EP1203831A2

EP1203831A2 - Thermal fatigue resistant stainless steel articles

Info

Publication number: EP1203831A2
Application number: EP01309345A
Authority: EP
Inventors: Michael M. Antony; John W Smythe
Original assignee: ATI Properties LLC
Current assignee: ATI Properties LLC
Priority date: 2000-11-03
Filing date: 2001-11-05
Publication date: 2002-05-08
Also published as: CA2361180A1; EP1203831A3

Abstract

A die casting die including a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, and 0 to 1.5 percent titanium. High strength, high toughness stainless steels having the indicated composition may exhibit favorable thermal fatigue resistance, Rockwell C hardness greater than 40, and Charpy impact toughness greater than 25 ft-lbs.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

FEDERALLY SPONSORED RESEARCH

Not Applicable

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

The present invention relates to articles including steels having favorable thermal fatigue properties. The present invention more particularly relates to articles including stainless steels having high thermal fatigue resistance, high hot hardness, high impact strength, and low thermal expansion properties. The articles including the stainless steels include, for example, die casting dies for production of parts from molten aluminum, zinc, magnesium, and brass, as well as other articles that may undergo thermal stress through high temperature cycling.

DESCRIPTION OF THE INVENTION BACKGROUND

Die casting is a process for the production of net shape or near net shape cast metal components. The process is accomplished by forcing molten metal, typically non-ferrous, under high pressure into a steel casting die or mold. Heat from the molten metal is conducted into the casting die, typically steel, which causes the molten metal to solidify. The casting die is then opened and the cast component is ejected from the die cavity. The die cavity is designed to impart the shape of the product to the solidifying metal. There are four principal alloy systems that commonly are die cast. These include zinc, magnesium, aluminum and copper (brass). The casting process is performed for each of these alloys at different molten metal temperatures. Typical die casting molten metal temperatures are 800°F (427°C) for zinc, 1200°F (649°C) for magnesium, 1250°F (677°C) for aluminum, and 1780°F (972°C) for copper.
Casting dies are subjected to significant stresses and erosive forces during the die casting process. Performance of a casting die typically is measured by the quality of the cast product and the life of the casting die. Casting die life may be measured in "shots", or the number of parts that may be made prior to failure of the casting die. A typical casting die normally produces from 20,000 to just over 200,000 parts during its useful life. The performance of the casting die depends on several factors, including casting temperature, casting shape, cooling rate, casting speed, and mold material. Generally, the hotter the casting metal that is injected, the hotter the casting die operating temperature, and the more rapidly the casting die metal is weakened or eroded. Casting die walls are required to dissipate the heat of the cast metal, and casting dies with complex shapes typically absorb and dissipate heat at different rates within different sections of the casting die. These differences produce temperature differentials within the casting die, resulting in thermal stresses in the die, thermal fatigue, and a reduction in die life. Similarly, higher casting rates result in a higher frequency of the die heating and cooling cycle, which also may reduce die performance. Casting dies typically fail by thermal fatigue or heat checking. Thermal fatigue occurs when small cracks develop on the die surface after repeated thermal cycling.
Other industrially important components undergo thermal stresses similar to those experienced by casting dies, and the overall performance of those components is a result of similar factors. Those components include, but are not limited to, core rods and core pins used to produce cavities in cast components or powder metal compacts, hot forging tools and dies, steel rolling mill rolls, and other hot tooling components.
