Q What are the proper procedures for pouring and reading a chill wedge to test carbide-forming tendency?
The chill wedge test is an old and trusted test for determining the carbide-forming tendency of both in-furnace and inoculated iron. The test is fast and simple, but errors in execution can lead to erroneous results and, in turn, poor decisions on molten metal processing. Follow these dos and don'ts to carry out the test properly.
* Use a standard chill wedge as described in ASTM A367-60 (re-approved 2005).
* Match the size of the chill wedge to the size of the casting based on the instructions in the ASTM standard.
* Train your operators to read the results of the chill wedge correctly (reading the width), and provide an acceptable range for the result. This training might also involve reading all three corners of the wedge if it is cast vertically.
* Use a standard gage that can be read the same way by all operators. You can make a visual template or fixture with actual wedges reading from zero to full chill--and everything in between--to simplify or eliminate use of a ruler.
* Use a clean refractory sampling spoon.
* Break the wedges in a consistent spot, and always break them perpendicular to the sample. Wedges sometimes break at a steep angle, making the measurements less consistent because it is hard to see the white line.
* Break the samples so there is no "end effect" associated with the mold design.
* Use the same sand fineness, type and resin for the wedge mold every time. See the ASTM standard for recommended materials.
* Pour all chill wedges at the same time after inoculating. The exact interval will vary among systems, but a standard rule of thumb is one minute.
* Pour chill wedges at the same temperature every time (the pouring temperature for the casting). This may require having one standard for colder jobs and a second standard for hotter jobs.
* Document and correlate your chill results (too much chill depth means hard iron, too little indicates soft iron) to actual casting results and any thermal analysis data you might gather.
* Act upon the data. If an accurate sample is taken and it indicates poor microstructure, then remedial action must be taken.
* Establish a standard on whether to water quench the solidified samples. Water quenching allows for quicker turnaround but might mean a harder to read sample with less clarity in the transition between zones along the fracture.
* Use a chill wedge mounted on a pattern plate if using in-stream or in-mold inoculation.
* Store your chill wedge molds in hot (above 100F) or cold (below 40F) conditions.
* Allow loose sand to enter the mold cavity of the chill wedge. This will affect the most important part of the sample--the edge. It is a good practice to store your chill molds upside-down.
* Use a graphite spoon to pour chill wedges. The graphite can have an inoculating effect.
* Use a cold spoon to sample the iron. It only takes a moment to preheat the spoon.
* Use wedge molds that are poorly compacted. A loose mold surface will affect cooling and reading of the sample.
* Misinterpret data because you are pouring multiple grades of iron or multiple types of castings. Variations in carbon equivalent, chromium content, etc., between types of iron can be massive, so a chill level that is acceptable on one ladle may not be appropriate for another.
* Use a damaged pattern to make the chill mold.
* Use a dirty or uneven chill plate if you are doing the plate version of the test.
* Pour your sample too slowly, or it will misrun.
Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience. If you need assistance with a technical issue, fax or email your question to: Casting Answers & Advice, c/o MODERN CASTING, at 847/824-7848 or firstname.lastname@example.org.
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|Title Annotation:||CASTING ANSWERS & ADVICE|
|Date:||Aug 1, 2007|
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