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
  • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys

Definitions

• This invention relates to a method of thermal mechanically treating cast austenitic heat resistant alloy structures to produce structures having superior strength and superior ductility at elevated temperatures and which also exhibit improved creep properties when exposed to carburizing or oxidizing environments at high temperatures.
• austenitic alloy steels exhibiting heat resistance and carburization resistance have been developed for use in pyrolysis furnaces for the thermal decomposition or organic compounds, such as the steam cracking of hydrocarbons.
• the pyrolysis furnace contains a series of heat-resistant alloy steel tubes in which the reaction occurs.
• the term "tube” as used herein also includes fittings, pipes and other parts used to contain carburizing and oxidizing materials at elevated temperatures.
• a microstructure develops which consists primarily of columnar grains oriented radially through the thickness of the tube wall.
• this type of grain structure encourages the nucleation and propagation of cracks, which once initiated, have a tendency to run throughout the thickness of the structure. Because of this serious detriment, it is highly desirable to develop a method of treating such structures so as to inhibit the initiation and propagation of such cracks. Furthermore, it would be even more desirable to inhibit the initiation and propagation of such cracks while improving other high temperature properties such as creep and ductility.
• a thermal mechanical treatment for improving the high temperature properties of cast austenitic heat-resistant chromium-containing alloy steel structures comprises (a) heating the structures to at least the temperature at which chromium carbides go into solution, but below the temperature where incipient melting occurs; (b) maintaining the structures at such a temperature long enough so that at least 50X, preferably at least 75%, for example substantially all, of the chromium carbides go into solution; (c) applying from about 15% to 60% plastic deformation by hot forming operations; and (d) cooling the structures to room temperature at such a rate-to allow complete recrystallization of the grains to occur.
• Austenitic alloy structures which can be treated in accordance with the invention are those structures which are fabricated by casting methods and which have been developed for high temperature application. Generally these structures are nickel-based or contain up to about 30 wt.% iron.
• the structure employed herein will contain from about 20 to about 30 wt.% chromium and about 0.25 to about 0.55 wt.% carbon, preferably about 0.3 to 0.5 wt.% carbon.
• the structure may also contain minor amounts of such elements as silicon, tungsten, molybdenum, manganese, niobium, hafnium, aluminum, yttrium, etc. as well as both tramp elements and minor amounts of impurities typically found in such alloys.
• the as-cast microstructure is modified such that a relatively coarse equiaxed grain structure is developed thereby minimizing the number of grain boundaries which are oriented transversely to the principal stress.
• relatively coarse equiaxed grains we mean equiaxed grains having a grain size of about AS TM 6 to 2 that is about 45 ⁇ m to about 180 A m.
• the resulting average grain size is from 80 ⁇ m to 100 ⁇ m. very fine grains are undesirable because they maximize grain boundary sliding during creep, thereby lowering the strength of the alloy and contributing to the nucleation and propagation of cracks.
• the structures are heated to a temperature at which the chromium carbides go into solution, but below the temperature where incipient melting occurs.
• incipient melting as used herein means those temperatures at which the lower melting phases of the alloys employed begin to melt.
• the structures are heated to a temperature of about 1050°C to 1 300°C, e.g. 1050°C to 1200°C; preferably about 1100 to 1200°C.
• the structures are maintained at that temperature for an effective amount of time.
• effective amount of time we mean that amount of time required to allow at least 50% of the chromium carbides to go into solution.
• hot forming operations While still maintaining the structures at such high temperatures, controlled plastic deformation is applied to the structures by hot forming operations -so that about 15% to 60% e.g. at least 50%, deformation occurs, preferably the deformation is applied in stages of about 10 to 15% per stage.
• hot forming operations suitable for use in the presen invention include rolling, extrusion, drawing and forging. In general, any hot forming operation is suitable which will cause deformation at the temperatures where chromium carbides go into solution. Below those temperatures the compatability stress in the vicinity of the carbide particles are not relaxed by creep in the matrix, instead cracks are generated as an alternative relaxation mechanism.
• the structures are transferred to a furnace and cooled at a rate not to exceed about 100 o C/hr to allow recrystallization of the grains to occur.
• Each of the coupons was deformed by about 60% by cold rolling and subsequently annealed in a tubular furnace at a temperature of about 1000°C + 5 0 C, except coupon E which was subjected to an additional annealing step at 800 o C. All annealing was performed in a high purity argon atmosphere. Table I below sets forth the temperatures and times for which each coupon was annealed.
• Coupons measuring 1.25 cm x 5 cm x 20 cm were taken from the wall of a cast austenitic tube having the same composition as that of the tube in the previous Comparative Examples. All the coupons were first heated for one hour at 1200°C and subjected to hot working at various temperatures by passing them through a single stand mill at least twice. Each pass caused about 10% reduction of the coupon. After deformation, the coupons were tested for creep rupture. Table II below sets forth the experimental conditions for each coupon and Table III below sets forth the conditions and creep data for each coupon.
• Hot rolling of the coupons represented in this Table IV resulted in massive cracking and fissuring. Therefore, hot-working must be accomplished at temperatures greater than 900 o C.
• a cast austenitic steel having the composition as the coupons set forth below is heated for 1 hour at 1200°C and subjected to deformation by extruding to cause a 30% reduction.
• the tube is annealed for 1 hour at 1100 0 C and cooled to room temperature at a rate less than 100°C/hr.
• the tube will be found to have superior high temperature strength and ductility as well as improved creep properties.

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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)
• Heat Treatment Of Articles (AREA)

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• 1982
  • 1982-11-09 CA CA000415183A patent/CA1196555A/fr not_active Expired
  • 1982-12-20 EP EP82306799A patent/EP0083191A3/fr not_active Withdrawn
  • 1982-12-28 JP JP22783282A patent/JPS58117826A/ja active Pending

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