US20250360557A1

US20250360557A1 - Systems and methods for improving iron-based camshaft fatigue life

Info

Publication number: US20250360557A1
Application number: US18/874,481
Authority: US
Inventors: Ben Wang; David RUTLEDGE; Chin-Pei Wang; Corey W. Trobaugh; Yong-Ching Chen; Greg Knight; Thomas Lambrosa; Lee Stark; Sophie R. Grimes
Original assignee: Cummins Inc
Current assignee: Cummins Inc
Priority date: 2022-06-20
Filing date: 2023-06-20
Publication date: 2025-11-27
Also published as: CN119317499A; WO2023249954A3; DE112023002692T5; WO2023249954A2

Abstract

A method of casting a camshaft including iron includes determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft. The method includes casting the camshaft including cooling the camshaft in a chiller based on the cooling rate profile. The method includes imparting the camshaft with a microstructure comprising carbide, ledeburite, pearlite, ausferrite, or combinations thereof.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63/353,743, filed Jun. 20, 2022, the content of which is herein incorporated by reference.

TECHNICAL FIELD

The present application relates generally to the field of camshafts for use in internal combustion engine systems, fuel systems, or the like.

BACKGROUND

Internal combustion engines include at least one cylinder which receives fuel and air and which combusts the fuel to produce mechanical energy. This mechanical energy is harvested via a piston which translates within the cylinder. Intake valves open to let the cylinder fill with air and exhaust valves open to allow combustion gases to leave. The opening and closing of the valves are controlled by one or more camshafts. In some applications, camshafts can undergo rolling contact fatigue (RCF), a wear mechanism that occurs when the camshafts are subjected to rolling stresses.

SUMMARY

Embodiments described herein relate generally to systems and methods for providing improved iron-based camshaft fatigue (e.g., rolling contact fatigue) life.
At least one aspect of the present disclosure is directed to a method of casting a camshaft including iron. The method comprises determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft. The method further comprises casting the camshaft including cooling the camshaft in a chiller based on the cooling rate profile. The method still further comprises imparting the camshaft with a microstructure including carbide and pearlite.
In some embodiments, the method comprises austempering the camshaft to homogenize the pearlite. In some embodiments, the cooling rate profile includes a cooling rate that changes over time. In some embodiments, the cooling rate profile includes a cooling rate that is constant over time. In some embodiments, the cooling rate profile is determined further based on a geometry of the chiller, a size of the chiller, a wall thickness of the chiller, a mass of the camshaft, a thickness of the camshaft, a size of the camshaft, a target hardness of the camshaft, or combinations thereof. In some embodiments, cooling the camshaft decreases an amount of graphite nodules in the microstructure of the camshaft. In some embodiments, casting the camshaft further includes pouring molten iron into a mold before cooling the camshaft such that the cooled camshaft comprises chilled ductile iron. In some embodiments, imparting the camshaft with the microstructure includes realizing the microstructure based on the chemical composition of the camshaft and the cooling of the camshaft according to the cooling rate profile.
At least one aspect of the present disclosure is directed to a method of casting a camshaft comprising iron. The method comprises determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft. The method comprises casting the camshaft including cooling the camshaft in a chiller based on the cooling rate profile. The method further comprises imparting the camshaft with a microstructure comprising carbide and ledeburite. The method still further comprises austempering the camshaft to treat the microstructure.
In some embodiments, the cooling rate profile includes a cooling rate that varies over time. In some embodiments, austempering the camshaft homogenizes the microstructure. In some embodiments, the microstructure further comprises pearlite, and austempering the camshaft transforms the pearlite into ausferrite. In some embodiments, cooling the camshaft includes treating the camshaft in the chiller at different temperatures over different periods of time. In some embodiments, cooling the camshaft is further based on a wall thickness of the chiller. In some embodiments, casting the camshaft further includes pouring molten iron into a mold before cooling the camshaft. In some embodiments, imparting the camshaft with the microstructure includes realizing the microstructure based on a chemical composition of the camshaft and the cooling rate profile.
At least one aspect of the present disclosure is directed to a method of casting a camshaft including iron. The method comprises determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft. The method comprises casting the camshaft including cooling the camshaft in a chiller based on the cooling rate profile. The method further comprises austempering the camshaft to impart the camshaft with a microstructure comprising carbide and ausferrite.
In some embodiments, the method still further comprises imparting the camshaft with the microstructure comprising carbide and pearlite. In some embodiments, austempering the camshaft homogenizes the pearlite in the microstructure to form the ausferrite.
Those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the disclosure will become apparent from the description, the drawings, and the claims, in which:

FIG. 1 illustrates a schematic diagram of a casting system, according to an embodiment.

FIG. 2 illustrates the casting system, according to an embodiment.

FIG. 3 is a flowchart showing an example method of casting a camshaft including iron.

FIG. 4 is a micrograph of a microstructure of a chilled ductile iron specimen, according to an embodiment.

FIG. 5 shows a microstructure of a chilled austempered ductile iron specimen, according to an embodiment.

FIG. 6A is a table comparing the RCF life of different chilled ductile iron specimens, according to an embodiment.

FIG. 6B is a plot comparing the RCF life of different chilled ductile iron specimens shown in FIG. 6A, according to an embodiment.

FIG. 7 shows a plot of cooling curves for different chiller wall thicknesses, according to an embodiment.

FIG. 8A is a table comparing the RCF life of different chilled ductile iron specimens, according to an embodiment.

FIG. 8B is a plot comparing the RCF life of different chilled ductile iron specimens shown in FIG. 8A, according to an embodiment.

