Forging Defect Atlas: Cracks, Segregation, Flakes (White Spots) – Causes, Standards & Prevention

Introduction

Forged components are widely used in oil & gas, power generation, aerospace, heavy machinery and automotive industries. Even minor metallurgical defects can lead to catastrophic in-service failure, fatigue fracture or unexpected downtime. Buyers and forging engineers must master defect identification, assessment criteria and control methods to reduce scrap and guarantee component reliability.

This article serves as a practical forging defect atlas, focusing on three high-risk defects: cracks, segregation, flakes (white spots). We cover visual & metallurgical features, root causes, international reference standards, real-case images guidance, and actionable prevention strategies.

1. Forging Cracks (Surface & Internal Cracks)

Defect Description & Image Reference

Forging cracks are sharp linear discontinuities, either exposed on the workpiece surface or hidden internally.

· Surface cracks: Usually appear at fillets, sharp corners, parting lines or flash trimming zones; visible by visual inspection, MT or ET.

· Internal cracks: Subsurface or deep inside the forging, invisible externally; only detectable via UT or macroetching.

Root Causes

1. Billet temperature too low, insufficient ductility during deformation

2. Overheating / burning of raw material, grain boundary oxidation

3. Excessive single-pass deformation, high tensile stress

4. Poor die design: sharp corners, abrupt metal flow transition

5. Fast post-forging cooling, large thermal stress

6. Raw material inherited microcracks or inclusions

Relevant International Standards

· ASTM E381: Standard method of macroetch testing for steel forgings, used to reveal cracks and flow discontinuities

· ASTM A388/A388M: Ultrasonic examination of steel forgings for internal crack detection

· ASTM E709: Magnetic particle testing for surface and near-surface cracks

· ISO 4969: Macroscopic examination by etching

Prevention Measures

1. Strictly control heating range, avoid overheating and burning; maintain proper start/finish forging temperature according to material grade

2. Optimize die design: increase fillet radii, smooth metal flow transition

3. Control single-pass reduction; adopt multi-pass forging for heavy deformation

4. Remove billet surface scale and original cracks before forging

5. Control cooling rate after forging; apply stress relief annealing to release residual stress

6. Implement 100% MT + UT inspection for critical pressure-bearing or aerospace forgings

2. Segregation (Chemical Segregation)

Defect Description & Image Reference

Segregation is non-uniform distribution of alloying elements (C, S, P, Cr, Mo, Ni) formed during ingot solidification. After hot forging, it remains as banded or concentrated zones. Segregation does not always show visible cracks, but creates uneven mechanical properties: local low toughness, poor transverse ductility, and higher risk of cracking during heat treatment.

Root Causes

1. Ingot solidification segregation, insufficient forging reduction ratio

2. Improper forging sequence, insufficient deformation to break cast dendritic structure

3. High impurity content in molten steel

4. Inadequate homogenization soaking before hot working

Relevant International Standards

· ASTM E381: Macroetch evaluation of segregation, banding and chemical inhomogeneity on forgings and blooms

· ASTM A609: Ultrasonic testing for carbon and low-alloy steel castings & forgings

· ISO 4969: Macroscopic etching examination

Prevention Measures

1. Select high-quality low-segregation billets / continuous cast material

2. Apply high-temperature homogenization soaking before forging

3. Guarantee sufficient forging reduction ratio (for critical parts ≥3:1, aerospace ≥4:1)

4. Adopt multi-directional forging to break dendritic segregation structure

5. Use macroetch sampling inspection per ASTM E381 to verify segregation grade

6. Limit sulfur & phosphorus content in steel melting

3. Flakes / White Spots

Defect Description & Image Reference

Flakes (white spots) are hydrogen-induced internal microcracks, mainly found in large cross-section alloy steel forgings (Ni-Cr-Mo series). On fracture surfaces, flakes look like silvery-white smooth round or elliptical patches. On transverse macro-etched samples, they appear as short, radial hairline cracks concentrated near the forging center. Flakes are extremely dangerous: they may not be detected after machining, and can trigger sudden brittle fracture under load. In most heavy forging specifications, flakes are unacceptable defects.

Root Causes

1. High hydrogen content in molten steel / billet

2. Fast cooling after hot forging; hydrogen cannot diffuse out and precipitates internally

3. Large cross-section forgings, slow hydrogen diffusion path

4. Missing dehydrogenation heat treatment

Relevant International Standards

· ASTM E381: Macroetch test for flake detection

· ASTM A388: Ultrasonic inspection to identify flake clusters inside heavy steel forgings

· AMS 2300: Aerospace metallurgical requirements for alloy steel forgings, hydrogen control

Prevention Measures

1. Low-hydrogen melting and vacuum degassing for alloy steel

2. Mandatory dehydrogenation annealing immediately after forging for large-section alloy steel

3. Slow cooling control in insulated furnaces after hot working

4. Hydrogen content testing of raw billets before production

5. UT screening + macroetch verification for high-risk heavy forgings

6. Avoid rapid quenching directly from high forging temperature

Additional Common Forging Defects

· Laps / Cold Shuts: Metal folds that fail to weld; stress concentration origin. Prevent via smooth die flow and proper billet volume.

· Scale Pits: Oxide scale pressed into surface; remove scale by high-pressure water or shot blasting before forging.

· Underfill: Die cavity not fully filled; optimize billet size, forging force and preform design.

Inspection & Quality Control Workflow

1. Raw material incoming check: chemical analysis, hydrogen test, UT

2. In-process: temperature logging, forging ratio control, FEM simulation of metal flow

3. Post-forging: controlled cooling, dehydrogenation / stress relief heat treatment

4. Final inspection: Visual, MT, UT, macroetch per ASTM E381 for critical parts

5. Metallurgical sampling and failure analysis when defects occur

Conclusion

Cracks, segregation and flakes are three of the most costly metallurgical defects in steel forgings. Most defects are traceable to raw material quality, heating parameters, forging reduction, cooling control and die design. By following ASTM / ISO inspection standards and implementing full-process metallurgical control, both forging manufacturers and purchasers can reduce defect rates and avoid catastrophic component failure.

When sourcing heavy forgings, buyers should clearly specify acceptance criteria, NDT scope and macroetch requirements in purchase contracts.