US3355280A - High strength, martensitic stainless steel - Google Patents

Description

United States Patent ()fiice 3,355,280 HIGH STRENGTH, MARTENSHIC STAINLESS STEEL Glenn W. Tuifnell, Ridgewood, and Frank W. Schaller, Ringwood, N.J., and Ralph B. G. Yeo, Mount Lebanon, Pittsburgh, Pa., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed June 25, 1965, Ser. No. 467,104 7 Claims. (Cl. 75-128) ABSTRACT OF THE DISCLOSURE A nickel-chrominum martensitic stainless steel characterized by yield strengths on the order of 100,000 p.s.i. and above together with good ductility and toughness. Nitrogen, if present, is specially controlled together with carbon content.

The present invention relates to stainless steels and, more particularly, to a low cost, martensitic stainless steel amenable to ease of processing and characterized by a combination of properties, including yield strength and toughness, such that the steels can be used for common constructional as wellv as for other purposes. I

Due regard being accorded the rather extensive and voluminous amount of work hitherto expended in connection with the stainless steels, there exists a need for an improved stainless steel of intermediate yield strength, to wit, 100,000 pounds per square inch (p.s.i.) and upwards to 125,000 or 150,000 p.s.i. (0.2% offset), the steel otherwise manifesting a broad range of commercially attractive characteristics. Specifically, at yield strengths of 100,000 p.s.i. such a steel should offer a minimum tensile elongation of 10% to 12% and preferably at least 15%, a minimum reduction in area of 50% and preferably at least 60% and the ability to absorb at least 40 foot-pounds (ft-lbs.) of impact energy and preferably at least 50 ft.-lbs. Equally important from commercial considerations, such properties should be exhibited by a stainless steel which is (1) in the form of plate, (2) highly corrosion resistant, (3) easy to weld, (4) devoid of deleterious delta ferrite and/or retained austenite, (5) of low cost and (6) which requires but the simplest, if any, of heat treatments, to wit, a normalizing treatment, without the necessity of recourse to more elaborate and/ or expensive. treatments, e.g., cold plastic deformation processing, quenching operations, etc. As will be illustrated, even the normalizing treatment can be dispensed with in accordance herewith.

Of the three broad classes of stainless steels, the austenitic, martensitic and ferritic, the former, as exemplified by the AISI 300 series, has found much the greater commercial acceptance and usage, a fact attributable to the outstanding corrosion resistance and fabricability characteristics thereof. However, the yield strengths (0.2% offset) of the austenitic type stainless steels are relatively low, being of the order of 35,000 or 40,000 p.s.i. in the non-hardened condition. These steels, practically speaking, are non-responsive to thermal treatment in the sense of increasing their strength or hardness. To be sure, the strength thereof can be substantially increased via the application of well known plastic deformation processing, such as cold rolling at room or lower temperatures, and yield strengths up to above 200,000 p.s.i. have been obtained by cold reducing about 40% or 50%. However, since cold rolling is normally applied in but one direction, the mechanical characteristics of the steels can become, as is known, undesirably anisotropic. Further, cold rolling is not without ditficulty and, of course, lends to increased cost.

3,355,280 Patented Nov. 28, 1967 What obtains regarding the austenitic stainless steels is generally inapposite regarding many of the maitensitic stainless steels, including a number of those falling within the A181 400 series classification. Generally speaking and apart from microstructure, the martensitic stainless steels differ from the austenitic (and also the ferritic) in that they undergo strengthening and hardening upon being subjected to heat treatment. Many of the martensitic stainless steels can be thermally hardened to yield strengths well above 200,000 p.s.i., e.g., AISI 440A, but the toughness characteristics are very poor, a factor which has well contributed to the commercial development of the ultra high strength, precipitation hardenable stainless steels. In addition, the high carbon contents of A181 440A (or 440B or 440C) render such steels more than diificultly weldable. It perhaps should be mentioned that strength levels of 200,000 p.s.i., can be obtained in various ways in stainless steels but the overall product cost would be of a magnitude much too high for applications herein contemplated.