FIG. 9A is a table comparing the RCF life of chilled ductile iron and 1080 steel specimens, according to an embodiment.

FIG. 9B is a plot comparing the RCF life of cast iron and 1080 steel specimens shown in FIG. 9A, according to an embodiment.

FIG. 10 illustrates the casting system, according to an embodiment.

FIG. 11 shows a plot of temperatures vs. time for different locations of the casting system, according to an embodiment.

FIG. 12 shows a plot of temperatures vs. time for different locations of the casting system, according to an embodiment.

FIG. 13A is a micrograph of a chilled ductile iron specimen subjected to a spallation surface analysis, according to an embodiment.

FIG. 13B is a micrograph of a chilled ductile iron specimen subjected to a spallation surface analysis, according to an embodiment.

FIG. 13C is a micrograph of a chilled ductile iron specimen subjected to a spallation surface analysis, according to an embodiment.

FIG. 14 shows a plot of volume fraction of graphite and plot of average diameter of graphite in different chilled ductile iron specimens as shown in FIGS. 13A-13C, according to an embodiment.

FIG. 15 shows a plot of area of subsurface cracking and a plot of RCF life of different chilled ductile iron specimens as shown in FIGS. 13A-13C, according to an embodiment.

FIG. 16A is a plot of depth hardness vs. depth of different chilled ductile iron specimens, according to an embodiment.

FIG. 16B is a plot of depth hardness vs. depth of the three different improved CDI specimens.

FIG. 17A is a plot comparing the area fractions of different phases as shown in FIGS. 17B-17E in different chilled ductile iron specimens, according to an embodiment.

FIG. 17B is a plot of area fraction of carbide in different chilled ductile iron specimens, according to an embodiment.

FIG. 17C is a plot of area fraction of dendrite in different chilled ductile iron specimens, according to an embodiment.

FIG. 17D is a plot of area fraction of rod-growth ledeburite in different chilled ductile iron specimens, according to an embodiment.

FIG. 17E is a plot of area fraction of lamellar-growth ledeburite in different chilled ductile iron specimens, according to an embodiment.

FIG. 18 a micrograph of a microstructure of a chilled ductile iron specimen, according to an embodiment.

It will be recognized that some or all of the figures are schematic representations for purposes of illustration. The figures are provided for the purpose of illustrating one or more implementations with the explicit understanding that they will not be used to limit the scope or the meaning of the claims.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various concepts related to, and implementations of, methods, apparatuses, and systems for improving iron-based camshaft fatigue life. The various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the described concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

I. Overview

Internal combustion engines include camshafts that may undergo rolling contact fatigue (RCF). Rolling contact fatigue can be caused by camshaft and follower spalling, Hertzian contact stress, and/or subsurface initiation. Steel is commonly used as a material for camshafts due to its hardness. However, camshafts made of steel (e.g., steel-based camshafts) may be more expensive than camshafts made of iron (e.g., iron-based camshafts).
Implementations described herein relate to methods of casting camshafts including iron. The method includes determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft. The method includes casting the camshaft, which includes cooling the camshaft in a chiller based on the cooling rate profile. The method includes imparting the camshaft with a microstructure including carbide and pearlite.
Implementations described herein relate to methods of casting camshafts including iron. The method includes determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft. The method includes casting the camshaft including cooling the camshaft in a chiller based on the cooling rate profile. The method includes imparting the camshaft with a microstructure comprising carbide and ledeburite. The method includes austempering the camshaft to treat the microstructure.
Implementations described herein relate to methods of casting camshafts including iron. The method includes determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft. The method includes casting the camshaft, which includes cooling the camshaft in a chiller based on the cooling rate profile. The method includes austempering the camshaft to impart the camshaft with a microstructure comprising carbide and ausferrite.
The methods described herein provide iron-based camshafts that have improved rolling contact fatigue life compared to that of steel-based camshafts or conventional iron-based camshafts. The improved iron-based camshafts may be less expensive than steel-based camshafts.

