EP0020357B1 - Lower bainite alloy steel article - Google Patents

Lower bainite alloy steel article Download PDF

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Publication number
EP0020357B1
EP0020357B1 EP79900923A EP79900923A EP0020357B1 EP 0020357 B1 EP0020357 B1 EP 0020357B1 EP 79900923 A EP79900923 A EP 79900923A EP 79900923 A EP79900923 A EP 79900923A EP 0020357 B1 EP0020357 B1 EP 0020357B1
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EP
European Patent Office
Prior art keywords
article
range
steel alloy
bainitic steel
temperature
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
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EP79900923A
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German (de)
English (en)
French (fr)
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EP0020357A1 (en
EP0020357A4 (en
Inventor
Stuart L. Rice
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Caterpillar Inc
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Caterpillar Tractor Co
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Publication of EP0020357A4 publication Critical patent/EP0020357A4/en
Publication of EP0020357A1 publication Critical patent/EP0020357A1/en
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium

Definitions

  • This invention relates generally to a low temperature bainitic alloy steel.
  • Carburized and hardened alloy steel gears are widely used for vehicle power trains in order to obtain a sufficient resistance to surface pitting and high bending loads, and thereby a generally desirable service life.
  • the heat treating and processing of such gears takes a long time, uses a considerable amount of energy and, accordingly, the gears are expensive. Drastic quenching of the gears is also often required, which results in considerable distortion.
  • the microstructure of the gears is inhomogeneous and the gears lack sufficient case toughness at the desired high hardness levels.
  • nitrided alloy steel gears are relatively brittle at relatively high hardness levels, for example, above a magnitude of about 58 on the Rockwell C hardness scale (Rc58), and do not exhibit a relatively uniform metallographic structure.
  • bainitic alloy steels are very desirable as a substitute for the above- mention martensitic steels.
  • low temperature bainite is more ductile than martensite at the same hardness level.
  • most prior art bainitic alloy steels have utilized controlled amounts of potentially critical and/or expensive materials such as chromium and nickel. Exemplary of the art in this area are the following U.S. Patents Nos.: 3,418,178 to S. A. Kulin et al on December 24, 1968; 3,303,061 to J. E. Wilson on February 7, 1967; 2,128,621 to B. R. Queneau on August 30, 1938; 3,298,827 to C. F.
  • U.S. Patent No. 1,924,099 issued to E. C. Bain et al on August 29, 1933 describes a process known as austempering. Such process involves the steps of: a) heating a steel article above an upper critical temperature to assure a change in the morphology of the article to sub--stantially 100% austenite; b) quenching the article below approximately 540°C. (1000°F.), but above the temperature of martensite formation or the so-called martensite start (M s ) line; and c) holding the steel article at such an intermediate temperature for a preselected period of time sufficient to convert the morphology of the article to a form other than 100% martensite.
  • TTT time-temperature-transformation
  • the alloy compositions and austempering processes resorted to have suffered two general deficiencies.
  • the quenching step has involved cooling at a rate such that the transforming start (T s ) curve of the alloy has been crossed and undesirable upper transformation products such as proeutectoid ferrite/carbide, pearlite, and upper bainite formed.
  • T s transforming start
  • undesirable upper transformation products such as proeutectoid ferrite/carbide, pearlite, and upper bainite formed.
  • the undesirable crossover of such nose portion results in a loss of toughness and hardness.
  • the heat treat holding times sufficient to obtain substantially complete transformation have been too long, for example, five hours or more. For general commercial applications, such an extended holding period is substantially impractical and represents a considerable waste of energy and time.
  • Seghezzi et al on September 8, 1970 is not desirable for an article thicker than about 12 mm (1/2") because the austempered morphology of the article would vary nonuni- formly across its cross section away from substantially complete lower bainite, and the hardness would undesirably drop below a magnitude level of about 55 to 58 on the Rockwell C scale.
  • the anchoring device of the Seghezzi patent also undesirably utilizes the potentially critical element chromium.
  • U.S. Patent No. 3,196,052 to K. G. Hann on June 28, 1961 is representative of another alloy steel for a thin wire that has substantially the same problems.
  • the present invention is directed to overcoming one or more of the problems as set forth above.
  • a bainitic steel alloy article consisting of carbon in the range of 0.65 to 0.75 Wt.%, manganese in the range of 0.60 to 0.70 Wt.%, silicon in the range of 1.20 to 2.00 Wt.%, molybdenum in the range of 0.50 to 0.60 Wt.%, optionally boron in the range of 0.0003 to 0.004 Wt.%, a plurality of residual elements individually limited to less than 0.20 Wt.% and the balance iron, said article having been heat treated to achieve a predominantly homogeneous and substantially complete low temperature bainite microstructure.
  • Figure 1 shows a diagrammatic time-temperature-transformation diagram for a bainitic alloy steel article of the present invention and including a heat treatment processing route.
