It is important to select the right type of steel to be used during construction of a new turbine runner or repair of an existing runner. Understanding the properties of the various materials, including the new stainless steel alloys, can help project owners draft proper specifications.
By Thomas Spicher
When a consultant is called upon to examine the condition of an existing runner, there generally has been some sort of problem. Cavitation, cracking, corrosion, or some combination thereof, are usually the primary concern. The reasons for these problems can be interrelated. When cracking occurs in the crown/blade fillet at the discharge edge of the runner, this may often be attributed to excessive heat from welding performed to repair cavitation damage that has added to the tensile stresses at the downstream edge of the blade. And cavitation damage is often exacerbated by coincidental corrosion.
Hydro turbine runners are commonly made of stainless steel alloys. To open the market to the most innovative and least expensive bids, owners' specifications for new or replacement runners are often vague as to the type of "stainless steel" or the method of manufacture.
Several types of stainless steel alloys are commercially available. The types - designated "martensitic" and "austenitic" or even duplex - have been used for runner fabrication, and the latter also have been used as corrosion- and cavitation-resistant overlays on carbon steel runners. Stainless steel relies on a thin passive film for its corrosion resistance. The film spontaneously forms on clean surfaces exposed to air and is rich in chromium oxides. Steel must have at least 10.5% chromium to exhibit mild atmospheric corrosion resistance, but for most purposes at least 12% chromium is needed. Increasing the chromium to 17% to 20%, as is typical of the standard austenitic stainless steel alloys, increases stability of the film and thus improves aqueous corrosion resistance.
|Cracking, shown here outlined with dye check, is a serious concern with turbine runners. Some materials used in manufacture of these runners are more susceptible to cracking than others.|
Runner manufacturers often prefer to provide a martensitic stainless steel due to a slight cost reduction. Prior to about 1970, a few runners were cast of martensitic stainless steel designated CA-15 in the U.S. This is a relatively high-carbon straight chromium heat treatable martensitic stainless steel. Type CA-15 offers aqueous corrosion resistance superior to carbon steel, with about twice the strength. But because it only contains 11.5% to 14% chromium, this steel exhibits marginal corrosion resistance in water, especially when the saline content is appreciable. Susceptibility to cracking, particularly at field welds, has proven to be the most undesirable trait of CA-15.
Beginning in the 1960s, a more crack-resistant martensitic steel designated CA-6NM became commercially available. CA-6NM was first used for North American hydroelectric projects at the Edmonston pumping plant and the 5,428-MW Churchill Falls complex in the Labrador region of Canada. CA-6NM still has only 11.5% to 14% chromium, but the addition of small amounts of nickel and molybdenum helps improve pitting resistance when compared to CA-15. The alloy balance of CA-6NM also increases notch toughness relative to CA-15 at the same strength level, with increased cavitation resistance over CA-15. The lower carbon content also permits useful welding of CA-6NM at lower preheat than needed for CA-15 steel.
These martensitics are nominally two to three times stronger in tensile strength than comparable austenitics, such as CF-3, CF-8, 304 or standard carbon steels. That strength has often resulted in thinner cross sections than comparable carbon steel or austenitic would require. Use of thinner cross sections can enhance the hydrodynamic flow conditions and efficiencies of a unit. But these thinner sections and reduced mass can result in increased levels of stresses due to vibration and increased potential for fatigue failures.
Cracking in a turbine runner is nominally the result of repeated vibration and fatigue added to whatever casting or fabrication stresses may be present. Crack-prone materials have a yield strength very close to the ultimate tensile strength, resulting in a very brittle condition. Fatigue life is not only dependent on ultimate tensile strength but also this difference between ultimate and yield strength. A difference of less than 10% results in low fatigue resistance and potential cracking from unusual single-impact events.
To produce the least expensive runners for Francis units, current manufacturing processes have been to produce cast or rolled blades that are attached to the massive crown portion by welding. The crown pieces are often cast and may vary from low-carbon steel to cast martensitic or austenitic material.
