US3661565A - Precipitation hardening steel - Google Patents

Abstract

A low carbon, precipitation hardening steel characterized by good room temperature and good high temperature properties, said steel containing as essential elements about 0.20 percent carbon, 1.50 percent chromium, 1.75 percent nickel, 2.00 percent molybdenum, 2.65 percent copper, and the balance substantially iron with residual elements in normal amounts.

Description

1 United States Patent 1151 3,661,565

Harvey 1 1 May 9, 1972 54] PRECIPITATION HARDENING STEEL 1,987,841 1/1935 Rittershausen 148/36 x 2,012,765 8/1935 Marthourey ..148/31 [72] Invent 2,327,490. 8/1943 Bagsar ..75/128R [73] Assignee: Metaltronics, lnc., Orchard Lake, Mich.

FOREIGN PATENTS OR APPLICATIONS [22] Filed: Aug. 4, 1969 387,865 2/1933 Great Britain ..75/125 1 1 pp 855,065 405,643 1/1934 Great Britain ..148/142 500,200 2/1939 Great Britain ..75/125 Dam 688,289 2 1940 Germany ..75 125 [63] Continuation-impart of Ser. No. 559,679, June 20,

1966, abandoned. Primary Eranii11er-Charles N. Lovell [52] US. Cl ..75/125, 75/1285, 75/1289, [57] ABSTRACT 148/31 148/36 A low carbon preci itation hardening steel characterized b [51] 1nt.C1 ..C22c 39/20, C22c 41/02 00d room i and 00d hi h tem ermure m [58] Field ofSearch ..75/123,125,128, 128.5,128.9; P g g P P P t1es, sa1d steel contammg as essentlal elements about 0.20 per- 148/142,31,36

cent carbon, 1.50 percent chromium, 1.75 percent mckel, [56] References Cited 2.00 percent molybdenum, 2.65 percent copper, and the balance substantially 11'01'1 w1th residual elements 1n normal UNITED STATES PATENTS amountsr 1,957,427 5/1934 Bucholtz ..75/125 4Claims, 1 Drawing Figure N 0) Ch O ois a TOTAL FERRlTE Accovms EFF-SGT PRECIPITATION HARDENING STEEL This application is a continuation in part of my copending application, Ser. No. 559,679 filed June 20, 1966, now abandoned, entitled Precipitation Hardening Steels.

The present invention relates to a precipitation hardening composition which posseses excellent room temperature properties as well as good physical properties at elevated temperatures. In addition the composition of the present invention is characterized by excellent machinability, high toughness, and minimum distortion on heat treatment. Furthermore the composition of the present invention is designed to provide for high physical properties at a cost which is a fraction of the cost of high strength precipitation hardening alloys presently available.

In general the low carbon, precipitation hardening compositions offer advantages in minimized distortion which cannot be duplicated by the more conventional, higher carbon, heat treating grades. Briefly, two steps are required to heat treat the precipitation hardening steels and the mechanism of hardening and the results are quite different from the higher carbon steels which are hardened and tempered.

The precipitation hardening compositions are first given a solution treatment which consists in cooling to room temperature from an elevated temperature range which is often from about l,800 to about 2,000 F. Following the solution treatment the precipitation hardening steels may be heated for aging or precipitation hardening which may consist in reheating at lower temperatures up to about 1,100 F. This results in precipitation hardening.

Many precipitation hardening composition have been attempted in the past and there are quite a few compositions which are capable of being precipitation hardening. As a practical matter, however, the difficulty with most compositions is that the precipitation of the age hardenable phase occurs selectively at the grain boundaries resulting in embrittlement and a marked loss in ductility. Accordingly, the composition of the present invention has been directed to the establishment of a substantially uniform matrix structure, wherein selective precipitation of the hardenable phase at the grain boundaries is substantially avoided.

The most widely used of the precipitation hardening alloys presently available are the so called Maraging" steels which contain up to 25 percent Nickel. However, these steels have limitations, a few of which are cited as follows:

1. The Maraging steels are very highly alloyed and require vacuum melting for optimum properties. As a result the Maraging steels are quite expensive and sell for about 3.00 per pound.

2. The Maraging steels have relatively poor high temperature properties and the hardness and strength fall off sharply at temperatures in excess of about 900 F.

3. Expedients such as refrigeration and cold working between solution hardening and aging is sometimes applied for maximum properties with the Maraging steels and this represents an item of additional processing cost and inconvenience.

