Cavitation resistant fluid impellers and method for making same

A fluid impeller for us in applications requiring superior cavitation erosion resistance. The impeller has a body fabricated from a castable metastable austenitic steel alloy which has a preferred chemical composition in the range of 17.5-18.5% chromium, 0.5-0.75% nickel, 0.45-55% silicon, 0.2-0.25% nitrogen, 15.5-16.0% manganese and 0.1%-0.12% carbon. Quantitative testing has shown cavitation resistance of four to six times that of standard boiler feed pump materials. A method for making cavitation resistant fluid impellers is also disclosed.

BACKGROUND OF THE INVENTION 
This invention relates generally to fluid impellers and more particularly 
to cavitation resistant fluid impellers made from castable cavitation 
resistant austenitic chromium-manganese alloy steels. 
Pump impellers frequently suffer cavitation damage for several reasons, 
including operation outside established hydraulic parameters. This damage 
is often a limiting factor in the life of the equipment. It may not be 
repairable by welding for reasons of inaccessibility. With a growing 
emphasis on enhanced reliability and longer life, there is a need in the 
pump industry for a casting alloy with significantly better cavitation 
resistance than the standard materials used to manufacture impellers. 
Other characteristics required for such a material to be commercially 
viable include machinability and weldability. 
For high speed applications, relatively high tensile and yield strength, 
and elongation will also be necessary. The mechanical properties of 
commonly used austenitic stainless steels, such as CF8M are: tensile 
strength 482 N/mm.sup.2 and yield strength 208 N/mm.sup.2 minimum. These 
low mechanical properties render such materials unsuitable for high speed 
impellers. 
The current state-of-the-art cavitation resistant material which has been 
used in pumps is a cobalt modified austenitic stainless steel known as 
Hydroloy.RTM.. Hydroloy.RTM. is described in U.S. Pat. No. 4,588,440, Co 
Containing Austenitic Stainless Steel with High Cavitation Erosion 
Resistance. One deficiency of Hydroloy.RTM. is susceptibility to hot short 
cracking. This characteristic contributes to poor castability. The 
presence of cobalt is also undesirable for some applications, particularly 
the nuclear industry. 
The foregoing illustrates limitations known to exist in present cavitation 
resistant alloy steels. Thus, it is apparent that it would be advantageous 
to provide an alternative directed to overcoming one or more of the 
limitations set forth above. Accordingly, a suitable alternative is 
provided including features more fully disclosed hereinafter. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention, this is accomplished by providing a 
fluid impeller for use in applications requiring a high degree of 
cavitation erosion resistance, the impeller having a body fabricated from 
a castable metastable austenitic steel alloy which has a chemical 
composition in the following range: 
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C Mn N Si Ni Cr 
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% min 0.08 14.0 0.3 17.0 
% max 0.12 16.0 0.45 1.0 1.0 18.5 
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the balance comprising iron and impurities.

