US3549426A - Method of forming an engine valve of a ferrous metal containing chromium and nickel by heating treating and deforming - Google Patents

Description

United States Patent METHOD OF FORMING AN ENGINE VALVE OF A FERROUS METAL CONTAINING CHROMIUM AND NICKEL BY HEATING TREATING AND DEFORMING Richard A. Kloske and William F. Barclay, Parma Heights, Ohio, assignors to Republic Steel Corporation, Cleveland, Ohio, a corporation of New Jersey No Drawing. Continuation-impart of application Ser. No. 607,406, Jan. 5, 1967. This application Nov. 29, 1967, Ser. No. 692,618

Int. Cl. CZld 7/02 US. Cl. 14812 3 Claims ABSTRACT OF THE DISCLOSURE Cold formable valve steel of composition 1923% chromium, 40-65% nickel, 6.28.5% manganese, 0- 1.5% silicon, 0.15-0.3% carbon, 0.15.0.3% nitrogen, 0-0.2% columbium and/or tantalum, 0-0.l% phosphorus, 0-0.1% sulphur, balance substantially iron, articles thereof and production. The above described alloy is utilized in making an engine poppet valve. A part made of the alloy is solution treated and quenched. Subsequently the part may be reheated for working. It is then extruded and upset to form a valve shape.

This application is a continuation-in-part of our copending application Ser. No. 607,406, filed Jan. 5, 1967.

This invention pertains to steels especially adapted for use in exhaust valves for internal combustion engines, and provides a steel of novel composition and improved properties therefor, which is further characterized over steels which are presently commercially acceptable for such applications, in being cold-formable, as by upsetting and extruding, into valve configuration. The invention also pertains to articles made of said steel and the production thereof.

The steel of the present invention is particularly adapted for use in exhaust valves for automobile and other internal combustion engines, the required combination of room and elevated temperature minimum properties for which are particularly severe. Typical minimum physical property requirements for a cold-formable steel for such applications are as follows: The elevated temperature rupture strength as measured by stress for 1% stretch at 1350 F. in 100 hours, should be 6000 psi. minimum, and as measured by stress for rupture at 1350 F. in 100 hours should be 10,000 psi minimum. As regards hardness, that at 1400 F. should be at least 90 Brinell, and that at room temperature should be at least 27 Rockwell C. The oxidation resistance as measured by weight loss in grams per sq. dm. per hour (g./dm. /h.) should not exceed 60, as determined by heating a specimen in molten lead oxide, Pb O at 1675" F. The room temperature impact strength as determined by the Charpy V-notch test should be at least 5 foot-pounds. And the cold-formability should be comparable to that of type 305 stainless steel (18Cr-12Ni0.12C).

The steel of the present invention not only has adequate cold-formability, but exceeds these minimum physical property requirements in every respect as shown by the test results hereinafter presented.

The steel of the invention is essentially a substantially austenitic, medium carbon, high nitrogen, chromium- Patented Dec. 22, 1970 nickel-manganese steel of the following broad and preferred composition ranges.

AMO UNT-WEIGHT PERCENT Balance, Substantially Fe.

1 Maximum.

A preferred specific composition is of substantially the analysis: 21% Cr5% Ni-7% Mn0.2% C0.2% N- (0-0.1% Cb-Fe. The steel of the invention preferably contains columbium in amount of about 0.05 to 0.2%, although tantalum may be substituted for part or all of the columbium.

The steel of the invention is of a balanced composition which is extremely critical with respect to the limits for each of the essential elements above specified. That is to say, our investigations have shown that the coldformability and other property requirements above stated, are obtained by so balancing the composition that the steel is completely or substantially austenitic at room temperature. Our investigations have further established that the critical structure-property relationships of the steel can be obtained only by carefully balancing the Ni, Mn, C and N values of the austenite.

The chromium content is set within critical limits of 19-23%, to assure adequate scale resistance and secondary hardening at service operating temperatures up to about 1400 F. When aged at about 13001400 F. the steel undergoes secondary or age-hardening by precititation of carbides and nitrides, and also phosphides if the steel contains an appreciable amount of phosphorus, i.e., in excess of about 0.04%. The sulphur content of the steel should not exceed about 0.1% and preferably not exceed 0.04%.

Silicon, which is usually present as a residual element in these steels, may be employed in amounts up to about 1.5%, the preferred upper limit being about 1%.

While boron is not an essential constituent of these steels, it may be tolerated in residual amounts up to about 0.008%. p

The nickel content should be maintained as high as is commercially feasible to insure cold-formability. Our investigations indicate that the lower limit for nickel content on the basis of cold-formability requirements occurs at about 4%. Also that such increase in coldformability as is obtained with nickel contents in excess of 66.5%, is insufiicient to warrant the increase in alloy costs.

