[1]United States Patent Office 3@366,472 3,366,472 STAIN'LESS STE4 EL Harry Tanczyn, Baltimore, and Paul A. Jennings, Boldw'n, Md., assignors to Armeo Steel Corporation, Middietown, Ohio, a corporation of Ohio No Drawi-ti,-. Cont-nuation of application Ser. No. 589,583, June 6, 1956. TL@is application Dec. 31, 1963, Ser. No. 334,925 12 Claims. (Cl. 75-128) ABSTRACT OF THE DISCLOSURE Heat-hardenable austenitic chromium - nickel - manganesevanadium-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 man-anese 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 copendin.- application Ser. No 589,583 filed June 6, 1956 and entitled Stainless Steel, now abandoned, and the invention relates to the fully au ic stainless stee an particularly concerns such stee s Nvhich may be har ened by heat-treating m--thods. An object of our invention is the provision of a stablyaustenit;c stainless steel which readily lends itself to heathardening at low temperatures, and as well, to a method fGr hardenin.- the same which is simple, direct and thorou,@bly practical and effective. Another object is to provide a fully austenitic stainless steel which is heat-hardenable and which, in hardened condition, displays advanta,-eous high strength qualities under sistained duty at either room temperature or at elevated temperature, the metal, both in thepre-hardened and in hardened condi'@ion, being noiimagnetic, capable of being readily welded and substantially free of detrimental and insoluble oxides and nitrides. Another object is to provide a heat-bardenable austenitic stainless steel prc-duct which can be readily fabricated while in prehardened condition, and subsequently hardened @@n ready and direct mailner so as to lend itself tO prolon.aed duty with retained high stren.-th when subjected to wide temperature variance from one part of the product to the oth.-r. Other objects and advantages of our invention in part will be obvious atid in part more fully pointed out during the col-irse of the description which follows. Accordingly, our invention resides in the combination of elements, composition of materials, and in the combination bf operational steps and the relation of each of the same to one or rnore of the others as described during the course of the followin.- disclosure, the scope of the application of all of wbich 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 inost of the austenitic stainless steels which are hardened by heat-treating meihc>ds 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 cbromiumnickel-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 chromium@nickel stainless steel which, after fabrication in relatively soft co-@idition, can tbereupo-@i be hardened at low temperature in simple, direct and certain manner, the resulting product displaying esPatented Jan. 30, 1968 2 sentially a martensitic structure with bardening copp@-r cast down in finely dispersed phase. The 0.2% yield stren.-th is high, amoiinting to at least about 180,000 p.s.i. Ustially, the unstable austenitic steels which have been evolved in an attempt to answer the requirements of the art are of the low-carbon type, wberein 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 io more ductile metal while in the pre-hardened condition. Reliance is placed upon other alloy additions to bring abotit requisite hardening. There are some stainless steels, however, which are _fully austenitic and yet which are hardenable by heat15 treatin, 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. Anoilier is the aluminum-bearing austenitic chromiumnickel stainless steel of Letters Patent No. 2,523,917 is20 sued to Peter Payson on Sept. 26, 1950. In these steels the hardening agents precipitate out of the austenite' The extent of the bardening 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 precipitation25 hardened steel. It was found, however, that when the steels of the prior art were subj-.cted to the exacting conditions of combined high temperattire and room temperature applications they failed to satisfy the requirements of the industry. The un30 stably austenitic steels did not possess satisfactory high temperature properties. And the presence of titanium, used as an important ingredient of the fully austenitic precipitationhardenable metals, presented important and serious disadvantages. In pouring the,metal a thick scum 35 formed, with the result that dirty metal was had. Insoluble oxides and nitrides erratically displayed themselves throughout the metal. Dirty r@letal also was had with the aluminum addition. Moreover, whea subjected to thermal stressing the age-hardened metal was observed to display 40 unsatisfactory stren.