4. In general the Maraging steels have relatively low impact strength which represents a serious deficiency or limitation for which these is a critical need for an improvement.

The deficiencies of the Maraging steels is illustrated by a survey which appeared in Business Week magazine of Apr. 18, 1964 which reported that even the most enthusiastic backers of Maraging steels admit some serious problems which must be solved before the Maraging steels find wider use. The present invention which is based on a systematic and extensive investigation of many compositions is submitted to correct for certain deficiencies in the Maraging steels.

The alloys of the present invention combine high strength and ductility at room and at elevated temperatures to a degree not previously obtainable with compositions of the prior art. Also the alloys of the present invention are considerably less expensive than the Maraging steels.

It is therefore an object of the present invention to provide precipitation hardenable steels which are characterized by excellent physical properties at room temperature at a cost which is considerably less than the high strength precipitation hardenable steels presently available.

Another object of this invention is to provide precipitation hardenable steels which are characterized by good elevated temperature properties, particularly in the temperature range above about 900 F up to about l,l00 F.

Other related objects of this invention include the provision of precipitation hardenable steels which are characterized by excellent machinability, minimized distortion on age hardening, and high ductility values.

Other objects of this invention will be apparent from the specification and examples.

I have discovered that the foregoing objects can be achieved with a steel of the following approximate composition, considering only the essential alloying elements:

Carbon 0.20

Chromium l.50

Nickel 1.75

Molybdenum 2.00

Copper 2.65

Iron Balance with normal residual elements.

As discussed more fully hereinafter, about 0.05 percent Nitrogen has been found to be beneficial to the machining properties and this may be added as nitrided ferrochrome. However Nitrogen is not essential to obtain the beneficial properties in accordance with the teachings of this invention.

Aluminum also contributes to precipitation hardening and may be added up to about 1.00 percent without serious adverse effects.

The balance is substantially iron with such elements as manganese, silicon, sulphur, and phosphorous in nonnal residual amounts.

The composition of the present invention may extend over a relatively wide range and retain its desirable characteristics for room temperature and elevated temperature properties. Broadly the composition may fall within the following limits of alloying elements:

Carbon About 0.10 to 0.25

Silicon 1.00 maximum Manganese 1.00 maximum Chromium 0.75 to 5.00 Molybdenum 1.00 to 4.00

Copper 1.25 to 3.50

Nickel 1.00 to 4.00

Aluminum 1.00 maximum Nitrogen 0.30 maximum The balance is substantially iron with residual elements in normal amounts.

The optimum solution treating temperature is from about 1,800' F to about 1,900 F followed preferably by air cooling to room temperature. A very satisfactory solution treatment consists in air cooling from the forging, pressing, or rolling operation without annealing. Needless to say this represents a savings in operating cost. In the solution treated condition, alloys of the present invention are about 38 to 43 Rockwell C in which condition they are readily machinable.

Aging or precipitation hardening may be accomplished by heating to about 950 to l, l 00 F for several hours followed by air cooling to room temperature. The foregoing treatment results in a hardness of about 48 to 52 Rockwell C as will be discussed more fully hereinafter.

The carbon content influences the hardness obtained on precipitation hardening. The carbon should be less than about 0.25 percent and preferably it should be in the range of about 0.10 to 0.20 percent.

The molybdenum content may be between 1.00 to 4.00 percent and preferably it should be between 1.50 to 3.00 percent.

Molybdenum contributes to precipitation hardening. However in amounts over about 3.00 percent the machinability is adversely affected.

Copper also contributes to precipitation hardening and also this element has an important beneficial effect on machinability. Copper, however, results in df'ficulties in hot working when present in amounts over about 3.50 percent and this element should preferably be in the range from about 1.50 to about 3.00 percent.

Nickel improves the forgibility of copper bearing steels and is added for this purpose as well as for contributing to precipitation hardening. Nickel may be added in the range of about 1.00 to 4.00 percent and should preferably be added in the range of about 1.25 to 3.00 percent. To ofi'set the detrimental effect of copper on hot workability, the nickel should preferably be present in amounts equivilent to at least about 90 percent of the copper content.

Chromium increases the hardenability or the ability to precipitation harden uniformly in large sections. While Chromium may be added within relatively wide limits within the range of about 0.75 to 5.00 percent with satisfactory results, a narrower range of 1.25 to 3.00 percent is suggested.

