US3366472A - Stainless steel - Google Patents

Description

United States Patent 3,366,472 STAINLESS STEEL Harry Tanczyn, Baltimore, and Paul A. Jennings, Baldwin, Md, assignors to Armco Steel Corporation, Middietown, Ohio, a corporation of Ohio No Drawing. Continuation of application Ser. No. 589,583, June 6, 1956. This application Dec. 31, 1963, Ser. No. 334,925

12 Claims. (Cl. 75128) ABSTRACT OF THE DISCLUSURE Heat-hardenable austenitic chromium nickel manganese-vanadium-carbon-nitrogen stainless steel. More particularly, stainless steel consisting essentially of chromium about 12% to 30%, nickel about 0.01% to 7%, manganese about 4% to 16%, with the sum of the nickel and manganese about 6% or more, vanadium about 0.50% to 2.00%, carbon about 0.20% to 1.50%, nitrogen .15% to about .75%, and remainder substantially all iron.

This application for patent is a continuation of our copending application Ser. No 589,583 filed June 6, 1956 and entitled Stainless Steel, now abandoned, and the invention relates to the fully austenitic stainless steels, and particularly concerns such steels which may be hardened by heat-treating methods.

An object of our invention is the provision of a stably austenitic stainless steel which readily lends itself to heathardening at low temperatures, and as well, to a method for hardening the same which is simple, direct and thoroughly practical and elfective.

Another object is to provide a fully austenitic stainless steel which is heat-hardenable and which, in hardened condition, displays advantageous high strength qualities under sustained duty at either room temperature or at elevated temperature, the metal, both in the pre-hardened and in hardened condition, being non-magnetic, capable of being readily welded and substantially free of detrimental and insoluble oxides and nitrides.

Another object is to provide a heat-hardenable austenitic stainless steel product which can be readily fabricated while in pre-hardened condition, and subsequently hardened in ready and direct manner so as to lend itself to prolonged duty with retained high strength when subjected to wide temperature variance from one part of the product to the other.

Other objects and advantages of our invention in part will be obvious and in part more fully pointed out during the course of the description which follows.

Accordingly, our invention resides in the combination of elements, composition of materials, and in the combination of operational steps and the relation of each of the same to one or more of the others as described during the course of the following disclosure, the scope of the application of all of which is more fully set forth in the claims at the end of this specification.

So that a more thorough understanding of certain features of our invention may be had, it is to be noted at this point that most of the austenitic stainless steels which are hardened by heat-treating methods rely on a structure which is unstably austenitic and in which a hardening element is precipitated with the grains of the martensite formed in heat-treating. Typical of these is the chromiumnickel-copper steel of Letters Patent No. 2,482,096 issued to William C. Clarke, Jr. on Sept. 20, 1949, for Alloy and Method. In that patent there is disclosed an age-hardenable chromiumfnickel stainless steel which, after fabrication in relatively soft condition, can thereupon be hardened at loW temperature in simple, direct and certain manner, the resulting product displaying es- 3,365,472 Patented Jan. 30, 1968 sentially a martensitic structure with hardening copper cast down in finely dispersed phase. The 0.2% yield strength is high, amounting to at least about 180,000 p.s.i.

Usually, the unstable austenitic steels which have been evolved in an attempt to answer the requirements of the art are of the low-carbon type, wherein carbon content ranges between 0.03% to aout 0.20%. The reasoning behind the low-carbon content is that, since carbon is a hardening element, its omission will result in a softer and more ductile metal while in the pre-hardened condition. Reliance is placed upon other alloy additions to bring about requisite hardening.

There are some stainless steels, however, which are fully austenitic and yet which are hardenable by heattreating methods. Among these is the titanium-bearing austenitic chromium-nickel stainless steel of Letters Patent No. 2,641,540 issued to Gunther Mohling on June 9, 1953. Another is the aluminum-bearing austenitic chromiumnickel stainless steel of Letters Patent No. 2,523,917 issued to Peter Payson on Sept. 26, 1950. In these steels the hardening agents precipitate out of the austenite. The extent of the hardening had gives 0.2% yield strengths on the order of 90,000 to 100,000 p.s.i., figures much less than those had with the unstable austenitic precipitationhardened steel.

