Steels maraging

Maraging steels are a particular class of extra-low-carbon (i.e., 0.03 wt.% C) and nickel-rich (i.e., 18 wt.% Ni 22 wt.%) iron alloys having an ultrahigh strength. Nickel is the major alloying element followed by cobalt, which is added up to 12 wt.% to accelerate precipitation reactions, molybdenum, and, to a lesser extent, titanium, aluminum, and copper. A typical example of a maraging steel is an iron alloy with the following composition 17 to 19 wt.% Ni, 7 to 9 wt.% Co, 4.5 to 5.0 wt.% Mo, and 0.6 to 0.9 wt.% Ti. Alloys of this type are 

For corrosion resistance, these steels (18% nickel, 9% cobalt, 3% molybdenum, 0.2% titanium and 0.02% carbon) are similar to the 13% chromium steels and, therefore, are suitable for mildly corrosive situations. Because of their very high strength after heat treatment (yield strength—1390 N/mm, elongation—15%, impact strength) maraging steels find some use in a very high-pressure equipment. 

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Metals having widely different properties, eg, copper or maraging steel, can be bonded readily. 

An important item in this array of matenals is the class known as maraging steels. This group of high nickel martensitic steels contain so Htde carbon that they are often referred to as carbon-free iron—nickel martensites (54). Carbon-free iron—nickel martensite with certain alloying elements is relatively soft and ductile and becomes hard, strong, and tough when subjected to an aging treatment at around 480°C. 

Maraging steels are a class of high strength steels of very low carbon content. Strengthening is achieved by the use of substitutional elements to produce age hardening in the martensitic iron-nickel matrix. The term maraging was thus coined from the words martensite and age hardening . 

Maraging steel type Ni Nominal composition (Vo) Co Mo Ti Fe Nominal yield strength (MNm- )... 

Data for the cast and the Cr-bearing alloys are contained in References 36 and 5 respectively. Discussion in this article is restricted to the 18 Vo Ni maraging steels. 

On cooling to room temperature after annealing, maraging steels transform completely to martensite. The as-annealed structure consists of packets of parallel lath-like martensite platelets arranged within a network of prior-austenite grain boundaries. The platelets have a high dislocation density but are not twinned. 

Maraging steels have been produced both by air and vacuum melting. Small amounts of impurities can decrease toughness significantly, sulphur in particular is detrimental and should be kept as low as possible. Silicon and manganese also have a detrimental effect on toughness and should be maintained below a combined level of 0-20%. Such elements as C, P, Bi, O2, Nj and Hj are kept at the lowest levels practicable. 

In atmospheric exposure 18% Ni maraging steel corrodes in a uniform manner , and becomes completely rust covered. Pit depths tend to be more shallow than for the low-alloy high-strength steels. Atmospheric corrosion rates in industrial (Bayonne, New Jersey) and marine (Kute Beach, North Carolina) atmospheres are compared with those for low-alloy steel in Figs. 3.29, 3.30 and 3.31. The corrosion rates drop substantially after the first year or two and in all cases the rates for maraging steel are about half the corrosion rate for HY80 and AISI 4340 steels. 

The corrosion rates for both maraging steel and the low alloy steels in seawater are similar initially, but from about 1 year onwards the maraging steels tend to corrode more slowly as indicated in Fig. 3.32. The corrosion rates for both low alloy and maraging steel increase with water velocity . During sea-water exposure the initial attack was confined to local anodic areas, whereas other areas (cathodic) remained almost free from attack the latter were covered with a calcareous deposit typical of cathodic areas in sea-water exposure. In time, the anodic rust areas covered the entire surface. ... 

Polarisation tests indicate that maraging steel does not exhibit passive behaviour in 3% NaCl, and that the polarisation curves are unaffected by changes in heat treatment. 

The 18% Ni maraging steels do not display passivity and normally undergo uniform surface attack in the common environments. Of more serious consequence, however, for all high strength steels, is the degree of susceptibility to stress corrosion cracking (s.c.c.). 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

Maraging steel welds are somewhat less resistant than base plate. U -bend exposure of 1 240 MNm strength welds survived for up to 2 years in seawater while at 1 380 MNm failures occurred in 2-18 months. ... 

Although tests on smooth specimens indicate that cathodic protection of maraging steel is possible, tests on specimens with pre-existing cracks indicate a greater sensitivity to hydrogen embrittlement during cathodic polarisation . The use of cathodic protection on actual structures must therefore be applied with caution, and the application of less negative potentials than are indicated to be feasible in smooth specimen tests is to be recommended if it is assumed that structures contain crack-like defects. 

Further evidence of the relative resistance of maraging steel is reproduced in Fig. 3.33 from Reference 24. Maraging steel is shown to be superior to a die steel and low alloy steel (both unidentified) in bent beam tests stressed at 75% of the yield strength in distilled water. Also shown is the beneficial effect in smooth surface tests of cold rolling. Shot peening has a similar beneficial effect . 

A further estimation of the corrosion resistance of maraging steel can be obtained from data on the rate of crack propagation. Although the rate of crack propagation has been found to be a function of stress intensity in some alloys, for many alloys and heat treatments there is a range of stress... 

It is notable that while it is possible to produce maraging steels with consistently uniform mechanical properties, the stress corrosion properties are subject to scatter, as indicated in Fig. 3.34. To a large extent this scatter is an indication of the greater sensitivity of s.c.c. resistance to metallurgical variables. Although the variation in cracking resistance is not well understood, and the reaction to certain treatments not always consistent, certain observations may be used to indicate guidelines for improved properties. 

Little data are available on hot corrosion behaviour. Figure 3.35 indicates maraging steel to have better resistance to air exposure at 535°C than a 5% Cr tool steel . Metallographic examination indicates that exposure to... 

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