US2467701A - Heat-resistant low alloy steels - Google Patents

Description

Patented Apr. 19, 1949 HEAT-RESISTANT LOW ALLOY STEELS John J. Ripich,Cleveland, Ohio No Drawing. Application December 21, 1945, Serial No. 636,497

Claims. 1

This invention relates generally to ferrous base alloys and particularly to that class of ferrous base alloys known as low alloy steels.

More particularly this invention relates to low alloy steels which are adapted for efiicient use at high temperatures and which resist distortion, deformation, corrosion, scaling and other deteriorations at high temperatures and which may be designated as heat resistant low alloy steels.

Still more particularly this invention relates to that class of heat resistant low alloy steels which are particularly well adapted for use in the art of melting, superheating, heat treating, conveying and die casting metals, and specifically such metals as magnesium, aluminum, zinc, type metal, solder and the like, and alloys of such metals.

More specifically, this invention relates to heat resistant low alloy steels which are especially well adapted for use in fabricating containers such as crucibles, ladles, pots, molds and the like, employed for melting, treating, conveying and casting magnesium, aluminum, zinc, type metal, solder and the like, and alloys thereof.

In the art of melting and alloying metals such as magnesium and aluminum and their alloys, great difficulty has heretofore been experienced in providing a material suitable for use in crucibles and similar containers for melting and alloying such metals.

In melting metals such as magnesium, for instance, in crucibles, it is the practice to first melt the magnesium and then super heat the molten magnesium either in the same or other crucibles. In superheating this magnesium, the temperature to which the molten magnesium is raised is frequently in the neighborhood of 1800 F. and the temperature of the combustion zone in the furnace to which the outer surface of the crucible is exposed, must necessarily be substantially in excess of that temperature.

The difficulties in providing suitable material for use in crucibles and other containers of the type specified, arise on the one hand from the deteriorating action of the heat and heating media applied to the exterior of the crucibles and on the other hand from the severe corrosive action of the molten metal upon the interior of the crucibles, at the relatively high temperatures used in melting, superheating and heat treating magnesium. A further difliculty involved in the provision of a suitable material for such crucibles is the fact that the lighter metals referred to, when in the molten state, are readily contaminated by absorbing various elements or impurities from the container. Another essential factor which must be taken into consideration in the provision of a material suitable for crucibles for melting and alloying light metals is the danger of explosion and the serious fire hazard resulting from the failure of a crucible, particularly in the case of magnesium.

Among the different metals which have been used for crucibles for melting and alloying magnesium and aluminum are the following: cast iron, low carbon steel, fire box steel, high chromium high nickel steel, and high chrome steel.

Each of these previously used materials presents certain technical or economic disadvantages. Cast iron, for example, presents the advantages of low cost, relative ease of fabrication and good corrosion resistance at normal temperatures. However, this material is characterized by certain disadvantages which are of major if not critical importance in high temperature applications involving, for example, contact with molten metals or alloys. Cast irons have a poor resistance to thermal shock, undesirable porosity, low structural strength which is reflected in relatively great wall thickness and concomitantly low thermal conductivity.

Low carbon steels, while possessing the advantages of greater physical strength and ductility than cast iron, are similarly characterized by a low resistance to thermal shock and in the case of low carbon steel castings include the additional disadvantages resulting from porosity caused by blow holes, fissures and the like.

Fire box steels, while possessing some heat resistance, are not satisfacory in respect to the corrosive action of the gaseous atmospheres encountered in many heat treating and melting techniques with the result that the wall thickness of the container made of this material is rapidly reduced due to scaling action resulting from the furnace atmosphere as well as the scaling action on the interior of the container resulting from contact with molten metal. In contact with molten aluminum or magnesium, for example, this scaling action is very severe. This type of steel also has generally poor resistance to distortion and deformation in high temperature service.

The high alloy stainless steels have been suggested and employed for the fabrication of melting crucibles and similar units. In spite of their excellent corrosion resistance, high temperature and creep resistance, these alloys in both cast and fabricated plate forms have proved to be disappointing in actual use not only because of difliculties in fabrication but also because, in service, the molten aluminum, magnesium, and the like, tend to leach out the nickel component of the alloy of the container. This has the double disadvantage of changing the analysis of and deleteriously affecting the steel and also deleteriously afiecting the molten metal undergoing heating because of the nickel pick-up by the moiten metal.

The high chrome steel alloys have not proved to be satisfactory due largely to their poor resistance to thermal shock. It is evident from the foregoing that for use in contact with the molten metals referred to, ordinary iron and steel castings do not have the desired characteristics and that high chromium high nickel alloy steels are unsatisfactory in performance and not economically feasible and that low carbon steels, and even reputedly heat resisting fire box steels, do not possess the desired durability when used under the drastic conditions involved with the molten metals referred to.