Casting dies, as well as other components that experience heating and cooling cycles, generally are produced by machining or other methods of forming metallic blocks. Materials considered suitable for these applications, and specifically for casting dies, should resist thermal fatigue and heat checking. This is particularly applicable to components that experience temperature changes that are frequent, rapid, and non-uniform throughout different sections of the component. Thermal fatigue is a condition in which stresses primarily are due to hindered thermal expansion or contraction. Rapid heating and/or cooling may create temperature gradients within the component causing the relatively cool sections to restrict the expansion of the metal of the hotter sections. This produces internal stresses. The internal stresses may exceed the mechanical stresses imposed on the component, resulting in thermal fatigue, which manifests itself as fine cracks in the casting die surface. These small cracks may result in rejection of the cast product. Thermal fatigue also may lead to catastrophic failure of the casting die over time. Thermal shock, on the other hand, is a sudden failure of a component that occurs as a result of a single, rapid temperature change or a single series of rapid cyclic temperature changes that induce stresses great enough to cause complete failure of the component.
To resist thermal fatigue and shock, casting dies preferably are formed of materials having a number of properties. These include low mean coefficient of thermal expansion, high thermal conductivity, high hardness, high impact strengths and resistance to austenite reversion as measured by retained hardness during thermal fatigue testing. A material's coefficient of thermal expansion is a measure of the increase in linear dimensions of the material accompanying an increase in temperature. The thermal conductivity of a material quantifies the rate of flow of thermal energy through the material in the presence of a temperature gradient. Because thermal fatigue and shock stresses occur internally in a component as one section of the component expands more than an adjacent section, low coefficient of thermal expansion and a high thermal conductivity reduce the internal stresses accompanying rapid heating or cooling. The other desirable properties, high hardness and resistance to austenite reversion, ensure that the material will be resistant to erosion during high flow rates of the molten casting material. Resistance to softening at elevated temperatures is necessary to prevent premature erosion during injection of the molten metal. Reversion of martensite to austenite may be evidenced by a reduced hardness after the casting die has experienced many heating/cooling cycles, as in a thermal fatigue test.
Casting dies commonly are made from hot work tool steels having the properties described above. The most common material used is H13ESR steel, UNS T20813, H13ESR steel has a nominal composition of 0.4 weight percent carbon, 5.25 weight percent chromium, 1.5 weight percent molybdenum, 1.0 weight percent vanadium, and balance iron. The hardness of H13ESR steel ranges from 42 to 50 Rockwell C depending upon the casting die geometry. H13ESR steel is generally considered to be the premium quality alloy for casting dies and presently is the alloy most extensively used for that purpose. In some applications, an H13ESR casting die may cost more than the casting machine.
Maraging steels are typically used in applications where high strength and high toughness are needed including casting dies having complex geometries that preclude removal of the Electrical Discharge Machining recast layer. Other materials used for casting dies include mold steels, certain high strength, high toughness stainless steels, and certain other alloy steels. Casting dies made from those materials, however, typically exhibit a shorter useful life than casting dies produced from H13ESR.
Thermal fatigue is generally regarded as the most significant failure mode limiting die life. To assess different casting die materials, laboratory tests have been developed to estimate the results of thermal fatigue. Thermal fatigue tests typically measure the total crack length and the number of cracks per unit length in an alloy specimen after repeated thermal cycles in a molten metal, such as aluminum. A plot of total crack length versus number of thermal cycles commonly shows a crack initiation period and a rapid crack growth area. The thermal fatigue resistance of the alloy may be defined by the number of thermal cycles occurring before an abrupt change in crack growth rate occurs. Thus, such testing provides a qualitative assessment of thermal fatigue resistance. A typical requirement for qualifying a new die material may be exposure to 15,000 heating and cooling cycles with no substantial evidence of the onset of the period of rapid crack growth.
Considering the commercial importance of die casting dies and other components subjected to thermal stresses, there exists a need for a material with improved resistance to thermal fatigue, a low mean coefficient of thermal expansion, high hot hardness, and favorable impact properties.
More specifically, there exists a need for a material that does not exhibit substantial evidence of the onset of thermal fatigue and substantially retains its initial hardness after repeated heating/cooling cycles, and that may be used to produce casting dies and other components such as core rods, core pins, rolling mill rolls, hot forging tooling, and other hot tooling components.

