Ultrasonic NDT can assess ductile iron quality.
Conventional quality control in the production of ductile iron castings includes testing of solidification structure (nodularity) and mechanical properties. The testing methods available to foundries, such as the metallographic exams of microstructural lugs for visual assessment of graphite morphology and the mechanical exams on tension test specimens cut from separately cast Y- and keel-blocks, are only capable of rates below 40 samples/hr.
From an economic standpoint, however, these methods are labor intensive, time consuming and costly. In addition, the information produced is not 100% reliable, as destructive testing doesn't guarantee the castings will be of the desired quality, meet the minimum mechanical properties required by ductile iron specification ASTM A-536 or fit the customer's specification.
To achieve 100% reliability, high-volume foundries perform additional nondestructive testing (NDT) methods, such as ultrasonic velocity (USV) and sonic resonance frequency (SRF), after cleaning and finishing castings. By applying these NDT methods, foundries ensure that low-nodularity castings do not reach their shipping docks. In addition, USV and SRF systems can be customized at foundries to ensure consistency for specific safety critical applications, such as automotive crankshafts and calipers.
In the late 70s and early 80s, researchers from England and the U.S. demonstrated the feasibility of these NDT methods in evaluating the graphite morphology and mechanical properties of ductile iron castings. Due to a lack of information on process control parameters and proper instrument selection, discrepancies in the influence of metallic matrix structure on USV and SRF, and a lack of data comparing USV and SRF over a wide range of graphite morphologies and metallic matrices obtained in either the as-cast or heat treated condition, foundries have not had confidence in these NDT techniques as surefire quality control methods. Thus, these methods are still to be embraced.
But recently, further research has been performed to renew NDT as a viable tool in the assessment ductile iron. The sonic methods have been proven to be more reliable than traditional testing of nodularity and mechanical properties and can be performed at a lower cost. In addition, portable sonic equipment is now readily available enabling foundries to perform these tests in less time, without highly-trained personnel and prior to pouring of the metal. Similar to the analysis performed by spectrometer, foundries are able to assess their solidified metal's nodularity and mechanical properties and correct any deficiencies before castings are made.
Following are steps to implement USV and SRF testing for NDT assessment of different grades of ductile iron. These steps have been developed through experiments using ductile iron samples cut from research and foundry tensile test specimens of various grades and microstructures. Both ascast and heat-treated specimens are analyzed.
USV to Assess Structure, Properties
Step 1 - Design your own microstructural lug that represents the overall solidification rate and wall thickness of the casting requiring USV testing. For the testing equipment, a simple-to-operate, handheld microprocessor-based USV gauge with one penetration probe (transducer) will suffice. This instrument operates by measuring the thickness of the specimen using a digital caliper. Input this measurement to the instrument via direct transmission [ILLUSTRATION FOR FIGURE 1 OMITTED]. When the ultrasonic probe is coupled to the center of the specimen, the sound velocity is displayed on the screen. Each test must be repeated 3-5 times until consistent results are obtained.
For consistency and to ensure that the USV reading reflects the graphite morphology produced by the casting's slowest cooling rate, use a holding fixture designed to specifically locate the transducer at the center of the test bar cross-section.
Step 2 - Collect and analyze the USV data as it compares to graphite nodularity and mechanical properties.
If USV testing is used as an express-analysis test for nodularity, the microstructural lug must be cut and evaluated using conventional metallography so a comparison base for nodularity can be established. When this test is done, measure USV on the same specimen by applying procedures recommended in step 1.
If USV testing is used to predict mechanical properties, one specimen must be cut from each standard tensile test bar. Upon completion of tensile testing, correlate the USV values measured in each tensile test bar to the mechanical property data obtained after mechanical testing. As a result, correlation between visually assessed nodularity, conventional tested mechanical properties and USV are established. Differences in USV for as-cast ductile iron with the same nodularity occur as a result of variations in the metallic structure of cast iron.
The next factor to consider is the structure of the metallic matrix. Figure 2 shows the effect of the metallic matrix on USV as measured in a 1-in. ductile iron Y-block test casting with greater than 85% nodularity. The structure of the metallic matrix has an effect on the sonic velocity, as increased ferrite content in as-cast iron results in decreased USV. Therefore, in as-cast iron containing 60-70% ferrite (grade 65-45-12) and 10-15% ferrite (grade 100-70-03), the minimum USV values were 0.22 in./microsec and 0.225 in./microsec, respectively. The difference observed between USV values resulted from the different amount of graphite present in these grades of ductile iron. These differences resulted in a decrease of USV with an increase in free graphite.