There are a few commercially produced, non-precipitation hardenable martensitic stainless steels which afford yield strengths of 100,000 to 150,000 p.s.i. For the most part, such steels are subjected to a three-stage heat treatment consisting of (depending upon the particular steel) an annealing treatment at a temperature of about 1200 F. to 1600 F. followed by cooling, a hardening and strengthening treatment consisting of austenitizing over a temperature range of about 1700 F. to 1900 F. followed by a cooling operation (often a liquid quench) and finally a tempering treatment at a temperature of about 400 F. to 1000 F. followed by cooling. In the annealed condition, for example, AISI 431 has a Rockwell hardness of about R 20 to 25. When hardened by austenitizing and quenching, the hardness is increased to about 40 to 45 R but toughness is drastically reduced to the order of about 10 to 15 ft.-lbs. (Izod), a prime factor responsible for the application of the tempering treatment. Thus, such steels, as reported in the Metals Handbook, eight-h edition, page 412, behave much in the manner as the carbon or low alloy steels, i.e., hardness (strength) and toughness are markedly influenced by carbon content.

Further, and most important, stainless steels such as A181 431 manifest an undesirably strong tendency to form delta ferrite and/or retained austenite, i.e., because of the difliculty in commercial practice in observing the extremely close tolerances necessitated by the chemistry of the steel, the rnicrostructure thereof is not consistently uniform from heat to heat and and too often is characterized by excessive amounts of austenite and/or the delta ferrite phase, the latter being conducive to embrittling. These objectionable processing difficulties are perhaps accountable, partially at least, for the reason that the production of A181 431 has not appreciably increased over the last twenty years. In addition, the high carbon content of A151 431 renders the alloy diflicultly weldable. A particular objective of the invention, therefore, is to overcome the difficulties attendant stainless steels of the A181 431 type and do so while minimizing processing operations; for example, eliminating the necessity for, inter alia, a tempering treatment.

Mention has been made above that the present invention is particularly concerned with stainless steels in the form of plate and attention should be addressed to this aspect. It is more than common, as is well known, to determine toughness characteristics, including tensile elongation, reduction in area and Charpy V-notch energy values, rom tests conducted on steel in the form of bar or rod. Also well known is the fact that such properties are, at times, higher, often substantially so, than those obtained from tests conducted on plate, particularly those performed on transverse sections. Even as to plate, it is documented that the ability to absorb impact energy is usually significantly higher when measured in the longitudinal direction as opposed to the direction transverse to rolling. Thus, since plate is an extremely common mill form, conventionally used, for example, in welding pressure vessels, and since the characteristics thereof are inherently self-imposed, it is with advantage, particularly in designing fabricated structures, to have available experimental data indicative of the expected performance of plate.

It has now been discovered that each of the aforediscussed characteristics can be combined into one low cost, easily processed stainless steel of novel composition in which various constituents thereof, notably chromium, nickel, carbon, nitrogen, silicon, manganese and aluminum, are maintained within special compositional ranges.

It is an object of the present invention to provide a novel stainless steel, a steel which lends itself to the minimum of processing.

Another object of the invention is to provide a new and improved martensitic stainless steel characterized by a yield strength of about 100,000 p.s.i. and above together with good toughness.

It is a further object of the invention to provide a novel, martensitic stainless steel which is weldable, corrosion resistant, of low cost and easily processed.

An additional object of the invention is to provide a martensitic stainless steel plate which manifests all of the aforedescribed characteristics.

Generally speaking and in accordance herewith, the present invention contemplates martensitic stainless steels consisting essentially of (by weight) from 12% to 16.5% chromium, from 3% to 6.5% nickel, the sum of the chrominum plus nickel not exceeding about 21.5 and advantageously not exceeding 21.25%, carbon up to 0.12%, up to 0.1% nitrogen, with the sum of the carbon and nitrogen not exceeding 0.13% and preferably not exceeding 0.12%, up to 1% and preferably not more than 0.75% manganese, up to 1% silicon, up to 0.15% aluminum, the balance being essentially iron. In referring to the iron content as constituting the balance or essentially the balance, it is to be understood, as will be appreciated by those skilled in the art, that the presence of other elements is not excluded, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely affect the basic characteristics of the steels. In this connection, elements such as sulfur, phosphorus, hydrogen and oxygen should be kept at levels as low as is consistent with good commercial steelmaking practice. However, where the optimum is required for resistance to pitting, up to 1.5% chromium can be replaced by molybdenum, the chromium not falling below 12%.

