Glossary · 22 terms
The words on your quote
Terms that turn up on hardened steel quotes, drawings and inspection reports, defined without the textbook padding. Each one has its own anchor if you need to link to it.
- Air hardening
A steel with enough alloy content to reach full hardness cooling in still or forced air, with no liquid quench. A2, D2 and H13 are air hardening. The practical benefit is distortion in the region of a quarter of what an oil quench produces.
- CBN (cubic boron nitride)
A cutting tool material second only to diamond in hardness that, unlike diamond, does not react chemically with iron at cutting temperature. It is what makes machining above about 55 HRC possible at all. Sold in two families: high-content metallic-binder grades for interrupted cuts such as milling, and low-content ceramic-binder grades for continuous turning.
- Comb cracks
A series of fine cracks running perpendicular to a cutting edge, caused by thermal cycling rather than mechanical load. The usual cause in hard milling is flood coolant on an interrupted cut: the tooth is heated in the cut and quenched out of it thousands of times a minute.
- CTQ (critical to quality)
The small number of features on a part that actually carry the function — a bore fit, a sealing face, a locating surface. Marking these and letting everything else sit at a general tolerance is the single largest lever a buyer has on the price of a hardened part.
- Dark layer
A zone of over-tempered, softer material sitting directly beneath a white layer. It is created by the same thermal event and is frequently the more damaging of the two, because a hard brittle skin on soft material is a poor structure to load.
- Dynamic milling
A toolpath strategy in which the CAM system continuously computes actual material engagement and modulates the path to hold it constant. It allows a small radial step with a large axial depth, which spreads wear along the whole flute rather than concentrating it near the tip.
- EXW (Ex Works)
An Incoterm where the buyer takes responsibility for the goods at the seller’s premises and arranges all transport, export clearance and insurance. The default quoting basis in this niche, because it separates the machining price from freight the buyer often already has a contract for.
- Hard milling
Finishing a part on a machining centre after it has been hardened, typically at 55-62 HRC, rather than machining soft and grinding afterwards. It removes a grinding setup and produces 3D geometry no grinder can, at the cost of tooling and machine rigidity requirements.
- Hard turning
Turning a hardened workpiece with CBN tooling instead of grinding it. Holds roughly ±0.005 mm on diameter and Ra 0.2-0.4, which covers a great many bearing seats and seal journals, and collapses three grinding setups into one lathe operation.
- HRC (Rockwell C)
The hardness scale used for hardened steel, measured by the depth a diamond cone penetrates under load. Tool steels typically work between 45 and 65 HRC. A Rockwell C indenter penetrates far deeper than a thin surface layer, so it reads the parent material and cannot detect surface damage.
- Machinability rating
A percentage comparing how readily a material cuts against a reference, conventionally 1% carbon tool steel in the annealed condition at 100%. D2 sits around 50-55%, A2 around 68%, O1 around 75%. Below about 50% means insert consumption is a real line item on a quote.
- MQL (minimum quantity lubrication)
A fine oil mist delivered at a few tens of millilitres per hour. Too little volume to quench a cutting edge, so it avoids the thermal shock that destroys CBN under flood coolant, while still providing lubricity and helping chip evacuation.
- Nital etch
A dilute nitric acid and alcohol etch used to reveal thermal damage on a ground or machined surface. Non-destructive, cheap, and the practical shop-floor method for detecting grinding burn or white layer that dimensional inspection cannot see.
- Pre-hardened steel
Steel supplied from the mill already quenched, tempered and stress relieved, typically at 28-36 HRC. P20, 1.2738 and 4140PH are the common grades. Machined and shipped with no heat treat step, no distortion allowance and no second finishing operation.
- Recast layer
A thin layer of melted and re-solidified material left on a wire EDM surface, typically 5-25 µm, harder and more brittle than the parent metal and carrying tensile stress. On a punch or die edge that sees impact it is where cracks start, so it is removed by skim passes or polishing.
- Retained austenite
Austenite that failed to transform to martensite during quenching and remains in the structure. It is unstable: it can transform later in service, causing a part to grow and lose accuracy. Multiple tempering cycles are the standard remedy, which is why a double or triple temper is normal practice on D2.
- Runout (TIR)
Total indicated runout — how far a rotating tool wobbles off its axis. A standard ER collet chuck typically holds 10-20 µm; shrink-fit and hydraulic holders hold 3-5 µm. In hard milling this is not a refinement: 15 µm of runout means one flute takes a third more chip than its neighbour and fails early.
- Stock allowance
The material deliberately left on a part before heat treatment so that distortion can be machined out afterwards. Typically 0.1-0.2 mm per side on small air hardening parts, up to 1 mm on large or oil-quenched ones. Too little scraps the block; too much doubles the finishing cost.
- Stress relief
A sub-critical heat treatment, typically 600-675 °C for about an hour per 25 mm of section, applied after rough machining. It lets locked-in stress from rolling and roughing release in a controlled way before hardening, rather than in the quench where it warps the part.
- Trochoidal milling
A roughing path that replaces a straight sweep with overlapping circular arcs, keeping the engagement angle roughly constant regardless of the geometry it moves through. It makes slotting in hardened steel viable by turning 100% radial engagement into a series of light arcs.
- White layer
A thin layer of untempered martensite, typically 1-20 µm deep, formed when a machined surface momentarily reaches austenitising temperature and is quenched by the cold bulk beneath it. It is hard, brittle, and sits above tensile residual stress — the combination that cuts fatigue life. It passes dimensional, hardness and roughness inspection.
- Wire EDM
Cutting with an electrical discharge between a thin travelling wire and the workpiece, submerged in deionised water. Hardness is irrelevant to it and it applies no cutting force, so it produces internal corners down to about 0.15 mm that no rotating cutter can reach. It only makes ruled surfaces.
Missing a term? Send it and it gets added. This list is maintained from real quotes and real drawings, not copied from a textbook — so gaps are likely and worth reporting.