-th characteristics. An important object of our invenlion, 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 heat45 hardening the same which itself is simple and direct, which steel is clean, is readily worked or formed in the prehardened condition, which is weldable with ease, and which displays high strength properties at bgth room and elevated temperatures. 50 In accordance with the practice of our invention, we provide a steel containing chromium to.-ether with sufficient nickel and/or man-anese to give an austenitic strueture. In our steel we employ a substantial quantity of both vanadium arid carbon. The carbon serves as an 55 austenite-former in the pre-hardened condition of the metal, the vanadium and carbon in particular critical amounts combine upon heat-hardening to form a hardeiiing carbide distributed throughout the austenite matrix. 60 In short, we provicie a fully austenitic staiiless steel containin.- chromium iii the anount of about 12% to 30% and high iti either man-anese or riickel, or both. Elustratively, P-ickel ranges from 0.01% to about 7% which iq generally similar manner the manganese content ranges 65 from about 0.01% to about 16.00%. Actually, we prefer to have both these elements present so as to take advantage of the specific properties of each of them. Ai,.d broadl@, the sum total of the combined maiganese a,-id nickel conlerts at lee,.st about 6% on up. No more nickel 70 is employed than necessary, however, becati-.e we find that the steel has a hi,-.%er solubility for carbides with the lower nickel content; nickel apparently preventiiig car-
[2]32366)472 3 bide solubility. Usually the nickel content of our steel ranges from about 5% to about 6%, while the manganese ranges fror@i about 4% to abotit 6%. Ordinarily, the carbon content of out steel ran-es from about 0.20% to about 1.50%. Preferably, th, more lim5 ited range of 0.30% to about 0.45% is employed for most steels, and even 0.20% to 0.50% for wrought products, althou-,h for castings we use a carbon cont,-nt of 0.80% or more. The va-,3adium content ran.-es from abolit 0.50% to about 2.00%, pre.Lerably within the more limited 10 ran.-e of about 0.70% to about 1.00%. Our steel responds to the broad compositio-ii range: Carbon 0.20% to 1.50%, mapganese 0.01% to 16.00%, chromium 12.00% to 30.00%, nickel 0.01% to 7.00%, with the sum of the rickel and manganese at least about 15 6.00%, vanad,.um 0.50% to 2.00%, nitro,-en 0.15% to 0.75%, phosphorus 0.050% maximur@i, s-olphur 0.35% maxi@num, silicon 1.25% maximum, and reniainder substantially all iron. The percentage figures are by wei.-ht. We find that our steel is almost fully non-magneticIt 20 can be hardened by heat-harde-@iing w-ethods, low temperature heat-treating with precipitation of a vanadium-carbon compound. Or it can be hardened at least to a certain e.-- @ nt by cold-working, particularlv when shaping the metal LC into products of intricate configuration. It is hot-workable2,5 Such alloy displqys hi.-h yield strength at room teriii)erature. This same steel also exhibits attractive mechar@ical strength properties when subjected fo duty at elevated tempera'tures. And @.t may be welded,@vith case. It is to be noted that the steel of our invention requires 0 the use of only small quantities -.f elements such as iiickel which are strategically important. And a further feattire of our steel is that titanium is or@iitted. For we find t.@at titanium and aluminum, elements heretofore freqtiently employed in a.al.-hardenable steels, tend toward the pro35 ductio-@l of a dirty metal. The titanium and aluminum, as noled above, readily form oxides and nitrides which a-e ine;ol,ble within the m,@lt, and remain as impurities. When the metal is worked from the billet these impurities, occurring at random vilhin the metal, serve as points of un40 predictable mechanical weakness. The steel of our invention is clean; we iind that any vanadium nitrides which are formed are readily soluble and go into the metal while the latter is in its molten conditio@-i. A preferred and more narrow ran.