II. Example Method of Casting a Camshaft

FIG. 1 illustrates a schematic diagram of a casting system 100. FIG. 2 illustrates the casting system 100. The casting system 100 includes one or more castings 105 (e.g., chilled ductile iron castings, chilled austempered ductile iron castings, etc.). The composition of the one or more castings 105 include iron (Fe). The one or more castings 105 can include carbon (C), chromium (Cr), copper (Cu), molybdenum (Mo), and/or nickel (Ni). For a low-alloy white cast iron, the one or more castings 105 can include less than 5 wt % chromium, copper, molybdenum, and nickel. For a high-alloy white cast iron, the one or more castings 105 can include greater than 5 wt % chromium, copper, molybdenum, and nickel.
The casting system 100 includes one or more chillers 110. The one or more chillers 110 surround the one or more castings 105. Each of the one or more chillers 110 has a wall thickness 115. According to various embodiments, each of the one or more chillers 110 can have a wall thickness 115 of between 2 mm and 15 mm (e.g., 2 mm, 2.5 mm, 3 mm. 3.5 mm, 4 mm, 4.5 mm, 5 mm, 5.5 mm, 6 mm, 6.5 mm, 7 mm, 7.5 mm, 8 mm, 8.5 mm, 9 mm, 9.5 mm, 10 mm, 10.5 mm, 11 mm, 11.5 mm, 12 mm, 12.5 mm, 13 mm, 13.5 mm, 14 mm, 14.5 mm, or 15 mm).
The one or more chillers 110 can include a first chiller having a first wall thickness and a second chiller having a second wall thickness. The first wall thickness can be greater than the second wall thickness. The first chiller can provide a greater cooling rate than the second chiller. The one or more chillers 110 cool the one or more castings 105. For example, the one or more chillers 110 can cool the one or more castings 105 based on a chemical composition of the one or more castings 105. The one or more chillers 110 can cool the one or more castings 105 based on a target bearing life of the one or more castings 105. Each of the one or more chillers 110 has a cavity. Each of the one or more chillers 110 can have a geometry that produces a camshaft lobe.
FIG. 3 illustrates a flowchart of an example process 300 (e.g., method, procedure, etc.) for casting a camshaft including iron (e.g., iron-based camshaft, iron-based camshaft lobe, etc.). The camshaft can be an engine camshaft or a fuel system camshaft. The camshaft can include chilled ductile iron (CDI) or chilled austempered ductile iron (CADI). Chilled ductile iron is formed by chilling or cooling molten iron. Chilled austempered ductile iron is formed by chilling molten iron to produce chilled ductile iron, and then austempering the chilled ductile iron to produce chilled austempered ductile iron. The process 300 can be used for forming camshafts, camshaft lobes, rollers, or other vehicle components.
The process 300 starts at 305 with determining a cooling rate profile based on a chemical composition of the camshaft (e.g., camshaft lobe) and a target bearing life of the camshaft. The cooling rate profile includes the rate of cooling of the camshaft. The rate of cooling can stay constant or vary over time. The cooling rate profile can be determined based on the geometry or size of the one or more chillers 110, the wall thickness 115 of the one or more chillers 110, the mass of the camshaft, the thickness of the camshaft, the size of the camshaft, and/or the target hardness of the camshaft. The cooling rate profile can be determined by the target microstructure of the camshaft.
The cooling rate can vary for a given chemical composition of the camshaft. The camshaft can have a chemical composition similar to or the same as the one or more castings 105. Each camshaft lobe can have a chemical composition similar to or the same as the one or more castings 105. For example, the composition of the camshaft or each camshaft lobe includes iron. The composition of the camshaft or each camshaft lobe can include carbon, chromium, copper, molybdenum, and/or nickel. For a low-alloy white cast iron, the camshaft can include less than 5 wt % chromium, copper, molybdenum, and nickel. For a high-alloy white cast iron, the camshaft can include greater than 5 wt % chromium, copper, molybdenum, and nickel.
The target bearing life of the camshaft includes the length of time the camshaft is expected to perform based on predefined or target operating conditions. Determining the cooling rate profile can include running a simulation based on desired or target properties of the camshaft. For example, the target properties of the camshaft can include the target bearing life of the camshaft. The simulation can be calibrated based on the properties and the chemical composition of the camshaft.
The process 300 continues to 310 with casting the camshaft. Casting the camshaft includes cooling the camshaft in a chiller based on the cooling rate profile. For example, cooling the camshaft in a chiller based on the cooling rate profile can include cooling the camshaft in one or more chillers 110 based on the cooling rate profile. Casting the camshaft can include pouring molten iron having a chemical composition similar to or the same as the one or more castings 105 into a mold to form the camshaft.
The molten iron can be cooled according to the cooling rate profile. For example, the molten iron can be cooled at a first temperature for a first period of time. The molten iron can be cooled at a second temperature for a second period of time. The molten iron can be cooled at a third temperature for a third period of time. The cooling rate profile can include different temperatures and different periods of times. For example, the cooling rate profile can include the first temperature, second temperature, and third temperature and the first period of time, second period of time, and third period of time. The cooling rate profile can include additional temperatures and additional periods of time. The cooling rate profile can include a series of temperature changes over time. Cooling the camshaft can decrease the presence of graphite nodules in the microstructure of the camshaft.
The process 300 continues to 315 with imparting the camshaft with a microstructure. The microstructure (e.g., primary microstructure) comprises phases including carbide, pearlite, and graphite, where the amount of each phase is measured in volume fraction or area fraction. The camshaft can include less than 1% graphite (e.g., 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.95%, etc.). The camshaft can include at least 50% carbide (e.g., 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, etc.). The camshaft can include an average of 50%-60% carbide. The camshaft can include 70% carbide locally (e.g., a volume of the camshaft that is less than the entire volume of the camshaft), while the remaining volume fraction of the camshaft can include pearlite (e.g., 49%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5%, etc.).
