The Working of Steel Part 15

Great advancement has been made in the heat treating and hardening of gears. In this advancement the chemical and metallurgical laboratory have played no small part. During this time, however, the condition of the blanks as they come to the machine shop to be machined has not received its share of attention.

There are two distinct types of gears, both types having their champions, namely, carburized and heat-treated. The difference between the two in the matter of steel composition is entirely in the carbon content, the carbon never running higher than 25-point in the carburizing type, while in the heat-treated gears the carbon is seldom lower than 35-point. The difference in the final gear is the hardness. The carburized gear is file hard on the surface, with a soft, tough and ductile core to withstand shock, while the heat-treated gear has a surface that can be touched by a file with a core of the same hardness as the outer surface.

ANNEALING WORK.--With the exception of several of the higher types of alloy steels, where the percentages of special elements run quite high, which causes a slight air-hardening action, the carburizing steels are soft enough for machining when air cooled from any temperature, including the finishing temperature at the hammer.

This condition has led many drop-forge and manufacturing concerns to consider annealing as an unnecessary operation and expense.

In many cases the drop forging has only been heated to a low temperature, often just until the piece showed color, to relieve the so-called hammer strains. While this has been only a compromise it has been better than no reheating at all, although it has not properly refined the grain, which is necessary for good machining conditions.

Annealing is heating to a temperature slightly above the highest critical point and cooling slowly either in the air or in the furnace.

Annealing is done to accomplish two purposes: (1) to relieve mechanical strains and (2) to soften and produce a maximum refinement of grain.

PROCESS OF CARBURIZING.--Carburizing imparts a sh.e.l.l of high-carbon content to a low-carbon steel. This produces what might be termed a "dual" steel, allowing for an outer sh.e.l.l which when hardened would withstand wear, and a soft ductile core to produce ductility and withstand shock. The operation is carried out by packing the work to be carburized in boxes with a material rich in carbon and maintaining the box so charged at a temperature in excess of the highest critical point for a length of time to produce the desired depth of carburized zone. Generally maintaining the temperature at 1,650 to 1,700 F. for 7 hr. will produce a carburized zone 1/32 in. deep.

Heating to a temperature slightly above the highest critical point and cooling suddenly in some quenching medium, such as water or oil hardens the steel. This treatment produces a maximum refinement with the maximum strength.

Drawing to a temperature below the highest critical point (the temperature being governed by the results required) relieves the hardening strains set up by quenching, as well as the reducing of the hardness and brittleness of hardened steel.

EFFECTS OF PROPER ANNEALING.--Proper annealing of low-carbon steels causes a complete solution or combination to take place between the ferrite and pearlite, producing a h.o.m.ogeneous ma.s.s of small grains of each, the grains of the pearlite being surrounded by grains of ferrite. A steel of this refinement will machine to good advantage, due to the fact that the cutting tool will at all times be in contact with metal of uniform composition.

While the alternate bands of ferrite and pearlite are microscopically sized, it has been found that with a Gleason or Fellows gear-cutting machine that rough cutting can be traced to poorly annealed steels, having either a p.r.o.nounced banded structure or a coa.r.s.e granular structure.

TEMPERATURE FOR ANNEALING.--Theoretically, annealing should be accomplished at a temperature at just slightly above the critical point. However, in practice the temperature is raised to a higher point in order to allow for the solution of the carbon and iron to be produced more rapidly, as the time required to produce complete solution is reduced as the temperature increases past the critical point.

For annealing the simpler types of low-carbon steels the following temperatures have been found to produce uniform machining conditions on account of producing uniform fine-grain pearlite structure:

0.15 to 0.25 per cent carbon, straight carbon steel.--Heat to 1,650F.

Hold at this temperature until the work is uniformly heated; pull from the furnace and cool in air.

0.15 to 0.25 per cent carbon, 1-1/2 per cent nickel, 1/2 per cent chromium steel.--Heat to 1,600F. Hold at this temperature until the work is uniformly heated; pull from the furnace and cool in air.

0.15 to 0.25 per cent carbon, 3-1/2 per cent nickel steel.--Heat to 1,575F. Hold at this temperature until the work is uniformly heated; pull from the furnace and cool in air.

CARE IN ANNEALING.--Not only will benefits in machining be found by careful annealing of forgings but the subsequent troubles in the hardening plant will be greatly reduced. The advantages in the hardening start with the carburizing operation, as a steel of uniform and fine grain size will carburize more uniformly, producing a more even hardness and less chances for soft spots. The holes in the gears will also "close in more uniformly," not causing some gears to require excessive grinding and others with just enough stock. Also all strains will have been removed from the forging, eliminating to a great extent distortion and the noisy gears which are the result.

With the steels used, for the heat-treated gears, always of a higher carbon content, treatment after forging is necessary for machining, as it would be impossible to get the required production from untreated forgings, especially in the alloy steels. The treatment is more delicate, due to the higher percentage of carbon and the natural increase in cement.i.te together with complex carbides which are present in some of the higher types of alloys.

