Martensitic stainless cast steel having high cavitation erosion resistance

Abstract

Martensitic stainless cast steel suitable for use as turbine elements for water power plants having high cavitation erosion resistance and consisting essentially of carbon of 0.1 wt % or less, silicon of 1.0 wt % or less, manganese of 2.0-9.0 (exclusive of 2.0) wt %, nickel of 0.5-8.0 wt %, chromium of 11.0-14.0 wt %, and the balance of essentially iron.

Description

The present invention relates to martensitic stainless cast steel suitable for use as water turbine elements for water power plants such as runner, guide vane and stay vane which are required to have high cavitation erosion resistance.

Output per unit power generator in thermal and atomic power generators has the trend of becoming larger and larger these days, but it is difficult for thermal and atomic power plants having such large output to weather through peak load of electric power. As one step to weather through such peak load, there has become popular construction of water power plants capable of adjusting output in a comparatively short time period, particularly construction of pumped-storage power plants capable of efficiently using excess power at night.

The water turbine employed in these pumped-storage power plants is the so-called "reversible pump turbine" which functions to perform both generating operation by day and pumping operation by night, and these power plants have the trend of having high head and high output for the purpose of efficiently using the construction site and reducing the construction cost per unit output, etc.

Cast steel (13-chromium cast steel) containing mainly chromium of about 13 wt% has conventionally been used as material for water turbine elements such as water turbine runner, guide vane and stay vane, but the condition under which water turbine elements are used toward high head and high output has become more and more severe. Namely, cavities are caused around the surface of runner blades because of high velocity of water flow and the surface of runner blades is damaged be repeated impulsive load generated when cavities collapse on the surface of runner blades. This is the so-called "cavitation erosion". Conventional materials were insufficient to resist this cavitation erosion. It is therefore desired in the trend of higher head and higher output to develop a material having improved mechanical strength and toughness and particularly excellent cavitation erosion resistance.

An object of the present invention is to provide martensitic stainless cast steel having high mechanical strength and toughness and excellent cavitation erosion resistance.

Another object of the present invention is to provide water turbine elements made of martensitic stainless cast steel having excellent cavitation erosion resistance, said water turbine elements being used in water power plants.

According to the present invention martensitic stainless cast steel is provided consisting essentially of carbon of 0.1 wt% or less, silicon of 1.0 wt% or less, manganese of 2.0-9.0 (exclusive of 2.0) wt%, nickel of 0.5-8.0 wt%, chromium of 11.0-14.0 wt%, and the balance of essentially iron, and having high cavitation erosion resistance.

According to the present invention water turbine elements are provided for use in water power plants, said water turbine elements being made of abovementioned martensitic stainless cast steel.

Martensitic stainless cast steel of the present invention has excellent cavitation erosion resistance and is excellent in mechanical strength and toughness. It can also be produced easily and industrially without using a special casting manner.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a perspective view showing a turbine runner for water power plant of the present invention.

FIG. 2 is a sectional view showing the turbine runner shown in FIG. 1.

It will be described below how additive elements should be contained and why these elements should be limited in amount to yield stainless cast steel of the present invention.

Carbon employed to yield stainless cast steel of the present invention serves to form stably martensite phase by heat treatment to enhance the strength of stainless cast steel. However, excess addition of carbon reduces the toughness of martensitic stainless cast steel and carbon should be therefore contained at most 0.1 wt%. It is preferable to add carbon in the amount of 0.05-0.1 wt%.

Silicon is added as deoxidizer together with manganese at the time of steel melting and serves to enhance the castability of cast steel. Excess addition of silicon reduces, like carbon, the toughness of stainless cast steel and silicon should be added at most 1.0 wt%. It is particularly preferable to compound silicon in the amount of 0.3-1.0 wt%.

