STAINLESS STEEL MATERIAL FOR SHIP PROPELLER

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a stainless steel material and more particularly, to a stainless steel material for manufacturing ship propellers. Through the blending of various specific-proportioned materials, relatively better fluidity can be obtained during the manufacture of propellers, that is conducive to the shaping of the propeller, and can bring better property of impact resistance than conventional stainless steel materials, quite suitable to be applied to ship propellers that need to be impacted by seawater for a long time.

2. Description of the Related Art

Taiwan is surrounded by the sea, and there are many small islands nearby. Based on this advantage in terrain, the development of this place from commercial trade to entertainment activities is around the ocean. In terms of economic development, many goods need to be imported and exported by marine transportation. There are also large cruise ships carrying tourists to and from other areas. Many coastal tourist areas focus on various water activities. It can be seen that the demand for the use of various ships is quite high. Therefore, many shipyards for large ships and medium to small ships are established in areas close to each port.

In the ship industry, there are not only shipyards for manufacturing hulls, but also many manufacturers of related components, such as ship propeller factories, ship hardware factories, ship information equipment factories and ship outfitting factories, and so on. For large container ships, cruise ships, warships, and medium to small fishing boats, yachts or jet skis, propellers are applied to all of them. The propeller rotates by being driven by a motor, so as to drive the ship to advance. Materials for manufacturing ship propellers can be plastic or metal. Since propellers are immersed in water for a long time, they need to have great corrosion resistance and impact resistance, thereby usually using high-strength and corrosion-resistant materials. In China Patent Publication No. 103643160 A and No. 116426725 A, 17-4 and 15-5 stainless steel materials are provided respectively. 17-4 and 15-5 stainless steel materials are precipitation hardening stainless steel that is widely used in aerospace, golf equipment, ship components or industrial components, but for casting, has disadvantages of poor fluidity and obvious chrome pitting, liable to cause failure due to defects on the forming surface and the resulting increase in manufacturing cost.

In order for relatively better fluidity during the manufacture, US Patent Publication No. 2004/0042926 A1 provides a stainless steel material with high silicon content. Although this material can achieve relatively higher tensile strength, it sacrifices impact resistance. If it is applied for manufacturing ship propellers, the risk of propeller breakage will be raised. Therefore, the inventor made effort to think about how to provide a material which has great fluidity for casting so as to lower the probability of surface defects on the formed product, and also has great impact resistance, so that it can be used as the material of ship propellers.

SUMMARY OF THE INVENTION

In view of the fact that the above-described existing stainless steel materials for ship propellers still have many shortcomings in actual implementation and use, the inventor uses rich professional knowledge and years of practical experience thereof to make an improvement, and creates the present invention accordingly.

It is a primary objective of the present invention to provide a stainless steel material for a ship propeller. Through adding appropriate-proportioned silicon (Si) and cobalt (Co) and adjusting the proportions of other materials, the stainless steel material has corrosion resistance, impact resistance and high strength, and during the manufacturing process of the ship propeller, can reduce the occurrence of chrome pitting on the surface, raising the success rate of manufacturing products to effectively lower the manufacturing cost.

To attain the above objective, the present invention provides a stainless steel material for a ship propeller, which includes 14.0-14.8 wt. % chromium (Cr), 5.4-6.0 wt. % nickel (Ni), 1.52-1.98 wt. % silicon (Si), 0.001-0.05 wt. % carbon (C), 0.3-0.7 wt. % manganese (Mn), 2.5-3.5 wt. % copper (Cu), 0.01-1.0 wt. % cobalt (Co) and 0.2-0.3 wt. % niobium (Nb), and the remaining part includes iron (Fe) and unavoidable impurities.

In an embodiment of the present invention, the stainless steel material is obtained by heating alloy including chromium (Cr), nickel (Ni), silicon (Si), carbon (C), manganese (Mn), copper (Cu), cobalt (Co), niobium (Nb) and iron (Fe) to a temperature of 1600° C.-1700° C. into a molten iron mixture, then putting the molten iron mixture into a ceramic mold with a temperature of 1100° C.-1150° C., opening the mold after cooling, then performing solution treatment under a temperature of 1050° C.-1100° C. for 1.5 hours, and then performing H1100 precipitation hardening.

In an embodiment of the present invention, the stainless steel material may further include up to 0.3 wt. % molybdenum (Mo).