It was found, however, that when the steels of the prior art were subjected to the exacting conditions of combined high temperature and room temperature applications they failed to satisfy the requirements of the industry. The unstably austenitic steels did not possess satisfactory high temperature properties. And the presence of titanium, used as an important ingredient of the fully austenitic precipitation-hardenable metals, presented important and serious disadvantages. In pouring themetal a thick scum formed, with the result that dirty metal was had. Insoluble oxides and nitrides erratically displayed themselves throughout the metal. Dirty metal also was had with the aluminum addition. Moreover, when subjected to thermal stressing the age-hardened metal was observed to display unsatisfactory strength characteristics.

An important object of our invention, therefore, is to provide a heat-hardenable, fully austenitic stainless steel, employing only a minimum of strategically important components, and as well, to provide a method of heathardening the same which itself is simple and direct, which steel is clean, is readily Worked or formed in the pre-hardened condition, which is weldable with ease, and which displays high strength properties at both room and elevated temperatures.

In accordance with the practice of our invention, we provide a steel containing chromium together with sufii cient nickel and/ or manganese to give an austenitic structure. In our steel we employ a substantial quantity of both vanadium and carbon. The carbon serves as an austenite-former in the pre-hardened condition of the metal, while the vanadium and carbon in particular critical amounts combine upon heat-hardening to form a hardening carbide distributed throughout the austenite matrix.

In short, we provide a fully austenitic stainless steel containing chromium in the amount of about 12% to 30% and high in either manganese or nickel, or both. Illustratively, nickel ranges from 0.01% to about 7% which in 7 generally similar manner the manganese content ranges 3 bide solubility. Usually the nickel content of our steel ranges from about 5% to about 6%, while the manganese ranges from about 4% to about 6%.

Ordinarily, the carbon content of our steel ranges from denum and tungsten for improved stress-rupture strength under elevated temperature duty. Thus molybdenum may be employed in amounts up to about 4.00%, tungsten in amounts up to 4.00%, columbium in amounts. up to about 0.20% to about 1.50%. Preferably, the more lim- 5 1.50%, and copper in amounts up to 4.00%. Preferably ited range of 0.30% to about 0.45% is employed for where one or more of these ingredients is employed, the most steels, and even 0.20% to 0.50% for Wrought prodmolybdenum is employed in the amount of 2.00% to ucts, although for castings we use a carbon content of 3.00%, tungsten in the amount of 2.00% to 3.00%, 0.80% or more. The vanadium content ranges from about columbium in the amount of 0.50% to 1.00%, and cop- 0.50% to about 2.00%,preferably within the more limited 10 per in the amount of 2.00% to 3.00%. Where desired, range of about 0.70% to about 1.00%. boron in an amount up to about 0.005% may be added Our steel responds to the broad composition range: Carto improve the hot-workability of the steel. bon 0.20% to 1.50%, manganese 0.01% to 16.00%, Most of the benefits of the narrow preferred range of chromium 12.00% to 30.00%, nickel 0.01% to 7.00%, our steel are had in the broader preferred range, with adwith the sum of the nickel and manganese at least about ditional ingredients, of carbon about 0.30% to 0.45%, 6.00%, vanadium 0.50% to 2.00%, nitrogen 0.15% to manganese about 4.00% to 9.00%, chromium about 0.75%, phosphorus 0.050% maximum, sulphur 0.35% 12.00% to 20.00%, nickel about 4.00% to 7.00%, vanadimaximum, silicon 1.25% maximum, and remainder subum about 0.70% to 1.00%, nitrogen about.0.l5% to stantially all iron. The percentage figures are by weight. 0.25%, tungsten up to about 3.0%, molybdenum up to We find that our steel is almost fully non-magnetic. It about 2.0%, copper up to about 3.0%, and remainder can be hardened by heat-hardening methods, low temperairon. ture heat-treating with precipitation of a vanadium-carbon To put the steel of our invention to practical use, it is compound. Or it can be hardened at least to a certain exheated at a temperature of about 2000 F. for such time, tent by cold-working, particularly when shaping the metal usually in the neighborhood of approximately one-half into products of intricate configuration. It is hot-workable. hour, as is required to form a solid solution. The harden- Such alloy displays high yield strength at room temperaing elements vanadium and carbon are held in this solid ture. This same steel also exhibits attractive mechanical solution. The steel is thereupon quenched in Water, down strength properties when subjected to duty at elevated temto room temperature. The metal is relatively soft and ducperatures, And it may be Welded with ease. tile and can be readily fashioned into desired intricate It is to be noted that the steel of our invention requires shapes. lllustratively, the steel thus produced may be the use of only small quantities of elements such as nickel worked into turbine discs and rotors. which are strategically important. And a further feature Once proper configuration and dimension is imparted of our steel is that titanium is omitted. For we find that to the articles fashioned of our steel, these articles are titanium and aluminum, elements heretofore frequently heat-hardened as by subjecting them to prolongedlow employed in age-hardenable steels, tend toward the pro heat treatment in the neighborhood of 1300 F. Although duction of a dirty metal. The titanium and aluminum, as we do not care to be bound by the explanation, We feel noted above, readily form oxides and nitrides which are that the combination of critical composition and heatinsoluble within the melt, and remain as impurities. When treatment effect precipitation of the vanadium and carbon the metal is worked from the billet these impurities, ocas vanadium-rich carbides throughout the steel, this in curring at random within the metal, serve as points of unfinely dispersed form. And we feel that the extent of the predictable mechanical Weakness. The steel of our invenprecipitation in substantial measure derives from the large tion is clean; we find that any vanadium nitrides which are amount of manganese and small amount of nickel emformed are readily soluble and go into the metal while 'ployed, manganese apparently fostering and nickel supthe latter is in its molten condition. pressing carbide solubility, as suggested above.