DETAILED DESCRIPTION 
The alloy described below has demonstrated cavitation resistance several 
times better than that of existing standard impeller materials. This new 
alloy also satisfies not desirable criteria, including castability, 
weldability, machinability, and low cost. 
This steel belongs to a class of alloys known as metastable austenitic 
steels. Both stainless and nonstainless grades of metastable austenitic 
steels have been produced. Austenite in metastable alloys can transform 
spontaneously into martensite either on cooling or as a result of 
deformation. This alloy has an austenitic structure upon water quenching 
from the solution annealing temperature but will transform to martensite 
on exposure to impact loading. The transformation which occurs in this 
class of materials is accompanied by an increase in hardness and has been 
exploited commercially in steels for wear and abrasion resistant 
applications. Hadfield manganese steels (a nonstainless type) are the best 
known of this class. 
The ease with which metastable alloys can be induced to transform to 
martensite is related to a characteristic known as stacking fault energy. 
Chemical composition can be adjusted to produce an alloy with low stacking 
fault energy which will readily develop fine cavitation induced twinning 
associated with the formation of a martensitic phase. The fine twinning is 
an efficient means of absorbing the incident cavitation impact energy. The 
relationship between low stacking fault energy and high resistance to 
cavitation was first identified by D. A. Woodward, 
Cavitation-Erosion-Induced Phase Transformations in Alloys, Metallurgical 
Transactions, Volume 3, May 1972. 
In this class of materials, the element nickel is known to promote a stable 
austenitic structure, whereas both manganese and nitrogen tend to promote 
the transformation of austenite to martensite. However, nitrogen has a 
tendency to cause bubbling during solidification. 
An old alloy, Tenelon, produced by United States Steel, has a composition: 
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C Mn N Si Ni Cr 
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% min 0.08 14.5 0.35 0.30 17.0 
% max 0.12 16.0 1.0 0.75 18.5 
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Tenelon is a wrought steel, not previously produced in cast form. 
Experimental efforts to develop a cast version of Tenelon have not been 
acceptable due to excessive porosity. 
The cavitation-resistant alloy (designated, generally "XM-31") according to 
this invention contains 17.5-18.5% chromium, 0.5-0.75% nickel, 0.45-0.55% 
silicon, 0.2-0.25% nitrogen, 15.5-16.0% manganese and 0.1%-0.12% carbon, 
the balance being iron and impurities. Preferably, phosphorus and sulfur 
are less than 0.02%. After the alloy is cast, the article is heat treated 
at 1050.degree. C. to 1100.degree. C. for one hour per inch of thickness, 
followed by a water quench. 
The preferred range of chemistry for the new alloy is: 
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C Mn N Si Ni Cr 
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% min 0.08 15.0 0.10 0.4 17.0 
% max 0.12 16.0 0.30 0.8 1.0 18.5 
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The alloy has a specific composition of critical elements as follows: 
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C Mn N Si Ni Cr 
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% min 0.10 15.5 0.20 0.45 0.5 17.5 
% max 0.12 16.0 0.25 0.55 0.75 18.5 
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We have determined that the manganese content is important to cavitation 
resistance. FIG. 2 shows the relationship between manganese and cavitation 
resistance. Preferably, the manganese content content is 16%. 
When casting articles using this new alloy, we have determined that olivine 
sand (MgFe).sub.2 SiO.sub.4 ! should be used for the molds. The metal 
bath should be kept at 1500.degree. C. to limit oxidation. Manganese in 
steel reduces solubility for nitrogen. Excess nitrogen in high manganese 
steel, which exceeds the solubility limit, promotes bubbling and gas 
defects as the casting solidifies. Consequently, nitrogen should be added 
to the melt just prior to casting. 
Quantitative laboratory cavitation test data was developed in accordance 
with ASTM G32-92 for several heats of the new alloy. Cavitation resistance 
was consistently superior, by a factor of about six, compared with the 
martensitic stainless alloy CA6NM which is the industry standard in boiler 
feed pumps and other demanding impeller applications where cavitation is a 
chronic problem. Cavitation resistance of the new material also exceeds by 
a factor of about four, that of 17-4PH and CA15Cu, both utilized in the 
pump industry as upgrades for CA6NM. The new alloy combines high 
mechanical properties, adequate for high energy pumps, with a level of 
cavitation resistance which far exceeds that of conventional materials. 
Table I and FIG. 1 summarize the results of cavitation tests carried out by 
the inventors. The table presents a comparison of the Brinell Hardness 
Number (BHN) and the Mean Depth of Penetration Rate (MDPR) for several 
alloys during cavitation testing. The composition of test sample XM31-2 
is: carbon 0.11%, manganese 15.3%, silicon 0.49% and chromium 18.39% and 
test sample XM31-3 is: carbon 0.11%, manganese 15.7%, silicon 0.51% and 
chromium 17.17%. 
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CAVITATION TEST RESULT SUMMARY 
Material BHN MDPR 
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XM31-3 260 0.00089 
Cast CA15Cu 388 0.00400 
17-4PH(cond. H1150) 255 0.00469 
Cast CA6NM(Dresser) 262 0.00651 
Cast CA6NM 262 0.00740 
Cast CA15 217 0.01110 
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The mechanical properties of the new alloy are: tensile strength 676-745 
N/mm.sup.2 yield strength 410-480 N/mm.sup.2 and elongation 43.2-53.7%. 
These properties are based upon testing of five different XM31 samples. It 
has also been determined that the new alloy can be welded using 
commercially available filler metals, and machined using standard 
techniques employed in the manufacture of pump impellers. 
The resulting alloy, described above, offers cavitation resistance far 
superior to that of conventional stainless casting alloys. It develops 
this high resistance by a strain hardening mechanism associated with the 
formation of cavitation induced twinning. This significantly delays the 
initiation of fatigue cracking. 
In the following claims, a blank means no minimum of the alloying agent 
specified.