CS275787B6 - Martensitic rustless steel for castings - Google Patents

Abstract

PURPOSE:To provide high mechanical strength and toughness, especially high cavitation errosion resistance to a martensitic stainless cast steel by specifying the contents of C, Si, Mn, Ni, Cr, etc. in the composition of the cast steel. CONSTITUTION:This cast steel consists of, by wt., <=0.1% C, <=1.0% Si, 2.0- 9.0% Mn, 0.9-8.0% Ni, 11.0-14.0% Cr and the balance essentially Fe or further contains <=2.0% Mn. It is melted with a high frequency or electric arc furnace. Since the cast steel has superior mechanical strength and toughness, especially superior cavitation erosion resistance, it is suitable or casting large-sized castings such as water turbine runners, guide vanes d stay vanes for equipment for hydroelectric power generation.

Description

4 / - CS 275787 3 6

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to martensitic stainless steel suitable for casting water turbine components for hydroelectric power plants, for example, for producing impellers, switchboards and reinforcing blades, which are required to have a high resistance to creep corrosion. In thermal and nuclear power plants, the unit power of the generator is still increasing, but it is very difficult to overcome peak loads. As one means of overcoming peak loads, hydroelectric power plants capable of adjusting output power over a relatively short period of time, particularly pumping power plant designs, which can effectively utilize excess power at night, have proven themselves.

The water turbine in these pumped-storage power plants is the so-called reverse pump turbine, which drives the generator by day and pumps water at night. These power plants tend to have high gradients and high performance to effectively utilize the building site and reduce construction costs per unit of performance.

Casting steel, which contains mainly chromium in an amount of about 13% by weight. is commonly used as a material for water turbine components such as an impeller, guide vanes, and a reinforcing blade; optionally, the steel also comprises nickel. However, the conditions under which high flow and high power working turbines are increasingly difficult. Vacuum areas are formed on the surface of the impeller feet due to the high flow rate of the water, and the surface of the blades is damaged by pulse loads when the vacuum tubes formed on their surface collapse. This results in cavitation corrosion. Conventional materials are unable to resist this cavitation corrosion. It is therefore desirable to develop a material with increased mechanical strength and toughness, and in particular with high cavitation corrosion resistance, in order to achieve a high water-power gradient and high performance. The subject of the invention is a martensitic stainless steel for casting up to 8.0 wt. % nickel and 11.0 to 14.0 wt. chromium. The subject matter of the invention comprises 0.05 to 0.1 wt. % carbon, 0.3 to 1.0 wt. silicon, manganese, the rest iron. which comprises 0.5 to 9.0 wt.

The inventive martensitic stainless steel has not only excellent resistance, cavitation corrosion, high mechanical strength and toughness, but can be easily manufactured on an industrial scale without any special casting measures.

The number of basic 5 alloying elements can be justified by their effect on the properties of the steel.

The carbon contained in the martensitic stainless steel according to the invention serves to form a stable martensitic phase by heat treatment and thus to increase the strength of the steel to the cast iron. However, excess carbon reduces the toughness so that the steel should contain 0.05 to 0.1% by weight. carbon. Silicon is added as a deoxidizing agent together with manganese in the smelting of steel to enhance castability of cast steel. However, too much silicon, like carbon, reduces the toughness of stainless steel, so that the amount of silicon should be between 0.3 and 1.0% by weight.

Manganese is a component that plays a particularly important role in enhancing the resistance of the stainless steel to the inventive casting against cavitation corrosion. This is because the total amount of manganese should be limited to between 2.0 and 9.0% by weight. is that the effect of the additive is not noticeable when the amount is less than 2.0 "by weight and that the epsilon and austenitic phase phases are formed in the steel to reduce the sustained elongation when the amount of manganese exceeds 9.0% by weight. most preferred is the addition of manganese in the range of 2.5 to 6.0% by weight.

Nickel is a solid phase dissolving component in a metal matrix, stabilizing the martensitic phase and increasing toughness. The total amount of nickel is limited to between 0.5% and 8.0% by weight, since when added less than the lower limit, the effect is too weak, while the addition of hardness deteriorates with the upper limit. the machinability of the martensitic stainless steel to the castings and the increase in the austenite content reduces the permanent elongation limit. Therefore, a nickel content of 1.0% to 6.0% by weight and most preferred 3.0% to 4.0% by weight is desirable.

Chromium is important as an alloying component promoting corrosion resistance. Amount of chromium below 11 wt. it has an inefficient effect in steel, whereas at an amount of more than 14 wt. chromium is formed in the ferrite delta matrix, which reduces the cavitation corrosion resistance. The total amount of chromium is preferably between 12.0 and 13.5% by weight.

The melting can be carried out in an induction or an arc furnace and casting can be carried out in a conventional manner, for example in a sand mold or a metal mold.