In achieving an optimum combination of properties, the stainless steels herein advantageously contain about 13% to chromium, about 4% to 6% and preferably 4.5% to 6% nickel, about 0.001 to 0.05% carbon, up to 0.05 nitrogen, the sum of the carbon plus nitrogen being not more than 0.07%, up to 0.1% aluminum (e.g., 0.01% to 0.1% aluminum) to provide good deoxidation characteristics during melting, up to 0.5% and preferably not more than 0.25% silicon, up to about 0.5% manganese and the balance essentially iron and normal impurities. When normalized in the temperature range of about 1450 F. to 1600 F., e.g., 1500 F., stainless steels within this more restricted compositional range and in plate form, say, one-half inch thick, afford yield strengths above 100,000 p.s.i., tensile elongations of about 13% or 14% and above (gage length of 1 inch), reductions in area of about 55% or 60% and higher and Charpy V-notch impact values upwards of about 45 to 50 ft.-lbs. at least in the longitudinal direction. In addition to providing good corrosion resistance, these steels exhibit good weldability and uniformity of results from heat to heat can be achieved consistently, i.e., the steels are notably absent of detrimental delta ferrite and/or retained austenite. These properties obtain not only in the normalized but also the as-rolled condition. Processing operations, albiet they be of the conventional type, such as cold plastic deformation or quenching or tempering are not at all required.

As to the respective constituents of the steels, chromium confers corrosion resistance and advantageously at least 13% chromium should be present since even at a level of 12%, resistance to various corrosive media is marginal. Amounts below 12% are simply unsatisfactory. On the other hand, excessive amounts of chromium are causative of or promote the formation of delta ferrite, a weak and embrittling phase which also gives rise to hot working difficulties. Thus, while the percentage of chromium can be as high as 16.5 it preferably should not exceed about 16% and to consistently achieve an optimum combination of processing characteristics and mechanical properties, it should not exceed about 15%.

Nickel counteracts the tendency for delta ferrite formation; however, excess nickel can lead to undesirable quantities of retained austenite on cooling from the austenitic condition. Stable or retained austenite is detrimental since various properties, notably yield strength, are adversely affected and to avoid the same, the nickel content should not exceed 6.5 and preferably is not greater than 6%. It should be mentioned, as will be appreciated by those skilled in the art, that at elevated temperatures, a completely austenitic structure is desired for good processing characteristics, particularly forgeability. However, the steels should transform at least substantially to martensite upon cooling to about room temperature and to that end the sum of the percentages of nickel and chromium should not exceed 21.5% and preferably should be less than 21.25%. Thus, a high martensitic transformation (M temperature is most desirable, a temperature on the order of above 200 F. and advantageously above 250 F.

Carbon and nitrogen also exert a strong influence in resisting the formation of the delta ferrite phase but, as with nickel, excessive amounts thereof, though relatively small, can impair strength characteristics, generally as a result of retained austenite, as will be shown herein. For the best overall combination of properties in the normalized condition, the total sum of carbon and nitrogen should not exceed 0.07%. in this connection, the carbon content can be below 0.01% and the desired combination of properties are readily attainable, a reflection of the fact that steels within the invention do not depend upon the presence of carbon to achieve a required level of properties.

Silicon and aluminum, being ferrite formers and thus capable of promoting delta ferrite, should not exceed 1% and 0.15%, respectively. It is preferred that the silicon content be less than 0.5 and most advantageously is not greater than 0.25%. Aluminum (also silicon) can adversely affect toughness and preferably does not exceed 0.1% although it is beneficial as an addition during melting in an amount sufiicient to provide good deoxidation. The use of aluminum for precipitation hardening purposes, e.g., 0.5%, is quite inconsistent with the invention.

In carrying the invention into practice and while vacuum processing can be used, a special virtue is that simple, standard air melting practice can be employed. Use of relatively high purity alloying ingredients is, of course, beneficial though not necessary, provided reasonable caution is exercised regarding the composition of, say, scrap material that might be utilized. The cast ingots formed from the steels can be soaked at about 2100 F. to 2300 F., e.g., about 2200 F., to provide thorough homogenization, one hour per inch of cross section being usually satisfactory. Thereafter, the steels are hot worked as by forging, rolling, etc. Suitable hot working temperatures ment to insure maximum transformation to the marten- 1O sitic condition. Temperatures at least as low as minus 300 F. can be used in this connection.

