Cam shaft manufacturing process

A cam shaft manufacturing process wherein a cam shaft is cast from a heat-treatable gray cast iron having a high carbide microstructure, is annealed while retaining the carbides and then cooled to room temperature. Surfaces, such as cam lobe surfaces, are then surface hardened and the cam shaft may thereafter be machined. The heat treating process comprises heating the cam shaft at 1600.degree. F. (871.degree. C.) in about twenty minutes, holding the cam shaft at 1600.degree. F. (871.degree. C.) for about twenty minutes and thereafter heat treating the cam shafts at a temperature of 1600.degree. to 1640.degree. F. (871.degree. to 893.degree. C.) for about eighty minutes. The cam shafts are then cooled rapidly to about 400.degree. F. (204.degree. C.), for example, within about an hour and a half, and thereafter air cooled.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to the manufacture of cam shafts having a series of 
cam lobes wherein the cam shafts are cast from a heat-treatable gray iron 
having a high alloy content. 
BACKGROUND ART 
In the manufacture of cam shafts for internal combustion engines, the cam 
shafts have been made by casting, or by steel forgings or by machining 
steel bar stock. The cam shafts have a very complex shape. Forging is a 
difficult process for making complex shapes to close tolerances. 
Accordingly, the forged cam shafts require some machining. Both of the 
forging and machining processes are quite expensive and difficult. 
Recently, a cam shaft has been made by casting a heat-treatable gray cast 
iron with the following composition: 
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Preferred Range General Range 
Analysis Percent Percent 
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Carbon 3.25-3.45 3.00-3.60 
Silicon 2.25-2.45 1.75-2.60 
Manganese 0.60-0.90 .50-.90 
Chromium 1.30-1.50 1.30-1.70 
Nickel plus copper 
0.40-0.60 .40-.80 
Molybdenum 0.40-0.50 .30-.75 
Sulfur 0.15-max .15 
Phosphorus 0.15-max .15 
Vanadium 0.25-0.40 .25-.60 
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This alloy in as-cast condition has significant carbides and a high 
hardness, pearlitic matrix. It is necessary to machine these castings 
somewhat to maintain tolerances. Thus, the castings must be annealed prior 
to machining. The as-cast hardness of the cam shaft was in the range of 
331-364 Brinell. These cam shafts were heat treated by raising the 
temperature to 1420.degree. F. (771.degree. C.) in four and one-half 
hours, holding that temperature for four to four and a half hours and then 
cooling slowly to atmospheric temperature in the oven. The cooling process 
typically took in excess of six or seven hours. 
Although the annealing decreased the Brinell hardness while retaining the 
carbides, it was found that the machinability was highly irregular and 
generally unsatisfactory. Some cam shafts thus heat-treated were virtually 
unmachinable. 
SUMMARY OF THE INVENTION 
According to the invention, a process for manufacturing a cam shaft having 
series of cam lobes from a heat-treatable gray cast iron comprises casting 
the cam shaft from a gray iron composition which includes elements 
selected from the group consisting of silicon, manganese, chromium, 
nickel, copper, molybdenum and vanadium, annealing the cast cam shaft to 
lower the hardness while retaining carbides, cooling the cam shaft, 
milling, surface hardening the cam lobe and thereafter machining the cam 
shaft. The improvement in applicant's invention comprises the 
heat-treating step in which the cam shaft is heated to a temperature in 
the range of 1550.degree. to 1700.degree. F. (843.degree.-927.degree. C.) 
in a relatively short period of time, for example, less than two hours, 
holding the cam shaft at the temperature for a relatively short period of 
time, for example, 1-4 hours, to enable the hardness to be reduced while 
retaining most of the carbides and thereafter cooling the cam shaft 
relatively quickly, for example, within about four hours. Preferably, the 
cam shaft is heated to about 1600.degree. F. (871.degree. C.) and held at 
that temperature for about thirty minutes, thereafter heated to a 
temperature within the range of 1600.degree. to 1640.degree. F. 
