How steel forgings are made
A forging starts as a cut billet or bar from a steel mill, heated in a gas furnace or induction heater and shaped under a hammer or press. In closed-die forging the metal is squeezed into shaped dies; surplus metal escapes as flash at the parting line and is trimmed off in a separate press. Open-die forging works the hot metal between flat or simple dies and is used for shafts, discs, rings and large blocks, often followed by heavy machining.
Forging aligns the grain of the steel along the shape of the part, which is why forgings are chosen for parts under fatigue or impact loads. The properties promised on the certificate come mostly from heat treatment after forging: normalising, or hardening and tempering, done in batch or continuous furnaces. A forge that buys billets with a mill certificate and then forges and heat treats them has changed the material's condition, so its own test results per heat-treatment lot matter more than the mill's paper.
Gujarat's Industries Commissionerate lists castings and forgings clusters in several towns; we do not name forging towns elsewhere because we have not found an official source for them.
What we check on steel forgings
| Check | How it is checked | What fails |
|---|---|---|
| Surface | Sampled forgings examined after shot blasting for laps, folds, seams, scale pits and flash remnants; magnetic-particle testing by the forge's certified technician witnessed where specified | Laps or seams at fillets and the parting line; cracks |
| Dimensions | Measured against the forging drawing (draft, fillet radii, die mismatch, flash trim) or, for machined forgings, the machining drawing and ISO 2768 class | Mismatch beyond tolerance; not enough stock left to machine |
| Internal soundness | Ultrasonic testing to ASTM A388 by the forge's certified technician, witnessed on forgings we select; calibration blocks, sensitivity setting and acceptance level checked against the order | Indications above the agreed acceptance level |
| Heat treatment | Furnace charts read for temperature, soak time and quench for each lot; thermocouple and furnace survey records checked; lot numbers on charts matched to parts | No chart for the lot shipped; soak too short; chart for a different load |
| Hardness | Brinell or Rockwell tests witnessed across the batch, including parts from different furnace positions | Spread suggesting uneven heat treatment; values outside the range |
| Mechanical tests | Tensile and impact tests from the specified test location witnessed at the forge's laboratory, or specimens sealed for an accredited lab | Test pieces cut from a different lot or bar |
| Grain flow | Macro-etch of a sectioned sample forging, where your specification calls for it, witnessed or sent to a lab | Grain flow broken by machining from bar where forging was ordered |
| Marking and traceability | Heat and lot numbers stamped or tagged on parts and traced to the EN 10204 certificate | Unmarked parts; numbers that do not appear on any certificate |
Ultrasonic testing and heat-treatment records
ASTM A388/A388M is a practice for contact, pulse-echo ultrasonic examination of steel forgings by straight-beam and angle-beam techniques. It tells the technician how to set up and scan; it does not on its own decide what is acceptable. The acceptance level must come from the product specification or your order, so a forge that reports "UT to ASTM A388: OK" without stating the acceptance criteria has not given you a meaningful result. We check that the procedure, reference block or DGS setting, scanning coverage and acceptance level match what you ordered, that the technician holds a valid certificate, and that the forgings tested are ones we selected.
Heat treatment is the step most often claimed and least often proved. A credible record shows the furnace, load number, parts in the load, temperatures against time and the quench medium, and links to the lot number on the certificate. Hardness readings across a batch, taken from parts at different positions in the furnace load, tell you whether the treatment was even. If the certificate says "quenched and tempered" but no chart exists for that lot, the finding is a Hold until the forge can prove it. See EN 10204 certificates for what a 3.1 must contain.
Common defects in steel forgings
| Defect | Why it matters | Class |
|---|---|---|
| Crack, or lap at a highly stressed fillet | Fatigue failure starts here | Critical |
| Heat treatment not done or not proven | Strength and toughness on the certificate do not describe the part | Critical |
| Internal flaw above the UT acceptance level | Hidden weakness in a load-bearing part | Critical |
| Wrong steel grade or mixed heats in one lot | Properties and traceability lost | Critical |
| Die mismatch or flash beyond tolerance | Machining stock missing; fit problems | Major |
| Decarburised or heavily pitted surface on as-forged faces | Lower surface hardness and fatigue life | Major |
| Hardness spread across the batch | Uneven treatment; some parts may be soft | Major |
| Light scale, grinding marks on non-functional faces | Appearance | Minor |
Packing, transit and your purchase order
Bright machined forgings need rust-preventive oil and VCI wrapping, with thread and bore protectors on finished features. Shafts should be supported along their length so they do not bend or knock together; heavy rings and discs strapped flat onto skids. Wooden crates must be ISPM 15 treated. The inspector checks weights, crate condition and container lashing.
Under EU CBAM, heading 7326 (other articles of iron or steel, which includes many forged parts) and heading 7307 (pipe fittings, including forged flanges) are on the Annex I list. Forgings classified as parts of machines or vehicles fall under those headings instead. Check classification with your customs adviser; CBAM is the importer's obligation.
Put these in your purchase order:
- Forging and machining drawings with revision, and tolerances for draft, mismatch and flash.
- Steel standard and grade, heat-treatment condition and hardness range.
- Mechanical tests, specimen location and number per lot; EN 10204 certificate type.
- UT procedure, scanning coverage and acceptance level; MPI extent and acceptance criteria.
- Grain-flow or macro-etch requirement, if any.
- Marking of heat and lot numbers; rust protection and packing.
Forgings for vehicles are also covered on engine and transmission components. Book an inspection anywhere in India.
Sources
- ASTM A388/A388M-26, Standard Practice for Ultrasonic Examination of Steel Forgings
- ASTM A788, Standard Specification for Steel Forgings, General Requirements
- Industries Commissionerate, Government of Gujarat: cluster development
- Regulation (EU) 2023/956 establishing a carbon border adjustment mechanism (EUR-Lex)
- European Commission: Carbon Border Adjustment Mechanism