US2234047A - High creep strength low alloy steel - Google Patents

Description

Patented Mar. 4, 1941 UNITED STATES HIGH CREEP STRENGTH LOW ALLQY STEEL Harry L. Frevert, Philadelphia, Pa., assignor to The Midvale Company, Philadelphia, Pa., a cor poration of Delaware No Drawing.

Application November 14, 1939,

Serial No. 304,329

4 Claims.

There are certain articles of manufacture, now made of steel, alloyed or not with one or more of the common steel-alloying elements, and which in use are subjected to high temperatures, commonly within the range of 800-1100 E, which it is highly desirable should have the qualities of high creep strength and freedom from brittleness. Creep strength" may be roughly defined as that quality enabling the article to resist continuous deformation under constant load at high temperature. It is also essential that these qualities should be maintained for a long period of time. It is well recognized that these articles subjected to such high temperatures, and particularly those which normally must function at such high temperatures continuously for prolonged periods of time, do not exhibit these qualities to a satisfactory degree, unless made of expensive, highly alloyed compositions.

The development of an inexpensive forging steel that will resist creep at temperatures practically up to incandescence is the object of my I invention. The problem presents unusual difficulties in view of the long time required experimentally to determine whether or not given compositions are durable with respect to the qualities sought, since the behavior of different compositions, found on long time tests to retain the desired qualities for markedlydififerent periods of time, do not, on short time tests, exhibit any variance sufiicient to predicate therefrom the length of time during which such qualities may be expected to be maintained.

In the manufacture of many such articles the use of straight low carbon steel has been, found to be open to fewer objections than most of the commercial alloy steels. Commercial alloy steels generally contain one or more of the common alloying elements in considerable proportions, such proportions being often of value and importance in securing other qualities than those above mentioned.

In view of the lack of superiority of commercial alloy steels over low carbon steel, in my investigation of the problem it occurred to me to determine the eiiect of the addition of very small amounts of different alloying elements. The result of my experimental work, which has necessistance.

the reverse to that exhibited by their addition in lower percentage. With respect to certain other elements it was found that increase of the amount added beyond a limited range does not improve the steel in respect to this quality, or does not improve it to any material extent, and that such increase is open to objections of so serious a nature as not to warrant the-use of the elements beyond the low percentage which approaches the optimum for the attainment of the best results. The elements that have been found to give cheaply the results sought, when all are added and when each is added in a certain small proportion, are manganese, silicon, chromium and molybdenum. The addition of increased proportions of these elements has been found to be not only ineffective, but even to impair the qualities produced by the addition of these elements in the amounts specified.

Metallurgists are of course quite familiar with alloy steels containing the particular combination of elements above specified, but the art is devoid of any discovery that this particular combination of elements, each added in very small percentage within a particular narrow range, is essential to.

secure the particular qualities mentioned in the forepart of this specification without sacrifice of other necessary qualities.

Considering in order the alloys specified, manganese is added in an amount which is not uncommon, namely between .50 and 1.25%. addition somewhat beyond the maximum specified does not decrease creep strength, but a larger amount makes the metal more difiicult to forge in large sizes and more sensitive to hardening under normal cooling conditions and causes the highly objectionable condition known in the art as banded ferrite. It is desirable not to add more than about one per cent.

The addition of silicon is of some importance but preferably it should not be present in a proportion above .4%. With a percentage of silicon beyond .4%, the creep resistance decreases at an accelerating rate.

The optimum proportion of chromium is about .6%, but within the range of .5 to 1% its effect in increasing creep resistance is pronounced. Increasing the percentage of chromium beyondthe maximum specified has the opposite effect.

The addition of as little as .15% molybdenum has a substantial effect in increasing creep recreases, the creep resistance also increases, although at a progressively decreasing rate. If molybdenum be added beyond 375% there is no increase of creep resistance commensurate with the added cost and the steel becomes more difficult to work.

The total of the specified four added alloying elements should not therefore exceed 3.4% and As the amount of molybdenum ln-.

It should be noted that the composition approaches more nearly that of carbon steel than do most alloy steels, although its improvement over carbon steel is striking. j

As indicating the pronounced effect of adding chromium and molybdenum to carbon steel containing manganese and silicon within the ranges above specified, the followingo'omparison of thecreep values of different compositions were obtamed: 1

Load producing 1%ll00,000-hrs creeprate Mn Si Cr Mo 800 F. 000 F. 1000 1 1100F (o) .20 .65 .55 11,000 0,500 3,000 1,820 (b) .24 1.05 .25 12,000 4,500 (c) .24 1.25 .18 25,200 14,000 .1 (:0- .25 .05 .20 .02 .20 44.500 25,500 0,000 2 .20 .50 .10 .45 .15 20,000 12,000 0,400 3,000

The results obtained on experimental composition v(d) were confirmedby a three-ton heat in which the composition (f)'-C .26, Mn .88, Si .17, Cr .60 and Mo .20-was nearly identical with composition ((1)4 Composition (a) does not meet requirements. Nor does composition (b), even though the percentage of manganese is increased to about what has been found to be the probably most effective proportion in the complete composition. When, however, the percentage of manganese is increased to about the maximum, and a very small proportion of molybdenum is added, a pronounced improvement results, as is illustrated by composition (0). When to this composition is added a verysmall percentage of chromium as in composition (d) then the improvement is quite remarkable. However, as shown in composition (e), a reduction in manganese, chromium and molybdenum produces a composition inferior to composition (0) which contains no chromium. This indicates that nearly twice the percentage of molybdenum will not compensate for low manganese and low chromium.