(871.degree.-893.degree. C.) for a period of eighty minutes, and then 
furnace cooled to a temperature of 400.degree. F. (204.degree. C.) in 
about one and one half hours. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The heat-treatable gray cast iron is generally a cast iron with a 
relatively high percentage of carbon and carbide-forming elements 
including chromium, molybdenum and vanadium. A graphitizing agent such as 
silicon and a sulfur scavenger such as manganese are desirably present in 
the composition. Pearlite stabilizers in the form of nickel and chromium 
are also added to the composition. A gray cast iron according to the 
invention has the following alloys: 
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Element Range - Percent 
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Total carbon 3.25-3.45 
Silicon 2.25-2.45 
Manganese 0.60-0.90 
Chromium 1.30-1.50 
Nickel plus copper 0.40-0.60 
Molybdenum 0.40-0.50 
Sulfur 0.15-max 
Phosphorus 0.15-max 
Vanadium 0.25-0.40 
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The Brinell hardness of the cam shaft as cast can vary but typically is in 
the range of 310 to 365 brinell. Subsequent to annealing the cam shaft 
hardness is reduced to approximately 270 to 320 Brinell. 
The temperature to which the cam shafts are heated is higher than normal 
annealing temperatures and is in the range of 1500.degree. to 1700.degree. 
F. (843.degree. to 927.degree. C.). The cam shafts are brought up to this 
temperature rapidly, for example, within 20 minutes to 1 hour, preferably 
within about 20 minutes. In a preferred embodiment, the cam shafts are 
brought up to a temperature of 1550.degree. to 1600.degree. F. 
(843.degree. to 871.degree. C.) in about 20 minutes. The cam shafts are 
thereafter held at this temperature to avoid thermal shock for a period of 
about 20 to 30 minutes and are thereafter subjected to an annealing 
temperature between 1550.degree. and 1700.degree. F. 
(843.degree.-927.degree. C.), preferably between 1600.degree. and 
1640.degree. F. (871.degree.-893.degree. C.) for a time of about one to 
four hours, preferably about 80 minutes. The time at which the cam shafts 
are held at the annealing temperature is selected so as to break down some 
of the iron carbides but retain the chromium carbides and/or iron-chromium 
carbides and to maintain the interstitial effect of vanadium carbide. 
The cooling from the annealing temperature takes place relatively rapidly, 
though not at quench rates, and to avoid thermal shock. The cooling takes 
place within one to four hours generally and preferably in about an hour 
and a half to about 400.degree. F. (204.degree. C.). 
The heat treating process, including cooling to 400.degree. F. (204.degree. 
C.) takes place in the lobes and other portions of the cam shafts can be 
surface hardened such as flame or induction hardening in a conventional 
manner. The cam shafts are typically straightened, if necessary, ground 
and drilled prior to the flame-hardening process. Subsequent to flame 
hardening, the cam shafts can be quenched to -20.degree. F. (-29.degree. 
C.) to transform any retained austenite into martensite. 
The cam shafts made according to the invention have been found to be 
particularly suitable for diesel engines.

SPECIFIC EXAMPLE 
A cam shaft was cast from a heat-treatable gray cast iron having the 
following composition: 
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Element Analysis 
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Carbon 3.35 
Silicon 2.35 
Manganese .70 
Chromium 1.40 
Nickel plus copper .50 
Molybdenum .50 
Sulfur .13 
Phosphorus .06 
Vanadium .30 
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The cam shaft had an as-cast brinell hardness in the range of 331 to 364. 
The cam shaft was heated to 1600.degree. F. (871.degree. C.) in 20 minutes 
in an electric furnace. The furnace temperature was then raised to 
1640.degree. F. (893.degree. C.) and held at that temperature for 80 
minutes. Subsequently, the temperature in the furnace was cooled to 
400.degree. F. (204.degree. C.) in one and a half hours. The cam shaft was 
then taken out of the furnace and allowed to air cool. The hardness of the 
cam shaft thus heat treated was in the range of 311 to 321 BHN. 
Subsequent to the heat treatment, the ends of the cam shaft were ground and 
drilled and the lobes of the cam shaft were flame hardened. Subsequent to 
the flame-hardening procedure, the cam shaft was quenched at a temperature 
of -20.degree. F. (-29.degree. C.) in a freezer until the cam shaft 
reaches this temperature. 
The cam shaft was then found to have good machinability in other areas 
other than the surface treated cam lobes. 
Reasonable variation and modification are possible within the scope of the 
foregoing disclosure and drawings without departing from the spirit of the 
invention.