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°-700° 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° C., cooled at the cooling rate of 150° C./h, and then heat-treated for tempering under the temperature of 650° 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° 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 μm was added to the specimen for 180 minutes in pure water of 25° 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/tρ×106 

where w represents the weight loss caused by cavitation erosion (g), t test time (min.) and ρ 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/                               pyramidmm2)    mm2)            (%)    (%)   cm2)                               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.