  • boron in the broad range of 0.0003 to 0.004 Wt.% is controllably added to the above-designated composition. More particularly, a boron range of 0.002 to 0.0035 Wt.% is preferred, and the most desirable amount is about 0.003 Wt.%.
  • carbon (c) is present in the relatively high amounts indiciated to impart the desired strength and hardness throughout the body of the article.
  • Carbon is an austenite former, and is present in the minimum amount stated to assure that a relatively uniform through-hardness value of a magnitude in excess of about R c 56 on the Rockwell C hardness scale can be obtained in the finished article. Below this minimum value the alloy would lack sufficient hardness and strength. Above the maximum stated value the alloy would become less ductile and/or too brittle, and the amount of carbon present would undesirably contribute to the formation of free carbides. Moreover, the range of carbon set forth assures that substantially complete transformation to low temperature bainite can be positively obtained.
  • Manganese (Mn) is also an austenite former and ferrite strengthener. Below the minimum established value the strength and hardness of the article produced would be lower than that desired, and there would not be enough manganese to tie up at least some of the sulfur usually present in residual amounts and to form manganese sulfide rather than undesirable iron sulfide. Above the maximum established value the ductility of the article would be lowered excessively.
  • Silicon (Si) is also a ferrite strengthener and is effective in the amounts indicated to assure the desired tensile strength and hardness of the final low temperature bainite alloy, as well as for grain size control. Below the stated minimum value there would be insufficient silicon for deoxidation purposes and for the desired level of hardness in the range of magnitude above about R c 56. Above the stated maximum value the toughness decreases to the point where excessive embrittlement occurs, and graphite tends to form.
  • Molybdenum (Mo) reduces graphitization, is a ferrite strengthener, and provides the desired hardenability characteristics to the low temperature bainite alloy.
  • the stated amounts of carbon, manganese, silicon, and molybdenum serve to lower the martensite start (M s ) transformation portion of the process route permitting the lower bainite transformation to occur at a relatively'low holding temperature for increased hardenability of the article and at a savings in energy.
  • these four elements optimize the position of the transformation start curve so that quenching does not have to be achieved at an excessive rate.
  • the nose portion of the transformation start curve is thereby desirably located to the right on the TTT diagram sufficient to allow quenching of articles of thicker cross section at a more practical rate that will minimize distortion of the article and still result in relatively uniform through-hardening thereof.
  • above the stated maximum value of molybdenum the transformation completed curve is located too far to the right on the TTT diagram, resulting in an extended required holding time of above two hours and a corresponding waste of energy.
  • Boron (B) improves bainite hardenability.
  • the addition of boron (B) is preferred because the boron plus molybdenum plus silicon conserve these elements and provide a more advantageous rightward position of the transformation start curve and to thereby permit more practical cooling rates for articles of various thickness during austempering. Boron and molybdenum and possibly silicon retard the polygonal ferrite reaction without retarding the bainitic ferrite reaction.
  • the boron acts as an intensifier from the standpoint that it intensifies the reaction of the other major elements. Boron is present in the minimum amount indicated to enable the proportions of molybdenum and/or manganese to be disproportionately reduced for economy, while simultaneously providing the desired morphology. However, going above the maximum value of about 0.004 Wt.% is believed detrimental to toughness.
  • Some undesirable residual elements such as ' sulfur (S) and phosphorus (P) are usually present in commercial steels.
  • other residual elements such as copper (Cu), chromium (Cr), titanium (Ti), etc. may also be present in relatively small amounts with some degree of benefit.
  • all of these residual elements should be individually limited to less than 0.20 Wt.%.
  • a typical TTT diagram for an alloy as claimed is as illustrated in Figure 1, including a transformation start curve 10 and a transformation complete curve 12.
  • a heat treatment processing route 14, therefor, is also shown, and it is to be noted that the processing route desirably avoids intersection with a nose portion 16 of the transformation start curve.
  • the processing route 14 for making an article of the composition claimed included the initial formation of a 76 mmx76 mm (3"x3") ingot and subsequently rolling and/or forging the ingot down to a 38 mmx38 mm (1-1/2"x1-1/2") bar.
  • the bar was heated to a preselected first temperature 18 within the austenite transformation temperature range.
  • the bar is heated in a salt bath, desirably a nontoxic, -electrically heated chloride salt bath to the approximate preselected first temperature point 18 noted in Figure 1.
  • the bar is maintained at such temperature for about 5 to 10 minutes to assure a substantially complete austenite microstructure.
  • the second step after heating the bar to the preselected first temperature 18 is to relatively rapidly cool or quench the heated bar as indicated in Figure 1 while missing the nose portion 16 of the transformation start curve 10 particular to the alloy steel composition of the present invention. If the heated bar is quenched toward a preselected second temperature 26 too slowly, a significant portion of the microstructure would be undesirably transformed to pearlite because the processing route would pass through a pearlite region 20 between curves 10 and 12 as indicated in Figure 1. If it is quenched at a slightly faster rate in a bath of a higher temperature, then an undsirable upper bainite microstructure could be formed because the processing route would pass through an upper bainite region 22 as shown in Figure 1.
