US2518715A - Heat-and corrosion-resisting alloy steel and articles thereof - Google Patents

Description

, range of about 0.1 to 0.7%.

Patented Aug. 15, 1950 HEAT- AND GORROSION-RESISTING ALLOY STEEL AND ARTICLES THEREOF .Peter Payson, New York, and Reinhold Schempp, Clay, N. Y., and Charles H. Savage, Morristown, N. J assignors to Crucible Steel Company of America, New York, N. Y., a corporation of New Jersey No Drawing. Application August 15, 1947,

Serial No. 768,928 x 3 Claims. (01. 75-126) This invention pertains to heatand corrosionresisting alloy steels, and provides an improved alloy steel of this character which combines to a superior degree the qualities of high strength and high hardness, with high resistance to stretching or elongation, and high resistance to corrosion, oxidation and scaling, at elevated temperatures.

The improved alloy steel of the invention is exceptionally well-adapted for such uses as valves and valve seats for internal combustion engines, hot work dies, die-casting dies, high speed and othencutting, shearing and forming tools, and related applications.

The steel of the invention comprises in its broader aspect an alloycontaining as essential constituents: about 22 to 26% chromium, together with about 2 to 6% in aggregate of one or more of the ferrite-forming elements molybdenum, tungsten and vanadium, and about 6 to in aggregate of one or more of the austeniteforming elements manganese, nickel and cobalt; or about 3 to 6% in aggregate of these elements, together with about 0. to' 0.25% nitrogen. The carbon content is not critical, but. for reasons explained below, is ordinarily held within the The balance of the alloy, except for the usual impurities within commercial tolerances, will ordinarily be substantially iron, although other elements may be present to the extent that they do not deleteriously affect to any appreciable degree the above-mentioned desirable combination of qualities of the alloy.

A preferred range of analysis for the improved alloy steel of the invention is: chronium, about 23.5 to 25.0%; ferrite-forming elements of the group molybdenum, tungsten and vanadium, about 2.5 to 5.0% in aggregate; austenite-forming elements of the group manganese, nickel and cobalt, about 6 to 8% in aggregate; carbon about 0.3 to 0.5%; and the balance substantially ron.

In the U. S. Reissue Patent 20,421, granted June 22, 1937, to Peter Payson, there is disclosed a series of heatand corrosion-resisting alloy steels containing: about 18 to 35% chromium; about 1 to 10% of either or both nickel and manganese; about 1 to 10% ofeither or both molybdenum and tungsten; carbon up to about 1% and the balance substantially iron, in relative proportions such that the steel may be hardened by heating within the temperature range of about 1200 to '1800 F., the so-hardened steel being characterized' in undergoing no substantial reduction in hardness after subsequent reheatings within said temperature range irrespective of the number or durations of such reheatings. As further pointed out in said reissue patent, the elements aforesaid are preferably combined in such relative proportions as to render the steel initially ferritic or magnetic, i. e., ferritic or magnetic in the as annealed, "as cast, as rolled or as forged" conditions. The heat-hardening of the steel is accompanied by a phase change from this initial magnetic to a non-magnetic condition. The nonmagnetic state resulting from the heat-hardening operation was found to be due to the formation of a non-magnetic constituent, known as the sigma phase. Also, as pointed out in said reissue patent, by appropriately selecting the relative proportions of the constituents referred to, and within the percentage limits ofeach above stated, individual analyses may be heat-hardened in the manner aforesaid as high as C Rockwell, hardnesses in excess of C 35 Rockwell being easily obtainable as shown by the various examples given in said reissue patent.