Annealing and/or austenitizing and/or tempering treatments hitherto employed in connection with prior art martensitic stainless steels, while not excluded from 15 the scope of the invention, are not mandatory in accord- 6 Heat Treatment BHeat Treatment A followed by heating to 800 F. for one hour and then air cooling to room temperature. Heat Treatment CHeated to 1800 F., held about one hour, then quenched.

Heat Treatment D-Heat Treatment C followed by heat-- ing to 800 F. for one hour, then air cooling to room temperature.

In Table I the yield strength (Y.S., 0.2% olfset) and ultimate tensile strength (U.T.S.) are given in pounds per square inch (p.s.i.), the tensile elongation (EL) (gage length of one inch) and reduction in area (R.A.) are given in percent and the Charpy V-notch (C.V.N.) impact values (average of 2 readings in each instance) are given in foot-pounds (ft-lbs).

TABLE I Ni, Q N, Si, Mn, Al, Heat Y.S, U.T. S., 101, RA, C.V.N, Alloy No Pep Per. p P P t; Pgr- Per- Per- Treatp.s.1 p.s.1. Per- Perft.-lbs

cent; cent; cent cent cent e cent ment Gent 001111 1 15,88 .019 0,023 0.042 0.41 0.0 0 .04 A 17, 153.000 66 56 V C 116, 700 147, 900 15 66 88 B 127, 900 158, 400 17 66 52 D 131, 500 155. 500 19 66 48 2 1 ,30 5, 0 0,034 0,023 0.057 0.45 0.49 0.033 A 1 17 17 65 7 0 128,100 166, 800 15 07 76 B 133, 300 165, 700 19 08 77 D 135, 100 166, 00 18 68 88 3 15.02 5 7() 09 0,014 (1104 0.49 0.49 0036 A 8 0 9 55 62 C 96. 000 202 000 17 57 60 B 108, 700 172, 100 26 65 91 8 0 039 it 33588 533 38 i3 3 94 .10 0.027 0.127 0.45 0 4 ,7 s 4 1 60 52 0 o 90, 000 200, 500 13 4s 50 B 118, 300 176, 300 29 65 80 D 127, 300 174 300 2 a- (18 7 5 1595 ,013 0.143 0.44 0.48 0.032 A 37.000 182300 34 80 C 38, 500 177, 300 35 110 B 43, 300 163, 100 35 46 73 D 46, 300 171, 300 35 49 107 51 3 15 Q05 0,020 0,070 0.51 0.48 0.0 A 1 00 183.300 1 52 C 133, 900 182, 000 15 59 1 E 134, 400 176, 500 17 67 67 4 37 AD 143,300 172,200 19 65 70 2 0 0,022 0.102 0.41 0. 5 0.0 1 8. 0 20.2 17 57 48 7 13 8 7 0 102,800 208,700 15 4s 47 B 130, 300 173, 500 24 60 80 D 140, 800 178, 100 20 06 76 1 Contained about 1.01% molybdenum.

employed. The steels are then air cooled to room tem- 50 perature, liquid quenching being quite unnecessary. Accordingly, the well known problems associated with a liquid quench are obviated.

For the purpose of giving those skilled in the art a better understanding of the invention, and/ or a better appreciation of the advantages thereof, the following illustrative description and data are given:

Several stainless steels having compositions given in Table I were prepared by air induction melting. The

alloying constituents were of good purity and final deoxidation just prior to pouring was accomplished with an aluminum addition of 0.1%. The ingots were soaked for about two hours at about 2200 F., forged at 2150 F. first on one diagonal and then on the opposite one, and,

finally, into one inch by three inches plate. Using 2 passes,

the plate was hot rolled to one-half inch thickness, the last pass being performed at about 1800" F. Tensile and Charpy V-notch specimens were prepared from fully heat treated blanks approximately one half inch in cross section and cut in a longitudinal direction. For purposes of test, the following four different heat treatments were used:

Heat Treatment A-Heated to about 1800 F., held for about one hour and then air cooled to room tempera ture, 7

2 Contained about 1.02% molybdenum.

The data in Table I (plate specimens) illustrate the advantages in maintaining the carbon content below about 0.12%, the total carbon plus nitrogen below about 0.13% and the sum of the chromium plus nickel below about 21.5%. For example, Alloy No. 5 having a carbon content of 0.13% and a carbon plus nitrogen content of 0.143% was of low yield strength, the magnitude of which remained unchanged notwithstanding the utilization of a second thermal treatment as with Heat Treatments B and D. The low yield strength was attributable to an excessive amount of retained austenite, examination revealing the presence of about 74% austenite.