In some embodiments, referring to FIGS. 17A-17E, the microstructure of the camshaft includes various combinations, at various volume/area fractions, of carbide, dendrite (including primary austenite and/or globular pearlite), graphite, and/or ledeburite (including lamellar-growth ledeburite and/or rod-growth ledeburite). In some embodiments, the camshaft includes at least 15% (in area fraction) lamellar-growth ledeburite (e.g., 15%, 20%, 25%, 30%, 35%, 40%, 45%, etc.). In some embodiments, the camshaft includes at least 25% lamellar-growth ledeburite. In some embodiments, the camshaft includes less than 35% (in area fraction) carbide (e.g., 30%, 25%, 20%, 15%, 10%, 5%, etc.). In some embodiments, the camshaft includes less than 25% carbide (e.g., 20%, 15%, 10%, 5%, etc.). In some embodiments, the camshaft includes less than 40% (in area fraction) dendrite (e.g., 35%, 30%, 25%, 20%, 15%, 10%, 5%, etc.). In some embodiments, the camshaft includes less than 30% dendrite (e.g., 25%, 20%, 15%, 10%, 5%, etc.). In some embodiments, the camshaft includes less than 0.05% (in volume fraction) of graphite (e.g., 0.04%, 0.035%, 0.03%, 0.025%, 0.02%, 0.015%, etc.). In some embodiments, such as for an improved CDI specimen with chromium cast in a chiller with a wall thickness of 5 mm (represented by “I-C2”) and an improved CDI specimen without chromium cast in a chiller with a wall thickness of 10 mm (represented by “B-C4”), the camshaft includes at least 25% lamellar-growth ledeburite, less than 25% carbide, less than 30% dendrite, and less than 0.05% graphite. In some embodiments, the I-C2 and the B-C4 specimens demonstrate higher RCF life than specimens with less wall thickness and with no chromium (e.g., specimen represented by “B-C1” as described below).
Imparting the camshaft with the microstructure can include realizing a microstructure for a given chemical composition of the camshaft through a processing treatment (e.g., cooling). For example, the manner in which the camshaft cools and the chemical composition of the camshaft can determine the microstructure of the camshaft. The microstructure can determine the RCF life or fatigue life of the camshaft (e.g., iron-based camshaft fatigue life, iron-based rolling contact fatigue life, rolling contact fatigue life, etc.).
According to various embodiments, the target bearing life of the camshaft corresponds to an L₁₀(e.g., L₁₀life, B₁₀life) of at least 2×10⁶life hours for smooth ball bearings with the microstructure provided herein. The L₁₀can include a minimum expected life or the bearing life associated with 90% reliability. For example, the target bearing life of the camshaft can correspond to an L₁₀of 2.35×10⁶life hours for smooth ball bearings with the microstructure including carbide and pearlite. The target bearing life of the camshaft can correspond to an L₁₀of at least 3×10⁷life hours for smooth ball bearings with the microstructure provided herein. For example, the target bearing life of the camshaft can correspond to an L₁₀of 3.076×10⁷life hours for smooth ball bearings with the microstructure including carbide and pearlite.
According to various embodiments, the target bearing life of the camshaft can correspond to an L₅₀(e.g., L₅₀life, B₅₀life) of at least 3×10⁷life hours for smooth ball bearings with the microstructure provided herein. The L₅₀life can include an average life or the bearing life associated with 50% reliability. For example, the target bearing life of the camshaft can correspond to an L₅₀of 3.6×10⁷life hours for smooth ball bearings with the microstructure including carbide and pearlite. The target bearing life of the camshaft can correspond to an L₅₀of at least 2×10⁸life hours for smooth ball bearings with the microstructure provided herein. For example, the target bearing life of the camshaft can correspond to an L₅₀of 2.59×10⁸life hours for smooth ball bearings with the microstructure including carbide and pearlite.
In some embodiments, the process 300 continues to 320 with austempering the camshaft. Austempering the camshaft homogenizes the pearlite contained in the microstructure of the camshaft. Austempering can improve the RCF life of chilled ductile iron. Austempering can transform the pearlite into ausferrite. Austempering can harden the camshaft and lower the hardness variation among different phases of the camshaft. Austempering can improve the wear resistance and fatigue performance of the camshaft. The austempering process can further harden the chilled ductile iron. The austempering process can further refine and homogenize the microstructure of the chilled ductile iron, thereby producing chilled austempered ductile iron.
FIG. 4 illustrates a micrograph 400 of the microstructure of chilled ductile iron. The microstructure of the chilled ductile iron includes carbide 405 and pearlite 410. The carbide 405 is represented by the dark phase and the pearlite 410 is represented by the light phase. The microstructure of the chilled ductile iron can include graphite nodules 415. The graphite nodules 415 can be involved with a failure mode (e.g., cracking) of the camshaft. The camshafts or the camshaft lobes can be made of such chilled ductile iron. The camshafts or the camshaft lobes can have a microstructure that includes the carbide 405 and the pearlite 410. The camshafts or the camshaft lobes made of chilled ductile iron can have an improved RCF compared to camshafts or camshaft lobes made of chilled iron or austempered iron. The camshafts or the camshaft lobes made of chilled ductile iron can have a similar RCF compared to camshafts or camshaft lobes made of steel. The camshafts or the camshaft lobes made of chilled ductile can provide wear resistance.
FIG. 5 illustrates the microstructure of chilled austempered ductile iron. The camshafts or the camshaft lobes can be made of such chilled austempered ductile iron. The camshafts or the camshaft lobes can have a microstructure that includes the carbide 405 and the pearlite 410. The camshafts or the camshaft lobes made of chilled austempered ductile iron can have an improved RCF compared to camshafts or camshaft lobes made of chilled iron or austempered iron. The camshafts or the camshaft lobes made of chilled austempered ductile iron can have a similar RCF compared to camshafts or camshaft lobes made of steel. The camshafts or the camshaft lobes made of chilled austempered ductile can provide wear resistance.