Aluminum has been added in small amounts up to about 1.00 percent. It contributes to precipitation hardening and facilitates nitriding. However aluminum narrows the pouring temperature range and difficulties were encountered in teeming high aluminum compositions. Aluminum is not an essential element. 1f added, the aluminum should preferably be less than about 0.50 percent.

The addition of 0.05 percent nitrogen, which was added as nitrided ferrochrome, results in an improvement in machinability in the solution treated condition. Nitrogen also contributes somewhat to precipitation hardening. To prevent bleeding ingots as well as from the standpoint of cost, the nitrogen should preferably be less than about 0.20 percent.

While a broad composition range for alloys of the present invention has been enumerated, it is advantageous in many instances to limit the composition to a narrower range of elements for a more economical and in some respects, a more effective use of the alloying elements. Therefore, a narrower, range of composition limits is as follows:

Carbon about 0.10 to 0.20

Silicon about 0.60 maximum Manganese 0.75 maximum Chromium 1.25 to 3.00

Nickel 1.25 to 3.00 Molybdenum 1.50 to 3.00

Copper 1.50 to 3.00

The balance is substantially iron with other elements in normal residual amounts.

EXAMPLE NO. 1

A heat of steel was made in accordance with the principles of this invention to the following analysis, all percentages being by weight:

Carbon 0.20 Silicon 0.22 Manganese 0.49 Chromium 2.76 Nickel 2.54 Molybdenum 2.78 Copper 2.22 Nitrogen 0.06

In the solution treated condition after air cooling from the forging operation the steel was 42 Rockwell C. On tempering or aging, typical hardness response is as follows:

TENSILE PROPERTIES Tensile properties in the solution treated and aged condition are as follows:

5 TABLE 1 Hardness Rock- Elonga- N 0. Condition well 0 Tensile Yield tion BA 1.. Solution treated... 42 219, 300 134,200 13 29. 4 2 do 42 210,700 138, 700 12 23. l 3 Aged 1,050 F..." 51 231,400 190,800 13 30.2 4 .do 51 233, 650 102, 300 13% 30. l 5. Aged1,150 F 45 104,450 165,350 13 35.7 6 .do 45 104, 060 166, 500 13% 34. 7

15 V NOTCH CHARPY IMPACT TABLE II Hardness VNotch Rockwell Charpy No. Treatment C Ft. Lbs. Deflection 1 Solution Treated 44 29 .169 2 Solution Treated 29 3 Aged 1050 F. 50 22 .034 4 Aged 1050" F. 21 5 Aged 1100* F. 46 22 .061 6 Aged 1100 F. 20 7 Aged 1150 F. 44 35 .100 8 Aged ll50 F. 37 9 Aged 1200* F. 38% 43 .160

10 Aged 1200 F. 42

The deflection was determined by piecing the broken halves of the Charpy specimen together and determining the deviation from a straight edge. Details of this method is reported in an article by R.F. Harvey and Dr. J.A. Berger entitled Evaluation of 5 percent Chromium Hot Work Tool Steels by Impact and Deflection," Metal Treatment and Drop Forging, May 1965.

The foregoing physical properties at room and at elevated temperatures will be recognized by those skilled in the art as being excellent. The physical properties at room temperature are not quite as high as, but are comparable with the more expensive Maraging steels. However the elevated temperature properties and the resistance to tempering of the composition of the present invention is definitely superior to the Maraging steels.

The size change on precipitation hardening is quite small and is of the order or magnitude of several hundredths of a thousandth of an inch per inch of length.

I have found the composition of the present invention to be remarkably free from heat checking which is a characteristic weakness of the conventional hot work tool steels. I have heated steel of the present invention to 2,000 F followed by water quenching and l have repeated this cycle 12 times without cracking. The higher carbon, conventional hot work steels cannot stand this severe treatment. In this connection, I have found steels of the composition of the present invention to be excellently suited for such hot work applications as dummy blocks and mandrels in extrusion tooling where conditions of water cooling is often encountered. Other hot work applications where resistance to heat checking is desireable includes forging dies, die casting dies, and extrusion dies.

Alloys of the present invention can be gas nitrided where file hard surfaces are desired. A two stage Floe process to eliminate the formation of an undesirable, brittle surface layer is recommended. The case depth is a function of nitriding conditions as follows:

TABLE III Case 1st Stage 2nd Stage The surface hardness after nitriding was 15-N 94 which is equivilent to Rockwell C 70.

