US20060266439A1

US20060266439A1 - Heat and corrosion resistant cast austenitic stainless steel alloy with improved high temperature strength

Info

Publication number: US20060266439A1
Application number: US11/495,671
Authority: US
Inventors: Philip Maziasz; John Shingledecker; Michael Pollard
Original assignee: Caterpillar Inc; UT Battelle LLC
Current assignee: Caterpillar Inc; UT Battelle LLC
Priority date: 2002-07-15
Filing date: 2006-07-31
Publication date: 2006-11-30
Also published as: RU2009107232A; CN101506399A; WO2008016395A1; EP2047007A1; KR20090035723A; CA2580933A1; JP2009545675A

Abstract

A heat and corrosion resistant cast austenitic stainless steel alloy which contains less than about 15% nickel. The alloy has a completely austenitic microstructure in an as-cast state, and a creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM E139 test conditions.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 10/195,724 filed Jul. 15, 2002, the disclosure of which is incorporated by reference herein.
This invention was made with government support under the terms of Contract No. DE-AC05-960R2264 awarded by the U.S. Department of Energy. The government may have certain rights in this invention.

TECHNICAL FIELD

This disclosure relates generally to cast austenitic stainless steel alloys of the CF8C type with improved high temperature strength. More particularly, this disclosure relates to CF8C type stainless steel alloys and articles cast therefrom having excellent high temperature creep strength, and aging resistance, and which exhibit a stable austenitic microstructure with substantially no delta ferrite after casting and high temperature aging.