Now the design is beginning to get more complicated. The high tensile strength of the martensitics incorporated into the design is now defeated by the welding and incorporation of other lower-strength materials. The band at the discharge end may be rolled or cast. The critical area of concern is the crown/blade/fillet portion. Fatigue cracking almost always initiates at an edge or anomaly within the material. The runner geometry will dictate the likely critical location of cracking initiation. Initiation of cracking in this crown area will usually cause the crack to deviate from normal (perpendicular) to the initiating stress toward the next most likely stress riser, such as a vent or flange fastener hole. In Kaplan, propeller or Pelton units, the area of greatest concern is the hub or blade flange.
Solutions to the material selection concerns
There are three primary solutions to concerns regarding material selection when specifying manufacture of new or replacement turbine runners for hydroelectric projects.
First, assure that all components within the runner are the same material.
The primary concern with mixing the alloys is that a suitable stress redistribution heat treatment may become damaging to some alloys. Even if all the components are martensitic, the optimum heat treatment would often cause distortion of those components because of the mixed geometry, section thickness and impressed stresses from casting, factory repairs or fabricating.
In addition, some manufacturers will offer to rebuild Francis runners with new blades while using the existing crown and/or band. If these items are carbon steel, consider having the newly designed blades also be made of carbon steel with limited stainless steel overlay in critical areas. This is likely to be a very time-intensive process, usually six months or longer including dismantle and reinstallation time. The value of this process is that the connection to the existing turbine shaft will be assured. The rotating rings can be turned and rebuilt with stainless steel, preferably Nitronic 60 (ER 218) overlay. The stationary rings should be replaced with aluminum/bronze or Nitronic 60 inserts.
|Cracking is more typical at the upper fillet of a turbine runner, where the maximum force is transmitted to the shaft. The coloration due to corrosion is an example of the relative corrosion resistance of CA-15, a martensitic stainless steel. An austenitic in the same location would have been spotless.|
For Francis units, integral rotating seal or wear rings eliminate the problem of broken or loosened rotating rings. That design solution is enhanced by use of a base material that is resistant to corrosion, galling and cavitation. As a general rule, all the austenitic stainless materials are more corrosion-resistant than the martensitic grades for hydro turbine applications. One material stands out for cavitation and galling resistance: Nitronic 60, ASTM CF-10SMnN in the cast form, UNS S21800 in the wrought form or AWS ER 218 as a welding wire. Costs for any specialty steel depends on volume utilized, and, while this material is roughly double the initial cost of a 304, the high strength, low galling potential and cavitation resistance make it highly attractive when life cycle cost analysis is applied. Also, the material cost is only a small portion of the total finished runner cost.
Second, do not allow welding with a non-compatible filler material. Welding a martensitic steel with an austenitic filler should not be acceptable.
One of the requirements of a large turbine runner is to be able to make repairs in place. With the martensitic base materials, crack repair and deep cavitation (15% or more of section thickness) require in-kind weld material repairs and a subsequent full heat treatment. This heat treatment requirement would involve dismantling and usually shipment to a suitable heat treating facility. Most suppliers of martensitic materials state that repairs may be easily done, usually with a 308 filler material. This is true if the damage is minimal or not in crack-prone locations. If the damage is in the upper or lower fillets and heat treating due to other major repairs is required, the austenitic material in those crack-prone locations will be sensitized and will likely crack in a short time.
Over many years of treating chronic maintenance problems, I have encountered several solutions. One is to treat the maintenance problems when the original technical specifications are written. This could be for original installations or when the unit is being upgraded or uprated. The Nitronic 60 material is ideal for seal or wear rings and threaded fasteners that engage any type of stainless steel where galling or loosening is often a problem. When installed to near yield with Locktite thread sealing material, Nitronic 60 threaded fasteners will provide secure, non-galling and reusable installations.
