For ring gears and pinions material of the following chemical composition is recommended: Carbon, 0.100 to 0.200 per cent; manganese, 0.350 to 0.650 per cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum per cent; chromium, 0.550 to 0.750 per cent; nickel, 0.400 to 0.600 per cent.
Care should be taken to see that this material is properly deoxidized either by the use of ferrovanadium or its equivalent. The advantage of using a material of the above type lies in the fact that it will produce a satisfactory finished part with a very simple treatment.
The heat treatment of ring gears and pinions is as follows: "Carburize at a temperature of from 1,650 to 1,700F. for a sufficient length of time to secure a depth of case of from 1/32 to 3/64 in., and quench directly from carburizing heat in oil. Reheat to a temperature of from 1,430 to 1,460F. and quench in oil. Temper in oil at a temperature of from 375 to 425F. The final quenching operation on a ring gear should be made on a fixture similar to the Gleason press to reduce distortion to a minimum."
One of the largest producers of ring gears and pinions in the automotive industry has been using this material and treatment for the last 2 years, and is of the opinion that he is now producing the highest quality product ever turned out by that plant.
On some designs of automobiles a large amount of trouble is experienced with the driving pinion. If the material and heat treatment specified will not give satisfaction, rather than to change the design it is possible to use the following a.n.a.lysis material, which will raise the cost of the finished part but will give excellent service: Carbon, 0.100 to 0.200 per cent; manganese, 0.350 to 0.650 per cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum per cent; nickel, 4.750 to 5.250 per cent.
The heat treatment of pinions produced from this material consists in carburizing at a temperature of from 1,600 to 1,650F. for a sufficient length of time to secure a depth of case from 1/32 to 3/64 in. The pinions are then quenched in oil from a temperature of 1,500 to 1,525F. to refine the grain of the core and quenched in oil from a temperature of from 1,340 to 1,360F. To refine and harden the case. The use of this material however, is recommended only in an emergency, as high-nickel steel is very susceptible to seams, secondary pipe and laminations.
The main criterion on rear-axle and pinion shafts, steering knuckles and arms and parts of this general type is resistance to fatigue and torsion. The material recommended for parts of this character is either S. A. E. No. 6135 or No. 3135 steel, which have the chemical composition given in Tables 9 and 7.
HEAT TREATMENT OF AXLES
Parts of this general type should be heat-treated to show the following minimum physical properties: Elastic limit, 115,000 lb. per square inch; elongation in 2 in., 16 per cent; reduction of area, 50 per cent; Brinell hardness, 277 to 321.
The heat treatment used to secure these physical properties consists in quenching from a temperature of from 1,520 to 1,540F. in water and tempering at a temperature of from 975 to 1,025F. Where the axle shaft is a forging, and in the case of steering knuckles and arms, this heat treatment should be preceded by normalizing the forgings at a temperature of from 1,550 to 1,600F. It will be noted that these physical properties correspond to those worked out for an ideal aviation engine crankshaft. If parts of this type are designed with proper sections, so that this range of physical properties can be used, the part in question will give maximum service.
One of the most important developments during the Liberty engine program was the fact that it is not necessary to use a high-a.n.a.lysis alloy steel to secure a finished part which will give proper service.
This fact should save the automotive industry millions of dollars on future production.
If the proper authority be given the metallurgical engineer to govern the handling of the steel from the time it is purchased until it is a.s.sembled into finished product, mild-a.n.a.lysis steels can be used and the quality of the finished product guaranteed.
It was only through the careful adherence to these fundamental principles that it was possible to produce 20,000 Liberty engines, which are considered to be the most highly stressed mechanism ever produced, without the failure of a single engine from defective material or heat treatment.
MAKING STEEL b.a.l.l.s
Steel b.a.l.l.s are made from rods or coils according to size, stock less than 9/16-in. comes in coils. Stock 5/8-in. and larger comes in rods. Ball stock is designated in thousandths so that 5/8-in.
rods are known as 0.625-in. stock.