SUMMARY OF THE INVENTION

The present invention addresses the above described needs by providing a casting die including a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, 0 to 1.5 percent titanium, and iron. Preferably, the casting die includes a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 11 to 16 percent cobalt, 3 to 6 percent molybdenum, 2.5 to 5 percent nickel, 0 to 1.5 percent titanium, and iron. The addition of chromium to the iron-base alloys appears to increase resistance to thermal fatigue and oxidation. The present invention also provides a die casting die including stainless steel consisting essentially of, by weight, 12 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, 0 to 1.5 percent titanium, iron, and incidental impurities.
The present inventors have found that high strength, high toughness stainless steel within the above compositional ranges may exhibit thermal fatigue resistance comparable to or better than H13ESR steel, room temperature Rockwell C hardness greater than 45, high retained hardness on thermal cycling, and high impact strength. As such, the stainless steels would be particularly useful in articles of manufacture such as, for example, die casting dies, core rods, core pins, hot forging tools, hot forging dies, steel rolling mill rolls, and in other applications in which thermal fatigue resistance is desirable. It is believed that under current market conditions the high strength, high toughness stainless steels of the present invention may result in significant savings compared to H13ESR steel.
The present invention also provides methods of producing die casting dies, and other forming components such as core rods, core pins, rolling mill rolls, hot forging tooling, and other hot tooling components, including stainless steels described above. Also provided is a method of making cast articles of manufacture using a die casting die constructed according to the present invention.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend additional details and advantages of the present invention upon making and/or using the stainless steels of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

The features and advantages of the present invention may be better understood by reference to the accompanying figures, in which:
Figure 1 is a graph comparing total crack length per corner for an H13ESR steel specimen and specimens of three die casting die alloys of the present invention after 5000, 10,000, 15,000, 20,000 and 25,000 cycles of heating in molten aluminum at approximately 1250°F (677°C) and cooling in room temperature water;
Figure 2(a) is a photomicrograph of thermal fatigue cracks in an H13ESR steel specimen after 25,000 cycles of heating in molten aluminum at approximately 1250°F (677°C) and cooling in room temperature water;
Figure 2(b) is a photomicrograph of thermal fatigue cracks in a specimen of alloy WG18 of the present invention after 25,000 cycles of heating in molten aluminum at approximately 1250°F (677°C) and cooling in room temperature water;
Figure 2(c) is a photomicrograph of thermal fatigue cracks in a specimen of die casting die alloy WG19 of the present invention after 25,000 cycles of heating in molten aluminum at approximately 1250°F (677°C) and cooling in room temperature water.
Figure 2(d) is a second photomicrograph of thermal fatigue cracks in an H13ESR steel specimen after 25,000 cycles of heating in molten aluminum at approximately 1250°F (677°C) and cooling in room temperature water.
Figure 3 is a graph of Rockwell C hardness measured at room temperature for an H13ESR steel specimen and specimens of three die casting alloys of the present invention prior to and during thermal fatigue testing after 5000, 10,000, 15,000, 20,000 and 25,000 cycles of heating in molten aluminum at approximately 1250°F (677°C) and cooling in room temperature water;
Figure 4 is a graph of the mean coefficient of thermal expansion of an H13ESR steel specimen and specimens of three die casting die alloys of the present invention at temperatures from 200°F (93°C) to 1000°F (538°C); and
Figure 5 is a bar graph comparing Charpy impact toughness measured at room temperature for an H13ESR steel specimen and specimens of three die casting die alloys of the present invention prior to thermal fatigue testing.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Improvements in casting die material would be highly beneficial. The cost of casting dies and other tooling are a major expense in the casting industry, and increased tool life would provide significant time and cost savings. The present invention provides high strength, high toughness die casting dies including stainless steels that are resistant to thermal fatigue and may be used in components, such as die casting dies, that are subjected to repeated cycles of heating and cooling. These steels may also be beneficially used in, for example, other forming components subjected to thermal stresses such as core rods, core pins, hot forging tools, hot forging dies, and steel rolling mill rolls.
The present inventors have found that an alloy's interaction with its environment plays an important role in thermal fatigue cracking. Typically, the surface of the metal in a component, when exposed to air, will react with oxygen in the air to produce metal oxides. These metal oxides may form a layer on the entire surface of the component. This surface oxidation process is accelerated at elevated temperatures. The inventors have discovered that thermal fatigue cracks are initiated in the surface oxide layers of a component and then penetrate into the underlying metal. The underlying metal may then be exposed to the same oxidizing atmosphere, and a metal oxide layer may be formed on the interior wall of the crack. Although not intending to be bound to any particular theory of thermal crack formation, oxidation of the underlying metal at the crack tip appears to accelerate the rate of enlargement of a crack formed by thermal fatigue. The oxidation of the metal at the crack tip may cause expansion of the surface metal on the wall of the crack, increasing the internal stresses at the crack tip and, therefore, causing the crack to enlarge. The ever larger and deeper crack exposes more of the underlying metal to the oxidizing environment. It is believed that minimization of the formation of an oxide layer may reduce the initiation and growth of thermal fatigue cracks.
With this observation in mind, the present inventors have developed die casting dies and other components subjected to thermal stresses using several high-strength, high-toughness stainless steels. These alloys are age or precipitation hardenable stainless steels comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, 0 to 1.5 percent titanium, and iron. An embodiment of the present invention includes a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 11 to 16 percent cobalt, 3 to 6 percent molybdenum, 2.5 to 5 percent nickel, and 0 to 1.5 percent titanium. Preferably, the stainless steels of the present invention will comprise, by weight, 14 to 16 percent chromium and 12 to 15 percent cobalt, which appears to further enhance their resistance to thermal fatigue. Stainless steels for the die casting dies and other articles of manufacture of the present invention that have been evaluated may be precipitation hardened to Rockwell C hardness of at least 40 and retain high hardness after exposure to numerous cycles of heating and cooling.

