GB2047269A - Heat Resisting Alloy - Google Patents

Abstract

A heat resisting alloy comprises the following by weight: Carbon 0.05% to 0.50% Nickel 18% to 28% Chromium 14% to 18% Silicon 0.1% to 2% Manganese 0.1% to 2% Niobium 0% to 6 x the carbon content Cerium 0% to 0.1% the balance being iron, provided that X=(4.5a+11b)-(1.1c+58d+61)</=0 where a is the percentage weight of the chromium content, b is the percentage weight of the silicon content, c is the percentage weight of the nickel content, and d is the percentage weight of the carbon content. Positive values for the quantity X derived from this formula indicate the percentage of sigma phase formation to be expected upon heating of the alloy for extended periods at 650 DEG C, so that if X is zero or has a negative value, no sigma phase formation is to be expected. To ensure stability of the alloy, X is preferably less than or equal to -1.

Description

SPECIFICATION Heat Resisting Alloys Cast and wrought austenitic, heat-resisting, nickel-chromium-iron alloys are extensively used in heat-treatment furnaces, petrochemical plant, cement mill furnaces and many other diverse applications.

One of the most popular types used in heat-treatment furnaces contains 0.3% carbon, about 37% nickel, 18% chromium, 1% silicon and 1% manganese although there can be quite wide variations in composition depending on whether (a) wrought or cast parts are being made because a lower carbon is necessary with the former and (b) which particular specification is being used since nickel, for example can range from 33 to 41 and chromium from 13-21 per cent.

This is in contrast with petrochemical plant where most of these steels are employed in the form of tubes and cast 0.4% carbon, 25% chromium, 20% nickel steels are the most used. However, higher nickel steels are used for certain critical parts such as manifolds and return bends whilst even tubular castings are made in 0.4% carbon, 35% nickel, 25% chromium type steels for certain high temperature furnaces. Often these latter materials are strengthened by alloying with small proportions of niobium and tungsten.

The use of these expensive high nickel steels for heat treatment furnace equipment and more limited use in petrochemical plant has hitherto been justified on the basis that nickel improves strength, carburisation resistance and perhaps oxidation or scaling resistance. The resistance of these steels to carburisation, i.e. carbon enrichment of the surface layers, is considered more important than scaling resistance which is not critical within limits. Thus the development of high carbon contents at the surface results not only in metal wastage but the differences in thermal expansion between carburised and uncarburised zones can impose a high level of tensile stress on the base metal such that deformation and eventually cracking occurs. A carburising environment is frequently experienced in both heat-treatment and petrochemical furnaces.

Steels having a high nickel content are known to show an enhanced resistance to carburisation and it is also well known that silicon contents of around 2% are also beneficial. Very high nickel steels also exhibit a good resistance to nitradation. Nickel does not easily form a nitride and hence such steels are used for containers and other parts needed in the ammonia gas nitriding of steel. Also, high nickel alloys are used in other ammonia cracking installations such as brazing furnaces.

Whilst, therefore there are undoubted and well proven benefits to be obtained from high nickel it is the purpose of the present invention to show that the advantages of high nickel can be over emphasised. It is the further object of this invention to show that completely satisfactory properties can be obtained at much less cost with lower nickel steels and in some instances, there may be an improvement in performance.

In developing lower nickel steels, it is important to ensure that the austenitic structure is stable and there is no formation of the embrittling sigma phase (Fe Cr) if the alloy is heated whether to relatively low temperatures (i.e. about 6500 C) or to higher temperatures of about 7500C to 8000C.

Formation of sigma phase is likely to occur on heating if the nickel and/or carbon content is too low and/or if the chromium and/or silicon content is too high.

According to the present invention, there is provided a heat resisting alloy comprising the following by weight: Carbon 0.05% to 0.50% Nickel 18% to 28% Chromium 14% to 18% Silicon 0.1% to 2% Manganese 0.1% to 2% Niobium 0% to 6xthe carbon content Cerium 0%to0.1% the balance being iron, and wherein X=(4.5a+11 b)-(1.1 c+58d+61) < 0 where a is the percentage weight of the chromium content, b is the percentage weight of the silicon content, c is the percentage weight of the nickel content, and d is the percentage weight of the carbon content.

