Ningbo Bohua Mechanical Parts Co., Ltd. · Linda, Export Sales · linda@ningbobohua.com · www.bohua-casting.com
Engineering guide
Case Study: A Large Pump Volute With Inclusions and Pinholes
A customer's large pump volute for hydropower stations had been cast by several gravity foundries, and industrial CT kept finding inclusions and pores. Bohua took on the part and tightened melt cleaning, degassing and release checks. After a little over a year of development, the castings met the customer's requirements and regular supply began.
On this page
The Problem: Castings That Failed CT Inspection
The part is a large pump volute used in pumps for hydropower stations. It is cast in A356, weighs about 10 to 15 kg and measures about 500 mm across. It had been made by several gravity foundries, but the results were not consistent.
The customer scanned the castings with industrial CT. The scans showed inclusions and pores inside the walls, and the castings did not meet the strength required for a large pump. The radiographs and CT images are covered by a confidentiality agreement, so this page describes the findings without images.
What Our Radiographs Showed in the Earlier Castings
We radiographed earlier castings on our own digital X-ray systems and found three kinds of indications. Each points to a different part of the process, so they were investigated separately.
| Indication | What it looked like | Main cause to investigate |
|---|---|---|
| Oxide inclusions (slag) | Irregular inclusions in the wall | Melt cleaning and dross removal were not complete; oxides were carried into the casting |
| Pinhole porosity | Dense, fine, dispersed pores | Dissolved hydrogen was not removed well enough before pouring |
| Shrinkage cavities | Larger irregular cavities in heavy sections | Feeding of the heavy sections during solidification |
A radiograph shows the shape and position of an indication. It does not prove the cause on its own, so each finding was checked against the melt records and the casting design.
What We Changed
The main change was in melt control. Every melt for this part is cleaned and rotary degassed, checked with a reduced pressure test and analyzed on the spectrometer before it is released, as described in our A356 melt-treatment guide.
- Melt cleaning and degassing: rotary degassing for 15 to 20 minutes at 480 to 520 rpm, with flux treatment and dross skimming.
- Release: no melt goes to the die before its reduced pressure test sample and spectrometer analysis pass.
- Temperature: continuous melt temperature from a silicon carbide sheathed thermocouple in the holding furnace; die temperatures spot-checked with infrared thermometers.
- Die and risers: the die and the risers were revised several times, under our mold process engineer Devin Julia, until the heavy sections fed soundly during solidification.
At Bohua
Development took a little over a year, including several die revisions. After the castings met the customer's requirements, the part moved into regular supply.
Questions to Ask Any Foundry About a Pump Volute
If a pressure-bearing casting has failed at other suppliers, ask how the new supplier will control the melt, not only how it will inspect the part.
- How is the melt cleaned and degassed, and for how long?
- What test releases a melt before pouring, and what is the pass criterion?
- Is the chemistry checked for every melt before production?
- Which radiographic standard and acceptance grade will be used, and in which zones?
- How will inclusions, hydrogen porosity and shrinkage be told apart when an indication is found?
For the inputs to include in a pump casing RFQ, see the pump housing RFQ guide and the pressure-tight pump casing RFQ page. Our inspection equipment is listed on the quality control page.
Send your pump casing drawing and inspection requirements
Submit the RFQ details, then email your drawing for a structured quote review. Not sure which process fits? Tell us what the part must do. In a free drawing review, our engineers recommend a process and suggest design changes where needed.
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