Casting Defects in Steels

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Introduction to Casting Quality Control

Metal casters strive to produce perfect castings, though few are completely free of defects. Modern foundries utilize sophisticated inspection equipment that can detect small differences in size and a wide variety of external and internal defects. The acceptability of imperfections varies by application; for instance, slight shrinkage on the back of a decorative wall plaque is acceptable, whereas similar shrinkage on a precision component cannot be tolerated. Regardless of the intended use, the goal of modern foundries remains zero defects in all castings.

The Impact of Casting Defects on Production

Scrap castings cause significant concern in the industry. Rejected parts result in reduced profits for companies and ultimately affect individual wages. Many foundries conduct daily scrap meetings attended by managers from all major departments. During these meetings, castings identified as scrap by inspectors are examined, with defects typically circled with chalk. The team analyzes the cause of each defect, and the manager whose department was responsible is directed to take corrective action to prevent similar defects in future castings.

The production of metal castings involves numerous variables, making defects often attributable to a combination of factors rather than a single cause. All pertinent data related to the production process (including sand and core properties, pouring temperature) must be documented to identify defects correctly. After proper identification, appropriate corrective actions can be implemented to eliminate the defect in future production.

Classification System for Casting Defects

The classification system presented in this article is based on the physical description of defects. This approach allows identification either through direct observation of the defective casting or from precise descriptions involving criteria such as shape, appearance, location, and dimensions. This morphology-based classification is more logical than cause-based systems since it requires no prior assumptions.

Seven basic categories of defects have been established, with one typical example presented for each category below.

1. Metallic Projections

Joint flash or fins: These present as flat projections of irregular thickness, often with lacy edges, perpendicular to one of the faces of the casting. They occur along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect.

Possible Causes:

• Clearance between two elements of the mold or between mold and core
• Poorly fit mold joint

Remedies:

• Care in pattern making, molding, and core making
• Control of dimensions
• Care in core setting and mold assembly
• Sealing of joints where possible

2. Cavities

Blowholes, pinholes: These appear as smooth-walled cavities, essentially spherical, often not contacting the external casting surface (blowholes). The largest cavities are typically isolated; the smallest (pinholes) appear in groups of varying dimensions. In specific cases, the casting section can be strewn with blowholes or pinholes. The interior walls can be shiny, oxidized or, in cast iron, covered with a thin layer of graphite. These defects can appear in all regions of the casting.

Possible Causes:

• Excessive gas content in metal bath
• Formation of carbon monoxide in steel and cast irons
• Excessive moisture in molds or cores
• Core binders liberating large amounts of gas
• Excessive additives containing hydrocarbons
• Blacking and washes liberating too much gas
• Insufficient evacuation of air and gas from the mold cavity
• Entrainment of air due to turbulence in the runner system

Remedies:

• Make adequate provision for evacuation of air and gas
• Increase permeability of mold and cores
• Avoid improper gating systems
• Ensure adequate baking of dry sand molds
• Control moisture levels in green sand molding
• Reduce amounts of binders and additives or change types
• Use blackings and washes that provide a reducing atmosphere
• Keep the sprue filled and reduce pouring height
• Increase static pressure by enlarging runner height

3. Discontinuities

Hot cracking: This presents as a crack often scarcely visible because the casting generally has not separated into fragments. The fracture surfaces may be discolored due to oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.

Possible Causes:

• Damage to the casting while hot due to rough handling
• Excessive temperature at shakeout

Remedies:

• Care in shakeout and handling while casting is hot
• Sufficient cooling of the casting in the mold
• For metallic molds: delay knockout, ensure mold alignment, use ejector pins

4. Defective Surface

Flow marks: These appear on otherwise sound castings as lines tracing the flow of liquid metal streams.

Possible Causes:

• Oxide films lodging at the surface, marking metal flow paths through the mold

Remedies:

• Increase mold temperature
• Lower pouring temperature
• Modify gate size and location (for permanent molding)
• Tilt the mold during pouring
• In die casting: vapor blast or sand blast mold surfaces perpendicular to the parting line

5. Incomplete Casting

Poured short: The upper portion of the casting is missing. Edges adjacent to the missing section are slightly rounded, while other contours conform to the pattern. The sprue, risers, and lateral vents are filled only to the same height above the parting line as the casting.

Possible Causes:

• Insufficient quantity of liquid metal in the ladle
• Premature interruption of pouring due to operator error

Remedies:

• Ensure sufficient metal in the ladle to fill the mold
• Check the gating system
• Train pouring crew and supervise pouring practices

6. Incorrect Dimensions or Shape

Distorted casting: Inadequate thickness extending over large areas of the cope or drag surfaces at the time the mold is rammed.

Possible Causes:

• Insufficient rigidity of the pattern or pattern plate to withstand ramming pressure, resulting in elastic deformation of the pattern and corresponding permanent deformation of the mold cavity

Remedy:

• Ensure adequate rigidity of patterns and pattern plates, especially when squeeze pressures are being increased

7. Inclusions or Structural Anomalies

Metallic Inclusions: These appear as metallic or intermetallic inclusions of various sizes distinctly different in structure, color, and properties from the base material. These defects most often appear after machining.

Possible Causes:

• Intermetallic formations between the melt and metallic impurities
• Charge materials or alloy additions not completely dissolved
• Exposed core wires or rods
• Insoluble intermetallic compounds forming and segregating during solidification

Remedies:

• Ensure clean charge materials and eliminate foreign metals
• Use small pieces of alloying material and master alloys
• Maintain sufficient bath temperature when making additions
• Avoid late additions too near to pouring time
• For nonferrous alloys, protect cast iron crucibles with suitable wash coating

Conclusion

Understanding the morphology of casting defects is essential for accurate identification and effective remediation in steel foundries. By adopting a systematic approach to defect classification based on physical characteristics, foundry professionals can more efficiently diagnose problems and implement appropriate corrective measures. This approach supports the industry's goal of achieving zero defects while maintaining production efficiency and profitability.