Rate Of Absorption





According to Guillet, the absorption of carbon is favored by those

special elements which exist as double carbides in steel. For example,

manganese exists as manganese carbide in combination with the iron

carbide. The elements that favor the absorption of carbon are:

manganese, tungsten, chromium and molybdenum those opposing it,

nickel, silicon, and aluminum. Guillet has worked out the effect

of the different elements on the rate of penetration in comparison

with steel that absorbed carbon at a given temperature, at an average

rate of 0.035 in. per hour.



His tables show that the following elements require an increased

time of exposure to the carburizing material in order to obtain

the same depth of penetration as with simple steel:



When steel contains Increased time of exposure

2.0 per cent nickel 28 per cent

7.0 per cent nickel 30 per cent

1.0 per cent titanium 12 per cent

2.0 per cent titanium 28 per cent

0.5 per cent silicon 50 per cent

1.0 per cent silicon 80 per cent

2.0 per cent silicon 122 per cent

5.0 per cent silicon No penetration

1.0 per cent aluminum 122 per cent

2.0 per cent aluminum 350 per cent



The following elements seem to assist the rate of penetration of

carbon, and the carburizing time may therefore be reduced as follows:



When steel contains Decreased time of exposure

0.5 per cent manganese 18 per cent

1.0 per cent manganese 25 per cent

1.0 per cent chromium 10 per cent

2.0 per cent chromium 18 per cent

0.5 per cent tungsten 0

1.0 per cent tungsten 0

2.0 per cent tungsten 25 per cent

1.0 per cent molybdenum 0

2.0 per cent molybdenum 18 per cent



The temperature at which carburization is accomplished is a very

important factor. Hence the necessity for a reliable pyrometer,

located so as to give the temperature just below the tops of the

pots. It must be remembered, however, that the pyrometer gives

the temperature of only one spot, and is therefore only an aid to

the operator, who must use his eyes for successful results.



The carbon content of the case generally is governed by the temperature

of the carburization. It generally proves advisable to have the

case contain between 0.90 per cent and 1.10 carbon; more carbon

than this gives rise to excess free cementite or carbide of iron,

which is detrimental, causing the case to be brittle and apt to chip.



T. G. Selleck gives a very useful table of temperatures and the

relative carbon contents of the case of steels carburized between

4 and 6 hrs. using a good charcoal carburizer. This data is as

follows:



TABLE 15.--CARBON CONTENT OBTAINED AT VARIOUS TEMPERATURES



At 1,500 deg.F., the surface carbon content will be 0.90 per cent

At 1,600 deg.F., the surface carbon content will be 1.00 per cent

At 1,650 deg.F., the surface carbon content will be 1.10 per cent

At 1,700 deg.F., the surface carbon content will be 1.25 per cent

At 1,750 deg.F., the surface carbon content will be 1.40 per cent

At 1,800 deg.F., the surface carbon content will be 1.75 per cent



To this very valuable table, it seems best to add the following

data, which we have used for a number of years. We do not know

the name of its author, but it has proved very valuable, and seems

to complete the above information. The table is self-explanatory,

giving depth of penetration of the carbon of the case at different

temperatures for different lengths of time:



---------------------------------------------------------

Temperature

Penetration -----------------------------

1,550 1,650 1,800

------------------------------------------------------

Penetration after 1/2 hr. 0.008 0.012 0.030

Penetration after 1 hr. 0.018 0.026 0.045

Penetration after 2 hr. 0.035 0.048 0.060

Penetration after 3 hr. 0.045 0.055 0.075

Penetration after 4 hr. 0.052 0.061 0.092

Penetration after 6 hr. 0.056 0.075 0.110

Penetration after 8 hr. 0.062 0.083 0.130

---------------------------------------------------------



From the tables given, we may calculate with a fair degree of certainty

the amount of carbon in the case, and its penetration. These figures

vary widely with different carburizers, and as pointed out immediately

above, with different alloy steels.





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