Iron-based alloy for internal combustion engine valve seat inserts and the like

[0001] This application claims the benefit under 35 USC 119(e) of copending U.S. provisional patent application Serial No. 60/137,651 filed Jun. 4, 1999.

[0002] The present invention relates to corrosion and wear resistant iron-based alloys, and particularly to alloys for use in valve seat inserts.

[0003] In diesel engines containing valve seat inserts, more restrictive exhaust emissions laws for diesel engines have driven changes in engine design including the need for high-pressure electronic fuel injection systems. Engines built with these systems have experienced significantly higher wear rates in their intake inserts and valves. This has driven a shift in material selection toward materials which offer improved wear resistance relative to the martensitic stainless steels which have traditionally been employed by the diesel industry.

[0004] Another emerging trend in diesel engine development is the use of EGR (exhaust gas recirculation) to further reduce exhaust emissions. With EGR, exhaust gas is routed back into the intake air stream. Since exhaust gas contains compounds of nitrogen, sulfur, chlorine, and other elements which potentially can form acids, the need for improved corrosion resistance for alloys used in intake insert applications is greatly increased for diesel engines using EGR. Acid can attack both insert and valves leading to premature engine failure. Earlier attempts to achieve improved corrosion resistance were typically accomplished through the use of martensitic stainless steels. Though these steels provide good corrosion resistance, martensitic stainless steels do not have adequate wear resistance to meet the requirements for valve seat inserts in modem diesel truck engines.

[0005] A desirable alloy would exhibit good corrosion and wear resistance suitable for use in intake insert applications in diesel engines using EGR.

[0006] The present invention provides an iron-based alloy with both good corrosion resistance and good wear resistance. The alloy is suitable for use in intake valve seat insert applications, even in diesel engines using EGR.

[0007] One aspect of the invention is an iron-based alloy comprising, in weight percentage: carbon from about 1.1 to about 1.4; chromium from about 11 to about 14.25; molybdenum from about 4.75 to about 6.25; tungsten from about 3.5 to about 4.5; cobalt from 0 to about 3; niobium from about 1.5 to about 2.5; vanadium from about 1 to about 1.75; copper from 0 to about 2.5; silicon up to about 1; nickel up to about 0.8; manganese up to about 0.6; and iron from about 61 to about 77.15.

[0008] Another aspect of the invention is a cast, iron-based alloy comprising, in weight percentage: carbon from about 1.1 to about 1.4; chromium from about 11 to about 14.25; molybdenum from about 4.75 to about 6.25; tungsten from about 3.5 to about 4.5; and the balance iron, other alloying elements, and incidental impurities.

[0009] In another aspect of the invention, parts such as valve seat inserts are made from the alloy, such as by casting or forming from a powder and sintering, or the alloy may be used to hard-face the parts.

[0010] The present invention is directed to an iron-based alloy with good corrosion resistance and wear resistance. The alloy is designed particularly for use in internal combustion engine valve seat inserts, but many other applications are feasible.

[0011] In order to accomplish both corrosion resistance and wear resistance, the alloy has a microstructure similar to martensitic tool steels. Martensitic tool steels for valve seat inserts exhibit a microstructure comprised of small secondary carbides in a fine equiaxed grain structure. In contrast, martensitic stainless steels used for valve seat inserts typically comprise large primary carbides in a coarser columnar casting structure. The finer equiaxed casting structures have greater wear resistant properties than coarser columnar structures. Although tool steels tend to exhibit good wear resistance, stainless steels typically exhibit greater corrosion resistance. The alloy of the present invention is particularly beneficial in that it exhibits both good wear resistance and good corrosion resistance.

[0012] The alloy exhibits a microstructure and hardness similar to cast tool steel with greatly improved corrosion resistance. The alloy of the invention may be made in cast, powdered metal, or hardfacing forms by conventional methods. The alloy is preferably manufactured by casting, which is a well-known process involving melting the alloy components and pouring the molten mixture into a mold. In a preferred embodiment, the alloy is used in the manufacture of valve seat inserts for use in heavy-duty diesel engines. The alloy may find advantageous utility in other applications including, but not limited to, valve seat inserts made for gasoline, natural gas, or alternatively fueled internal combustion engines. Valve seat inserts may be manufactured by conventional methods known in the art.

[0013] The alloy of the invention exhibits good corrosion resistance while maintaining the fine-grained martensitic tool steel microstructure which provides excellent wear resistance. In addition to good corrosion and wear resistance, the alloy also possesses good high-temperature properties.