When the quantity X in the formula given above rises above zero, its value represents the percentage of sigma phase formed when the alloy is heated for extended time periods at 6500 C, so that clearly, when X equals zero or has a negative value, there is no sigma phase formation. To ensure the stability of the alloy, it is preferred that Xis less than or equal to -1.

Where niobium is contained in the alloy, preferably in the range 5 to 6 (i.e. nominally 5.5) times the carbon content, to improve its strength, the carbon content available for combination with the chromium is correspondingly reduced, and thus the formation of sigma phase is promoted, unless the chromium content of the alloy is restricted to 17% by weight for stability. Moreover, if the carbon content is in any case low, i.e. 0.05% to 0.1%, as is the case in wrought steel alloys, then it is desirable for stability to restrict the silicon content to a maximum of about 1% by weight.

The addition of up to 0.1% nitrogen to alloys containing niobium also assists in achieving a stable alloy during heating. The use of niobium in an alloy according to the invention whilst improving its strength can drastically impair its resistance to scaling. The addition of up to 0.1 % cerium to the alloy, particularly if it contains niobium can improve its resistance to oxidation i.e. to scaling.

The use of silicon in the alloy tends to improve its resistance to carburisation, and the addition of cerium also may provide further improvement in this respect since the adherent oxide promoted by the cerium addition should act as a barrier to carbon penetration.

It will be appreciated that in addition to the elements discussed above, an alloy according to the invention will also, in practice, contain impurities such as up to 0.05% each of sulphur and/or phosphorus.

A preferred alloy according to the invention, contains 25% nickel, 1 7% chromium, 1% silicon, 1 manganese, and 0.3% carbon for cast steel or 0.05% to 0.1% carbon for wrought steel.

Examples The following steel alloy was sand-cast in plate form, the analysis by percentage weight being: Steel No. C Si Mn Ni Cr Ce 1 (type 17-25 Ce) 0.36 1.81 1.04 26.5 17.2 0.05 Bars of 0.252 inch diameter were cut from the plate and used in a stress-rupture test to establish strength data, a stress of 3 tons f/in2 (46.3 N/mm2) being applied thereto at a temperature of 9000 C.

The following results were obtained: Life (in hours) Elongation (%) 134.5 22.4 133.9 not determinable The strength of the alloy thus tested in this example is accordingly similar to the strengths of alloys having a higher nickel content such as the 37% Ni-1 8% Cr alloy steel mentioned above.

The following steels were also sand-cast, the analysis of each by percentage weight being as follows: Steel No. C Si Mn Ni Cr Nb Ce 2 (type 17-25) 0.36 1.81 1.04 26.5 172 Nil Nil 3 (type 18-37) 0.4 0.87 0.88 36.7 18.65 Nil Nil Specimens of each of steel Nos. 1, 2 and 3 were then subjected to a scaling test, being heated in air for 259 hours at 9800C for assessment of the scaling behaviour of each with periodic removal and weighing of the specimens during that time. the following results were obtained:: Total change in Total weight of specimen weight detached scale Scaling Steel No. (g) (g) (g/m2/h) 1 0.0001 0.0152 0.08 2 Weight loss 0.0593 0.28 3 ,, ,, 0.0504 0.26 It should be noted that if the specimen gained in weight over the 259 hour test the detached scale weight was added to give the total value, whereas if the specimen lost weight, then either the weight loss or the weight of detached scale, whichever was the larger, is shown.

It will therefore be seen that in the absence of cerium the 17-25 steel (No. 2) according to the invention has similar scaling properties to the 1 8-37 steel (No. 3) included for comparison. The addition of cerium to the 1 7-25 Ce steel (No. 3) produces a considerable improvement over the 17-25 steel (No. 2) Indeed, 25% Ni-1 7% Cr steel has been found to have adequate oxidation (scaling) and carburisation resistance at temperatures up to 10000 C.

Bars of the 17-25 Ce steel (Steel No. 1) were heated for 696 hours at 80000C and the following tensile properties were then obtained: UTS Yield Strength Elongation % tons ffln2 tons ffln2 on 1.26 inch (32 mm) (N/mm2) (N/mm2) gauge length 31.8(491) 20.6 (318) 6.0 32.0 (494) 20.8 (321) 7.0 These values are similar to those obtainable from 18% Cr37% Ni steel.

