Steels_ Metallurgy and Applications, Third Edition

Engineering steels 263

Figure 3.39 Fracture toughness of maraging steels and other high-strength martensitic
grades (After Magnee et al. 33)

3. Low levels of oxygen, nitrogen and hydrogen.


4. Low levels of inclusions.

As indicated in Figure 3.39, the toughness of the maraging grades is significantly

better than that of tempered high-carbon martensitic steels. 33

The casting grade 17Ni 1600 is normally solution treated by homogenizing

for four hours at 1150~ followed by air cooling. Maraging then takes place

at 480~ for three hours, which is similar to the conditions used for the lower

strength wrought grades. However, a more complex heat treatment is sometimes

employed in order to improve toughness. This involves the following:

11500C 4 hours + 5950C 4 hours + 820~ 1 hour + 480~ 3 hours

Corrosion behaviour

The corrosion rate of maraging steels in both marine and industrial atmospheres

is almost half of that shown by high-strength, low-alloy steels. In sea water,

both types of steels show similar corrosion rates initially, but after six months

the maraging steels corrode more slowly. The International Nickel Co. 32 have

published Kiscc data for exposure in aqueous environments, with and without

NaC1, which show that the 18% Ni maraging steels compare favourably with

other high-strength steels.

Applications

From the foregoing remarks, it is apparent that maraging steels are capable of

producing very high strengths with good fracture toughness but at a cost far in

excess of that of conventional, low-alloy engineering steels. They are therefore