BACKGROUND

There is a need for high strength, oxidation resistant and crack resistant cast alloys for use in components subject to extreme temperature environments. Advanced diesel engines must continue to have high fuel efficiency as well as reduced exhaust emissions, without sacrificing durability and reliability. More demanding duty cycles require exhaust manifolds and turbocharger housing materials to withstand temperatures above 750° C. Such materials must withstand both prolonged, steady high-temperature exposure as well as more rapid and severe thermal cycling. New emissions reduction technology and transient power excursions can push temperatures in these critical components even higher. The current material of choice for diesel engine components is Silicon-Molybdenum (SiMo) cast iron. However, it is being pushed beyond its high temperature strength and corrosion limitations. Nickel based super alloys are candidate materials for other high temperature applications like gas turbines, due to its excellent high temperature properties. However, the cost of nickel makes nickel based super alloys expensive, and turbine manufacturers are considering lower-cost alternatives for casings and large structural components. These material issues are not unique to diesel engines and combustion turbines. Distributed power applications that utilize advanced natural gas reciprocating engines need low-cost high-temperature capable materials as expectations for efficiency and service temperature increase. Any new material for these applications should have low cost and good high temperature creep and fatigue resistance.
Because these components are made by casting processes, any new material should have good casting characteristics like melt fluidity, hot tear resistance, and weldability. A significant factor in the cost of producing a casting is the post casting stress relief or solution heat treatment typically required for stainless steel castings. Eliminating the need for post casting heat treatments can result in substantial time and money savings for casting manufacturers. These cost savings can be even higher for large components like steam turbine casings where large furnaces have to be used. Therefore, any new material should have the desired properties in the as-cast state, that is, without the need for post casting heat treatment.
CF8C is a commercially available cast austenitic stainless steel that is relatively inexpensive. However, standard practice is to solution treat CF8C castings at 1050° C., which like discussed above, may increase its cost for some applications. Currently-available cast austenitic stainless CF8C steels may include from 18 wt. % to 21 wt. % chromium, 9 wt. % to 12 wt. % nickel and smaller amounts of carbon, silicon, manganese, phosphorous, sulfur and niobium. CF8C typically includes about 2 wt. % silicon, about 1.5 wt. % manganese and about 0.04 wt. % sulfur. CF8C is a niobium stabilized grade of austenitic stainless steel most suitable for applications at temperatures below 500° C. In the standard form CF8C has poor strength at temperatures above 600° C. It also does not provide adequate cyclic oxidation resistance at temperatures exceeding 700° C., does not provide sufficient ductility, does not have the requisite long-term stability of the original microstructure after high temperature aging, and lacks long-term resistance to cracking during severe thermal cycling.
In austenitic stainless steel castings, such as CF8C, delta ferrite is present in the as-cast microstructure. This delta-ferrite in the microstructure transforms to sigma (σ) phase during prolonged high-temperature exposure, decreasing the ductility of the material, particularly at lower or ambient temperatures. The absence of delta ferrite and sigma phase in the microstructure in the as-cast state and after prolonged exposure to high temperatures (high temperature aging) is an important advantage to preserve the as-cast properties of the material during the lifetime of a component created with the material.
A class of stainless steel alloys is described in U.S. Pat. No. 5,340,534 issued to Magee (hereinafter, “the '534 patent”.) The '534 patent seeks to improve the galling resistance and corrosion resistance of stainless steel alloys. A concentration of silicon above 2.25% is an important contributor to the improved galling resistance of the alloy. Silicon is also important for metal fluidity of casting steels. However, silicon promotes the formation of ferrite, sigma phases and niobium rich lanes or other silicide phases in the steel, and ferrite volume measurements indicate ferrite volumes between 2.3 and 7 percent in different heats of alloys described in the '534 patent. As described earlier, presence of ferrite and sigma phases deteriorates the properties of steels exposed to high temperatures. Another class of stainless steel alloys is described in U.S. Pat. No. 4,341,555 issued to Douthett et al. (hereinafter, “the '555 patent”.) In the alloys described in the '555 patent, the concentration of carbon is restricted to 0.06%, and a concentration of molybdenum is kept between 2 and 4.5% for good pitting and acid corrosion resistance. The alloys described in the '555 patent rely on post casting stress relief heat treatments to improve their mechanical properties.
It is, therefore, desirable to have a modified CF8C type steel alloy which has good casting characteristics, improved strength and creep properties at temperatures above 600° C. in the as-cast state, and which exhibits a stable and completely austenitic microstructure after casting and high temperature aging, so that the improved strength and ductility of the material is maintained over the lifetime of the alloy. Completely austenitic microstructure refers to a nearly 100% austenitic microstructure which is substantially free of delta ferrite and sigma phases of steel.
The disclosed system is directed to overcoming one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure is directed to a heat and corrosion resistant cast austenitic stainless steel alloy which contains less than about 15% nickel. The alloy has a completely austenitic microstructure in an as-cast state, and a creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM E139 test conditions.
In another aspect, the present disclosure is directed to a heat and corrosion resistant cast austenitic stainless steel alloy containing less than 15% nickel. The alloy has a creep rupture life exceeding 3,000 hrs and a minimum creep rate of less than 1×10⁻³at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions. The alloy also has a 0.2% yield strength exceeding 130 MPa at 750° C. in the as-cast state, and a decrease in 0.2% yield strength from 750 to 900° C. of less than 20%; and, a completely austenitic microstructure after casting.
In another aspect, the present disclosure is directed to an article made of a heat and corrosion resistant cast austenitic stainless steel alloy which contains less than about 15% nickel and has a completely austenitic microstructure. The article also shows no detectable ferromagnetic phases like ferrite or martensite when measured with a measurement device after casting and after high temperature aging for 3000 hrs at 750° C. The article has a creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM El 39 test conditions and a creep rupture life exceeding 2000 hours and a minimum creep rate less than 5×10⁻³at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions.
The present disclosure also discloses a heat and corrosion resistant cast austenitic stainless steel alloy which has a completely austenitic microstructure in the as-cast state. The alloy includes about 0.05 weight percent to about 0.15 weight percent of carbon, about 1.5 weight percent to about 3.5 weight percent copper, about 0.25 weight percent to about 1.0 weight percent tungsten, and about 0.6 weight percent to about 1.5 weight percent of niobium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is an SEM micrograph of the microstructure of an exemplary polished and etched as-cast CF8C alloy.
FIG. 1 b is an SEM micrograph of the microstructure of an exemplary polished and etched as-cast CF8C-Plus alloy.
FIG. 2 a shows the microstructure of an exemplary CF8C alloy before high temperature aging.
FIG. 2 b shows the microstructure of an exemplary CF8C alloy after high temperature aging.
FIG. 3 a shows the microstructure of an exemplary CF8C-Plus alloy before high temperature aging.
FIG. 3 b shows the microstructure of an exemplary CF8C-Plus alloy after high temperature aging.
FIG. 4 a is a TEM image of the microstructure of an exemplary CF8C alloy after creep testing at 850° C. and 35 MPa.
FIG. 4 b is a TEM image of the microstructure of an exemplary CF8C-Plus alloy after creep testing at 850° C. and 35 MPa.

