Ejecución de pruebas del Sprint 5

Technical data illustrating the properties of heat resistant alloys are very helpful guides in selecting an alloy suitable for a given application. However the behavior of alloys during long exposure to the many environments and temperatures that may be encountered cannot be completely documented nor described by laboratory tests. Experience obtained from many actual installations is most helpful. One must develop the judgment needed to determine which of the many factors involved are the most important.

A few points to consider.

Temperature is often the first—and sometimes the only—data point given when we are asked for suggestions regarding alloy selection. One cannot successfully chose an alloy based on temperature alone. Nevertheless one simple first guide to alloy selection is knowing the maximum temperature at which a given alloy may have useful long term engineering properties. Picking oxidation in air, or strength, as a limiting factor one might rate alloys as follows, in plate form. Thin sheet will have a lower limiting temperature due to proportionally greater losses to oxidation.

Carbon steel, such as ASTM A 387 Grade 22 (2¼ Cr, 1 Mo). Typically considered 950°F (510°C), above which 304H is stronger.

409 ferritic stainless (UNS S40900, Werkstoff Nr. and EN 1.4512) 1200°F (650°C), limited by oxidation. Subject to embrittlement after several years’ service above about 600°F (316°C).

Formable, weldable.

410S low carbon martensitic stainless (UNS S41008, W.Nr. 1.4000) 1200°F (650°C), limited by oxidation. Subject to “885°F” embritlement after long service above about 600°F (326°C).

410 martensitic stainless (UNS S41000, W. Nr. 1.4024) 1200°F (649°C), limited by oxidation.

Subject to embrittlement after several years’ service above about 600°F (316°C). Can be hardened by heat treatment, difficult to weld.

304/304H & 316 stainless (S30400/S30409, W.Nr. 1.4301 & S31600, 1.4401) 1500°F (816°C). If product contamination by scale particles is a consideration, consider a 1200°F (649°C) limitation, and move up to RA309 for 1500°F (816°C) service.

321 (S32100, W.Nr. & EN 1.4541) stainless has about a 100°F (55°C) advantage over 304, and is used to 1600°F (1202°C). In Europe 316Ti (W.Nr. & EN 1.4571) is used to 1650°F (899°C), whether because of technical advantage over 321 or difference in philosophy we do not know at this time.

RA309 (S30908, W.Nr. & EN 1.4833) is useful to about 1850-1900°F (1010-1038°C) above which our customers seem dissatisfied with its oxidation performance.

RA800H/AT (UNS N08811) is a little more oxidation resistant, still we’d suggest keeping it below 2000°F (1093°C)

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RA 253 MA^® (UNS S30815, W.Nr. 1.4893, EN 1.4835) has superior oxidation resistance to a fairly definite upper limit of 2000°F (1093°C). Above this temperature the oxidation resistance may be adequate but no longer exceptional.

RA310 (S31008, W.Nr. & EN 1.4845) is reasonably oxidation resistant to about 2150°F (1177°C), although the strength is quite low.

RA330^® (N08330) combines useful oxidation resistance and fairly high melting point so that it will tolerate more extreme temperature abuse than any other fabricable austenitic grade with which we are familiar. RA330 muffles are regularly used at 2100-2150°F (1149-1177°C).

In one exceptional case an 11 gage (3mm) wall RA330 muffle provided six months service brazing with 65% palladium 35% cobalt filler at 2370°F (1300°C).

RA 353 MA^® (S35315, EN 1.4854 ) has a melting point similar to that of RA330, with better oxidation resistance in laboratory tests. Field experience at this time is with muffles and calciners. Based on its chemistry and test results we would expect it to tolerate extreme temperature at least as well as does RA330.

RA333^® (N06333, W.Nr. 2.4608) in open air use is limited more by its incipient melting point than by oxidation. Temperatures to 2200°F (1204°C) may be considered, though stagnant conditions might not be desirable. We have no experience with this grade at 2300°F (1260°C).

RA600 (N06600, W.Nr. 2.4816) excellent carburization resistance. Oxidation resistance does not drop off rapidly with temperature. RA600 is used at the same high temperatures as RA330, although somewhat more creep deformation may occur in service.

