Exhaust valve alloy

Description

[0001] The present invention is concerned with components of diesel exhaust systems particularly of diesel exhaust systems in large marine diesel engines.

BACKGROUND OF THE INVENTION

[0002] In the past twenty years or so, a number of suggestions have been made concerning alloys suitable for diesel engine exhaust valves and other components of diesel engine exhaust systems. The documents set forth in Table I contain many of these suggestions.

TABLE I

Patent No.	Inventor	Date
U.S. 3,573,901	Economy	1971
U.S. 3,972,713	Muzyka et al	1976
U.S. 4,379,120	Whitney et al	1983
U.S. 4,631,169	Isobe et al	1986
U.S. 4,714,501	Yamanaka et al	1987
U.S. 4,715,909	Minami et al	1987
Japanese Appln. 85/101,589	--	1985
Alloy Digest Pyromet 31, Dec. 1977

As valid and as useful as these suggestions may be, they have not solved satisfactorily a continuing problem of severe hot erosion and corrosion which exists when diesel exhaust components are exposed to hot (e.g. greater than 650°C) diesel exhaust streams resulting from internal combustion in marine atmospheres of poor grades of marine diesel fuels.

[0003] Specifications for marine fuels have been established by the International Organization for Standardization, Petroleum Products-Fuels (Class F), Specifications of Marine Fuels, ISO8217:1987(E) (1987) and the International Council on Combustion Engines (CIMAC). Specifications for the poorest grades of marine diesel fuel as specified by these organizations is set forth in Table II.

TABLE II

	ISO 8217 RMH/RML 55	CIMAC 12/13
Density at 15°C kg/l	0.991	0.991/1.010
Kinematic viscosity at 80°C		130
(centi-stokes) at 100°C	55	55
Flash point °C	60	60
Pour point °C Winter quality	30	30
Summer quality	30	30
Conradson carbon res. w%	22	22
Ash (max) w%	0.2	0.2
Water content (max) v%	1.0	1.0
Sulfur content (max) w%	5.0	5.0
Vanadium content (max) wppm	600	600
Aluminum content (max) wppm	--	30

The grades of marine diesel fuels as set forth in Table II are distinguished from diesel fuels employed in most automotive, truck and land-based vehicle and machinery usages in not being distillate fuels. Thus, contaminants, particularly sulfur and vanadium which are to a great extent not present in distillate fuels, are generally characteristic of localities at which a diesel powered ship may be bunkered. In short, depending upon where a ship picks up fuel, the diesel fuel taken on may grade from reasonable to very poor especially with respect to vanadium content. Accordingly, the exhaust system of a marine diesel engine should comprise components which are resistant to the products of combustion in a marine atmosphere of a representative grade of fuel likely to be encountered. As far as is known, the prior art has not solved the problem of providing such components in a satisfactory manner.

[0004] For purposes of this specification and claims, the term "marine atmosphere" means atmospheric air which contains aerosol size or larger particles of sea salt generally produced by the action of wind on the crests of ocean waves. Wind tears at the crest of ocean waves dislodging particles of water containing sea salt. Some of these particles fall back into the ocean as spray while others tend to remain in suspension in the air either as liquid or as solid particles of dried out (or semi-dried out) solid. Thus, marine atmospheres contain not only high amounts of water vapor, but also significant amounts of sodium, potassium and other metals principally in the form of chlorides. For all practical purposes, a marine diesel engine is continuously burning fuel with air which contains these metals.

STATEMENT OF THE INVENTION

[0005] The present invention contemplates components of the exhaust systems of marine diesel engines particularly exhaust valves having at least exhaust contacting surfaces made of an alloy comprising 0.02-0.07% carbon, 24-32% chromium, 0.7-3.0% titanium, 0.7-1.5% aluminum, 0.7-1.5% niobium, 0 to 0.1% zirconium, 0-0.006% boron, 0-1% iron, balance essentially nickel, together with conventional amounts of incidental and impurity elements. More specifically, entire exhaust components of marine diesel engines are made of the alloy of the present invention which can be in cast/­wrought form or can be made by powder metallurgical or other methods. Once the alloy is made and formed into the exhaust syo tem component desired, it can be heat treated by age-hardening in the range of about 675°C to about 725°C for about 2 to about 48 hours preferably after solution treatment at a temperature in excess of about 1000°C. Those skilled in the art will appreciate that other aging treatments can be employed. For example, age-hardening can be accomplished by slow cooling through the age-hardening temperature range or by use of two or more steps where the alloy is held at each step for a particular temperature and a particular length of time.

[0006] In order that the art be particularly apprised of the intent of applicant in setting forth the range of alloying elements as stated hereinbefore, specific amounts of each element constituting the alloy of, and employed in, the present invention are set forth in Table III with the understanding that alloy ranges within the scope of the present invention can be constructed from each and every value set forth in Table III.

