US3437478A - Free-machining austenitic stainless steels - Google Patents

Free-machining austenitic stainless steels Download PDF

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US3437478A
US3437478A US455863A US3437478DA US3437478A US 3437478 A US3437478 A US 3437478A US 455863 A US455863 A US 455863A US 3437478D A US3437478D A US 3437478DA US 3437478 A US3437478 A US 3437478A
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sulfur
manganese
steels
machinability
percent
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Arthur Moskowitz
Curtis W Kovach
Ralph G Wells
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Crucible Steel Company of America
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Crucible Steel Company of America
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

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  • This invention relates to chromium-nickel austenitic stainless steels having improved physical properties, most notably, significantly improved machinability.
  • steels of this type containing up to 0.50% carbon, from about 0.25 to about 0.45% sulfur, from about 2.0 to about 7.0% manganese, from about 16 to about 30% chromium, from about 5 to 26% nickel, and wherein the manganese-to-sulfur ratio is about at least 8 to 1, are vastly improved with respect to machinability without impairing the corrosion resistance of the alloy to the expected extent at the achieved level of machinability.
  • Optional elements that may be included in the alloy are silicon up to about 3%, molybdenum up to about 4%, zirconium up to about 1%, copper up to about 4%, selenium, tellurium or lead up to about 0.5% each, columbium, tantalum or titanium up to about 2.0% total, nitrogen up to about 0.35%, and phosphorus up to about 0.50%.
  • This invention relates to chromium-nickel austenitic stainless steels, and particularly to improved steels having enhanced free-machining properties.
  • the structure of the contemplated class of stainless steels is predominantly austenitic in ordinary temperatures, Such steels are non-magnetic and are not hardenable by heat treatment, but are hardenable by cold working. Work hardening results from strain hardening and transformation of the steel structure from the relatively softer austenite to a relatively harder martensitic phase during working. Thus, work hardening is a function of the stability of the austenite and the latter, in turn, is largely dependent upon the composition of the steel, i.e., a balancing of ferriteand austenite-promoting alloying elements.
  • the principal alloying element in all stainless steels and the one conferring the stainless property on the iron base is, of course, chromium.
  • This element is a strong ferritizer and, to offset the effect thereof in the austenitic stainless steels, the element nickel, which is an austenite promoter, is chiefly used to achieve the desired austenite structure and stability.
  • Other elements are also used for such structural balancing, as well as to confer other desired propertie on the steels.
  • Such elements are, for example, the ferritizers molybdenum and silicon, and the austenite promoters manganese, carbon, nitrogen and copper.
  • Manganese for example, has been used in substantial quantities, for example, about 5.5 to 10%, in some austenitic stainles steels, as AISI Types 201 and 202, as a partial substitute for the more expensive and scarcer element nickel, although in most manganesecontaining austenitic stainless steels, manganese is limited to a maximum of about 2%, and in actual practice is used in amounts substantially less than 2%.
  • the austenitic stainless steels are especially useful by reason of the wide range of mechanical properties which are obtainable by cold-working.
  • the austenitic stainless steels of leaner alloy content such as AISI Type United States Patent ()fi ice 3,437,478 Patented Apr. 8, 1969 301, the common 177 steel (17% chromium, 7% nickel), have highest work hardening rates because these steels, containing relatively small amounts of nickel, have an austenitic structure of lesser stability than others of the austenitic steels having a richer alloy content, for example, AISI Type 309 containing about 23% chromium and 12 to 15% nickel.
  • machinability is an important property in the application of austenitic stainless steels for many purposes. Elements such as sulfur, selenium, tellurium, lead and phosphorus have been added to certain austenitic stainless steels to improve machinability.
  • AISI Type 303 the common 188 stainless steel (AISI Type 302 containing about 18% chromium and 8% nickel together with a maximum of 2% manganese) to which has been added from about 0.15 to 0.35% sulfur.
  • sulfur is an effective additive to such steels for machinability enhancement, it also decreases the corrosion resistance of the steels and makes it difficult to obtain highest surface finishes. Consequently, it is desirable to use the least amount of sulfur compatible with the necessary machinability required for the end application for which the steel is intended.
  • a preferred embodiment of the invention comprises a free-machining austenitic stainless steel containing about 17 to 19% chromium, about 6.5 to 10% nickel, up to about .15% carbon, up to about 1% silicon, up to about .50% phosphorus, up to about .60% manganese or zirconium, and, in particular, about .30 to .40% sulfur, together with about 3 to 4.5% manganese.
  • FIGURE 1 is a graph relating the effect of sulfur content upon machinability of austenitic stainless steels
  • FIGURE 2 is a graph representing the effect, on a rectangular coordinate scale, of manganese-sulfur ratio upon machinability of austenitic stainless steels;
  • FIGURE 3 is a graph showing correlation between test and calculated drill machinability ratings
  • FIGURE 4 is a graph illustrating, on a semi-logarithmic scale, the FIGURE 2 relationship between manganese-sulfur ratio and machinability;
  • FIGURES 5A and 5B are photomicrographic illustrations of austenitic stainless steels which contain, respectively, desirably small sulfide inclusions, and harmfully large sulfide inclusions;
  • FIGURE 5 comprises graphs illustrating the detrimental effect, upon the drill machinability rating of austenitic stainless steels, of large sulfide inclusions which are formed at high sulfur levels in austenitic stainless steels containing different amounts of manganese;
  • FIGURE 6 is a ternary diagram graphically illustrating the effect of steel composition upon the appearance therein of large sulfide inclusions, and the relationship of steel composition and large sulfide content to machinability.