FIG. 6A illustrates a table 600 comparing the RCF life of different chilled ductile iron specimens. The different chilled ductile iron specimens include a baseline CDI, an improved CDI without chromium cast in a chiller with a wall thickness of 2.5 mm (represented by “2.5 no Cr”), and an improved CDI without chromium cast in a chiller with a wall thickness of 5 mm (represented by “5 no Cr”). The testing was performed at 3.9 GPa using smooth ball bearings. The CDI specimens can correspond to camshaft lobes.
The target bearing life of the baseline CDI can correspond to an L₁₀of 3.3×10⁵life hours for smooth ball bearings. The target bearing life of the baseline CDI can correspond to an L₅₀of 1.4×10⁷life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 2.5 mm can correspond to an L₁₀of 2.35×10⁶life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 2.5 mm can correspond to an L₅₀of 3.6×10⁷life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 5 mm can correspond to an L₁₀of 3.076×10⁷life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 5 mm can correspond to an L₅₀of 2.59×10⁸life hours for smooth ball bearings.
FIG. 6B illustrates a plot 650 comparing the RCF life of different chilled ductile iron specimens. The plot 650 includes the different chilled ductile iron specimens illustrated in table 600. The different chilled ductile iron specimens include the baseline CDI, the improved CDI without chromium cast in a chiller with a wall thickness of 2.5 (represented by “2.5 no Cr” or “B-C1” as described below) and the improved CDI without chromium cast in a chiller with a wall thickness of 5 mm (represented by “5 no Cr” or “B-C2” as described below). The RCF life of the different chilled ductile iron specimens are plotted in a Weibull distribution using maximum likelihood estimation.
FIG. 7 illustrates a plot 700 of cooling curves for different chiller wall thicknesses. The cooling curve can represent the cooling rate profile. The 2.5 mm curve represents a specimen or casting cooled by a chiller with a wall thickness of 2.5 mm. The 5 mm curve represents a specimen or casting cooled by a chiller with a wall thickness of 5 mm. The 7.5 mm curve represents a specimen or casting cooled by a chiller with a wall thickness of 7.5 mm. The 10 mm curve represents a specimen or casting cooled by a chiller with a wall thickness of 10 mm.
The cooling rate varies over the solidification time. The cooling rate profile can include the varying cooling rates or changing cooling rates. The cooling rate profile can include a cooling rate that is constant or that changes over time. The cooling rate profile can be determined based on the geometry or size of the one or more chillers 110, the wall thickness 115 of the one or more chillers 110, the mass of the camshaft, the thickness of the camshaft, the size of the camshaft, and/or the target hardness of the camshaft. The cooling rate profile can be determined by the target microstructure of the camshaft. The cooling rate can vary for a given chemical composition of the specimen.
FIG. 8A illustrates a table 800 comparing the RCF life of different chilled ductile iron specimens. The different chilled ductile iron specimens include a baseline CDI, an improved CDI without chromium cast in a chiller with a wall thickness of 2.5 mm (represented by “B-C1”), an improved CDI without chromium cast in a chiller with a wall thickness of 5 mm (represented by “B-C2”), and an improved CDI with chromium cast in a chiller with a wall thickness of 5 mm (represented by “I-C2”). In some embodiments, the amount of chromium in the I-C2 specimen has a range of 0.5 wt % to 1 wt %. The testing was performed at 3.6 GPa using smooth ball bearings. The CDI specimens can correspond to camshaft lobes.
The target bearing life of the baseline CDI can correspond to a Bio of 3.3×10⁵life hours for smooth ball bearings. The target bearing life of the baseline CDI can correspond to a B₅₀of 1.4×10⁷life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 2.5 mm (“B-C1”) can correspond to a B₁₀of 2.35×10⁶life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 2.5 mm can correspond to a B₅₀of 3.6×10⁷life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 5 mm (“B-C2”) can correspond to a B₁₀of 3.07×10⁷life hours for smooth ball bearings. The target bearing life of the CDI without chromium cast in a chiller with a wall thickness of 5 mm can correspond to a B₅₀of 2.59×10⁸life hours for smooth ball bearings. The target bearing life of the CDI with chromium cast in a chiller with a wall thickness of 5 mm (“I-C2”) can correspond to a B₁₀of 1.48×10⁸life hours for smooth ball bearings. The target bearing life of the CDI with chromium cast in a chiller with a wall thickness of 5 mm can correspond to a B₅₀of 5.13×10⁸life hours for smooth ball bearings.
FIG. 8B illustrates a plot 850 comparing the RCF life of different chilled ductile iron specimens. The plot 850 includes the different chilled ductile iron specimens illustrated in table 800. The different chilled ductile iron specimens include the baseline CDI, the improved CDI without chromium cast in a chiller with a wall thickness of 2.5 mm (“B-C1”), the improved CDI without chromium cast in a chiller with a wall thickness of 5 mm (“B-C2”), and the improved CDI with chromium cast in a chiller with a wall thickness of 5 mm (“I-C2”). The RCF life of the different chilled ductile iron specimens are plotted in a Weibull distribution using maximum likelihood estimation.
FIG. 9A illustrates a table 900 comparing the RCF life of chilled ductile iron and 1080 steel specimens. The chilled ductile iron specimen can include an improved CDI without chromium cast in a chiller with a wall thickness of 10 mm (represented by “B-C4”). The 1080 steel is represented by “IH 1080”. The testing was performed at 3.6 GPa using roughened ball bearings. The specimens can correspond to camshaft lobes.
The target bearing life of the cast iron can correspond to a B₁₀of 7.97×10⁶life hours for roughened ball bearings. The target bearing life of the cast iron can correspond to a B₅₀of 3×10⁷life hours for roughened ball bearings. The target bearing life of the 1080 steel can correspond to a B₁₀of 1.52×10⁷life hours for roughened ball bearings. The target bearing life of the cast iron can correspond to a B₅₀of 5.0×10⁷life hours for roughened ball bearings.