TABLE III

Element	% by Weight
Carbon	0.02	0.03	0.04	0.05	0.06	0.07
Chromium	24	25	26	28	30	32
Titanium	0.7	1.2	1.8	2.0	2.4	3.0
Aluminum	0.7	0.9	1.1	1.3	1.5	--
Niobium	0.7	0.9	1.1	1.3	1.5	--
Zirconium	0	0.02	0.04	0.06	0.08	0.1
Boron	0	0.001	0.002	0.003	0.004	0.006
Iron	0	0.2	0.4	0.6	0.8	1.0
Nickel	Balance Essentially

In formulating alloys and exhaust system components made of the alloys of the present invention, it is vital that the iron content of the alloys be maintained at less than about 1% by weight. In other words, iron should not exceed that amount which may be introduced inadvertently by use of scrap in preparing a melting charge or otherwise. As will be discussed hereinafter, the corrosion rate of alloys similar to those of the invention but containing in excess of 1% iron is significantly increased compared to that of alloys of the invention when tested at temperatures in excess of about 700°C in contact with synthetic ash containing vanadium in oxide form. Residual fuel ash deposits resulting from combustion of diesel fuel as described in Table II generally have compositions as set forth in weight percent in Table IV.

TABLE IV

Component	Range	Typical
V₂O₅	38-75%	42%
Na₂O	4-11%	11%
CaO	4- 9%	7%
Fe₂O₃	1-10%	7%
NiO	5-11%	9%
SO₃	11-22%	18%

Ash corrosion test results specified herein were obtained with a synthetic ash containing 40% V₂O₅, 10% NaVO₃, 20% Na₂SO₄, 15% NiSO₄ and 15% CaSO₄ at temperatures of 650°C and 750°C. As will be made evident by data hereinafter, iron is very detrimental to corrosion resistance in this synthetic ash at temperatures above about 700°C.

EXAMPLES OF THE INVENTION

[0007] Five casts of Example 1 and four casts of Example 2 were made having average compositions in percent by weight as set forth in Table V.

TABLE V

Element	Example 1	Example 2
C	0.03	0.03
Al	1.32	0.77
Cr	24.9	29.9
Nb	1.33	0.72
Ti	2.67	1.60
Zr	0.08	0.07
B	0.004	0.004
Ni	Bal.	Bal.

The casts of the alloys of Example 1 and Example 2 were forged to bar stock and made into marine diesel exhaust valves. The alloys were heat treated for 2 hours at 1080°C followed by air cooling and, subsequently were aged for 16 hours at 700°C followed by air cooling. These valves were tested in actual marine engine usage using residual fuel SSF₇ with a sulfur content of 3.6% max., a vanadium content of 380 ppm max., an aluminum content of 3 ppm max. and a maximum ash of 0.1% (all by weight) and found to be very satisfactory. The tests of these valves gave full indication of practical utility in marine diesel engine service.

[0008] Prior to production of casts of the alloys of Examples 1 and 2 laboratory heats of the current best alloy of choice for marine diesel exhaust valves (Alloy A) and Examples 1 and 2 were cast and forged and heat treated as specified hereinbefore. Actual compositions of these laboratory heats, in percent by weight, are set forth in Table VI.

TABLE VI

Element	Alloy A	Example 1	Example 2
C	--	--	--
Si	0.05	0.02	0.01
Mn	0.01	0.01	0.01
Al	1.55	1.35	0.77
Co	0.01	0.02	0.01
Cr	19.37	24.90	31.24
Fe	0.04	0.10	0.05
Mo	--	0.02	0.01
Nb	--	1.20	0.74
Ni	Bal. (76.36)	Bal. (69.81)	Bal. (65.52)
Ta	--	0.01	--
Ti	2.48	2.55	1.62
V	0.01	0.01	0.01
W	0.02	0.01	--
Zr	0.065	0.001	0.001
Cu	0.03	0.01	0.01
P	0.004	0.002	0.002

Tensile properties of forged and machined samples of the alloys specified in Table VI are set forth in Table VII.

TABLE VII

Alloy	Test Temp. (°C)	Stress N/mm²			Elong. %	R of A %
		0.1% PS	0.2% PS	TS
A	RT	797	818	1271	25.4	39.1
	500	722	745	1109	23.2	44.5
	550	704	732	1085	23.2	41.0
	600	706	728	1086	20.5	32.1
	650	687	712	1047	17.9	23.5
	700	655	691	967	17.9	21.1
	750	612	632	829	17.9	23.0
Ex. 1	RT	860	875	1274	25.9	38.6
	500	780	799	1135	21.4	31.1
	550	794	813	1163	17.9	30.3
	600	751	779	1135	13.4	17.3
	650	757	780	1098	9.8	14.3
	700	733	760	1008	7.1	9.7
	750	614	651	872	6.3	10.2
Ex. 2	RT	580	592	1083	31.3	45.6
	500	576	588	932	25.9	44.2
	550	554	570	912	23.2	36.4
	600	570	581	908	18.8	26.1
	650	556	579	862	11.6	14.5
	700	535	549	784	10.7	13.6
	750	460	471	670	12.5	14.0

Table VII shows that the tensile properties of the alloys of Examples 1 and 2 (especially Example 1) are more than adequate for use as marine diesel engine exhaust valves as compared to the tensile properties of prior art Alloy A. The tensile properties of the laboratory heats of Examples 1 and 2 reasonably matched the tensile properties of the comparable heats used to make exhaust valves when tested at room temperature and 550°C.