A sample of steel No. 1 was treated for 250 hours at 9800C in a high energiser carburising compound and subsequently polished and examined by electron X-ray microprobe analysis. The carbon penetration found was as follows:- Depth (/lmJ Carbon (%J 150 0.64 250 0.86 350 0.69 550 0.55 750 0.57 950 0.44 1450 0.41 1950 0.21 This result compares very favourably with those obtained on more highly alloyed steels.

So far as the stability of the alloys are concerned, the following formula provides an indication of the likelihood of sigma phase formation at a temperature of about 6500C, the formula representing the slope of a line dividing stable from unstable alloy compositions on an isothermal ternary plot: % sigma phase, given by X=4.5a+11 b-1.1 c-58d-61 where a is the percentage weight of the chromium content, b is the percentage weight of the silicon content, c is the percentage weight of the nickel content, and d is the percentage weight of the carbon content.

If the value of X for a given alloy composition is greater than zero then sigma phase formation at 6500C is to be expected, whereas if the value for X is zero, or negative, then the alloy will be stable and free from sigma phase at that temperature, the degree of stability increasing with increasing negative values.

The probable behaviour of some typical alloys at 6500C is as follows: Type C Si Ni Cr % Sigma expected No. 4 low carbon 0.1 1.5 25 18 3.2 No. 5 high carbon 0.3 1.5 25 1 8 Nil (X=-8.4) No. 6 low carbon 0.1 1.5 25 17 Nil(X=-1.3) No. 7 low carbon 0.1 1.0 25 18 Nil(X=-2.3) No 8 low carbon 0.05 1.5 25 18 6.1 (with Nb) No.9 higher carbon 0.15 1.5 25 18 0.3 (with Nb) No. 10 higher carbon 0.15 1.5 25 17 Nil (X=-4.2) (with Nb) In other words alloy No. 4, a low carbon wrought alloy with a relatively high chromium content is suspect, whereas the higher carbon, cast alloy No 5 is stable.Reduction of the chromium content of alloy No. 4 to 17%, i.e. alloy No. 6 or the reduction of the silicon content of alloy No. 4 to 1%, i.e. alloy No. 7 would, however, produce a stable alloy.

If niobium were added to alloy Nos. 4 and 5, it can be presumed that the carbon available to combine with the chromium would be approximately halved, giving alloy Nos. 8 and 9, of which alloy No. 8 is unstable, having an expectation of 6.1% sigma phase formation, and alloy No. 9 is more stable but is still suspect having an expectation of 0.3% sigma phase formation. Reduction of the chromium content of for example alloy No. 9 to 17% (i.e. alloy No. 10) would of course improve its stability, by giving a value of X=-4.2, and this is preferable to a reduction in the silicon content, which would have a similar effect, because silicon improves both the scaling and carburisation resistance of the alloy.

It has been ascertained that a lower chromium content, i.e. 1 6% to 17%, is not detrimental, particularly if cerium is added to improve the scaling resistance.

Claims (7)

Claims

1. A heat resisting alloy comprising the following by weight: Carbon 0.05% to 0.50% Nickel 18% to 28% Chromium 14% to 18% Silicon 0.1% to 2% Manganese 0.1% to 2% Niobium 0% to 6xthe carbon content Cerium 0%to0.1% the balance being iron, and wherein X=(4.5a+11 b)-(1.1 c+58 d+61) < 0 where a is the percentage weight of the chromium content, b is the percentage weight of the silicon content, c is the percentage weight of the nickel content, and d is the percentage weight of the carbon content.

2. A heat resisting alloy as claimed in claim 1, wherein X < -1.

3. A heat resisting alloy as claimed in claim 1 or claim 2, in which the percentage weight of the niobium content is in the range of 5 to 6 times the carbon content, and the percentage weight of the chromium content is not greater than 17%.

4. A heat resisting alloy as claimed in claim 3, which also contains up to 0.1% by weight of nitrogen.

5. A heat resisting alloy as claimed in any of claims 1 to 4, in which the weight of carbon content is in the range 0.05% to 0.1%, and the weight of the silicon content does not exceed 1%.

6. A heat resisting alloy as claimed in any of claims 1 to 5, containing 25% nickel, 1 7% chromium, up to 1% silicon, and up to 1% manganese.

7. A heat resisting alloy as claimed in claim 6, further containing 0.05% cerium.