After casting, cooling is carried out at a rate such that no cracks are formed, the cooling rate being dependent on the shape and dimension of the steel casting; it is desirable for the tempering to proceed from 500 to 700 ° C. Examples:

The materials according to the invention, the chemical compositions of which are given in Examples 1 and 2 in Table 1, were melted in an induction furnace and heat treated so that their thermal treatment corresponds to the cooling of the large casting in air. The samples were then further heat treated at 1050 ° C, cooled at 150 ° C / h and then heat treated and tempered at 650 ° C.

The samples produced were tested for tensile strength Rm, plastic deformation limit Rp 02, Z contraction, elongation A5, KCV2 notch (Charpy hammer, V-shaped notch 2 mm at 20 ° C) and Vickers hardness HV.

The results are summarized in Table 2.

Thereafter, the samples were treated for 130 minutes in 25 ° C pure water with electro-vibration with a frequency of 6.5 kHz and a range of 100 µm, then the weight loss caused by cavitation corrosion was determined; the cavitation coefficient was then calculated according to the following equation: KS = w / (t. j5) .10 6 where w is the weight loss due to cavitation corrosion | g |, t is the test time in minutes and j5 is the density | kg.m ^ | .

Chemical control materials listed in Table 1 under numbers 1-7 (Controls) were melted, cast and heat treated in the same manner as the inventive materials and sampled. The properties of these samples were then tested as well as the properties of the samples of the steels of the invention. The results are also summarized in Table 2. 3 CS 275787 B 6 l ·? Ň

Table 1

Composition (% w / w) C N Si. Cr Ni Mn Mo Nb Cu Fe Examples 1 0,05 - 0,31 13,44 1,97 5,33 - - remainder 2 0,06 - 0,32 12,95 3,26 5,15 - - - II Kontrol samples 1 0.05 - 0.32 13.09 1.81 0.59 0.56 - -, 1 2 0.04 - 0.32 13.27 3.52 0.57 0.55 - - II 3 0.05 - 0.31 13.42 1.94 1.48 - - - · '4 0.06 - 0.33 13.05 1.91 3.03 3.53 - - II 5 0.05 - 0 , 30 13.20 3.45 1.51 - - - II 6 0.06 - 0.34 13.17 3.43 2.98 3.58 - - II 7 0.05 - 0.30 13.12 5 , 93 0.58 1.56 - - II

Table 2

Tensile strength Rm (MPa) Interposition.deformationRpO2 (MPa) Z (¾) ductility A5 (¾) KCV2 '(J.cm-Z) toughness T correlation according to Vi ck HV K 5 Examples 1 865 592 78 , 5 15.0 202 274 44.31 2 989 601 72.5 15.6 172 316 34.12 Controls samples 1 X 756 537 69.1 16.7 228 243 64.30 2 863 546 62.5 14, 0 152 270 55.16 3 705 523 77.6 17.3 305 233 63.06 4 827 509 64.7 16.9 13 268 45.28 5 782 595 79.2 15.1 301 254 58.27 6 952 553 62.5 15.0 20 302 33.85, 7,914 503 64.0 15.8 147 304 49.20

Claims (2)

As can be seen from Table 2, martensitic stainless steel samples according to the invention have a cavitation coefficient of less than 45 as opposed to control samples; it is thus evident that the steel samples according to the invention have a particularly excellent cavitation corrosion resistance, chromium steel with a content of 13% by weight. chromium, corresponding to control samples 1 and 2, which has hitherto been commonly used as a structural material for conventional water turbine components and whose cavitation coefficient KS is greater than 55. It is also apparent that the strength and the tensile strength of the steel of the present invention are equal to or better than control samples from superficial steels. Control Sample No. 6 has excellent cavitation corrosion resistance but high impact strength. Thus, it is not suitable as a construction material for component turbo turbines such as impellers, guide vanes and reinforcing vanes, which have to have high toughness. As already mentioned, the inventive martensitic stainless steel has an excellent cavitation corrosion resistance, high strength and toughness. It is easy to manufacture on an industrial scale without any special casting precautions. It is therefore very suitable as a material for bolts for ships and for water turbine components for hydroelectric power plants, such as impellers, reinforcing blades and vanes. FIG. 1 is a perspective view of a turbine impeller made of martensitic stainless steel according to the invention and used in a hydropower plant. FIG. 2 shows a cross-section through the impeller of FIG. 1 to which other components of the turbine are assigned. FIGS. 1 and 2 show the top 2 of the turbine, the vanes 2, the reinforcing vanes, and the patents of the martensitic stainless steel, 11.0 to 14.0 wt. % of chromium, characterized in that 0.3 to 1.0 wt. % silicon, 2.0 to 9.0 wt. CLAIMS Containing 0.5 to 8.0% by weight % nickel and contains 0.05 to 0.1 wt. carbon, manganese, the rest iron. 1 drawing