In respect of Alloys Nos. 3 and 4, the thermal treatment at 800 F. (B and D) aflorded yield strengths in excess of 100,000 p.s.i., Alloy No. 3 containing 0.09% carbon and 0.104% carbon plus nitrogen with Alloy No. 4 containing 0.10% carbon and 0.127% carbon plus nitrogen. Lower normalizing temperatures (Table II, infra) accomplished the same purpose as did refrigeration in liquid nitrogen at minus 320 P. On the other hand, Alloys Nos. 1 and 2 of low carbon and low carbon plus nitrogen exhibited a satisfactory yield strength level coupled with the desired combination of other properties without the need of quenching or of thermally heating subsequent to normalizing. Further, as will be seen in Table II herein, the desired yield strength level obtains over a broad range of normalizing temperatures, i.e., 1500 F. to 1800 F.

Each of the Alloys Nos. 1 through 7 was normalized at temperatures other than 1800" F., to wit, 1500 F., 1600 F. and 1700 F. (Heat Treatments E, F and G, Table II). The data resulting therefrom are given in Table II, the data of Heat Treatment A being included for convenience.

TABLE II Cr, Ni, 0, N, O+N, Si, Mn, Al, Heat Y.S., U.T.S., EL, R.A., C.V.N., ft.-1bs. Alloy perperperperperperperpertreatp.s.i. p.s.i. perper- No. cent cent cent cent cent cent cent cent ment cent cent 1 15.88 0.019 0.023 0.042 0.41 0.50 0.049 E 119,900 156,200 15 F 119, 500 156, 500 15 G 115,300 154, 000 15 A 117, 000 153, 000 15 2 15.80 5.20 0.034 0.023 0.057 0.45 0.49 0.033 E 124,200 167,700 15 F 117, 100 176, 500 16 G 104, 400 171, 500 15 A 108, 600 172. 500 17 3 15.02 5.70 0.09 0.014 0.104 0.49 0.49 0.036 E 115,700 180,100 14 F 101, 600 187, 300 G 64, 300 204, 400 14 A 88, 800 204. 800 19 4 15.60 5 0.10 0.027 0.127 0.45 0.48 0.039 E 112,900 183,800 11 F 108,900 100, 900 13 G ,600 204,000 13 A 68, 700 209. 700 16 5 15.95 6.15 0.13 0.013 0.143 0.44 0.48 0.032 E 39,800 187,200 17 F ,000 187,000 20 G 30, 000 182. 300 21 A 37. 600 182, 300 34 6 13.75 5.15 0.05 0.020 0.070 0.51 0.48 0.035 E 138,100 132,800 14 F 119, 000 186, 700 13 G 115, 200 185, 200 13 g A 124, 000 183, 200 13 7 13 80 5.70 0 08 0.022 0.102 0.47 0.45 0.037 E 121,700 183,500 13 F 107, 500 195, 500 13 G 84, 100 210, 600 14 A 108, 200 203, 200 17 The data in Table II reflect that with carbon contents of 0.05% and below and carbon plus nitrogen contents of 0.07% or less (Alloys Nos. 1, 2 and 6), a wide range of normalizing temperatures (1500 F. to 1800" F.) can be employed. Thus, close temperature control is not necessary and this is an additional processing advantage regarding alloys falling within the most advantageous alloy ranges described above herein. With higher carbon and/ or carbon plus nitrogen contents, closer observance of normalizing temperature should be maintained (absent subsequent thermal treating). In this regard, temperatures of less than 1700 F. and preferably not over 1600" F. should be used. This is illustrated by Alloys Nos. 3 and 4. Regardless of normalizing temperature, the yield strength of Alloy No. 5 remained extremely low.

Additional data on plate specimens are given in Table III, the data being set forth to further illustrate various points of the invention. The steels were prepared and tested in the manner described in connection with the steels of Table I.

pact energy with increasing contents of carbon, a point further illustrated by Alloys Nos. 13 and 14 at a different level of nickel. In additional testing, Alloy No. 11 exhibited a 15 ft.-lb. impact transition temperature of minus 225 F. in the transverse direction (minus 200 F.

in the longitudinal direction) and had a Charpy V-notch impact strength of 22 ft.-lbs. at minus 40 F. (29 ft.-lbs. in the longitudinal direction). These data illustrate that the stainless steels contemplated herein can also be used for cryogenic applications.

