Classifications

• • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment

Definitions

• This invention relates to a process for improving steel by a high-temperature heat treatment. Still more particularly, the invention concerns a heat treatment which comprises heating steel to a minimum temperature of 2400 F. for at least 10 minutes. Still more particularly, the inven tion concerns heating a steel within the range of about 2400 to about 2500 F. for 10 to 20 minutes.
• dendritic crystals When steel solidifies from the liquid state, dendritic crystals are usually formed and as a result, dendrites are an unavoidable characteristic of cast steel.
• the core of dendrite crystals is of higher purity than the last metal solidified.
• steels When hot worked into products, steels often exhibit a condition known as banding due to the dendritic segregation. Banding is in most respects undesirable since it enhances directionality in mechanical properties and decreases uniformity of response to heat treating. The elimination or reduction in the severity of banding is, therefore, highly desirable.
• any method of heating and any rate of heating can be used. It is not necessary to start at room temperature, but higher temperature can follow hot-working so as to utilize heat already present in the steel. It is necessary, however, that the steel be exposed to a temperature of at least about 2400 F. for a minimum of 10 minutes. Following such exposure to temperature, the steel is cooled, normally by air cooling. However, quenching in water or water spray cooling is also satisfactory.
• the product In a common application of the process of the invention as applied to produce strip or plate, the product would be initially reduced by conventional hot-working to a thickness about 25% greater than the final product. As the strip or plate emerges from the final hot-rolling pass, it is passed through a furnace to bring its temperature throughout to at least 2400 F. and the steel is maintained at substantially this temperature for at least about 10 minutes. The strip or plate would then be passed through rolls to flatten it and reduce it to final dimension. Thereafter, the steel can be handled in any conventional manner. As an alternative, the strip or plate could be hotworked oversize and subsequently in an independent operation reheated to at least 2400 F. for at least 10 minutes and then given a finishing pass to produce a product with good surface and close dimensional tolerance.
• Steel treated as described above can be subsequently heated in any conventional manner. For example, it can be annealed, normalized or hardened without return of the banded condition.
• treatments involving reaustenitizing can be repeated indefinitely without destroying the beneficial effect of the high-temperature treatment.
• the process is applicable to any steel which can withstand a 10-minute heating at 2400 F. without melting, burning or suffering damage of overheating. This includes all constructional carbon and alloy steels. Cast steel with dendritic segregation, as well as banded wrought steel, will benefit from the high-temperature treatment of the invention.
• the treatment described herein has other benefits in addition to reducing banding.
• One such additional advantage is refinement of the steels inherent grain size, i.e. the development of relatively fine grain size upon subsequent conventional miscellaneous heat treatments.
• the term inherent grain size refers to the grain coarsening behavior of the steel as it occurs over a range of temperature and time.
• inherently fine grain steels are those whose austenite grains do not coarsen when heat treated conventionally.
• the treatment of the invention may be used to reduce banding where banding is a problem or to achieve finer grain heat treated microstructures or both.
• the advantages of finer grain sizes are well known and include increased yield strength and enhanced toughness.
• a significant grain refining efiect has consistently been obtained in silicon-killed steels which, as usually processed, are coarse grained.
• the grain refining of silicon-killed steels by the present method can often achieve as fine a grain structure as aluminum-killed steel.
• steels substantially free of aluminum and other grain refining elements classified as inherently coarse grained steels, i.e. having grains coarser than ASTM #5 upon austenitizing at temperatures of 1700 F. or lower, when treated in accordance with the invention become inherently fine grained, i.e. capable of developing a finegrained structure of ASTM #6 and finer when austenitized at 1700 F. or below.
• a grain refining effect may also be found in aluminum-killed steels in which more or less than the optimum amount of aluminum is present.
• a still additional beneficial effect obtained in connection with the high-temperature treatment involves the conversion of long thin inclusions of sulphide or silicate types into less harmful disconnected inclusion particles. In this manner, cracking often associated with such inclusions is minimized.
• An 0.5-inch plate of commercially-manufactured steel having the composition described below was heated at 2400 F. for 10 minutes and air cooled.
• the high temperature treatment in accordance with the invention results in more uniform ductility in the longitudinal and transverse directions. Moreover, strength and toughness are improved as well. In addition, the tendency toward lower ductility and notch toughness in the transverse direction is reduced by the higher temperature treatment.
• a grain-refining, high temperature treatment for reducing banding and capable of promoting coalescence of inclusion stringers in a wrought article comprising a steel subject to dendritic segregation on solidification, comprising subjecting the article to be treated, prior to subsequent heat treatment of the article, to a temperature of at least about 2400 F. and less than a temperature siifiicient to cause incipient melting or burning of the article, and maintaining the article within said temperature range for about 10 to 20 minutes.
• a method in accordance with claim 1 wherein said high temperature heat treatment consists of heating said steel article to a temperature in the range of 2400 to about 2500 F. for 10 to 20 minutes.
• the article comprises a steel selected from the group consisting of carbon and alloy constructional steels.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

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Cited By (2)

Citations (2)

• 1967
  • 1967-02-23 US US617829A patent/US3507710A/en not_active Expired - Lifetime
• 1968
  • 1968-02-19 DE DE19681583999 patent/DE1583999A1/de active Pending
  • 1968-02-19 BE BE710962D patent/BE710962A/xx unknown
  • 1968-02-21 FR FR1555137D patent/FR1555137A/fr not_active Expired

Patent Citations (2)

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