Quench cracking
Sharp, oxide-free cracks after hardening: how to tell a quench crack from a forging lap or a hydrogen crack by magnetic particle, penetrant and sectioning, and the quench and temper controls that stop them.
| Defect family | Heat treatment |
|---|---|
| Related service | Heat Treatment |
| Reviewed |
Standards commonly cited
Grades this affects
What you see
Straight or slightly curved cracks that appear after quenching — under the head fillet, along the shank, across a head recess, at a thread root, or radially at the corner of a nut. They are open and sharp-tipped, and after plating they often show as a dark line that bleeds. Cracked parts may ring dull instead of clean, and a cracked lot normally shows several pieces cracked in the same geometric position.
How to confirm it
Magnetic particle inspection to ISO 9934-1 / ASTM E1444, or liquid penetrant to ISO 3452-1 / ASTM E165, will find the crack; a metallographic section identifies the mechanism. A true quench crack runs in from the surface, follows prior-austenite grain boundaries, is unbranched, and has no oxide or decarburization on its flanks because it opened after the part left the furnace. A forging lap has a rounded tip with oxidised, decarburized flanks. A hydrogen crack appears after plating rather than after quenching. Check the adjacent microstructure and hardness at the same time — untempered martensite is the usual companion.
Related calculator: Hardness by steel grade — reference tableRoot causes — most common first
- 1Quench severity mismatched to the grade — water or brine used on an alloy or high-carbon steel that only needed oil.
- 2Delayed or missing temper: martensite left untempered overnight cracks by itself, so tempering has to follow the quench without delay.
- 3Austenitising too hot or too long, coarsening the grain and lowering Ms so transformation stress arrives at a more brittle structure.
- 4Geometry and stress raisers — sharp head fillets, unradiused recess corners, abrupt section changes, deep die marks, stamped identification in a high-stress zone.
- 5Carbon or hardenability at the top of the allowed band, which raises crack risk sharply on the same cycle that ran safely last month.
- 6Non-uniform entry into the quench: nested parts, uneven agitation, or quench oil that is too hot, wet or contaminated, giving locally different cooling rates.
Process controls that prevent it
- Match quench medium and agitation to the grade and section, and control the oil — temperature, water content and contamination all change the cooling curve.
- Temper immediately after quenching and treat the quench-to-temper interval as a controlled parameter with a recorded limit.
- Hold austenitising temperature and time to the minimum that transforms the section; small fasteners rarely need long soaks.
- Fix the geometry where cracks repeat — radius the head fillet, deburr the recess, move stamped marks out of the high-stress zone.
- Load baskets so parts cannot nest or shield each other, and keep loading density constant between lots.
- Crack-check the first-off and a sample of every lot on crack-sensitive grades rather than waiting for the customer to find them.
What to send us for diagnosis
- The cracked parts as found and uncleaned; if they were plated, add unplated pieces from the same lot.
- The grade, the drawing and the exact crack location on the part — a marked photograph is enough.
- The thermal route: austenitising temperature and time, quench medium and its temperature, and the interval before tempering.
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Part types this shows up on
Questions engineers ask
Can a quench crack be repaired?+
No. It is a separation through hardened material, and neither tempering nor plating closes it. The lot has to be sorted by magnetic particle or penetrant inspection and the cracked parts scrapped.
Why did the crack only appear after plating?+
Plating revealed it rather than caused it — pickling opens and cleans the crack flanks and the bright coating makes the line visible. A section settles it: a quench crack has oxide-free flanks and was present before plating, while hydrogen cracking is intergranular and appears in the hours and days after.
Which fasteners crack most often?+
High-carbon and high-hardenability grades in parts with sharp internal corners: socket screws with deep recesses, thin-walled nuts and spring parts. The pattern is nearly always geometry plus a quench faster than the grade needed.
Guidance based on published standards and general fastener metallurgy. Values are typical ranges that depend on the part, the material and the governing specification — the customer drawing and the applicable standard always prevail. This page does not replace a qualified failure analysis of your own parts.