This is the highest temperature to which steel can be heated to obtain maximum hardness.
Percentage carbon Critical temperature for hardening and annealing in °C
0,10 915 – 980
Figure 3.9: Critical temperatures in steels
Did you know?
The points where one phase changes to another are called critical points by heat treaters and transformation points by are abbreviations for a French term ‘arrêt de chauffage’ which means
‘heating ends’.
Decalescence point
• The temperature where carbon steel changes from pearlite to austenite when heated (700 °C for 0,83% carbon steel).
Recalescence point
• The temperature where carbon steel changes back from austenite to pearlite when the steel is cooled (700 °C for 0,83% carbon steel).
Critical temperature of ordinary carbon steel
• When a steel or iron sample is heat treated, it undergoes structural changes.
At this stage, the nature of the structural changes is not as important as the temperature at which they take place. When an artefact consisting of ordinary medium-carbon steel is heat treated, internal structural changes take place at 700 °C and it takes up an entirely new form at a temperature of about 800 °C.
For this particular sample, 700 °C is regarded as the lower critical temperature and 800 °C as the higher critical temperature. The critical temperature of any type of sample which is subjected to heat treatment must be known for the process to be performed successfully.
The following points should be considered:
• The lower critical temperature for all ordinary carbon steels is 700 °C.
• The higher critical temperature fluctuates in accordance with the carbon content of the steel.
• Steel or iron with a carbon content of 0,87% has only one critical temperature, i.e. 700 °C.
Changes during the hardening of carbon tool steel
Carbon steel which has been fully annealed consists mainly of two parts: One of the elements is iron or ferrite (derived from the word ferrous, meaning containing or resembling iron) and the other is a carbide of iron known as cementite.
When carbon steel in the fully annealed state is heated, usually to a temperature between 680 °C and 720 °C (depending on the carbon content), the alternate bands or layers of ferrite and cementite form many alternating layers side-by-side, like layers of bread and meat in an endless sandwich. The layers in the steel are so thin that they are only visible under a microscope. They look like alternating sheets of white and black paper (viewed from the edge). The pearlite layers begins to merge into each other. The temperature at which this occurs is known as the lower critical point (AC1).
The merging process continues until the pearlite is thoroughly ‘dissolved’, forming what is known as austenite. If the temperature of the steel continues to rise, the pearlite and any excess ferrite or cementite will also begin to dissolve into austenite until finally only austenite will be present. The merging process continues until the pearlite is completely dissolved to form austenite. If the temperature of the steel continues to rise and there is (apart from the pearlite) any remaining ferrite or cementite present, it will also ‘dissolve’ until eventually only austenite is present.
The temperature at which the excess ferrite or cementite is completely dissolved into austenite is called the upper critical point (AC3).
If the steel is now suddenly cooled by plunging it into a bath of cold water or oil, a new structure is formed when the austenite is transformed into martensite and this provides the steel with the property of hardness.
Did you know?
Cementite is the silvery speckle in white cast iron after it is fractured (and is intensely hard). A mixture of a certain proportion of these two elements is called pearlite because under the microscope it frequently has the appearance of ‘mother of pearl’, hence the name.
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Figure 3.10: The iron-carbon equilibrium diagram
Steel characteristic changes at critical temperatures (AC1, AC2, AC3) The change that takes place when heating steel is of great importance when
explaining the reasons for the effect of the different heat-treatment processes on the metal. This aspect often causes considerable confusion.
If a steel bar containing, say, 0,3% carbon is gradually heated in a furnace and the time limit of the heating process is observed, it will be noted that the temperature rises uniformly at first. When, however, the temperature reaches 700 °C (a dull, red heat) it will remain stationary for a while and will then rise at a slower rate until reaching 800 °C (a bright red colour). Hereafter, the temperature will continue to rise constantly if the heating can be maintained, as was initially the case.
This first arrest point (AC1) is called a critical point or point of decalescence.
Now observe a piece of steel heated to 900 °C (a bright reddish-yellow colour) and then allowed to cool in dim light so that the colour can be observed effectively.
On cooling, the steel will lose its brilliance first. The cooling will continue normally until the temperature is reached (AR1) which more or less coincides with the temperature where the steel experienced the arrest point during heating (AC1). At this point, it will appear as if the steel has stopped cooling and, by taking careful note, the steel will appear to have an extra glow as if it was heated. The rate of cooling will continue normally after this point; so noting that time is important during the cooling process.
Figure 3.11 shows a graph representing the steps for the duration of heating and cooling of steel against time taken. We must remember that the point at which this arrest in temperature drop takes place is known as the recalescence point (AR1) and indicates that the change in the internal structure of the steel has taken place. In all heat treatment, time and temperature are both important in producing the desired change in the steel.
Figure 3.11: The temperature – time graph