  • the austenite microstructure of the bar or similar article directly to a lower bainite microstructure by choosing a preselected cooling rate 28 sufficient for avoiding crossing of the transformation start curve 10 until reaching a preselected lower range of temperatures.
  • the lower end of such lower bainite range is defined by a preselected second temperature 26 located within a band of temperatures adjacent the M s line 24 for the composition of elements selected in order to maximum the final hardness of the article.
  • the preselected second temperature 26 be limited to less than about 15°C (30°F) above or below the M s .line.
  • the preselected second temperature should be above the M s line if it is desired to substantially avoid transformation to martensite.
  • the M temperature is typically about 270°C (520OF and the preselected second temperature 26 chosen was 260°C (500°F).
  • the upper end of the lower bainite range is defined by a preselected third temperature 30 of about 350°C (660°F). If the preselected third temperature is raised to a higher temperature, the alloy may be transformed at least in part into an upper bainite microstructure with its undesirable coarser grain structure.
  • the bar is preferably quenched in a second salt bath having the preselected second temperature 26.
  • the bar is quenched in a nontoxic, electrically heated nitrate-nitrite salt bath at the preselected cooling rate 28 indicated in Figure 1.
  • the time scale along the bottom of Figure 1 is of advantageous logarithmic form, so that in this way the cooling rate 28 approximates a substantially straight line throughout a significant portion of the first 10 seconds or so of the processing route 14.
  • the second salt bath was maintained at a quenchant temperature of about 260°C (500°F), and approximately 0.6 Wt.% water is added to the salt bath for greater quench severity.
  • the third step of the processing route 14 is to hold or maintain the bar at a relatively stable temperature between the above-mentioned lower and upper temperature reference lines 26 and 30 for a preselected period of time just prior to the transformation start curve 10 and thereafter to the transformation complete curve 12 to complete the transformation of the alloy steel to a substantially complete low temperature bainite microstructure.
  • this term it is meant that there is less than 10 Vol.% of retained austenite, substantially no pearlite, and less than about 10 Vol.% transformation to martensite in the subject lower bainite alloy steel. Since the hardness of the article increases as the holding temperature approaches the M s line 24, it is desirable to maintain the bar or article at or adjacent the preselected second temperature 26.
  • Such isothermal heat treat transformation is complete with the processing route 14 reaches the transformation complete curve 12.
  • the transformation start and complete curves 10 and 12 define the left and right time-indicating boundaries of a lower bainite transformation region 32, while the lines 26 and 30 define the lower and upper temperature boundaries of the same region which varies in a range of about 250°C (482°F) to 350°C (660°F) for the subject alloy.
  • the time scale along the bottom of the lower bainite transformation region 12 indicates that the length of holding time required for the first example alloy steel is only about 1800 seconds. This is a great improvement over the extended holding time period of prior art.
  • the holding time can be reduced to about 800 seconds by raising the temperatures of the second salt bath and the subsequent holding temperature to a piont adjacent the line 30 in order to save energy and time.
  • the fourth step of the processing route 14 not shown in the drawing is to remove the article from the second salt bath and allow air cooling thereof at substantially ambient temperatures. Such step is taken after the transformation complete curve 12 has been breached by the processing route.
  • the through-hardened, low temperature bainitic alloy steel of the present invention exhibits physical properties that could be extremely useful for a wide number of applications including gears, bushings, bearings and the like. Particularly, it exhibits the potential for use in a power train gear having a plurality of teeth thereon for increasing gear static strength to a level of magnitude of 50%, reducing gear distortion by a level of magnitude of 75%, and maintaining equivalent pitting resistance when compared to conventional carburized and hardened steel gears at a minimal increase in cost.
  • the subject lower bainite alloy steel is economical to produce, yet is adaptable to manufacturing procedures requiring no natural gas, for example. Moreover, the entire thermal transformation time is less than about two hours.
  • the preselected amount of boron indicated is believed to increase the time available to lower the temperature of the articie being quenched to the transformation temperature, and the amount of molybdenum has a significant effect in reducing the holding time required to complete isothermal transformation to lower bainite. Significantly too, all of the preselected elements, except boron, lower the M s line.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Articles (AREA)
EP79900923A 1978-11-15 1978-11-15 Lower bainite alloy steel article Expired EP0020357B1 (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/US1978/000151 WO1980001083A1 (en) 1978-11-15 1978-11-15 Lower bainite alloy steel article and method of making same

