Forging and forming
Open die forging: what goes wrong and where to inspect
Large shafts, rolls, discs and blocks are worked between flat dies by hydraulic presses or hammers, refining the cast structure of the ingot into a tough wrought one.
01How the process works
A heated ingot or billet is upset, drawn and turned between open dies until it reaches the required shape, with enough total reduction to close internal porosity and refine grain. Forgings are rough machined, heat treated (normalised, quenched and tempered) and ultrasonically tested.
The quality of an open die forging is decided by ingot quality, forging reduction, heating practice and heat treatment. Internal discontinuities are found by ultrasonic testing; properties are proven by tests on prolongations or sacrificial forgings.
- Ingot or billet. Steel made, cast and identified by heat.
- Heating. Furnace heating to forging temperature.
- Forging. Upsetting and drawing to shape with defined reduction.
- Heat treatment. Normalising, quench and temper.
- Machining and testing. Rough machining, UT, test coupons, dimensional check.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Internal bursts and porosity | Low reduction, ingot quality | UT |
| Surface laps and folds | Forging practice | MT, visual |
| Coarse grain | Overheating, low reduction | Metallography, UT attenuation |
| Low toughness | Heat treatment | Impact tests |
| Flakes (hydrogen) | Steel hydrogen content, cooling | UT |
| Wrong section from prolongation | Test location not per spec | Witness of test marking |
03Where an inspector should look
Before production
- Steelmaking route and ingot size
- Reduction ratio and test locations
- UT method and acceptance class
During production
- Heating and forging temperatures
- Forging sequence
- Heat treatment charts
After production
- UT after heat treatment
- Mechanical and impact tests
- Dimensions and marking
04Standards and references
EN 10250 (open die forgings), EN 10228 (NDT of forgings), ASTM A668, A388 (UT of heavy forgings).
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.