US2435624A - Weather resistant steel and articles - Google Patents

Description

Feb. 10, 1948. R. FRANKs 2,435,624

WEATHER RESISTANT STEEL AND ARTICLES Filed June 3, 1944 37. CR; 006% C.

BHN- |3| 7C# A, 37. CR; 0.067. C. No CR; 0.097. C. SHN-|28 -BHN-lo'/ INVENTOR RUSSELL FRANKS ATTORNEY Patented Feb. 10, 1948 UNxTr-:D srA'rEs PATENT oFFlcE.

waarna usrs'r aan ANT CLES STEEL AND ammalarsi-ammi, mum-te Electro Metallurgical Company, a corporation plasma .nml s, 1m. serial N. ssaus atures and to articles fabricated from such steel f suitable for use under conditions wherein they are subjected to impact shock or multi-dimensional stress at subnormal temperatures. This latter class of articles includes. for instance,

' pressure vessels and machine elements used at high altitudes or in arctic regions or in refrigerating apparatus.

The principal object of the invention is to provide a mild steel and articles made therefrom characterized by resistance to progressive rusting,

`by goodproperties of strength, ductllity and toughness, by retention of toughness at subzero temperatures and by ease of fabrication by roll. l ing, bending, drawing, welding and other conventional methods. A

It has been widely recognized for many years that the rusting of iron and steel structures results in a tremendous loss of wealth not only through the destruction of the structural steel itself but also through the expense involved in maintenance of such structures. Painting. resorted to as a means of inhibiting rusting, is an unsatisfactory expedient at best. Any break in a paint film on a painted carbon steel structure exposed to the weather becomes the center of a rapidly widening area of rusting. The rusting proceeds under the paint illm and in time the latter falls away. Therefore, painting on a large structure is a never-ending task and expense.

Although there are available so-called stainless or rustless steels and irons, these materials owe the principal part of their very great re sistance to corrosion `to a substantial content of chromium, the chromium content being upwards of These highly alloyed steels are far too expensive to use for non-ornamental parts of large structures subject to corrosive conditions no more severe than ordinary weathering.

Steels containing about 4% to 0% chromium are widely used to resist mild eorrosives. The steels are less expensive than the stainless or rustless steels, and their resistance tocorrosion s cuan.. (ci. 'is-12e) tionally, these steels upon prolonged exposure to the weather are susceptible to the formation of surface pits, the eifects of which to a degree offset the advantages of the resistance of the steels to generalized surface corrosion. vA further disadvantage of the steels which renders their use for the fabrication of structures of the type under discussion commercially impractical is that their cost is very materially greater than that of ordinary structural steels.

Medium chromium steels containing about 3% chromium and more than 0.1% carbon have been investigated in the past. These steels have a metallographic structure profoundly different from that of plain carbon steels, and they are .air hardening, that ls harden on cooling in still air fromtemperatures above the upper critical point. Because of their air hardening characteristic they are not amenable to fabrication into by mild corrosives is good. However, the engineering properties of these steels are such that they are ill suited to use in the fabrication of structwes of the kinds under discussion. Addiarticles of the kind under discussion by standard methods of hot working such as hot rolling or by welding since they can not be permitted to cool rapidly from the temperatures employed in such operations.

In recent years `a number of low alloyed steels containing fractions of 1% of chromium and either copper or phosphorus have come into use for engineering and structural purposes. 'I'he principal advantage of these steels lies in their h ighstrength. 'I'hey are also appreciably more resistant to progressive rusting than the ordinary carbon structural steels, but their improved resistance to rusting alone would .not justify the use of these steels for the fabrication of structures exposed to the weaher.

The invention comprises steels, and articles made therefrom. containing between 2,75% and 3.5% chromium, 0.01% to 0.08% carbon, 0.1% to 0.6% silicon, 0.1% to 0.6% manganese, remalnder iron and the incidental impurities commonly present in steels of good quality, which steels are resistant to deterioration on prolonged exposure to the weather, are readily workable by ordinary methods of fabrication, and are strong at low temperatures.k The invention also includes a method of working such steels.

