Steel founders share knowledge for improved production.
The technical program featured 43 presentations. Among those garnering strong interest were a study on cracks in steel castings and a computational study of feeding rules and yield improvement techniques, as highlighted below.
Correcting Crack Defects
"Cracks are one of the most common causes of casting scrap, rework, customer reject and casting failures," said Rod Duncan, SFSA Casteel Technical Assoc. While little information on the subject exists to assist steel foundrymen in preventing cracks, he said that 90% of all cracks could be eliminated if adequate resources were committed to their identification and prevention.
In addition to his categorization of cracks (hot tears, hot cracks, check and cold cracks, "rock candy" aluminum nitride and carbon-nitride cracks, under riser cracking, quench cracking, hydrogen cracking, craze cracking and parting line cracks), he outlined the process for successfully eliminating cracks.
1. Evaluate the pattern of crack occurrence. At what point in the process did it occur or first become visible? Is it related to a particular design feature? What is its frequency of occurrence in relation to similar/dissimilar castings in that heat? Compared to similar castings in other heats? Was the treating or welding process not performed according to established procedures or specifications? Is there an unusual concentration of certain elements in the analysis? (Key contributors to cracks include carbon, sulfur, aluminum, hydrogen, nitrogen and boron.)
2. Closely observe the visible crack where it intersects the casting surface to determine the crack surface features. Different cracking mechanisms have distinct appearances both on the casting surface and on the crack face itself.
3. Open the crack with a press or wedge to reveal the unique crack face features, which can be used to further identify the cracking mechanism. The origin of the crack should be identified where possible. It is often indicated by patterns of crack face oxidation or crack progression chevrons that point to the crack origin. Close inspection of the origin should be made, using a 20X scope when appropriate. The objective is to detect any pre-existing condition at the origin that may have led to crack formation (such as an inclusion, cold lap, micro hot tear, etc.)
4. If there is doubt as to the cracking mechanism, the crack can be sectioned in a plane perpendicular to its face and mounted, polished and observed at 100-40OX in an optical microscope. In some circumstances, such as in solidification-regulated cracks, a macroetch of a polished section will reveal segregation points.
Once the crack mechanism is identified, the appropriate process variables related to the formation can be determined. "Only then can the proper remedial action be taken," said Duncan.
Feeding, Yield Improvement
C. Beckermann, X. Shen, J. Gu and R. Hardin of the Univ. of Iowa described their computational investigation of SFSA feeding guidelines (which are used to produce 40% of all steel castings) and potential yield improvement techniques.
Using a commercial casting simulation by relating the predicted Niyama values to porosity and the desired soundness level, the researchers established a methodology to determine feeding distances.
While examining the commercial simulation vs. SFSA rules, they found that the present rules are overly conservative in some cases. They recommended that further research be performed on the SFSA guidelines.
Also, while investigating active and passive yield improvement techniques via simulation, they gained insight into the limitations of chills and riser-insulating materials.
Their research also examined directional solidification techniques for improving casting yield. By stacking castings and employing a hypothetical semicontinuous process, the possibility of more remarkable yield improvement was demonstrated. "The results are preliminary," the authors concluded, "but they demonstrate that entirely new concepts in casting practice are possible."
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|Title Annotation:||Steel Founders' Society of America|
|Author:||Lessiter, Michael J.|
|Date:||Feb 1, 1998|
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