  • a first series of 36 experimental steel heats was prepared wherein the steel comprised a base composition of essentially 18-8 austenitic stainless steel, and wherein varying contents of manganese and sulfur were utilized.
  • the compositions of these experimental steel heats are 4 ing time and the test bar drilling time and multiplying by 100. Accordingly, test bars with good drill machinability showed a drilling time less than the standard and therefore have a drill machinability rating greater than 100.
  • the test drill machinability ratings so determined are set forth in Table I. 5 given, for the several test speclmens, in Table I.
  • the several test bars were then tested for machinability in the above condition, the test being in the form of a drill machinability test of the sample bars, to each of which a drill machinability rating was assigned by comparison of the observed drill machinability with that of a comparison standard bar comprising AISI Type 303 stainless steel in a similar heat treated condition and to which standard test bar a drill machinability rating of 100 was assigned.
  • the drill test was made in a direction perpendicular to the longitudinal axis of the test bar.
  • the drill used was a Cleveland Twist Drill No. 3197 high speed steel drill sharpened to a point With a 118 included angle.
  • a vertical drill press was utilized and operated at a uniform speed of 460 r.p.m.
  • Graph A of FIGURE 1 represents the effect of sulfur upon machinability, expressed as the difference between the actual test drill machinability rating and the drill machinability rating calculated for all factors except sulfur.
  • Graph B of FIGURE 2 represents the effect of the manganese-sulfur ratio upon machinability expressed as the difference between actual test drill machinability rating and the drill machinability rating calculated for all factors except the manganese-sulfur ratio.
  • Equation 2 based upon the f(S) and the (Mn/S) relationships illustrated in FIGURES l and 2, the constant K was found to have a value of 33, and the equation gave good correlation of calculated drill machinability rating with test drill machinability rating, as illustrated by Graph C of FIGURE 3.
  • FIGURE 2 shows that, whereas increasing the manganese-sulfur ratio up to values of about 4 or 5 to 1 results in a rapid and substantially uniform rate of improvement of machinability, the rate of improvement decreases at higher values. Best machinability is obtained at manganese-sulfur ratios over about 8 to 1 or 12 to 1-with little or no further improvement by the use of greater values.
  • FIGURE 4 The change in the effect of manganese-sulfur ratio upon machinability is more clearly seen in FIGURE 4 where these factors are plotted on a semi-logarithmic scale. From the latter figure, it is seen that a drastic change in the elfect of manganese-sulfur ratio upon drill machinability takes place at a manganese-sulfur ratio of about 8 to 1.
  • Graph D of FIGURE 4 represents the probable average relationship between manganese-sulfur ratios up to 8 to 1, and the actual vs. calculated drill machinability difference for the Table I steels. The observed scatter in the data is bounded within a scatter band defined by dotted line Graphs E and F of FIGURE 4.
  • Graph G of FIGURE 4 represents the probable average relationship between manganese-sulfur ratios over 8 to 1, and the difference between test and calculated drill machinability ratings of the Table I steels. The observed variation in this relationship is encompassed within the scatter band described by dotted line Graphs H and I of FIGURE 4.
  • the invention contemplates the provision of austenitic stainless steels containing sulfur as a freemachining additive, for example, in amounts from about .25 to about .40 or .45 preferably from about .25 to about .35 and wherein manganese is preferably added in sufficient quantity as to produce a minimum manganese to sulfur ratio of about 8 to 1, preferably about 10 to 1 all in order to realize the benefits 0f the relationship of the manganese-sulfur ratio upon machinability as illustrated in FIGURE 4.
  • the uniformly distributed dark particles of FIGURE 5A are sulfideinclusions having a mean maximum dimension of about 20 10'" inch or less.
  • FIG- URE 5B a photorni-crograph of a steel, Heat No.
  • test steel compositions were selected, the steels having manganese contents of about 4 to 5% (averaging 4.41% and wherein the sulfur content was varied between about .15% and about .45%.
  • the drill machinability ratings for these steels were determined in accordance with the aforesaid test procedure.
  • the compositions of such steels, together with the asquenched hardness and the observed drill machinability ratings thereof, are set forth in Table II.
  • Mn S Si Ni Cr Mo Cu (BHN) Rating An additional series of test steels were prepared, where- 15 in the steels had a manganese content of from about 5 to about 10%, averaging 7.27%. These additional steels were also tested for machinability as aforesaid, and the compositions, hardness and drill machinability ratings thereof are set forth in Table III.
  • Table IV The data of Table IV are graphically depicted by the ternary diagram of FIGURE 7, wherein the apices of the diagram represent, respectively, sulfur, manganese and the ironapproxiamtely 16 to 17% chromiumnickel base alloy.
  • One coordinate shows a sulfur content variation from O to about .80% and another shows manganese varying from to about 12%.
  • the nickel content of the alloys varies with the manganese content, as given in Table IV, nickel being lowered with increasing manganese content in order to maintain the desired austenite-ferrite balance.
  • test steel compositions A distinct separation of the test steel compositions is shown, by the thus-plotted data, as consisting of those compositions which contain large sulfide inclusions on the one hand, and on the other hand, those which do not contain large inclusions. This division is represented by Graph L of FIGURE 7; those compositions containing two or more large inclusions falling above Graph L and those containing fewer or no such inclusions falling therebelow.
  • a further graph, M, of FIGURE 7 is established by determining for steel compositions having various manganese contents, the sulfur level at which the observed drill machinability rating commences to decrease, in the manner illustrated by Graphs I and K of FIGURE 6.
  • Graph M of FIGURE 7, so established is generally parallel to Graph L, so that the conclusion may be drawn that the incidence of the large sulfide inclusions, as delimited by Graph L, may be correlated with machinability.