- It was also found, in outs de tests. by prospective users of compositions-essentially the same as (d) and (I). that after load ng at 102 Fjfor 2200 hrs. the metal or composition has not undergone any deterioration. its elastic limit, ductility and Charpy impact values having as a matter of fact ncreased. Tests before and after loading at 1022 F. for 2200 hrs. follow:

Charo T. 8. E. L. m lbs.

Elong.

Before testing at 1022 F 000 m After testing at 1022 F was waco

coaoo steels otherwise useful for high temperature service it does not undergo embrittlement.

Tests to determine the load required to fracture this material over long periods of time produced the following results:

5 Load to produce Temperature, F. rup in 100,000hrs. P. S. I.

Fractures produced in rupturing bars incident to determining, by extrapolation, the above figures all showed ductile transcrystalline fractures as opposed to the brittle, intercrystalline fracture of steels which fail suddenly and without warning at such temperatures. These tests were made on material simply air quenched from 1550 F. and drawn at 1200 F., which treatment confers a fine microstructure without free ferrite.

Theonly element not specified-that I have discovered to have a beneficial efiect in attaining the object of theinvention is tungsten, which is much more expensive than molybdenum andto produce the same effect as molybdenum must be added in much larger proportion. In specifying, in the claims, after enumerating the elements above described, that the balance of the composition is substantially iron, it is not intended to exclude the addition of an inconsiderable amount of any other element that is inefi'ective to materially reduce the efiiciency of the elements specified.

It will be noted that the total percentages of alloying elements other than the carbon in compositions (d) and (f) are respectively 1.9? and 1.85%. The total percentage of these four ele- 'ments' should not exceed about 3.4% or be less than 1.25% and preferably should be between 1.5 and. 2.5%. In all cases the percentage of either manganese or chromium should not be much less than, and should usually exceed, that of the molybdenum; and the percentage of silicon should generally be less than that of any of the other elements except carbon, and always less than that of the manganese or the chromium.

For many purposes the composition has slight advantage over carbon steel of equivalent strength, but for many other uses, the creep resistance, the maintenance of that resistance with prolonged use, and freedom from cold embrittlement after prolonged high temperature exposure, that characterize my improved. composition, are of the utmost importance and value. Among articles of manufacture for such use may be mentioned ovens, autoclaves, and other reaction chambers and vessels, turbine spindles and'other turbine elements (wherein very slight creep? is a serious matter), heat exchangers, tubing and piping, bolts and nuts.

What I claim and desire to protect by Letters Patent is:

1. An alloy having a high and durable creep strength and freedom from cold embrittlement after prolonged high temperature exposure conmium'is in excess of the percentage oi molybdenum and in excess of the percentage of silicon, the balance of the composition being substantially all iron.

2. An alloy having a high and durable creep strength and freedom from cold embrlttlement after prolonged high temperature exposure containing carbon not over .5% and the addition elements manganese, silicon, chromium and molybdenum, the manganese being present in a proportion betweem .65 and 1.5%, the chromium in a proportion between .5 and .9%, the

' silicon in a proportion between .15 and .40%,

and the molybdenum in a proportion between .15 and and having the further characteristic that the percentage of manganese is not less than the percentage of chromium plus the percentage of either of the remaining two addition elements and that the percentage of chromium is in excess of the percentage of molybdenum and in excess of the percentage of silicon, the balance of the composition being substantially all iron.

3. An alloy having a high and durable creep strength and freedom from cold embrittlement after prolonged high temperature exposure containing carbon not over .5% and the addition elements manganese, silicon, chromium and molybdenum, the manganese being present in a proportion between .85 and 1.1%, the chromium in a proportion between .5 and .9%, the silicon in a proportion between .15 and 3% and the molybdenum in a proportion between .15 and 4% and being further characterized by the fact that the percentage of manganese is not substantially less than the percentage of chromium plus the percentage of the remaining two addition elements and that the percentage of chrois not substantially less than approximately the percentage of silicon and molybdenum combined, the balance of the composition being substantially all iron.

4. An alloy steel having a high and durable creep strength and freedom from cold embrittlement after prolonged high temperature exposure containing carbon less than 50% and the additional elements manganese, silicon, chromium and molybdenum in a total percentage between 1.5 and-3.4 per cent., the relative proportions of the addition elements being so adjusted that the percentage of manganese is not less than the percentage of chromium plus the percentage of molybdenum and that the percentage of chromium substantially exceeds the percentage of either molybdenum or silicon, the balance of the composition being substantially all iron.

HARRY L. FREVERT.