Examples of stainless steels for the die casting dies and other articles of manufacture of the present invention and premium quality H13ESR steel were evaluated for thermal fatigue resistance (as measured by crack initiation and growth), hardness retention after exposure to heating and cooling cycles, mean coefficient of thermal expansion, and Charpy impact strength. The composition of the evaluated alloys within the present invention and the prior art H13ESR steel are provided in Table I.

Alloy

3 Composition
Specimen No.	%C	%Ni	%Co	%Cr	%Mo	%V	%Ti	%Fe and incidental impurities
WG16	0.004	4.0	15.0	15.0	2.9	-	1.0	Balance
WG18	0.025	4.0	12.0	14.0	5.0	-	-	Balance
WG19	0.001	2.5	15.0	15.0	5.0	-	-	Balance
ESRH13	0.4	-	-	5.2	1.5	1.0	-	Balance

The higher chromium content of the alloys of the present invention relative to the prior art steel increases resistance to formation of a surface layer of metal oxides. Assuming the inventors are correct that the presence of the surface oxide layer contributes to crack initiation, increased chromium content should enhance the thermal fatigue resistance of the alloys. For the testing, each alloy listed in of Table 1 was produced by a similar process, including providing an initial heat having the desired chemical composition by vacuum induction melting, followed by vacuum arc remelting to ensure that the alloy was chemically and physically homogenous. The alloys were also age or precipitation hardened at relatively low aging temperatures to increase strength. The specimen tested for thermal fatigue and thermal expansion properties were solution annealed for one hour at a temperature at 1650°F followed by an air cooling. After the air cool, these specimen were then additionally quenched in liquid nitrogen for four hours. The liquid nitrogen quench further increased the hardness and the room temperature tensile strength of the alloys of the present invention. After the liquid nitrogen quench, the alloys were allowed to return to ambient temperature in air, and then age hardened in a 1000°F environment for three hours. To finish the hardening process the alloys were air cooled.
Additional specimen were alternatively solution annealed at 1900°F and tested for thermal fatigue. These specimen solution annealed at the higher temperatures performed similar, if not better, than the specimen solution annealed at 1650°F.
Age or precipitation hardening is a heat-treating process designed to produce a uniform dispersion of a fine, hard, coherent precipitate in a softer, more ductile matrix. The age hardening process is also relatively independent of component size, and uniform levels of high strength are possible throughout even large components.
The procedure and results of the thermal fatigue testing were as follows.