[0014] The chromium level and carbon content are believed to contribute to the beneficial properties of the alloy. Chromium (Cr) is present in the alloy in an amount of at least about 11 weight percent of the total alloy; preferably, at least about 11.5 weight percent; more preferably, at least about 12 weight percent; even more preferably, at least about 12.75 weight percent. The chromium content is less than about 14.25 weight percent; preferably, less than about 14.15 weight percent; more preferably less than about 14 weight percent; even more preferably less than about 13.50 weight percent; most preferably about 13 weight percent. This relatively high chromium level provides the beneficial corrosion resistance properties of stainless steel.

[0015] The good wear resistance properties are believed to be attributed to the microstructure and hardness of the alloy. The relatively low carbon concentration inhibits the formation of primary carbides and instead results in the formation of secondary carbides which, as discussed above, are finer. A primary carbide typically forms before the bulk material freezes. In contrast, secondary carbides freeze after the bulk material freezes.

[0016] Carbon (C) is present in the alloy in an amount ranging from about 1.1 to about 1.4 weight percent of the total alloy; preferably, at least about 1.15 weight percent; preferably, at most about 1.35 weight percent; most preferably, about 1.25 weight percent.

[0017] Molybdenum (Mo) is present in the alloy in an amount ranging from about 4.75 to about 6.25 weight percent of the total alloy; preferably, at least about 5 weight percent; more preferably, at least about 5.25 weight percent; preferably, at most about 6.15 weight percent; more preferably at most about 6 weight percent.

[0018] Tungsten (W) is present in the alloy in an amount ranging from about 3.5 to about 4.55 weight percent of the total alloy; preferably, at least about 3.75 weight percent; preferably, at most about 4.5 weight percent.

[0019] Cobalt (Co) may be present in the alloy in an amount less than about 3 weight percent of the total alloy; preferably, at most about 2.5 weight percent; more preferably, at most about 2.45 weight percent; even more preferably, at most about 2.25 weight percent; preferably, at least about 1.5 weight percent; more preferably, at least about 1.75 weight percent; even more preferably, at least about 1.95 weight percent; most preferably, about 2 weight percent cobalt is employed. Cobalt is believed to improve hot hardness. Alternatively, copper may be substituted for cobalt.

[0020] Copper (Cu) may be present in the alloy in an amount less than about 2.5 weight percent of the total alloy; preferably, about 2 weight percent if no cobalt is employed.

[0021] Niobium (Nb) is present in the alloy in an amount ranging from about 1.5 to about 2.55 weight percent of the total alloy; preferably, at least about 1.75 weight percent; preferably, at most about 2.5 weight percent. Niobium is added to provide microstructure grain refinement. Preferably, the Niobium content is about 2 weight percent.

[0022] Vanadium (V) may be present in the alloy in an amount ranging from about 1 to about 1.75 weight percent of the total alloy; preferably, at least about 1.15 weight percent.

[0023] Silicon (Si) may be present in the alloy in an amount up to about 1 weight percent of the total alloy.

[0024] Nickel (Ni) may be present in the alloy in an amount up to about 0.8 weight percent of the total alloy.

[0025] Manganese (Mn) may be present in the alloy in an amount up to about 0.6 weight percent of the total alloy.

[0026] Boron (B) may be present in the alloy in less than about 1.0 weight percent of the total alloy. Preferably, no boron is included.

[0027] The balance of the alloy is iron (Fe), and incidental impurities. Generally, the alloy contains an amount ranging from about 61 to about 77 weight percent iron.

[0028] A cast alloy is prepared having the following composition.

	TABLE 1
	Element	Weight Percent
	Carbon	1.25
	Silicon	0.6
	Chromium	13.15
	Molybdenum	5.75
	Tungsten	4.0
	Cobalt	2.05
	Vanadium	1.25
	Niobium	2.05
	Manganese	0.3
	Nickel	0.55
	Iron	69.05

[0029] The hardness of an alloy having the composition shown in Table 1 was tested at room temperature and at various elevated temperatures.

[0030] The Rockwell C hardness of the resulting alloy at room temperature is 45. For comparison, the hardness of a martensitic stainless steel (J125) and a martensitic tool steel (J120V) is also measured. J125 and J120V are available from L. E. Jones Company. The Rockwell C hardness of J125 and J120V at room temperature is 40 and 44, respectively. The hot hardness results are shown below.

Hardness (HV10)
	Temperature	Invention	J125	J120V
	75 F. (24 C.)	583	397	536
	200 F. (93 C.)	548	389	530
	400 F. (204 C.)	508	358	493
	600 F. (315 C.)	484	344	465
	800 F. (427 C.)	445	306	416
	1000 F. (538 C.)	348	215	344
	1200 F. (649 C.)	198	119	209
	1400 F. (760 C.)	100	47	104

[0031] The alloy of the invention possesses excellent hot hardness with results which are comparable to or exceed those of tool steel at all elevated temperature levels tested.

