Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/058—Alloys based on nickel or cobalt based on nickel with chromium without Mo and W
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01L—CYCLICALLY OPERATING VALVES FOR MACHINES OR ENGINES
  • F01L3/00—Lift-valve, i.e. cut-off apparatus with closure members having at least a component of their opening and closing motion perpendicular to the closing faces; Parts or accessories thereof
  • F01L3/02—Selecting particular materials for valve-members or valve-seats; Valve-members or valve-seats composed of two or more materials

Definitions

• the present invention relates generally to corrosion and heat resistant alloys and, more particularly, is directed to a Fe-Ni-Cr alloy useful for diesel engine components, primarily exhaust valves.
• the alloy features a favorable balance of low cost, high-temperature monotonic and fatigue strength, corrosion resistance, and metallurgical stability.
• the alloy of the present invention may also be usefully employed in the manufacture of other diesel engine parts such as, for example, exhaust train components which experience similarly aggressive environments. 2 Description of the Prior Art
• the aforementioned alloys exhibit some shortcoirrings.
• the aforementioned alloys exhibit some shortcoirrings.
• Pyromet® 31V features a relatively high Ni content and was also found to precipitate a potentially embrittling acicular alpha ( )-Cr phase after extended' exposures to service temperatures of 760°C (1400°F).
• the 40 Ni alloy is low cost but contains only moderate amounts of Cr, thus impairing corrosion resistance. Furthermore, the alloy seems to be prone to unwanted eta ( ⁇ )-phase (Ni 3 Ti) precipitation upon extended exposure harming ductility.
• alloy HI ® 461 which features a dispersion of primary TiC carbides in addition to the customary gamma prime ( ⁇ ')-strengthening. It was felt, however, that further performance enhancements at the same moderate cost level were still needed in order to achieve even further improvements in engine performance and reliability.
• the present invention is directed to an improved alloy particularly suited for diesel engine exhaust valves and the like which features an attractive balance of low cost, high-temperature monotonic and fatigue strength, corrosion and abrasion resistance, metallurgical stability, and ease of fabrication.
• the alloy according to the present invention is characterized by a composition in weight percent of about 0.15-0.65% C, 40-49% Ni, 18-22% Cr, 1.2-1.8% Al, 2.0-3.0% Ti, 0.9-7.8% Nb, not more than 1% Co and Mo each, the balance being Fe and inevitable impurities, whereby the Ti : Al ratio (wt.%) must not exceed 2 : 1, and the
• Nb : C ratio (wt.%) is adjusted to lie within the range of 6 : 1 and 12 : 1 (or 0.8 : 1 to 1.5 : 1 on an atomic basis).
• a further presently preferred Nb range is 0.9-6.5 wt.% with a Nb : C ratio of between 6:1 and 10:1 on a wt.% basis (or 0.8:1 to 1.3:1 on an atomic basis).
• Nb may be partially substituted for Ta on an equiatomic basis. In this case, the ratio of the combined atomic percentage (Nb + Ta) to C present should be adjusted to lie within the range of 0.8 - 1.5.
• the alloy may also contain certain elements essential for deoxidation/desulfurization and improved hot workability in the following amounts: up to 2.0% Mn, up to 0.01% B, and up to 0.3% Zr. Silicon additions up to 1.0 wt.% are also beneficial to improve the alloy's oxidation resistance.
• the C content is limited to 0.25-0.55%
• the Ni content is 42-48%
• the Cr content is 19-21%
• the Al content is 1.4-1.7%
• the Ti content is 2.3-2.7%
• the Nb content is 1.8- 5.5%
• the balance being Fe and inevitable impurities
• the Nb : C weight % ratio is adjusted to lie within the range of 7 : 1 and 10 : 1
• the Ti : Al weight % ratio is less than or equal to 2 : 1.
• a still more preferred Nb range is about 2.5-3.0%.
• the microstructure of the alloy of the present invention features, even after extended exposures to valve operating temperatures in the vicinity of 760 °C (1400 °F), essentially a uniform dispersion of micron size Nb-rich primary MC type carbides, fine discrete Cr-rich secondary M 2 C 6 type carbides in the austenite grain boundaries, and submicroscopic intragranular ⁇ ' precipitates. Moreover, the microstructure of a preferred embodiment of the invention features less than 5 vol.% of any acicular phase.
• the present invention further includes diesel engine valves, particularly exhaust valves, as well as other exhaust train components, manufactured from the above described alloy.
