially an alloy of iron and carbon, in the first instance it is important to recognise that iron can appear in different forms depending on the temperature. Between room temperature and 1535°C it can appear as four different structures, and the range of temperatures over which these changes occur vary according to how much carbon is added to the iron. This can be shown quite simply by referring to part of the ‘Iron-carbon thermal equilibrium diagram’ shown in Figure 2.

Figure 2

As the diagram shows, at high temperatures above 900°C higher percentages of carbon are soluble in the iron than at room temperature ie the structure looks like a pure metal (all the grains are similar). This occurrence is due to changes in the atomic structure, which occur as the temperature of iron is raised to its melting point. The diagram also shows the two constituents of a steel’s grain structure at room temperature with carbon contents in excess of 0.05%, that is iron and a lamellar structure of iron and iron carbide, known as pearlite.

The heat treatment of steels

The first stage in any heat treatment operation, whether for the purpose of softening or hardening of a steel is to heat it into the range where all the carbon is held in solid solution ie the ‘’ range (above 910°C). It is by varying the cooling rate and introducing secondary heating/cooling treatments that the mechanical properties of a steel can be altered. The physics of transformation are beyond the scope of this handout but suffice it to say that if the steel is quenched rapidly from this temperature it will achieve its hardest condition and the degree of hardness is determined by the percentage of carbon on the steel, hence the term ‘hardening’. If the steel is allowed to cool at the slowest possible rate ie in a furnace, a treatment known as ‘annealing’, it achieves its softest possible condition. ‘Normalising’ is a variation on annealing, the difference being the cooling rate where the steel would be allowed to cool outside the furnace still air.

Clearly, a fully hardened steel will be very brittle and therefore have limited application. However, if the steel is reheated into the temperature range 400 to 550°C and subjected to various cooling mediums, a process known as ‘tempering’ the hardness is replaced by a much tougher condition.

Adding other elements such as nickel, chromium, molybdenum etc also affects the critical cooling rate and the grain structure that is retained at room temperature. Therefore, the properties of low alloy steels are usually superior to those of plain carbon steels that have undergone similar treatments.

Table 1 below provides a summary of the factors affecting cooling rate and hardenability and hence the mechanical properties of the steel.

Factors affecting cooling rate / Factors affecting hardenability
Quenching medium / Temperature to which component is heated
Temperature and agitation of quenching medium / Prior treatment of component
Component size and shape / Heating rate
Surface contamination / Composition
Temperature to which component is heated / Grain structure

Table 1