Manganese is a component to act a particularly important role of enhancing the cavitation erosion resistance of stainless cast steel of the present invention. The reason why the compounded amount of manganese should be limited from 2.0 wt% to 9.0 wt% (exclusive of 2.0 wt%) is that effect is not made remarkable when less than 2.0 wt% and that epsilon and austenite phases are formed in cast steel to reduce proof stress when over 9.0 wt%. It is practically preferable to add manganese in the amount of 2.5-6.0 wt%.

Nickel is a component to dissolve in matrix in a solid state to make a martensite phase stable and enhance toughness. The compounded amount of nickel is limited from 0.5 wt% to 8.0 wt%, because effect of addition is made low when less 0.5 wt% and because increase of hardness makes the machinability of martensitic stainless cast steel worse remarkably and increase of residual austenite reduces proof stress when over 8.0 wt%. It is practically preferable to add nickel in the amount of 1.0-6.0 wt% and more preferably in the amount of 3.0-4.0 wt%.

Chromium is important to enhance corrosion resistance. The reason why chromium should be added ranging from 11.0 wt% to 14.0 wt% is that effect of addition is not enough when less than 11.0 wt% and that delta ferrite is formed in matrix in relation with the amount of nickel to thereby reduce cavitation erosion resistance when over 14.0 wt%. The compounded amount of chromium preferably ranges from 12.0 wt% to 13.5 wt%.

In addition to above-mentioned components, stainless cast steel of the present invention may further include one or more components selected from the group consisting of molybdenum, copper, niobium and nitrogen.

Molybdenum is an important element in enhancing the cavitation erosion resistance, mechanical strength and temper softening resistance of martensitic stainless cast steel, and in preventing the temper brittleness. The amount of molybdenum is 2.0 wt% or less, preferably in the range of 0.5-2.0 wt% and more preferably in the range of 0.5-1.6 wt%. Impact value is reduced when over 2.0 wt%.

Copper serves to enhance the cavitation erosion resistance of martensitic stainless case steel of the present invention. Copper is added ranging from 0.1 wt% to 0.5 wt%. Addition effect is low when less than 0.1 wt% and toughness is reduced when over 0.5 wt%.

Niobium is a component to make fine the grain size of cast steel to enhance proof stress and cavitation erosion resistance. The added amount of niobium ranges from 0.01 wt% to 0.1 wt%. Addition effect is not enough when less than 0.01 wt% and ferrite is formed in matrix to reduce the cavitation erosion resistance of cast steel when over 0.1 wt%. Same effect can be obtained by adding at least one or more components selected from vanadium, titanium, hafnium, tantalum and zirconium, instead of or in addition to niobium.

Nitrogen serves to enhance cavitation erosion and corrosion resistances of cast steel. The added amount of nitrogen is in the range of 0.02-0.15 wt%. Addition effect is not enough when less than 0.01 wt% and pin-holes and belo-holes are caused in cast steel when over 0.2 wt%. It is preferable that the amount sum of nitrogen and carbon is in the range of 0.02-0.15 wt%.

There will be briefly described a method of manufacturing stainless cast steel of the present invention. Melting can be carried out by induction furnace or electric-arc furnace, for example, and casting may be achieved by the usual manner such as sand casting and metal mold casing.

After casting, cooling is carried out at a cooling rate of causing no crack, said cooling rate depending upon shape and size of cast steel, and it is preferable that tempering is carried out of the temperature of 500.degree.-700.degree. C.

Examples and controls will be described to prove the effect of the present invention.

EXAMPLES

Materials having chemical compositions shown in Examples 1-56 of Table 1 were melted in the induction furnace and heat-treated to have heat history corresponding to the as-cast cooling of large scale cast product. These samples were further solution-treated at the temperature of 1,050.degree. C., cooled at the cooling rate of 150.degree. C./h, and then heat-treated for tempering under the temperature of 650.degree. C., to thereby produce various specimens.

Specimens thus produced were examined about their tensile stress, 0.2% proof stress, elongation, reduction of area, impact value (Charpy 2 mmV notch, 20.degree. C.), diamond pyramid hardness and cavitation erosion index (C.E.I.). Results thus obtained are shown in Table 2.

