GEK Wiki / Metal Alloys for High Temp Operation
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Metal Alloys for High Temp Operation

Page history last edited by jim mason 6 years, 7 months ago

return to Practical Engineering

 

A good summary of various stainless steel alloys and their properties at high temp is here:  http://www.azom.com/Details.asp?ArticleID=1175 .  The main table of relevance from this article is reproduced below.  However, there are many caveats due to carbide precipitation in mid temp ranges, and other alloy modifications with "L", "H", or other variants.

 

General summary for gasifier thermal applications, in ascending order of preference and price, is (with caveats):  304 - 321 - 310 - 253MA (2111HTR).  Articles on each of these alloys are linked below.  At the end of this page are explanations of Carbide Precipitation and Sigma Phase Embrittlement which are important considerations for thermal applications of SS alloys.

 

Table 1. Maximum service temperatures in dry air, based on scaling resistance (ref: ASM Metals Handbook)

Grade

Intermittent (°C)

Continuous (°C)

304

870

925

309

980

1095

310

1035

1150

316

870

925

321

870

925

410

815

705

416

760

675

420

735

620

430

870

815

2111HTR

1150

1150

 


General explanation of L and H grades

Within the usual designations of the common austenitic grades of stainless steel, such as 304 and 316, there are "sub-grades" - "L" and "H" variants - with particular applications. L is lower carbon content.  H is higher carbon conent  http://www.azom.com/article.aspx?ArticleID=1258   The lower carbon content helps prevent carbide precipitation formation during welding or high temp apps.  The higher carbon content makes for a stronger alloy.

 

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Article on 304, 304L, 304H

http://www.azom.com/article.aspx?ArticleID=965

 http://www.azom.com/article.aspx?ArticleID=2867

 

Grade 304 displays good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C. Continuous use of 304 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important. Grade 304L is more resistant to carbide precipitation and can be heated into the above temperature range.

Grade 304H has higher strength at elevated temperatures so is often used for structural and pressure-containing applications at temperatures above about 500°C and up to about 800°C. 304H will become sensitised in the temperature range of 425-860°C; this is not a problem for high temperature applications, but will result in reduced aqueous corrosion resistance.

 

 

Article on 321 and 347
 http://www.azom.com/article.aspx?ArticleID=967

 

Grades 321 and 347 are the basic austenitic 18/8 steel (Grade 304) stabilised by Titanium (321) or Niobium (347) additions. These grades are used because they are not sensitive to intergranular corrosion after heating within the carbide precipitation range of 425-850°C. Grade 321 is the grade of choice for applications in the temperature range of up to about 900°C, combining high strength, resistance to scaling and phase stability with resistance to subsequent aqueous corrosion.

Grade 321H is a modification of 321 with a higher carbon content, to provide improved high temperature strength.

 

 

Article on 310

http://www.azom.com/article.aspx?ArticleID=966

 

Good resistance to oxidation in intermittent service in air at temperatures up to 1040°C and 1150°C in continuous service. Good resistance to thermal fatigue and cyclic heating. Widely used where sulphur dioxide gas is encountered at elevated temperatures. Continuous use in 425-860°C range not recommended due to carbide precipitation, if subsequent aqueous corrosion resistance is needed, but often performs well in temperatures fluctuating above and below this range.

Grade 310 is generally used at temperatures starting from about 800 or 900°C - above the temperatures at which 304H and 321 are effective.

 

 

Article on 253MA (2111HTR) 
http://www.azom.com/article.aspx?ArticleID=959

253MA is a grade combining excellent service properties at high temperatures with ease of fabrication. It resists oxidation at temperatures up to 1150°C and can provide superior service to Grade 310 in carbon, nitrogen and sulphur containing atmospheres.

 

other alloys of interest

 

3CR12

A lower cost is required, and the reduced corrosion resistance and resulting discolouration are acceptable.

http://www.azom.com/article.aspx?ArticleID=968

430

A lower cost is required, and the reduced corrosion resistance and fabrication characteristics are acceptable.

 

 

 

 

 

Carbide Precipitation and Sigma Phase embrittlement
 from http://www.azom.com/article.aspx?ArticleID=1177

 

 

Carbide Precipitation or Intergranular Corrosion 

See article: intergranular corrosion.