Steel in accordance with said Payson reissue patent has been employed extensively in the automotive industrles in this country in the form of poppet valves, particularly exhaust valves, for the internal combustion engines of automobiles of the small or pleasure car variety. A representative analysis which has been extensively employed for such purposes is the following: chromium, 24%; nickel, 4.8%; molybdenum, 2.8%; manganese, 0.9%; silicon, 0.3%; carbon, 0.4%; and balance iron, except for the usual impurities within commercial tolerances. However, valves made of this and similar analyses have not proved altogether satisfactory in truck and bus engines, in which the service conditions in'respect to temperature and stress are much more severe than they are in automobiles of the small or pleasure car types. Under the more severe conditions of truck and bus engine operation, valves made from steel of such analyses,v have become stretched or permanently elongated; A valve that has thus become stretched or elon Now, we have discovered that an alloy steel of I the above-mentioned limited range of analysis of the improved steel of the present invention possesses, in additionto the heat-hardening and corrosion-resisting properties of the steel of said Payson patent, unexpectedly improved character- 'istics with respect thereto, as regards resistance to stretching or elongation, when employed as an internal combustion engine valve, and under the severe service conditions of truck or bus engine operation.

A test has been devised for measuring the resistance of steels to stretching under such conditions. A tensile test piece, much like the conventional shape, with threaded ends and reduced gage section, is machined and, in addition, the ends of the test piece are ground parallel and "square with the axis of the test piece. The overall length of the test piece is measured very carefully by means of a dial indicator reading to 0.0001 inch over a surface plate. The gage section is also measured for cross-section and length. The test piece is then placed in a furnace mounted between the heads of a universal testing machine as is usual for elevated temperature tensile testing. The test piece is heated to 1400 F. and held at this temperature with close regulation for the duration of the test.v

After the test piece has become heated through at the testing temperature, a load of 10,000 p. s. 1. (pounds per square inch) is applied to it and maintained constant over a period of 8 hours (or longer for special tests). At the end of this time, the load on the test piece is reduced to zero, and the test piece is removed from the furnace and allowed to cool to room temperature. It is then again measured for overall length by means of the dial indicator and surface plate, and the percentage stretch, or permanent elongation is calculated on the basis of the original length of the gage section. Stretch data obtained by this test on the steel of this invention will be given subsequently.

For satisfactory service, an internal combustion engine valve should have a hardness of about C 38 to 45 Rockwell in the stem and at the tappet end. If the hardnes is lower, the valve wears unsatisfactorily in service causing the engine to operate inefficiently. If the hardness of the steel is higher, there is considerable risk of breakage of the valve in service causing a breakdown of the engine.

There is another requirement of an automotive exhaust valve steel and that is corrosion-resistance sufficient to withstand serious corrosion by the products of combustion of the leaded gasoline now being used extensively. To measure this resistance, it is customary to expose samples of the steel to molten lead oxide at 1675 F. for 1 hour and determine its loss in weight per unit of area.

Finally, it is of course desirable that the steel for valves be as low in cost as possible so that valves can be produced which are economical for use in automotive engines.

It is now known that the structure of heathardenable steels of the character aforesaid in their soft condition, that is, "as annealed, and frequently as cast, as rolled or as forged, consists of ferrite, austenite and carbides. Because of the high alloy content of the steel, particularly the chromium content, the ferrite of the steel is not altogether stable at temperatures position of the steel. The optimum temperature for the formation of sigma from ferrite, and thus the temperature for hardening the steel, is about 1400 F. It is also known that the hard constituent, sigma, may form from austenite in F. depends on the prior condition of the ferrite;

particularly in regard to the relative amount of straining of the ferrite. It is known that ferrite that is severely strained can be converted to sigma much more readily than ferrite that is completely free from strains. For this reason, the as rolled or as forged material is much more readily hardened than the as annealed material of the same composition. The annealing procedure for this type of steel consists of heating it at 1800 F., or higher, for about 30 minutes or longer, and then cooling it fairly rapidly in air, or even by a quench in water.

The austenite of this steel because of its high alloy content is believed to be substantially stable at all temperatures down to room temperature, and below, except perhaps in regard to solution and precipitation of carbides, which have little effect on the final properties of the steel.

The carbides in these heat-hardenable steels are primarily chromium iron carbides which contain some molybdenum or tungsten, since the latter are more active carbide formers than chr0- mium. It is believed that as the molybdenum and tungsten in the analysis increases, less of the chromium is tied up with the carbide and is available for ferrite which is the source of sigma formation.

