Heating Methods for Metals and Alloys | Metallurgy

The first step in any heat treatment cycle is to heat the parts to a predetermined temperature.

The following are the principles of heating methods of metals and alloys:

1. Heat the parts in a furnace which has already been heated to the required heat treating temperature. The parts get heated up at a fast rate. Fig. 2.1 (a) illustrates one heating curve for the surface of the part and the other for the centre of the part, if it is a thick part, or and if it is made of material of relatively bad conductor of heat. Some soaking time is needed here so that the centre also attains the desired heating temperature and transforms to a homogeneous structure.

A still more rapid heating may be obtained if the heated furnace is well above the required temperature, and when the surface of the part is about to attain the desired temperature of heating, the temperature of the furnace may be allowed to fall to this desired heating temperature as illustrated in Fig. 2.1 (b), or remove the part from the furnace when its surface has attained the temperature, and put in section of furnace which is at the desired heating temperature. Advantage is taken of this latter principle particularly in continuous furnace practice.

Where the temperature of the furnace is kept well above the desired temperature and the passage through the furnace is regulated so that the piece being treated will reach the desired temperature at the outgoing end of the furnace. If the part is thick, then a small section at the end of the continuous furnace is kept heated at the constant heat treating temperature, while the remaining front portion of the furnace is kept at a higher temperature.

The passage of the part through- the furnace is so regulated that no sooner the surface of the part attains the desired heat treating temperature during its motion through high temperature zone, it moves forward to the zone of the furnace kept heated to the heat treating temperature (end part of the furnace), which heats the centre of the part too to the desired heat treatment temperature.

Quite often, a batch furnace may be kept heated to a much higher temperature. When large amount of cold parts are charged into it, it loses heat to the charge and the temperature is allowed to fall to the required heat treating temperature, and made to maintain at that temperature till the parts get heated up to the required heat treating temperature. In practice, much more rapid heating may be employed, but in such cases, the safety of the practice must generally be determined by experiment. Generally, the safe rule of heating time of one hour per 25 millimeter is employed.

2. Heat the parts along with the furnace at a required rate of heating, the principle of which is illustrated in Fig. 2.1 (c). It is a much slower rate of heating than 2.1 (a), or 2.1 (b).

On a temperature-time graph, a constant rate of heating is shown by a straight line with a slope such as shown by line OP for the heating of the furnace in Fig. 2.1 (c). If the slope is higher, then the rate of heating is higher. The actual rate of heating of the parts may not be always constant, but varying with time as illustrated by separate curves on the temperature-time graph for the surface and the centre of a part in say Fig. 2.1 (a) and (b). In such cases, the rate of heating at any instant, or temperature is given by the slope of the curve (tan θ) at that point such as point x in Fig. 2.1 (a) and y in Fig. 2.1 (b) at same temperature T₁.

The rate is higher in case 2.1 (b) than in 2.1 (a) as the slope is higher in former. Generally in practice, instead of instant heating rate, the average heating rate is used, which is calculated based on the total temperature interval divided by the time taken for that temperature rise, that is, the rate of heating,

Of the methods illustrated in Fig. 2.1 (a), (b) and (c), the maximum difference of temperature between the surface and the centre at any instant is in case 2.1 (b), which is because it is the highest of the three rates of heating, and the difference is minimum in case 2.1 (c), though the latter takes much more time in heating to the required temperature. The average slope is maximum of the curve of case 2.1 (b).

3. Step Heating:

Heat the parts in a furnace heated to a temperature much below the heat treating temperature and when, the centre of the parts also attains the first stage heating temperature as illustrated by curve Tc in Fig. 2.1 (d), the heating of the parts along with the furnace is done to the desired heat treatment temperature.

Many times, the heating of the parts may be done in more than one step in between. As high speed steels have very low thermal conductivity, large (of dia over 30 mm) tools of intricate shapes made from such a steel (say having composition- 0.75% C, 18% W, 4% Cr, 1% V are heated in three stages, i.e. heating is done in three separate furnaces (one after the other) kept at different temperatures.

1. Preheat the parts in a batch furnace kept at a temperature of around 350-400°C.

2. Preheat in a molten salt bath furnace kept at a temperature of 800°C, holding time is around 15-30 second per mm of diameter, or section.

3. Heat at the required heating temperature 1260°C to 1290°C in a molten salt bath (normally of BaCl₂) furnace, watching the time till the tool changes its colour to that of molten salt bath (that is, when it becomes only dimly visible on the bath surface-normally in 2-5 minutes) as illustrated in Fig. 2.1 (e).