Neither high nickel nor nickel plus manganese is employed or required in applicants steel for imparting an austenitic structure, this being achieved more efficiently and cheaply by the relatively high contents of the interstitials, carbon and nitrogen. Thus applicants steel differs fundamentally in this respect from low carbon austenitic steels of, for example, 0.05% max. carbon, which are strengthened exclusively by nitrogen additions, and which require a minimum of about 14% nickel plus manganese for imparting a fully austenitic structure, and also a minimum of about 8% manganese for preventing ingot porosity. In applicants steel manganese is employed primarily for maintaining a high nitrogen content in the steel as an austenitizing agent. For applicants steel to contain the above specified amount of about O.l50.3% nitrogen, manganese is required in minimum of about 5% up to not more and 7.5% depending on the particular analysis.

As above stated, austenits stability is achieved in applicants steel primarily by the interstitial additions, carbon and nitrogen. However, since progressively increasing additions of these elements rapidly strengthen the austenite, thereby correspondingly reducing the coldformability of the steel, it should preferably contain just sufiicient carbon plus nitrogen to maintain a fully austenitic structure.

In order to demonstrate the properties of steels according to the invention with respect to the properties above discussed, compositions according to the following Table I were melted and thereafter tested as discussed below.

TABLE I.NOMINAL AND ANALYZED COMPOSITIONS OF THE EXPERIMENTAL COLD FORMING VALVE STEELS [21-5-7/Cr-Ni-Mn steels: varying C, N, P, Ob content] Alloy content (weight percent) I-l'cat Compo- No. sition Cr Ni Mn Si O N 1 Cb B40 {NominaL 21. 5.0 7. 0 0. 0.20 AetuaL 21. 35 5. 35 6. 55 0. 49 0. 194 B51 {Nommalu 2. 5. 0 7. 0 0. 5 0. ActuaL 21. 80 5. l0 6. 50 0. 66 0. 197 B 41 {NominaL 21. 0 5. 0 7. 0 0. 5 0. 4O Actual. 21. 6 5. 10 6. 9O 0. 51 0. 37 B52 {Nominaln 21. 0 5. 0 7. 0 0. 5 0. A0tual 21. 5. O0 6. 55 0. 59 0. 289 B53 {NomiuaL 21. 0 5. 0 7. 0 0. 5 0.20 Actual. 21. 8O 5. 15 6. 80 0. 60 0. 22 B54 {NominaL 21.0 5. 0 7. 0 O. 5 0. 20 Actual 22. 30 5. 15 6. 70 0. 49 0. 20 B42 {N0rninal 21. 0 5. 0 7. 0 0. 5 0. 15 Actual 21.7 5.15 6.90 0.56 0.136 0.116

Specimens of each of the above steels were tested for hardness, creep strain and impact strength in various heat-treated and otherwise processed conditions, with results as s hown in the following Table II.

4 additions of phosphorus and columbium, the test data clearly demonstrates the critical effects of the interstitial contents.

Room temperature tensile properties of the aboveexemplified steels according to the invention in the condition as solution treated at about 2100 F. and water quenched are shown in the following Table III.

TABLE III.IENSILE PROPERTIES OF NOMINAL 21 Cr- 5N1 7Ml1 STEEL WITH VARYING C, N, P, Ob

Heat Alloy content 0.2% Y.S. U.T.S. El, RA, N 0. (wt. percent) K s.i. K s.i. percent percent Machined tensile specimens for the above tests Were ground to 0.505-inch in diameter and a 2-inch gauge length was employed for measuring elongation.

From the Table III data it will be seen that all of the steels exemplified have in the as-annealed, i.e., solutiontreated and quenched condition, relatively low yield strengths and high ductilities, the latter as measured both by elongation and area reduction, conductive to good coldforming properties. From this data it will further be seen that steels according to the invention, as solution treated and quenched, have 0.2% otfset yield strengths of not more than about 75 k.s.i., together with tensile elongations in excess of about 35% and area reductions of more than about It is evident from this data that these steels are characterized by a high degree of plastic deformability at room temperature. As shown by Table II, all were reduced by cold rolling without difficulty. However, for cold upsetting and extrusion into internal combustion engine valves, the steels having area reductions of at least about 60% with low yield strengths were found most suitable as closely approximating to cold-formability of type 305 stainless steel, the annealed properties of which are TABLE II.MECHANICAL PROPERTIES OF THE EXPERIMENTAL COLD FORMING VALVE STEELS [21-5-7/Cr-Ni-Mn steels; varying C, N, P, Cb content] Creep Hardness Rockwell C strain 2 1,350 F., 1,400 F. Cold Aged 10,000 p.s.i., Brinell 70 F, rolled (at 1,350 F.) percent Hardimpact Heat Nominal alloy Solution Ground strain/test ness strength No. content, wt. percent treated 1 surface reduction 100 hr. 1,000 hr. duration HN 4 ft. lbs.

