45
which is extremely hard and much stronger than the original soft material. Unfortunately
it is also very brittle. In effect, we have traded off the steel’s ductility for its increased
strength. The rapid cooling also introduces strains to the part. The change in the shape
2
of the stress-strain curve as a result of quenching a ductile, medium-carbon steel is
shown in Figure 2-12 (not to scale). While the increased strength is desirable, the se-
vere brittleness of a fully quenched steel usually makes it unusable without tempering.
TEMPERING Subsequent to quenching, the same part can be reheated to a lower
temperature (400–1300F {200–700C}), heat-soaked, and then allowed to cool slowly.
This will cause some of the martensite to convert to ferrite and cementite, which reduces
the strength somewhat but restores some ductility. A great deal of flexibility is possible
in terms of tailoring the resulting combination of properties by varying time and tem-
perature during the tempering process. The knowledgeable materials engineer or met-
allurgist can achieve a wide variety of properties to suit any application. Figure 2-12
Stress
also shows a stress-strain curve for the same steel after tempering.
ANNEALING The quenching and tempering process is reversible by annealing. The quenched
part is heated above the critical temperature (as for quenching) but now allowed to cool tempered
slowly to room temperature. This restores the solution conditions and mechanical properties of the unhardened alloy. Annealing is often used even if no hardening has been previously done in order to eliminate any residual stresses and strains introduced by the annealed
forces applied in forming the part. It effectively puts the part back into a “relaxed” and soft state, restoring its original stress-strain curve as shown in Figure 2-12.
E
NORMALIZING Many tables of commercial steel data indicate that the steel has 1
been normalized after rolling or forming into its stock shape. Normalizing is similar to annealing but involves a shorter soak time at elevated temperature and a more rapid Strain
cooling rate. The result is a somewhat stronger and harder steel than a fully annealed one but one that is closer to the annealed condition than to any tempered condition.
F I G U R E 2 - 12
Stress-Strain Curves for
Annealed, Quenched,
Surface (Case) Hardening
and Tempered Steel
When a part is large or thick, it is difficult to obtain uniform hardness within its inte-rior by through hardening. An alternative is to harden only the surface, leaving the core soft. This also avoids the distortion associated with quenching a large, through-heated part. If the steel has sufficient carbon content, its surface can be heated, quenched, and tempered as would be done for through hardening. For low-carbon (mild) steels other techniques are needed to obtain a hardened condition. These involve heating the part in a special atmosphere rich in either carbon, nitrogen or both and then quenching it, a process called carburizing, nitriding, or cyaniding. In all situations, the result is a hard surface (i.e., case) on a soft core, referred to as being case-hardened.
Carburizing heats low-carbon steel in a carbon monoxide gas atmosphere, causing the surface to take up carbon in solution. Nitriding heats low-carbon steel in a nitrogen-gas atmosphere and forms hard iron nitrides in the surface layers. Cyaniding heats the part in a cyanide salt bath at about 1 500F (800C), and the low-carbon steel takes up both carbides and nitrides from the salt.
For medium- and high-carbon steels no artificial atmosphere is needed, as the steel has sufficient carbon for hardening. Two methods are in common use. Flame hard-
46
MACHINE DESIGN -
An Integrated Approach
ening passes an oxyacetylene flame over the surface to be hardened and follows it with a water jet for quenching. This results in a somewhat deeper hardened case than obtainable from the artificial-atmosphere methods. Induction hardening uses electric 2
coils to rapidly heat the part surface, which is then quenched before the core can get hot.
Case hardening by any appropriate method is a very desirable hardening treatment for many applications. It is often advantageous to retain the full ductility (and thus the toughness) of the core material for better energy absorption capacity while also obtaining high hardness on the surface in order to reduce wear and increase surface strength. Large machine parts such as cams and gears are examples of elements that can benefit more from case hardening than from through hardening, as heat distortion is minimized and the tough, ductile core can better absorb impact energy.
Heat Treating Nonferrous Materials
Some nonferrous alloys are hardenable and others are not. Some of the aluminum alloys can be precipitation hardened, also called age hardening. An example is aluminum alloyed with up to about 4.5% copper. This material can be hot-worked (rolled, forged, etc.) at a particular temperature and then heated and held at a higher temperature to force a random dispersion of the copper in the solid solution. It is then quenched to capture the supersaturated solution at normal temperature. The part is subsequently reheated to a temperature below the quenching temperature and held for an extended period of time while some of the supersaturated solution precipitates out and increases the material’s hardness.
Other aluminum alloys, magnesium, titanium, and a few copper alloys are amenable to similar heat treatment. The strengths of the hardened aluminum alloys approach those of medium-carbon steels. Since all aluminum is about 1/3 the density of steel, the stronger aluminum alloys can offer better strength-to-weight ratios than low-carbon (mild) steels.
Mechanical Forming and Hardening
COLD WORKING The mechanical working of metals at room temperature to change their shape or size will also work-harden them and increase their strength at the expense of ductility. Cold working can result from the rolling process in which metal bars are progressively reduced in thickness by being squeezed between rollers, or from any operation that takes the ductile metal beyond the yield point and permanently deforms it.
Figure 2-13 shows the process as it affects the material’s stress-strain curve. As the load is increased from the origin at O beyond the yield point y to point B, a permanent set OA is introduced.
If the load is removed at that point, the stored elastic energy is recovered and the material returns to zero stress at point A along a new elastic line BA parallel to the original elastic slope E. If the load is now reapplied and brought to point C, again yielding the material, the new stress-strain curve is ABCf. Note that there is now a new yield point y’ which is at a higher stress than before. The material has strain-hardened, increasing its yield strength and reducing its ductility. This process can be repeated until the material becomes brittle and fractures.