[0032]

Temperature	Invention	J125	J120V
75 F. (24 C.)	169 ksi (1165 MPa)	139 ksi	198 ksi (1365 MPa)
		(958 MPa)
600 F. (315 C.)	141 ksi (972 MPa)	125 ksi	153 ksi (1055 MPa)
		(862 MPa)
800 F. (427 C.)	136 ksi (938 MPa)	120 ksi	139 ksi (958 MPa)
		(827 MPa)

[0033]

	Temperature	Invention	J125	J120V
	25-200 F. (4-93 C.)	9.93	11.00	11.08
	25-400 F. (4-204 C.)	10.89	12.25	12.24
	25-600 F. (4-315 C.)	11.57	12.91	12.76

[0034] All CTE values are expressed in (10⁶ mm/mm C.).

[0035]

	Temperature	Invention	J125	J120V
	25 F. (4 C.)	16.8	15.4	23.4
	200 F. (93 C.)	20.8	18.8	28.3
	400 F. (204 C.)	23.9	20.1	27.8
	600 F. (315 C.)	24.9	23.7	27.6

[0036] All thermal conductivity values are expressed in W/mK.

[0037] Sulfidation Test: The sulfidation test involves heating a pre-weighed cylindrical test specimen ( (12.7 mm) diameter X (12.7 mm) length) to a temperature of 1500 F. (816 C.) for 100 hours while submersing the specimen in a powder mixture as follows:

[0038] 10 parts CaSO₄, 6 parts BaSO₄, 2 parts Na₂SO₄, 1 part Graphite

[0039] When the 100-hour bake is complete, the specimens are carefully cleaned and re-weighed. The difference in weight per unit surface area before and after test is then calculated. Sulfidation test results for the alloy of the invention, J125, and J120V were as follows:

	Weight Loss (g/cm2):	Invention:	0.05 g.
		J125:	0.08 g.
		J120V:	54.7 g.

[0040] The sulfidation test results show that the alloy of the invention and J125 experienced little to no corrosion while J120V corroded very heavily. Thus, the alloy of the invention provides good sulfidation resistance compared to a tool steel.

[0041] Oxidation Test: The oxidation test involves heating pre-weighed test specimens exposed to air to an elevated temperature such as 900 F. (482 C.), and 1300 F. (704 C.), and 1500 F. (816 C.) for 50 minutes. After cooling in air, the part is re-weighed to determine weight gain due to oxidation. Results were as follows:

[0042] Weight gain is expressed in g/cm² and in parentheses in mg/(hr m²).

Temperature	Invention	J125	J120V
900 F. (482 C.)	<0.1 (1.2)	<0.1 (1.2)	1.3 (15.6)
1300 F. (704 C.)	0.8 (9.6)	0.4 (4.8)	2.8 (33.6)
1500 F. (816 C.)	5.7 (68.4)	0.6 (7.2)	12.4 (148.8)

[0043] The results of this test show that the alloy of the invention exhibits a substantial improvement over J120V, particularly at the 900 F. and 1300 F. temperatures. Improvement at these temperatures is most relevant since insert temperatures are typically 900 F. (482 C.) or below.

[0044] A second oxidation test involves heating preweighed specimens exposed to air to an elevated temperature of 800 F. (427 C.) for 100 hours. After cooling in a vacuum, the part is reweighed to determine weight gain due to oxidation. Results were as follows:

[0045] Weight gain is expressed in mg/(hr m²).

	Temperature	Invention	J120V
	800 F. (427 C.)	0.4	3.9

[0046] Wear tests were performed on a wear test fixture. The fixture simulates constant speed, sliding wear without lubrication. A stationary (9.5 mm) wide test block is held against a (12.7 mm) diameter cylinder rotating at 1725 RPM at loads of 4.5 pounds (2.05 kg), 9 pounds (4.10 kg), and 13.5 pounds (6.15 kg). Cylinder material is Eatonite 6, a commonly used valve facing material. Results in terms of weight loss on the test block are as follows:

Test Block Weight Loss (milligrams)
	Invention		J120V
	Load (kg)	J125	Test 1	Test 2	Test 1	Test 2
	2.05	50	25	16	15	20
	4.10	65	40	30	29	27
	6.15	160	50	32	27	31

[0047] These results show the alloy of the invention has improved wear resistance relative to stainless steels like J125 which is traditionally employed in the diesel industry.