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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium

Definitions

• This invention relates generally to an alloy steel and more particularly to a nitriding grade alloy steel and articles made therefrom.
• Nitrogen case hardening more commonly referred to as nitriding, increases surface hardness, wear resistance, and resistance to certain types of corrosion and surface stresses that improve the fatigue resistance of a nitrided part. Accordingly, nitrided alloy steel articles are often used for g.ears, couplings, shafts and other applications that require resistance to wear and high stress loading.
• °ne group of alloy steels suitable for nitriding have a composition as follows:
• a more economical group of hardenable alloy steels that have been nitrided after heat treating are the AISI/SAE 4100 series alloy steel.
• AISI/SAE 4140H alloy steel has been found to be useful in the manufacture of various gears that require a
• AISI/SAE 4140H alloy steel has a specified composition as follows:
• parts having the above composition are first forged, or rolled from billets, and are quenched and tempered, then machined and nitrided.
• AISI/SAE 4140H alloy steel has been useful in certain nitriding applications, it also has some disadvantages. For example, this steel contains molybdenum, an expensive alloying element. Further, it has been found that articles having the AISI/SAE 4140H composition are prone to quench cracking and therefore generally require an oil quench. Still further, the nitrided case hardness of AISI/SAE 4140H is generally limited to about Rockwell C (Re) 55 or less.
• an alloy steel according to the present invention provides a composition that is economical, adaptable to a variety of quench mediums, maintains high core hardness after tempering and has improved nitriding characteristics.
• the initial cost of the steel is reduced due to the deletion of molybdenum.
• articles manufactured of the alloy steel composition of the present invention may be quenched in either a water or oil medium. Further, after tempering, such articles retain a useful core hardness within a controlled range of from Re 25 to Re 32 depending upon tempering temperature. Also, it has been found that for equivalent case depths, nitriding time of articles having the new alloy composition may be decreased by as much as 40% compared to articles having the AISI/SAE 4140H composition.
• a nitriding grade alloy steel has a composition, by weight percent, of 0.20 to 0.40 carbon, 0.50 to 1.60 manganese, 0.40 to 1.50 chromium, 0.07 to 0.30 aluminum, 0.03 to 0.20 vanadium, and 0.10 to 0.40 silicon with the balance being essentially iron.
• an article formed from an alloy steel having the above composition has a surface hardness, measured on the Rockwell 15-N scale, of at least 89 and a core hardness of at least Rockwell C 25 after the article has been quenched, tempered and nitrided.
• the alloy steel composition according to the present invention provides a unique combination of hardenability, resistance to loss of hardness during tempering and greatly enhanced response to nitriding. These characteristics are achieved by the use of small, carefully controlled amounts of aluminum and vanadium. Further, the unique properties are achieved without the need for- expensive nickel or molybdenum additions and ⁇ hence provides an economical material.
• Fig. 1 is a graph illustrating the improved hardenability characteristics of a nitriding grade alloy steel according to the present invention
• Fig. 2 is a graph illustrating the improved nitride response and resistance to loss of hardness during tempering of the alloy steel; and.
• Fig. 3 is a graph comparing the nitriding 5 response of the alloy steel according- to the present invention with a typical nitriding grade alloy steel.
• the carbon content is at least 0.20% by weight to maintain adequate core hardness after tempering and is no more than about 0.40% by weight to assure resistance to quench cracking ⁇ _5 and an adequate response to nitriding. It has been found that if the carbon content is more than about 0.34% by weight, water quenching may cause cracking or distortion in complex-shaped articles and, in such cases, a less drastic quench medium such as oil may be
• alloy steel articles formed from the more desirable or preferred ranges described herein may
• Manganese contributes to the deep hardenability and is therefore present in all hardenable alloy steel grades.
• the disclosed alloy is
• 30 steel contains manganese in an amount of at least 0.50% to assure adequate core hardness and contains no more than about 1.60% to prevent cracking.
• a narrower range of manganese from 1.00% to
• Chromium contributes to the hardenability of the present steel alloy and is also an excellent nitride former thereby enhancing nitriding characteristics. To best realize these effects a minimum of 0.40% chromium is required " , and preferably at least 0.90% chromium should be present. To avoid embrittlement, the amount of chromium should be limited to a maximum of 1.50%, and preferably no more than about 1.20%.
• Aluminum an essential ingredient of the present invention, contributes to hardenability and is a good nitrider former. As will be shown by examples, aluminum should be present in an amount of at least 0.07%, and preferably at least 0.10%.
• Vanadium is also an essential ingredient in the present alloy steel composition, and must be _ present in an amount of at least 0.03% to realize a consistently measurable enhancement of case and core hardness. Vanadium, in amounts greater than 0.20% does not significantly enhance the nitride response or the hardenability of the material. For these reasons, the limits of vanadium are at least 0.03% and no more than 0.20%; and preferably from 0.05% to 0.10% to make the best economic use of this ingredient.
• the remainder of the alloy steel composition is essentially iron except for nonessential or residual amounts of elements which may be present in small amounts.
• silicon in the recognized commercially specified amounts is used for deoxidation of the molten steel.
• silicon may be present in an amount of at least 0.10%.
• lead may be added in an amount up to about 0.15% to improve free machining characteristics of the material.
• Phosphorous in an amount over .05% may cause embrittlement, and preferably the upper limit should not exceed 0.035%.
• Manufactured articles such as shafts, couplings and gears, having the above stated composition, are preferably formed by forging or rolling and then hardened by heating to a temperature of about 870°C (1600°F) for a period of about one hour and quenched in either water or oil followed by machining to a desired final dimension.
• nitriding grade alloy steel compositions generally required oil quenching.
