Casting answers & advice.

Q How do I properly pour separately cast test bars?

A To provide certification that the cast iron metal and pouring process have produced castings that meet customer requirements, it is common for metalcasters to pour keel block or "Y" block molds at prescribed intervals of the casting production cycle from which test bars can be excised. This separately cast test bar represents the mechanical properties that can be expected from the casting poured from the same metal.

During initial heats, when the casting process practices are being established, tensile and yield strength tests should be made frequently to establish standard practices and correlate mechanical properties and microstructure of the cast component to pouring practice. Tensile bars, or "coupons," are typically machined from Y or keel block molds made to specific dimensions. These test bars should accompany the castings through any subsequent heat treating process. All test bar molds and their respective cast bars are made to or referenced to an ASTM specification (A48, A536, etc). The cast bars are sized to a range of nominal thicknesses and/ or cooling rates equivalent to represent the nominal section thickness and cooling rate of the casting they represent. Any exceptions by the metalcaster on bar size and/or grade of iron must be conveyed to the purchaser.

The Test Bar Mold

Test bar molds should be made of the same material as the casting mold (e.g. the same type of sand, grain size and sand properties) and the molds should be bonded with the same type of sand binders. Using a different mold media can change the cooling rate, which could significantly affect the mechanical properties of the test bar. Test bars should never be made in a mold with defects. The bar molds should be inspected for mold quality prior to pouring to make sure that the mold has good compaction with an absence of loose sand or other defects. The bar molds should be at room temperature when poured. All molds must include a tracking system so that each bar can be identified with the casting it represents. This information can be molded into the bar mold, scribed, or tagged.

Pouring separately cast bar molds by hand (using pouring cups) or directly from the ladles requires training and care to ensure top quality test bars free from inclusions or cold iron. Particular care is needed to produce a clean pour when pouring ductile iron test bars from large ladles. All pouring conditions should match that of the casting pour (e.g. melt temperature, metal treatment and chemistry). The following tips will help produce test bars that fully represent the castings.

Ductile Iron--The production of ductile iron is a time-temperature process, and the cycle from treatment to end of pour is critical to maintaining quality iron, free from magnesium fade. All cast bars, microstructure lugs and chemistry coupons should be sampled at the end of the pour to ensure that the end metal (the most faded treated iron) has the same integrity and treatment throughout the pour. The test mold pour weight must be calculated onto the pour weight and cycle time so that there is sufficient metal in the ladle after pouring the castings to produce quality test bars without slag inclusions or faded iron that will not meet specifications. This can be a particular concern for job shops that pour multiple large molds where true pouring weights may not be known. Having several hundred pounds of metal left after a six- or eight-ton pour off may include faded iron that is not suitable for test bars.

The pouring ladle should have a skimming device, such as a ceramic blanket or manual rake, to keep the slag back from the spout. A bottom pour ladle will ensure cleaner iron.

When pouring the test bar mold, great care must be exercised to prevent turbulence and splashing. Either excessive turbulence or dribbling the iron into the bar mold too slowly can result in cold iron and inclusions.

When late inoculation is used (either in the pouring basin or sprue/runner of the casting mold), the metal used for the test bar mold must have the same treatment. If the casting mold also has filters, filters must be incorporated in the test bar mold as well.

In those shops pouring both ductile and gray iron, a dedicated ladle should be used for pouring ductile iron test bars.

At the start of the day's heat, the ladles should be repaired and free of slag and solidified iron, and slag build up should be chipped and removed from the ladle during the day.

The poured test bars should be cooled until black in color (approximately 900F [480C]) before shakeout.

Gray Iron--Unlike ductile iron, gray iron bars are usually poured at the beginning of the pour. If inoculation fade is a concern, the more potent inoculants are used either at the tap or at the mold.

Gray iron test bars are often poured using a smaller pouring cup or ladle because the molds are typically smaller in size and it is easier to control the pour using a cup. The cups should be dedicated for pouring gray iron only.

The cups must be kept clean and free from slag and solidified metal.

When sampling, the cup should be immersed in a slag free area of the molten metal, after the ladle is cleared by raking. The cup should be washed (immersed) several times before pouring the test bar mold.

The cup should contain enough metal to fill the mold and the pour fast enough to choke the metal in the pouring basin.

Test bars, which represent the castings, must be cooled to a black color (around 900F [480C]) before shakeout.

If bars are shaken out hotter, it must be agreed upon by the metalcaster and casting purchaser.