Practical Heat Treatment of Steel

Method of Heating 

Steel should be heated slowly and uniformly, as the temperature to which steel is  heated is one of the factors determining the success of any heat treatment. It is   to use heating equipment which permits rigid control of the temperature. Muffle furnaces ovens usually heated by oil. gas or electricity, are clean to use, capable of high degree of heat  and can be left at any given temperature Without fear of overheating.

The use of the black  smith's fire is not a good method of heating, as temperature control and uniform   difficult to maintain. Unless great care is taken, the metal may become overheated and burnt.

When using a blacksmith’s fire heat the jobs, if possible, in a steel tube and not directly in  the flame. Baths heated by oil or gas and containing low melting point alloys or molten salts  provide a good method of heat-treating steel.

The heating is clean uniform and under strict  control. Irregular shapes can be heated throughout without overheating. Various salts may  be used in the bath according to the range of temperature required.   

Measurement of Temperature  

For measuring the temperature of bath,  furnace or oven, some form of pyrometer is  used.

The “ Thermo-electric ” pyrometer embodies the “thermo-couple ” principle i.e. if two  wires or the ends of strips of dissimilar metal  are joined and the junction is heated, a small  electrical pressure is generated.

With increase in  temperature there is a corresponding increase  in electric pressure. Connected to the free ends  of the wires or strips, is a millivoltmeter, which  has a scale calibrated to read degrees centigrade.  The instrument is situated close to the heating  apparatus.

The hot junction, protected by a  porcelain tube, is located inside the furnace  or bath.   

When the open fire ‘method of heating is employed, the temperature of the work is  judged by its temperature color, or if the temperature is within the normal tempering  range, by color of the oxide film which forms on its surface.

Although the results obtained  are satisfactory for forging operations, and the hardening and tempering of a single tool  such as a cold chisel, the heat treatment of steel should be done by more accurate methods of  measuring temperature.   

Temperature and Approximate Color Guide

 The colors and corresponding temperatures are given in the table set out below  but it must be emphasized that experience is necessary to identify the colors accurately.

Quenching   

The usual quenching media in the order of rapidity of effect are brine, water, oil  and air. Work which is to be hardened all over should be plunged straight“ into the  quenching medium. If the work has to be partially quenched, it is advisable to move it, up and down to prevent the formation of a sharp hardening line which might subsequently  be the cause of a crack. If the shape of the Work is irregular, quench the larger portion  first. Cracks and distortion during quenching are often due to internal strains set up  by previous forging, bending or other cold working, which can be avoided by normalizing  prior to hardening.

Successful hardening of steel will largely depend upon the following factors :

a) Control over the rate of heating specially to prevent cracking of thick and  irregular sections. 
(b) Thorough and uniform heating to the correct hardening temperature.
(c) Correct heat absorbing capacity, viscosity and temperature of quenching media.     

Tempering Temperatures 

The higher the tempering temperature, the lesser the hardness. but the greater the  toughness of the steel. Thus the purpose of the tool or component must be considered.  The tempering temperature may be approximately determined by the “temper color" appears on the bright surface of steel when slowly heated. Each color corresponds to  a particular temperature.

“ One heat” Method of  Hardening and’ Tempering

9. This is the method used for tools which require a hard point or cutting edge  with the remainder of the tool left tough to Withstand hammering or shock, e.g., cold  a  chisels, center punches etc. The procedure for making, hardening and tempering a carbon“  steel chisel by this method is as follows :

Heat the metal to a light cherry red color and forge it to shape.

1. Normalize and when cold, grind roughly to shape.  
2. Heat the cutting end half of the chisel to cherry red, then plunge vertically  into water to about a quarter of the length, moving it up and down a little  to prevent a sharp line between the quenched and unquenched portions which  would cause “water line” cracks.   
3. When the point of the chisel is black, withdraw it from the water, polish  the point with emery‘ or sand stone and observe the temper colors formed  as the heat flows down from the unquenched portion.  
4. When the desired color (purple brown) appears, instantly quench the tip,  and wait until all visible red heat has died away, before submerging the  remainder of the tool.   Grind the cutting edge, exercising care not to overheat the stool.                  

 Annealing Temperatures   

10) The temperature for annealing should be above the upper critical point for steel  containing upto 0-87% Of carbon and below this point for steels containing a higher percentage of carbon. For annealing single item by the Smith’s fire method the item should  be heated to a cherry red and allowed to cool as slowly as possible with, the "fire" or in  hot ashes and must be excluded from draughts.