Electrostrictive vibration whose frequency was 6.5 kHz and travelling distance 100 .mu.m was added to the specimen for 180 minutes in pure water of 25.degree. C. to measure the weight loss caused by cavitation erosion (g), and cavitation erosion index (C.E.I.) was obtained from the following equation:

C.E.I.=w/t.rho..times.10.sup.6 where w represents the weight loss caused by cavitation erosion (g), t test time (min.) and .rho. specific gravity. Controls:

Materials having chemical compositions shown in Controls 1-7 of Table 1 were melted, cast and heat-treated by same manner as in above Examples to produce specimens. Specimens thus produced were examined about their properties same as those of specimens in above Examples. Results thus obtained are also shown in Table 2.

TABLE 1__________________________________________________________________________composition (% by weight)C N Si Cr Ni Mn Mo Nb Cu Fe__________________________________________________________________________Example1 0.05 -- 0.31 13.44 1.97 5.33 -- -- -- balance2 0.06 -- 0.34 133.3 1.99 2.99 1.73 -- -- "3 0.06 -- 0.32 12.95 3.26 5.15 -- -- -- "4 0.06 -- 0.33 12.87 3.34 2.95 1.69 -- -- "5 0.06 -- 0.33 13.41 0.51 8.74 1.13 -- -- "6 0.06 -- 0.31 13.16 1.45 7.09 0.17 -- -- "7 0.05 -- 0.32 13.04 5.16 2.51 1.12 -- -- "8 0.05 -- 0.31 13.01 2.66 5.16 0.55 -- -- "9 0.05 -- 0.31 13.07 2.69 5.92 0.53 -- -- "10 0.05 -- 0.31 13.11 3.03 5.05 0.56 -- -- "11 0.05 -- 0.29 13.18 3.09 5.44 0.54 -- -- "12 0.05 -- 0.29 12.97 3.51 5.06 0.55 -- -- "13 0.05 -- 0.34 13.08 3.58 5.77 0.61 -- -- "14 0.05 -- 0.32 13.03 3.59 6.06 0.63 -- -- "15 0.06 -- 0.33 13.04 4.16 5.56 0.64 -- -- "16 0.06 -- 0.34 12.96 1.10 7.09 1.00 -- -- "17 0.07 -- 0.32 13.13 4.76 3.66 0.59 -- -- "18 0.06 -- 0.34 13.19 4.86 4.32 0.59 -- -- "19 0.06 -- 0.33 13.25 5.27 3.11 0.63 -- -- "20 0.06 -- 0.35 13.23 5.28 3.43 0.62 -- -- "21 0.06 -- 0.35 13.03 5.84 2.99 0.64 -- -- "22 0.058 -- 0.33 13.54 3.65 2.57 0.198 -- 0.10 "23 0.057 -- 0.32 13.67 3.60 2.58 0.193 -- 0.30 "24 0.060 -- 0.33 13.93 3.59 4.50 -- -- 0.10 "25 0.057 -- 0.34 13.64 3.58 4.55 -- -- 0.33 "26 0.05 -- 0.34 13.18 2.44 5.46 0.56 0.02 -- "27 0.05 -- 0.32 13.23 2.42 5.97 0.54 0.03 -- "28 0.06 -- 