Where poor machining conditions in heat-treated steels are present they are generally due to incomplete solution of cement.i.te rather than bands of free ferrite, as in the case of case-hardening steels.

This segregation of carbon, as it is sometimes referred to, causes hard spots which, in the forming of the tooth, cause the cutter to ride over the hard metal, producing high spots on the face of the tooth, which are as detrimental to satisfactory gear cutting as the drops or low spots produced on the face of the teeth when the pearlite is coa.r.s.e-grained or in a banded condition.

In the simpler carburized steels it is not necessary to test the forgings for hardness after annealing, but with the high percentages of alloys in the carburizing steels and the heat-treated steels a hardness test is essential.

To obtain the best results in machining, the microstructure of the metal should be determined and a hardness range set that covers the variations in structure that produce good machining results.

By careful control of the heat-treating operation and with the aid of the Brinell hardness tester and the microscope it is possible to continually give forgings that will machine uniformly and be soft enough to give desired production. The following gives a few of the hardness numerals on steel used in gear manufacture that produce good machining qualities:

0.20 per cent carbon, 3 per cent nickel, 1-1/4; per cent chromium--Brinell 156 to 170.

0.50 per cent carbon, 3 per cent nickel, 1 per cent chromium--Brinell 179 to 187.

0.50 per cent carbon chrome-vanadium--Brinell 170 to 179.

THE INFLUENCE OF SIZE

The size of the piece influences the physical properties obtained in steel by heat treatment. This has been worked out by E. J. Janitzky, metallurgical engineer of the Illinois Steel Company, as follows:

[Ill.u.s.tration: FIG. 55.--Effect of size on heating.]

"With an increase in the ma.s.s of steel there is a corresponding decrease in both the minimum surface hardness and depth hardness, when quenched from the same temperature, under identical conditions of the quenching medium. In other words, the physical properties obtained are a function of the surface of the metal quenched for a given ma.s.s of steel. Keeping this primary a.s.sumption in mind, it is possible to predict what physical properties may be developed in heat treating by calculating the surface per unit ma.s.s for different shapes and sizes. It may be pointed out that the figures and chart that follow are not results of actual tests, but are derived by calculation. They indicate the mathematical relation, which, based on the fact that the physical properties of steel are determined not alone by the rate which heat is lost per unit of surface, but by the rate which heat is lost per unit of weight in relation to the surface exposed for that unit. The unit of weight has for the different shaped bodies and their sizes a certain surface which determines their physical properties.

"For example, the surface corresponding to 1 lb. of steel has been computed for spheres, rounds and flats. For the sphere with a unit weight of 1 lb. the portion is a cone with the apex at the center of the sphere and the base the curved surface of the sphere (surface exposed to quenching). For rounds, a unit weight of 1 lb. may be taken as a disk or cylinder, the base and top surfaces naturally do not enter into calculation. For a flat, a prismatic or cylindrical volume may be taken to represent the unit weight. The surfaces that are considered in this instance are the top and base of the section, as these surfaces are the ones exposed to cooling."

The results of the calculations are as follows:

TABLE 20.--SPHERE

Diameter Surface per of sphere pound of steel _X_ _Y_ 8 in. 2.648 sq. in.

6 in. 3.531 sq. in.

4 in. 5.294 sq. in.

3 in. 7.062 sq. in.

2 in. 10.61 sq. in.

_XY_ = 21.185.

TABLE 21.--ROUND

Diameter Surface per of round pound of steel _X_ _Y_ 8.0 in. 1.765 sq. in.

6.0 in. 2.354 sq. in.

5.0 in. 2.829 sq. in.

4.0 in. 3.531 sq. in.

3.0 in. 4.708 sq. in.

2.0 in. 7.062 sq. in.

1.0 in. 14.125 sq. in.

0.5 in. 28.25 sq. in.

0.25 in. 56.5 sq. in.

_XY_ = 14.124.

TABLE 22.--FLAT

Thickness Surface per of flat pound of steel _X_ _Y_ 8.0 in. 0.8828 sq. in.

6.0 in. 1.177 sq. in.

5.0 in. 1.412 sq. in.

4.0 in. 1.765 sq. in.

3.0 in. 2.345 sq. in.

2.0 in. 3.531 sq. in.

1.0 in. 7.062 sq. in.

0.5 in. 14.124 sq. in.

0.25 in. 28.248 sq. in.

_XY_ = 7.062.

Having once determined the physical qualities of a certain specimen, and found its position on the curve we have the means to predict the decrease of physical qualities on larger specimens which receive the same heat treatment.

When the surfaces of the unit weight as outlined in the foregoing tables are plotted as ordinates and the corresponding diameters as abscissae, the resulting curve is a hyperbola and follows the law _XY = C_. In making these calculations the radii or one-half of the thickness need only to be taken into consideration as the heat is conducted from the center of the body to the surface, following the shortest path.