US1942779A - Alloy steel - Google Patents

Description

Patented Jan. 9, 1934 UNITED STATES PATENT OFFICE No Drawing. Application December 3, 1928,

Serial No. 323,572, and in Germany January 19,

Alloys of iron and aluminum in themselves are known.. Their use is however very limited, as although they have good properties, these are accompanied by a greater number of unfavorable ones, depending upon the amount of the content in aluminum.

4% aluminum steel has been worked up into dynamo and transformer plates in ,order to utilize that property of aluminum which has a similar effect to silicon on the magnetic losses of soft iron. The use of this has however been given up as the aluminum plates in other respects had not properties of the same value as the silicon sheets and were moreover more expensive.

German Patent No, 431,314, class 18b describes alloys which it is claimed possess the special advantages of a high electrical resistance and great resistance-to heat. They contain from,8 to 16% of aluminum, 6 to 18% of chromium or 6 to 8% of manganese. These alloys belong to the class of high heat resisting alloys, which in general show a high percentage content of chromium and nickel, with Or without other constituents, as for example, manganese, cobalt and in a few cases also aluminum between 8 and 60%.

Apart from theirhigh price these high alloy steels cannot be used for many purposes, for instance boiler tubes, on account of their Martensite structure in the rolling or forging state, and the difiiculty of working them. I

Separate technological tests, as for example the hard bending test for steam tubes (according to the resolutions of the German Steam Boiler 0 Commission on regulations with regard to material and work for stationary boilers, of June18, 1926), as also the rolling, etc. of the beforementioned high alloy materials, meet with almost insuperable difiiculties, apart from the difficult and almost impossible production of tubes.

High alloys of iron-aluminum, having content of more than 10% aluminum, and which are heat resisting up to more than 900 C., are also known, but these alloys are only used for castings, as they cannot be rolled or forged In several prior patent specifications ironaluminum alloys are described, all of which have high aluminum content (never under 5%) and high nickel and chromium contents. For example:

2 Claims. (01. 75-1 United States patent specification 1,633,805

Per cent Aluminum .10 to Chromium 5 Nickel United States patent specification, 1,633,826

I Per cent Aluminum 8 to 15 Nickel 25 Chromium"--. 7 to 15 British patent specification 220,006 Up to 40% chromium with high manganese, nickel and tungsten contents. The addition of cobalt, copper and aluminum in small quan: titles is only incidentally mentioned as alloy con stituents. British patent-specification 215,231 I Per cent Chromium -1 15 to 35 Aluminum 5 to 12 begins to scale in an oxidizing atmosphere as low as 600 C3 A soft low percentage alloy which would not showscaling at 700to 800? 0. would be an advance, as these temperatures are for many productions partlytreatment and partly working temperatures.

The present invention is based on the results of exhaustive investigations carefully carried out over a long period in this direction, and which have led to a material which satisfies for example the requirements of modern high capacity steam boilers. The intended increase of steam temperature to about 600 C. has hitherto been pre-.

vented for the want of suitable material, which not only had to be scale resisting but whichcould be rolled without difficulty and which in every.

other respect would satisfy the various treatment and working requirements of the above mentioned technical tes s. The material of the invention has prppertie which the high alloy, high temperature resisting steels, apart from their high cost, did not possess.

The essence of the invention lies in the recognition that 2.7% of aluminum is sufiicient as an alloy to make ordinary steel scale-resisting up to about 800 C., and that aluminum content as high as that made known by the before-mentioned proposed alloys, must be regarded for many purposes as disturbing or even as actually prejudicial. The economical progress shown in the price alone, will be easily recognized.

In contradistinction to the high alloy, high heat resisting steelsythis low alloy aluminumsteel may be subjected without difliculty and without special precautionary measures to the most diverse punching and rolling processes (such as the Ehrhardt, Stiefel, Mannesmann etc.) and may for example be rolled into tubes.

Thus the invention relates to a steel alloy which is used in the rolled, drawn, punched or forged state and which is scale-proof up to 800 C., said steel comprising a low alloyed soft steel containing about 2.7 to 4%, or preferably 3 to 4% of aluminum. The balance of the alloy consists principally of iron in addition to the usual nonmetallic constituents, which preferably are present in the relatively low amounts as hereinafter specified. Furthermore, it may contain, as will be seen more in detail from the following specification, such elements, which in respect to the solubility of the alpha-iron for the carbon, operate in a contrary manner to aluminum in a total amount up to about 2%. Each constituent of said type may be present in amounts lower than 1%. Silicon may be used in small amounts. Other alloying constituents are not intended to be present in the alloy in appreciable amounts which would influence the character ofthe alloy.

A further advance is the discovery that this steel, which is in itself coarse grained, may by suitable heat treatment be improved,-that is, made tough and ductile. By heating, with subsequent quenching, from temperatures above 700 C. up to about 1000 C. and re-heating to 500 up to 750 C. a tough steel may be obtained. The technical tests on boiler tubes are more easily met and the steel may be drawn out for tubes of smaller diameters. It should be here mentioned, that unlike the usual heating, the first heating in the region of the solid solution should preferably last longer, in order to ensure the complete diffusion of the carbon in the interior of the aluminum-ferrite. As above a certain limit of concentration, which lies below about 2% aluminum, the gamma (7) phase is entirely suppressed and according to experiments made by the Kaiser-Wilhelm-Institute for the investigation of iron, only one holding point'(Ac at 735 Ar at 732 C.) could be detected. It is not necessary in the improving process to quench the steel above 800 C.; as, with content above 2% aluminum, it is impossible to reach the field of the solid solution, but through the quenching abetter distribution of the pearlite is obtained and besides even with longer subjection to heat, scaling is prevented. A further advantage of the steel produced by the new process is, that with it, such processes of treatment as hitherto necessitated special intermediate treatment for the which do not need to be specially tough, the following alloy is preferably chosen:

These low alloy aluminum steels may be rolled into plates, bars-or shapes and made into tubes of large and small dimensions without diificulty, by the most diverse punching and rolling processes. They are scale-resisting up to 800 C., and are suitable, for instance, for gas generator retorts, for tubes in which a gaseous or fluid medium is to be heated, and for other purposes. With a tensile strength of about 40 to 50 kgs/mm in the rolled state, the elongation was from 10 to 18%. They are easily electrically welded and are worked without difllculty.