I. Experimental Procedure

The thermal fatigue testing was conducted to predict the performance of the alloys of the present invention in components, such as a die casting die, that are subjected to repeated and numerous heating and cooling cycles. The test procedure was designed to repeatedly submerge multiple test specimens in a molten metal for a desired period and then cool the specimens in air or in a quenching bath. Molten aluminum at approximately 1250°F (677°C) was used to heat the specimens. The testing equipment allowed testing of up to six specimens at one time, providing a good comparison of the alloys under identical test conditions. The test specimen dimensions were 0.5 inch (12.7 mm) square by 6 inches (15.2 mm) long, with a 0.75 inch (19.1 mm) deep threaded hole at one end.
Each stainless steel alloy was annealed and then aged for 3 hours at 1000°F (538°C). The H13ESR sample was additionally oil quenched from 1875°F (1024°C), and then double tempered at 1100°F (593°C). Prior to testing, the finished specimens were heated to 700°F (371°C) in air for one hour, and air cooled to room temperature. This process produced a thin oxide layer that resisted soldering of the molten aluminum to the specimen.
The thermal fatigue testing apparatus consisted of three parts: a gas-fired crucible to heat the molten aluminum, a mechanical fixture or arm to manipulate the specimen, and a water bath to cool the specimen. The crucible contained molten alloy 384 aluminum at a temperature of approximately 1250°F (677°C). This is a typical holding temperature for the metal used in die castings. The water bath consisted of tap water and a commercial die lubricant. During the test, water was replenished from the tap at a very low rate, and the lubricant was continuously dripped into the water bath to maintain the proper ratios of the mixture.
The specimens were first dipped into the molten aluminum to a depth of 5 in. (12.7 cm) and held for seven seconds. The specimens were then lifted over the water bath and immersed in the water for 10 seconds. The specimens were then lifted out of the water and held above the crucible of molten aluminum for five seconds. This completed one cycle. The cycle was then repeated.
The performance of each specimen was monitored over the course of the thermal fatigue testing. The Rockwell C hardness of each specimen was measured before testing and after every 5000 cycles. The specimens were also evaluated for thermal fatigue cracks. Prior to each evaluation, two opposite faces of each specimen were polished using silicon carbide paper on a granite surface plate. After polishing, the corners of each specimen were examined under a stereomicroscope at a magnification of 90X. The area examined was approximately 1-3/8 in. (34.9 mm) long and was located approximately 1-3/8 in. (34.9 mm) from the bottom end of the specimen to avoid any end effects occurring due to the method of testing. Each corner was examined along the entire approximately 1-3/8 in (34.9 mm) length. The number of cracks and the lengths of each crack for each specimen were recorded. The Rockwell hardness was then measured at the center of one of the cleaned faces for each specimen. The specimen was subsequently exposed to additional series of 5000 cycles and reexamined as described above. The testing of the specimens was conducted for 25,000 cycles.