[0009] The dynamic Young's moduli of 19 mm, forged bar samples of alloys of Examples 1 and 2 heat treated as specified hereinbefore were in units of N/mm² x 10³, about 217.7 at room temperature and decreased to about 182.7 at 600°C. These values are significantly higher than the dynamic Young's moduli of Alloy A at these temperatures. Mean coefficients of thermal expansion from 20°C to temperatures as high as 600°C for alloys of Examples 1 and 2 were marginally lower than such coefficients of thermal expansion for Alloy A. All physical and mechanical tests of Alloy A and the alloys of Examples 1 and 2 indicated that from strength, thermal expansion, thermal conductivity and abrasion and fatigue resistance standpoints Alloy A and the alloys of Examples 1 and 2 were equally well adapted for marine diesel exhaust valve usage.

[0010] The significant difference between Alloy A and the alloys of Examples 1 and 2 is corrosion resistance in the environment in which a marine engine diesel exhaust valve is operating. Calculated levels of strain, even allowing for excess strain, in marine diesel engine exhaust valves indicate that such valves made of Alloy A are not likely to fail. However, experience has proven that this prediction is faulty because corrosion promotes crack initiation under low cycle and high cycle fatigue conditions. Once a crack or cracks is initiated, failure of an exhaust valve quickly follows.

[0011] Table VIII sets forth data concerning relative corrosion of samples of Alloy A and the alloys of Examples 1 and 2 under laboratory conditions in contact with air and synthetic ash (as defined hereinbefore) when tested at 650°C for 500 hours with the air containing 0.2% sulfur oxide as the dioxide or trioxide.

TABLE VIII

Alloy	Corrosion Loss Per Side (µm)*
Alloy A	136 ± 50
Example 1	74 ± 15
Example 2	57 ± 21

*Mean plus or minus two standard deviations

It is vital to note, in contrast to many teachings of the prior art such as set forth in Table I that the alloys of the invention must contain less than about 1% iron in order to exhibit the excellent corrosion resistance as indicated in Table VIII at temperatures in excess of 700°C. Under corrosion testing conditions similar to those used in obtaining the data set forth in Table VIII, gn alloy of the invention containing 25% Or, 2.5% Ti, 1.25% Al, 1.25% Nb, balance essentially nickel, and two alloys outside the invention containing respectively 10% and 20% iron in replacement of nickel provided data as set forth in Table IX.

TABLE IX

% Fe	Descaled Weight Loss* (mg/cm²)	Surface Loss* (mm)	Maximum Penetration* (mm)
0	49.0	0.06	0.085
	48.5	0.00	0.085
10	58.9	0.07	0.095
	61.5	0.07	0.095
20	82.8	0.10	0.125
	89.0	0.11	0.135

*These data are for a 50 hour exposure

When exposure times to synthetic ash and air are extended beyond 50 hours at 750°C, e.g. for 300 hours, the descaled weight loss of the alloys containing iron are even greater in proportion to the descaled weight loss of the iron-free alloy of the invention than disclosed in Table IX. The same phenomenon is evident when the criterion of corrosion is total depth of attack.

[0012] As disclosed, the alloys of the present invention are particularly useful as exhaust valves and parts, e.g. facings, coatings, external portion of composite exhaust valves and other exhaust contacting items for marine diesel engines and other diesel engines employing non-distillate diesel fuel, particularly such fuel contaminated with vanadium. It is to be noted that while alloys of the invention are contemplated to contain from 24 to 32% chromium, best results are obtained with alloys containing 24-28% chromium as is evident by the relatively better tensile characteristics disclosed in Table VII for the alloy of Example 1 compared to the alloy of Example 2.

Claims

1. An alloy particularly adapted to be employed at high temperatures under corrosive conditions comprising, in percent by weight, 0.02-0.07% carbon, 24-32% chromium, 0.7-3.0% titanium, 0.7-1.5% aluminum, 0.7-1.5% niobium, 0-1% zirconium, 0-0.006% boron, 0-1% iron, balance essentially nickel, together with conventional amounts of incidental and impurity elements.

2. An alloy as in claim 1 containing about 24-28% chromium.

3. An alloy as in claim 1 containing about 25% chromium.

4. A component of a marine diesel exhaust system comprising, at least in part of, the alloy of any one of claims 1 to 3.

5. A component as in claim 4 comprising an exhaust valve.

6. An exhaust valve as in claim 5 made substantially entirely of the alloy of any of claims 1, 2 or 3.