2. A second way of carrying out the process is specially suitable for articles which should be tougher. A steel article produced according to the previously described process is subjected in the rolled state to an annealing or improving process, in which it is for example heated at 750 simply heating at 600 to 700 C., which on account of the low cost, is preferable for objects not needing great ductility.

3. By means of another way of carrying out the process, a finer structure, and above all, more evenly distributed carbon contents, are at once obtained. For this, certain additions are re- .quired as alloy elements, which in respect of the solvent properties of the alpha (0:) iron for the carbon, act in a contrary manner to aluminum.

Exhaustive experiments have shown, that with quite small additions 'of chromium or manganese, or of both together, a finer grained structure may be obtained in the cast metal and consequently in the forged or annealed state also. Tungsten, molybdenum, vanadium, titanium etc. alone or jointly, act more strongly in this direction. These elements also act on the tensile properties in known manner. One or more elements should therefore be chosen, which, besides, producing a finer grain, also give the desired tensile strength. For instance the aluminum steel is alloyed with molybdenum or vanadium, or with both together, if a high range of heat resistance is required. Titanium is to be preferred for im-' parting shearing strength.

The forged state structure of this steel is considerably finer grained and more even, than without these additions. The alloy elements are given in each case.

4, In a further manner of carrying out the process, where it is desired to obtain great ductility, these steels are quenched down from about 800 C. and re-heated at 500 to 750 C., as mentioned earlier. A certain degree of toughness may be obtained by simply heating up these steels at 600 to 700 0., instead of quenching and tempering them.

The following tables show a few values, which clearly indicate the improvement in the toughness depending on the alloy additions and the improving treatment. The high elastic limit as compared with the tensile strength is worthyof note, as it often reaches 80 per cent.

a e a a P n i ti Hand 5 421E133 g g g g g ,1 Treatment, degrees C.

Efi iii gas a 8 Q E Q Alloued with 36. 4 46. 5 15, 0 Annealed at 940. 326% 35.4 44. 2 30. 0 Qiggi ched from 940, tempered a4 6 44. 2 28.4 Quenched from 800, tempered.

650. 2.7% aluminum 0 48. 2 21. 4 As delivered. 0.8% chrom1um 44. 7 27. 6 Quenched from 940, tempered 650. 2.8% aluminum. 0.8% chromium. 39.4 52.5163 As delivered. 0.1% molybdenum. 35.8 51.828.8 Quenched from 940, tempered 0.1% tungsten 650.

47.5 59.4142 As delivered. 35. 4 54. 8 l8. 3 Annealed 940. 44.1 56.1250 Quenched from 940, tempered 36% aluminum 0.7% chromium... 39.7 54.024 2 Quenched from 800", tempered 3% molybdenum 650.

38. 7 52.2 29 2 Qugggzhed from 940, tempered 7 48. 8 59. 9 20. 4 Annealed at 680.

Below are further explanations with regard to the tables referring to the new process and the new material:

The following remarks apply to other structures of such steels:

A fine grained micro-structure after quenching, similar to austenite, of a steel with 3% aluminum and 0.3% tungsten is extraordinarily brittle in this condition. The carbon is completely in solution; the fracture is coarse crystalline. After tempering this quenched steel the carbon in the separate austenite fields separates out as cementite, the lamella: of which are rather widely separated, so that a considerably higher carbon content, with more active etching, is shown. The elongation of the test bars (1=5d), which was 1.2% in the quenched state, has risen to 30.5%; and the contractidn from 6.2 to 70.8%.

The object of the inventionis fdrther to produce material according to the various forms of carrying out the new process, suitable for making articles capable of resisting scale formation up to 800 0., especially tubes, the material consisting of low alloyed soft steel containing about 2.7% to 4% aluminum. In particular the invention comprises the use of this new material, utilizing its special properties, in particular as material for the making of such objects as may be produced by known rolling or drawing processes, as for the production of drawn or rolled tubes. Finally a part of the invention is the means to produce in particular superheater tubes, steam boiler and stay tubes, as well as other steam boiler components, especially for high pressure boilers, from a low alloy soft steel containing about 2.7 to 4% aluminum.

What I claim and desire to secure by Letters Patent of the United States is:

l. A rolled, .drawn, punched or forged steel material, scale proof up to 800 C., which is made of a low alloyed soft steel containing about 2.8% of aluminum, about 0.8% of chromium, about 0.1% of molybdenum. and about 0.1% of tungsten, while the balance consists of iron besides theusual non-metallic constituents and is free from further alloying constituents.

2. A rolled, drawn, punched or forged steel material, scale proof up to 800 C., which is made of a low alloyed soft steel containing from about 2.7% to about 3.6% of aluminum, from about 0.7% to about 0.9% of chromium, from about 0.1% to about 0.5% of molybdenum, and from about 0.1% to'0.5% of tungsten, while the bal-