Naval ships are the most important equipment of the navy, as well as the platforms for maritime transportation and battles. The materials used to build naval ships should be able to withstand the corrosion of sea water and the ocean atmosphere, have high specific strength, plasticity and toughness, as well as excellent processing technology. Due to reason that the overall structure of the ship is complicated, the amount of material used is large with many varieties and specifications. Selecting materials with excellent performance to make warships is the basis to ensure the integrity and advancement of naval equipment.
Titanium alloy which is used in building vessels and warships, mainly based on the following features of titanium alloy: the wide range of strength, excellent mechanical properties, the unique physical properties (including magnetic), high specific strength and structure validity, excellent corrosion resistance and erosion resistance, good shock resistance, good machinability and weld ability, reasonable cost and high effectiveness. Experience in using a large number of marine piping material, confirmed that the service life of the pipeline system in the traditional material is limited, among them, the corrosion can produced in steel pipe system when using 1 ~ 2 years, The service life of the CuNi piping system is about 6 ~ 8 years, and titanium alloy pipe in the pipe system, pumps, valves, heat exchangers and other equipment can serve more than 40 years. As Marine titanium alloy needs to be used in marine environment for a long time, the strength, stress corrosion fracture toughness and weld ability of the alloy should be taken into consideration when designing marine titanium alloy.
Russia is the first country in the world to develop and use marine titanium alloy. It is also the first country with the widest range and largest quantity of marine titanium alloy, mainly includes pt-7m, pt-1m, pt-3v, 37, 5V titanium alloy and its corresponding welding wires. It has formed marine titanium alloy products of different strength levels such as 490, 585, 686, 785 MPa. The titanium alloy has been successfully applied in the following part of the ship and equipment, such as deepwater riser, supply pipe, pump, filters, the sea line, drinking water pipe, drilling pipe and groundwater, heat exchanger, independent diesel engine fire pump and fire extinguishing system, deepwater equipment shell, well outside the system flexible pipe, pressure vessel, platform of high-strength fastening joint flexible stretching part, process solution of piping and vessel etc, and successively in the former Soviet union series atomic icebreaker "Lenin," North Pole ", "Russia", "The Soviet Union" and other types of war ships. The titanium steam generator on a series of has been safely used for 20-40 years without any serious damage.
The titanium alloy used by U.S. is based on the aviation material by selecting the titanium alloys with corrosion resistance, weld ability and stress corrosion resistance in the corrosive environment of sea water are selected, including pure titanium, ti-0.3mo-0.8ni, ti-3al-2.5v, ti-6al-4v, ti-6al-4v ELI, ti-3al-8v-6cr-4mo-4zr. In addition, other Marine titanium alloys such as ti-5al-1zr-1sn-1v-0.8mo-0.1si and ti-6al-2nb-1ta-0.8mo have also been developed according to the characteristics of marine titanium alloy. The use of high-performance titanium alloy on the ship has a significant effect on improving the mobility, stability and effectiveness of the ship and reducing the hull mass. The certification tests was conducted by U.S. navy in the 1990s for the following ships, including the nuclear-powered aircraft carrier (CVN), guided missile cruiser (CG - 47), the missile shield who ship (FFG - 7), the detection of agents, ship two habitats (MCM), water and land, land boat (LSD41CV), boats, hovercraft lu (LVCA), amphibious assault landing ship (LHD), fast combat ammunition resupply vessels (AOE - 6), double hull surveillance ship T - AGOS19 (SWATH), coastal craft (MHC - 51), missile destroyer, The DDG - 51). The titanium alloy parts which as manufactured in sea cooling systems, sea water systems and fire extinguishing systems, structural parts, propellers, sewage treatment systems, electrical components, fasteners, etc. are applied in these ships or will soon be in use of high-performance titanium alloy.
The research and application of China's marine titanium alloy began in the 1960s. After decades of development, a relatively complete marine titanium alloy system has been formed, which can meet the requirements of different strength levels for ships, submarines and deep-sea submersivers. Application fields include hull structure, propulsion system, power system, electronic information system, auxiliary system, special device, etc. According to the grade of yield strength, the yield strength below 490 MPa is low strength titanium alloy, with excellent plasticity; 490 ~ 790 MPa is medium strength titanium alloy, higher than 790 MPa is high strength titanium alloy, mainly used in ship power engineering heat and corrosion resistant parts and ship special machinery.
