Get documents about "Surface Hardening by Carburising
Document Sample
Surface Hardening by Carburising
1.0 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.
2.0 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
3.0 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:
Put each steel specimen in the stainless steel box
containing the charcoal.
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).
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.
Take out the specimen and cool it with water.
After cooling, each specimen is cut in half, grinded
(with 120, 180, 240, 320, 400, 600 paper) and
polished.
Each specimen is then put under the microscope
and the case depth is calculated.
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.
4.0 Results:
The results are as tabulated below:
Temperatures Time Depth of carbon (cm/magnification)
used (degrees) (hours) Mag. 100x Mag. 180x AVG.
900 5 0.06 0.05 0.055
900 6 0.1 0.083 0.915
900 9 0.12 0.01 0.065
920 3 0.06 0.047 0.0535
920 6 0.08 0.062 0.0745
920 9 0.155 0.12 0.1375
950 3 0.18 0.14 0.16
950 6 0.23 0.24 0.22
950 9 0.37 0.37 0.37
Temperature Time hardness
(degrees) (hours) Outer surface 5 mm from 10 mm from center
surface surface
900 5 53.9 19.6 18.5 16.5
900 6 53.9 15.5 14.4 10
900 9 53.9 29.2 9.3
920 5 53.9 20.7 12.8 11.9
920 6 63.9 50.9 47.5 48.5
920 9 53.9 31.5 18.2 16.7
950 5 53.9 24.8 17.5 16.8
950 6 53.9 17.4 15.2 10.2
950 9 53.9 40.4 18.4 17.1
HRC VS Distance
70
60
900 3hr
50 900 6hr
HRC
40 900 9hr
30 920 3hr
920 6hr
20
920 9hr
10 950 3hr
0 950 6hr
0 2 4 6 8 10 12 14 16 950 9hr
Distance (mm)
Time VS Case Depth
0.4
0.35
0.3
Time (hr)
0.25 900
0.2 920
0.15 950
0.1
0.05
0
0 2 4 6 8 10
Case Depth (mm)
5.0 Discussion
As we can see from the results table, for the case depth,
as we go down the table, 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.
6.0 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.