We have discovered, in accordance with the present invention, that by properly balancing the austenite-forming elements, carbon, manganese, nickel, cobalt and nitrogen, with the ferriteand carbide-forming elements chromium, molybdenum, tungsten and vanadium, within the limits of composition which permit the steel to be hardened satisfactorily for exhaust valve service, we can increase the stretch resistance of the steel appreciably. We have also discovered that this stretch resistance is not proportional to the hardness of the steel; indeed, in many instances, we have found that the stretch resistance is better if the steel after it is forged is first annealed before it is hardened, as will be shown subsequently.

Finally, since cost is an important consideration in the manufacture of automotive exhaust valves, it is appropriate to point out that of the austenite formers in the steel of this invention, cobalt is the most expensive; and of the ferrite and carbide formers, vanadium and tungsten are the most expensive; and it is desirable therefore to hold these elements to a minimum. Nitrogen is usually introduced in the steel by means of high nitrogen ferro-chromium, although it may also be added by means of nitrogen compounds, of which calcium cyanamide is a good example. The control of nitrogen in this complex steel is not very easy, and even though the nitrogen addition is not very expensive, it is preferred to use carbon, manganese and nickel to provide the proper balance of austenite formers. Mandanese is less expensive than nickel and should be used preferably. .The cost of chromium depends on the carbon content of the term-chromium used, the lower the carbon, the higher the cost. and therefore it is preferable to use a medium carbon content ratherthan a very low amount. Chromium is less expensive than molybdenum, but

a minimum of molybdenum is necessary in the steel in order to give it satisfactory hardening characteristics.

The effect of carbon on the heat-hardening of steel according to this invention, is shown in the following Table I from which it is evident that as the carbon increases the attainable hardness decreases:

TABLE I Efiect of carbon on heat"hardening [All pieces heated at 1400 F. for lfihoum] Analysis-Percent H Rockweli 0 Mn Ni Cr M0 Ba]. 0"

0.27 0.91 4.94 23. 46 2.78 Fe 43 0.40 0.90 5.05 23. 52 2.76 Fe 42 0.52 0.93 4.94 23.16 2.57 Fe 37 0.27 0.89 6.20 23.50 2.81 Fe 39 0.43 0.86 6.14 23.58 2.64 Fe 36 0.28 0.93 6.91 23.52 2.68 Fe 38 0.37 0.93 6.85 23.90 2.60 Fe 36 0.43 0.94 6.84 24.02 2.59 Fe 33 TABLE II Efleet of manganese, nickel, cobalt and nitrogen on heat hardening [All pieces heated at 1400 F. for 16 hours.)

Analysis-Per cent Hume. Bar Rockwell 0 Mn Ni 00 N or Mo Bel. 0

PART A-EFFECT 0F MANGANESE ALONE 7066 0.52 4.00 23.94 2.64 Fe 20 7067 0.54 6.78 24.28 2.49 Fe 31 PART B-EFFECT OF MANGANESE WITH NICKEL Pew-rea s 882222.!

m nswe r: $9 8382 Analysis-Per cent Hardness 133i Rockwell OMnNiCoNCrMoBaL mm n-hrrno'r or NICKEL WITH HIGH MANGANESE 23.04 2.54 Fe 40 My 24.07 2.44 Fe 40 5224 2421 2.54 Fe 44 23.48 2.00 Fe 43 PART E-EFFEOT 0F NICKEL WITH COBALT 0.30 0.92 4.94 1.58...... 23.50 2.84 Fe 43 0.33 0.94 4.88 4.80--- 23.53 2.82 Fe 48 0.30 0.38 5.01 5.79 23.42 2.88 Fe 43 0.35 0.94 520 0.05 22.49 3.90 Fe 47 0.30 0.55 4.88 294- 2270 3.75 Fe 41 0.35 0.58 492 5.93 2290 3.50 Fe 33 0.32 0.95 477 1.21---" 22.83 530 Fe 50 0.37 0.86 4.72 2.86.-- 22.90 5.36 Fe 40 0.35 0.33 4.79 7.09---- 22.40 5.04 Fe 38 PART F-FFFECT 0F NITROGEN WITH MANGANESE.

