Blind risers: why they don't always work; part 1 of 3.
This puncture hole allows the atmospheric pressure to push on the liquid metal in the blind riser so that it may feed the casting. Without the firecracker core, the riser will not feed the casting even if all the thermal gradients for directional solidification are present.
Blind riser failure can be troublesome because it is usually not easily detected, especially in the case of side blind risers. Often the neck area of the casting will appear solid after riser removal and shrinkage occur at a location behind or away from the riser contact. What actually happens in many cases is that the casting feeds the side blind riser instead of the riser feeding the casting.
The major causes of blind riser failure include:
* open risers feeding blind risers
* "firecracker" core failure
* gas due to plugged core vents
* riser necks freezing prematurely
* cold metal in the riser
Technological change has contributed to some of these problems but, conversely, technological innovations have been introduced that help the foundry engineer cope with the problems. The purpose of this series is to examine the cause and effect of each of the major problems experienced with blind risers and to show they may be avoided.
Blind Riser Problems
A blind riser that has worked properly will have a clearly visible, pencil-sized hole in the top. When the riser is sectioned, a dendritic shrink pattern will be seen, beginning at the firecracker core and extending down toward the neck of the riser.
A blind riser that has failed to work properly usually will show the shape of the firecracker core and either no shrink cavity or a small rounded cavity of no more than the 6% volumetric shrinkage that occurs as the steel cools from a liquid to a solid state. This is illustrated in Fig. 1.
The failed riser will usually appear solid at the interface of the casting surface and the riser neck, although shrinkage indications can often be found at this location by magnetic particle or dye penetrant testing. The fact that the shrinkage is not visually apparent in most cases makes this a very serious defect, one that may not become known until the customer machines the casting.
The major causes of blind riser failure can all be anticipated during the pattern construction and sampling stage if one knows what to expect. Suspect castings should be sectioned by saw cutting or radiographed to check for the existence of internal shrinkage.
In spite of all the precautions and checking that takes place during the sampling stage, it is still possible to have sporadic blind riser failure during production if proper process control measures are not maintained.
For this reason it is necessary to instruct riser removal personnel to recognize riser anomalies that signal potential problems. Blind riser problems most commonly seen in carbon, low-alloy and stainless steel castings are discussed here along with design and process control precautions that must be taken to avoid problems.
Feeding Blind Risers
One of the most common problems experienced with blind risers is that of an open riser, usually located in the cope, feeding a parting line or drag blind riser. This can usually be identified in the first sample castings, and once corrected will seldom be seen again in the same casting.
In this case the open riser will usually be thoroughly drained because it feeds, or tries to feed, a large portion of the casting plus the blind riser. The blind riser is usually solid with no evidence of an opening under the firecracker core. The casting itself may have an open shrinkage defect under the open riser and will likely have an internal shrinkage defect near the blind riser.
This occurs because of the lack of sufficient end effect between the section of the casting which is to be fed by the open riser and the section of the casting being fed by the blind riser. The lack of an end effect results in insufficient thermal gradients to allow the risers to feed as planned. To cure this problem, it is necessary to provide an end effect, usually with the use of metal chills. The chill must be placed in a location that will isolate the section of the casting to be fed by the open riser from the section of the casting to be fed by the blind riser as shown in Fig. 2.
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|Title Annotation:||Cast Facts|
|Author:||Rowe, Charles A.|
|Date:||Feb 1, 1991|
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