0.34 13.07 2.93 5.05 0.55 0.03 -- "29 0.05 -- 0.31 13.22 2.95 5.47 0.56 0.03 -- "30 0.06 -- 0.32 13.24 2.92 5.46 1.52 0.03 -- "31 0.05 -- 0.30 13.20 3.42 4.98 -- 0.05 -- "32 0.05 -- 0.32 13.17 3.43 4.97 1.53 0.03 -- "33 0.05 -- 0.36 13.06 3.55 5.65 0.63 0.05 -- "34 0.06 -- 0.34 12.86 3.64 5.19 0.65 0.05 -- "35 0.06 -- 0.34 13.05 4.15 5.54 0.63 0.05 -- "36 0.06 -- 0.31 13.00 2.68 5.49 0.51 0.03 0.18 "37 0.05 -- 0.28 13.06 2.60 6.00 0.52 0.04 0.19 "38 0.06 -- 0.31 13.22 3.07 5.00 0.47 0.04 0.19 "39 0.05 -- 0.28 13.05 3.01 5.46 0.50 0.04 0.19 "40 0.05 -- 0.33 13.16 3.51 5.15 0.58 0.04 0.17 "41 0.06 -- 0.36 13.13 3.56 5.40 0.57 0.05 0.18 "42 0.06 -- 0.33 13.06 3.66 5.97 0.63 0.05 0.20 "43 0.06 -- 0.34 12.95 4.08 5.50 0.63 0.05 0.20 "44 0.06 -- 0.32 12.09 3,74 6.16 0.52 0.06 0.20 "45 0.06 -- 0.33 13.32 4.86 3.94 0.58 0.05 0.20 "46 0.06 -- 0.35 13.26 4.74 4.12 0.57 0.05 0.20 "47 0.06 -- 0.34 13.01 5.30 3.04 0.63 0.05 0.20 "48 0.06 -- 0.35 13.11 5.33 3.49 0.59 0.05 0.20 "49 0.06 -- 0.36 13.03 5.82 3.09 0.62 0.05 0.20 "50 -- 0.12 0.31 13.22 3.56 5.17 -- -- -- "51 0.02 0.06 0.33 13.17 2.02 3.04 1.63 -- -- "52 0.04 0.02 0.33 13.44 1.51 7.15 0.21 -- -- "53 0.02 0.07 0.33 13.11 5.24 2.56 1.13 -- -- "54 0.02 0.05 0.32 13.27 3.49 4.93 -- 0.05 -- "55 0.02 0.06 0.30 13.39 3.48 4.94 1.55 0.03 -- "56 0.02 0.07 0.33 13.26 3.55 2.63 0.20 -- 0.11 "Control1 0.05 -- 0.32 13.09 1.81 0.59 0.56 -- -- "2 0.04 -- 0.32 13.27 3.52 0.57 0.55 -- -- "3 0.05 -- 0.31 13.42 1.94 1.48 -- -- -- "4 0.06 -- 0.33 13.05 1.91 3.03 3.53 -- -- "5 0.05 -- 0.30 13.20 3.45 1.51 -- -- -- "6 0.06 -- 0.34 13.17 3.43 2.98 3.58 -- -- "7 0.05 -- 0.30 13.12 5.93 0.58 1.56 -- -- "__________________________________________________________________________ TABLE 2______________________________________ulti- impactmate valueten- 0.2% (2sile proof re- mmV dia-stress stress duc- elon- notch) mound(kg/ (kg/ tion gation (kg-m/ pyramidmm.sup.2) mm.sup.2) (%) (%) cm.sup.2) hardness C.E.I.