Extensive experimentations in this field indicated that the most desirable steel for this purpose would be a steel capable of being produced substantially as economically as ordinary low carbon or mild open hearth steel and alloyed with small amounts of elements which would add the desired heat and distortion resisting properties to the steel and yet would not materially increase the difficulty or cost of production and fabrication.

It is accordingly a prime object of this invention to provide an improved low alloy steel particularly suitable for producin articles and equipment used in high temperature fields and especially suitable for use in crucibles, ladies, pots, molds, and other types of containers for melting, alloying, conveying and confining molten metals.

It is a further object of this invention to provide such a low alloy steel which is well suited for containers of the above type when used for melting, superheating, alloying, conveying and die casting such metals as magnesium, aluminum, zinc and the like and alloys of such metals.

A further object of this invention is to provide a steel which is resistant to corrosion, scaling and erosion when exposed to gas or oil fuel flames in metal melting or metal heating furnaces.

A further object of this invention is to provide a steel which has a high resistance to deformai tion, distortion and progressive change of shape under such loads as are imposed on crucibles confining metals such as magnesium, aluminum and the like, at the temperatures necessary for the meltin and alloying of these metals.

It is a further object of this invention to provide a low alloy steel which will not contaminate or deteriorate or adversely affect the properties or qualities otherwise inherent in the metals being melted or alloyed in crucibles formed of this low alloy steel.

It is a further object of this invention to provide a low alloy steel which has a maximum resistance to corrosion by molten magnesium and its alloys and which has a maximum resistance to a progressive eating away when confining molten metals of this class, thus avoiding the destruction of the containers from the inside.

It is a further object of this invention to provide respondlngly less expansion and contraction strains in the wall of the crucible, and making possible a reduction in the weights of crucibles without lowering the factor of safety.

It is a further object of this invention to provide a low alloy steel having a high resistance to thermal shock such as results from the sudden application of a high heat flame to the crucible or a sudden pouring of molten metal into the crucible, or a sudden cooling of the crucible.

It is a further object of this invention to provide a low alloy steel having the above characteristics which has a high ductility and readily lends itself to forming by the usual hot and cold working methods and tools, and which can readily be welded by the usual welding processes.

It is a further object of this invention to provide a steel having the qualities and characteristics set forth above which may be made at a cost reasonably commenstuate with the cost of ordinary low carbon steel.

It is a further object of this invention to provide a low alloy steel which is de-oxidized to such an extent as to avoid segregation and provide a homogeneous austenitic fine grain structure.

With the above and other objects in view, I have made a large number of experiments and have caused to be produced a large number of heats of various analyses within the general sphere of my invention by well qualified steel producing plants under careful supervision. These heats have generally been large in volume and have been rolled into plates of varying thicknesses suitable for manufacturing crucibles and the like. Samples from all of these heats were subjected to complete chemical analyses and careful records of these analyses were made for use as a basis for comparing performances of crucibles made of the difierent alloys. Samples of the plates were also subjected to physical tests and photomicrographic examination and were further subjected to various accelerated tests to determine their heat distortion and corrosion resisting characteristics and suitability for services lntended, similar tests under identical conditions being made simultaneously on other metals previously used for crucibles. The quantity of metal in these heats was in most cases sufficient to permit me to manufacture crucibles varying in capacity from fifty to one thousand pounds and crucibles in numbers upwards of fifty from each heat.

These crucibles were furnished to different foundries producing aluminum and magnesium castings in large quantities, and their performance, in each case, was carefully checked and recorded by competent metallurgists.

As a result of these extended experiments and refinements, I have discovered a low alloy steel which satisfies the difficultly attainable criteria set forth above; and for the purposes of more specifically defining my invention I state that the simplest form of the low alloy steel embodying this invention includes as major essential metallurgical components iron and the elements set forth below in the ranges indicated:

Percent Carbon .08 to .15 Manganese .90 to 1.25 Phosphorus .09 to .13 Aluminum .025 to .070

This steel is preferably produced by the open hearth process, or may be produced by the electric furnace or Bessemer processes, in a manner sim- 5 ilar to that used in the production of ordinary mild or low carbon steel.