Regardless of the base material, the optimum overlay material for Kaplan or propeller blade tips is Nitronic 60. It equals or exceeds the cavitation resistance of any other commonly used materials. Semi-spherical or spherical throat rings should be Nitronic 60 to avoid damage to the unit should contact occur. The shock from blade tip contact can cause secondary damage to head cover fasteners, wicket gate shafts and linkages. Providing this galling resistance may allow tightening the gap at the blade tips, which reduces leakage, enhances efficiency and reduces damage to fish passing through the unit. When correcting initial contour flaws, the Nitronic 60 allows ease of application without compromising strength, corrosion or cavitation resistance of the base material or weld application.
For carbon steel or austenitic runners, Nitronic 60 should be the first choice for cavitation repair or initial overlay in critical areas. For martensitic runners, Nitronic 60 has comparable tensile strength and twice the yield strength of other austenitic materials. Tests by the U.S. Army Corps of Engineers are detailed in an excellent comparison document for basic materials used in waterways.1 In this document, the austenitic materials rate considerably higher in most environments for corrosion/electrolytic problem avoidance. There is one alloy that stands out for galling resistance, corrosion resistance, strength and cavitation resistance: Nitronic 60 austenitic stainless steel.
Construction of runners may be by casting, wrought materials welded together, or a combination of these basic two types. When later welding is required, the austenitic material requires no further heat treatment, while the martensitic materials require a full heat treatment for any major repairs (ASTM A743), which could include severe cavitation damage or any through-blade cracking.
Welding repairs should incorporate contour and entrance angle modifications to minimize the potential for future damage, rather than rely upon exotic material changes that can accelerate electrolytic material loss. Crack repairs of significant depth or cavitation repairs in martensitics should not involve austenitics at all if there is an option. If no post welding heat treatment is possible for martensitic repair of martensitic runners, the highest strength austenitic should be chosen. In most cases, this would be Nitronic 60 (AWS 218). The acceptable martensitic weld material would be 410NiMo wire or electrode.
And third, accept martensitics only in those cases where conditions indicate the strength of the material is necessary to provide a usable design, such as very-high-head reversible units or a Pelton (Turgo) unit with unusually high head. Even this aspect is questionable and could be addressed with the higher-strength austenitic, such as Nitronic 60 in either cast or wrought form.
Nitronic 60, CF-10SMnN, is subject to foundry problems, if the mold material is not properly treated, because of manganese in the alloy. A special sand is also available that avoids those problems. A capable foundry that is alerted to these possible problems should have no difficulty.
In establishing a cavitation performance guarantee for new runners, it is wise to identify material loss from any source within this guarantee. This forces the civil design to address suitable rock traps or other hard particle separation as well as identifies the metallurgy that will withstand the aqueous conditions and material combinations.
Pelton units will often provide many decades of cavitation-free performance if the water is free of abrasive particles. Clean water can sometime be provided to run-of-river installations by use of a Coanda-type intake. This loses a few feet of head in the separation process but will reduce maintenance outages significantly. As with the other types of runners, contour and, in the case of Peltons, sharp accurate cusps between the buckets are essential for good performance. As rounding of the cusps occurs, side flows detract from efficiency and add the potential for cavitation at the bucket edges.
Turgo units experience stresses due to the blades passing the nozzle(s). These stresses can ultimately result in cracking. Cracking in any of the impulse runners must be addressed immediately or catastrophic failure may result.
Life cycle costing, which includes those costs of outages and maintenance, should make the small savings from usage of martensitics immeasurable.
1Kumar, A., A.A. Odeh and J.R. Myers, "Mechanical Properties and Corrosion Behavior of Stainless Steels for Locks, Dams, and Hydroelectric Plant Applications," Technical Report REMR-EM-6, U.S. Army Corps of Engineers' Construction Engineering Research Laboratory, Springfield, Va., 1989.
Tom Spicher is a consultant with Hydro Y.E.S. and Black & Veatch. While working at 6,809-MW Grand Coulee for 14 years, he developed methods for reducing the effects of cracking and cavitation in hydro turbines runners and encountered problems with certain martensitic stainless steels.