Casting defects and troubleshooting

How to use this library

Each entry describes the typical appearance of a defect, its possible causes, the process phases in which it may originate, how it is detected, and common preventive and corrective measures. Filter by process phase to see which defects a given stage can produce.

1 shrinkage cavity: irregular, rough-walled, located in the last region to solidify · 2 gas porosity: rounded, smooth-walled pores, often near the surface

Defect reference

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Misrun and incomplete filling

Definition and appearance: Part of the casting is missing; edges and thin walls are rounded and smooth where the metal froze before filling the cavity.
Possible causes: Possible causes include low metal or shell temperature, slow or interrupted pouring, insufficient metallostatic head, low shell permeability with trapped air, or walls thinner than the alloy can fill over the given flow length.
Where it may originate: Wax patterns and assembly Ceramic shell Melting and casting
Detection: Visual inspection; dimensional check of thin edges.
Prevention and correction: Raise shell preheat or superheat within the alloy's limits, shorten flow paths by gating thin areas directly, vent or thin the shell locally, review minimum wall thickness with the designer.

Cold shut

Definition and appearance: A smooth-edged line or seam where two metal fronts met without fusing; it may look like a crack with rounded edges.
Possible causes: Possible causes: metal streams cooled or oxidised before meeting, interrupted or too slow pouring, unfavourable gating that splits the flow around cores or windows.
Where it may originate: Melting and casting
Detection: Visual inspection; liquid penetrant or magnetic particle testing reveals tight seams.
Prevention and correction: Increase pouring rate and temperature, relocate or add gates so that fronts meet hot, protect oxidation-sensitive alloys during pouring.

Gas porosity

Definition and appearance: Rounded, smooth-walled cavities, isolated or in clusters, at or just below the surface or distributed through the section.
Possible causes: Possible causes: dissolved hydrogen or nitrogen rejected on solidification, air entrained by turbulent filling, gas from incompletely fired shells or residual wax, insufficient deoxidation.
Where it may originate: Ceramic shell Melting and casting
Detection: Radiography or computed tomography; surface-breaking pores by visual or penetrant testing, often only after machining.
Prevention and correction: Dry and clean charge materials, controlled melting time and atmosphere, degassing or vacuum melting where appropriate, complete shell burnout, calm filling.

Shrinkage porosity and cavities

Definition and appearance: Irregular, rough-walled or dendritic cavities in the last regions to solidify: heavy sections, junctions and under gates; may appear as a surface sink or as internal micro-porosity.
Possible causes: Possible causes: liquid and solidification contraction not compensated by feed metal because the feed path froze first, isolated hot spots, undersized gates or feeders, excessive pouring temperature, alloys with a long freezing range.
Where it may originate: Engineering and tooling Wax patterns and assembly Melting and casting
Detection: Radiography, computed tomography or sectioning; pressure or leak testing for pressure-tight parts; exposed by machining.
Prevention and correction: Design for directional solidification with uniform or tapering walls, gate into the heaviest section, add feeders or chills, verify with solidification simulation; hot isostatic pressing can close internal porosity where specified.

Hot tears and cracks

Definition and appearance: Irregular, branching, often oxidised cracks at section changes and internal corners (hot tears); straight, bright cracks formed in the solid state during cooling, cut-off or heat treatment (cold or quench cracks).
Possible causes: Possible causes: contraction restrained by a rigid shell, cores or the gating system while the metal is still weak; sharp internal corners; abrupt thickness changes; alloy composition with a wide freezing range; thermal shock during cut-off or quenching.
Where it may originate: Engineering and tooling Ceramic shell Melting and casting
Detection: Visual, liquid penetrant or magnetic particle testing; radiography for internal tears.
Prevention and correction: Generous fillet radii and gradual transitions, shell or gating changes that reduce restraint, composition control within the specification, controlled cooling and preheating before cut-off of crack-sensitive alloys.

Non-metallic and ceramic inclusions

Definition and appearance: Ceramic fragments, slag, dross or oxide films embedded in the surface or interior; after blasting they often appear as irregular pits.
Possible causes: Possible causes: loose shell particles from spalled prime coat or from undercut wax joints, debris entering the shell after firing, slag or furnace lining carried over, re-oxidation during turbulent pouring.
Where it may originate: Wax patterns and assembly Ceramic shell Melting and casting
Detection: Visual inspection after blasting, penetrant testing, radiography or CT; metallography for identification.
Prevention and correction: Smooth, fully sealed wax joints, shells handled and stored cup-down, shell cavities cleaned before pouring, skimming and filtration, bottom-up or low-turbulence filling.