Among the elements which are present in my improved low alloy steel, in addition to those set forth in the above table, are silicon, sulphur, copper, nickel, chromium and molybdenum. Some of these elements and impurities have desirable advantages when they are present in quantities within certain ranges and positive disadvantages when present in quantities beyond these ranges. The range in which the presence of these elements is permissible is as follows: Silicon in amounts ranging from 03% to 08% is desirable but is objectionable if present in amounts over .08%. Sulphur is not objectionable if present in amounts less than 05%. The presence of copper is undesirable but is not seriously objectionable if it is present in amounts less than .10%. Nickel in low amounts is desirable but is objectionable if present in amounts of more than .10%. Chromium is not objectionable if present in amounts less than .10% but additional amounts cause fabricating difficulties. Molybdenum in small amounts is advantageous but is objectionable in amounts in excess of 10%.

The complete chemical analysis of my improved low alloy steel as commerically produced with the essential elements and all of the usual residual and incidental elements includes:

Percent Carbon .08 to .15 Manganese .90 to 1.25 Phosphorus .09 to .13 Aluminum .025 to .070 Silicon .03 to .08 Sulphur .05 maximum Copper .10 maximum Nickel .10 maximum Chromium .10 maximum Molybdenum .10 maximum Iron, of course, constitutes the base of the alloy and is present in a predominant amount, usually constituting in excess of 96% and preferably in excess of 97% of the alloy.

In addition to the iron and other elements specifically set forth in the above table there may be and usually are present in this alloy certain other elements and compounds and also certain inclusions and impurities incident to usual steel making practice such as iron oxides, iron silicates, manganese sulphides, aluminum oxides, and the like.

In order to test the above specifications and establish the fact that they would consistently produce steel of the desired characteristics, several heats were made in open hearth furnaces and poured into ladles in which the metal was aluminum-treated to produce a controlled austenitic grain size, and from which the metal was cast into ingots and subsequently rolled into plates of varying thicknesses from 1% to 2%. Complete chemical analyses were made of the metal produced from each heat and all elements were found to be within the ranges indicated. Physical tests were made on specimens taken from the above heats and the tests indicated that this steel, at room temperature, has an ultimate tensile strength of approximately 75,000 pounds per square inch, and a yield strength of approximately 48,000 pounds per square inch, and a high ductility as indicated by an elongation of approximately 32% in 2 inches. The high yield strength of this'steel allows me to reduce the 6 crucible thickness approximately while still maintaining the same factor of safety obtained in crucibles made of other competitive materials having lower yield points.

Photomicrographic examinations of this im-' proved alloy indicated that the steel has inherently fine and homogeneous austenitic grain structure.

Comparative sag tests were made on steel of the above analysis and other steels frequently used in the manufacture of crucibles. These sag tests consisted of preparing specimens of identical cross-sections and lengths of steel-of the above analysis and other steels and supporting r them in horizontal positions in the manner of a cantilever and simultaneously subjecting the samples to temperatures of the order of those found in crucible heating furnaces. This exposure to high heat was continued for a substantial period of time and the extents of the deflection of the extreme ends of the samples from the original axis were recorded. These sag tests indicated that steel of the above analysis had a greater resistance to sag under high heat than any of the other steels heretofore used for crucibles excepting only the high chromium high nickel steels.

Comparative solubility tests were likewise made on steel of the above analysis and other metals. These tests consisted of immersing specimens of known weight and identical form in baths of molten aluminum under like conditions for identical periods of time. The loss of weight of each specimen tested, as a result of the solution or 1 absorption of this metal by the molten aluminum was recorded. These comparative solubility tests showed that the low alloy steel of the above analysis had a greater resistance to solubility in molten aluminum than any of the other metals 40 used for crucibles referred to at the beginning of these specifications.

Comparative corrosion and scaling tests were also made, these tests including simultaneously subjecting samples of identical sizes to high heat for protracted" periods of time and observing the nature of the corrosion and the loss in Weight due to the corrosive action on and into the body of the metal. These tests indicated that the corrosion and scaling effect on low alloy steel of the above analysis proved to be substantially less than on any of the other metals tested.

Careful laboratory tests disclosed that the linear co-eflicient of expansion of this improved low alloy steel is relatively low in the higher temperature ranges, and is superior in this respect to high alloy heat resisting steels.

The plates made from these several proving heats were fabricated by the usual forming and welding operations into several hundred crucibles and similar containers all of which were put into service in foundries where they were used in large scale production. These crucibles were carefully examined periodically during service by skilled metallurgists and comparative records of their performance kept. The examinations of the crucibles included inspection for distortion, erosion, fissures and scaling and other indications of deterioration, and the performance records included a recording of the number of heats, quantity ofmetal heated, temperature of the heats, and similar data on these containers and comparative data on containers made of other materials.

