: The Working Of Steel

Nickel may be considered as the toughest among the non-rare alloys

now used in steel manufacture. Originally nickel was added to give

increased strength and toughness over that obtained with the ordinary

rolled structural steel and little attempt was made to utilize its

great possibilities so far as heat treatment was concerned.

The difficulties experienced have been a tendency towards laminated

during manufacture and great liability to seam, both

arising from improper melting practice. When extra care is exercised

in the manufacture, particularly in the melting and rolling, many

of these difficulties can be overcome.

The electric steel furnace, of modern construction, is a very important

step forward in the melting of nickel steel; neither the crucible

process nor basic or acid open-hearth furnaces give such good results.

Great care must be exercised in reheating the billet for rolling

so that the steel is correctly soaked. The rolling must not be

forced; too big reduction per pass should not be indulged in, as

this sets up a tendency towards seams.

Nickel steel has remarkably good mechanical qualities when suitably

heat-treated, and it is preeminently adapted for case-hardening. It

is not difficult to machine low-nickel steel, consequently it is

in great favor where easy machining properties are of importance.

Nickel influences the strength and ductility of steel by being

dissolved directly in the iron or ferrite; in this respect differing

from chromium, tungsten and vanadium. The addition of each 1 per

cent nickel up to 5 per cent will cause an approximate increase of

from 4,000 to 6,000 lb. per square inch in the tensile strength and

elastic limit over the corresponding steel and without any decrease

in ductility. The static strength of nickel steel is affected to

some degree by the percentage of carbon; for instance, steel with

0.25 per cent carbon and 3.5 per cent nickel has a tensile strength,

in its normal state, equal to a straight carbon steel of 0.5 per

cent with a proportionately greater elastic limit and retaining

all the advantages of the ductility of the lower carbon.

To bring out the full qualities of nickel it must be heat-treated,

otherwise there is no object in using nickel as an alloy with carbon

steel as the additional cost is not justified by increased strength.

Nickel has a peculiar effect upon the critical ranges of steel,

the critical range being lowered by the percentage of nickel; in

this respect it is similar to manganese.

Nickel can be alloyed with steel in various percentages, each percentage

having a very definite effect on the microstructure. For instance, a

steel with 0.2 per cent carbon and 2 per cent nickel has a pearlitic

structure but the grain is much finer than if the straight carbon

were used. With the same carbon content and say 5 per cent nickel,

the structure would still be pearlitic, but much finer and denser,

therefore capable of withstanding shock, and having greater dynamic

strength. With about 0.2 per cent carbon and 8 per cent nickel, the

steel is nearing the stage between pearlite and martensite, and

the structure is extremely fine, the ferrite and pearlite having

a very pronounced tendency to mimic a purely martensite structure.

Steel with 0.2 per cent carbon and 15 per cent nickel is entirely

martensite. Higher percentages of nickel change the martensitic

structure to austenite, the steel then being non-magnetic. The

higher percentages, that is 30 to 35 per cent nickel, are used

for valve seats, valve heads, and valve stems, as the alloy is a

poor conductor of heat and is particularly free from any tendency

towards corrosion or pitting from the action of waste gases of

the internal-combustion engine.

Nickel steels having 3-1/2 per cent nickel and 0.15 to 0.20 per

cent carbon are excellent for case-hardening purposes, giving hard

surfaces and tough interiors.

To obtain the full effect of nickel as an alloy, it is essential

that the correct percentage of carbon be used. High nickel and

low carbon will not be more efficient than lower nickel and higher

carbon, but the cost will be much greater. Generally speaking,

heat-treated nickel alloy steels are about two to three times stronger

than the same steel annealed. This point is very important as many

instances have been found where nickel steel is incorrectly used,

being employed when in the annealed or normal state.