US3165401A - Alloy steel for cast parts resistant to high temperatures and corrosion - Google Patents

Description

United States Patent 3,165,401 ALLOY STEEL FQR CAST PARTS RESISTANT TO HIGH TEMPERATURES AND CORROSIGN Emil Reusch, Fort Wayne, Ind., assignor to International Harvester Company, Chicago, 111., a corporation of New Jersey No Drawing. Filed Mar. 20, 1957, Ser. No. 647,216

Claims. (Cl. 75-128) This invention relates to a new and improved stainless steel, and more particularly to a steel suitable for the casting of internal combustion engine parts, valves and the like, gas turbine parts, etc., which must possess qualities of high strength, high hardness, and high resistance to corrosion oxidation and scaling at elevated temperatures. In the effort of increasing the output of internal combustion engines the trend has constantly been to higher operating temperatures. To avoid certain undesirable results of such increased temperatures it has been necessary to add to the fuel, anti-detonants which, however, greatly increase the corrosive effects on parts which come into contact with the products of combustion. In the conventional internal combustion engine, one of these parts which is seriously affected is the engine exhaust valve. The materials heretofore used have proven to be unsatisfactory in meeting the aforementioned conditions prevalent in present day higher efliciency engines. Exhaust valves made from conventional materials, forged or extruded, usually stretched over their whole length, cupped in. the head portion and cracked at the stem close to the head of the valve, which failures in many instances occurred as a direct result of the high temperatures and the lack of the material to withstand the effect of the forces acting on the valve. The increase of the corrosive eifect showed up mainly in eating away the material in the neck portion and on the outer rim of the valve. It is a fact, therefore, that a good valve material that will meet present dayrequirements must possess the following characteristics: high hot hardness, high hot strength, good wear resistance, high creep resistance, low thermal expansion; high shock resistance, low ductility, high toughness, high oxidation and corrosion resistance, high thermal conduc tivity and good scaling resistance.

Valves heretofore made by the process of forging or ex truding (known as wrought valves) have been observed to more readily stretch, during operation, to the breaking point or generally become quickly brittle resulting in premature failures due to cracking or distorting of the rim of the valve head. In addition to the inadequacies of the material utilized it is also apparent that such failures are further abetted by the forging or extruding techniques which generally limits the chemicalcomposition and sets up undesirable grain patterns in the finished product.

It is a prime object of this invention therefore to provide an improved alloy steel having chemical properties ideally suited for casting, to provide manufactured parts having inherent resistance characteristics with respect to high temperatures and products of combustion resulting from the operation of an internal combustion engine.

A still further-object is to provide an improved material particularly suited to the manufacture and use of an improved cast valve.

Another object is to provide an improved stainless steel having a chemical combination which particularly meets, to a maximum degree, all of the desirable characteristics of a valve element as expressed above.

A morespecific object is to provide an improved alloy steel particularly suited to parts subjected to the conditions and products of combustion of an engine, the mate rial having among other things a silicon content which particularly suits the same to the manufacture of cast parts, such as exhaust valves, etc.

A further object is to provide a steel alloy material particularly suited to cast parts subjected to high temperatures and corrosion, the material having favorable work hardening characteristics which improve the inherently favorable characteristic of the cast article during use.

In the development of the improved cast alloy steel certain resultant factors have been taken intoconsideration. Chromium is of course particularly desirable in that it provides high corrosion resistance. Up to chromium may be particularly beneficial in this respect but over this figure brittleness generally results. Applicants steel therefore utilizes a chromium range of approximately 20 to 22% which assures high corrosive resistance without the possible undesirable characteristics which additional chromium might produce. Carbon of course will increase the hardness of an austenitic structure but it also may, when united with chromium increase the susceptibility to corrosion. This results when the carbon at higher temperatures becomes unstable and is rejected from solid solutions in the form of chromium-rich carbide. Thus adjacent areas of the metal become impoverished in chromium with resultant increase in corrosion. In the present material a range of .90% to 1.20% carbon has been found to be particularly effective.

The important characteristics of hot hardness and hot strength are not found in predominantly chromium-ironcarbon alloys and in addition therefore an austenizing element is necessary where the aforesaid properties are of pronounced importance. Nickel is a well known element for this purpose. Nickel may however have an adverse effect on corrosion resistance while operating in the presence of lead compounds of amounts required for improved engine operation. Furthermore nickel is of course also a very scarce product. By supplanting a substantial quantity of nickel ordinarily required for austenitic quality, with manganese, an austenitic steel is obtained and the adverse effect of nickel upon corrosion resistance in the combustion products of leaded fuels is largely dispelled. Accordingly in my alloy steel the nickel content is reduced to the smallest percentage 1.00% to 2%.