-e of analysis accord- 45 ing to the Dractices of our invention is as follows: Carbon 0.30% to 0.45%, manganese 4.00% to 6.00%, ch.romium 14.00% to 17.00%, nickel 5.00% to 6.00%, vanadium 0.70% to 1.00%, nitrogen 0.15% to 0.25%, phosphorus 0.050% maximum, sulphur 0.050% maximlm, sili- 50 con 0.70% maximum, and remainder subs'lantially all iron. In a certain sense, our invention lies in the discovery that vihen presenl in critical percenta,-e, carbon and vaiiadium coact to render the auSteDitiC composition harden55 able by heat-treatment, with resultin.- fabricated articles dis-olayin,-, fohowin- hardenin,-, high mechanical stren@ths at room temperature and at elevated temperatures. A further advanta,-e of our invention is that the ingredients of our steel are so compatible with each other 60 as to lend themselves to the ready introduction of certain other elements to acheive certain specific objectives. Illustratively, copper may be included for improved resistance to corrosion, columbium for better creep, and molybSample c Mn * ------------ .40 8.09 * ------------- .40 8.48 c ------------- .43 8.66 D ------------- .42 8.42 E ------------- .39 6.27 F ------------- .35 4.70 G ------------- .33 4.79 4 denum and tungsten for improved stress-rupture strength upder elevated temperature dlity. Thus molybdentim may be employed in amounts up to abotit 4.00%, tungsten in amolints Lip to 4.00%, columbium in amounts up to 1.50%, and copper in amounts up to 4.00%. Preferably where o.,ie or more of these ingredients is employed, the rrolybdenum is employed in the amount of 2.00% to 3.00%, tungsten in the amount of 2.00% to 3.00%, columbium in the amour-t of 0.50% to 1.00%, and copper in the amount of 2.00% to 3.00%. Where desired, boron i-.i an amount up to about 0.005% may be added to improve the hot-workability of the steel. Most of the benefits of the narrow preferred ran.-e of our steel are had in the broader preferred range, with additional in.-redie-@its, of carbon about 0.30% to 0.45%, man.- arese about 4.00% to 9.00%, chromium about 12.00% to 20.00%, iiickel about 4.00% to 7.00%, vanadium abolit 0.70% to 1.00%, nitrogen about.0.15% to 0.25%, tun.-sten up to about 3.0%, molybderum up to about 2.0%, copper up to abolit 3.0%, and r6mainder iron. To put the steel of our invention to practicall use, it is heated at a temperature of about 2000' F. for such t@me, usually in the neighborhood of approximately one-half hour, as is required to form a solid solution. Tlle bardening elements vanadium and carbon are held in this solid solution. The steel is thereupon qii,,nched in water, down to room temperature. The metal is relatively soft and dlictile and can be readily fasbioned into desired intricate shapes. Illustratively, the steel t'@ius produced may be wor'zzed into turbine discs and rotors. Once proper configuratio-@i and dimension is imparted to the articles fashioned of our steel, these articles are heat-hardened as by subjectin.- them to prolon,,ied low heat treatment in the neighborhood of 1300' F. Although we do not care to be bound by the explanation, we feel that the combination of critical composition and heattrea:tment effect precipitation of the vanadium and carbon as vanadium-rich carbides throughout the steel, this in finely dispersed form. Ard we feel that the extent of the precipitation in substantial measure derives from the large amount of manganese and small amount of nickel employed, manganese apparently fosterin-. and nickel suppressin.- carbide solubility, as stiggested above. In a matter of explanatiop, the severe requirements demanded of such articles as turbi@le discs or rotors when placed in service are satisfied with the steel of our invention. As the discs and rot6rs come up to speed, and thereupon up to the temperature encountered in such service, high mechanical stresses are successfully withstood. Although the turbine blades heat up quickly, rising to about 1500' F the rbtor heats :up much more slowly, remaining at but"little more than room temperature at.the hub. Our steel displays high strength properties at both room temperatures and at elevated temperatures, as well. Moreover, the steel is resistant to the thermal stresses and strains set up by the severe temperature gradients in the turbine discs or rotors. We also find that the steel is quite suitable for jet engi-.ie parts, iings, bolts and vanes. - And even in heavy sections, the steel is readily weldable. As iuustrative of the mechanical properties of our steel, tests were conducted on specimen compositions shown in the following table: TABLEI Cr Ni v N MO w Cu 12.41 4.15 ---------- .20 .99 ---------- ---------- 12.65 4.13 .92 .18 ---------- ---------- ---------- 15.23 6.14 .97 .24 1.89 ---------- 2.88 19.65 6.61 .95 .19 2.02 ----- 16.00 6.00 1.00 .21 ---------- 16.10 6.09 1.01 .18 1.60 .60 2.62 16.17 5.94 1.01 .20 1.52 1.58 -------