______________________________________Ex-am-ple1 86.5 59.2 78.5 15.0 20.2 274 44.312 81.5 51.9 73.1 17.3 27.9 264 44.733 98.9 60.1 72.5 15.6 17.2 316 34.124 91.9 55.7 73.7 14.3 21.2 291 38.695 100.2 51.7 64.8 15.0 5.3 318 27.756 105.5 42.1 64.8 17.9 10.7 326 26.467 97.1 52.1 62.7 15.5 23.9 302 28.848 100.2 61.9 67.0 13.6 14.3 330 34.999 102.9 53.3 70.0 15.1 10.2 332 30.7910 102.6 64.2 69.8 14.6 13.3 331 34.1511 103.7 53.9 63.9 14.7 11.1 320 33.6312 102.2 62.0 66.2 12.8 11.0 334 33.0413 109.7 59.0 61.7 15.1 9.84 330 28.8814 113.2 52.6 51.0 13.8 9.09 335 25.7915 114.4 50.0 57.7 15.7 9.76 344 25.8816 102.7 68.4 58.5 14.1 5.70 325 34.2717 98.9 62.0 71.0 12.2 13.70 312 40.4118 101.6 62.0 68.3 14.0 13.64 319 31.6519 96.4 60.4 71.8 13.8 15.33 310 39.9920 99.0 60.7 65.4 13.4 12.15 316 35.6421 99.5 63.3 66.1 12.2 11.80 313 34.5422 84.3 61.8 71.8 12.9 23.2 268 39.5223 83.2 63.5 70.4 12.2 23.4 267 39.9224 102.1 56.6 66.9 13.9 6.9 330 28.1125 101.0 63.9 60.9 11.6 8.8 320 28.0726 100.5 63.7 66.9 13.5 12.6 322 35.8227 105.1 53.8 66.2 14.6 10.5 327 29.8828 103.5 62.5 67.0 13.7 11.6 337 33.7129 105.6 55.3 64.0 15.2 10.6 338 30.1630 107.1 61.4 64.1 15.8 10.6 341 27.5031 106.4 68.0 64.7 14.4 12.8 327 29.8532 108.0 66.7 65.5 15.6 11.5 335 29.2833 109.7 55.7 61.8 16.1 10.34 327 25.7934 116.9 51.5 51.8 13.8 9.26 346 23.9435 113.1 49.1 57.7 14.3 9.92 338 26.1936 101.6 59.4 68.4 13.9 12.5 330 34.3937 103.4 54.6 64.0 15.1 10.0 332 31.4738 102.9 61.5 67.7 13.5 11.7 329 35.0939 106.1 58.2 64.0 15.7 10.7 335 26.9840 106.1 67.9 64.0 13.4 12.93 325 30.4541 108.9 57.2 59.3 15.4 10.64 325 27.5342 115.7 50.0 49.2 14.5 9.18 334 24.1743 113.3 48.7 57.6 15.4 9.59 332 33.8244 111.3 48.7 57.0 14.6 9.17 335 27.8045 100.0 64.1 68.3 12.6 15.23 320 35.7846 100.6 62.3 67.5 13.9 12.93 317 35.1947 97.9 58.0 70.4 13.9 13.11 315 36.7748 100.2 57.4 69.8 14.2 11.11 322 33.2349 98.6 61.2 71.2 15.1 12.06 313 35.4450 95.6 56.8 73.1 15.8 17.9 314 35.2651 80.7 50.3 73.3 16.2 26.8 266 42.8952 99.2 45.7 65.2 17.4 11.1 324 28.7753 95.4 51.1 63.9 14.8 19.7 311 29.2354 103.9 65.2 66.7 14.1 12.8 313 30.4655 106.3 62.0 64.6 14.5 10.8 329 29.3456 82.6 59.3 70.8 12.2 22.2 265 40.24Con-trol1 75.6 53.7 69.1 16.7 22.8 243 64.302 86.3 54.6 62.5 14.0 15.2 270 55.163 70.5 52.3 77.6 17.3 30.5 233 63.864 82.7 50.9 64.7 16.9 1.3 268 45.285 78.2 59.5 79.2 15.1 30.1 254 58.276 95.2 55.3 62.5 15.0 2.0 302 33.857 91.4 50.3 65.8 15.8 14.7 304 49.20______________________________________

