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

by Fred H. Colvin; K. A. Juthe

First taking up the question of flame shields, the illustration, Fig. 102, is a typical installation that shows the main features for application to a forging machine or drop-hammer, oil-burning furnace, or for an arched-over, coal furnace where the flame blows out the front. This shield consists of a frame covered with sheet metal and held by brackets about 6 in. in front of the furnace. It will be noted that slotted holes make this frame adjustable for height, and it should be lowered as far as possible when in use, so that the work may just pass under it and into the furnace openings.

Immediately below the furnace openings, and close to the furnace frame will be noted a blast pipe carrying air from the forge-shop fan. This has a row of small holes drilled in its upper side for the entire length, and these direct a curtain of cold air vertically across the furnace openings, forcing all of the flame, or a greater portion of it, to rise behind the shield. Since the shield extends above the furnace top there is no escape for this flame until it has passed high enough to be of no further discomfort to the workman.

In this case fan-blast air is used for cooling, and this is cheaper and more satisfactory because a great volume may be used. However, where high-pressure air is used for atomizing the oil at the burner, and nothing else is available, this may be employed—though naturally a comparatively small pipe will be needed, in which minute holes are drilled, else the volume of air used will be too great for the compressor economically to supply. Steam may also be employed for like service.

FIGS. 102 to 108.—Protective devices for furnace fronts.

The latest shields of this type are all made double, as illustrated, with an inner sheet of metal an inch or two inside of the front. In the illustration, A, Fig. 102, this inner sheet is smaller, but some are now built the same size as the front and bolted to it with pipe spacers between. The advantage of the double sheet is that the inner one bears the brunt of the flame, and, if needs be, burns up before the outer; while, if due to a heavy fire it should be heated red at any point, the outer sheet will still be much cooler and act as an additional shield to the furnace man.

Heavy Forging Practice.—In heavy forging practice where the metal is being worked at a welding heat, the amount of flame that will issue from an open-front furnace is so great that a plain, sheet-steel front will neither afford sufficient protection nor stand up in service. For such a place a water-cooled front is often used. The general type of this front is illustrated in Fig. 103, and appears to have found considerable favor, for numbers of its kind are scattered throughout the country.

In this case the shield is placed at a slight angle from the vertical, and along the top edge is a water pipe with a row of small holes through which sprays of water are thrown against it. This water runs down in a thin sheet over the shield, cooling it, and is collected in a trough connected with a run-off pipe at the bottom. The lower blast-pipe arrangement is similar to the one first described.

There are several serious objections to this form of shield that should lead to its replacement by a better type; the first is that with a very hot fire, portions in the center may become so rapidly heated that the steam generated will part the sheet of water and cause it to flow from that point in an inverted V, and that section will then quickly become red hot. Another feature is that after the water and fire are shut down for the night the heat of the furnace can be great enough to cause serious warping of the surface of the shield so that the water will no longer cover it in a thin, uniform sheet.

After rigging up a big furnace with a shield of this type several years ago, its most serious object was found in the increase of the water bill of the plant. This was already of large proportions, but it had suddenly jumped to the extent of several hundred dollars. Investigation soon disclosed the fact that this water shield was one of the main causes of the added cost of water. A little estimating of the amount of water that can flow through a 1/2-in. pipe under 30-lb. pressure, in the course of a day, will show that this amount at 10 cts. per 1,000 gal., can count up rather rapidly.