QUALITY · August 20, 2026 · 9 MIN READ

Forging Defect Analysis: Nine Common Defects, Their Root Causes and How to Detect Them

Roughly 70 % of hot forging rejects trace to four causes: billet temperature outside the window, insufficient metal in the blocker, worn flash land, and laps from a badly placed parting line. Each of the nine common defects below has a distinct signature on the fracture or the surface, which means it can be identified before a metallurgical report and corrected on the next shift.

WHERE THIS APPLIES AT FORGEPRESS CHINA

ForgePress China ships a dimensional and visual inspection report with each lot and sections through any rejected feature on request, 0.1–500 kg per part. Capacity: HKM3 hot-die forging line 1,000–5,000 t, part weight 0.1–500 kg, maximum envelope 1,200 × 800 mm, as-forged to ISO 8062 class C with critical dimensions machined to ±0.2 mm, process control to ISO 9001, first article 4–8 weeks from die design release.

KEY NUMBERS

Laps, underfills and shear cracks account for the majority of dimensional and visual rejects in closed-die hot forging.
Decarburisation depth on a properly controlled furnace is 0.1–0.3 mm; above 0.5 mm indicates excessive time at temperature.
UT for internal voids typically detects discontinuities above 2–3 mm equivalent; smaller flaws need RT or sectioning.
Die-related defects repeat at a fixed position on every part; process-related defects drift with temperature and billet batch.

The diagnostic split: die-fixed versus process-drifting

Before any metallurgy, ask whether the defect appears in the same place on every part. If it does, it is die-fixed: wrong radius, worn land, misaligned insert, blocked ejector. If it moves between parts or between batches, it is process-drifting: temperature, billet mass, lubrication, press condition.

This one question resolves most disputes in a few minutes and decides who pays for the correction — the tooling budget or the process budget.

Defect reference table

Defect	Visual signature	Root cause	Detection
Lap / fold	Straight or curved seam, no decarburisation inside	Poor billet placement, wrong blocker, sharp die step	Visual + magnetic particle
Underfill	Rounded, unfilled corner or rib	Insufficient metal, low temperature, low tonnage, worn die	Visual + first-article scan
Cold shut	Seam with oxide inside, visible at section	Two metal fronts meeting below welding temperature	Sectioning, UT
Shear crack at flash	Crack parallel to flash line	Finish temperature too low, die clearance wrong	Visual + PT
Quench crack	Crack after heat treatment, often at section change	Fast quench, sharp corner, high carbon	PT after heat treatment
Decarburisation	Soft, light-coloured skin	Too long at temperature, oxidising atmosphere	Hardness traverse, microscopy
Grain growth / overheating	Coarse fracture, brittle	Billet above 1,270 °C	Macro etch, fracture inspection
Internal void / pipe	No external sign	Centreline shrinkage in billet, insufficient reduction	UT, sectioning
Scale pit	Pitted surface after machining	Furnace atmosphere, long soak, water on billet	Visual after descaling

Detection column assumes a normal inspection plan. Radiography is reserved for safety-critical parts because of cost.

The three defects that are almost always process, not die

Cold shuts, shear cracks at the flash and decarburisation all point to temperature. Cold shuts form when two metal fronts meet below the welding temperature, which is what happens when a billet cools during a long transfer or when the blocker does not distribute metal to the far corner. Shear cracks at the flash line are the classic signature of finishing below the recrystallisation range. Decarburisation is time-at-temperature: 0.1–0.3 mm is normal, 0.5 mm or more means the furnace is soaking too long or running oxidising.

None of these is fixed by changing the die. They are fixed by shortening the transfer, raising the finish temperature, or correcting the furnace atmosphere.

The two defects that are almost always die, not process

Underfill in a fixed location and laps along a fixed line are tooling signatures. A corner that fails to fill on every part is either short of metal at the blocker stage or has a die radius too tight for the metal to flow into. A lap on a fixed line means the die has a step or an abrupt section change that folds the surface back on itself.

Both are corrected by die modification — opening the radius, adding a preform step, or reworking the blocker — and both will be reproduced exactly by a new die cut to the same drawing. Fix the design, not just the die.

What to do with the evidence

Photograph the defect with a scale and record its position relative to a datum, not "on the corner".
Record the process card values for that batch: start and finish temperature, billet mass, press, die number, shot count.
Cut one section through the defect before repairing anything; the oxide pattern inside a seam distinguishes a lap from a crack.
Compare with the previous good batch — if the good batch used a different billet heat number, the billet is a candidate cause.

Frequently asked questions

What is the difference between a lap and a crack?

A lap is a fold of surface metal back onto itself; the seam contains oxide and the metal underneath is decarburised along the fold. A crack has fresh metal faces with no oxide film and usually follows a grain boundary or a stress concentration. Magnetic particle inspection distinguishes them quickly: a lap gives a broad, weak indication, a crack gives a sharp, strong one.

Why do my forgings crack after heat treatment but not before?

Quench cracking. It appears at section changes, sharp corners and case-hardened surfaces because those are where the cooling rate difference generates the highest tensile stress. Raising the quench temperature, switching to a slower polymer, or adding a radius at the section change removes most of it.

How much decarburisation is acceptable?

For most structural forgings, 0.1–0.3 mm on the as-forged surface is normal and is removed by machining allowance. If the drawing specifies a decarburisation-free surface, that has to be stated as a controlled-atmosphere requirement, not assumed.