IE63312B1 - Ferritic steel alloy - Google Patents

Abstract

A heat-formable, ferritic steel alloy having 20 to 25% of chromium, 5 to 8% of aluminium, not more than 0.01% of phosphorus, not more than 0.01% of magnesium, not more than 0.5% of manganese and not more than 0.005% of sulphur, the remainder being iron, including unavoidable impurities, is proposed, according to the invention the said alloy furthermore containing 0.03 to 0.08% of yttrium, 0.004 to 0.008% of nitrogen, 0.02 to 0.040% of carbon and about equal amounts of 0.035 to 0.07% of titanium and 0.035 to 0.07% of zirconium, with the proviso that the sum of the contents of titanium and zirconium in percent is 1.75 to 3.5 times as large as the sum of the contents of carbon and nitrogen in percent. The alloy has substantially better properties in use than the base alloy.

Description

-1- 63 51 2 DESCRIPTION The invention relates to a heat-deformable ferritic steel alloyhaving the basic composition: to 25 % chromium to 8 % aluminium max. 0.01 % phosphorus max. 0.01 % magnesium max. 0.5 % manganese max. 0.005 ,% sulphur residue iron, including unavoidable impurities.

Such alloys are used for the production of electric heatingelements and highly heat-resistant catalyst supports. They form 15 firmly adhering oxide layers and therefore have very satisfactoryresistance to scaling. Of course, there has been no lack ofattempts to further improve the basic composition by the additionof further elements or by reducing the unavoidable impurities due to manufacture. 20 For example, GB A 2 070 642 suggests also adding up to 2%yttrium, hafnium, zirconium, cerium or lanthanum and also 0.1 to2% titanium, to give the cast structure a fine grain and improve * heat deformability. The additions of yttrium, hafnium, zirconium 25 and mixed metal (CE + La) are preferably up to 1% in each case(claims 7, 8, 10 and 11). The best results are achieved with0.34% Ti and 0.46% Nb (Example B). However, it has been foundthat the teaching of GB A 2 070 642 still has disadvantages. 2 If the fact is ignored that some of the suggested additives arevery expensive and have a very adverse effect on the economics ofthe steel alloy in proportions of up to 1%, it must be pointedout that although titanium in the prescribed quantity effects animprovement in mechanical properties, at the same time behaviourunder cyclically changing oxidation conditions deteriorates. Inthe VIW Test* the service life dropped from 5000 changes for thetitanium-free sample to 2800 changes for the comparison samplecontaining 0.47% titanium. This is due to the enrichment oftitanium oxide in the outer oxide layer, which causes the oxidelayers to peel off (cf. Corrosion Science, Vol. 24, No. 7, 1984,pp. 613-627). * In the VIW Test small test coils of wire 0.4 mm in thicknessare heated by the direct passage of current in air. Thecurrent supply is switched on and off for 2 minutes inconstant alternation. The maximum temperature reached ismeasured optically and kept constant with a constantswitching frequency by changing the voltage applied,throughout the duration of the test (K.E. Volk: Nickel andNickel Alloys, page 145, Springer Publishers, 1970).

Moreover, in the case of a sample containing 0.47% Ti, heavy,irregularly distributed titanium carbide precipitations wereobserved in the hot working stage; these cause considerablydiffering mechanical properties and make uniform cold workingdifficult or even impossible. 3 The addition of niobium to ferritic iron-chromium alloys not onlycauses a slight increase in 475°C embrittlement (cf. Boron,Calcium, Columbium and Zirconium in Iron and Steel, page 199, John Wiley & Sons, New York, 1966), but also produces a 5 deterioration in resistance under cyclically changing oxidationconditions. With increasing temperature various niobium oxides(Nb, NbC>2, Nb2C>5) occur, something which is connected with anincreasing volume, growth stresses and the peeling-off of oxidesresulting therefrom. Moreover, the high temperature version of 10 niobium oxide is not very stable, since its melting point is comparatively low - i.e., approximately 1500°C (cf. P. Kofstad:High Temperature Oxidation of Metals, page 215, John Wiley & Sons, New York, 1966). 15 According to EP B 0 091 526, the basic alloy contains, in addition to 0.002 to 0.06% rare earths and a maximum of 0.04% phosphorus and 0.03% sulphur, also zirconium and niobium forstabilization and improvement in creep rupture strength.Accordingly, up to 1.068 and 1.928% respectively of Zr and Nb 20 should be added, in dependence on the carbon content (max. 0.05%)and the nitrogen content (max. 0.05%), and also up to 0.364 and1.209% Nb respectively, if carbon and nitrogen should becompletely absent (cf. Claim 1). 25 Even steel alloys produced in accordance with that teaching still have disadvantages. If rare earths are added, the formation of relatively low-melting oxides must be expected, so that the steel alloy can be used only up to certain maximum temperatures.

