Question

Write the steps about the procedure of finding casting defects in deatils at least five steps?

Typed only!

Answer 1

Step 1 – Identify the Defect

Foundry personnel have a tendency to identify a defect based on cause like slag defect or sand inclusions. While this is an acceptable method, after the diagnosis is done, defects be classified based on appearance rather than cause.

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. This would suggest that metal and molding sand were not primary causes of defect. For this reason, no trials were run with modified base iron metallurgy or modified molding sand properties.

Instead, CWC focused on studying the pouring temperature, pouring time and method, Mg-treatment and inoculation method (tablet in manual pour, in-stream in automatic pouring unit). Table 1 summarizes the key variables and factors considered. Detailed fractional factorial design experiments were designed based on well-known statistical methods. Most foundry casting defects are caused by interacting variables (for example, low pouring temperature + specific chemistry).

Factorial design is a tool that allows experimentation on many factors simultaneously. In this case study, researchers ran 2 level factorial design with three factors requiring a total of eight runs. Three factors considered in the experiment were: ladle type (regular ladle and insulated ladle), temperature (lower pouring temperature and higher pouring temperature) and pour cup (D-shaped cup: conical cup with one side flat- and offset-basin cup).

Step 3 – Gating Design and Filtration Review

Often, foundry personnel 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 constants 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 pouring time was controlled by manual pouring operation. It is strongly recommended that computer simulations of solidification and flow are conducted to review the performance of the gating system. Oxidation of iron and formation of inclusions will likely increase as the velocity 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 runner and ingate system.

There are two aspects of filter sizing: the primary sizing is related to ensuring that the filter does not act as the choke. The standard rule of thumb is the cross-sectional area of the filter should be at least 4-6 times that of the choke.

The secondary sizing requirement is related to filter capacity, or the volume of ductile iron that can be passed through the filter prior to blockage. Typically, this capacity is around 20-40 lb./square inch of filter.

Filter pore size can be classified as fine, medium and large. In most applications, medium or large openings are preferred. The filter supplier can provide appropriate sizing sheets for particular castings and filter types.

In the case of this project, the filter was considered adequate for the specific application. Excellent guidelines are available for sizing and placing filters in gating system.

Step 4 – Preliminary Trials

It is important that trials be conducted with just a handful of variables. Proper experimental designs are required to ensure interaction effects are captured (effect of pouring time, temperature and chemistry together, for example). Pours were grouped by heat, and at least 10-20 molds were poured per heat. It is important to measure and document all variables related to the casting.

Step 5 – Production Trials

Production trial volumes and details are important in a quality assurance program. Production trials for automotive applications typically require thousands of castings. Some foundries might review data for a whole shift or for several heats to ensure repeatability and reliability of quality. Data tracking includes pouring temperatures, pouring times, microstructure, chemistry, lab tests related to mechanical properties, and other information.

During the preliminary and production trials, the pouring was done automatically but the metal transfer was manual (by forklift) and, therefore, took more time, resulting in a higher than normal temperature loss. Following the five-step process, key recommendations to reduce the defect included:

-Increase the pouring temperatures.
-Control pouring temperatures (by improving ladle insulation).
-Reduce magnesium additions.
-Increase the bismuth addition after MgFeSi treatment.
-Improve the automatic pouring ability to pour in the center of the cup.
-Reduce velocity and turbulence of the metal in the mold.
-Operate the automatic pouring system with fully automatic ladle filling.