The accompanying drawing is based on photomicrographs taken at an original magnification l of 250 diameters. In preparing the drawing, the photomicrographs were somewhat enlarged, and

the lines in the drawing are somewhat exaggerated in thickness. In the drawing:

Figs, 1A and 1B are representations of the microstructure of a low carbon 3% chromium steel and oi' a plain low carbon steel, respectively, in one condition of heat treatment;

Figs. 2A and 2B are representations of theA microstructure of a 3% chromium steel containing more carbon than thesteel of Fig. 1A and of a low carbon, 4.5% chromium steel, respectively, in the samelconditlon of heat treatment;

an i

gigs. 3A and 3B are representations. of the microstructure of the steels illustrated in Figs.

LA and 1B but in a different condition of heat treatment.

I have discovered in the course of tests running continuously for periods as long as ten years that the steels of the invention are strongly resistant to progressive rusting by weathering and have notendency to form pits on prolonged exposure to the weather. Moreover, they behave metallurgically like mild plain carbon steel. They even when unpainted, is remarkable. For instance, over a year exposure of unpainted test samples to the weather in two successive industrial neighborhoods, a mild steel gauge sheet containing,0.l5% carbon, remainder iron -and incidental impurities, lost anaverage of 0.11 gram per square inch per year and. in the ten years suffered a decrease in thickness averaging 49% and in places amounting to 62%; while a mild steel 20 gauge sheet containing 0.07% carbon, 3.05% chromium, remainder iron and incidental impurities, lost an average of but 0.023 'gram per square inch per year, or about l5 of the average loss of the plain carbon steel, and suilered a decrease in thickness in ten years averaging over the sample only 8% and being at no point greater than 20%. The loss in weight of a similarly exposed test sample of steel containing 5.60% chromium and 0.07% carbon was at a rate of 0.021 grams per square inch per year, immaterially different from that of the 3% chromium steel, and this sample was deeply pitted.

These test results, as well as other similar results, clearly indicate that the low carbon 2.75% to 3.25% chromium steels of the invention are importantly more resistant to weathering than plain carbon steels and that raising the chromium content to about 6% imparts no material additional resistance to progressive rusting and may even be Adetrimental from the standpoint of resistance to weathering since steels containing about 4% t0 6% chromium are susceptible to pitting on prolonged exposure to the weather.

The steels of the invention may be readily worked by the ordinary methods employed in with medium chromium steels. Extensis@ tests have indicated that the percentage of chromium and the percentage of carbon present in the steels of the invention arecritical. Steels containing no more than about-3.25% chromium and 0.08% carbon may be rapidly cooled in air from temperatures above the upper critical point Without hardening. Ii however, the chromium content is raised only to about 4%, tests have shown that the steel will harden considerably on air cooling from above the upper criticalpoint even though the carbon content is as low as 0.08%. Similarly, steel containing 0.1% carbon or more will harden on air cooling from temperatures above the upper critical point even 1i. the chromium content is no more than 3.25%. Accordingly, to achieve one of the most important benets of the invention, both the upper limit of chromium and the upper limit of carbon must be observed strictly.

In Figs. 1 and 2 of the drawing there are reproduced photomicrographs of a low carbon 3% chromium steel containing 0.06% carbon (Fig. 1A), a plain `carbon steel (Fig. 1B) containing 0.09% carbon. a 3% chromium steel containing 0.16% carbon (Fig. 2A), and a low carbon 4.5% chromium steel containing 0.07%, carbon (Fig. 2B), allof which steels arein the condition resulting from heating 10 minutes at 1650 F. followed by cooling in still air. In this condition the Brinell hardness (BHN") of the low carbon 3% chromium steel is 131 and that of the plain low carbon steel is 107, while the Brinell hardness of the low carbon 4.5% chromium steel is 311 and that of the 3% chromium steel containing 0.16%'carbon is 293. 'I'hese values show that the hardness of thelow. carbon 3% chromium steel in this condition of heat treatment is only slightly greater than that of the plain low carbon steel but that an increase oi' ychromium or carbon toproportions outside the ranges of those elements recited above results in pronounced air hardening.

