35561-wedriesday T 4. The Cast Oil-Field Fitting-Case study: (15) A cast iron, T

Question

35561-wedriesday T 4. The Cast Oil-Field Fitting-Case study: (15) A cast iron, T-type fitting is being produced for the oil driling industry using air-set or no-bake sand for both mold and core. Silica sand has been used in combination with a catalyzed-oil/urethane binder. The Figure 2 presents a cross section of the mold section of the finished casting (b). The final casting contains several significant defects. Gas bubbles are observed in the bottom section of the horizontal tee. A penetration defect is observed near the bottom of the inside diameter, and there is an enlargement of the casting at location C. Answer the following questions: with the core in practice (a) and a cross What is the most likely source of the gas bubbles? and why they are present only at the location a. b. What factors led to the enlargement of the casting at point C? What would you recommend to c. What factors may have caused the penetration defeet? Why is the defect present near the (15) noted? What might you recommend as a solution? correct this problem? bottom of the casting but not near the top? Mold Core Penetration Mold enlargement) la) ibi

Explanation / Answer

a) When a metal is poured into a sand mould containing cores, the gas present in the core expands and attempts to escape. The resin binders (urethrane) used in core manufacture starts to breakdown and generate additional gas. The gaas can escape from the core via the core prints, but if the corre prints are too small or if the mould and the core have low permeability, the gas pressure will build up inside the core. If the pressure reaches the level where it exceeds the opposing pressure of the molten metal, a bubble can be formed that floats up towardds the top of the casting. The gas pressure in an enclosed core may take some time to build up, so any bubble is released after some freezing has already occured. Thus core blows are usually trapped under a substantial thickness of solidified skin. They may be located above the core which has caused the blow, but often they are sufficiently large and mobile to migrate to highest portion of casting, and can make this region completely hollow.

The solution to this problem include:

1. Ensuring that cores are properly vented; i.e., there is a means for gas to escape to atmosphere.

2. Using sand binders which are low in volatile content and which break down slowly.

3. Filling rapidly to a hydrostatic pressure in the liquid metal above that of the pressure of gas in the core, thus suppressing the expansion of the gas out into the liquid.

b) Swell is an enlargement of the mould cavity by localized metal pressure. It occurs due to insufficient and soft ramming, low strenngth mould and core and when mould is not supported properly. From figure (a) we see bottom gating system is used, which could lead to large accumulation of molten metal at point C and hence causing a swell at that location. Use of Parting line gating system may avoid this problem to a larger extent. The corrections for this problem may also include:

1. Sand should be rammed evenly and properly.

2. Increased strength of mould and core.

c) When fluidity of the metal is high, it may penetrate into sand mould or core, causing casting surface to consist of a mixture of sand grains and metal. It occurs when molten metal flows between sand particles in mould. It occurs due to lower strength of the moulding sand, large size of moulding, high permeability of sand, soft ramming. At the entry of the molten metal through the bottom gating system, i.e. near point C, the fuildity is very high, thus penetrating into the sand mould.

The solution to this problem may be:

1. Use of fine grains with low permeability

2. Appropriate ramming.

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