The Working of Steel / Annealing, Heat Treating and Hardening of Carbon and Alloy Steel

by Fred H. Colvin; K. A. Juthe

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 1½ 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°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°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, ¾ 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°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.