The eilect of chromium and carbon contents greater than 3.25% and 0.08% respectively on metallographic structure is evident from a comparison of the photomicrographs reproduced in Figs. 1 and 2 of the drawing. The structure of the low-carbon 3% chromium steel (Fig. 1A) is predominantly ferritlc (light areas) with patches of a pearlitic constituent (dark areas). Comparison with the plain low carbon steel (Fig. 1B) shows that the structures oi' these, two steels are very similar, the carbon steel also containing principaliy ferrite with some pearlite areas. However. in the case of the steel containing 3% chromium and 0.16% carbon (Fig. 2A) a very different structure is revealed. Here the structure is primarily pseudomartensitic containing a small amount of chromium ferrite (light areas) and is similar to that of the 4.5% chromium steel containing 0.07% carbon (Fig. 2B).

The ductility of the steels of the invention, as measured by elongation and reduction of area in tensile tests, is equal `to or better than the ductility of plain carbon steels, and the. yield strength and tensile strength of the steels o i the invention are considerably higher than the yield strength and tensile strength oi' the ordinary carbon steels. For example, in the hot rolled condition the yield strength at 0.2% onset of an ordinary carbon steel containing 0.09% carbon is in-the neighborhood oi 47,000 'pounds per square inch while the yield strength at 0.2%

bon and 8.03% chromium isabout 61,500 pounds per square inch. The tensile strength oi the plain carbon steel is about 58,000' pounds per square inch, while the tensile strength oi the 3% chromium steel is about 81,000 pounds per square inch. These values are typical of those measured in the direction of rolling.

For .the best combination oi.'` ductility and toughness. 'the steels of the inventionmay be heat treated by heating them to a temperature just below the lower critical point, for example, between about 1250 F. and 1400 F., and cooling in air from -this temperature. The eiiect of this .subcritical heat treatment on the microstruce aan einer or shot roues steel comming 0.045% car- .5 f arethussuchastcmakethememinentiysuitable i'or the fabrication o! articles which are reture of the. steels of thev invention is illustrated in Fig. 3A oi' the drawing. In Fig. -3 there are reproduced photomicrpgraphs of a 3% chromium steel containing 0.06% carbon (Fig. 3A) and of a plain carbon steel containing0.09% carbon (Fig. 3B) both of which steels are in the condition resulting from heating 2 hours at 1340 F. and cooling in air. `In both instances 1t is evident that the carbides present in the original pearlitic constituents .of the steels have been vspheroidized.r By this heat treatment both the impact strength and the ductility of the steels of the invention are materially improved with only a slight decrease in the yield strength.

Ordinary carbon steels, even of very low carbon content, lose their shock resistance and become notch brittle at even moderately subnormal temperatures, for instance below 40 F. This is a matter of some concern, and has led to the use o1' noni'errous metals or special steels, usually austenitic steels`for many articles which otherwise would be made of carbon steel. In strong contrast to plain carbon steel, the steels of the invention have good impact strength at subzero temperatures. yTheir retention -of toughness is particularly notable after the sub- .critical heat treatment just described. For instance, a steel containing 3.07% chromium and 0.075% carbon had an impact strength at room temperature, after having been heated 2 hours at 1340 F. and air cooled, of 86 foot pounds as measured Iby the Izod impact test. Tests at 32` F., 40 F., 90 F., -150 F., and 190 F. indicate that the impact strength of this steel in this condition 'of heat treatment is substantially unchanged at temperatures at least as low as -190 F. Thus, the steel had an impact strength at -l90 F. of 76 foot pounds, and at the other temperatures just recited had an impact strength o! 81 to 83 foot pounds. Even in the condition resulting from cooling in air from a temperature above the upper criticalpoint, the impact v strength ot the steel is in the neighborhood of about foot pounds at -190 F. 'Ihe impact strength of the same steel after having been heated above the upper critical point, air cooled, then reheated to just below thelower critical POlnt and air cooled was of the same order as when the steel was merely given the subcritical heat treatment.