DETAILED DESCRIPTION

CF8C is the traditional cast equivalent of type 347 stainless steel. The chemistry of CF8C-Plus is based on the composition of CF8C with precise additions of nickel (Ni), manganese (Mn), and nitrogen (N) combined with a reduction in silicon (Si) and adjustments of other minor alloying elements. These alloy modifications were made to improve the high-temperature mechanical properties and the casting characteristics of the CF8C steel using inexpensive alloying elements without the need for post casting heat treatments.

	TABLE I


	Weight percentage (%)

	Compositional		Example
	range		alloy

	Min	Max	CF8C	CF8C+

Carbon (C)	0.05	0.15	0.07	0.1
Chromium (Cr)	18	25	19	19
Nickel (Ni)	10	15	10	12.5
Niobium (Nb)	0.1	1.5	0.8	0.8
Nitrogen (N)	0.05	0.5		0.25
Manganese (Mn)	0.5	10	1	4
Sulphur (S)	0	0.05	trace	trace
Molybdenum (Mo)	0	1	0.3	0.3
Phosphorous (P)	0	0.04	trace	trace
Copper (Cu)	0	3.5
Silicon (Si)	0.2	1	1	0.5
Titanium (Ti)	0	0.2
Cobalt (Co)	0	5
Aluminum (Al)	0	3
Boron (B)	0	0.01
Tungsten (W)	0	3
Vanadium (V)	0	3