  • the Graphs L and M are not coincident, that is, the machinability of the steels does not decrease simultaneously with the appearance of small numbers of large sulfides, such a decrease being observed only when a sufiiciently large number of large sulfides is formed as to constitute such a large fraction of the total volume of the sulfides present as will, in effect, counteract the benefit of adding more sulfur.
  • Graph M of FIG- URE 7 represents the dividing line between these opposing effects.
  • the invention contemplates the provision of austenitic stainless steels wherein sulfur and manganese are so balanced with one another as to fall below the Graph M of FIGURE 7 and, preferably, below Graph Lthereof.
  • manganese may be present in the new steels up to about 8.0%, although, because of the aforementioned difiiculty in obtaining controllable sulfur recovery, an upper manganese limit of about 7% is set for the new steels, and manganese is preferably included in maximum amount of about 4.5 to 5.0%, in order to permit the use of higher sulfur contents (for better machinability) without encountering the danger of large sulfide formation and the consequent deterioration of machinability.
  • sulfur is preferably limited to a maximum of about .40%, that element may be used in somewhat larger amounts. Inspection of FIGURE 7 shows that, at the lowest contemplated manganese levels, sulfur may be present up to about 0.55 or 0.60 percent without encountering the large sulfide-decreased machinability area delimited by Graph M. However, at such high sulfur levels, not only are the aforesaid deleterious effects of sulfur most pronounced, but the rapidly contracting range of permissible manganese contents become so small as to make practical melting procedures unreliable or impossible to achieve. Consequently, sulfur may be used in amounts as great as .45%.
  • the manganese and sulfur contents of the inventive steels are not only balanced within the ranges aforesaid, in accordance with the showing of FIGURE 7, but, further, are limited to compositions to the right of line N-O of FIGURE 7, which line represents a minimum manganese content of 8 times the particularly contemplated sulfur range of .25 to .40%.
  • Carbon is limited, on the high side of its range, as shown above, in order to avoid the formation, upon annealing, of large quantities of carbides which deleteriously affect the corrosion resistance of the steels.
  • Both carbon and nitrogen are, of course, potent austenite stabilizers and can be adjusted within the respected ranges of each in order to obtain a more or less stably austenitic structure as desired.
  • At least about 5% nickel is required in the steels of the invention for adjustment of the chemical balance so that the steels are austenitic during hot working and so that they have desirable cold working properties.
  • Cost considerations limit the maximum nickel content of the steels of the invention to the values shown hereinabove in Table V.
  • the range of nickel content when nickel is used near the lower end of its specified range, it is contemplated that manganese is to be used on the high side of its range if more stably austenitic structures are to be obtained.
  • the principles of the invention still apply in respect of leaner alloy steels having an austenitic structure of lesser stability.
  • Molybdenum and zirconium are common additions to austenite stainless steels.
  • molybdenum is often added to these steels because of its function in expanding the passivity range and the tendency to improve corrosion resistance, particularly chloride pit corrosion resistance. Accordingly, molybdenum may be included in the new steels in usual amounts up to about 4%.
  • Molybdenum is, of course, a strong ferritizing element so that steels wherein that element is present must be balanced in respect of the austenite promoting elements in order to obtain a structure of the desired characteristics.
  • Selenium, tellurium, lead and phosphorus are wellknown free-machining additives and, consequently, may be utilized individually or in combination in the steels of this present invention, for example, in amounts up to about .50% of each of these elements.
  • Selenium is particularly desirable in this regard in view of its lesser effect, as compared to sulfur, in decreasing corrosion resistance and in promoting the formation of non-metallic inclusions.
  • the austenitic stainless steels are particularly susceptible to sensitization, which is generally considered a precipitation of harmful grain boundary constituents.
  • the elements columbium, tantalum and titanium are commonly added to these steels to minimize this disadvantage. They may, accordingly, be added to the steels of this invention for a similar purpose.
  • Copper is an occasional alloying addition to austenitic stainless steels for its effect in enhancing corrosion resistance, for example, in oil-cracking applications. Copper is also considered an inexpensive austenite-promoting alloy ingredient and is occasionally used for such purpose in austenitic stainless steels. That element, accordingly, may be added to the inventive steels as shown in Table V.
  • the invention brings to the art a highly useful new class of steels having all of the known advantages of austenitic stainless steels, together with enhanced free-machining properties which are obtained with a minimal appearance of the heretofore generally experienced disadvantage accompanying the use of sulfur as a free-machining element.
  • this free-machining additive is realized, without detrimentally affecting the wide range of usefulness of the steels in which it is incorporated.
  • An austenitic stainless steel of enhanced freemachining properties consisting essentially of, by weight percent
  • a free-machining austenitic stainless steel consisting essentially of, by weight percent
  • a free-machining austenitic steel consisting essentially of, by weight percent
  • the steel also containing at least about 0.25 percent sulfur, the manganese-to-sulfur ratio being at least about 8 to 1, and the sulfur content being selected so as to fall below the Graph L of FIGURE 7.
  • a wrought free-machining austenitic chromiumnickel-manganese stainless steel article consisting essentially of up to .25 percent carbon, from about 16-19 percent chromium, from about 6.5 to about 10 percent nickel, from about 0.25 to about 0.40 percent sulfur, wherein the manganese content is from over 2 to about 7 percent and is at least about 8 times the sulfur content, balance iron and wherein substantially all of the sulfur is present in the form of uniformly distributed sulfide particles having a maximum dimension less than about .010 inch.