1 Manganese has been substituted for larger proportions of nickel. It contributes markedly to strength and hardness and tends to decrease the critical cooling rate of the material. Further with increasing carbon lower ductility results. Manganese being highly resistant to sulphur and lead bearing atmospheres is also very desirable. In my alloy I utilize a manganese content of 5.00% to 8.00%. Above this figure a possibility exists of delta ferrite formation and resultant brittleness.

Nitrogen contributes greatly to hot strength and toughness in my material as well as serving to retain the austenitic structure under the combined action of thermal and mechanical forces. This material in combination with manganese substitutes for my decrease in the nickel content. It further restricts grain growth while main taining its important characteristic of being an austenizing element; The chromium and manganese of my material increase the solubility of the nitrogen which exists in the range of 20% to 0.40%.

Wrought steel alloys normally contain less than 50% silicon. This is particularly true of a conventional valve material where the silicon content is held below .25% With a low silicon content of this type the advantages of a cast material cannot be obtained. High silicon content is important in promoting castability of my alloy. Too low a silicon content will also result in poor weldability and poor scaling resistance. Too high a silicon content may cause serious brittleness. In our cast valve material I have adapted a range of .60% to 1.20% silicon to obtain with the high carbon, high hot hardnessand a high degree of scale resistance without adversely aifecting the corrosion resistance. Furthermore the high silicon also provided better castability of the material.

and age hardening. The valve in the engine is repeatedly heated and cooled which to a great extent no doubt transforms the structure of the material in the same way that results from age hardening. In addition, however,

that these changes area result of a combination of work;

The steel then consists of the following: certain structural change take place in the valve which Percent also are believed to definitely result from the phenom- Carbon 0.90 to 1.20 enon of work hardening. Not only does the valve de- Silicon 0.60 to 1.20 'velop greater hardness but also the mechanical proper- Manganese 5.00 to 8.00 ties'are definitely improved and it is felt that this is due Ch i 15,00 to 25,00 1 to the steady impounding and the state of stress under Ni k l 00 to which the valve works in the engine. In other Words Ni 0 20 to 4 this material consistently improves during the operation Iron Balance of the engine whereas other valve materials known in the art deteriorate during operation. High hardness in- Some of the properties of this material in the as cast 15 creases the amount of elastic deformation that can be condition are as follows: tolerated. The combination of high hardness with low Tensile Properties Room 1,000 F. 1,200 F. 1,400 F., 1,500? F. 1,600 F.

Temp.

Ultimate Strength, p.s.l 106,650 76,150 66,850 52,400 44,300 33,335 Yield Strength (2% p.s.i. 86,500 56,100 50,000 44,900 37, 500 27,850 Elongation, Percentin4D. 4.29 5. 72 5.72 4.75 4.6 6.4 Reduction of Area, perceut 3. 39 5. 61 6. 12 5. 82 7. 21 6.73

Hardness after test At Room Temperature Hardness Room 1,000 F. 1,200 F. 1,400 F. 1,500 F. 1,600 F.

Temp.