RA601 (N06601, W.Nr. 2.4851) has deformed more than RA333 in 2150F (1177C) applications and should have a somewhat lower maximum temperature use.

RA X (N06002, W.Nr. 2.4665) is designed for gas turbine combustors where the hot gases continually sweep over the metal surface. Due to its 9% molybdenum content this grade may be subject to catastrophic oxidation under stagnant conditions, or in open air above roughly 2150°F (1177°C). temperature. One cure for this is to paint braze stop-off on the parts or fixture. Nicrobraze^® Orange Stop-Off, from Wall Colmonoy Corp. is used for this purpose. Alloys commonly used as fixturing for vacuum heat treating tool or stainless steel include RA330, RA600 and RA601.

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Selecting The Alloy—atmosphere, continued

Air—those alloys useful in just plain hot air are also suited for oxidizing products of combustion of natural gas and even coal. Generally oxidation and strength are the only issues. “Oxidation” usually refers to metal wastage, but concern about product contamination from scale is an occasional issue. For example, glass forming operations take place around 1100-1400°F (600-760°C) where 304 stainless might suit as a structural element. But because scale from this stainless gets onto the glass, RA330 is used for its much higher oxidation resistance. With respect to oxidizing products of combustion of coal, a major end use of RA 253 MA is for coal nozzles in powdered coal fired utility boilers.

Oxidizing products of combustion from heavy fuel oils may be corrosive due mostly to small amounts of vanadium in the oil, particularly in oil from Venezuela. The vanadium pentoxide formed is very corrosive at red heat. The only alloy said to resist V2O5 hot corrosion is the cast 50Cr-48Ni alloy (“50-50”), UNS R20501. Short of that, one might consider RA333, although it is definitely not as resistant as the 50%Cr alloy.

While oxidizing products of combustion of coal can readily be handled by alloys such as RA 253 MA, or higher nickel grades if one prefers, reducing products of coal combustion are another matter. In reducing atmospheres, which do occur in certain areas of the current generation of low-NOx burners, sulphidation from both coal and oil fuels can be a serious matter. We would not suggest use of any alloy with higher nickel than RA310. That is, limit nickel content of the alloy to about 20%, to minimize sulphidation attack.

In all other atmospheres there may be some potential for carburization or hot corrosion. If the atmosphere really is hydrogen, argon or nitrogen then no reaction with the alloy should occur. But sometimes the atmosphere as it exists in the furnace is unintentionally different than the pure gas pumped into the furnace. A classic case is a coil annealing cover for carbon steel. The atmosphere is nominally nitrogen-hydrogen, which would be quite neutral.

But residual palm oil from cold rolling steel sheet vaporizes and makes the atmosphere carburizing enough to deposit soot inside the cover. This “inert” atmosphere will also mildly carburize the cover itself, usually RA309 or RA330. When steel rod coils are annealed the carbon potential of the atmosphere is controlled to 0.4%C, to be neutral to the AISI 4140 or 1045 steel rod being annealed. This atmosphere is actually carburizing to Ni-Cr-Fe heat resistant alloy, and tends to embrittle RA 253 MA.

A less common situation is sulphidation of alloy fixturing used to anneal copper cathodes, electrolytically refined copper. The cathodes have residual copper sulphate from the electrolyte used in this process. This is the source of sulphur, which will attack nickel alloy furnace fans, in particular, used for annealing cathodes in a reducing or neutral atmosphere.

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Intentionally carburizing atmospheres are commonly used in heat treatment of steel parts.

Resistance to embrittlement from carbon absorbtion is largely conferred by the total chromium , nickel and silicon content of the heat resistant alloy. Wrought alloys commonly used to resist carburization include RA330, RA333, RA600 and RA601. Of the lower nickel grades only RA85H, at 3 1/2% silicon, has useful carburization resistance in heat treat furnaces. RA800H/AT is too coarse grained, and too low in silicon, for practical use in such applications. RA X has good carburization resistance and is occasionally used.

In high chlorine, or fluorine, atmospheres high nickel alloys are preferred. RA600 is the usual choice. When some amount of oxygen is also present, with only moderate halogen levels, RA601 may be useful. Under oxidizing conditions chromium, molybdenum and tungsten form highly volatile oxychlorides.

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