Classifications

• • 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
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
  • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
• • 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

• the present invention relates to a process for heat treating ferrous-base alloys and more particularly to a process wherein cast steels of the maraging type are subjected to a sequence of heat treating operations whereby the capability of such steels to resist stress-corrosion cracking is markedly improved.
• the present invention contemplates subjecting a cast maraging steel containing about 14% to about 17.5% nickel, about 8% to about 12% cobalt, about 4% to about molybdenum, up to about 0.45% or 0.5% titanium, up to about 0.5% aluminum, e.g., about 0.03% to about 0.45% aluminum, up to about 0.05% carbon, up to 0.1% zirconium and the balance being essentially iron to a heat treatment cycle which includes (a) heating the steel at a temperature of about 1700 F. to about 1850 F.
• the steels can also contain up to about 3.5% chromium, which can be used to replace an equivalent percentage of nickel on a weight to weight basis. However, the sum of the nickel plus chromium should not exceed 17.5% and the nickel content should not be less than about 14%.
• Small amounts of supplemental or auxiliary elements can be present including up to about 2% tungsten, up to about 0.5 columbium, up to about 0.45% vanadium, up to about 0.5 tantalum, up to about 3% copper and up to about 0.3% beryllium.
• the supplemental elements which can contribute to the hardness or strength of the steels should not exceed a total of about 4% and preferably should not exceed a total of 3%.
• Deoxidizing and/or malleabilizing elements can also be present in small amounts, including up to about 0.05 boron, e.g., up to 0.03% boron, up to 0.2% of each of silicon and manganese, rare earth elements and other such constituents such as lithium, magnesium and uranium.
• impurities such as sulfur, phosphorus, nitrogen, oxygen, antimony, tin, selenium, tellurium, arsenic and bismuth should be kept at low levels consistent with good commercial steelmaking practice.
• the steels can be prepared in the manner described in US. Patent No. 3,132,937.
• a particular satisfactory alloy composition is as follows: 16% to 17.5% nickel, 9.5% to 11.5% cobalt, 4.4% to 5% molybdenum, 0.1% to 0.45 titanium, 0.03% to 0.45% aluminum, up to about 0.03% carbon, up to about 0.1% zirconium, with the balance being essentially iron.
• the compositions herein described are those contemplated in US. Patent No. 3,132,937.
• the tem perature of the homogenization treatment should not fall below that necessary to insure the occurrence of recrystallization and a temperature of at least 1750 F. is preferred. While the complete theory is not yet at hand which might explain the phenomenon responsible for the striking improvement in stress-corrosion cracking behavior obtainable in accordance herewith, it is considered that the unusually small grain size, about ASTM No. 6 or finsteel development described in US. Patent No. 3,132,937, this is quite unexpected. In that patent, the recommended optimum homogenizing temperature was 2100 F., it being preferred not to depart more than 50 F. therefrom.
• the maximum homogenizing temperature should not exceed 1850 F. and a temperature range for the first stage of the heat treatment cycle is preferably from 1750 F. to 1825 F.
• the austenite to go back into solution such that it transforms to martensite on cooling before aging. While a solution anneal over the range of 1350 F. to 1600 F. is satisfactory, a temperature range of about 1400 F. to 1550" F. with a holding time of about one to eight hours is preferred. Following the solution anneal, the steels are cooled until a martensitic structure is obtained and are thereafter aged.
• a temperature of from 800 F. to about 1000 F. can be employed although it is preferred to use a temperature within a range of about 850 F. to 950 F.
• temperatures appreciably below 800 F. require an inordinately long time to achieve a sufficient aging response and even then a full aging might not be achieved.
• temperatures of above about 1000 F. tend to promote austenite reversion which can result in lower strengths and hardness.
• the conditioning treatment is most essential, for without it, a fine grain structure is not obtained and stresscorrosion characteristics are poor. It would appear that this treatment results in a condition whereby retained austenite is present or the martensite formed as a result of the first cooling step reverts to austenite. Proceeding further, it is considered that this austenite forms additional nuclei for the ultimate obtaining of a fine grain size, the austenite co-existing with other phases.