Iron (Fe)	Balance	Balance	Balance

Table I is directed towards the maximum and minimum ranges of the compositional elements made in accordance with the present disclosure. Table I also includes (in column labeled “Example alloy”) an example of an embodiment of an alloy made in accordance with the present disclosure. Embodiments covered by the present disclosure include alloys with any subset of compositional ranges falling within the minimum and maximum ranges shown in Table I. It should be noted that allowable ranges of cobalt (Co), vanadium (V), and titanium (Ti) may not significantly alter the performance of the resulting material. Specifically, based upon current information, Co may range from 0 to about 5 weight percent, V may range from 0 to about 3 weight percent, and Ti may range from 0 to about 0.2 weight percent without significantly altering the performance of the alloys.
To study the effect of these modifications on the mechanical properties and creep behavior of the materials, mechanical testing was carried out, and the test results of the modified alloy (named CF8C-Plus) samples are compared with those of the traditional CF8C steel alloy. Samples of the traditional CF8C and the CF8C-Plus material were cast for experiments using centrifugal casting. Table I also shows the composition of CF8C and CF8C-Plus steel alloys that were used for these studies.
FIG. 1 a shows the microstructure of an exemplary polished and etched as-cast CF8C alloy, and FIG. 1 b shows the microstructure of an exemplary polished and etched as-cast CF8C-Plus alloy The microstructure of the as-cast CF8C alloy includes an austenite matrix with delta ferrite 10 pools in the interdentrite core regions, and niobium carbide (NbC) 12 in the interdentritic regions. In contrast, the microstructure of the as-cast CF8C-Plus alloy does not show any delta ferrite 10. The microstructure of CF8C-Plus alloy is fully austenitic with a mixture of chromium carbide (Cr₂₃C₆) and NbC 12 in the interdentritic regions. A digital Fisher® Feritscope® was used to measure the ferrite content of both the CF8C and CF8C-Plus steel castings. The CF8C had a ferrite number of about 16.8+/−1.1, which is equivalent to about 14% delta ferrite, and the CF8C-Plus did not register any detectable ferromagnetic behavior, meaning it has less than about 0.1% delta ferrite. Both these macroscopic measurements and microscopic studies indicate that the CF8C-Plus material in the as-cast state is substantially free of delta ferrite 10 in the as-cast state.
To investigate the microstructural evolution of CF8C and CF8C-Plus steel during aging, sand cast keel bars were encapsulated in quartz tubes, evacuated, and backfilled with argon. These were aged in an air box furnace at 750° C. for 3,000 hours. These specimens were polished and etched for optical microscopy using an etchant composed of glycerin, hydrochloric acid, nitric acid, and acetic acid having a volumetric ratio of 3:3:1:1. Scanning Electron Microscopy (SEM) analysis was performed on polished and unetched specimens using backscatter electron (BSE) imaging, and x-ray energy dispersive spectroscopy (XEDS) was performed on areas of interest.
FIG. 2 a shows a BSE image of the microstructure of an exemplary CFSC alloy before high temperature aging, and FIG. 2 b shows a BSE image of the microstructure of an exemplary CF8C alloy after high temperature aging at 750° C. for 3,000 hours. Comparison of FIGS. 2 a and 2 b indicate a change in BSE image contrast in delta ferrite 10 of the aged material. XEDS analysis of these regions in the aged material indicates that they are enriched in silicon (Si) and chromium (Cr) compared to the delta-ferrite 10 found in the as-cast structure. Comparison of FIGS. 2 a and 2 b indicates that delta-ferrite 10 has transformed into sigma phase 14 after high temperature aging. Based on the chemical composition of the phase and the knowledge that delta-ferrite 10 can transform rapidly to sigma phase 14 in stainless steels, it is concluded that aging for 3,000 hours at 750° C. transforms a majority of the delta ferrite 10 in CF8C steel into sigma phase 14. Electron diffraction patterns from these regions were studied using transmission electron microscopy (TEM) and confirmed the presence of body centered tetragonal (bct) sigma phase.
FIG. 3 a shows a BSE image of the microstructure of an exemplary CF8C-Plus alloy before high temperature aging, and FIG. 3 b shows a BSE image of the microstructure of an exemplary CF8C-Plus alloy after high temperature aging at 750° C. for 3,000 hours. In contrast to the CF8C alloys shown in FIG. 2 a and FIG. 2 b, the CF8C-Plus steel alloys in FIG. 3 a and FIG. 3 b do not show the formation of any delta ferrite 10 or sigma phases 14 after high temperature aging. The structure of CF8C-Plus alloy before and after high temperature aging samples is austenitic with interdendritic carbides 16. No obvious change was observed in carbide size or morphology after aging. These studies indicate that in contrast to CF8C alloy, the CF8C-Plus alloy is substantially free of delta ferrite 10 or sigma phases 14 of steel after high temperature aging at 750° C. for 3,000 hours.
From the centrifugal castings, tensile, creep, and fatigue specimens were machined in both the hoop and longitudinal orientations. Room temperature and elevated temperature tensile tests were performed in accordance with ASTM E8 and E21. Air creep testing was performed in accordance with ASTM E139 at constant load in lever-arm type creep machines with extensometers attached to the shoulders of the specimens to measure creep deformation. Low cycle fatigue (LCF) and creep-fatigue (C-F) testing was performed in servo-hydraulic test systems in strain control using induction heating in accordance with ASTM E606. For the creep-fatigue tests, a strain hold was imposed at maximum tensile strain during the cycle.

Table II compares the average tensile properties, namely 0.2% offset yield strength (YS), ultimate tensile strength (UTS), and ductility as measured as the percentage elongation at fracture (Elong.), and percentage reduction of cross sectional area at fracture (RA), for CF8C, and CF8C-Plus (CF8C+) steels as a function of temperature. The average yield strength of CF8C-Plus changes very little above 700° C., while that of CF8C steel shows significant weakening. The average ultimate tensile strength of the CF8C-Plus steel is higher for the entire temperature range compared to CF8C. This increase is significantly higher (>70 Mpa) at temperatures above 700° C. The ductility, as measured by elongation and reduction of area, of CF8C-Plus steel are both higher than that of CF8C steel above 700° C.

	TABLE II


	Tensile tests (ASTM E8 &E12)

Ductility

Temp

YS (Mpa)

UTS (Mpa)

Elong. (%)

RA (%)

(C.)	CF8C	CF8C+	CF8C	CF8C+	CF8C	CF8C+	CF8C	CF8C+

25	252	273	555	587	41.0	42.5	41.0	34.1
200	184	194	425	473	37.5	40.0	54.0	36.0
400	162	169	415	476	32.0	41.4	55.0	42.5
600	138	143	330	398	34.4	40.0	40.5	45.5
700	136	135	237	324	20.5	32.0	24.0	38.5
750		129		333		29.8		42.5
800	121	136	151	255	11.0	27.4	16.5	43.2
850		132		243		33.4		56.0
900	66	120	73	170	25.5	49.5	43.5	65.5

Table III compares the average creep rupture life of CF8C and CF8C-Plus (CF8C+) alloys at different stresses and temperatures. As seen in the table, the creep rupture life of CF8C-Plus steel is over an order of magnitude higher than that of CF8C steel in all cases. The creep ductility of CF8C-Plus steel, both as measured as a percentage change in elongation and percentage change in area, also shows a significant improvement over that of CF8C steel. In most cases, this improvement in ductility over CF8C steel is over 100%. The minimum creep rate of CF8C-Plus steel also shows a significant decrease over that of CF8C. In most cases, this decrease in minimum creep rate is over an order of magnitude lower than that of CF8C.