Precipitation hardening alloys of the present invention are freely machinable in the solution treated condition as illustrated by the following tabulated results of a facing test made on a 6 inch diameter disc of the steel of the present invention in the solution treated condition at 42 Rockwell C.

TABLE IV Machining tests were made with a 15inch square tool bit of M 2 high speed steel with 10 side rake, 8 back rake, 8 side relief, and 8 end relief. The nose radius was 0.005 in. No lubricant was used.

RPM Depth of Cut Failure 212 0.050 in. 208 ftJmin. 328 0.010 174 fL/min. 328 0.025 160 ftJmin.

In a further machining test, 5/16 inch diameter high speed steel drills at a speed of 328 rpm was used. A constant load of 52 inch pounds was applied and the depth of the drilled hole was measured after drilling for exactly 60 seconds. The results are tabulated in Table V as follows: 5

TABLE V Hardness Depth Alloy Rockwell C Drilled Machinability Present 42 0.130 100 Invention M81 P 20 30 0.092 10 45 Mold Steel Maraging Steel 40 0.079 61 l8 7: Nickel EXAMPLE NO. 2

A further confirmatory test was made on another heat of steel made in accordance with the teachings of this invention to the following composition:

Carbon 0.19

Silicon 0.20

Manganese 0.55

Chromium 2.87

Nickel 2.44

Molybdenum 2.60 0 Copper 2.48 6 Nitrogen 0.04

Aluminum 0.18

Keyhole Charpy tests were made on this composition with the following results as tabulated in Table VI. 5

1050 F. 2 hr. 51.4 33.7 0.140

In developing precipitation hardening alloys with ultimate physical properties, I have found that it is essential that a balance is maintained between the austenite forming elements and the ferrite forming elements. The various elements vary in their alloying effect in accordance with the following factors tabulated in Table VII.

TABLE VII Relative Effect of Alloying Elements Austemite Forming Ferrite Forming Carbon 35 Chromium l Nitrogen 35 Silicon 5 Nickel l Molybdenum 2 Copper 1 Manganese 1 Aluminum 10 TABLE VIII EXAMPLE NO. I

Austenite Forming Elements Ferrite Forming Elements Carbon 0.20X35 7.0 Chromium 2.76Xl 2.76 Nitrogen 0.06X35 2.1 Silicon 0.22X5 1.10 Nickel 2.54X1 2.54 Molybdenum 2.78X2 5.56 Copper 2.22Xl 2.22 Manganese 0.49 l .49 Total Austenite Total Ferrite Alloying Effect 13.86 Alloying Effect 9.91

Total austenite alloying eifect 13.86 40 Total ferrite alloying effect 9.91

l have found that the total austenite alloying effect should be in the range of about 8 to 16 and the total ferrite alloying effect should be in the range of about 7 to 14. Also the ratio of the total austenite alloying effect to the total ferrite alloying effect should be in the range of about 1.0 to 1.65. This is a critical ratio which should be held for optimum results.

The FIGURE represents the extent of the useful area of compositions according to the teachings of this invention. Area abcd represents graphically this useful area of compositions in terms of the total austenite alloying effect the total ferrite alloying effect.

The presently available Maraging steels containing 18 percent nickel are expensive and sell for about 3.00 per pound or more. Metallurgists and the metalworking industry recognize the critical need for a low cost Maraging type, precipitation hardening steel. The present invention is designed to fullfill this urgent need.

It is realized that all of the elements in the composition of the present invention have been used previously. However, no one has previously made or described a low carbon, precipitation hardening steel of the composition of the present invention. In this connection, it is realized that others have made steels containing copper, nickel, molybdenum etc. William F. Finkl, for example, in U.S. Pat. No. 2,104,980 describes an alloy die block steel having the following analysis taken from the example of his invention:

The Finkl analysis differs considerably from the composition of the present invention. It will be noted also that the I Finkl composition and patent are directed to a higher carbon,

quench hardening analysis.

There is no mention of precipitation hardening in the Finkl patent and the relatively higher carbon composition of Pink] is not capable of precipitation hardening to any appreciable or useful degree.

- On the basis of the total austenite alloying effect and the total ferrite alloying effect, the ratio of these in the example cited by Finkl is 30.5 to 1.9 respectively or 16.1 which is quite remote from the teachings and objectives of the present inven tion.

While several illustrations of the present invention have been described, it willbe understood that this invention may be otherwise embodied within the scope of the following claims.