[3]3,366,472 5 Sample A comprised an austenitic stainless steel with mangan ese and nickel but with no vanadium, this for ready comparison. The steels E, F and G respond to the preferre d range of composition of our steel set forth above. We conditioned the steels of Table I for testing by sub- 5 jectin.- them to solution heat-treatment at about 2050' F. for a period of about one-half hour, followed by waterquenchin .- to room temperature. Thereupon we subjected the steels +o prolonged heat-hardening at comparatively low temperatures, this for sufficient time to bring about the 10 required p.-ecipitation in dispersed phase through the metal of the hardenin,a additive. In the particular test specimens the steel was held for ten hours at 1300' F. followed by waterquenchin,-. An annealing fumace was employed for this low-temperature hardening treatment. The advanta- 15 geous mechanical properties characteristic of our steel are strikinaly disclosed in the following Table ][I: The important enhancement of mechanical properties had with our steel (Samples B through G) as contrasted with the prior ar-t specimen A is strikingly disclosed in Table H. Important increase in hardness is observed upon a.-e-hardening treatment. Illustratively, specimens B 35 throu,-h G, in a solution-treated, pre-hardened condition, have Rockwell Hardness of B95 to C30, while, following heat-hardening, the same steels display a Rockwell Hardness ran.-e of C30-50. The valuable high temperature characteristics of the 40 steels of Table I in the solution-treated and heat-hardened condition as hereinbefore described, are disclosed in Table III: TABLE II:I.-STRESS RUPTURE PROPERTIES 45 6 for this test. The respective results of the 1000-hr. life test are seen to be 43,OGO p.s.i., 41,500 p.s.i. and 43,000 p.s.i. At 13501 F. the single sample tested (Sample E) revealed good stress rupture properties. Thus it will be seen that we provide in our invention a steel which enjoys the many desirable qualities and advantages set forth above. It is clean and free of insoluble oxides and nitrides. Moreover, it is non-ma,anetic. Our steel, while readily worked and formed in the unhardened condition, readily lends itself to hardening by simple low-temperature and heat-treatment. This hardenin- is certain and predictable, whether the pre-hardened steel be in cast, welded or wrou.-ht 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, stren.-th which, as is evident from consideration of the results of Tables 11 aind 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 resistapce to corrosion can be achieved, better creep obtained, and even higher stress rupture strength u-,lder prolonged high temperature duty with one or more of these additives. All of the valuable qualities noted are obtained throu.ah the use of but smar quantities of elements having strate.@ic importance. As coneems the savinas of strate-ic materials, * -------------- 120,000 40,000 ---------- 5.0 10.0 --------- - ---------- * -------------- 154,300 107,100 94,000 26.5 27.8 --------- - 12 * -------------- 155,200 108,800 96,000 20.5 32.4 --------- - 16 * -------------- 166,000 132,000 112,000 21.0 28.0 --------- - ---------- * -------------- 161,000 128,000 99,000 22.0 33.0 C36 19 * -------------- 152,300 98,900 71,800 22.0 38.0 C35 24 * -------------- 146,700 91,300 67,500 26.0 43.5 C34 35 Sample Temperature Loads for 100-Hr. Fracture (p.s.i.) it wiU be seen that the hi.-h temperature strengths are obF.) Life 1,000-Hr. Life tained with the use of a minimum of nickel; that nickel in amount less than man-anese actually is preferred. B --------- 1,200 48, OW 34,500 Whe intend the fore.-oing description to be considered c --------- 1,200 58,000 43,000 as purely illustrative inasmuch as many modifications of D -------- 1,200 62,000 47,000 5 0 * --------- 1,200 60,000 43,000 the disclosed embodiments will su-gest themselves to * --------- 1,200 58,000 43,000 * -------- 1,200 58,000 41,500 those skilled in the art to which the invention relates. * --- 1,350 32,000 21,000 We