A preferred and more narrow range of analysis accord- In a matter of explanation, the severe requirements deing to the practices of our invention is as f llows: Carmanded of such articles as turbine discs or rotors when" boil 030% to 045%, manganese 400% to 600%, I placed in service are satisfied with the steel of our inl 14-00% to nickel 500% to 600%, Vana' vention. As the discs and rotors come up to speed, and dull 070% to Q mtmgen 015% to 9? thereupon up to the temperature encountered in such'serv- Phorus 0-O50% mfxlmumr Su1PhurQ-O50% maxlmflm ice, high mechanical stresses are successfully withstood. con 0.70% maximum, and remainder substantially all Although the turbine bl d h t up quickly, rising to Iron h about 1500 F., the rotor heats up much more slowly, reg g gg gg ii r ggi ggggg f gi gg a 53 3;: maining at but little more than room temperature at the b .1 hub. Our steel displays high strength properties at both rum coact to render the austenitic composition harden- 55 t n d t 1 t d t t 11 able by heat-treatment, with resulting fabricated articles room tempera mes i e eva e empera as we displaying, following hardening high mechanical stwngths Moreover, the steel is reslstant to the thermal stresses and at room temperature and at elevated temperatures. stralfls up by the Severe mperature gradients in the A further advantage of our invention is that the turbine discs or rotors. We also find that the steel is quite gredients of our steel are so compatible with each other sultable for l englile P Hugs, bolts and n -A as to lend themselves to the ready introduction of cereven 1H heavy sectlons, the Steel 15 y Weldabletain other elements to acheive certain specific objectives. AS illustrative of the mechanical Properties of our Steel, Illustratively, copper may be included for improved resistests were conducted on specimen compositions shown in ance to corrosion, columbium for better creep, and molybthe following table:

TABLE I 0 Mn Cr Ni V I N I Mo Sample A comprised an austenitic stainless steel with manganese and nickel but with no vanadium, this for ready comparison. The steels E, F and G respond to the preferred range of composition of our steel set forth above.