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EP0020357A4 EP0020357A4 (en) 1980-11-28
EP0020357A1 EP0020357A1 (en) 1981-01-07
EP0020357B1 true EP0020357B1 (en) 1984-07-18

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EP (1) EP0020357B1 (enrdf_load_stackoverflow)
JP (1) JPS55500910A (enrdf_load_stackoverflow)
DE (1) DE2862430D1 (enrdf_load_stackoverflow)
WO (1) WO1980001083A1 (enrdf_load_stackoverflow)

Families Citing this family (7)

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Publication number Priority date Publication date Assignee Title
JP3018268B2 (ja) * 1993-05-25 2000-03-13 新日本製鐵株式会社 伸線加工性に優れた高炭素鋼線材または鋼線およびその製造方法
US5658402A (en) * 1993-05-25 1997-08-19 Nippon Steel Corporation High-carbon steel wire rod and wire excellent in drawability and methods of producing the same
SE9404110D0 (sv) * 1994-11-25 1994-11-25 Hoeganaes Ab Manganese containing materials having high tensile strength
JP3997662B2 (ja) 1999-08-09 2007-10-24 株式会社ジェイテクト 転がり軸受
JP4554714B2 (ja) * 2009-02-04 2010-09-29 ファナック株式会社 流体軸受構造および軸受溝作成方法
CN112267009A (zh) * 2020-10-23 2021-01-26 江苏大学 一种水溶性淬火液及其制备方法、处理盾构机刀圈的方法和淬火盾构机刀圈
CN114525388A (zh) * 2022-02-21 2022-05-24 刘鑫 一种上贝氏体层状组织钢及其制备方法

Citations (6)

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Publication number Priority date Publication date Assignee Title
US1924099A (en) * 1931-11-20 1933-08-29 United States Steel Corp Thermally hardening steel
GB457872A (en) * 1934-03-14 1936-12-02 Child Harold Wills Improvements in or relating to molybdenum alloy steels
US2791500A (en) * 1954-03-19 1957-05-07 Int Nickel Co High strength aircraft landing gear steel alloy elements
US3806378A (en) * 1972-12-20 1974-04-23 Bethlehem Steel Corp As-worked bainitic ferrous alloy and method
USRE28523E (en) * 1963-11-12 1975-08-19 High strength alloy steel compositions and process of producing high strength steel including hot-cold working
GB2019436A (en) * 1978-04-21 1979-10-31 Hilti Ag Steel Alloys, Articles Made Therefrom, and Method of Making Such Alloys

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US2513395A (en) * 1946-07-18 1950-07-04 United Steel Companies Ltd Boron-treated molybdenum steel
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US3366471A (en) * 1963-11-12 1968-01-30 Republic Steel Corp High strength alloy steel compositions and process of producing high strength steel including hot-cold working
GB1080304A (en) * 1965-03-12 1967-08-23 Natural Res Dev Corp Ausforming high-strength alloy steels
DE1558505A1 (de) * 1967-01-23 1970-04-16 Hilti Ag Verankerungsmittel
AT309495B (de) * 1970-01-09 1973-08-27 Boehler & Co Ag Geb Kettensägenstahl

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1924099A (en) * 1931-11-20 1933-08-29 United States Steel Corp Thermally hardening steel
GB457872A (en) * 1934-03-14 1936-12-02 Child Harold Wills Improvements in or relating to molybdenum alloy steels
US2791500A (en) * 1954-03-19 1957-05-07 Int Nickel Co High strength aircraft landing gear steel alloy elements
USRE28523E (en) * 1963-11-12 1975-08-19 High strength alloy steel compositions and process of producing high strength steel including hot-cold working
US3806378A (en) * 1972-12-20 1974-04-23 Bethlehem Steel Corp As-worked bainitic ferrous alloy and method
GB2019436A (en) * 1978-04-21 1979-10-31 Hilti Ag Steel Alloys, Articles Made Therefrom, and Method of Making Such Alloys

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
HARTEREI-TECHNISCHE-MITTEILUNGEN vol. 29 (June 1974) no. 2, pages 71-79. F. HENGERER et al. "Zwischenstufenumwandlung von Wälzlagerstählen" *
Heat Treatment of ferrous Alloys, C.R. Brooks, pp. 229-232 *
MACHINE MODERNE, vol. 63 (December 1969), nr. 729 Q.D. MEHRKAN "Austempering et Martempering en bain de sel", pages 3-11 *

Also Published As

Publication number Publication date
EP0020357A1 (en) 1981-01-07
EP0020357A4 (en) 1980-11-28
JPS55500910A (enrdf_load_stackoverflow) 1980-11-06
WO1980001083A1 (en) 1980-05-29
DE2862430D1 (en) 1984-08-23

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