The steels of the invention may be welded by fusion-deposition methods using as filler metal rods oi' substantially the same composition. Tests have indicated that sound, strong welds are readily produced and that the welds retain their desirable properties at temperatures at least as lowas 40 F. v

The properties oi' the 'steels of the invention quired to withstand exposure to the weather or of articles which are required tofwithstand lmpact or multi-dimensional stress'at subaero tem peratures. Because the -steels can be made in the open hearth furnace and because they may be fabricated into structures by standard methods with standardequipment, the increase incest oi' the steels when compared to ordinary carbon steel isreduced to a minimum, and their resistance to progressive rusting makes possible sufilcicnt saving in maintenance expense to more than otl'set its additional cost.

I claim: 1. Weather-resistant mild lsteel substantially free from air hardenability, consisting substantially of chromium ferrite and chromium pearl. ite. and composed of 2.75% to 3.25% chromium, 0.01% to 0.08% carbon. 0.1% to 0.6% silicon, 0.1% to 0.6% manganese, remainder iron and incidental-impurities. l

2. Steel articles required to withstand corrosive attack on prolonged exposure to weather composed of thesteel claimed in claim 1.

3. Steel as claimed in claim 1, in the condition produced by heating at a high temperature not far below the lower critical point and thereafter rapidly cooling.

4. Weather-resistant steel fencing composed of the steel claimed in claim 1.

5. Weather-resistant thin sheet metal rooi'- ing and siding composed of the steel claimed in claim 1.

6. Weather-resistant thin sheet steel con- .tainers' composed oi the steel claimed in claim 1.

7. Steel articles for use at subzero temperatures, which articles maintain an impact touuhness oi' at least 2li-foot pounds Izod at temperatures down to -200f F., composed oithe steel claimed in claim l.

8. Method of making weather-resistant mild steel sheet. strip, and wire which comprises 'hot working from above to just below the critical temperature a chromium ferrite-pearlite material composed of 2.75% to 3.25% chromium, 0.01% to 0.08% carbon, 0.1% to 0.6% silicon, 0.1% to 0.6% manganese,` remainder iron and -incidental impurities. such composition being characterized by freedom from air hardenabllity:

heating the worked material to an elevated temperature not far below the critical temperature:

and air cooling the so heated material.

I RUSSELL FRANKS. n nnrrzmcss crrnn The following references are of record in the ille of this patent:

UNITED STATES PATEIHIB Number Name Date 1,287,153 Weituenkorn Dec. 10, 1918 1,110,395 Lucertinl Sept.' l5. 1914 2,206,370 Scherer July 2, 1940 2,315,156 Larrabee Mar. 30.1948

FOREIGN PATENTS Number Country Date 797,842 France Hay 5, 1886 OTHER REFERENCES Bauens. steel `ma ns neat' armament. voi 1.

page 115.

' Patent No. 2,4a5;e24;

ce1-mem of comelio l February 10, 1948.

RUSSELL FRAN KS It is hereby rti'iiem'l that errors appear in the Carintied specication of the above nmxbergogatent requu'mg correctlon a's follows: olumn 5, line 52, for "--190 F3. rea. the said Letters Patent should be read with these corrections therein thetthe same may conform to the record of the case in the Patent Oce. v

Signed and sealed this 27th`dey of April, A. D. w48.

THOMAS F. mmf,

Autista/nt Uazmssmr of Patents.

F.; column 6, line 32, claim 5, for the word metalf read ated; and that

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