As apparent from Table 2, each specimen of Examples according to the present invention is less than 45 in C.E.I. as compared with that of Controls, and it can particularly be understood that each specimen of Examples has remarkably excellent cavitation erosion resistance as compared with 13-chromium steels (Controls 1 and 2) which has widely been used as structural material for conventional water turbine elements and whose C.E.I. is over 55. It can also be understood that Example specimens are equal to or more excellent in mechanical strength and toughness than Control specimens.

The specimen of Control 6 is excellent in cavitation erosion resistance, but remarkably low in impact value. It is therefore unsuitable for use as structural material for water turbine elements such as runner, stay vane and guide vane which are needed to have high toughness.

As described above, martensitic stainless cast steel according to the present invention has excellent cavitation erosion resistance and is excellent in mechanical strength and toughness. It can also be manufactured easily and industrially without using a special casting manner. Therefore, it is most suitable for use as propeller material for ships as well as material for water power plant turbine elements such as runner, stay vane and guide vane.

FIG. 1 is a perspective view showing a runner of turbine made of stainless cast steel of the present invention and employed for water power plants. FIG. 2 is a sectional view of runner shown in FIG. 1 and including other turbine elements. In FIGS. 1 and 2 numeral 1 represents a crown, 2 blades, 3 a shroud, 4 a stay vane and 5 a guide vane.

Claims

1. Martensitic stainless cast steel having high cavitation erosion resistance and consisting essentially of carbon of 0.1 wt% or less, silicon of 1.0 wt% or less, manganese of 2.0-9.0 (exclusive of 2.0) wt%, nickel of 0.5-8.0 wt%, chromium of 11.0-14.0 wt%, molybdenum of 2.0 wt% or less and the balance of essentially iron.

2. Martensitic stainless cast steel having high cavitation erosion resistance and consisting essentially of carbon of 0.1 wt% or less, silicon of 1.0 wt% or less, manganese of 2.0-9.0 (exclusive of 2.0) wt%, nickel of 0.5-8.0 wt%, chromium of 11.0-14.0 wt%, niobium of 0.01-0.1 wt% and the balance of essentially iron.

3. Martensitic stainless cast steel having high cavitation erosion resistance and consisting essentially of carbon of 0.1 wt% or less, silicon of 1.0 wt% or less, manganese of 2.0-9.0 (exclusive of 2.0) wt%, nickel of 0.5-8.0 wt%, chromium of 11.0-14.0 wt%, copper of 0.1-0.5 wt% and the balance of essentially iron.

4. Martensitic stainless cast steel having high cavitation erosion resistance and consisting essentially of carbon of 0.1 wt% or less, silicon of 1.0 wt% or less, manganese of 2.0-9.0 (exclusive of 2.0) wt%, nickel of 0.5-8.0 wt%, chromium of 11.0-14.0 wt%, nitrogen of 0.02-0.15 wt% and the balance of essentially iron.

5. Martensitic stainless cast steel according to claim 4 wherein the amount sum of nitrogen and carbon is in the range of 0.02-0.15 wt%.

6. Martenistic stainless cast steel according to claim 1 wherein molybdenum is in the range of 0.5-2.0 wt%.

7. Martensitic stainless cast steel according to claim 6 wherein molybdenum is in the range of 0.5-1.6 wt%.

8. Martensitic stainless cast steel according to claim 1, 6, 7, 5, 2, 3, or 4 wherein carbon is in the range of 0.05-0.1 wt% and silicon in the range of 0.3-1.0 wt%.

9. Martensitic stainless cast steel according to claim 4 wherein manganese is in the range of 2.5-6.0 wt%, nickel in the range of 1.0-6.0 wt%, and chromium in the range of 12.0-13.5 wt%.

10. Martensitic stainless cast steel according to claim 3 wherein nickel is in the range of 3.0-4.0 wt%.

11. Martensitic stainless cast steel according to claim 5, 2, 3, 4 further containing molybdenum which is 2.0 wt% or less.

12. Martensitic stainless cast steel according to claim 5, 3, 4 further containing niobium in the range of 0.01-0.1 (exclusive of 0.1) wt%.

13. Martensitic stainless cast steel according to claim 5-4 further containing copper in the range of 0.1-0.5 wt%.

14. A turbine element for water power plants made of martensitic stainless cast steel according to claim 1, 6, 7, 5, 2, 3 or 4.

15. A turbine element according to claim 14 wherein said said turbine element is runner, stay vane or guide vane.

16. Martensitic stainless cast steel according to claim 1 wherein molybdenum is in the range of 0.5-2.0 wt%.

17. Martensitic stainless cast steel according to claim 16 wherein molybdenum is in the range of 0.5-1.6 wt%.