Surface Hardening by Carburising

Introduction
    The service conditions of many steel components such as cams, gears, and shafts make it necessary for them to posses both hard, wear resistant surface and at the same time, tough, shock resistant cores. This situation can be best dealt with by introducing low carbon steel with suitable core properties. In this case the carbon penetrates the surface to a regulated depth (case depth) causing the material to be harder.

Basic Theory
    Surface hardening by carburizing can be classified into three kinds; solid carburizing, liquid carburizing and gas carburizing. In each of these three processes, certain chemicals and methods are used, for the solid carburizing we use charcoal, and in liquid carburizing we use Cyanide (CN) but in gas carburizing we use carbon monoxide gas (CO). The best way among these three ways is characterized according to depth, which goes from gas to solid. But in our case, we’ll be using solid carburizing because chemicals used in liquid and gas carburizing are poisonous and apparatus are of high cost.
Carburizing is solid media involves packing the work into heat resisting boxes by which the distance between them is 50mm. The boxes are heated to a temperature equal the carburizing temperature and maintained at that temperature for a period of time according to the case depth required.
The entrapped air between carbon molecules reacts with carbon to form CO gas according to this equation:
2C + O2 -> 2CO

The actual carburizing process depends on carbon monoxide gas to carry carbon atoms to the surface of the work piece.
And at the surface of the work piece, carbon is released according to this equation:
2CO -> CO2 + C

These carbon atoms are dissolved on the surface of the steel.

The carburizing process is affected by three factors:

1. Temperature
   
2. Time
   
3. Energizers: such as BaCO3, CaCO3 and NaOH
   

These energizers speed up or aid the process of carburizing by adding CO2, which later reacts with charcoal producing CO, where it carries C atoms to the surface of steel as explained above, according to these reaction:
BaCO3 -> BaO + CO2 & CO2 + C -> 2CO

Experimental Apparatus & Methods
    The materials needed in this experiment are 0.15% C steel specimens (the specimens are divided into three categories; 870ºC, 900ºC and 925ºC, and each specimen is left in the furnace for five different timings, 4, 16 ,22,24 and 30 hours), Stainless steel box containing solid carbon powder (Charcoal). As for the apparatus, we need a heating furnace, timing equipment, a microscope supplied with a monitor.

The procedure of this experiment is easy to follow, and it goes as:

1. Put each steel specimen in the stainless steel box containing the charcoal.
   
2. Put the box containing the specimen in the heating furnace at the specified temperature and leave it for the specified time (at 950ºC, steel changes from BCC at room temperature to FCC).
   
3. Turn off the furnace and take out the specimen out of the box and put it into another furnace at a temperature of 850ºC for half and hour.
   
4. Take out the specimen and cool it with water.
   
5. After cooling, each specimen is cut in half, grinded (with 120, 180, 240, 320, 400, 600 paper) and polished.
   
6. Each specimen is then put under the microscope and the case depth is calculated.
   
7. The last step in this experiment is to do the hardness test for each specimen with the Rockwell C scale (HRC) where we use the diamond cone as the indenter.
   

For the calculation of the case depth, the length of the carbon layer is measured on the monitor, and then it is divided over the magnification to give the true depth.
Another method may be applied is to follow a simple and easy rule: case depth = K(t), where K is a constant and t is the time.

In heat treatment, sometimes specimens are treated at two different temperatures each for half an hour (760ºC and 880ºC). One half hour is for case hardening and the other is for refining. Moreover, refining is done to get a high toughness from inside and increase the fatigue life.

Discussion
    As we can see, for the case depth, i.e. increasing each time and temperature, the case depth increases for each step more than the previous one. Thus, if we do the hardness test, as tabulated, we can see that hardness increases as we go down the table for the 5mm, 10mm,15mm (the center) positions but stays almost constant for the case (or surface). The case reaches a max value at 920ºC 6hrs and then goes back to the constant value.
Moreover, at the three other positions, the strength increases as the temperature and time increases, and this is because the concentration of carbon going from the case to the center decreases, thus causes the strength to decrease.

Conclusion
    As the concentration of carbon increases, the strength increases. As we saw, the strength is higher for the specimen at the case than at the 10 mm, and at 10 mm higher than at 5 mm, and at 5 mm higher than the centre. Thus, as a conclusion, the strength increases wherever there is a higher accumulation of carbon.

Also, surface hardening makes steel components hard, with a wear resistant surface and at the same time, tough and with shock resistant cores. Whereas time, temperature, and case produced by surface hardening are directly proportional, and as time and temperature increase, the case produced increase in depth.