	TABLE III


	Creep strength (ASTM E139)

Creep Resistance

Creep

Creep ductility (%)

Minimum Creep

Stress

Temp

rupture life

Elong. (%)

RA (%)

Rate (%/hr)

(Mpa)	(C.)	CF8C	CF8C+	CF8C	CF8C+	CF8C	CF8C+	CF8C	CF8C+

35	850	1159	24100	8.4	7.8	8.3	13.5
75	850		104		28.6		60.9		1.8E−02
100	750	87.4	2443.5	5.9	25.6	4.9	44.6	1.0E−02	2.2E−03
140	750	3.7	120.5	8.2	31.6	12.5	51.1	9.6E−01	1.1E−02
180	650	88.5	3913.2	8.1	20.5	12.4	43.3	2.2E−02	1.0E−04

FIG. 4 a is a TEM image of the microstructure of CF8C after 493 hours of creep testing at 850° C. and 35 MPa. FIG. 4 b is the TEM image of the microstructure of CF8C-Plus after over 20,000 hours of creep testing at 850° C. and 35 MPa. Comparison of FIGS. 4 a and 4 b shows that the NbC 12 precipitates in the CF8C-Plus steel were less than about 50 nanometers in average diameter (as shown in FIG. 4 b,) while the average diameter of these precipitates was over about 250 nanometers, with much larger spacing after only 493 hours of testing, in CF8C alloys (as shown in FIG. 4 a.)
To study the effect on low cycle fatigue, fully reversed (R-ratio=−1) strain controlled low cycle fatigue tests were run at 650° C. and 800° C. at constant frequency. Table IV compares the low cycle fatigue life of CF8C and CF8C-Plus (CF8C+) steels at different strain ranges at the two different temperatures. At 650° C. both materials show similar behavior at high strains, but CF8C-Plus alloys show significant improvement in cycles to failure for the lowest strain ranges. A similar result is found at 800° C.

TABLE IV

650 C. 850

Strain range CF8C CF8C+ CF8C CF8C+

0.3 30161 >255567 8535 30471

0.5 7333 22545 3392 3826

0.7 7647 4099 2624 2203

1 1794 2396 1394 717
Additionally, low cycle fatigue tests were run at 750° C. with an R-ratio of 0 to 0.45% strain (0 to 0.45% total strain) at a strain rate of 0.001/sec. For these creep-fatigue experiments, a 180 second hold time at the maximum strain (0.45%) was utilized. Table V shows the results for these tests. For the low cycle fatigue tests, the cycles to failure for the CF8C was 50% that of the CF8C-Plus. Both materials had a reduction in cycle life when the 180 second peak strain hold was added, but the CF8C showed a more dramatic reduction (75%) compared to the CF8C (60%). The creep-fatigue cycle life of the CF8C-Plus steel was three times that of the CF8C steel.

TABLE V

Approximate

Low cycle cycles to

fatigue failure

life CF8C CF8C+

Continuous cycle 5000 9750

180 Sec peak 1100 4000

strain hold

The effects of further alloying elements in CF8C-Plus material was also studied. Four separate alloying additions, B, W, Cu, and Al, were chosen for evaluation on CF8C-Plus steel. Fifteen pound lab-scale heats of CF8C-Plus with minor alloy additions were produced by induction melting with an argon cover gas and cast into graphite blocks (152 mm 102 mm×25.4 mm). One heat was cast to the CF8C-Plus composition, and four other heats contained a single alloy addition each. The approximate measured compositions of these five castings (wt %) are given in Table VI. The column titled ‘CF8C+’ lists the approximate composition of an embodiment of an alloy made in accordance with the present disclosure. This alloy is used as the baseline to compare the effect of additional alloying elements in the alloy. The columns titled ‘CF8C+B’, ‘CF8C+W’, ‘CF8C+Cu’, and ‘CF8C+Al’ list the composition of the alloys obtained by adding about 0.005 weight percent of boron, about 0.45 weight percent of tungsten, about 2.5 weight percent of copper, and about 1.3 weight percent of aluminum, respectively, to the composition of the CF8C-Plus alloy.