For testing heat-treated ductile iron castings, a full ferritizing anneal performed on the 60-40-18 grade ductile iron (as is usually performed) results in a metallic matrix that is 100% ferritic. A normalizing is performed to obtain 10070-03 grade ductile iron with a fully pearlitic metallic matrix containing up to 5% ferrite. The actual values of USV are decreased in the heat-treated castings as compared with those obtained in as-cast condition. This is due to the progressive coarsening of the graphite nodules during heat treatments.
The testing also determined that as the nodularity of ductile iron is increased and the metallic matrix remains steady, the USV will increase. For example, the minimum USV for 60-40-18 grade, annealed, ferritic ductile iron with 85% and 95% nodularity is 0.217 in./microsec and 0.22 in./microsec, respectively. For normalized pearlitic ductile iron grade 100-70-03 with a nodularity of 85% and 95%, this measurement increased to 0.219 in./microsec and 0.222 in./microsec, respectively. Therefore, different grades of ductile iron with the same graphite nodularity require different minimum USVs to attain standard specification for ASTM A-536. In this case, the standard depends upon the ferrite/pearlite ratio. For more accurate assessment of the structure and properties of ductile iron castings, it also is necessary to know whether the matrix structure has been obtained in the as-cast state or by heat treatment.
Step 3 - Develop USV testing instructions and work with customers to agree upon an acceptable limit for USV values for different grades of ductile iron. The same USV measurement methods must be used for random testing of graphite nodularity and certification of ductile iron castings.
Once the USV standards have been established, it is important to maintain consistency while measuring the castings in production. After the castings have been cleaned and shotblast, place the ultrasonic probe on the same area of the casting (as agreed upon with the customer) for every measurement. In addition, this area must correspond to the specimen that is used for visual assessment of microstructure. Compare the USV readings taken from the castings against corresponding microstructures and the jointly agreed upon standards as established in the initial testing.
Dynamic Elastic Properties via SRF NDT Method
For castings such as crankshafts or gears, design engineers will specify elastic properties - modulus of elasticity, shear modulus and Poisson's ratio - and their calculations of load-deflection, thermoplastic stresses and fracture mechanics as critical specifications for castings. To measure the elastic properties of ductile iron, use the portable impulse excitation NDT technique SRF [ILLUSTRATION FOR FIGURE 3 OMITTED].
The SRF test apparatus consists of an impulse tool, a transducer and an analyzing system. When a test specimen (cylindrical or rectangular) is tapped by a light external mechanical impulse, the transducer converts mechanical vibration during the free relaxation of the test specimen into an electrical signal that transfers to the electronic analyzing system. Since the resonant or natural frequency is shape- and size-dependent, the obtained resonant frequency data and the dimensions and mass of the specimens are used to calculate the dynamic elastic modulus (DEM). For the tapping, use a steel ball 0.2 in. in diameter at the end of a flexible 4 in. polymer rod.
This NDT method is simple and fast and the impulse excitation for evaluating DEM is adaptable to environmental testing and elevated temperatures. An internal reference crystal ensures accuracy to better than 0.005%, including any effects due to temperature, supply voltage fluctuations and aging.
In contrast to the previously published data, testing has found that DEM values vary with different grades of ductile iron. For example, ductile iron with the same nodularity and increasing ferrite content demonstrates a decrease in the DEM [ILLUSTRATION FOR FIGURE 4 OMITTED]. This is due to an increase in the amount of free graphite in iron, which contains a larger amount of ferrite.
In addition, the data obtained indicates that the density of as-cast ductile iron gradually reduces with increases in ferrite content. This explains why the DEM of pearlitic, grades of ductile iron is greater than those of ferritic iron and why, as the density of ductile iron increases, so does resonant frequency and DEM.
The data collected in this experiment also has shown that the density values are greater in as-cast ductile iron than in heat-treated ductile iron and the DEM decreases in heat treated ductile iron. In addition, the DEM values increase with increasing tensile strength.
The testing on ductile iron casting specimens established several advantages that NDT testing via USV and SRF can provide. These advantages include:
* accuracy - USV- and SRF-based NDT are more accurate than traditional testing of nodularity because visual metallographic testing examines only the surface in one dimension, while the NDT methods examine the volume of material in 3-D;
* less time - USV and SRF testing of the mechanical and microstructural properties of ductile iron castings are done in significantly shorter time with less cost than traditional methods;
* lower cost equipment - Currently available portable instrumentation is inexpensive and allows foundries to perform USV and SRF tests in less than 60 sec without highly trained personnel.
By establishing simple USV and SRF NDT methods, foundries can develop quality assurance before a casting is poured, instead of after.
This article was adapted from a paper (98-012) presented at the 1998 AFS Casting Congress and is available from the AFS Library at 800/537-4237. The co-author of the paper is A. Voorobiev, Moscow Institute of Steel and Alloys.
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|Title Annotation:||nondestructive testing|
|Author:||Lerner, Yury S.|
|Date:||Nov 1, 1998|
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