Steel for making b.a.l.l.s of average size is made up of:
Carbon 0.95 to 1.05 per cent Silicon 0.20 to 0.35 per cent Manganese 0.30 to 0.45 per cent Chromium 0.35 to 0.45 per cent Sulphur and phosphorus not to exceed 0.025 per cent
For the larger sizes a typical a.n.a.lysis is:
Carbon 1.02 per cent Silicon 0.21 per cent Manganese 0.40 per cent Chromium 0.65 per cent Sulphur 0.026 per cent Phosphorus 0.014 per cent
b.a.l.l.s 5/8 in. and below are formed cold on upsetting or heading machines, the stock use is as follows:
TABLE 14.--SIZES OF STOCK FOR FORMING b.a.l.l.s ON HEADER ------------------------------------------------------- Diameter of | Diameter of | Diameter of | Diameter of ball, inch | stock inch | ball, inch | stock, inch -------------|-------------|-------------|------------- 1/8 | 0.100 | 5/16 | 0.235 5/32 | 0.120 | 3/8 | 0.275 3/16 | 0.145 | 7/16 | 0.320 7/32 | 0.170 | 1/2 | 0.365 1/4 | 0.190 | 9/16 | 0.395 9/32 | 0.220 | 5/8 | 0.440 -------------------------------------------------------
For larger b.a.l.l.s the blanks are hot-forged from straight bars.
They are usually forged in multiples of four under a spring hammer and then separated by a suitable punching or shearing die in a press adjoining the hammer. The dimensions are:
----------------------------------------------------------- Diameter of ball, | Diameter of die, | Diameter of stock, inch | inch | inch -------------------|------------------|-------------------- 3/4 | 0.775 | 0.625 7/8 | 0.905 | 0.729 1 | 1.035 | 0.823 -----------------------------------------------------------
Before hardening, the b.a.l.l.s are annealed to relieve the stresses of forging and grinding, this being done by pa.s.sing them through a revolving retort made of nichrome or other heat-resisting substance.
The annealing temperature is 1,300F.
The hardening temperature is from 1,425 to 1,475F. according to size and composition of steel. Small b.a.l.l.s, 5/16 and under, are quenched in oil, the larger sizes in water. In some special cases brine is used. Quenching small b.a.l.l.s in water is too great a shock as the small volume is cooled clear through almost instantly. The larger b.a.l.l.s have metal enough to cool more slowly.
b.a.l.l.s which are cooled in either water or brine are boiled in water for 2 hr. to relieve internal stresses, after which the b.a.l.l.s are finished by dry-grinding and oil-grinding.
The ball makers have an interesting method of testing stock for seams which do not show in the rod or wire. The Hoover Steel Ball Company cut off pieces of rod or wire 7/16 in. long and subject them to an end pressure of from 20,000 to 50,000 lb. A pressure of 20,000 lb. compresses the piece to 3/16 in. and the 50,000 lb.
pressure to 3/32 in. This opens any seam which may exist but a solid bar shows no seam.
Another method which has proved very successful is to pa.s.s the bar or rod to be tested through a solenoid electro-magnet. With suitable instruments it is claimed that this is an almost infallible test as the instruments show at once when a seam or flaw is present in the bar.
CHAPTER V
THE FORGING OF STEEL
So much depends upon the forging of steel that this operation must be carefully supervised. This is especially true because of the tendency to place unskilled and ignorant men as furnace-tenders and hammer men. The main points to be supervised are the slow and careful heating to the proper temperature; forging must be continued at a proper rate to the correct temperature. The bar of stock from which a forging was made may have had a fairly good structure, but if the details of the working are not carefully watched, a seamy, split article of no value may easily result.
HEATING.--Although it is possible to work steels cold, to an extent depending upon their ductility, and although such operations are commonly performed, "forging" usually means working _heated_ steel.
_Heating_ is therefore a vital part of the process.
Heating should be done slowly in a soaking heat. A soft "lazy"
flame with excess carbon is necessary to avoid burning the corners of the bar or billet, and heavily scaling the surface. If the temperature is not raised slowly, the outer part of the metal may be at welding heat while the inner part is several hundred degrees colder and comparatively hard and brittle.