We conditioned the steels of Table I for testing by subjecting them to solution heat-treatment at about 2050 F. for a period of about one-half hour, followed by waterquenching to room temperature. Thereupon we subjected the steels to prolonged heat-hardening at comparatively low temperatures, this for sufiicient time to bring about the required precipitation in dispersed phase through the metal of the hardening additive. In the particular test specimens the steel was held for ten hours at 1300 F. followed by water-quenching. An annealing furnace was employed for this low-temperature hardening treatment. The advantageous mechanical properties characteristic of our steel are strikingly disclosed in the following Table H:

TABLE II.MECHANICAL PROPERTIES condition, readily lends itself to hardening by simple low-temperature and heat-treatment. This hardening is certain and predictable, whether the pre-hardened steel be in cast, welded or wrought condition and substantially regardless of the intricacy or delicacy of the fabrication imparted thereto.

Our steel displays, in hardened condition, high mechanical strength at room temperatures, strength which, as is TABLE IIL-STRESS RUPIURE PROPERTIES Sample Temperature Loads for 100111. Fracture (p.s.i.)

( F.) Lite 1,000-Hr. Life B- 1,200 48,000 34,500 0 1, 200 58, 000 43, 000 D 1, 200 62, 000 47, 000 1, 200 000 43, 000 l, 200 58, 000 43, 000 1, 200 58, 000 41, 500 1, 350 32, 000 21, 000 1, 350 33, 000 22, 000 1, 350 33, 000 22, 000

From the data given in Table 111 it is readily apparent that our steel in the solution-treated and heat-hardened condition possesses excellent properties up to 1200 F. and more. The steel of Sample B (austenitic chromiumnickel-manganese steel of substantial vanadium and carbon contents (sustains a load of 48,000 p.s.i. for 100 hours at 1200 F. and 34,500 p.s.i. for 1000 hours. Samples C and D, similar steels containing molybdenum with or without copper, exhibit substantially better stress rupture properties, namely 58,000 to 62,000 for the 100-hr. life test and 43,000 to 47,000 p.s.i. for the 1000-hr. test. At even the higher temperature of 1350 F., good stress rupture values are had for the steels containing molybdenum with or without the copper.

With the steels which additionally contain tungsten (Sample E) and the further ingredient molybdenum (Sample G) and copper with the molybdenum and tungsten (Sample F) uniformly good stress rupture values are had at 1200 F. The former steel sustains a load of 60,000 p.s.i. for 100 hours while the latter sustains 58,000 p.s.i.

evident from consideration of the results of Tables II and III, is substantially retained at temperatures ranging as high as 1350 F. or more. The metal is hot-workable. And important ductility is observed at room temperatures. Moreover, the fundamental composition of our steel is fully compatible with the inclusions therein of certain other ingredients for specific purposes, particularly one or more of molybdenum, tungsten, columbium and copper. Illustratively, increased resistance to corrosion can be achieved, better creep obtained, and even higher stress rupture strength under prolonged high temperature duty with one or more of these additives.

All of the valuable qualities noted are obtained through the use of but small quantities of elements having strategic importance. As concerns the savings of strategic materials, it will be seen that the high temperature strengths are obtained with the use of a minimum of nickel; that nickel in amount less than manganese actually is preferred.

Whe intend the foregoing description to be considered as purely illustrative inasmuch as many modifications of the disclosed embodiments will suggest themselves to those skilled in the art to which the invention relates.

We claim as our invention:

1. A heat-hardenable austenitic stainless steel consisting essentially of chromium about 12% to 30%, nickel about 0.01% to 7%, manganese about 4% to 16%, with the sum of the nickel and manganese about 6% or more, vanadium about 0.50% to 2.00%, carbon about 0.20% to 1.50%, molybdenum up to about 4.00%, tungsten up to about 4.00%, columbium up to about 1.50%, copper up to about 4.00%, nitrogen .15 to about 0.75%, boron up to about 0.005%, and remainder substantially all iron.

2. A heat-hardenable austenitic stainless steel consisting essentially of chromium about 14% to 17%, nickel about 5% to 6%, manganese about 4% to 6%, vanadium about 0.70% to 1.00%, carbon about 0.30% to 0.45%, nitrogen about 0.15% to 0.25%, and remainder substantially all iron.