According to the Applicants7 observations, even phosphorus 4 contents of up to 0.04% and sulphur contents of up to 0.03%cannot be tolerated. More particularly, however, considerabledisadvantages must be accepted with zirconium contents up toapproximately 1% and niobium contents up to approximately 2%. inthe case of niobium this can be gathered from the comments madeon GB A 2 070 642. With higher zirconium contents theimprovement in resistivity to oxidation quickly decreases, andeven turns to the opposite (H. Pfeiffer and H. Thomas: Scale-freeAlloys, page 260, Springer Publishers, 1963). Moreover, withzirconium contents greatly above the dissolving capacity of theferritic iron matrix, coarsely dispersed precipitations ofzirconium nitrides, zirconium carbides and zirconium carbonitrides occur which do not produce any lasting inhibitionof grain growth or any appreciable enhancement of strength.

It is an object of the invention to further improve the long-familiar basic alloy, while at the same time obviating thedisadvantages found in the prior art. More particularly, graingrowth is to be clearly limited and service life in the cyclicoxidation test appreciably improved. To solve this problemaccording to the invention the alloy contains, according to Claim 1: 20 to 25 % chromium 5 to 8 % aluminium max. 0.01 % phosphorus max. 0.01 % magnesium max. 0.5 % manganese max. 0.005 ;% sulphur 0.03 to 0.08 % yttrium 5 0.004 to 0.008% nitrogen 0.020 to 0.040% carbon and in substantially equal portions 0.035 to 0.07 % titanium 0.035 to 0.07 % zirconium wherein' the percentual total of the Ti and Zr contents is 1.75 to 3.5 times as great as the potential total of the C and Ncontents, residue iron, including unavoidable impurities.

Advantageous further developments of the inventive idea are setforth in the subclaims.

Further details and advantages of the inventive idea will bedescribed in greater detail in conjunction with Figs. 1 to 5.

Figs, la and lb show in a heavily simplified manner the test device used Fig Fig Fig shows the service life values achieved shows the values of high-temperature strength achieved shows the service life values achieved under tensile stressing, and 6 Figs. 5a and 5b show values of grain growth and bending valuesrespectively.

Fig. la shows in a highly simplified manner a device for testingthe service life of- a horizontally disposed helically woundheating conductor (1) which is clamped at the end in a holder (2)and is connected to a voltage source (3). In the present casethe heating conductor consisted of a coil 50 mm in length having12 windings and an internal diameter of 3 mm. The diameter ofthe wire was 0.4 mm. The heating conductor was alternatelyswitched on and off for two minutes at a time, the temperaturereached during the heating phase being measured without contactby means of a radiation pyrometer and controlled to a constantvalue by changing the voltage applied.

Such tests are continued in a normal air atmosphere until theheating conductor burns through, the number of cycles forming adirect measure of service life. The varying amount of scalingunavoidable in all materials results in the metal cross-rsection available for the conduction of the electric current becomingsmaller and smaller in the course of time; at the same time theelectric resistance is correspondingly increased and the giventest temperature with an unchanged switching rhythm can bemaintained only if the voltage is reduced. The test apparatusused had an automatic temperature controlling device, so that thetest temperature given for the heating phase would be maintainedthroughout the whole duration of the test, until the wire burntthrough, independently of the progressive scaling of the heating conductor.

Fig- lb shows, also in a highly simplified manner, a device fortesting the service life of a vertically suspended heatingconductor wire (4) of one meter in length which is suspended byits top end in a holder (5), is loaded by a variable weight (6)and is connected to a voltage source (7).

Using this apparatus, a heating conductor wire 0.4 mm in thickness was alternately switched on and off for two minutes.