Brinell (750 Kg. 5 mm. Ball), At dilierent temperatures; 289 212 173 159 182 89 Hardness after test At Room Temperature As indicated in my stated objects my improved matemodulus will result in superior wear resistance of the rial is particularly adapted for casting and thus grain 35 material. Indetermining the best suited elastic modulus structure advantages can be achieved which are not posand the higher possible hardness, my material composisible in forged or extruded (wrought) materials. A tion was chosen to have low atomic weight sum and high wrought valve for instance stretches and distorts in melting temperature. In this respect my' casting matethe rim portion which leads toburning or it becomes rial shows a great difference when compared with the brittle and breaks under the high temperature condition. 40 commercially available valve materials (wrought mate- It is believed that this undesirable condition, in addition rials). to conventional materials used, results from the grain Another important feature'of thismaterial is'its work structure obtained incident to the forging or extruding hardening ability. Hardness of the cast material is not Operation diminished due to elevated operating temperatures but Grains in cast material are very coarse by wrought in contradiction, it is increased-due to work-hardening. material standards. Coarse grain size, however, is bene- Thus a valve made from my material may go into an ficial since most failures are intergranular, and coarse engine at 30Rockwell C? and during operation in the grained materials, having less grain boundary, are less engine the Rockwell hardness increases from 30 to 41-45 prone to integranular failure. This is of particular RC. importance in my valve material since this material is Thus, it is readily apparent that the'objects of the inoperating at high temperature at which the grain vention have been fully achieved; Changes, and modifiboundaries generally become weaker and failure more cations in the composition may be made which'do not likely will result. Furthermore it has become apparent depart from the spirit of the invention, or from the scope that stress orientation in the crystal structure of a cast of the appended claims. 7 valve is considerably more favorable to the conditions What is claimed is: to which it is subjected than in a forged valve. The 1. A stainless steel having high resistance to high tempresent material therefore is particularly suited for castperatures andproducts of internal combustion, containing ing. While it is especially adapted for exhaust valves about 0.90% to 1.20% carbon, about 0.60% to 1.00% it must be realized that it has particular adaptability for silicon, 5% to 8% manganese, 20% to 22% chromium, any parts which must operate at high temperature and 1% to 2% nickel, about 0.20% to 0.40% nitrogen, and under conditions where hot corrosive gases are incident the remainder substantially iron. p to the operation. 2. A stainless steel valve having'high resistance to high Another desirable feature of my material is its hardentemperatures and products of internal combustion, conability. It is capable .of Work and age hardening to over taining about 0.90% to l.20% carbon, at least 0.60% to 40 Rockwell C. On quenching from 2150 F. it has a 1.00% silicon, 5% to 8% manganese, 20% to 25% chrohardness of about 30 Rockwell C and on reheating to mium, 1% to 2% nickel, about 0.20% to 0.40% nitrogen, 1400 F. and holding it there for 8 hours, its hardness and the remainder substantially iron. increases to 42 to 45 Rockwell C hardness. 3. A stainlesssteel having high resistance to high tem- The terms above, namely, work and age hardening peratures and products of internal combustion, containhave particular significance in the present valve steel. ing about 0.90% to 1.20% carbon,'atleast.0.60% to The cast valve herein described is placed into theengine 1.20% silicon, 5% to 8% manganese, 20% to 22% chroin the as-cast condition without hardening by precipitamium, 1% to 2% nickel, about 0.20% to 0.40% nitrogen, tion or by other means. During the use in the engine and the remainder substantially iron. significant changes taken place in valve and it is felt 4. A stainless steel material particularly adapted for casting internal combustionengine valves which during use are subjected to high temperatures and corrosive atmospheres containing about 0.90% to.1.20% carbon, 0.60% to 1.20% silicon, about 5% to 8% manganese, 20% to 25% chromium, 1% to 2% nickel, 0.20% to 0.40% nitrogen, and the remainder substantially iron.

5. A stainless steel material particularly adapted for casting internal combustion engine valves which during use are subjected to high temperatures and corrosive atmospheres containing about 0.90% to 1.20% carbon, 0.60% to 1.20% silicon, about 5% to 8% manganese, 15% to 22% chromium, 1% to 2% nickel, 0.20% to 0.40% nitrogen, and the remainder substantially iron.

6. A stainless steel material particularly adapted for casting parts which during use are subjected to high temperatures and corrosive atmospheres containing about 0.90% to 1.60% carbon, not less than 0.60% and not more than 1.20% silicon, about 5% to 8% manganese, 20% to 25 chromium, not less than 1% and not exceeding 2% nickel, 0.20% to 0.40% nitrogen, and the remainder substantially iron.

7. A stainless steel material for producing cast internal combustion engine valves containing not less than 0.90% and not more than 1.20% carbon, about 0.60% to 1.00% silicon, 5% to 8% manganese, 15% to 25% chromium, 1% to 2% nickel, 0.20% to 0.40% nitrogen, and the remainder substantially iron.

8. An austenitic stainless steel consisting essentially of about 0.90% to 1.50% carbon, 0.60% to 1.20% silicon, 5% to 8% manganese, 15% to 25% chromium, 1% t 2% nickel, 0.20% to 0.40% nitrogen and the remainder substantially all iron.

9. An austenitic alloy consisting essentially of about 0.9% to 1.0% carbon, 0.6% to 1.0% silicon, to 8% manganese, 15% to 25% chromium, 1% to 2% nickel, 0.2% to 0.4% nitrogen and the remainder substantially iron.

I 10. An austenitic iron-base valve having high strength and hardness and good resistance to corrosion in the presence of leaded fuel combustion products at valve operating temperatures and containing as'essential alloying elements carbon from about 0.9% to about 1.0%,

manganese from about 5.0% to about 8.0% silicon from about 0.6% to about 1.20%, chromium from about 15.0% to about 25.0%, nickel from about 1.0% to about 2.0%, nitrogen from about 0.2% to about 0.4%, the balance being iron with incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A STAINLESS STEEL HAVING HIGH RESISTANCE TO HIGH TEMPERATURES AND PRODUCTS OF INTERNAL COMBUSTION, CONTAINING ABOUT 0.90% TO 1.20% CARBON, ABOUT 0.60% TO 1.00% SILICON, 5% TO 8% MANGANESE, 20% TO 22% CHROMIUM, 1% TO 2% NICKEL, ABOUT 0.20% TO 0.40% NITROGEN, AND THE REMAINDER SUBSTANTIALLY IRON.