The above refers to m.u.f.fle furnaces. If the heating is done in a small blacksmith"s forge, the fire should be kept clean, and remade at intervals of about two hours. Ashes and cinders should be cleaned from the center down to the tuyere and oily waste and wood used to start a new fire. As this kindles a layer of c.o.ke from the old fire is put on top, and another layer of green coal (screened and dampened blacksmiths" coal) as a cover. When the green coal on top has been c.o.ked the fire is ready for use. As the fuel burns out in the center, the c.o.ke forming around the edge is pushed inward, and its place taken by more green coal. Thus the fire is made up of three parts; the center where c.o.ke is burning and the iron heating; a zone where c.o.ke is forming, and the outside bank of green coal.
STEEL WORKED IN AUSTENITIC STATE.--As a general rule steel should be worked when it is in the austenitic state. (See page 108.) It is then soft and ductile.
As the steel is heated above the critical temperature the size of the austenite crystals tends to grow rapidly. When forging starts, however, these grains are broken up. The growth is continually destroyed by the hammering, which should consequently be continued down to the upper critical temperature when the austenite crystals break up into ferrite and cement.i.te. The size of the final grains will be much smaller and hence a more uniform structure will result if the "mother" austenite was also fine grained. A final steel will be composed of pearlite; ferrite and pearlite; or cement.i.te and pearlite, according to the carbon content.
The ultimate object is to secure a fine, uniform grain throughout the piece and this can be secured by uniform heating and by thoroughly rolling it or working it at a temperature just down to its critical point. If this is correctly done the fracture will be fine and silky. Steel which has been overheated slightly and the forging stopped at too high a temperature will show a "granular" fracture.
A badly overheated or "burned" steel will have iridescent colors on a fresh fracture, it will be brittle both hot and cold, and absolutely ruined.
STEEL CAN BE WORKED COLD.--As noted above, steel can be worked cold, as in the case of cold-rolled steel. Heat treatment of cold-worked steel is a very delicate operation. Cold working hardens and strengthens steel. It also introduces internal stresses. Heat-treatments are designed to eliminate the stresses without losing the hardness and strength. This is done by tempering at a low heat. Avoid the "blue" range (350 to 750C.). Tempering for a considerable time just under the critical is liable to cause great brittleness. Annealing (reheating through the critical) destroys the effect of cold work.
FORGING
HIGH-SPEED STEEL.--Heat very slowly and carefully to from 1,800 to 2,000F. and forge thoroughly and uniformly. If the forging operation is prolonged do not continue forging the tool when the steel begins to stiffen under the hammer. Do not forge below 1,700F.
(a dark lemon or orange color). Reheat frequently rather than prolong the hammering at the low heats.
After finishing the forging allow the tool to cool as slowly as possible in lime or dry ashes; avoid placing the tool on the damp ground or in a draught of air. Use a good clean fire for heating.
Do not allow the tool to soak at the forging heat. Do not heat any more of the tool than is necessary in order to forge it to the desired shape.
CARBON TOOL STEEL.--Heat to a bright red, about 1,500 to 1,550F.
Do not hammer steel when it cools down to a dark cherry red, or just below its hardening point, as this creates surface cracks.
OIL-HARDENING STEEL.--Heat slowly and uniformly to 1,450F. and forge thoroughly. Do not under any circ.u.mstances attempt to harden at the forging heat. After cooling from forging reheat to about 1,450F. and cool slowly so as to remove forging strains.
CHROME-NICKEL STEEL.--Forging heat of chrome-nickel steel depends very largely on the percentage of each element contained in the steel. Steel containing from 1/2 to 1 per cent chromium and from 1-1/2 to 3-1/2 per cent nickel, with a carbon content equal to the chromium, should be heated very slowly and uniformly to approximately 1,600 F., or salmon color. After forging, reheat the steel to about 1,450 and cool slowly so as to remove forging strains. Do not attempt to harden the steel before such annealing.