Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2000-04-04
2002-07-23
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06422090
ABSTRACT:
BACKGROUND OF THE DISCLOSURE
1. Field of the Invention
The invention relates to apparatus, and an accompanying method for use therein, for a thermodynamic material testing system that is capable of controllably inducing very large strains in metallic structures, specifically in crystalline metallic specimens. Additionally, the system can also simultaneously direct resistance heat or conductively cool such a specimen then under test, under controlled conditions, in order to establish isothermal planes at a desired substantially uniform temperature throughout a work zone in the specimen. The invention is particularly, though not exclusively, suited for simulating the performance of high-speed multi-stand rolling mills and in properly configuring those mills to produce metallic material with very fine-grained crystalline structures.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products.
Such materials, with relatively few exceptions, solidify in ordered structures that have atoms arranged in a pattern that repeats itself periodically in three dimensions. Whenever an ordered crystalline structure that forms an entire piece of solid material has a single orientation, the material is viewed as being a single crystal. Polycrystalline aggregates, which are formed of assemblages of large numbers of relatively small crystals, each being a so-called “grain”, are the most common form of metals. Within a pure metallic material, each grain has the same composition and structure as that of all of its neighbors, but differs from them in size, shape and orientation. Orientation differences result in the appearance of definite grain boundaries at interfaces between adjacent crystals. These differences significantly affect the properties of the material and to a great extent more so than do the grains themselves.
One important property of a metallic material is its material strength. Commercially speaking, materials with relatively high strengths are extremely. important. A predominant reason is simply that an item can be manufactured to contain less material and hence with generally less weight, if a higher rather than a lower strength material is used. Based on the relative material strength, the weight savings can be appreciable. Oftentimes and not surprisingly, with currently available materials, these weight advantages may well be offset, in certain applications, by increased material cost.
As the grain size of a metallic material decreases, the strength of that material increases. Inclusion of high angle grain boundaries into the material further enhances material strength. Given this, for many decades, considerable effort has been expended in the art to devise and implement techniques for reducing grain sizes, and particularly those techniques that can be used with relatively low cost materials. Clearly, those techniques that readily lend themselves to use in mass production are in greatest demand and hence very eagerly sought by material manufacturers.
Currently, metallic materials are typically fabricated through rolling, forging or extruding-based techniques into sheet, strip or wire, i.e., intermediate products, which are thereafter appropriately and ultimately formed into a shape of a final product. Each of these techniques is governed by interaction of a number of different process parameters all of which significantly influence physical characteristics, such as grain size and shape, and grain boundary orientation, of the final material produced thereby.
Compressively deforming a polycrystalline metal causes its grains to distort. For example, when a metal is rolled, its grains are elongated in a direction of material travel through a rolling mill and cross-wise of longitudinal axes of the rolls, i.e., perpendicular to a roll-metal interface. When a short cylindrical piece of material is compressively deformed, such as through forging, in a direction parallel to a longitudinal cylindrical axis of the material, the length of the cylinder decreases while its diameter increases as the material flows radially outward while it is being deformed; hence, its grains flow elongate here too. Such mechanical working of a metal occurring below its recrystallization temperature, typically referred to as “cold/warm” working, increases internal energy within the crystalline structure. New grains can be formed when additional energy, typically through post-deformation thermal treatment though occurring below the recrystallization temperature, is imparted to the crystalline structure of the material.
The volume of a metallic material at a constant temperature and pressure is itself basically constant. Hence, with the exception of heavily distorted crystalline structures, deforming such a material to change its shape will not appreciably change its volume. When such a material, in the form of a sheet or ingot, is deformed in a rolling mill—i.e., when a roll stand imparts sufficient pressure to the material in excess of the material yield strength, the resulting material will be reduced in thickness but its length will increase. The material will continue to flow elongate as its thickness is increasingly reduced through successive rolling operations.
Grain size and material strength are directly determined through the amount of strain imparted to a metallic material. Deformation causes strain. As the strain increases—up to a point where recrystallization occurs, the microstructure in the material moves, grain refinement occurs and concomitantly grain size decreases. Owing to reduced grain size, the strength of the material increases. Currently, most production rolled steel has a grain size in the 10-50 micron range. If material grain size can be reduced to approximately 1 micron from, e.g., 10 microns, then strength of the resulting material will likely double, if not increase further. Obviously, producing material with such fine grains and high material strengths has profound commercial importance.
In that regard, if lighter materials, such as aluminum, can be processed to yield very small grains and hence significantly enhanced material strengths, then such materials could be used to fabricate parts heretofore formed of heavier materials, such as steel alloys, that possess the same strength. Alternatively, in other applications where material weight may not be an important factor but material cost certainly is, then if relatively low cost materials, such as common low cost steel alloys, could be processed in a fashion that would yield smaller grain sizes and hence increased material strength than heretofore realized in the art, then substantial cost savings will likely result if these materials could replace relatively high-cost high-strength materials.
Another technique commonly used to increase material strength in structural steels has been to alloy various expensive elements into these steels. However, the resulting alloys still tend to be extremely costly and hence economically unsuitable for use in many applications. If the grain size of low-cost steels could be reduced to the point at which such steels possess material strength comparable to that of such alloys, these steels could form a rather cost-effective alternative to such alloys.
Hence, a continual effort has occurred in the art over the course of many years for techniques that can significantly reduce material grain size in order to yield relatively low cost materials that have increased strength.
Mathematically, true strain (&egr;), for a compressive deformation of a specimen, is defined as −ln(h
0
/h) where h is final specimen height and h
o
is initial specimen height, and true strain rate is d&egr;/dt or −(1/h)(dh/dt). Strain is linearly cumulative, provided the specimen material does not recrystallize during an entire deformation process. In that regard, if &egr;
i
represents an amount of strain resulting from deformation i to a specimen, then total strain imparted to that specimen, &egr;
t
, is simply a sum of the individu
Davis Octavia
Dynamic Systems Inc.
Fuller Benjamin R.
Michaelson Peter L.
Michaelson & Wallace
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