FIG. 9B illustrates a plot 950 comparing the RCF life of cast iron and 1080 steel specimens. The plot 950 includes the cast iron and 1080 steel specimens illustrated in table 900. The RCF life of the cast iron and 1080 steel specimens are plotted in a Weibull distribution using least squares estimation.
FIG. 10 illustrates the casting system 100, according to an embodiment. The casting system 100 can include one or more risers 1005. For example, the casting system 100 includes a first riser (riser 1), a second riser (riser 2), a third riser (riser 3), and a fourth riser (riser 4). The casting system 100 can include a sprue well 1010. The casting system 100 can include one or more cavities. For example, the casting system 100 includes a first cavity (cavity 1), a second cavity (cavity 2), a third cavity (cavity 3), and a fourth cavity (cavity 4). The first cavity has a wall thickness of 2.5 mm. The second cavity has a wall thickness of 5 mm. The third cavity has a wall thickness of 7.5 mm. The third cavity has a wall thickness of 10 mm.
FIG. 11 shows a plot 1100 of temperatures vs. time for different locations of the casting system 100. One or more thermal couples can be located at the different locations of the casting system 100. The temperatures at various points in time of a thermal couple located in the sprue well 1010 are plotted. The temperatures at various points in time of a thermal couple located in the first riser are plotted. The temperatures at various points in time of a thermal couple located in the second riser are plotted. The temperatures at various points in time of a thermal couple located in the fourth riser are plotted.
FIG. 12 shows a plot 1200 of temperatures vs. time for different locations of the casting system 100. The temperatures at various points in time of a thermal couple located at the bottom of the first cavity are plotted. The temperatures at various points in time of a thermal couple located at the top of the second cavity are plotted. The temperatures at various points in time of a thermal couple located at the bottom of the third cavity are plotted. The temperatures at various points in time of a thermal couple located at the top of the fourth cavity are plotted.
FIGS. 13A-18 demonstrate various testing results and analyses pertaining to the improved CDI specimens including the B-C1, the B-C2, the I-C2, and the B-C4, which were obtained by cooling the specimens according to the cooling curves illustrated in FIG. 7 . For example, the B-C1 specimen was cooled according to the cooling curve corresponding to the chiller with the wall thickness of 2.5 mm; the B-C2 and the I-C2 specimens were cooled according to the cooling curve corresponding to the chiller with the wall thickness of 5 mm; and the B-C4 specimen was cooled according to the cooling curve corresponding to the chiller with the wall thickness of 10 mm.
FIGS. 13A-13C show three-dimensional micrographs of three improved CDI specimens each subjected to a spallation surface analysis. The micrographs may be obtained by X-ray microscopy (XRM), for example. FIG. 13A corresponds to the improved CDI specimen without chromium cast in a chiller with a wall thickness of 2.5 mm (“B-C1”), FIG. 13B corresponds to the improved CDI specimen without chromium cast in a chiller with a wall thickness of 5 mm (“B-C2”), and FIG. 13C corresponds to the improved CDI specimen with chromium cast in a chiller with a wall thickness of 5 mm (“I-C2”). The improved CDI specimens illustrated in FIGS. 13A-13C are analyzed and compared based on features including sizes and volume fractions of graphite nodules (e.g., graphite) near spalls, areas of subsurface cracks caused by spalling, and the RCF life.
FIG. 14 shows a plot 1400 of variation in the volume fraction (in %) of graphite nodules between different improved CDI specimens including the B-C1, the B-C2, and the I-C2, and a plot 1450 of variation in the average diameter of the graphite nodules between the different specimens near the spalls created during the spallation surface analysis. The volume fraction and the average diameter of the graphite nodules are calculated based on the CT images used to construct FIGS. 13A-13C. It is noted that graphite nodules with submicron sizes are excluded from the plot 1450 shown in FIG. 14 due to limit in scan resolution of the CT images.
As shown in the plot 1450, all three specimens include graphite nodules having substantially similar or the same average sizes near the spalls. For example, the average diameter of the graphite nodules varies from about 0.007 mm (or 7 μm) to about 0.01 mm (or 10 μm) between the different specimens. As shown in the plot 1400, the volume fraction of the graphite nodules is significantly higher in the B-C1 specimen than in the B-C2 specimen, which has a volume fraction similar to the I-C2 specimen. For example, a difference in the volume fraction of the graphite nodules between the B-C1 specimen and the B-C2 specimen is about 0.2%, while a difference in the volume fraction of the graphite nodules between the B-C2 specimen and the I-C2 specimen is about 0.025%, where both the B-C2 and the I-C2 specimens include less than 0.05% of the graphite nodules.
In addition, the variation in the volume fraction of the graphite nodules between the B-C1, the B-C2, and the I-C2 specimens is generally inversely related to the variation in the RCF life of the same specimens as shown in FIGS. 8A and 8B. For example, the B-C1 specimen with the highest volume fraction of the graphite nodules has the shortest RCF life (e.g., the B₁₀life), while the I-C2 specimen with the lowest volume fraction of the graphite nodules has the longest RCF life (e.g., the B₁₀life).
FIG. 15 shows a plot 1500 of variation in the area of subsurface cracks between the different improved CDI specimens including the B-C1, the B-C2, and the I-C2, and a plot 1550 of variation in the RCF life between the different specimens. As shown in the plot 1500, the B-C2 specimen demonstrates a lower area of subsurface cracks than the B-C1 specimen, where the specimens are without chromium and cast in chillers with different wall thicknesses. Furthermore, the I-C2 specimen, which includes chromium cast in a chiller with the same wall thickness as the B-C2 specimen, demonstrates a lower area of subsurface cracks than the B-C2 specimen. As shown in plot 1550, the variation in RCF life between the three different specimens demonstrates a trend inverse to that of the plot 1500. For example, the I-C2 specimen with the lowest area of subsurface cracks has the highest RCF life, and the B-C1 specimen with the highest area of subsurface cracks has the lowest RCF life (also see FIGS. 8A and 8B).