- AND NICKEL 7005 .47 2.82 1.41 0.04 24.24 2.45 Fe 27 7170 0.51 2.39 1.40 0.22 24.32 2.15 Fe 40 7822 0.24094 3.51 0.04 23.74 2.79 Fe '39 7174 0.47 0.04 2.74 0.20 24.04 2.32 Fe 43 7705 0.40 0.90 5.05--- 0.04 23.52 270 Fe 42 5005 0.39 1.01 4.91 0.15 23.44 3.01 Fe 43 5006 0.43 1.05 4.95---" 0.28 23.37 2.93 Fe e743 0.32 0.85 477 1.21 0.04 22.83 5.30 Fe 50 5001 0.41 0.99 4.93 0.13 23.34 5.01 Fe 45 5002 0.37 0.97 4.86-.-" 0.22 23.51 5.02 Fe '41 is shown in parts B, C and D of Table II that adequate hardening is obtained when mixtures of -manganese and nickel in amounts from about 5 to 10% for the sum of the two are present in the steel along with about 23.5% chromium and about 2.5% or more of molybdenum. Also, it may be seen that as the sum of these two elements increases in a steel with a given amount of chromium and molybdenum, the hardening of the steel reaches a. maximum and then decreases.

The addition of the supplementary austeniteforming element, cobalt, to steel having a mixture of manganese and nickel in excess of about 5% has the effect of lowering the hardness attainable in a steel with a given amount of chromium and molybdenum, as shown in Part E of Table II.

Part F of Table II shows that when the strong austenite former, nitrogen, is added to steel in which the sum of the elements manganese and nickel is less than 5%, the hardening of the steel can be made adequate if nitrogen is present in amounts around 0.2%. However, when nitrogen in amounts around 0.2% is added to steel in which the sum of manganese and nickel is over about 5%, it causes the hardening of the steel to decrease.

The following Table III shows generally that the hardening of the steel increases with increase 01 chromium:

TABLE 1::

Eflect of chromium on heat hardening [All pieces heated at 1400 F. for 16 hours.)

However, as shown above in Table III, the minimum chromium necessary to give the steel a hardness of "C" 38 Rockwell or more depends on the manganese plus nickel content and the molybdenum content, but generally a minimum of about 22% or more of chromium is required. Furthermore, for adequate corrosion resistance, the steel should have a minimum of about 22% chromium. n the other hand, when chromium is over about 26%, the steel is dimcult to forge and has inadequate toughness when hardened and therefore would be undesirable for exhaust valves extruded or upset from rolled steel.

The eflect of increasing amounts of the ferriteforming elements molybdenum, tungsten and vanadium, on the hardening of the steel is shown in the following Table IV:

TABLE IV Eflect of molybdenum, tungsten, vanadium on heat hardening [All pieces heated at l400 F. for 16 hours.)

Analysis-Percent Hard- B Anal-75iS Pernt Hardness. Percent at will B8! Rockwell Elonga- 0 Mn Ni 00 01' Mo w v B01. 0 Mn Ni Cr Mo ML "0" tion 7824.-- .24 .87 4.98--- 3-6 1-75 Fe 36 8390- 0.30 0.85 4.89 23.05 2.14 Fe 44 1.2 7823-.-" .25 .02 4.91 2 2 23 Fe 31 8636 0.42 0.01 4.10 22.52 2.00 Fe 43 1.0 7m---" .24 .89 4.88---" 23-48 -7 e 42 50 8637.- 0.42 0.03 4.15 24.13 3.03 Fe 43 1.2 0:100.--" .42 .05 4.s0 23.51 3.00 Fe 44 0025.. 0.40 0.02 4.18 24.54 2.88 Fe 43 1.0 0000"--- .42 .02 4.00 23.62 5.21 Fe 40 0501"--- .32 .01 4.89 4.80 22.10 2.00 Fe 30 man" 81 H5 4.82 21% 275m" L51 Fe 40 However, when the amount of austenite forming 0500--- .33 .04 4.04 4.10 22.22 4.22 Fe 36 elements is increased and necessary compensa- $4 21-91 538 R 38 55 tions are made by increase of ferrite-forming .1 13 34 3"". 3.20 a; .1 6 elements, the stretch resistance is appreciably .l e a g 33 31 g 2% improved, as shown in the following Table VI. 6 g 20.00 3.11 2.00 .84 0 40 TABLE 45 6 Stretch resistance of improved steels As shown in the above Table IV, with given amounts of nickel-like elements and chromium, the steel increases in hardness with increase of these ferrite-forming elements. The minimum amount required depends on the balance of the composition. With about 6% of manganese plus nickel and about 23.5% chromium, the steel requires a minimum of about 2.5% molybdenum in order to harden to "C" 38- Rockwell or higher. When the steel does not have the proper amounts of manganese plus nickel, or chromium, to harden adequately, an increase in the ferrite-forming elements will cause increased hardening. In this respect, vanadium appears to be more effective than molybdenum in increasing hardening, and