II. Results

a. Thermal Fatigue

Figure 1 shows the results of crack measurement during thermal fatigue testing for the alloys of the present invention and the prior art H13ESR steel. Thermal fatigue resistance is generally defined as the number of cycles an alloy is exposed to before an abrupt change in crack growth rate occurs. A benchmark for applications requiring exposure to repeated heating/cooling cycles was established as the occurrence of less thermal fatigue cracking than H13ESR after exposure to at least 15,000 cycles in a Crucible thermal fatigue test rig. Specimen no. WG18 displayed the best thermal fatigue resistance of all the alloys tested. Specimen no. WG18 met the benchmark for applications requiring exposure to repeated heating/cooling cycles by exhibiting no significant thermal fatigue cracking after exposure to 15,000 cycles. Specimen no. WG18 also continued to exhibit a strong resistance to thermal fatigue cracking after 20,000 cycles. At the conclusion of the 25,000 cycle thermal fatigue test, specimen no. WG18 had substantially less than 0.1 inch of total crack length per corner.
Specimen no. WG19 also met the benchmark by exhibiting no significant thermal fatigue cracking after exposure to 15,000 cycles. Even after completion of the thermal fatigue testing, specimen no. WG19 exhibited only approximately 0.12 inches of total crack length per corner. It should be noted that specimen no. WG19 fell into the molten aluminum during the first 5000 cycles, and it stayed there for approximately one hour until it was retrieved. However, only a slight drop in specimen hardness occurred, and it was decided to continue testing of the specimen.
The H13ESR specimen and specimen no. WG16 began to display a marked increase in total length of cracks per corner after 15,000 cycles, and both specimens exhibited greater than 0.4 inches of total crack length by the conclusion of the testing. These two alloys showed significantly less resistance to thermal fatigue testing than the WG18 and WG 19 alloys.
Specimen nos. WG18 and WG19 exhibited high resistance to thermal fatigue and, overall, performed better than any other specimen. Based on the results of the testing, it is believed that high strength, high toughness stainless steels within the invention, which include 13 to 16 weight percent chromium, would exhibit sufficient oxidation resistance to provide significant resistance to thermal fatigue cracking. Also based on the experimental results, it is preferred that the alloys of the invention include 14 to 16 weight percent chromium so as to provide even further increases in resistance to oxidation and, therefore, thermal fatigue. It is noted that even though specimen nos. WG18 and WG19 included, by weight, 14 and 15 percent chromium, both of those specimens showed evidence of light oxidation at the crack tips.
All of the specimens experienced crack initiation at the corners. These cracks generally were oriented normal to the surface of the alloy. Photomicrographs of cracks formed during testing are shown in Figures 2(a)-(d). In all cases, the cracks were predominantly transgranular and no evidence of reaction between the molten aluminum and the test specimen was found. Figure 2(a) is a photomicrograph of thermal fatigue cracks in the specimen of prior art alloy H13ESR after 25,000 cycles of the thermal fatigue testing. For comparison, Figure 2(b) is a photomicrograph of thermal fatigue cracks in specimen no. WG18 after 25,000 cycles. As can be seen by comparing Figures 2(a) and 2(b), specimen no. WG18 showed only isolated cracking with relatively little branching of the cracks. The corners of the prior art H13ESR specimen included more numerous cracks with relatively wide openings and more significant branching after the thermal fatigue testing. A comparison of Figures 2(c) and 2(d) shows the improved performance of specimen no. WG19 relative to the H13ESR steel specimen. These photomicrographs more closely show a single crack in each specimen. While each alloy displayed some oxidation at the crack tip, the thickness of the oxidation layer varied considerably. Of all tested alloys, the smallest amount of oxidation occurred on specimen no. WG19, and the prior art alloy displayed the heaviest oxidation. It appears that the rate of enlargement of the crack was accelerated by oxidation and formation of the oxide layer at the tip of the crack.
Additional tests were conducted on specimen with approximately the same alloy composition as WG18, WG19 and H13ESR. These specimen, however, were solution annealed at a temperature of 1900°F for one hour. This higher temperature annealing temperature is preferred to reduce embrittlement of the stainless steel which may occur with lower temperature annealing. These specimen were analyzed for total crack length after 25,000 cycles of thermal fatigue testing as described above. The results of this thermal fatigue testing are also shown in Figure 1.

b. Hardness

Hardness is an important property for casting dies since it may be used to predict the dies resistance to erosion during casting. Erosion of the casting die can occur because the molten metal is forced into the casting die under high pressure and at high flow rates.
Hardness values for each of the tested alloys were determined prior to and during the thermal fatigue testing described above. Figure 3 is a graph of the Rockwell C hardness ("HRc") measured at room temperature for the H13ESR specimen and the specimens of the three tested alloys of the present invention prior to and after 5000, 10,000, 15,000, 20,000 and 25,000 cycles of heating and cooling as described above. Prior to testing, each specimen, except for specimen no. WG16, was hardened to between HRc 45-50. The WG16 specimen was hardened to HRc 40 prior to testing. The specimen were hardened according to the procedure described earlier. All of the alloys experienced some softening during testing. Specimen nos. WG18 and WG19 did not soften appreciably compared to H13ESR. Interestingly, the high strength stainless steels of the present invention appeared to reach a steady-state hardness, while the hardness of H13ESR continued to drop throughout the testing. Heat No. WG16 started with a relatively low hardness and dropped to the lowest hardness level of the three stainless alloys of the present invention.

