Cracks

How to Solve Cracks and Cold Shuts in Investment Casting Products
This guide provides a comprehensive, step-by-step approach — from root cause analysis to process control and inspection — for eliminating cracks and cold shuts in precision casting.

I. Defect Identification and Differentiation
1. Cracks
Appearance:
Long, irregular linear defects; may penetrate or stay on the surface; inner walls often oxidized and dark.

Hot Tears: Occur during solidification due to restricted shrinkage; often at hot spots or section transitions.

Cold Cracks: Form in the solid state, usually caused by internal stress or thermal shock during heat treatment.

Typical Locations:
Hot spots, corners, riser necks, rib roots, and complex internal sections.

2. Cold Shuts
Appearance:
Silver-gray linear seams on the surface; two metal streams fail to fuse completely, leaving an oxidized interface.

Typical Locations:
Flow junctions, thin-wall intersections, gating ends, and distant areas from the sprue.

Difference:
Cold shuts are usually shallow and continuous; cracks are deeper and rougher.

II. Main Causes and Countermeasures for Cracks
1. Casting Design and Gating System
Causes:

Unbalanced gating leads to large temperature gradients and uneven solidification.

Hot spots without feeding paths create stress concentration.

Countermeasures:

Optimize gating for smooth, directional filling.

Provide risers or chills to ensure sequential solidification.

Avoid sharp corners or sudden thickness changes; use fillets (R ≥ 3 mm).

2. Shell and Firing
Causes:

Low shell strength or high thermal expansion; under- or over-firing leads to brittleness.

Uneven shell thickness causes uneven cooling and constraint.

Countermeasures:

Use silica sol shells with low thermal expansion.

Control firing temperature/time (typically 850–1100 °C).

Keep shell thickness uniform (6–10 layers).

Pour while shell is hot to avoid moisture absorption and stress buildup.

3. Alloy and Melting
Causes:

Improper alloy composition (excess S, P, Pb) lowers hot ductility.

Gas or slag inclusion causes brittle microstructure.

Overheating enlarges grain size and increases hot tear susceptibility.

Countermeasures:

Maintain alloy composition and purity within specification.

Ensure clean melt — dry all tools, refine, and degas.

Control pouring temperature:

Stainless Steel: 1550–1600 °C

Carbon Steel: 1520–1570 °C

Aluminum Alloy: 700–740 °C

Shell temperature: 900–1100 °C (steel), 450–550 °C (aluminum)

4. Cooling and Knockout
Causes:

Rapid cooling or premature shell removal causes thermal stress.

Tight metal–shell bonding restricts contraction.

Countermeasures:

Cool gradually; extend hold time for thick parts.

Use sand or furnace cooling.

Avoid forced shell removal or heavy vibration.

III. Main Causes and Countermeasures for Cold Shuts
1. Pouring Temperature and Speed
Causes:

Pour temperature too low or metal cools too fast.

Pouring too slow causes poor flowability.

Countermeasures:

Raise pouring and shell temperature by 10–20 °C above normal.

Keep pouring continuous, without interruptions.

Optimize gating ratio (sprue : runner : ingate ≈ 1 : 2 : 2).

2. Gating and Flow Direction
Causes:

Opposing metal streams collide, creating turbulence and oxide film layers.

Countermeasures:

Use bottom gating or same-direction flow systems.

Add vent holes or wax vents at the end of filling.

Verify flow pattern with mold flow simulation (CAE).

3. Metal Fluidity
Causes:

Poor alloy composition, gas/slag contamination, or excessive holding time.

Countermeasures:

Degas immediately before pouring.

Maintain uniform melt temperature.

Add flow-enhancing elements (Si, Mg, Ni, depending on alloy).

IV. Inspection and Verification Methods
Item Method Purpose
Surface Defects Penetrant Testing (PT) Detect microcracks and cold shut lines
Internal Defects X-ray / CT Evaluate crack depth and direction
Metallography Optical Microscopy Identify hot/cold crack vs. cold shut structure
Residual Stress Strain gauge / hole-drilling Locate stress concentration zones

V. Summary of Preventive Key Points
Stable filling, sequential solidification, and smooth thickness transitions.

Control temperature window: metal, shell, and environment.

Reduce constraint: match metal and shell expansion.

Clean melting: avoid oxidation and inclusions.

Use mold flow analysis for critical components.

Close feedback loop: map defect locations for process improvement.

VI. On-Site Quick Troubleshooting Checklist
Confirm defect type — smooth cold shut or oxidized hot crack.

Review pouring/shell temperature and time curves.

Check shell firing log and hot strength test bars.

Verify alloy composition and refining record.

Inspect gating cross-sections and pouring rate.

Check cooling and knockout procedure.

For recurring defects, perform CAE analysis and tooling redesign.

VII. Reference Control Table
Alloy Type Pour Temp (°C) Shell Temp (°C) Pour Rate Typical Issues & Solutions
Carbon Steel 1520–1570 950–1050 Steady Hot cracks: small fillets; Cold shuts: poor flow
Stainless Steel 1550–1600 1000–1100 Steady Hot cracks: low Si; Cold shuts: low pour temp
Aluminum Alloy 700–740 450–550 Fast Cold shuts: oxide film; Cracks: overheating/cooling imbalance
Copper Alloy 1080–1150 700–800 Smooth Cold shuts: Zn oxidation; Cracks: thermal stress at hot spots

VIII. Post-Processing and Remediation
Cracks: Avoid welding in critical areas. If repaired, apply post-weld heat treatment to relieve stress.

Cold Shuts: Minor surface defects may be removed by machining; reject if in load-bearing regions.

Prevention First: Implement a “defect mapping + process traceability” system for continuous improvement.