• Fig. 1(a) is a graph of ultimate tensile strength versus temperature for Alloys 1-5 of the present invention and several comparative alloys of the prior art;
• Fig. 1(b) is a graph of tensile elongation versus temperature for Alloys 1-5 of the present invention and several comparative alloys;
• Fig. 2 is a graph of rotating beam fatigue strength versus cycles to failure showing fatigue data for Alloys 1-5 of the present invention and several comparative alloys;
• Fig. 3 is a graph of hardness versus temperature for Alloy 2 of the present invention and two comparative alloys;
• Figs. 4a - 4c are bar graphs depicting hot salt corrosion attack on alloys of the present invention and several comparative alloys;
• Figs. 5a and 5b are bar graphs showing Charpy impact strength of alloys of the invention and comparative alloys;
• Fig. 6 is a scanning electron photomicrograph of Alloy 2 of the invention after 2,500 hours' exposure to a temperature of 1400°F (760°C). DETAILED DESCRIPTION OF THE INVENTION
• the chemical composition of the alloy is limited as described below.
• C: 0.15-0.65 wt.% Carbon (C) in the amounts present combines during melting with Nb and
• Nickel (Ni) stabilizes the austenitic matrix phase and is essential for the formation of the strengthening ⁇ ' phase (Ni 3 (Al,Ti)) utilized to impart heat resistance on the alloy.
• Ni is, on a cost basis, a relatively expensive alloying element (in comparison with Fe) and, hence, limited to less than 49 wt.%.
• the lower bound of 40 wt.% is determined by metallurgical stability considerations, i.e., the increasing propensity of the alloy to form harmful TCP (topologically close packed) phases, in particular sigma ( ⁇ ) phase, upon extended service.
• Chromium (Cr) is of paramount importance in imparting high- temperature oxidation and corrosion resistance to the alloy. Controlled laboratory tests simulating hot salt corrosion in an engine environment have shown that a minimum amount of 18 wt.% Cr is needed to attain satisfactory corrosion resistance. When Cr is added in amounts greater than 22 wt.%, however, the alloy becomes prone to massive intragranular precipitation of acicular phases, ⁇ or ⁇ -Cr upon extended exposures to 760 °C, thus harming ductility and toughness. Cr contents in the above defined range can also be used to promote the precipitation of discrete secondary grain boundary carbides of the M 23 C 6 type, thus increasing stress rupture strength.
• Al 1.2-1.8 wt.%
• Aluminum (Al) is the primary hardening element leading in the above amounts present to the formation of ⁇ ' (Ni 3 (Al,Ti)).
• ⁇ ' Ni 3 (Al,Ti)
• Al contents below 1.2 wt.% the volume fraction of ⁇ ' is too small to meet the monotonic and fatigue strength targets.
• Contents of Al greater than 1.8 wt.% result, however, in increasing hot workability problems when forming the valves.
• Ti 2.0-3.0 wt.%
• Titanium (Ti) is, next to Al, of utmost importance for the formation of ⁇ '. Moreover, by virtue of increasing the anti-phase boundary energy of ⁇ ', alloying with Ti also results in a stronger precipitate, thus improving the overall strength of the alloy. On the other hand, exceedingly large amounts of Ti lead to phase instability, i.e., the precipitation of eta ( ⁇ )-phase (Ni Ti). This ⁇ -phase is generally considered harmful for ductility. Hence, the Ti : Al wt.%> ratio is limited to 2 : 1. The total combined amount of hardener elements (Al+Ti) is adjusted to balance strength requirements with fabricability of the alloy.
• Nb 0.9-7.8 wt.%
• the primary purpose- of alloying with niobium (Nb) is to cause precipitation of primary Nb-rich MC carbides.
• These Nb-rich carbides are more effective than Ti-rich MC carbides in increasing the abrasion resistance of the alloy owing to their higher hot hardness.
• the Nb content is carefully balanced against the C content.
• C weight ratios less than 6.5 : 1 or 6 : 1 (or 0.8 : 1 on an atomic basis) the primary carbides become increasingly Ti-rich, thus diminishing the positive effect on the abrasion resistance.
• Nb C ratios greater than 12 : 1 (or 1.5 : 1 on an atomic basis), the uncombined Nb tends to overalloythe austenitic matrix, thus raising the solvus temperature of harmful TCP phases above the valve operating temperature.
• the Nb : C weight % ratio should reside within the range of 6 : 1 to 12 : 1 or within the range of 0.8 : 1 to 1.5 : 1 on an atomic basis.
• a presently preferred broad range for Nb is about 0.9 to 7.8 wt.%, with a preferred intermediate range of 0.9 to 6.5 wt.% Nb and a narrow range of 1.8 to 5.5 wt.% Nb, or a more narrow range of 2.5 to 3.0 wt.% Nb.
• Nb also improves the weldability of ⁇ '-hardened superalloys and, likewise, increases corrosion resistance in sulfidizing environments, such as those encountered in diesel engines.
• Nb may be partially substituted for tantalum (Ta) on an equiatomic basis, cost permitting.
• Ta also strongly stabilizes the primary MC carbide and is surmised to be equally beneficial to hot hardness and abrasion resistance.
