FH Flame hardening
Flame hardening · Tratamiento térmico · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “Flame harden rolling surface” states a flame hardening requirement.
What controls
Localized surface hardening in which heating is done with an oxy-fuel torch, followed by quenching with water or a coolant spray. It pursues the same goal as induction hardening (a hard case over a tough core in a specific area) but with less control over temperature and hardened extent, and with greater dependence on operator skill and on the procedure. In exchange, it works on large parts or awkward geometries where no induction coil is feasible.
How to verify
Surface hardness with a portable hardness tester at several points spread across the whole hardened area, not just in the middle. Case depth and extent by sectioning or macro-etching a production part or a representative test coupon. The record of the procedure applied (parameters, operator, sequence) is part of the evidence, because the process is less repeatable than others.
Workshop guideline values
- General
- Case depths in the order of 2–5 mm on large steel and cast iron parts.
- Accuracy
- Case depths in the order of 1.5–3 mm with a guided procedure and hardness checks at several points.
- High precision
- Uncommon: when requirements are demanding it is replaced by induction hardening, which is far more repeatable.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Treating it as equivalent to induction hardening in repeatability and control of the hardened area.
- Checking hardness at a single point of an area hardened by hand.
- Applying it without a written procedure and expecting consistent results across operators and shifts.
How to get it in the workshop / if it comes out of tolerance
Expect more scatter than with induction: in hardness, in depth and at the boundary of the hardened area. The procedure (torch distance, travel speed, flow rate, sequence) has to be defined and followed, because it is the only real control there is. It is common on castings and large weldments. Leave grinding stock and bear in mind that the surface comes out with an oxide scale.
Uneven hardness along the area: inconsistent torch travel speed or varying standoff distance; this is a matter of procedure and of guiding fixtures. Local overheating with possible surface melting: reduce dwell time. Cracks: quench too severe or geometry with stress risers. If the part is critical and the scatter cannot be controlled, the right treatment is probably induction hardening.
Related symbols
Other symbols in Heat treatment
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