B40 0. 2C, 0. 1P 12. 3 15. 4 41. 8 39. 8 38. 3 17. 0/75 hr 95. 6 4 B51 0. 2C, 0. 2N, 0. 1 12. 1 15. 6 44. 6 40. 0 38. 4 0 57/100 hr 124. 5 4 B41 0. 4C, 23. 5 26. 5 48. 2 34. 9 33. 5 24 2/57 hr 108. 9 17 B52 0. 3C, 0 IN 17.1 19.4 45. 4 34. 8 34. 6 24 3/93 hr 97. 2 9 B53 0. 2C, 0. 2N 12. 4 13. 4 44. 9 37. 9 35. 7 0 29/100 hr 128. 3 8 B54 0. 2C, 0. 2N, 0.07 01) 16.0 20. 8 45. 1 37. 5 35. 7 0 15/100111 s- 135. 4 8 B42 0.15C, 0.1N 12. 9 14. 0 39. 9 37. 7 37. 4 19.2/26 hr 97.2 4

1 Solution treatment: 1 hr. at 2,050 E, Water quenched.

2 Creep sample thermo-mechanical history: solutlon treated as in 1 above, ground to 0.750-inhistory: same as 1 above, aged 98 hrs. at 1,350 F.

2 passes, stress relief annealed 2 hrs. at 1,350 F.

3 Hardness, impact sample thenno-mechanieal 4 BHN, 1,000 kg. load, 10 mm. dia. chromium carbide indenter.

From the above data it will be seen that all of the above steels meet most of the above-stated minimum requirements for the ideal steel for internal combustion engine exhaust valves. Steels B53 and B54 meet all of the requirements as regards minimum room and elevated temperature hardness, creep strain and impact strength. It will be observed that these two steels had aged room temperature hardnesses in the range of 35-38 Rc(Rockwell C), well in excess of the specified minimum of 27 Rc. It will further be noted that these steels had by far the lowest creep strain values of 0.29 and 0.15%, respectively, far below the specified minimum of 1%. In addition, steels B53 and B54 had by far the highest 1400 F. hardness values of 128 and 135 BHN, well above the specified minimum of 90 BHN.

Since all of these steels were melted to a nominal composition of 21% Cl5%, Ni-7% Mn with varying amounts of carbon and nitrogen plus in some instances optional dia., cold-rolled to 0.5-iu. thickness in 37,000 p.s.i. yield and 85,000 p.s.i. tensile strength, with 55% elongation in 2 inches and 65% area reduction. As shown by Table III the steels most closely approximating these values as regards low yield strength and high area reduction values are steels B42, 1351, B53 and B54.

Although the steels of the invention are cold-formable into valves at room temperature, the required forming pressures are greatly reduced by preheating to about 450 1600 F., or more generally below the temperature at which recrystallization would otherwise occur during forming, the preferred temperature range being below about 1280 F. This might properly be termed warm forming in contrast to the conventional hot forming of valve steels at a forging heat of about 19002200 F.

Steels according to the invention are preferably produced by melting the ingredients in the electric arc furnace, teeming in a molten state into ladles, following by casting into ingot molds. The stripped ingots are reheated to about 1900-2200 F., and for va ve applications, hot rolled into bars and air-cooled to room temperature. For cold or warm extrusion, the bars are solution-treated at 2050-2150 F., preferably 2100" F., usually for about one hour or until all carbides are in solution, and thereupon cooled to room temperature with suflicient rapidity, as by water quenching, to retain carbides in solution, vin which state the hardness is about -20 Rc. The bar stock is then cut into lengths for valve components and cold or warm upset into valve shapes, the resultant hardness being about 40-50 Re. The valves are then stress-relieved at about 1300-1400 F., preferably at 1350 F.,for about two hours and air cooled to room temperature, the resultant hardness being at least 35 Rc. The valves are placed in service in internal combustion engines having a service temperature for exhaust valves of about 1350 F. While in service the valves undergo secondary or age hardening such as to maintain the room temperature hardness above 30 Rc.