Japanese Marine titanium alloys mainly include pure titanium, ti-6al-4v and ti-6al-4v ELI, which are mainly used in the pressure-resistant shell of deep submersible and various civilian cruise ships and fishing boats.
Existing problems and solutions in use
3.1 crevice corrosion
Corrosion problem: there are small cracks in the dismountable joints (flanges, threaded joints, etc.) used in ships. There will be high concentration of chloride ions and fluorine ions with erosion in these cracks, which requires titanium alloy to have good resistance to high temperature crack corrosion in the seawater environment. In titanium alloy heat exchangers and seawater desalination equipment, the working environment is 90 ~ 250℃ and the medium pH value is 1.5 ~ 4.0 (especially in salt and kerosene deposits), and the crevices corrosion is very fatal.
(1) By adding Pd and Ru to titanium alloy can effectively improve the crack corrosion resistance of titanium alloy.
(2) To conduct surface treatment containing Ru/Pd on the surface of titanium alloy, such as infiltration of Ru/Pd elements on the surface of titanium alloy or micro-arc oxidation to form oxides containing Ru/Pd elements on the surface of titanium alloy. In order to reduce the cost of surface treatment, gradient coatings containing Ru/Pd elements can also be prepared on the surface of titanium alloys.
(3) To avoid hydrogen hygrosaturation and hydrogen cracking of titanium alloy when cathodic protection technology is used to protect the steel structure, the potential is between -800 mV and -1050 mV.
When titanium is connected with steel and copper, it is easy to have galvanic corrosion. Protection measures include:
1. Conduct thermal oxidation, micro-arc oxidation and anodic oxidation of the pipe, and form an oxidation layer or a ceramic layer on the metal surface to achieve insulation. The oxide film formed by oxidation treatment of titanium tube can reduce the cathode polarization effect of titanium alloy by 80% ~ 90%, and the life of oxide film is the same as that of titanium tube.
2. Use asphaltene rubber for insulation treatment at the interface where the pipe, valve nozzle and steel and copper equipment come into contact;
3. Protect the middle flange at the connection between the titanium tube and the stainless steel nozzle.
welding corrosion of large structural parts
Protective measures include:
1. The shell, high-pressure pressure vessel, tensile parts and other large structural parts used on the ship need to use high-strength titanium alloy based on the requirements of pressure resistance performance, and the connection of the high-strength titanium alloy thick wall welding parts should have excellent operability in sea water, and no heat treatment is required after welding.
2. TC4 titanium alloy and low-gap TC4 titanium alloy are used in the United States and Europe (oxygen content is controlled within 0.13%); The pt-3v, 37, 5V marine titanium alloy was developed by the central research institute of Prometheus structural material in Russia. China has developed TA24 and TA31 titanium alloys.
3. For large structural parts, heat treatment cannot be carried out after welding, and tensile stress exists on the surface, which greatly reduces corrosion resistance and fatigue resistance of titanium alloy components. The surface of welding parts can be treated by ultrasonic shock, and the tensile stress on the surface of welding parts can be changed into compressive stress, so as to improve the fatigue performance.
Development prospect of China Marine titanium alloy
For material research, the performance research of titanium alloy in marine environment needs to be carried out by China in marine titanium alloy including:
(1) The influence of working conditions (temperature, pH value, solution composition), stress mode (free state, reverse bending, torsion) and alternating load on corrosion fatigue, fatigue crack growth rate and stress corrosion fracture toughness of titanium alloy in air and sea water;
(2) The influence of processing technology on the texture type (radial texture, axial texture) and texture type on the properties of pipe processing technology (flaring, flattening, blasting) of titanium alloy pipe;
(3) The study on corrosion resistance mechanism of titanium alloy by Pd and Ru trace elements in H2S and Cl- environment;
(4) The tensile and fatigue performance evaluation of titanium alloy under irradiation environment;
(5) The research on welding deformation control and correction of different welding forms of corrosion-resistant titanium alloy.