In Table IV there are set forth data obtained on bar specimens in both the as-rolled (A.R.) and normalized (Heat Treatment E) conditions,

TABLE III Alloy Cr, Ni, O, N, C+N, Si, Mn, Heat Y.S U.T.S., EL, R.A., C.V.N., No Percent Percent Percent Percent Percent Percent Percent Treatp.s.i. p.s.i. Percent Percent it.-lbs.

ment

15.45 2. 0.05 0. 016 0.066 0. 16 0. 43 E 107, 700 140, 000 16 60. 5 32. 5 15. 0 6. 0 0. 08 0. 107 0.097 0. 07 0.40 E 177,800 183, 600 17 61. 5 47 15. 0 6. 0 0. 09 0.014 0. 104 0. 07 0. 40 E 120, 800 187, 800 16 59 41 15. 02 5. 0. 09 0. 014 0. 104 0. 49 O. 49 E 115, 700 186, 100 14 53 43 15. 35 5. 20 0. 10 0. 0026 0.1026 0. 24 0. 40 E 111, 600 181, 700 14 56 35 15.0 6.0 0. 12 0. 019 0. 139 0.07 0. 40 E 104, 800 189,000 14 54. 5 33 15. 6. 15 0. 13 0.013 0. 143 0. 44 0. 48 E 39, 800 187, 200 17 31 15.6 3.85 0. 104 0.0135 0. 1175 0.26 0.40 E 142, 000 181,300 14 53. 5 23 15. 1 8. 8 0. 02 0.022 0.042 0. 23 0.40 E 118, 000 147. 16 63. 5 52 N 01; more than 0.1% aluminum was present in any of the steels.

TABLE IV Alloy Alloy Cr, N 1, C, N, C+N, Si, Mn, Comm Y.S., U.T.S., EL, R.A., C.V.N.,

No. Percent Percent Percent Percent Percent Percent Percent tion p.s.i p.s.i. Percent Percent it.-lbs.

15 14. 71 4. 40 0. 011 0.016 0.027 0. 21 0. 54 A.R. 124, 800 153, 700 15 65. 68 E 119, 200 150, 200 16 68. 0 76 E 115, 300 142, 900 16 70. 0 93 19 14. 69 5. 35 0. 008 0. 015 0. 023 0. 24 0. 50 A. R. 116, 900 145, 000 16 70. 0 88 E 114, 500 141, 400 17 70.5 89

21 14. 93 6. 05 0.014 0. 0325 0. 0465 O. 23 0. 50 A. R 112, 300 160, 500 16 64. 5 71. 5

E 91, 600 165, 700 18 68. 0 68 24 14. 63 6. 80 0.019 0.067 0.086 0. 16 0. 47 A.R. 88. 400 161, 500 19 54. 5 102 E 83, 500 163, 300 19 69. 0 95 Each of the alloys contained less than 0.1% aluminum.

It will be noted from Table IV that yield strengths well above 100,000 p.s.i. were obtained in the as-rolled as well as normalized condition together with good toughness characteristics. Any overall difference in the mechanical properties set forth due to the steels being in the normalized versus the as-rolled (hot worked) condition is negligible. Alloy N0. 24 contained too much nickel (6.80%). The yield strength of Alloy No. 23 was under the minimum contemplated herein and it will be noted that the steel contained the maximum permissible amount of nickel and, for thisamount of nickel, a relatively high amount of carbon ,plus nitrogen, to wit, 0.081%. Since nickel, carbon and nitrogen are all austenite formers, it is beneficial when using a nickel percentage at the high end of the range to use an amount of carbon plus nitrogen which is relatively at the lower end of" its range. Refrigeration and/or cold working would result in an increase in yield strength of Alloy No. 23 since a greater amount of austenite would be transformed to' martensite. Reducing'the nickel and carbon plus nitrogen slightly, Alloy No. 22, would provide a higher minimum yield strength in the absence of refrigeration or other processing.