c. Thermal Expansion

Coefficient of thermal expansion also is an important property for casting dies. As discussed above, as the material expands or contracts during heating, internal stresses are produced that may lead to initiation of thermal fatigue cracks. Thus, the occurrence of thermal fatigue cracks may be limited in materials with relatively low coefficients of thermal expansion.
Thermal expansion properties were determined prior to and during thermal fatigue testing. Figure 4 is a graph of the mean coefficient of thermal expansion of the H13ESR specimen and the specimens of the three alloys of the invention at temperatures from 200°F (93°C) to 1000°F (538°C). The coefficient of thermal expansion was determined according to ASTM testing procedure E228. All of the specimens were hardened as described above before hardness was measured.
As can be seen in Figure 4, specimen no. WG19 displayed the lowest coefficient of thermal expansion, followed by specimen no. WG18 and H13ESR. Specimen no. WG16 displayed a considerably higher coefficient of thermal expansion than the other alloys of the present invention. It is presumed that this result is due to the presence of retained austenite in the specimen. Austenitic alloys generally have much higher coefficients of thermal expansion than martensitic alloys.

d. Impact Strength

Impact strength is a measure of the ability of a material to withstand shock loading and generally represents the impact stress necessary to fracture the material. A material with higher impact strength will be able to better withstand the mechanical stresses imparted on a component in an industrial application such as casting. Thus, a higher impact strength may provide a component with added protection against crack propagation and initiation when exposed to thermal stresses.
Figure 5 is a bar graph comparing Charpy impact toughness measured at room temperature for each tested alloy prior to thermal fatigue testing. The Charpy impact toughness test is a method that is widely used to test the notch-impact strength of metals. A notched specimen of the test material is placed in a device and broken by the impact of a swinging pendulum, and the amount of energy necessary to fracture the material is calculated from the arc of the follow-through swing of the pendulum.
The values in Figure 5 represent the typical impact strength of the specimen at peak hardness. It would be expected that thermal cycling would reduce impact strength to some extent. The stainless steel specimens of the present invention displayed remarkably higher impact strength than the H13ESR specimen. The Charpy impact strengths ranged from 28 to 33 ft-lbs for the three alloys of the present invention as compared to 10 ft-lbs for H13ESR.
Accordingly, thermal fatigue testing demonstrated that high strength, high toughness stainless steels within the present invention exhibit improved properties relative to conventional H13ESR steel. Specimen nos. WG18 and WG19, for example, exhibited improved thermal fatigue resistance relative to the prior art material, and the WG18 and WG19 specimens also did not appreciably soften after repeated exposure to cycles of heating and cooling. Stainless steels within the present invention also exhibited coefficients of thermal expansion comparable to or lower than that of the H13ESR material. All of the alloy samples of the present invention evaluated were substantially tougher than the prior art material. Thus, the high strength, high toughness stainless steels alloys of the present invention, including the specific embodiments described above, may exhibit thermal fatigue properties comparable or better than the material commonly employed in the production of die casting dies.
Under current market conditions, it is expected that the stainless steels of the present invention would result in significant savings compared to H13ESR material. Thus, it is believed that the stainless steels of the present invention may be readily applied in die casting applications and in other applications subjected to thermal stresses, while also resulting in significant cost savings.
It is to be understood that the present description illustrates those aspects of the invention relevant to a clear understanding of the invention. Certain aspects of the invention that would be apparent to those of ordinary skill in the art and that, therefore, would not facilitate a better understanding of the invention have not been presented in order to simplify the present description. Although the present invention has been described in connection with certain embodiments, those of ordinary skill in the art will, upon considering the foregoing description, recognize that many modifications and variations of the invention may be employed. It is intended that all such variations and modifications of the invention are covered by the foregoing description and the following claims.