FIG. 16A shows a plot 1600 comparing hardness (e.g., Vickers Hardness or HV) vs. depth of three different improved CDI specimens including the B-C1, the B-C2, and the B-C4, which were all without chromium and cast in chillers with different wall thicknesses as described in detail above. Datapoints of the plot 1600 were obtained by implementing a hardness test (e.g., a Vickers hardness test) with an applied load of HV 500 gf according to the ASTM Standard E92. The average hardness (in HV) was measured from surface (i.e., 0 mm depth) to a depth of 2 mm (in finished camshaft).
FIG. 16B shows a plot 1650 comparing hardness (e.g., HV) vs. depth of the three different improved CDI specimens including the B-C1, the B-C2, and the B-C4. Similar to the plot 1600, the average hardness in the plot 1650 was measured from surface (i.e., 0 mm depth) to a depth of 2 mm (in finished camshaft). However, different from the plot 1600, datapoints of the plot 1650 were obtained by implementing a hardness test with an applied load of HV 100 gf according to the ASTM Standard E92. The error bar shown in each of FIGS. 16A and 16B corresponds to the 95% confidence interval (CI).
As shown in both of the plots 1600 and 1650, the hardness (e.g., the depth hardness) over the measured depth ranks from the lowest to the highest in this order: the B-C4 specimen, the B-C2 specimen, and the B-C1 specimen. Specifically, the B-C4 specimen has an average hardness of 700 HV at the minimum measured from the surface to the greatest depth (e.g., 2 mm), with no measurement of less than 600 HV. In contrast, the I-C2 specimen (not shown in the plots 1600 and 1650), when subjected to the same hardness test and the same applied load, has an average hardness of 600 HV at the minimum with no measurement of less than 550 HV.
Surface hardness (e.g., in Hardness Rockwell C scale or HRC) of the B-C4 specimen and the I-C2 specimen are also compared. The surface hardness was measured by implementing a hardness test (e.g., Rockwell hardness test) according to ASTM Standard E18. Specifically, the surface hardness of the B-C4 specimen is 57 HRC at the minimum and the surface hardness of the I-C2 specimen is 55 HRC at the minimum.
FIG. 17A shows a plot 1700 comparing area fractions (in %) of various microstructural phases (e.g., phases) present in different improved CDI specimens including the B-C1, the B-C2, the I-C2, and the B-C4, after performing a spallation surface analysis. Each of the specimens includes at least four different phases, such as a primary carbide phase (e.g., carbide; “C”) 1702, a dendrite phase (e.g., dendrite; “D”) 1704, which includes a primary austenite phase (e.g., austenite) and/or a globular pearlite phase (e.g., pearlite), a rod-growth ledeburite phase (e.g., rod-growth ledeburite; “R”) 1706, and a lamellar-growth ledeburite phase (e.g., lamellar-growth ledeburite; “L”) 1708.
As shown, the amount (i.e., the area fraction) of the primary carbide phase 1702 generally decreases and the amount of the lamellar-growth ledeburite phase 1708 generally increases as the chiller wall thickness increases. For example, the I-C2 specimen (with a 5-mm chiller wall thickness) includes less than 25% of the primary carbide phase 1702 but greater than 25% of the lamellar-growth ledeburite phase 1708, while the B-C4 specimen (with a 10-mm chiller wall thickness) includes less than 15% of the primary carbide phase 1702 and more than 40% of the lamellar-growth ledeburite phase 1708.
Furthermore, cross-referencing with the RCF life of the specimens shown in FIGS. 8A-8B and 9A-9B, a decrease in the amount of the primary carbide phase 1702 and an increase in the amount of the lamellar-growth ledeburite phase 1708 are indicative of a specimen with a relatively higher RCF life. For example, compared to the B-C1 specimen, which has the lowest RCF life, the B-C2, the I-C2, and the B-C4 specimens each include less of the primary carbide phase 1702 (e.g., less than 30% as shown in FIG. 17A) and more of the lamellar-growth ledeburite phase 1708 (e.g., greater than 15% as shown in FIG. 17A).
In some embodiments, the I-C2 specimen and the B-C4 specimen have similar amounts of the dendrite phase 1704, such as both being between 25% and 30%. In some embodiments, the I-C2 specimen and the B-C4 specimen both include graphite nodules as well, though the amount (e.g., volume fraction) of the graphite nodules in each specimen is relatively lower than the other phases as shown in FIG. 17A. For example, the amount of the graphite nodules is less than 0.05% and less than 0.03% for the I-C2 specimen and the B-C4 specimen, respectively.
FIGS. 17B-17E show a series of charts each comparing the area fraction of each microstructural phase described in the plot 1700 for the B-C1, the B-C2, the I-C2, and the B-C4 specimens. For example, a chart 1720 of FIG. 17B compares the area fraction of the primary carbide phase 1702; a chart 1740 of FIG. 17C compares the area fraction of the dendrite phase 1704; a chart 1760 of FIG. 17D compares the area fraction of a rod-growth ledeburite phase 1706; and a chart 1780 of FIG. 17E compares the area fraction of a lamellar-growth ledeburite phase 1708. As shown in FIGS. 17B and 17E, the amount of the lamellar-growth ledeburite phase 1708 generally varies inversely with the amount of the primary carbide phase 1702 among the specimens. Furthermore, the amount of the lamellar-growth ledeburite phase 1708 generally varies in a similar trend among the specimens, i.e., the specimens (e.g., I-C2 and B-C4) with greater amounts of lamellar-growth ledeburite phase 1708 demonstrate longer RCF life.
FIG. 18 is a micrograph 1800 showing the microstructural phases of the B-C1 specimen after undergoing the spallation surface analysis provided herein. As shown, the specimen includes a surface 1810 and spallation 1820 below the surface 1810. The specimen includes a primary carbide phase 1830, a primary austenite (e.g., a dendrite) phase 1840, a lamellar-growth ledeburite phase 1850, and a rod-growth ledeburite phase 1860 near the spallation 1820. The presence and the relative amounts of the different phases described in FIGS. 17A-17E and 18 result from the manner in which the various specimens were cooled according to the cooling curves demonstrated in FIG. 7 and may further correlate with one or more of the material properties, such as hardness (both depth and surface hardness), of the specimens demonstrated herein.