Data obtained as a result of weldability tests have also shown that the stainless steels contemplated herein can be welded quite satisfactorily. In this connection and using the X-weld test, steel plates having the composition .of Alloy No. 11 given in Table III were welded using the manual, inert gas, tungsten-arc process. The filler wire employed was of a composition essentially matching that of the base plate and contained about 13.7% chromium, about 5% nickel, about 0.07% carbon, about 0.20% silicon, about 0.34% manganese, the balance being essentially iron and impurities.

Subsequent to welding, a three-inch long X-weld was cut transversely into four sections, polished, etched and examined magnifications) for defects. The structure was sound throughout the length of the weld and no indication of porosity or cracking was observed. Specimens were then obtained from the sections for tensile and other tests. The mechanical properties obtained from the weld deposit, normalized at 1500" F. were as follows: a yield strength of 136,200 p.s.i., an ultimate tensile strength of 169,400 p.s.i., a tensile elongation of 14.5%, a reduction in area of 64.5% and a Charpy V-notch impact value (average of two tests) of 67.1 foot-pounds.

With regard to metallographic examination of both the as-welded and normalized materials, no areas of delta ferrite, retained austenite or localized carbide precipitation were noted. Further, a microhardness examination using the Vickers, 136 diamond indentor with a 500 gram load was made on each test specimen transversely across the weld. The hardness of the as-welded specimen was about 400 D.P.H. which represented but a slight in crease over the 365 D.P.H. in the heat-aifected zone.

Hardness in the normalized condition was about 365 D.P.H. for the entire specimen. Such results denote satisfactory as-welded properties, there being no deterioration within the heat-affected zone.

The martensitic stainless steels of the invention are suitable for diverse use, particularly applications requiring steels having a good combination of strength and toughness, e.g., pressure vessels, suction press rolls, transportation equipment, such as truck frames, etc. The steels can also be used in applications requiring a reasonably high hardness, e.g., a Rockwell hardness of about R 35 to 45. In this connection, carbon contents above 0.05% are beneficial, e.g., 0.07% to 0.12% carbon. Items of cutlery, particularly knives, would be an example where such hardness values would be desirable. The steels can be provided in common mill forms, including plate, bar, rod, etc., or in the form of castings. Corrosion resistance of the steels compares more than favorably with the prior art martensitic stainless steel AISI 431. Tests conducted for a period of four months in ammonium nitrate, for example, have given excellent results.

While liquid quenching and tempering are not necessary, they can be utilized as explained before herein. In this connection, thermal treatments over the range of about 700 F. to about 900 F. result in hardening of steels contemplated herein and which contain relatively low amounts of carbon or carbon plus nitrogen. In other words, such steels do not temper, i.e., they do not soften. Rather, they increase in hardness. This behavior, in the light of prior art martensitic stainless steels, is deemed unusual and the complete theory which might explain the same is not yet at hand.

As used herein and as will be appreciated by those skilled in the art, the term martensite includes the low temperature transformation and decomposition products of austenite. Less than 10% and preferably less than 5% or 3% of delta ferrite should be present. The optimum is a steel devoid of delta ferrite. In addition, austenite, retained or otherwise, should be kept to a minimum, to Wit, not more than 5% and preferably less than about 3%.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be Within the purview and scope of the invention and appended claims.

We claim:

1. A low cost, easily processed, stainless steel in the normalized condition having a martensitic microstructure with less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, a tensile elongation of at least about 14%, a reduction in area of at least 55% and a Charpy V-notch impact value of about at least 40 foot-pounds when in the form of plate up to a thickness of one-half inch, the plate having been normalized at a temperature of about 1500" F., said steel consisting essentially of about 13% to 16% chromium, about 4% to 6% nickel, the sum of the chromium plus nickel not exceeding 21.25%, carbon in an amount up to 0.05%, nitrogen in an amount up to 0.05%, the sum of the carbon plus nitrogen not exceeding 0.07%, up to 0.5% manganese, up to 0.5% silicon, up to 0.1% aluminum and the balance essentially iron.

2. A low cost, easily processed, stainless steel in the normalized condition having a martensitic microstructure With less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, a tensile elongation of at least about 14%, a reduction in area of at least 55% and a Charpy V-notch impact value of about at least 40 foot-pounds when in the form of plate up to a thickness of one-half inchfthe plate having been normalized at a temperature of about 1500 F., said steel consisting essentially of about 13% to 15% chromium, about 4.5% to 6% nickel, about 0.001% to 0.05% carbon, nitrogen in an amount up to 0.05 the sum of the carbon plus nitrogen not exceeding 0.07%, up to 0.5 manganese, up to about 0.25% silicon, about 0.01% to 0.1% aluminum and the balance essentially iron.