In this case also, as with the apparatus illustrated in Fig. la,the temperature reached during the heating phase was measuredwithout contact and controlled to a constant value.

The results reproduced in Figs. 2 to 5 relate to a comparisonalloy (sample 1) and an alloy modified according to the invention(sample 2), having the following composition: Sample 1 Sample 2 Cr 20.10 20.45 A1 4.91 5.05 P 0.009 0.007 Mg < 0.01 < 0.01 Mn 0.22 0.15 S 0.003 0.002 Y - 0.04 N 0.010 0.007 C 0.045 0.037 Ti - 0.07 Zr 0.16 0.06 Fe residue residui 8 Fig. 2 shows the service life values obtained in an apparatus as shown in Fig. la, expressed by the number of cycles reached up to burning-through. The samples were alternately switched on and off for 2 minutes and the temperature reached during the heating v 5 phase measured without contact; changing the voltage applied ensured that a constant test temperature of 1200°C was maintainedin each cycle throughout the whole duration of the test. Sample1 withstood 5343 cycles, while sample 2 burnt through only at6213 cycles. This corresponds to an increase of over 15%. 10 In Fig. 3 the averaged heat resistivity values are plotted; itcan be seen that the alloy modified according to the inventionhas higher values of heat resistivity over the whole range of testing temperatures. 15 Fig. 4 shows the number of cycles achieved in a test according toFig. 1, in dependence on the voltage applied. Under all loadingsthe modified alloy shows a clearly increased service life. Theservice life obtained is 6 times the voltage of 2 N/mm2, nearly 5 20 times with 3 N/mm2 and still 3.5 times even at 4 N/mm2.

The ductility of a material after long-term use at elevatedtemperatures is an important constructional feature. Thedecrease in ductility of ferritic Fe-Cr-Al alloys is due to the 25 heavy grain growth at elevated temperatures. Fig. 5a shows the grain size values in μιη for sample 1 after 6.5 days ageing at 950to 1050°C (upper curve). The grain size values are also shownfor the modified alloy after an ageing time of 13 days at 950, 1050 and 1150°C (lower curve). As can be clearly seen, the 9 modified alloyclearly has a finer grain than the comparisonalloy even with twice as long a duration of ageing.

It is therefore not surprising that the bending values (number of5 bendings through 180 °C up to rupture) as shown in Fig. 5b are, due to its finer-grained structure, clearly,higher in the case ofthe alloy modified according to the invention than the values ofthe samples of the comparison alloy aged at 950, 1075 and 1175°Cfor 13 and 6.5 days respectively. The comparison shows that the 10 modified alloy has a substantially higher ductility than thecomparison alloy.

Wording on drawings: Figs. 2 and 4: Zyklen = cycles; Fig. 5b:Biegezahl = number of bendings.

Claims (6)

1. 0

1. A heat-deformable, >0 to 25 % 5 to 8 % max. 0.01 % max. 0.01 % max. 0.5 % max. 0.005% 0.03 to 0.08 % 0.004 to 0.008% 0.020 to 0.040% CLAIMS : ferritic steel alloy comprising: chromium aluminium phosphorusmagnesium manganese sulphuryttriumnitrogen carbon and in substantially equal portions 0.035 to 0.07 % titanium 15 0.035 to 0.07 % zirconium wherein the percentual total of the Ti and Zr contents is 1.75 to 3.5 times as great as the potential total of the C and Ncontents, residue iron, including unavoidable impurities. 20

2. An alloy according to claim 1, characterized in that thequantitative ratio of the additions of titanium and zirconiumlies in the range of 0.6 to 1.4. 11

3. An alloy according to claims 1 or 2, characterized in that titanium and zirconium are completely or partially replaced by hafnium and/or tantalum or vanadium.

4. 54. An alloy according to claim 3, characterized in that at leasttwo of the elements Ti, Zr, Hf, Ta and V are added in each case.

5. An alloy according to claim 4, characterized in that at leastthree of the aforementioned elements are used in each case. 10

6. A heat-deformable, ferritic steel alloy according to claim 1,substantially as hereinbefore described with particular referenceto the accompanying drawings. F. R. KELLY & CO. , AGENTS FOR THE APPLICANTS