Thermal measuring and testing – Determination of inherent thermal property
Reexamination Certificate
1999-07-13
2001-05-29
Bennett, G. Bradley (Department: 2859)
Thermal measuring and testing
Determination of inherent thermal property
C374S044000, C374S045000, C374S145000
Reexamination Certificate
active
06238087
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to heat treating and more particularly, to a method and apparatus for determining a series of heat transfer coefficients for characterizing a quench for purposes such as use in computer programs for predicting hardness characteristics of a steel part subject to the quench, and for selecting a quench for producing desired hardness characteristics on a part.
BACKGROUND ART
Heat treatment for hardening a steel part involves heating the part to a high temperature, typically to reach the austenite condition, then subjecting the part to a quench which rapidly cools the part for achieving desired hardness characteristics on at least the outer surface of the part, and in many cases, also at one or more internal locations between the outer surface and the core of the part. Numerous fluid mediums are commonly used for quenching, a particular quench typically being selected based on experience and various factors including the hardness characteristics one is trying to achieve, and size, shape and composition of the part to be quenched. Examples of commonly used quench mediums include water, oil, aqueous polymer solutions and mixtures of the above mediums. Air blasts can also be used. Additionally, liquid quench mediums can be in a still state or in varying degrees of agitation. Other factors observed to affect characteristics of a particular quench include level of impurities in the quench medium, volume of the quench medium, tank size, pump flow rates (for agitation), and the like.
Currently, a variety of methods for determining hardenability of steels are known. Reference for instance, the well known Jominy End Quench Test. The Jominy End Quench Test involves quenching one end of a cylindrical steel specimen with a water quench, measuring the hardness of the specimen at one-sixteenth inch increments from the quenched end, then preparing a plot of the hardness measurements versus distance from the quenched end. The Jominy test is specifically done with a water quench and only quenches the flat, blunt end of the specimen, to produce uniaxial heat flow through the specimen. The test holds the specimen shape and size and the quench medium constant, so that hardenability of various tested steel compositions can be determined and compared. However, since the Jominy test utilizes only a water end quench, which provides only uniaxial heat flow, it has been found to be less than an ideal tool for predicting hardness of actual parts having shapes and/or sizes that differ substantially from the test specimens. The Jominy test is also less than an ideal tool when a different quench is used, for instance, wherein a different quench medium is used, and wherein most of a part, or an entire part, is quenched instead of just the end.
It is also known to determine hardness of steel parts experimentally, by actually making a sample part and subjecting the part to a desired quench. Then, hardness measurements are taken at a desired location or locations on the part to determine whether the quench achieved the desired hardness characteristics. For this determination, the part can be sectioned at one or more locations of interest and the hardness measured on a sectional surface or surfaces to ascertain hardness at internal locations on the part. However, this can be an expensive and time consuming process, depending on factors such as the complexity of the part, and the number of experiments required to determine a quench which produces the desired hardness characteristics.
Other tools for predicting hardness of steel parts of different shapes and sizes include the various well known commercially available hardness prediction software programs such as the HEARTS program available from CRC Research Institute, Inc. of Santa Clara, Calif. Programs such as the HEARTS program are operable using finite element analysis or similar techniques for predicting a hardness traverse for a part when provided with certain predetermined inputs including chemistry of the part, dimensions of the part, geometric characteristics of the part and a characterization of a particular quench to be used, which characterization is typically a series of convection heat transfer coefficients for the quench.
A limitation of the known hardness prediction programs, however, is that the accuracy of the hardness traverse output has been found to be only as good as the accuracy of the inputted information. In particular, it has been found that an outputted hardness traverse is significantly negatively affected by inaccurate heat transfer coefficient inputs. This is problematic because only limited reliable heat transfer coefficient data is available, necessitating in many instances the use of known data for one kind of quench for determining hardness produced by different quenches, and the use of estimates, which are accurate only within the capabilities of the estimator.
It is also known to determine heat transfer coefficients for a quench experimentally. In this regard, it is known that the hardness of a part at a particular location is a function of the cooling rate at that location, which in turn is a function of the heat transfer coefficient at the interface between the surface of the part and the quench medium, which heat transfer coefficient fluctuates with temperature. By placing thermocouples at desired locations in a part, measurements of the rate of temperature decline at the locations can be taken over time using well known formulas. This data can then be used to calculate a series of heat transfer coefficients for the quench versus time. The series of heat transfer coefficients can then be inputted into the HEARTS program along with the other inputs discussed above, and a reasonably accurate predicted hardness traverse outputted.
However, collecting temperature data in the above-described manner is time consuming and expensive. Also, it is not uncommon to experience equipment failure such as a thermocouple being destroyed or broken loose due to the initial high temperatures involved, the sudden temperature drop experienced during the quench, handling, and other causes.
Therefore, what is required is a tool which enables more easily determining heat transfer coefficients for characterizing a quench for use in predicting desired hardness characteristics produced on a part subject to the quench, and for determining whether a particular quench will produce desired hardness characteristics on a part, without requiring actually quenching the part and the attendant problems just discussed.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.
DISCLOSURE OF THE INVENTION
In one aspect of the present invention, a method is disclosed for characterizing a quench for producing a desired hardness characteristic on a subject part, the subject part having at least one distinguishing characteristic effecting the hardness characteristic, the method comprising the step of: determining a difference between information representative of the desired hardness characteristic and information representative of a hardness characteristic produced by a known quench on at least one specimen part having the at least one distinguishing characteristic; and if the difference is less than a predetermined value, then producing information representative of a characterization of the known quench; and if the difference is greater than or equal to the predetermined value, then determining and producing information representative of a characterization of a new quench based on the difference.
According to a preferred aspect of the present invention, the quench characterizations are each a series of heat transfer coefficients for the respective quenches. The preferred distinguishing characteristics of the subject part and the specimen parts include, but are not limited to, geometric configuration, size and composition.
According to another aspect of the present invention, apparatus for characterizing a quench for producing a desired hardness characteristic on a subjec
Burris Kenneth W.
Chuzhoy Leonid
Clements Thomas E.
Bennett G. Bradley
Caterpillar Inc.
Morrison John W.
Verbitsky Gail
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