In an embodiment of the present invention, the stainless steel material may further include up to 0.05 wt. % vanadium (V).

In an embodiment of the present invention, the stainless steel material may further include up to 0.05 wt. % nitrogen (N).

In an embodiment of the present invention, the stainless steel material may further include up to 0.04 wt. % phosphorus (P).

In an embodiment of the present invention, the stainless steel material may further include up to 0.03 wt. % sulfur(S).

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE is a Schaeffler diagram of preferred embodiments of the present invention and other stainless steel materials.

DETAILED DESCRIPTION OF THE INVENTION

The composition of the stainless steel material for a ship propeller in the present invention includes 14.0-14.8 wt. % chromium (Cr), 5.4-6.0 wt. % nickel (Ni), 1.52-1.98 wt. % silicon (Si), 0.001-0.05 wt. % carbon (C), 0.3-0.7 wt. % manganese (Mn), 0.01-0.3 wt. % molybdenum (Mo), 0.01-0.05 wt. % vanadium (V), 2.5-3.5 wt. % copper (Cu), 0.01-1.0 wt. % cobalt (Co), 0.2-0.3 wt. % niobium (Nb), 0.005-0.05 wt. % nitrogen (N), 0.01-0.04 wt. % phosphorus (P) and 0.001-0.03 wt. % sulfur(S). The remaining part is iron (Fe).

When implementing the present invention, the manufacture is performed based on the G5121 standard for stainless steel castings in the Japanese Industrial Standards (JIS). At first, a mold for a ship propeller is manufactured. After a wax pattern for the mold is made, dipping, stuccoing and drying processes are repeated for 5 times. After the wax is removed by steam dewaxing, a ceramic mold can be obtained.

After that, alloy containing chromium, nickel, silicon, carbon, manganese, molybdenum, vanadium, copper, cobalt, niobium, nitrogen, phosphorus, sulfur and iron is heated to a temperature of 1620° C. into a molten iron mixture. Then the mold is heated to a temperature of 1150° C. The molten iron mixture is put into the mold by pouring. After cooling, the mold is removed by a de-shelling process, and a sand blasting process is performed so that a casting is obtained. Then, heat treatment is performed for improving properties such as hardness, strength, toughness and corrosion resistance. The product is put in a vacuum furnace for solution treatment under a temperature of 1050° C. for 1.5 hours. At last, H1100 precipitation hardening is performed, so that the ship propeller of the present invention is obtained. The surface can be further processed for improving the flatness, roughness and luster of the product surface.

In the embodiments of the present invention, a plurality of stainless steel materials are prepared in different proportions, then processed into tensile test bars that comply with the American Society for Testing and Materials (ASTM) standard E8/E8M for metal tensile testing and impact test pieces that comply with the Japanese industrial specifications (JIS) standard Z2202 for metal impact testing, and compared in mechanical properties with the conventional 15-5 stainless steel and 17-4 stainless steel.

The composition of the embodiments X71, X72, X81 and X82 of the present invention is as shown in Table 1.

Carbon can primarily improve the hardness of the stainless steel material, but too much carbon will lead to the precipitation of too many carbides, that will reduce impact toughness and affect corrosion resistance. Therefore, the carbon content is limited to 0.001-0.05 wt. %. The hardness of the stainless steel material is improved by other elements.

Molybdenum and vanadium will combine with carbon to precipitate carbides so as to increase the hardness of the stainless steel material, and molybdenum can also increase the corrosion resistance of the stainless steel material. In order to maintain certain impact resistance, the molybdenum content is limited to no more than 0.3 wt. %, and the vanadium content is limited to no more than 0.05 wt. %, so as to achieve the optimum proportions. However, in the present invention, it is optional to include or not include molybdenum and vanadium.

Phosphorus and sulfur are not indicated in the composition shown in Table 1. Although sulfur can make the stainless steel material relatively easier to be processed, it also represents the decline of mechanical properties. Therefore, the sulfur content is limited to no more than 0.03 wt. %. In general, the lower the phosphorus content, the better. Considering the cost of purification, the phosphorus content is usually limited to no more than 0.04 wt. %. Although nitrogen can improve the strength of the stainless steel material, it will lower the impact resistance. The excessive nitrogen content will also cause nitrogen pore defects, so the nitrogen content is limited to no more than 0.05 wt. %.