3. A low cost, easily processed, stainless steel in the normalized condition having a martensitic microstructure with less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch in combination with good toughness, said steel consisting essentially of about 13% to 15% chromium, about 4% to 6% nickel, about 0.001% to 0.03% carbon, up to 0.05% nitrogen, the sum of the carbon plus nitrogen not exceeding 0.07%, up to 0.5% manganese, up to 0.25% silicon, about 0.01% to about 0.1% aluminum and the balance essentially iron.

4. A stainless steel in the normalized condition having a martensitic microstructure with less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, said steel consisting essentially of about 12% to 16.5% chromium, about 3% to 6% nickel, the sum of the chromium plus nickel not exceeding about 21.25 carbon up to about 0.12%, nitrogen in an amount up to about 0.1%, the sum of the carbon plus nitrogen not exceeding about 0.13%, up to about 1.5% molybdenum, up to about 1% manganese, up to about 1% silicon, up to about 0.15% aluminum, and the balance essentially iron.

5. A stainless steel in the normalized condition having a martensitic microstructure with less than 5% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, said steel consisting essentially of about about 13%- to 16% chromium, about 4% to 6% nickel, the sum of the chromium plus nickel not exceeding about 21.25%, carbon up to about 0.12%, nitrogen in an amount up to about 0.1%, the sum of the carbon plus nitrogen not exceeding about 0.13%, up to about 1% manganese, up to about 0.5 silicon, up to about 0.15% aluminum, and the balance essentially iron. I

6. An alloy as set forth in claim 5 containing 14% to 16% chromium, up to 0.25% silicon and a martensitic microstructure containing less than about 3% austenite. 7. A stainless steel having a martensitic microstruct-ure with less than 5% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, said steel consisting essentially of 13% to 15% chromium, 4% to 6% nickel, carbon up to 0.12%, nitrogen in an amount up to about 0.1%, the sum of the carbon plus nitrogen not exceeding about 0.13%, up to about 1.5% molybdenum, up to about 1% manganese, up to about 1% silicon, up to about 0.15 aluminum, and the balance essentially iron.

References Cited UNITED STATES PATENTS 2,903,386 9/1959 Waxweiler -128 X 3,253,966 5/1966 Malagari et al. 75-128 X 3,258,370 6/1966 Floreen et a1. 75-128 X 3,259,528 7/1966 Carlsen 75-128 X 3,288,611 11/1966 Lula et a1. 75-128 CHARLES N. LOVEIJL, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,355,280 November 28, 1967 Glenn W. Tuffnell et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below: Column 2, line 50, cancel "and", first occurrence. Column 4, line 7, "albiet" should read albeit Columns 5 and 6, TABLE I, eighth column, line 1 thereof, "0.050" should read 0.50 Columns 7 and 8, TABLE III, fifth column, line 2 there of, "0.107" should read 0.017 same TABLE III, tenth column, line 2 thereof, "177,800" should read 117,800

same TABLE III, eleventh column, line 9 thereof, "147.100"

should read 147,100

Signed and sealed this 16th day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

Claims (1)

1. 7. A STAINLESS STEEL HAVING A MARTENSITIC MICROSTRUCTURE WITH LESS THAN 5% DELTA FERRITE AND LESS THAN ABOUT 5% AUTENITE AND CHARACTERIZED BY A YIELD STRENGTH OF AT LENGTH 100,000 POUNDS PER SQUARE INCH, SAID STEEL CONSISTING ESSENTIALLY OF 13% TO 15% CHROMIUM, 4% TO 6% NICKEL, CARBON UP TO 0.12%, NITROGEN IN AN AMOUNT UP TO ABOUT 0.1%, THE SUM OF THE CARBON PLUS NITROGEN NOT EXCEEDING ABOUT 0.13%, UP TO ABOUT 1.5% MOLYBDENUM, UP TO ABOUT 1% MANGANESE, UP TO ABUT 1% SILICON, UP TO ABOUT 0.15% ALUMINUM, AND THE BALANCE ESSTENTIALLY IRON.