• Co not more than 1 wt.%
• Co Co
• Mo not more than 1 wt.%
• molybdenum (Mo) at levels exceeding 1 wt.% impairs corrosion resistance in sulfur-containing environments at valve operating temperatures.
• Mn not more than 2 wt.%
• Mn manganese
• B not more than 0.01 wt.%
• B Boron (B) effectively improves the hot workability and creep rupture strength if present in small amounts. Excessive amounts of B, however, harm hot workability. Zr: not more than 0.3 wt.%
• zirconium Like boron, zirconium (Zr) is also effective in improving the hot workability and creep rupture strength if present in small amounts. Zr in excessive amounts, however, harms hot workability. Si: not more than 1.0 wt.%
• Si is an element effective in improving the oxidation resistance of the alloy. However, excessive additions of Si deteriorate the ductility of the material.
• Fe balance Iron (Fe) is essentially a matrix-forming element and comprises the balance of the alloy including unavoidable or incidental impurities and trace elements in residual amounts.
• a more narrow, presently preferred alloy composition according to the invention consists essentially of in % by weight: 0.25-0.55% C, 42-48% Ni, 19-21% Cr, 1.4-1.7% Al, 2.3-2.7% Ti, 1.8-5.5% Nb, the balance essentially Fe and incidental impurities, and wherein the Nb : C weight % ratio is about 7 : 1 to 10 : 1.
• the Nb range may be further narrowed to about 2.5-3.0 wt.%.
• Alloy 1 through Alloy 5 and two comparative alloys mimicking HI ® 461 and the 40 Ni alloy were vacuum induction melted and cast into 22 kg (50 lb.) ingots.
• a conventional Ca + Mg deoxidation practice was used.
• Hot hardness tests up to 760°C (1400°F) using a Rockwell A tester and converting the hardness numbers to Rockwell C are reported in Fig. 3 to rank the alloys in terms of their abrasion resistance.
• the highest hot hardness was measured on an alloy of this invention, thus demonstrating a superior abrasion resistance of this alloy over the comparative alloys. It can, hence, be expected that hardfacing the alloy of this invention will not be necessary.
• Hot salt corrosion tests (an 80-hour standard, and a 250-hour aggravated test) in a mixture of CaSO 4 : BaS0 4 : Na 2 SO 4 : C in a ratio of 10 : 6 : 2:1, respectively, at a temperature of 870 °C (1598°F) are reported in Figs. 4a, 4b and 4c.
• the longer the bar graphs on Figs. 4a-c the poorer the corrosion resistance of the particular alloy tested.
• Each alloy tested is listed in the box appearing on each of Figs.
• alloy HI 461 is identified with the letter "(A)", alloy 40 Ni as “(B)", alloy 1 of the invention as “(C)”, alloy 2 of the invention as “(D)”, and alloy 751 as “(E)”.
• alloys 2-5 of the present invention are identified as follows: alloy 2 as “(D)”, alloy 3 as “(G)”, alloy 4 as "(H)” and “(1)” (duplicate test) and alloy 5 as "(J)".
• Metallurgical stability tests by means of long-term exposures to 760 °C (1400°F) up to 2,500 hours and subsequent Charpy impact testing as a sensitive indicator of potential embrittlement are reported in Fig. 5, assisted by metallographic evaluation of the exposed lnicrostructures shown in Fig. 6.
• the alloys of the present invention exhibit at least an equivalent retention of toughness as the comparative alloys upon long-term exposures. This is consistent with the metallographic inspections of Fig. 6 in that only minuscule amounts, if any, of intragranular acicular phase formed during aging.
• grain boundary carbides remained discrete in nature and, thus, in a preferred morphology.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• General Engineering & Computer Science (AREA)
• Heat Treatment Of Steel (AREA)
• Cylinder Crankcases Of Internal Combustion Engines (AREA)
• Powder Metallurgy (AREA)
• Pressure Welding/Diffusion-Bonding (AREA)
• Exhaust Silencers (AREA)

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• 2001
  • 2001-08-22 US US09/934,892 patent/US6372181B1/en not_active Expired - Lifetime
  • 2001-08-23 EP EP01964346A patent/EP1313888B1/en not_active Expired - Lifetime
  • 2001-08-23 AT AT01964346T patent/ATE267271T1/de active
  • 2001-08-23 AU AU2001285211A patent/AU2001285211A1/en not_active Abandoned
  • 2001-08-23 CA CA002420346A patent/CA2420346A1/en not_active Abandoned
  • 2001-08-23 WO PCT/US2001/026277 patent/WO2002016661A2/en not_active Ceased
  • 2001-08-23 DE DE60103410T patent/DE60103410T2/de not_active Expired - Lifetime
  • 2001-08-23 JP JP2002522331A patent/JP3905034B2/ja not_active Expired - Fee Related

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