3. A heat-hardenable austenitic stainless steel consisting essentially of chromium about 12% to 20%, nickel about 4% to 7%, manganese about 4% to 9%, vanadium about 0.7% to 1.00, carbon about 0.30% to 0.45%, nitrogen about 0.15% to 0.25%, tungsten up to about 3.0%, molybdenum up to about 2.0%, copper up to about 3.0%, and remainder iron.

' 4. A heat-hardenable austenitic stainless steel consisting essentially of chromium about 12% to 30%, nickel about 4% to 7%, manganese about 4% to 16%, with the sum of the nickel and manganese about 6% or more, vanadium about 0.50% to 2.00%, carbon about 0.20% to 1.50%, at least one ingredient of the group consisting of molybdenum about 2.00% to 3.00%, tungsten about 2.00% to 3.00%, columbium about 0.50% to 1.00%, and copper about 2.00% to 3.00%, together with nitrogen .15% to about 0.75%, and remainder substantially all iron.

5. A heat-hardened austenitic stainless steel essentially consisting of carbon about 0.20% to 1.50%, chromium about 12% to 30%, manganese about 4% to 16.00%, nickel about 0.01% to 7%, With the sum of the nickel and manganese being about 6% or more, vanadium about 0.50% to 2.00%, phosphorus about 0.050% maximum, sulphur about 0.35% maximum, silicon about 1.25% maximum, nitrogen .15% to .75 and the remainder substantially all iron.

6. A heat-hardened austenitic stainless steel essentially consisting of chromium about 12% to 20%, nickel about 4% to 7%, manganese about 4% to 9%, vanadium about 0.7% to 1.00%, carbon about 0.30% to 0.45%, nitrogen about 0.15% to 0.25%, tungsten up to about 3.0%, molybdenum up to about 2.0%, copper up to about 3.0%, and the remainder substantially all iron.

7. Heat-hardened, non-magnetic austenitic stainless steel parts for jet engines and the like such as rings, bolts, vanes, discs and rotors essentially consisting of carbon 0.30% to 0.45%, manganese about 4.00% to 6.00%, chromium about 14.00% to 17.00%, nickel about 5.00% to 6.00%, vanadium about .70% to 1.00%, nitrogen about 0.15% to 0.25%, phosphorus about 0.050% maximum, silicon about .70% maximum, and the remainder sub stantially all iron.

8. A heat-hardenable austenitic stainless steel consisting essentially of chromium about 12% to 30%, nickel about 0.01% to 7%, manganese about 4% to 16%, with the sum of the nickel and manganese about 6% or more, vanadium about 0.50% to 2.00%, carbon about 0.20%

to 1.50%, nitrogen .15% to about .75%, and remainder substantially all iron.

9. A heat-hardenable austenitic stainless steel consisting essentially of chromium about 12% to 20%, nickel about 4% to 7%, manganese about 4% to 9%, vanadium about .7% to 1.0, carbon about .20% to 50%, nitrogen about 0.15 to 0.25 and remainder substantially all iron.

10. A heat-hardenable austenitic stainless steel consisting essentially of chromium about 12% to nickel about 0.01% to 7%, manganese about 4% to 16%, with the sum of the nickel and manganese about 6% or more, vanadium about 0.50% to 2.00%, carbon about .80% to 1.50%, nitrogen .15% to about .75%, and remainder substantially all iron.

11. A heat-hardenable austenitic stainless sttel consisting essentially of chromium about 12% to 30%, nickel about 0.01% to 7%, manganese about 4% to 16%, with the sum of the nickel and manganese about 6% or more, vanadium about 0.50% to 2.00%, carbon about .20% to .50%, nitrogen .15 to about .75%, and remainder substantially all iron.

12. A heat hardenable austenitic steel consisting essentially of, by Weight, carbon about .4 to about 1%, manganese about 8 to about 16%, chromium about 20 to about 25%, nickel about 1.5 to about 6%, vanadium about .50 to about 2%, nitrogen about .15 to about .4%, balance essentially iron.

References Cited UNITED STATES PATENTS HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner. P. WEINSTEIN, Assistant Examiner.