• the less restrictive requirement for the present material with respect to quench practice is a result of the contribution that aluminum makes to both nitrideability and hardenability, particularly in combination with the lower carbon limits of the preferred range.
• the increased freedom to select quench medium is therefore a valuable benefit of the present invention.
• test samples 1 and 3 contain relatively low amounts of aluminum, and test samples 2 and 4 contain aluminum in an amount close to the upper end of the preferred range.
• Vanadium in an amount representing the lower end of the preferred range, was added to test samples 3 and 4.
• the test samples were heated and quenched in accordance with the standard ASTM End Quench Test for Hardenability of Steel (A255) and the following hardness values were measured in 1/16 inch (1.59 mm) increments from the quenched ends.
• Fig. 1 illustrates the improved hardenability characteristics of test samples 2 and 4, having 0.18% aluminum over test samples 1 and 3 which respectively
• OMPI contain only 0.056% and 0.072% aluminum. While it is recognized that aluminum in amounts less than that present in test samples 2 and 4 may contribute somewhat to hardenability, it has been found that the influence of aluminum in amounts less than about 0.07% is inconsistent. Since aluminum readily combines with oxygen and nitrogen, the amount of aluminum available for hardening will depend on melt practices which influence the amount of free oxygen and nitrogen in the steel. For these reasons, 0.07% is considered to be the practical lower limit for consistent hardenability.
• test bars for nitriding from the four taps were tempered.
• the nitriding grade alloy steel of the present invention can be tempered at relatively high temperatures without adversely reducing core hardness.
• Each of the four tests bars, having the composition as identified in Table I above, were heated to a temperature of 593°C Q.100°F) and held at that temperature for three hours.
• Test sample 2 containing 0.18% aluminum and no vanadium, shows much higher surface hardness (3 to 7 points Rc) and substantially the same core hardness as test sample 1 which likewise contains no vanadium and has a lower aluminum content (0.056%).
• Both of the test samples containing about .05% vanadium, tests samples 3 and 4 have higher hardness than corresponding samples 1 and 2 which respectively have essentially the same base composition and aluminum addition but contain no vanadium.
• the highest hardness was measured on test sample 4 which contained .18% aluminum and 0.048% vanadium.
• the lowest hardness was measured on test sample 1 which had no vanadium and the lowest aluminum content, 0.056%.
• sample 4 containing 0.18% aluminum and about 0.05% vanadium had higher surface hardness, and at least as high core hardness, than samples 1-3 which contained lower amounts of aluminum or vanadium.
• Nitriding response is measured by the amount of time that a test sample must be held at an elevated temperature in an atmosphere containing raw ammonia to develop a predetermined nitride case depth.
• the measured composition of the three test pieces and their respective physical properties are listed below in Table III.
• the first test piece was formed of AISI/SAE 4140 material. This material has been commercially used by applicant and is generally recognized as a desirable nitriding grade alloy steel in heat treated and tempered, nitrided article applications.
• the second test piece, identified in Table III as test sample 5 contains aluminum and vanadium in the preferred range of the present invention.
• test sample 6 is similar to sample 5 except that sample 6 has added aluminum to increase the aluminum content to near the maximum amount desired in the more broadly contemplated range of the present invention.
• Two sets of the three above-described test pieces were prepared. Both sets were heated, quenched, and tempered as previously described for the test samples in Table II. After tempering, one set of the test pieces was nitrided in an atmosphere of raw ammonia gas with a dissociated ammonia carrier gas at a te perature of 526°C (980°F) for a period of 21 hours. The second set of test pieces was similarly nitrided for a period of 48 hours.
• ** core hardness was measured as a Vickers Hardness number (DPH) and converted to a corresponding value on the Rockwell C scale.
• case depth is defined as the distance from the surface at which a Vickers hardness of 423 was measured.
• OMPI From the graph shown in Fig. 3, it can be seen that the 0.27 mm case depth of the prior art AISI/SAE 4140 material after 21 hours nitriding time can be realized in about 15 hours with the composition of the present invention. Similarly, a 0.38 " mm case depth in the new material, having the preferred range of aluminum, can be reached in only about 25 hours whereas the prior art material required 48 hours. Thus it can be seen that nitriding time can be significantly reduced for articles having the composition of the present invention. For a case depth of 0.3 mm the nitriding time can be reduced on the order of 40% thereby effecting significant cost savings.
• the present invention is particularly useful in the manufacture of oil or water quenched, nitrided articles such as gears, shafts, bushings and similar parts where maximum nitriding response is desired.
• the alloy steel composition of the present invention is economical to produce in that it does not require expensive alloy additions.
• the new alloy steel is especially desirable in applications where nitrided articles and first hardened and then subsequently tempered and machined to final dimensions prior to nitriding. When applied in such instances, the new alloy steel provides a unique combination of hardenability, retention of high core hardness after tempering, and additional processing cost savings as a result of the material's excellent nitriding response.

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• Chemical & Material Sciences (AREA)
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• Mechanical Engineering (AREA)
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• 1984
  • 1984-02-13 AU AU25715/84A patent/AU2571584A/en not_active Abandoned
  • 1984-02-13 JP JP59500984A patent/JPH0617540B2/ja not_active Expired - Lifetime
  • 1984-02-13 WO PCT/US1984/000205 patent/WO1985003527A1/en active IP Right Grant
  • 1984-02-13 DE DE8484900944T patent/DE3482597D1/de not_active Expired - Fee Related
  • 1984-02-13 BR BR8407278A patent/BR8407278A/pt not_active IP Right Cessation
  • 1984-02-13 EP EP84900944A patent/EP0172165B1/de not_active Expired - Lifetime
• 1985
  • 1985-01-29 CA CA000473063A patent/CA1243507A/en not_active Expired

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