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Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3645722A (en) * 1969-09-04 1972-02-29 Carpenter Technology Corp Free machining stainless steel alloy
US3888659A (en) * 1968-05-29 1975-06-10 Allegheny Ludlum Ind Inc Free machining austenitic stainless steel
US4613367A (en) * 1985-06-14 1986-09-23 Crucible Materials Corporation Low carbon plus nitrogen, free-machining austenitic stainless steel
US5788922A (en) * 1996-05-02 1998-08-04 Crs Holdings, Inc. Free-machining austenitic stainless steel
US20070187458A1 (en) * 2006-02-16 2007-08-16 Stoody Company Stainless steel weld overlays with enhanced wear resistance
US20090282952A1 (en) * 2008-05-14 2009-11-19 Potzu Forging Co., Ltd. Cold forged stainless tool and method for making the same
CN109504916A (zh) * 2018-12-22 2019-03-22 中南大学 一种含铜钛高强度高耐蚀奥氏体不锈钢及其制备方法
US20190309228A1 (en) * 2018-04-04 2019-10-10 Nova Chemicals (International) S.A. Reduced fouling from the convection section of a cracker
CN113528963A (zh) * 2021-07-16 2021-10-22 浙江青山钢铁有限公司 易切削高耐腐蚀的奥氏体不锈钢盘条及其制备方法