Crucibles made of this steel proved to be particularly Well suited for containers used for melting, superheating, alloying, conveying and die casting such metals as magnesium, aluminum, zinc and the like and their alloys. Crucibles made from this material proved to have little or no objectionable deformation, distortion or progressive change of shape when used for the melting and alloying of magnesium and aluminum. Crucibles made of this material proved further to be highly resistant to the formation of incipient cracks and fissures, scaling and erosion when exposed to flames in metal melting or metal heat treating furnaces. C'rucibles made of this metal had excellent resistance to thermal shock and no precautions were required to avoid failures resulting from sudden temperature changes. Crucibles made of this metal further had a superior resistance to corrosion by molten magnesium and did not contaminate or otherwise adversely affect the properties of the metal being melted or alloyed in these crucibles; in other words, there was no objectionable pick up by the molten metal of elements from the crucible wall. Crucibles made from the metal specified proved to have a useful life two or three times as long as the useful life of crucibles made from the best prior known metal and the hazard of sudden unexpected failure during the life of the crucible was completely eliminated.

This low alloy steel, having the composition herein set forth, readily lends itself to flame cutting, and to forming by the usual hot and cold working methods and tools, and to A. S. M. E. class 1 welding, subject to X-ray inspection, by the usual welding processes, when the steel is used for the fabrication of crucibles or similar articles. I have discovered that the limits of the ranges specified for the different elements set forth in my preferred analysis are important and that any substantial variations from the ranges set forth produce a steel which is inferior to steel conforming to the specifications given, and does not completely fulfill the objects hereinbefore set forth.

I shall briefly point out the properties which are contributed by the different elements specifically included in this improved steel and the disadvantages or defects resulting from the inclusion of amounts of the stated elements substantially beyond the limits given.

Carbon is included to add strengthto the steel. 1

If the carbon content is less than .08% the steel is relatively low in tensile strength and if the carbon content isv over 15% the ductility is reduced resulting in brittleness andthe formation of incipient cracks and fissures. The weldability of the steel is adversely affected by higher carbon content.

Manganese is added to improve the tensile strength and resistance to creep at elevated temperatures. If the amount of the manganese present is less than .90% the steel is deficient in physical strength and resistance to distortion under heat. If the manganese is present in an amount in excess of 1.25%, plates of the metal containing such excess of manganese are difficult to form and have atendency to crack under welding and localized heating.

Phosphorus is essential in increasing the yield strength, improving machinability and: adding resistance to corrosion. I have found, however, that the total percentage of carbon and phos-* phorus, with the other elements as specified, should not exceed 28%, otherwise this lowalloy steel will be brittle. Steel with phosphorus below .09% is. lacking in desirable yield strength.

When phosphorus is over 13%, Or when the total percentage of carbon and phosphorus exceeds 28%, the brittleness and thermal shock resistance is adversely affected. I have found that the total percentage of carbon and phosphorus can exceed but slightly 28% in material 1 thick and over which is not subjected to the severe forming employed with thinner material. Steel less than thick is difficult to fabricate when the total percentage of carbon and phosphorus is 28%. Therefore I have determined that, for the best formability, material under A thickness should analyse on the low side of the range of that specified for carbon and phosphorus.

Silicon is used in the steel for partial deoxidizing. In the melting process in the furnace most of the silicon is burned out. It may therefore be desirable to add a little more silicon in order to aid in the de-oxidizing of the steel. If the steel contains over .08% silicon, with other elements as specified, it becomes relatively more brittle and has a reduced elongation. Such an excess of silicon also interferes with the ready workability of the steel.

Sulphur which is unavoidably present in steel should not exceed .05% otherwise it segregates excessively in the finished steel plates giving planes of weaknesses and additionally making the steel more brittle than is desirable. An ex=- cess of sulphur over this amount increases objectionable scaling condition particularly in the presence of flames in oil fired furnaces. High sulphur also adversely affects welding character istics.

Copper is a residual or incidental element which accumulates from the steel scrap used as raw material. Copper is not needed as an essen tial functioning element in this improved steel and in the ideal steel, copper may be omitted entirely but this is not commercially feasible. The objection to copper is that, in the steel, as a. result of the subsequent alternate heating and cooling in use, it segregates about the grain boundaries whereby selective oxidation and melting forms pits and fissures causing premature failure of the crucible. Careful experiments have been made on the presence of this element copper in this steel and I find that a maximum of .10% is not particularly objectionable and that in such limited amounts, no objectionable segregation occurs.