With the exception of the ability to be cold-formed into a finished piece, the most severe mechanical property requirement a cold-forming valve steel must satisfy, is the ability to resist further deformation by creep at the internal service temperature. The major alloy design problem is economically to utilize the residual strengthening resulting from the forming operation together with precipitation hardening at the service temperature to transform a steel having good cold-forming characteristics at lower temperatures into one which will resist deformation at an intermediate temperature. To achieve such a transformation in properties, a favorable structural revision must occur at the service temperature. As the following discussion will show, the carbon and nitrogen contents of the applicants steel critically affects the microstructural changes accompanying aging and, hence, the mechanical properties of the aged steel.

At a carbon content in excess of 0.3%, much of the carbide phase of the steel remains undissolved after the solution treatment anneal. On aging at 1350 F. after cold reduction, the dissolved carbon precipitates at slip bands and as large globular particles randomly dispersed throughout the austenite. These modes of prepicitation impart resistance neither to softening nor to deformation by creep at the service temperature. Thus, ahigh concentration of undissolved carbides leads to an ineffective precipitate structure.

A total carbon plus nitrogen content of less than about 0.3% is insufficient to maintain a completelyiaustenitic structure in these steels. Consequently, some delta ferrite is present in their microstructure after solution Itreatment. During aging after cold-forming, carbides precipitate prefenentially within these ferrite bands leaving e austenite matrix relatively depleted of precipitate particles. Although this banding of the precipitate phase results in a high hardness which is stable at 1350 F., it presents no obstacle to deformation by creep in the precipitate-poor austenite.

0n the other hand, a solution treated steel of the invention containing about 0.2% each of carbon and nitrogen, is found to completely dissolve the carbide phase without introducing large amounts of high temperature ferrite.

When tested for weight loss due to oxidation, in molten lead oxide at 1675 F., steels B40, B41, B54 and B42 were found to meet the required condition above stated of a weight loss of less than 60 g./dm. /hr. As shown by this and the other conditions above stated, steel B54 meets all of the requirements for valve steel application.

A good commercial melting range for steel according to the invention is about: 0.180.28% carbon, 0.15- 0.30% nitrogen, 0-1.5% silicon, 20-22% chromium, 45-65% nickel, 6.58.5% manganese and ODS-0.20% columbium and/or tantalum, columbium being preferred.

What is claimed is:

1. The method of producing an internal combustion engine poppet valve from a steel consisting essentially of about: 19 to 23% chromium, 4 to 6.5% nickel, 6.2 to 8.5% manganese, 0.15 to 0.3% each of carbon and nitrogen, up to 1.5% silicon, up to 0.2% of an element of the group columbium and tantalum and combinations thereof, up to 0.1% each of sulphur and phosphorus, up to 0.008 boron, balance substantially iron, which comprises: solution treating said steel at temperature sufiiciently high to place all carbides in solution and cooling thence to ambient temperature at a rate sufiiciently rapid to retain said carbides in solution, thence reheating said steel to temperature below its recrystallization temperature and extruding and upsetting the metal into the shape of said valve.

2. The method of producing an internal combustion engine poppet valve from a steel consisting essentially of about: 19 to 23% chromium, 4 to 6.5% nickel, 6.2 to 8.5% manganese, 0.15Lto 0.3% each of carbon and nitrogen, up to 1.5% silicon, up to 0.2% of an element of the group columbium and tantalum and combinations thereof, up to 0.1% each of sulphur and phosphorus, up to 0.008 boron, balance substantially iron, which comprises: hot rolling said steel into bar stock, solution treating and quenching said bar stock to retain all carbides in solution, cutting said bar stock into lengths suitable for forming into said valves, reheating said lengths to temperature below the recrystallization temperature of the steel, and thence extruding and upsetting said lengths into said valves.

3. The method of producing internal combustion engine poppet valves which comprises: hot rolling into bars a steel consisting essentially of 19-23% chromium, 4- 6.5% nickel, 6.2-8.5% manganese, 0.15-0.3% each of carbon and nitrogen, up to 1.5 silicon, up to 0.1% each of sulphur and phosphorus, balance substantially iron, solution treating said bars and cutting to bar lengths suitable for forming into said valves, reheating said bar lengths to temperature below the recrystallization temperature of said steel, and thence extruding in part and upsetting in part said bar lengths into the shape of said valves.

References Cited UNITED STATES PATENTS 3,311,511 3/1967 Goller 148-12 3,376,780. 4/1968 Tanczyn 14812 3,461,001 8/1969 Kuberg 14-12 HYLAND BIZOT, Primary Examiner