40 THE NAVY OCEAN ENGINEERING PROGRAM 



more than 5 percent nickel. The special vacuum arc melting and special 

 carbon deoxidation technology of this steel assures ultra-cleanUness and 

 freedom from nonmetallic inclusions and such embrittling interstitial ele- 

 ments as oxygen, nitrogen, or hydrogen. 



Maraging steels in the 180,000 to 210,000 psi range represent a new 

 family of steels that have little similarity to the quenched-and-tempered 

 steels. The strengthening mechanism involves a precipitation of micro- 

 scopic particles throughout the matrix as the result of heat treatments 

 known as "aging." When the steel cools from austenitizing temperatures in 

 the range of 1500° to 1900°F, it changes to an almost carbon-free soft and 

 ductile martensite. Maraging steels have an advantage over quenched-and- 

 tempered steels for small deep submergence pressure hulls, because such 

 hulls are small enough to fit into the heat-treating furnaces, which permits 

 aging after fabrication. Quenched-and-tempered steel pressure hulls are 

 generally welded in the heat-treated condition, without postwelding heat 

 treatment; this arrangement presents the problem of introducing excessive 

 residual stresses. 



Titanium Alloys 



For the past five years the Navy has been supporting exploratory de- 

 velopment of titanium alloys. Because of its high specific strength (low 

 density compared to steel and its 95,000 to 140,000 psi yield strength), 

 titanium is an attractive material for deep submergence vehicles. 



Unalloyed titanium exists in the alpha (hexagonal close packed) crystal 

 form up to 1620°F. Above this temperature, it becomes a beta (body- 

 centered cubic) phase. Alloying elements can be added to strengthen the 

 alpha structure or to modify the room temperature structure to an all-beta 

 or mixed alpha-beta structure. The type of structure existing at room tem- 

 perature determines whether the titanium alloy will respond to heat treat- 

 ment, and whether it is sensitive to thermal embrittlement, which interferes 

 with weldability. 



A 7Al-2Cb-lTa titanium was tentatively selected as the hull material 

 for the Navy's first Deep Submergence Rescue Vehicle, with capability of 

 descending to 6000 ft. Evaluation of the 7-2-1 titanium revealed that the 

 alloy was susceptible to stress corrosion cracking in sea water at relatively 

 low stress levels so HY 130/150 steel was selected for the hull material. 

 The HY 130/150 steel has a depth capabihty of approximately 5000 ft, 

 while that 7-2-1 titanium is 6000 ft. 



