What Are the Causes of Defects in Valve Castings During Production?

1.Gas Porosity

Gas porosity refers to small voids formed when gases fail to escape during the metal solidification process. These pores have smooth inner walls and contain gases, making them highly reflective to ultrasonic waves. However, due to their typically spherical or ellipsoidal shape (i.e., point-like defects), the amplitude of the reflected ultrasonic wave may be limited. In steel ingots, gas pores are often flattened into planar defects after forging or rolling, making them easier to detect via ultrasonic testing.

2.Shrinkage Cavities and Porosity

As castings or ingots cool and solidify, they shrink in volume. In areas where solidification finishes last, a lack of liquid metal replenishment can lead to voids. Large, concentrated voids are known as shrinkage cavities, while smaller, dispersed ones are called porosity. These typically occur in the central, last-to-solidify areas and often have rough inner surfaces, accompanied by impurities and small gas pores. Due to thermal contraction, shrinkage is inevitable, but its form, size, and location vary depending on the processing method. If not removed during forging, such defects become residual shrinkage cavities within the final forged product.

Valves

3.Slag Inclusions

During melting, slag or fragments of refractory material from the furnace may fall into the molten metal. If they are entrapped during pouring, they form slag inclusion defects. These are rarely isolated and often appear densely clustered or dispersed at various depths. They are volumetric defects but may also exhibit a certain linearity.

4.Inclusions

These include non-metallic inclusions such as oxides and sulfides formed during melting, or metallic inclusions resulting from unmelted alloying elements, like tungsten or molybdenum, which have high densities and melting points.

5.Segregation

Segregation refers to uneven distribution of alloying elements during smelting or metal melting, leading to localized variations in composition. Mechanical properties in segregated areas differ from those of the base metal. When the difference exceeds acceptable limits, it is considered a defect.

6.Casting Cracks

Cracks in castings are primarily caused by internal stresses during cooling and solidification that exceed the material’s strength limit. These are related to the design of the casting and the casting process. High impurity levels can also increase crack sensitivity (e.g., high sulfur content causes hot brittleness, while high phosphorus leads to cold brittleness). In steel ingots, axial intergranular cracks may form, and if they are not welded together during forging, they remain as internal cracks in the final product.

gate valve

7.Cold Shut

This is a laminar defect specific to castings, mainly due to casting design issues. It occurs when splashing, turbulence, pouring interruptions, or converging metal flows create semi-solidified films that do not fuse properly, resulting in diaphragm-like area-type defects within the casting.

8.Skin Formation (Laminations)

During steelmaking, if pouring from the ladle into the ingot mold is interrupted or paused, the previously poured molten metal quickly forms an oxide film upon contact with air. When pouring resumes, the new molten metal breaks through this film, folding it into the ingot and creating a layered (area-type) defect. This cannot be eliminated during subsequent forging.

9.Anisotropy

Due to varying cooling rates from the surface to the center during solidification, different crystal structures form in castings or ingots. This results in anisotropy in mechanical properties and also affects acoustic properties, causing variations in sound velocity and attenuation. The presence of anisotropy negatively impacts the evaluation of defect size and location during ultrasonic testing.

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