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the diagnosis is done, the International Atlas of Cast- ing Defects recommends that unknown defects be classified based on appearance rather than cause. Using the photos and descriptions given in the atlas, the foundry decided these defects appear to be G121 (inclusions) or B123 (pinholes) (Figures 2 and 3). Te key advantage of this


classification system is that foundries are allowed to have multiple labels for the defect and leaves it more open to investigate different causes for the defect. Detailed optical microgra-


phy confirmed the defects were dross and pinholes related to magnesium vapors. It is essen- tial to verify the casting defects through such detailed analysis, prior to exploring cause and remediation steps. In addition to optical microscopy, SEM (scanning electron micros- copy) along with spectroscopy was used.


Step 2 – Experimental Design


When the castings were


poured with the manual ladles, the frequency of inclusion defects was very low. When the castings were poured with the automatic pouring system, the frequency of inclusion defects was very high. Also, it was observed that on a same day at the same time, under similar sets of green sand parameters, frequency of the inclusion defects was still higher with the automatic pouring system compared to manual pour. Tis would suggest that metal and molding sand were not pri- mary causes of defect. For this reason, no trials were run with modified base iron metallurgy or modified molding sand properties. Instead, CWC focused on study- ing the pouring temperature, pouring time and method, Mg-treatment and inoculation method (tablet in manual


Fig 1. After analysis, defects were identified as dross and pinholes (see box region). Cold shut (misruns) were also found associated with these defects.


2 level factorial design with three factors requiring a total of eight runs. Tree factors considered in the experi- ment were: ladle type (regular ladle and insulated ladle), temperature (lower pouring temperature and higher pour- ing temperature) and pour cup (D-shaped cup: conical cup with one side flat- and offset- basin cup). Detailed factorial design/experimental design is provided in Table 2.


Step 3 – Gating Design and Filtration Review


Fig 2. After analysis, defects were identified pinholes (inset shows casting defect region). Cold shut (misruns) were also found associated with these defects.


Fig 3. After analysis, defects were identified as dross and pinholes (see box region). Cold shut (misruns) were also found associated with these defects.


pour, in-stream in automatic pouring unit). Table 1 summarizes the key vari- ables and factors considered. Detailed fractional factorial design experiments were designed based on well-known statistical methods. Most foundry casting defects are caused by interact- ing variables (for example, low pouring temperature + specific chemistry). Factorial design is a tool that allows


experimentation on many factors simulta- neously. In this case study, researchers ran


Often, foundry person- nel jump to modifying the gating system when they observe slag/dross defects. While turbulence in the gating system may be an important factor, the gating system is one of the few con- stants in the multi-variable production environment of the foundry. Since this gating system worked well in the manual pouring system, no major modifications were proposed. It was noticed that the cross-sectional area of the sprue base and runner was very large and the pour- ing time was controlled by manual pouring operation. It is strongly recommended that computer simulations of solidification and flow are conducted to review the per- formance of the gating sys- tem. Oxidation of iron and formation of inclusions will likely increase as the veloc- ity of the metal increases. Slowing down the flow and


keeping the gating system full may show a reduction in surface inclusions. Filters should be considered an


“insurance” policy rather than the main function of keeping external slag and dross away. Inefficient dross removal practice can lead to filter blockages, quickly leading to misruns and slow pours. Filters are often considered “flow modifiers” as significant dross in castings are related to turbulence in the


July 2018 MODERN CASTING | 33


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