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are more resistant than the medium carbon low alloy steels. Processing techniques that improve the fracture toughness, such as vacuum melting, proper hot working, and keeping residual impurities low, also improve the resistance to stress corrosion cracking. The alloys are available in the form of sheet, plate, bar, and die forgings. Most applications use bar or forgings.

Maraging steels are either air melted followed by VAR or vacuum induction melted followed by VAR. Aerospace grades are tripled melted using air, vacuum induction and vacuum arc remelting, to minimize the residual elements carbon, manganese, sulfur, and phosphorous and the gases oxygen, nitrogen, and hydrogen. Carbon and sulfur are the most deleterious impurities because they tend to form brittle carbide, sulfide, carbonitride, and carbosulfide inclusions that can crack when the metal is strained, lowering the fracture toughness and ductility.

The maraging steels are readily hot worked by conventional rolling and forging operations. As the titanium content increases, hot working becomes more difficult due to increased hot strength and either higher loads or higher temperatures are required. The precipitation of TiC at the grain boundaries, which can form during slow cooling through the temperature range of 2000-1380° F, must be avoided. They also have good cold forming characteristics in spite of relatively high hardness in the annealed condition. Their machinability is similar to 4330 at equivalent hardness. As a result of their low carbon contents, weldability is excellent. The maraging steels can be readily welded by GTAW in either the annealed or the aged conditions. During welding, avoid prolonged dwell times at high temperatures, avoid preheat, keep interpass temperatures low (250° F), avoid slow cooling, and keep welds as clean as possible. Welding of aged material should be followed by aging at 900° F to strengthen the weld area.

Heat treatment consists of solution annealing, air cooling, and then aging. Solution annealing is usually conducted at 1500° F for 1 h. Since the nickel content is so high, austenite transforms to martensite on cooling from the austenitic temperature. The martensite start temperature (Ms) is about 310° F and the martensite finish temperature (Mf) is about 210° F. The formation of martensite is not affected by cooling rate and thick sections can be air cooled and

Aging Temperature (° F)

Fig. 5.19. Effect of Aging Temperature on 18Ni(250) Maraging Steel20

Aging Temperature (° F)

Fig. 5.19. Effect of Aging Temperature on 18Ni(250) Maraging Steel20

still be fully martensitic. Since the martensitic transformation involves only an austenite-to-martensite transformation of iron-nickel and does not involve carbon to any considerable extent, the martensite formed is relatively ductile. Before aging, maraging steels have yield strengths in the range of 95-120 ksi. The effect of aging temperature on 18Ni(250) is shown in Fig. 5.19.

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