Deep-sea manned/unmanned submersibles are the necessary high-tech equipment for ocean exploration. It is used to carry crews or equipment to various deep-sea complex environments for efficient exploration, scientific investigation and resource exploitation [1
]. Deep-sea submersibles contain multiple complex systems. Buoyancy regulation system is one of the important subsystems of the submersible. The adjustable ballast tank is one of the key components, which can ensure that the submersible has a good ability of depth setting and weight fine-tuning [3
]. Through the high-pressure seawater pump, seawater is pumped in and out of the ballast tank, and the buoyancy balance of the submersible in seawater is adjusted within a certain range.
Reducing the weight for better performances such as range, speed, and payload is a significant consideration for the designers of the submersibles, as the weight-to-displacement ratio is used to evaluate the structural efficiency of underwater vehicles [1
]. The weight of the submersible is distributed among the main components. At present, a spherical adjustable ballast tank with its promising application to a sea depth of 11,000 m and a volume of 300 L is designed, where domestic ultra-high strength maraging steel 18Ni(350) was used for first time for this purpose, as shown in Figure 1
. The adjustable ballast tank is composed of upper and lower hemispheres, which are connected by a Huff clamp. An O-ring seals the hemispheres at the end. Fine-tuning makes the weight of the submersible maintain positive buoyancy or produce enough negative buoyancy for safely sitting at bottom. A flat cover is beneficial for installation of different cabin piercing parts. Sealing is carried out through a radial O-ring and is evenly locked with the sphere by screws. The selection of maraging steel for the current ballast tank is exactly based on the principle of strength enhancement for lower weight. This is also the first time that this material has been used in deep-sea pressure vessels. Some of the candidate materials for underwater pressure hulls, such as titanium and high strength steel, and their main properties can be found in literature [6
]. The candidate material for pressure hulls using 18Ni grade maraging steels has been preliminarily investigated in terms of their application history, performance, and manufacturing capability by the authors [9
]. The damage tolerance related performances of 18Ni grade maraging steels, including yield ratio (σy
) and fracture toughness, have been evaluated. There are basically four wrought commercial maraging steels of the 18 percent nickel family i.e., 18Ni (200), 18Ni (250), 18Ni (300), and 18Ni (350) with yield strength ranging from 1400 MPa to 2400 MPa [10
]. In the development of ‘MIR’ submersibles in the 1980s [11
], new techniques to produce high strength, high nickel-content steel 18Ni(250) for pressure hulls are applied for its two pressure spheres. Up to now, other grades of 18Ni series of maraging steels have no application experience in deep-sea pressure hulls.
The increase in strength is very powerful in reducing the weight of pressure hulls, but the only concern is that increased strength generally leads to a decrease in toughness [12
] while it is generally not considered in the design of deep-sea compressive components. Plane strain fracture toughness is an important factor to represent the crack resistance property of the material. Wherein, 18Ni(250) with yield strength of 1700 MPa has the plane-strain fracture toughness level of 85–110 MPa·m1/2
, but the value of 18Ni(350) with yield strength of 2400 MPa rapidly reduces to 30–50 MPa·m1/2
]. Improving the toughness and plasticity of maraging steel can be done in various ways, such as reducing harmful elements or gas content in steel by a double vacuum smelting process, controlling inclusions morphology, adjusting microstructure by special processing, and heat treatment processes [14
]. Results have shown that when the harmful elements in 18Ni(350) maraging steel are reduced to 10−5
magnitude at the same time, the number and volume of inclusions are greatly reduced, which is an important reason for the remarkable improvement of fracture toughness of ultra-pure 18Ni(350) maraging steel. The influence of the toughness on the resistance to cracking for deep-sea components needs to be examined.
The hydraulic pressure test on the adjustable ballast tank with its sealing flat cover was carried out to examine its performance. The ultimate goal of pressure test was to check whether it can endure the external pressure of 126.5 MPa (i.e., 115 MPa for 11,000 m deep-sea environments times a safety factor of 1.1 for pressure testing. Note, the actual pressure value for the 11,000 m deep-sea environment is 113.8 MPa and this value has been used in the design calculation, but here for the test, a more conservative value of 115 MPa is used). There are sensor mounting holes and high pressure pipe mounting holes on the sealing cover. However, the flat cover collapsed during the loading process of external pressure in the high-pressure chamber. The pressure was high, which was the trigger of the collapse, but still considerably below the design limit of the hull. The failure can be caused by unexpected defective material properties, the possible stress concentration resulting from design/processing, or inappropriate installation method. In this paper, the mechanical properties of the actual materials used in the collapsed flat cover are re-examined by sampling and testing of the broken parts. Non-metallic inclusions analysis, micro-structure analysis, and fracture surface analysis are conducted to acquire the possible fracture cause. Furthermore, finite element analysis based on fracture mechanics is conducted to understand the ultimate cause of destruction. The analysis results demonstrate the importance of material selection for engineering components based on the comprehensive properties of the materials.
5. Summary and Conclusions
In this paper, a destroyed flat cover made of 18Ni (350) used in an adjustable ballast tank during high pressure testing was analyzed. To find out the cause of the damage, the mechanical properties of the material used in the flat cover were re-examined by sampling and testing of the broken parts. Non-metallic inclusions analysis, microstructure analysis, and fracture surface analysis were conducted to find out the possible fracture cause. Finite element analysis based on fracture mechanics to understand the ultimate cause of destruction was conducted. The following conclusions and recommendations can be made:
(1) Maraging steels are expected to still have high toughness when improving their ultra-high strength level. 18Ni (250) grade is currently an acceptable candidate material for deep-sea pressure hulls, which has achieved satisfactory application experience. To obtain better weight-to-displacement ratio, the designers applied 18Ni (350) for an adjustable ballast tanks used in a deep-sea submersible. The strength of 18Ni (350) is higher than that of 18Ni (250), however its toughness is lower. In the current design guideline for deep-sea pressure hulls, the requirements for material toughness have not been emphasized. The trial of applying 18Ni (350) was made in the present study but unexpected failure of the flat cover occurred when load was relatively small.
(2) Non-metal inclusions analysis, microstructure analysis, and fracture surface analysis showed that the original state of the material meets the design requirements with fewer kinds of non-metallic inclusions. The material has low impact toughness, which is one of the reasons for the cracking of the flat cover during the test.
(3) From the design point of view, it is suggested to optimize the transition from right angle to chamfer at variable cross-sections, and to increase the thickness of the outer edge liner of the cover appropriately, which will reduce the possibility of damage. During processing, the surface finish of structural parts should be ensured, especially the root surface of the variable cross-section, so as to avoid surface processing defects.
(4) The reliability of deep-sea pressure hulls, especially the pressure hulls of manned submersibles, guarantees the safety of personnel and equipment. There must be attention paid to the comprehensive performance of materials. From the point of view of material selection, the application of 18Ni (350) maraging steel to the pressured structure in deep-sea environments is a risky choice. On the other hand, the 18Ni series of maraging steels have broad application prospects in deep-sea environments, so improving the toughness and plasticity together with increasing the strength level will be an important research direction [23