In industrial practice, the first method of heating (Fig. 2.1 a) or its variation (Fig. 2.1 b), is most commonly used and preferred as the total time of heating a part is less, which gives the following advantages:

1. Less oxidation and scale formation

2. Less decarburisation

3. Less grain growth

4. The productivity of furnace increases

5. As holding time of material in heat treatment section is reduced, meaning thereby, more production at less cost.

6. Low fuel/power consumption as the furnace need not be heated every time with the charge as in method 2.1 (c).

This method, being cheaper, is the most desired method from the point of view of industrial production. However, too rapid a heating rate may set up high internal stresses, particularly if irregular and heavy sections are involved and thus, may become many times undesirable. For example, if a thick cylinder is put in a furnace already at high temperature.

There shall be a large difference of temperature at any instant between the surface layers and the centre of the part depending on following factors:

1. Thickness or Diameter of the Cylinder:

The larger is the diameter/thickness greater shall be the difference in temperatures between the surface layers and the centre of the part.

2. Thermal Conductivity:

A material of low thermal conductivity conducts the heat slowly to the interior of the part, i.e. poor thermal conductors have greater difference of temperature between the surface layers and the centre. The thermal conductivity of the steel becomes less as the carbon content and the amount of alloying elements increase in steel.

Table 2.1 illustrates the thermal conductivity of some steels:

3. Temperature of the Furnace:

Higher is the temperature of the furnace (as the furnace might have been kept at a very high temperature than the heat treating temperature to quicken the process of heating), more is the difference of temperature between the surface and the centre of the part.

4. Medium of the Furnace (Heat Transfer Characteristics of the Medium):

Salt, or molten baths generally have higher heat t ran star coefficient and therefore heat the parts more rapidly than an air furnace.

All these factors can cause non-uniform heating of say, steel, which leads to a large gap in the heating curves of the surface and of the centre. Fig 2.2 (b) illustrates that after heating time of 100 minutes, a difference of temperature of around 400°C exists between the surface and the centre of a 800 mm diameter steel cylinder. The surface layers being at high temperature expand much more than the centre of the cylinder, which is at a much lower temperature.

The central part inhibits the expansion of the outer layers, whereas the surface layers try to force the central part to expand to greater extent than caused by heating so far.

Thus, the surface layers put the central part under tension, that is, the central part develop tensile nature of internal stresses, whereas the surface layers being under compression (due to restraints of central part) develop compressive nature of internal stresses. When the temperature becomes higher than temperature of phase transformation, the new phase forms to cause contraction (pearlite changing to austenite in steel).

This contraction is also non-uniform, which may complicate the nature, magnitude and distribution of stresses in the steel part. More non-uniform is the heating of the part, larger the magnitude of the stresses developed Wherever magnitude of these internal stresses exceeds the yield strength of the material, distortion (due to yielding) takes place and when its magnitude becomes more than the tensile strength of the material at that temperature, the cracks may develop in the steel heavy cylinder.

Now, Table 2.2 illustrates the tensile strength, yield strength of 0.35% C steel at various temperatures. The recrystallisation temperature of the steel is around 650°C, that is, above this temperature the steel recrystallises to remove these internal stresses by local plastic deformation because then the steel is ductile. Thus above 600°C or so, the steel parts may be heated at a fast rate with negligible distortion and cracks. In temperature interval 500° to 600°G, the tensile as well as yield strengths have low values (Table 2.2).

The development of internal stress in this temperature interval due to non-uniform expansion (due to non-uniform heating, or high rate of heating) and or phase transformations (in alloy steels) becomes dangerous particularly if its magnitude exceeds yield strength or tensile strength to cause distortion or cracks respectively in the steel parts.

To avoid any such chance, the heating of massive and intricate shaped articles particularly, is done, in steps as illustrated in Fig. 2.1 (d) and 2.1 (e). Commonly the steel is heated in a furnace kept in between 500°C to 600°C, held at this temperature for good time so that the temperature over the whole section of steel part becomes uniform.

As the rate of heating is slow, due to step-temperature being low, hardly internal stresses are developed, more because the temperature becomes uniform by holding at the step-temperature for a long time. Fig. 2.1 (e) illustrates that heavy, intricate and expensive high speed steel is heated in three steps to avoid any distortion and cracks, during heating to very high temperature of austenitising. Normally, the heating rate above the step is increased to have smaller heating time from the temperature of step to the final heat-treating temperature to avoid more oxidation, decarburisation and grain growth.

For plain carbon steels and low alloy steels, heating up time in an oil fired furnace to hardening temperature, 850°C as a function of bar diameter based on empirical calculations and actual trials.