Claims

A die casting die for casting an article, the die casting die including a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, and 0 to 1.5 percent titanium.
The die casting die of claim 1, wherein the stainless steel comprises, by weight, 11 to 16 percent cobalt, 3 to 6 percent molybdenum, and 2.5 to 5 percent nickel.
The die casting die of claim 2, wherein the stainless steel comprises, by weight, 12 to 15 percent cobalt and 14 to 15 percent chromium.
The die casting die of claim 3, wherein the stainless steel comprises, by weight, 5 percent molybdenum.
The die casting die of claim 1, wherein the die casting die has a Rockwell C hardness greater than 45.
The die casting die of claim 1, wherein the die casting die has a Charpy impact toughness greater than 25 ft-lbs.
The die casting die of claim 1, wherein the die casting die is precipitation hardened.
The die casting die of claim 7, wherein the die casting die is precipitation hardened by a process comprising:

solution annealing the die casting die at a temperature between 1600°F to 2000°F;

quenching the die casting die in water;

submerging the die casting die in liquid nitrogen;

air cooling the die casting die; and

age hardening the die casting die at about 1000°F.
The die casting die of claim 7, wherein solution annealing the die casting die comprises annealing the die casting die at a temperature of about 1900°F.
A die casting die for casting an article, the die casting die including a stainless steel consisting essentially of, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, 0 to 1.5 percent titanium, iron, and incidental impurities.
The die casting die of claim 10, consisting essentially of, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 11 to 16 percent cobalt, 3 to 6 percent molybdenum, 2.5 to 5 percent nickel, 0 to 1.5 percent titanium, iron, and incidental impurities.
The die casting die of claim 10, consisting essentially of, by weight, 14 to 15 percent chromium, 0 to less than 0.1 percent carbon, 12 to 15 percent cobalt, 3 to 6 percent molybdenum, 2.5 to 5 percent nickel, 0 to 1.5 percent titanium, iron, and incidental impurities.
An article of manufacture including a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, and 0 to 1.5 percent titanium, wherein the article of manufacture is selected from a core rod, a core pin, a hot forging tool, a hot forging die, and a rolling mill roll.
The article of manufacture of claim 13, wherein the stainless steel comprises, by weight, 10 to 16 percent cobalt, 3 to 6 percent molybdenum, and 2.5 to 5 percent nickel.
The article of manufacture of claim 13, wherein the stainless steel comprises, by weight, 14 to 15 percent chromium and 12 to 15 percent cobalt.
The article of manufacture of claim 13, wherein the stainless steel consists essentially of, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 16 percent cobalt, 3 to 6 percent molybdenum, 2.5 to 5 percent nickel, 0 to 1.5 percent titanium, iron, and incidental impurities.
The article of manufacture of claim 13, wherein the article of manufacture is precipitation hardened.
A method for providing an article of manufacture, wherein the article of manufacture is selected from a casting die, a core rod, a core pin, a hot forging tool, a hot forging die, and a rolling mill roll, the method comprising:

providing a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, and 0 to 1.5 percent titanium; and

fabricating the article of manufacture from the stainless steel, wherein the article of manufacture is selected from a casting die, a core rod, a core pin, a hot forging tool, a hot forging die, and a rolling mill roll.
The method of claim 18, wherein the stainless steel comprises, by weight, 10 to 16 percent cobalt, 3 to 6 percent molybdenum, and 2.5 to 5 percent nickel.
The method of claim 18, wherein the stainless steel comprises, by weight, 14 to 15 percent chromium and 12 to 15 percent cobalt.
The method of claim 20, wherein the stainless steel comprises, by weight, 5 percent molybdenum.
The method of claim 18, further comprising:

solution annealing the article at a temperature between 1600°F to 2000°F;

quenching the article in water;

submerging the article in liquid nitrogen;

air cooling the article; and

age hardening the article at about 1000°F.
The method of claim 22, wherein solution annealing the article of manufacture is at a temperature of about 1900°F.
A method of producing a cast article of manufacture, comprising:

providing a die casting die including a stainless steel comprising, by weight, 13 to 16 percent chromium, 0 to less than 0.1 percent carbon, 10 to 17 percent cobalt, 2 to 6 percent molybdenum, 2 to 5 percent nickel, and 0 to 1.5 percent titanium; and

casting the article of manufacture in the die casting die.