What is claimed is:

l. A precipitation hardening composition consisting essentially of about 0.10 to 0.25 percent carbon; 1.00 percent maximum silicon: 1.00 percent maximum manganese; 0.75 to 5.00 percent chromium; 1.00 to 4.00 percent nickel; 1.00 to 4.00 percent molybdenum; 1.25 to 3.50 percent copper; 1.00 percent maximum aluminum; and 0.30 percent maximum nitrogen; and the balance being iron; said precipitation hardening composition having a ratio of total austenite alloying effect to total ferrite alloying effect of from about 1.00 to 1.65.

2. A precipitation hardened composition, consisting essentially of about 0.10 to 0.20 percent carbon, 0.60 percent maximum silicon, 0.75 percent maximum manganese, 1.25 to 3.00 percent chromium, 1.25 to 3.00 percent nickel, 1.50 to 3.00 percent molybdenum, and 1.50 to 3.00 percent copper, with the remainder essentially iron; said composition being characterized by an essentially uniform precipitation hardened structure in which precipitation of the age hardenable phase at the grain boundaries is substantially avoided.

3. A precipitation hardenable composition consisting essentially of about 0.10 to 0.20 percent carbon, 0.60 percent maximum silicon, 0.75 percent maximum manganese, 1.25 to 3.00 percent chromium, 1.25 to 3.00 percent nickel, 1.50 to 3.00 percent molybdenum, 1.50 to 3.00 percent copper, and about 0.05 percent nitrogen with the balance being substantially iron.

4. A low carbon alloy composition, precipitation hardenable to a hardness in excess of Rockwell C 50 on aging at about 1,050" F, said alloy composition consisting essentially of about 0.10 to 0.25 percent carbon, 1.00 percent maximum silicon, 1.00 percent maximum manganese, 0.75 to 5.00 percent chromium, 1.00 to 4.00 percent nickel, 1.00 to 4.00 percent molybdenum, and 1.25 to 3.50 percent copper, the balance iron with incidental impurities; said alloy composition being characterized by a total austenite alloying effect offrom about 8 to 16 and a total ferrite alloying effect of from about 7 to 14.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORREQTEN Patent No. 3,661, 565 Dated May 9, 1972 Inventor(s) Richard F. Harvey It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the grant only, insert the attached pages, which are columns 5, 6, 7 and 8.

Signed and sealed this 27th day of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

ROBERT GOTTSCHALK v Attestlng Officer Commissioner of Patents FORM O-1050 (10-69) USCOMM-DC 60376-P69 W LE5, GOVERNMENT PRINTING OFFICE: 969 0'366-334.

Claims (4)

1. 2.00 PERCENT MOLYBDENUM, 2.65 PERCENT COPPER, AND THE BALANCE SUBSTANTIALLY IRON WITH RESIDUAL ELEMENTS IN NORMAL AMOUNTS.
2. 2. A precipitation hardened composition, consisting essentially of about 0.10 to 0.20 percent carbon, 0.60 percent maximum silicon, 0.75 percent maximum manganese, 1.25 to 3.00 percent chromium, 1.25 to 3.00 percent nickel, 1.50 to 3.00 percent molybdenum, and 1.50 to 3.00 percent copper, with the remainder essentially iron; said composition being characterized by an essentially uniform precipitation hardened structure in which precipitation of the age hardenable phase at the grain boundaries is substantially avoided.
3. 3. A precipitation hardenable composition consisting essentially of about 0.10 to 0.20 percent carbon, 0.60 percent maximum silicon, 0.75 percent maximum manganese, 1.25 to 3.00 percent chromium, 1.25 to 3.00 percent nickel, 1.50 to 3.00 percent molybdenum, 1.50 to 3.00 percent copper, and about 0.05 percent nitrogen with the balance being substantially iron.
4. 4. A low carbon alloy composition, precipitation hardenable to a hardness in excess of Rockwell C 50 on aging at about 1,050* F, said alloy composition consisting essentially of about 0.10 to 0.25 percent carbon, 1.00 percent maximum silicon, 1.00 percent maximum manganese, 0.75 to 5.00 percent chromium, 1.00 to 4.00 percent nickel, 1.00 to 4.00 percent molybdenum, and 1.25 to 3.50 percent copper, the balance iron with incidental impurities; said alloy composition being characterized by a total austenite alloying effect of from about 8 to 16 and a total ferrite alloying effect of from about 7 to 14.

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