Nickel is another residual element which may be and usually is present in steel made from scrap as raw material and this is not particularly ob-' jectionable if present in amounts less than .10%. Any nickel in excess of this is seriously objection able for use in crucibles used for melting or alloying aluminum or magnesium because these metals pick up, dissolve or absorb nickel from the wall of the crucible if the nickel is present in excess of the amount stated. This absorption of nickel by magnesium and aluminum results in a serious deterioration of the properties of these metals or their alloys and makes it impossible to meet government specifications.

Chromium is another residual element usually present in steel made from scrap and a high chromium content is objectionable because of the inability of the steel containing this chromium to resist thermal shock and also because it hardens the steel, causing fabricating and welding. difficulties. An amount of chromium not in excess of .10% is not objectionable.

Molybdenum is another element which may be present in steel made from scrap. This element as is known, increases the tensile strength and yield point of steel but, with the other elements specified, any substantial amount of molybdenum increases the hardness and difficulty in forming and for this reason I prefer to limit the amount of this element, when present, to less than .10%.

Aluminum is added to the molten steel in the ladle into which the steel is poured from the open hearth furnace. The aluminum is added to control the austenitic grain size and supplements the de-oxidizing action of the silicon. It is important to note that silicon must be present in the steel before the aluminum is added. If no silicon is present the full effect of the aluminum is lost as a part of the aluminum is used to deoxidize the steel instead of producing a controlled grain size. This aluminum thus added to the steel produces a very fine and homogeneous austenitic grain structure. Suffi-cient aluminum is added to leave the indicated amount of aluminum in the finished steel thus making certain that the steel has the desired controlled grain size. An amount of aluminum in excess of the maximum amount given is objectionable in the steel because of the increased solubility of such a steel in the molten aluminum, and because of the diificulties encountered in welding steel containing excessive amounts of alumina oxide inclusions.

It should be pointed out that the efiicaceous performance of the low alloy stee1 produced in accordance with this invention is very largely the result of the fine austenitic grain structure produced by the aluminum addition to the steel in combination with the elements as specified.

While the advantages of this improved low alloy steel have been particularly pointed out with reference to crucibles used for magnesium and aluminum, it is important to note that this low alloy steel has proved to have superior corrosion and distortion resisting properties under high heat in crucibles used for other metals and when used for other items, such as annealing boxes, salt bath containers, molds and the like, and industrial furnace parts exposed to high temperature oxidation and distortion.

It should also be noted that while the properties of this alloy have been principally described with reference to a rolled form of this steel, it is equally eflicacious in its resistance to distortion and corrosion when produced in cast form.

It will be understood that the presence of elements in this alloy in minute amounts beyond the limits of the ranges set forth may occur in some constituent elements with-out seriously deleteriously affecting the properties of the alloy as set forth especially in the relatively greater metal thicknesses of this alloy and the ranges set forth in this application are intended to serve the purpose of identifying the alloy and I desire not to be precisely limited to the exact ranges set forth.

What I claim is:

1. A ferrous base alloy characterized by high temperature corrosion and distortion resistance, said alloy consisting essentially of .08% up to .15% carbon, .90% up to 1.25% manganese, .09% up to .13% phosphorus, .025% up to .070% aluminum, balance iron and incidental impurities.

2. A ferrous base alloy characterized by high temperature corrosion and distortion resistance, said alloy consisting essentially of .08% up to 15% carbon, .90% up to 1.25% manganese, .09% up to .13% phosphorus, .025% up to .070% aluminum, .03% up to .08% silicon, balance iron and incidental impurities.

3. A ferrous base alloy characterized by high temperature corrosion and distortion resistance, said alloy consisting essentially of .08% up to .15% carbon, .90% up to 1.25% manganese, .09% up to .13% phosphorus, .025% up to .070% aluminum, .03% up to .08% silicon, sulphur up to .05%, balance iron and incidental impurities.

4. A ferrous base alloy characterized by high temperature corrosion and distortion resistance, said alloy consisting essentially of .08% up to .15% carbon, .90% up to 1.25% manganese, .09% up to 13% phosphorus, .025% up to .070% aluminum, and up to .10% each of one or more of the group consisting of copper, nickel, chromium, molybdenum, balance iron and incidental impurities.

5. A ferrous base alloy characterized by high temperature corrosion and distortion resistance, said alloy consisting essentially of .08% up to .15% carbon, .90% up to 1.25% manganese, .09% up to .13% phosphorus, .025% up to .0'70% a1uminum, .03% up to .025% silicon, and up to .10% each of one or more of the group consisting of molybdenum, nickel, chromium and copper, balance iron and incidental impurities.

JOHN J. RIPICH.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Country Date Great Britain May 17, 1932 OTHER REFERENCES Number