III. Construction of Example Embodiments

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed but rather as descriptions of features specific to particular implementations. Certain features described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.
As utilized herein, the terms “substantially” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.
The terms “coupled,” “connected,” and the like, as used herein, mean the joining of two components directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two components or the two components and any additional intermediate components being integrally formed as a single unitary body with one another, with the two components, or with the two components and any additional intermediate components being attached to one another.
The terms “fluidly coupled,” “in fluid communication,” and the like, as used herein, mean the two components or objects have a pathway formed between the two components or objects in which a fluid (e.g., exhaust, water, air, gaseous reductant, gaseous ammonia, etc.) may flow, either with or without intervening components or objects. Examples of fluid couplings or configurations for enabling fluid communication may include piping, channels, or any other suitable components for enabling the flow of a fluid from one component or object to another.
It is important to note that the construction and arrangement of the system shown in the various example implementations is illustrative only and not restrictive in character. All changes and modifications that come within the spirit and/or scope of the described implementations are desired to be protected. It should be understood that some features may not be necessary, and implementations lacking the various features may be contemplated as within the scope of the application, the scope being defined by the claims that follow. When the language “a portion” is used, the item can include a portion and/or the entire item, unless specifically stated to the contrary.
Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, Z, X and Y, X and Z, Y and Z, or X, Y, and Z (i.e., any combination of X, Y, and Z). Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y, and at least one of Z to each be present, unless otherwise indicated.
Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, Z, X and Y, X and Z, Y and Z, or X, Y, and Z (i.e., any combination of X, Y, and Z). Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y, and at least one of Z to each be present, unless otherwise indicated.

Claims

1. A method of casting a camshaft comprising iron, comprising:

determining a cooling rate profile based on a chemical composition of the camshaft and a target bearing life of the camshaft;

casting the camshaft including cooling the camshaft in a chiller based on the cooling rate profile; and

imparting the camshaft with a microstructure comprising ledeburite, carbide, and pearlite.

2. The method of claim 1, further comprising austempering the camshaft to homogenize the pearlite.

3. The method of claim 1, wherein the cooling rate profile includes a cooling rate that changes over time.

4. The method of claim 1, wherein the cooling rate profile includes a cooling rate that is constant over time.

5. The method of claim 1, wherein the cooling rate profile is determined further based on a geometry of the chiller, a size of the chiller, a wall thickness of the chiller, a mass of the camshaft, a thickness of the camshaft, a size of the camshaft, a target hardness of the camshaft, or combinations thereof.

6. The method of claim 1, wherein cooling the camshaft decreases an amount of graphite nodules in the microstructure of the camshaft.

7. The method of claim 1, wherein casting the camshaft further includes pouring molten iron into a mold before cooling the camshaft such that the cooled camshaft comprises chilled ductile iron.

8. The method of claim 1, wherein imparting the camshaft with the microstructure includes realizing the microstructure based on the chemical composition of the camshaft and the cooling of the camshaft according to the cooling rate profile.

9. A method of casting a camshaft comprising iron, comprising:

casting the camshaft including cooling the camshaft in a chiller based on the cooling rate profile;

imparting the camshaft with a microstructure comprising carbide and ledeburite; and

austempering the camshaft to treat the microstructure.

10. The method of claim 9, wherein the cooling rate profile includes a cooling rate that varies over time.

11. The method of claim 9, wherein imparting the camshaft with the microstructure produces a first amount of carbide and a second amount of ledeburite, the second amount being greater than the first amount.

12. The method of claim 9, wherein imparting the camshaft with the microstructure forms pearlite in the microstructure.

13. The method of claim 12, wherein austempering the camshaft transforms the pearlite in the microstructure into ausferrite.

14. The method of claim 9, wherein cooling the camshaft includes treating the camshaft in the chiller at different temperatures over different periods of time.

15. The method of claim 9, wherein cooling the camshaft is further based on a wall thickness of the chiller.

16. The method of claim 9, wherein casting the camshaft further includes pouring molten iron into a mold before cooling the camshaft.

17. The method of claim 9, wherein imparting the camshaft with the microstructure includes realizing the microstructure based on the chemical composition of the camshaft and the cooling rate profile.

18. A method of casting a camshaft comprising iron, comprising:

imparting the camshaft with a microstructure comprising ledeburite, carbide, and pearlite; and

austempering the camshaft to form ausferrite in the microstructure.

19. The method of claim 18, wherein imparting the camshaft with the microstructure forms lamellar-growth ledeburite and rod-growth ledeburite in the microstructure.

20. The method of claim 18, wherein austempering the camshaft homogenizes the pearlite in the microstructure to form the ausferrite.