 

Intergranular corrosion is a form of relatively rapid and localised corrosion associated with a defective microstructure known as carbide precipitation. When austenitic steels have been exposed for a period of time in the range of approximately 425 to 850°C, or when the steel has been heated to higher temperatures and allowed to cool through that temperature range at a relatively slow rate (such as occurs after welding or air cooling after annealing), the chromium and carbon in the steel combine to form chromium carbide particles along the grain boundaries throughout the steel. Formation of these carbide particles in the grain boundaries depletes the surrounding metal of chromium and reduces its corrosion resistance, allowing the steel to corrode preferentially along the grain boundaries. Steel in this condition is said to be "sensitised".


It should be noted that carbide precipitation depends upon carbon content, temperature and time at temperature. The most critical temperature range is around 700°C, at which 0.06% carbon steels will precipitate carbides in about 2 minutes, whereas 0.02% carbon steels are effectively immune from this problem.


It is possible to reclaim steel which suffers from carbide precipitation by heating it above 1000°C, followed by water quenching to retain the carbon and chromium in solution and so prevent the formation of carbides. Most structures which are welded or heated cannot be given this heat treatment and therefore special grades of steel have been designed to avoid this problem. These are the stabilised grades 321 (stabilised with titanium) and 347 (stabilised with niobium). Titanium and niobium each have much higher affinities for carbon than chromium and therefore titanium carbides, niobium carbides and tantalum carbides form instead of chromium carbides, leaving the chromium in solution and ensuring full corrosion resistance.


Another method used to overcome intergranular corrosion is to use the extra low carbon grades such as Grades 316L and 304L; these have extremely low carbon levels (generally less than 0.03%) and are therefore considerably more resistant to the precipitation of carbide. Many environments do not cause intergranular corrosion in sensitised austenitic stainless steels, for example, glacial acetic acid at room temperature, alkaline salt solution such as sodium carbonate, potable water and most inland bodies of fresh water. For such environments, it would not be necessary to be concerned about sensitisation. There is also generally no problem in light gauge steel since it usually cools very quickly following welding or other exposure to high temperatures.


It is also the case that the presence of grain boundary carbides is not harmful to the high temperature strength of stainless steels. Grades which are specifically intended for these applications often intentionally have high carbon contents as this increases their high temperature strength and creep resistance. These are the "H" variants such as grades 304H, 316H, 321H and 347H, and  also 310. All of these have carbon contents deliberately in the range in which precipitation will occur.

 

 

 

Signa Phase Embrittlement

 

A further problem that some stainless steels have in high temperature applications is the formation of sigma phase. The formation of sigma phase in austenitic steels is dependent on both time and temperature and is different for each type of steel. In general Grade 304 stainless steel is practically immune to sigma phase formation, but not so those grades with higher chromium contents (Grade 310) with molybdenum (Grades 316 and 317) or with higher silicon contents (Grade 314). These grades are all prone to sigma phase formation if exposed for long periods to a temperature of about 590 to 870°C. Sigma phase embrittlement refers to the formation of a precipitate in the steel microstructure over a long period of time within this particular temperature range. The effect of the formation of this phase is to make the steel extremely brittle and failure can occur because of brittle fracture. Once the steel has become embrittled with sigma it is possible to reclaim it by heating the steel to a temperature above the sigma formation temperature range, however this is not always practical.

 

Because sigma phase embrittlement is a serious problem with the high silicon grade 314, this is now unpopular and largely replaced by high nickel alloys or by stainless steels resistant to sigma phase embrittlement, particularly 2111HTR (UNS S30815).

 

Grade 310 is also fairly susceptible to sigma phase formation in the temperature range 590 to 870°C, so this "heat resistant" grade may not be suitable for exposure at this comparatively low temperature range and Grade 321 is often a better choice.

 

 

Corrosion Properties of Stainless Alloys

http://www.azom.com/article.aspx?ArticleID=1177

 

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Another good summary of metal alloys and their performance under various thermal and industrial conditions is here:

http://www.vici.com/ref/mat_met.php

 

A detailed review of the properties of Inconel, here:

http://docs.twpinc.com/Inconel-alloy-600-Sept-2008.pdf

 

 

Comments (1)

Dave S. said

at 10:49 pm on May 2, 2009

Try some Inconel 600, if you're having temperature/oxidation/corrosion/abrasion problems; it's an austenitic nickel-chromium alloy (with a little iron, manganese, and copper thrown in). Used for things like gas turbine blades, it'll stand up to nearly anything you throw at it, even if you throw everything at it all at once. Prices might be high, but for critical pieces of kit like an injector nozzle, it might be just the thing you need.

Heck, if it's good enough for a nuclear reactor, it's good enough for anything else.

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