• the minimum temperature for the second stage heating should not fall below a temperature of about 1000 F.; otherwise, insufficient nuclei would be present for the formation of new grains and a temperature of at least 1050 F. or 1075 F. is most advantageous since such temperatures promote finer grains, e.g., ASTM No. 8.
• temperatures above 1250 F. can lead to inferior results since such temperatures seemingly tend to contribute to coarser grains and thus such temperatures should be avoided. It is advantageous that the second stage heat treating operation be conducted over a temperature of about 1050 F. to 1150 F. for about one hour to six hours.
• the steels can be cooled to room temperature before subjecting them to the third heating operation of the four-stage cycle. However, if desired, the steels can be immediately brought to the temperature range over which the solution annealing operation is conducted.
• the yield and ultimate tensile strengths are seriously impaired should this step be omitted and this is deemed attributable to an austenite effect.
• the retained or reverted austenite brought about by the conditioning treatment is characterized by stability. In other words, if the steels are cooled from the conditioning treatment and then aged (or are directly aged), excessive amounts of austenite are still present because being stable the austenite does not transform to martensite. Now this austenite is much softer and weaker than, say, martensite and, as a consequence, lower strengths ensue.
• Heat Treatment A a heat treatment in accordance with the invention
• Heat Treatment B the latter consisting of homogenizing at 1800 F. for four hours, air cooling to room temperature, heating to a temperature of 1100 F. and holding for about four hours, transferring to a furnace at 1500 F. and holding thereat for about one hour, air cooling to room temperature, aging at 900 F. for about three hours and finally air cooling to room temperature.
• Table III illustrates the significant loss in toughness characteristics as reflected by the substantial drop in tensile elongation and severe loss in reduction of area resulting from the application of modified Heat Treatment B.
• Steels Nos. 1 and 2 having a section size of inch when subjected to Heat Treatment A in accordance with the invention manifested the same order of magnitude in mechanical properties as those obtained in connection with Alloy No. 3 having a thickness of three inches.
• no appreciable sacrifice in mechanical characteristics is experienced in achieving the high resistance to stress-corrosion cracking in accordance herewith.
• a distinct advantage of the present invention is that cast steels of the maraging type are greatly less susceptible to stress-corrosion cracking in marine environments, particularly sea water and ambient atmospheres.
• the cast steels can be used in such applications as gun parts, tools, rolls, dies, etc.
• a process for improving the resistance to stresscorrosion cracking of martensitic cast steels containing about 14% to about 17.5% nickel, about 8% to about 12% cobalt, about 4% to about 5% molybdenum, up to about 0.5% titanium, up to about 0.5% aluminum, up to about 0.05% carbon, up to 0.1% zirconium, up to about 3.5% chromium with the sum of nickel plus chromium not exceeding 17.5%, up to about 2% tungsten,
• a process for improving the resistance to stresscorrosion cracking of martensitic cast steels containing about 14% to about 17.5% nickel, about 8% to about 12% cobalt, about 4% to about 5% molybdenum, up to about 0.5% titanium, up to about 0.5% aluminum, up to about 0.05% carbon, the balance essentially iron which comprises subjecting the steels to a homogenization heat treatment within the temperature range of1750" F. to 1825 F. for a period sufiicient to achieve a good homogencous structure, cooling the said steel to a temperature sufiiciently low to provide a martensitic structure, heating the steels within a temperature range of about 1050 F. to 1150 F.

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

Priority Applications (4)

Applications Claiming Priority (1)

Publications (1)

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Cited By (7)

Citations (4)

• 1965
  • 1965-07-12 US US471458A patent/US3341372A/en not_active Expired - Lifetime
• 1966
  • 1966-06-30 GB GB29479/66A patent/GB1096978A/en not_active Expired
  • 1966-07-07 CH CH986466A patent/CH461110A/fr unknown
  • 1966-07-12 BE BE683978D patent/BE683978A/xx unknown

Patent Citations (4)

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