the other two. ,vanadium. might be desirable it relatively inexone-third that of vanadium and about one-half that of tungsten, from the economic viewpoint, it is desirable to use molybdenum in preference to Additions of tungsten and pensive scrap containing these elements is available.

It may be stated then, in summary, that minimum amounts of the austenite-forming elements, such as manganese, nickel, etc., are required in the steel of the invention together with minimum amounts of chromium and the ferrite-forming elements, such as molybdenum, tungsten, vanadium, to'give it the ability to harden to the desired minimum hardness 01' C 38 Rockwell; that the hardening decreases with increasing amounts of austenite-forming elements beyond a miximum value; and that when the austeniteforming elements are present in large amounts, the required hardness can be attained by adding more chromium and ferrite-forming elements to compensate for the larger amounts of austeniteforming elements.

We have also discovered that the desired stretch resistance of the steel is improved when the austenite-forming elements are increased beyond the minimum amount necessary to give the steel adequate hardness, and when the chromium and ferrite-forming elements are present in sumcient amount to compensate for the austeniteforming elements. For example, it is shown in the following Table V that the stretch resistance is poor, that is, over 1% in 8 hours, under the test conditions, when the austenite-forming elements are present in amount suihcient to cause the steel to harden satisfactorily:

TABLE V Stretch resistance of conventional steels [Samples hardened at l400 F. for 16 hours and tested at 1400 F. with stress of 7000 p. s. i. for 8 hours.)

[Samples hardened at 1400" F. for 16 hours and tested at 1400 F. with stress of 7000 p. s. i. for 8 hours] Analysis-Percent Hard- Pep ness, cent Bar Rock- Elem 0 Mn Ni 00 N Cr Mo w Bel. $5 gation 0.38 0.87 6. 47 24. 27 4.15 Fe 45 0.4 8651. 0.46 0.90 6. 47 24. 70 4. 62 Fe 44 0. 5 8742..." 0.35 0. 88 4.92 5 93 22. 86 3.60 Fe 4 38 0.3 8745 0.37 0. 88 4. 87 6. 28 21. 93 5. 04 Fe 38 0.3 5003"- 0. 40 1. 01 6.36 0. 17 23. 66 5. 03 Fe 44 0. 2 5006"-" 0. 43 1.05 4. 95 0.28 23.373. 98 Fe 40 0.1 s400- 0. 41,0. 87 4. 95 23. l2. 70l2. 39 Fe 40 0.3

It is further shown in the following Table VII that when these improved steels are annealed before they are hardened in order to limit the hardening of the steel and thus improve its toughness as hardened, the stretch resistance is also appreciably better than that obtained with the conventional steel.