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4576641A (en) * 1982-09-02 1986-03-18 The United States Of America As Represented By The United States Department Of Energy Austenitic alloy and reactor components made thereof
US5482674A (en) * 1994-07-07 1996-01-09 Crs Holdings, Inc. Free-machining austenitic stainless steel

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US2484903A (en) * 1948-09-24 1949-10-18 Crucible Steel Company Heat and corrosion resisting alloy steel
US2496245A (en) * 1948-04-06 1950-01-31 Armco Steel Corp Internal-combustion engine valve
US2557862A (en) * 1947-11-19 1951-06-19 Armco Steel Corp Internal-combustion engine valve
US2687955A (en) * 1951-11-05 1954-08-31 Armco Steel Corp Cold-workable stainless steel and articles
US2697035A (en) * 1951-12-03 1954-12-14 Armco Steel Corp Free-machining stainless steel and method
US2891958A (en) * 1958-05-29 1959-06-23 Reilly Tar & Chem Corp Certain alkyl n-pyridylthiopicolinamides and alkyl n-pyridylthiosonicotinamides and process

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DE592299C (de) * 1932-12-29 1934-02-05 Cie Des Forges De Chatillon Co Austenitische Staehle mit erhoehter Bearbeitbarkeit

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US2557862A (en) * 1947-11-19 1951-06-19 Armco Steel Corp Internal-combustion engine valve
US2496245A (en) * 1948-04-06 1950-01-31 Armco Steel Corp Internal-combustion engine valve
US2484903A (en) * 1948-09-24 1949-10-18 Crucible Steel Company Heat and corrosion resisting alloy steel
US2687955A (en) * 1951-11-05 1954-08-31 Armco Steel Corp Cold-workable stainless steel and articles
US2697035A (en) * 1951-12-03 1954-12-14 Armco Steel Corp Free-machining stainless steel and method
US2891958A (en) * 1958-05-29 1959-06-23 Reilly Tar & Chem Corp Certain alkyl n-pyridylthiopicolinamides and alkyl n-pyridylthiosonicotinamides and process

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3888659A (en) * 1968-05-29 1975-06-10 Allegheny Ludlum Ind Inc Free machining austenitic stainless steel
US3645722A (en) * 1969-09-04 1972-02-29 Carpenter Technology Corp Free machining stainless steel alloy
US4613367A (en) * 1985-06-14 1986-09-23 Crucible Materials Corporation Low carbon plus nitrogen, free-machining austenitic stainless steel
US5788922A (en) * 1996-05-02 1998-08-04 Crs Holdings, Inc. Free-machining austenitic stainless steel
US20070187458A1 (en) * 2006-02-16 2007-08-16 Stoody Company Stainless steel weld overlays with enhanced wear resistance
WO2007097939A2 (en) * 2006-02-16 2007-08-30 Stoody Company Stainless steel weld overlays with enhanced wear resistance
WO2007097939A3 (en) * 2006-02-16 2008-07-17 Stoody Co Stainless steel weld overlays with enhanced wear resistance
US8124007B2 (en) 2006-02-16 2012-02-28 Stoody Company Stainless steel weld overlays with enhanced wear resistance
US20090282952A1 (en) * 2008-05-14 2009-11-19 Potzu Forging Co., Ltd. Cold forged stainless tool and method for making the same
US20190309228A1 (en) * 2018-04-04 2019-10-10 Nova Chemicals (International) S.A. Reduced fouling from the convection section of a cracker
CN109504916A (zh) * 2018-12-22 2019-03-22 中南大学 一种含铜钛高强度高耐蚀奥氏体不锈钢及其制备方法
CN113528963A (zh) * 2021-07-16 2021-10-22 浙江青山钢铁有限公司 易切削高耐腐蚀的奥氏体不锈钢盘条及其制备方法

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NL6606700A (de) 1966-11-15
DE1783104C2 (de) 1974-03-28
GB1094409A (en) 1967-12-13
ES326678A1 (es) 1967-07-01
NO117149B (de) 1969-07-07
DE1783104B1 (de) 1973-08-23
FR1584963A (de) 1970-01-09

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