Based on the proportions of the composition of the embodiments in Table 1, their nickel equivalent (Nieq) and chromium equivalent (Creq) are the values as shown in Table 2. Referring to the sole FIGURE as well, the FIGURE is a Schaeffler diagram of the conventional 15-5 and 17-4 stainless steel materials and the embodiments of the present invention.

The formula for the nickel equivalent is:

Ni ⁢ eq = Ni + Co + 0.5 ( manganese ) + 0.3 ( copper ) + 25 ⁢ ( nitrogen ) + 30 ⁢ ( carbon ) .

The formula for the chromium equivalent is:

Cr ⁢ eq = Cr + 2 ⁢ ( silicon ) + 1.5 ( molybdenum ) + 5 ⁢ ( vanadium ) + 5.5 ( aluminum ) + 1.75 ( niobium ) + 1.5 ( titanium ) + 0.75 ( tungsten ) .

It can be seen from the sole FIGURE that the embodiments X71, X72, X81 and X82, according to their positions in the Schaeffler diagram, are basically stainless steel materials with three-phase structure of Austenite+Martensite+Ferrite, but relatively closer to the metallographic structure of Austenite, that indicates relatively better impact toughness.

In the stainless steel material, manganese and sulfur will combine into manganese sulfide (MnS), that reduces the chance of producing iron sulfide (FeS). Therefore, in the embodiments, 0.3-0.7 wt. % manganese is added, that can reduce the occurrence of hot tearing during casting. But if too much manganese is added, the metallographic structure of the stainless steel material will enter the Austenite+Ferrite zone, that will lower the hardness instead.

Taking three samples of each of the conventional stainless steel materials 15-5, 17-4 and the embodiments X71, X72, X81, X82 for tensile test and then taking one sample of each of them for impact test, the test results are as shown in Table 3.

In Table 3, YS represents yield strength, TS represents tensile strength, and EL represents elongation.

For the conventional stainless steel material 15-5 or 17-4, the silicon content is usually only up to 1 wt. %. It is shown in Table 1 that the silicon content of the present invention is increased to 1.52-1.98 wt. %, that can increase the fluidity of the molten iron mixture during casting, conducive to the shaping of the propeller and reducing the occurrence of chrome pitting on the surface of the propeller.

For preventing the chromium equivalent from being too high and thereby causing decline of mechanical properties, the chromium content is limited to 14.0-14.8 wt. %. At the same time, the nickel content is limited to 5.4-6.0 wt. %. Besides, cobalt content of 0.01-1.0 wt. % is added to further raise the impact resistance of the stainless steel material. It can be seen from Table 3 that compared with the conventional stainless steel materials 15-5 and 17-4, the average of the impact toughness of the embodiments X71, X72, X81 and X82 attains 45.3 J, that is much higher than the impact toughness 26.1 J and 9.9 J of the 15-5 and 17-4 stainless steel materials. In addition, the embodiments X71, X72, X81 and X82 are close in yield strength and tensile strength values to 15-5 and 17-4 stainless steel materials, that represents the embodiments are not weaker in strength than 15-5 and 17-4 stainless steel materials, also having great resistance to deformation at the same time. Therefore, the stainless steel material of the present invention is more satisfactory for ship propellers, which can be used in water for a long time and provides sufficient impact resistance to extend the service life.

It can be known from the above description that compared with the existing technique, the present invention has the following advantages.

• • 1. Through the adjustment of the silicon content, the stainless steel material for ship propellers of the present invention is improved in fluidity during casting, that is conducive to the forming of the appearance of the propeller during pouring, and also reduces surface defects caused by chrome pitting, thereby raising the success rate of manufacturing ship propellers to effectively lower the manufacturing cost.
  • 2. Through the addition of an appropriate quantity of cobalt, the stainless steel material for ship propellers of the present invention is raised in impact resistance. The impact toughness is obviously better than the conventional stainless steel materials, thereby more suitable for manufacturing ship propellers.

In conclusion, the stainless steel material for ship propellers of the present invention can indeed attain the expected usage effects through the above disclosed embodiments. However, the above disclosed diagram and description are only the preferred embodiments of the present invention. The manners and constituent elements disclosed in the above embodiments are only taken as examples for description, not intended to limit the scope of the present invention. The substitution or variation of other equivalent elements should be included within the scope of the following claims of the present invention.