	TABLE VI


	Approximate weight percent

	CF8C +
CF8C+	B	CF8C + W	CF8C + Cu	CF8C + Al

C	0.1	0.1	0.1	0.09	0.01
Mn	4.25	4.25	4.16	4.26	4.44
Si	0.5	0.5	0.5	0.5	0.6
Ni	12.7	12.7	12.6	12.9	12.3
Cr	19.3	19.4	18.9	19.1	19.2
Mo	0.25	0.25	0.25	0.25	0.26
V	0.008	0.01	0.008	0.008	0.008
Nb	0.78	0.79	0.75	0.77	0.8
N	0.28	0.22	0.26	0.25	0.26
Fe	Bal	Bal	Bal	Bal	Bal
Addition		.005B	.45W	2.5Cu	1.3Al

No post-casting stress-relief or solution annealing treatment was given to these castings. Tensile bars were machined from the casting blocks, and tensile tests and creep tests were performed on these materials. The test condition chosen to screen all the specimens was 850° C. and 75 MPa. Alloy samples that had creep rupture lives comparable to the CF8C-Plus material were then tested at 750° C. and 140 MPa.

Table VII compares the tensile test and creep test results of the four alloy additions to the CF8C material. As the results indicate, samples with Al and B additions exhibited worse creep life than CF8C-Plus material, and were not, therefore, chosen for creep testing at 750° C. and 140 MPa. The results indicate that the alloys with the Cu and W additions performed better than the base CF8C-Plus material in high temperature creep.

	TABLE VII


	Tensile tests (ASTM E8 &E12)	Creep strength (ASTM E139)

YS (Mpa)

UTS (Mpa)

Creep Rupture life (hrs)

Alloy	25 C.	850 C.	25 C.	850 C.	(850 C., 75 MPa)	(750 C., 140 MPa)

CF8C-Plus	291	151	655	231	170	320
CF8C-Plus + B	267	145	608	220	85	—
CF8C-Plus + W	298	153	604	233	180	340
CF8C-Plus + Cu	283	153	667	231	190	450
CF8C-Plus + Al	227	123	555	176	30	—

Based upon these results, both Cu and W together were added to the CF8C-Plus material to obtain and alloy having the approximate composition (in weight percent) of 0.09C, 3.9Mn, 0.46Si, 13.1Ni, 20.1Cr, 0.28Mo, 0.008V, 0.77Nb, 0.28N, 2.94Cu, 1W, and the balance Fe. Tensile bars were machined from the casting blocks, and tensile tests and creep tests were repeated on this new alloy.

Table VIII lists the results of these tensile and creep tests. Unexpectedly, the inventors found that adding both the Cu and W together introduced synergistic effects that decreased the creep rate and increased the creep rupture life of the material significantly. Microscopic analyses of this alloy indicated that its microstructure was substantially free of delta ferrite 10 or sigma phases 14 of steel in the as-cast and the post high temperature aged microstructure.

	TABLE VIII


	Creep strength (ASTM E139)

Creep

Tensile tests (ASTM E8 &E12)

Creep

resistance

Temp

YS

UTS

Ductility

Stress

Temp

rupture

Creep ductility

Min. creep

(C.)	(Mpa)	(Mpa)	Elong. (%)	RA (%)	(Mpa)	(C.)	life (hrs)	Elong. (%)	RA (%)	rate (%/hr)

25	252	585	40.0	39.0	75	850	80.4	22.4	43.2	2.80E−03
200	165	475	38.0	41.5
400	145	440	40.5	44.5
600	135	375	37.0	41.0	100	750	3904.4	19.1	32.4	5.10E−04
700	140	310	20.0	18.5
750	150	275	12.0	13.5
800	140	235	13.0	19.5	140	750	289.6	21.1	39.2	2.50E−03
850	150	225	14.0	17.0
900	120	155	36.0	39.0