Shell-related surface defects

Definition and appearance: Positive metal such as fins and veins from shell cracks, bulges from shell deflection, scabs where the prime coat lifted, and metal penetration between ceramic grains.
Possible causes: Possible causes: shell cracked during drying or dewaxing, insufficient shell strength for the metal pressure, prime coat adhesion lost through poor wetting, drying conditions or wax surface contamination.
Where it may originate: Ceramic shell
Detection: Visual inspection; dimensional inspection for bulges.
Prevention and correction: Controlled slurry properties and drying climate, pattern cleaning and etching before the first dip, adequate number of back-up coats, fast autoclave pressurisation.

Defects originating in the wax pattern

Definition and appearance: Sink marks, flow lines, knit lines, air bubbles, parting-line flash and handling damage that are reproduced exactly in the metal.
Possible causes: Possible causes: unsuitable wax or die temperature, injection pressure or dwell time, inadequate venting, worn or mismatched die halves, patterns ejected too warm or stored without support.
Where it may originate: Wax patterns and assembly
Detection: Pattern inspection before assembly; visual inspection of the casting.
Prevention and correction: Documented injection parameters per die, wax chills or cored heavy sections, die maintenance, setters or fixtures for dimensionally critical patterns.

Dimensional distortion

Definition and appearance: Warping, bowing, twisting or out-of-flatness; dimensions outside tolerance although the die is correct.
Possible causes: Possible causes: pattern distortion in storage or assembly, non-uniform contraction of unequal sections, restraint by the gating system, stress relief or sagging during heat treatment, handling while hot.
Where it may originate: Wax patterns and assembly Melting and casting Finishing and machining
Detection: Dimensional inspection: CMM, 3D scanning, flatness and straightness gauges.
Prevention and correction: Symmetrical design and ribs, tie bars removed after heat treatment, supported loading in the furnace, straightening or coining as an agreed operation, die compensation after first samples.

Surface roughness and pitting

Definition and appearance: Surface rougher than agreed, orange-peel texture, small pits or adhering ceramic.
Possible causes: Possible causes: coarse or poorly wetted prime coat, stucco penetration, rough die or pattern surface, metal-mould reaction at high shell or metal temperature, carbon pick-up reactions in stainless steels, over-aggressive blasting.
Where it may originate: Wax patterns and assembly Ceramic shell Melting and casting
Detection: Visual and tactile comparison with reference specimens; profilometer measurement of Ra or Rz.
Prevention and correction: Fine prime slurry with controlled viscosity, die polishing, lowest practical shell and pouring temperature, agreed blasting medium and reference samples.

Oxidation, scale and decarburisation

Definition and appearance: Scale, discoloured or depleted surface layers; lower surface hardness on carbon and low-alloy steels; alpha case on titanium.
Possible causes: Possible causes: reaction of hot metal with air and shell during slow cooling, oxidising furnace atmosphere in heat treatment, long times at temperature.
Where it may originate: Melting and casting Finishing and machining
Detection: Hardness traverse and metallographic section; visual inspection.
Prevention and correction: Protective or reducing conditions during cooling, controlled-atmosphere or vacuum heat treatment, machining allowance on functional surfaces, specified maximum decarburisation depth.

Metallurgical non-conformities

Definition and appearance: Composition, hardness, strength, toughness, ferrite content or corrosion test results outside the specification; unwanted phases, carbides or coarse grain in the microstructure.
Possible causes: Possible causes: charge calculation or alloy addition errors, element losses during melting, incorrect heat-treatment temperature, time or quench delay, slow cooling of heavy sections, mixed heats.
Where it may originate: Melting and casting Finishing and machining
Detection: Spectrometric analysis, hardness and mechanical tests, metallography, ferrite measurement, corrosion tests where specified.
Prevention and correction: Analysis before tapping with correction, qualified heat-treatment procedures with calibrated furnaces, heat and batch traceability; re-heat-treatment is possible in many cases if the specification allows it.

From symptom to root cause

A sound investigation follows a sequence:

Describe the defect precisely: location on the part and on the cluster, appearance, frequency, affected heats and shells.
Classify it by appearance, using metallography or scanning electron microscopy where the type is uncertain. Shrinkage and gas porosity, for example, call for opposite countermeasures.
Correlate occurrence with process records: slurry data, drying conditions, shell and metal temperatures, wax batches, operators and equipment.
Test the most probable cause by a controlled change of one parameter at a time.
Confirm the effect over a sufficient number of parts and update the process documentation.

Several defect types share causes, and one corrective action can worsen another defect: raising pouring temperature to cure misruns, for instance, increases the risk of shrinkage and surface reaction. Changes are therefore evaluated for their side effects.

Acceptance of discontinuities

Not every discontinuity is a defect in the contractual sense. Material and inspection standards define acceptance levels for surface and internal discontinuities, and a casting that meets the agreed level is conforming. Defining these levels, and the zones to which they apply, is part of specifying the part; see non-destructive testing and surface finish.