Surface Carburizing





Carburizing, commonly called case-hardening, is the art of producing

a high-carbon surface, or case, upon a low carbon steel article.

Wrenches, locomotive link motions, gun mechanisms, balls and ball

races, automobile gears and many other devices are thereby given

a high-carbon case capable of assuming extreme hardness, while

the interior body of metal, the core, remains soft and tough.



The simplest method is to heat the piece to be hardened to a bright

red, dip it in cyanide of potassium (or cover it by sprinkling

the cyanide over it), keep it hot until the melted cyanide covers

it thoroughly, and quench in water. Carbon and nitrogen enter the

outer skin of the steel and harden this skin but leave the center

soft. The hard surface or case varies in thickness according to

the size of the piece, the materials used and the length of time

which the piece remains at the carburizing temperature. Cyanide

case-hardening is used only where a light or thin skin is sufficient.

It gives a thickness of about 0.002 in.



In some cases of cyanide carburizing, the piece is heated in cyanide

to the desired temperature and then quenched. For a thicker case

the steel is packed in carbon materials of various kinds such as

burnt leather scraps, charcoal, granulated bone or some of the

many carbonizing compounds.



Machined or forged steel parts are packed with case-hardening material

in metal boxes and subjected to a red heat. Under such conditions,

carbon is absorbed by the steel surfaces, and a carburized case is

produced capable of responding to ordinary hardening and tempering

operations, the core meanwhile retaining its original softness and

toughness.



Such case-hardened parts are stronger, cheaper, and more serviceable

than similar parts made of tool steel. The tough core resists breakage

by shock. The hardened case resists wear from friction. The low cost

of material, the ease of manufacture, and the lessened breakage

in quenching all serve to promote cheap production.



For successful carburizing, the following points should be carefully

observed:



The utmost care should be used in the selection of pots for carburizing;

they should be as free as possible from both scaling and warping.

These two requirements eliminate the cast iron pot, although many

are used, thus leaving us to select from malleable castings, wrought

iron, cast steel, and special alloys, such as nichrome or silchrome.

If first cost is not important, it will prove cheaper in the end

to use pots of some special alloy.



[Illustrations: FIGS. 27 to 30.--Case-hardening or carburizing boxes.]






The pots should be standardized to suit the product. Pots should be

made as small as possible in width, and space gained by increasing

the height; for it takes about 1-1/2 hr. to heat the average small

pot of 4 in. in width, between 3 and 4 hr. to heat to the center

of an 8-in. box, and 5 to 6 hr. to heat to the center of a 12-in.

box; and the longer the time required to heat to the center, the

more uneven the carburizing.



The work is packed in the box surrounded by materials which will

give up carbon when heated. It must be packed so that each piece

is separate from the others and does not touch the box, with a

sufficient amount of carburizing material surrounding each. Figures



27 to 31 show the kind of boxes used and the way the work should be

packed. Figure 31 shows a later type of box in which the edges can

be easily luted. Figure 30 shows test wires broken periodically to

determine the depth of case. Figure 28 shows the minimum clearance

which should be used in packing and Fig. 29 the way in which the

outer pieces receive the heat first and likewise take up the carbon

before those in the center. This is why a slow, soaking heat is

necessary in handling large quantities of work, so as to allow

the heat and carbon to soak in equally.



While it has been claimed that iron below its critical temperature

will absorb some carbon, Giolitti has shown that this absorption

is very slow. In order to produce quick and intense carburization

the iron should preferably be above its upper critical temperature

or 1,600 deg.F.,--therefore the carbon absorbed immediately goes into

austenite, or solid solution. It is also certain that the higher

the temperature the quicker will carbon be absorbed, and the deeper

it will penetrate into the steel, that is, the deeper the case.

At Sheffield, England, where wrought iron is packed in charcoal and

heated for days to convert it into blister steel, the temperatures

are from 1,750 to 1,830 deg.F. Charcoal by itself carburizes slowly,

consequently commercial compounds also contain certain energizers

which give rapid penetration at lower temperatures.



The most important thing in carburizing is the human element. Most

careful vigilance should be kept when packing and unpacking, and the

operator should be instructed in the necessity for clean compound

free from scale, moisture, fire clay, sand, floor sweepings, etc.

From just such causes, many a good carburizer has been unjustly

condemned. It is essential with most carburizers to use about 25 to

50 per cent of used material, in order to prevent undue shrinking

during heating; therefore the necessity of properly screening used

material and carefully inspecting it for foreign substances before

it is used again. It is right here that the greatest carelessness

is generally encountered.



Don't pack the work to be carburized too closely; leave at least

1 in. from the bottom, 3/4 in. from the sides, and 1 in. from the

top of pots, and for a 6-hr. run, have the pieces at least 1/2

in. apart. This gives the heat a chance to thoroughly permeate

the pot, and the carburizing material a chance to shrink without

allowing carburized pieces to touch and cause soft spots.



Good case-hardening pots and annealing tubes can be made from the

desired size of wrought iron pipe. The ends are capped or welded,

and a slot is cut in the side of the pot, equal to one quarter of

its circumference, and about 7/8 of its length. Another piece of

the same diameter pipe cut lengthwise into thirds forms a cover

for this pot. We then have a cheap, substantial pot, non-warping,

with a minimum tendency to scale, but the pot is difficult to seal

tightly. This idea is especially adaptable when long, narrow pots

are desired.



When pots are packed and the carburizer thoroughly tamped down,

the covers of the pot are put on and sealed with fire clay which

has a little salt mixed into it. The more perfect the seal the

more we can get out of the carburizer. The rates of penetration

depend on temperature and the presence of proper gas in the required

volume. Any pressure we can cause will, of course, have a tendency

to increase the rate of penetration.



If you have a wide furnace, do not load it full at one time. Put

one-half your load in first, in the center of the furnace, and

heat until pots show a low red, about 1,325 to 1,350 deg.F. Then fill

the furnace by putting the cold pots on the outside or, the section

nearest the source of heat. This will give the work in the slowest

portion of the furnace a chance to come to heat at the same time

as the pots that are nearest the sources of heat.



To obtain an even heating of the pots and lessen their tendency

to warp and scale, and to cause the contents of the furnace to

heat up evenly, we should use a reducing fire and fill the heating

chamber with flame. This can be accomplished by partially closing

the waste gas vents and reducing slightly the amount of air used

by the burners. A short flame will then be noticed issuing from

the partially closed vents. Thus, while maintaining the temperature

of the heating chamber, we will have a lower temperature in the

combustion chamber, which will naturally increase its longevity.



Sometimes it is advisable to cool the work in the pots. This saves

compound, and causes a more gradual diffusion of the carbon between

the case and the core, and is very desirable condition, inasmuch

as abrupt cases are inclined to chip out.



The most satisfactory steel to carburize contains between 0.10

and 0.20 per cent carbon, less than 0.35 per cent manganese, less

than 0.04 per cent phosphorus and sulphur, and low silicon. But

steel of this composition does not seem to satisfy our progressive

engineers, and many alloy steels are now on the market, these,

although more or less difficult to machine, give when carburized

the various qualities demanded, such as a very hard case, very tough

core, or very hard case and tough core. However, the additional

elements also have a great effect both on the rate of penetration

during the carburizing operation, and on the final treatment,

consequently such alloy steels require very careful supervision

during the entire heat treating operations.





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