INDUSTRIAL APPLICABILITY

The disclosed heat and corrosion resistant cast austenitic stainless steel alloy can be used for the production of any articles exposed to extreme temperatures and/or extreme thermal cycling conditions. The disclosed alloy can be used for components in engines and power systems. However, the present disclosure is not limited to these applications, as other applications will become apparent to those skilled in the art.
By employing the stainless steel alloys of the present disclosure, manufacturers can provide a more reliable and durable high temperature component. The absence of delta-ferrite 10 in the microstructure after casting produces a stable austenitic microstructure in CF8C-Plus. Delta-ferrite 10 transforms to sigma phase 14 during prolonged high-temperature exposure causing embrittlement. CF8C-Plus has a nearly 100% austenite microstructure, substantially free of delta ferrite and sigma phase.
The improved creep ductility combined with a lower creep rate for CF8C-Plus steel results in increased low cycle fatigue life and creep rupture strength. Increased low cycle fatigue life and creep rupture strength allows components made of CF8C+ to be long lasting. The increased creep strength and fatigue life of the disclosed CF8C-Plus steel alloys over traditional CF8C material is unexpected because both these materials are castings, and therefore, deformation processes are not involved in creating a dislocation structure upon cooling. Potential reasons for the significant improvement of low cycle fatigue life and creep rupture life of CF8C-Plus over traditional CF8C alloys are that the presence of Mn alters the stacking fault energy of the CF8C-Plus alloy giving rise to higher energy stacking faults, and the presence of manganese and nitrogen in the alloy composition helps nucleation of NbC. The size and density of the NbC 12 precipitates in the matrix may also contribute to the observed improvement in fatigue life and creep rupture life. The presence of these fine particulates of NbC 12 could likely pin dislocations, improving the creep rupture life of CF8C-Plus alloys. The increased fatigue and creep rupture life, the decreased creep strain rate, and the lower decrease in 0.2% yield strength at high temperatures may allow engine and turbine manufacturers to increase power density by allowing engines and turbines to run at higher temperatures, thereby providing possible increase in fuel efficiency.
Engine and turbine manufacturers may also reduce the weight of components as a result of the increased power density by thinner section designs allowed by increased high temperature strength and corrosion resistance compared to conventional high-silicon molybdenum ductile irons. Further, the stainless steel alloys of the present disclosure provide superior performance over other cast stainless steels for a comparable or lower cost. Finally, stainless steel alloys disclosed herein will assist manufacturers in meeting emission regulations for diesel, turbine and gasoline engine applications.
While only certain embodiments have been set forth, alternative embodiments and various modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of the present disclosure.

Claims

1. A heat and corrosion resistant cast austenitic stainless steel alloy comprising;

less than about 15% nickel;

completely austenitic microstructure in the as-cast state; and,

creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM El 39 test conditions.

2. The cast austenitic stainless steel alloy of claim 1 further including,

creep rupture life exceeding 2000 hours, when creep tested in the as-cast state under ASTM E139 test conditions at a stress of 100 MPa and a temperature of 750° C.

3. The cast austenitic stainless steel alloy of claim 2 further including,

minimum creep rate lower than 5×10⁻⁵, when creep tested in the as-cast state under ASTM E139 test conditions at a stress of 100 MPa and a temperature of 750° C.

4. The cast austenitic stainless steel alloy of claim 1 further including,

about 0.07 weight percent to about 0.15 weight percent carbon,

about 0.2 weight percent to about 1 weight percent silicon,

about 0.1 weight percent to about 1 weight percent molybdenum, and

about 0.6 weight percent to about 1.25 weight percent niobium.

5. The cast austenitic stainless steel alloy of claim 4 further including,

about 0.5 weight percent to about 10 weight percent manganese,

about 18 weight percent to about 25 weight percent chromium, and

about 0.05 weight percent to 0.5 weight percent nitrogen.

6. The cast austenitic stainless steel alloy of claim 1 further including,

completely austenitic microstructure after high temperature aging for 3000 hrs at 750° C.

7. The cast austenitic stainless steel alloy of claim 1 further including,

0.2% yield strength at 700° C. exceeding 125 MPa in the as-cast state, and a decrease in 0.2% yield strength from 700 to 900° C. of less than 20%.

8. The cast austenitic stainless steel alloy of claim 7 further including,

an ultimate tensile strength in the as-cast state exceeding 300 MPa at 700° C. and 125 MPa at 900° C.

