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 cementite 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 cementite 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 coarse-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.





Carburizing Material Case-hardening Treatments For Various Steels facebooktwittergoogle_plusredditpinterestlinkedinmail

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