TABLE VII Stretch resistance of improved steels [Samples annealed and then hardened at l400 F. for 16 hours and tested at 1400 F. with stress of 7000 p. s. 1. for 8 hours] Analysis-Per Cent g Per B Roe]; Cent at we Elonga- Mn Ni N Or M0 E81. 0"

873l 0.35 0.94 6.35 24.33 4.56 Fe 39 0.3 8840""- 0.38 0.87 6.47 24.27 4.15 Fe 39 0.2 8395.--" 0.42 0.85 4.89 23.57 3.85 Fe 40 0.4 8306 .42 0.82 4.90 23.62 5.21 Fe 44 0.4 8653..." 0.38 0.93 6.85 25.08 4.12 Fe 39 0.4 8994 0.36 2.48 4.88 23.28 4.63 Fe 42 0.5 8996.--" 0.35 3.84 4.88"", 23.35 5.49 Fe 45 0.2 5002... 0.37 0.97 4.86 0.22 23.51 5.02 Fe 38 0.2

It has been shown therefore that a steel of satisfactory hardness and appreciably superior stretch resistance over that of the conventional heat-hardening steels can be produced by properly balancing the composition within the following limits:

so that the permanent elongation of the steel as hardened within the range of about C 38 to 45 Rockwell will be under 1.0% when tested at 1400 F. with a stress of 7000 p. s. i. for 8 hours.

We claim:

1. The method of making a heat-hardened and corrosion-resisting alloy steel, characterized, in the hardened condition, in possessing high strength, high hardness, high resistance against stretching and elongation, and high resistance against corrosion, oxidation and scaling at velevated temperatures, said method comprising: casting an alloy steel containing 22 to 26% chromium, 6 to 10% of austenite-forming elements of the group manganese, nickel, and cobalt, 2 to 6% of ferrite-forming elements of the group molybdenum, tungsten and vanadium, carbon about 0.1 to 0.7%, the balance principally iron; thereafter hot-forming said steel above about 1800 F.; thereafter annealing said steel by cooling fairly rapidly from above about 1800 F.; and thereafter hardening said steel by heating within the approximate temperature range of about 1200 to 1800 F. until hardened.

2. A heatand corrosion-resisting alloy steel containing about: 22 to 26% chromium; 3 to 6% of austenite-forming elements of the group manganese, nickel and cobalt; 0.15 to 0.25% nitrogen; 2 to 6% of ferrite-forming elements of the group molybdenum, tungsten and vanadium; 0.1 to 0.7% carbon; and the balance substantially all iron, characterized in being hardened by heating said steel and holding within the approximate temperature range of 1200 to 1800 F. until hardened, and in thereafter undergoing no substan-- tial reduction in hardness after heating to temperatures as high as about 1400 F., said steel being further characterized, in the hardened condition, in high resistance against stretching and elongation.

3. An article made of a heat-hardened and corrosion-resisting alloy steel, said steel being in the condition obtained by heating at about 1400 F. for upwards of one hour, said steel containing about: 22 to 26% chromium; 3 to 6% of austenite-forming elements of the group man REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,009,974 Payson July 30, 1935 2,229,065 Franks Jan. 21, 1941 2,380,821 Breeler et a1 July 31, 1945 2,484,903 Payson Oct. 18, 1945

Claims (1)

1. 2. A HEAT- AND CORROSION-RESISTING ALLOY STEEL CONTAINING ABOUT: 22 TO 26% CHROMIUM; 3 TO 6% OF AUSTENTITE-FORMING ELEMENTS OF THE GROUP MANGANESE, NICKEL AND COBALT; 0.15 TO 0.25% NITROGEN; 2 TO 6% OF FERRITE-FORMING ELEMENTS OF THE GROUP MOLYBDENUM, TUNGSTEN AND VANADIUM; 0.1 TO 0.7% CARBON; AND THE BALANCE SUBSTANTIALLY ALL IRON, CHARACTERIZED IN BEING HARDENED BY HEATING SAID STEEL AND HOLDING WITHIN THE APPROXIMATE TEMPERATURE RANGE OF 1200 TO 1800*F. UNTIL HARDENED, AND IN THEREAFTER UNDERGOING NO SUBSTANTIAL REDUCTION IN HARDNESS AFTER HEATING TO TEMPERATURES AS HIGH AS ABOUT 1400*F., SAID STEEL BEING FURTHER CHARACTERIZED, IN THE HARDENED CONDITION, IN HIGH RESISTANCE AGAINST STRETCHING AND ELONGATION.