9. The cast austenitic stainless steel alloy of claim 1 further including,

a creep ductility, expressed as a percentage change in elongation at rupture, exceeding 5%, when creep tested in the as-cast state at 35 MPa and 850° C., and

a creep ductility, expressed as a percentage reduction in area at rupture, exceeding 10%, when creep tested in the as-cast state at 35 MPa and 850° C.

10. The cast austenitic stainless steel alloy of claim 1 further including,

a low cycle fatigue life at a strain range of 0.3% exceeding 200,000 cycles at 650° C., and exceeding 20,000 cycles at 800° C., when tested in the as-cast state under fully reversed (R-ratio=−1) strain controlled tests under constant frequency.

11. An article made from the cast austenitic stainless steel alloy of claim 1.

12. A heat and corrosion resistant cast austenitic stainless steel alloy comprising;

less than about 15% nickel;

a creep rupture life exceeding 3,000 hrs and a minimum creep rate less than 1×10⁻³at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions;

a 0.2% yield strength exceeding 130 MPa at 750° C. in the as-cast state;

less than 20% decrease in 0.2% yield strength from 750 to 900° C.; and

completely austenitic microstructure after casting.

13. The cast austenitic stainless steel alloy of claim 12 further including,

niobium carbide precipitates in the microstructure that are less than or equal to about 50 nanometers after 20,000 hours of creep test at a stress of 35 MPa and a temperature of 850° C. under ASTM E139 test conditions.

14. The cast austenitic stainless steel alloy of claim 12 further including,

about 0.05 weight percent to about 0.15 weight percent carbon,

about 1.5 weight percent to about 3.5 weight percent copper, and

about 0.2 weight percent to about 1 weight percent silicon.

15. The cast austenitic stainless steel alloy of claim 14 further including,

about 0.1 weight percent to about 1 weight percent molybdenum,

about 0.1 weight percent to about 1.5 weight percent niobium, and

about 0.25 weight percent to about 1.0 weight percent tungsten

16. The cast austenitic stainless steel alloy of claim 12 further including,

a creep rupture life exceeding 200 hours and a minimum creep rate less than 5×10⁻³at a stress of 140 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions.

17. An article made from the cast austenitic stainless steel alloy of claim 12.

18. An article made of a heat and corrosion resistant cast austenitic stainless steel alloy comprising;

less than about 15% nickel;

completely austenitic microstructure;

no detectable ferromagnetic behavior when measured with a measurement device after casting and after high temperature aging for 3000 hrs at 750° C.;

a creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM E139 test conditions; and,

a creep rupture life exceeding 2000 hours and a minimum creep rate less than 5×10⁻³at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions.

19. The article of claim 18 further including,

a creep rupture life exceeding 3,000 hrs and a minimum creep rate less than 1×10⁻³at a stress of 100 MPa and a temperature of 750° C., when creep tested in the as-cast state under ASTM E139 test conditions,

a 0.2% yield strength at 750° C. exceeding 130 MPa in the as-cast state, and

less than 20% decrease in 0.2% yield strength from 750 to 900° C.

20. The article of claim 18 further including,

about 0.07 weight percent to about 0.15 weight percent carbon,

about 18 weight percent to about 25 weight percent chromium,

about 0.65 weight percent to about 1.5 weight percent niobium,

about 0.2 weight percent to 0.5 weight percent nitrogen,

about 0.5 weight percent to about 10 weight percent manganese,

less than or equal to about 3.5 weight percent copper,

about 0.1 weight percent to about 0.45 weight percent molybdenum,

less than or equal to about 2 weight percent tungsten, and

about 0.1 weight percent to about 1.0 weight percent silicon.

21. A heat and corrosion resistant cast austenitic stainless steel alloy comprising;

a completely austenitic microstructure in the as-cast state;

about 0.05 weight percent to about 0.15 weight percent of carbon;

about 1.5 weight percent to about 3.5 weight percent copper;

about 0.25 weight percent to about 1.0 weight percent tungsten; and

about 0.6 weight percent to about 1.5 weight percent of niobium.

22. The cast austenitic stainless steel alloy of claim 21 further including,

about 0.1 weight percent to about 1.0 weight percent silicon, and

about 0.2 weight percent to about 0.5 weight percent nitrogen.