Heat treatment is a controlled process of heating and cooling metals to alter their physical and mechanical properties without changing the part shape. It covers annealing (softening), normalizing (grain refinement), quenching and tempering (hardening + stress relief), case hardening (carburizing/nitriding for surface hardness), and solution treatment + aging (for aluminum alloys).
China's heat treatment industry serves every sector from automotive fasteners (billions of parts/year) to aerospace landing gear components. The key challenge for overseas buyers is inconsistency — heat treatment is invisible after the fact: a poorly hardened part looks identical to a correctly processed one until it fails in service. Knowing what to specify, check, and verify is critical.
| Process Type | Temp Range | Cooling Medium | Hardness Achievable (HRC) | Typical Case Depth |
|---|---|---|---|---|
| Through Hardening (steel) | 800–1050 °C | Oil, water, polymer | 45–65 HRC | Full section |
| Tempering | 150–650 °C | Air (still or forced) | 20–55 HRC | Full section |
| Carburizing (gas) | 880–950 °C | Oil quenching | 58–64 HRC (case) | 0.3–3.0 mm |
| Nitriding (gas/plasma) | 480–560 °C | Slow furnace cool | 62–70 HRC (case, HV scale) | 0.1–0.6 mm |
| Induction Hardening | 850–1000 °C (local) | Water/polymer spray | 50–62 HRC | 1.0–6.0 mm |
| Vacuum Hardening | 1000–1100 °C | High-pressure gas (N₂/He) | 58–65 HRC | Full section |
| Solution + Aging (Al) | 480–540 °C (solution) | Water quench + 170–200 °C age | 12–18 HRC (Brinell 80–150 HB) | Full section |
| Annealing (steel) | 650–900 °C | Furnace slow cool | 15–25 HRC | Full section |
China's largest commercial heat treatment cluster with hundreds of "heavy color" (热处理) shops. Most have vacuum furnaces (up to 1,200 × 1,000 mm chamber) + salt bath lines. Strong in tool steel (SKD11, Cr12MoV, H13) heat treatment for plastic injection molds and die casting dies. Quick turnaround: 24–48 hours for standard jobs. Prices from ¥2–5/kg ($0.30–0.70/kg). Quality: variable — audit your shop personally.
Hardware, fastener, and automotive component heat treatment hub. Continuous mesh-belt furnaces for mass production (1–10 tons/day throughput). Carburizing and carbonitriding for automotive fasteners — many shops have IATF 16949 certification. More consistent quality than Guangdong due to larger automated lines. Prices ¥1.5–3/kg ($0.20–0.45/kg) for mesh-belt production.
Premium precision heat treatment — aerospace and medical device tier. Vacuum furnaces with 6-bar or 10-bar gas quench capabilities (for HSS and powder metallurgy steels). Deep cryogenic treatment (−120 °C to −196 °C) lines for tool stability. Nadcap and AS9100 certified shops available. Prices ¥8–20/kg ($1.10–2.80/kg).
Risk warning: The biggest unknown in Chinese heat treatment is temperature uniformity. Many smaller shops do not perform periodic furnace calibration (SAT/TUS per AMS 2750). Always request the latest furnace calibration certificate before placing an order. We've seen 50 °C variance across a furnace zone in uncalibrated shops — that's enough to turn a batch of H13 tool steel into scrap.
After austenitizing, the time between opening the furnace door and full immersion in quench medium is critical. For high-carbon and alloy steels, a delay of more than 10–15 seconds can cause premature pearlite formation at thin sections, resulting in soft spots. Chinese shops with manual transfer (trolley + basket) are more prone to this than automated lines. If your parts have thin cross-sections (< 5 mm), specify the maximum quench delay in your PO.
When steel is heated in air, surface carbon combines with oxygen to form CO₂, reducing surface hardness by 3–8 HRC. For hardened and tempered parts, this means a soft skin that wears rapidly. Mitigation: specify inert atmosphere (N₂) or vacuum for surface-critical parts. For batch furnaces without atmosphere control, request minimum machining allowance of 0.3–0.5 mm per side for decarburized layer removal post-HT.
For tool and die steels (D2, A2, M2), retained austenite after quenching leads to dimensional instability — the part grows over time as austenite transforms to martensite at room temperature. Chinese shops that skip sub-zero treatment (−70 °C to −80 °C) are common. For precision tooling, specify cryogenic treatment and verify with X-ray diffraction or magnetic analysis.
| Process | Typical Cost (per kg) | Batch Minimum | Typical Lead Time |
|---|---|---|---|
| Through harden + temper (normal) | $0.20–0.60 | $30–80 | 1–2 days |
| Carburizing (gas) | $0.40–1.00 | $50–150 | 2–4 days |
| Nitriding (gas or plasma) | $0.60–2.00 | $80–200 | 1–3 days |
| Vacuum hardening | $1.20–3.50 | $100–300 | 1–2 days |
| Induction hardening (per piece) | $0.50–5.00/pc | $50–150 | 1 day |
| Cryogenic (−80 to −120 °C) | $0.30–0.80 | $50–100 | +1 day |
| Solution + aging (aluminum) | $0.30–1.00 | $50–150 | 1–2 days |
Pricing note: Chinese heat treatment pricing is typically ¥1–10/kg ($0.15–1.50/kg). Heavy parts (gear blanks, shafts) are cheaper per kg. Small precision parts (mold inserts, punches) cost more due to fixturing and handling.
Absolutely not. Untempered martensite is brittle — it has high hardness but near-zero toughness. Quenched steel must be tempered immediately (within 4 hours) to transform some martensite to tempered martensite + relieve internal stresses. A typical H13 die steel: quenched to 55–58 HRC, then double tempered at 540 °C to 48–52 HRC with optimal toughness. Skipping temper means the part risks catastrophic failure under load.
Black oxide (blackening) is a chemical conversion coating — it creates a thin Fe₃O₄ magnetite layer on steel surfaces, typically at 135–145 °C in a caustic soda bath. While it requires heat, it does not change the steel's mechanical properties; it's purely a cosmetic and mild corrosion protection treatment. Do not confuse black oxide with heat treatment when specifying mechanical requirements.
Nondestructive methods: portable hardness tester (Leeb or Telebrineller) on a flat ground surface, eddy current testing for case depth estimation, and Barkhausen noise analysis for grinding burn detection. For critical parts, include a test coupon (same steel grade, same cross-section, processed alongside production parts) in every batch that can be destructively tested. Insist on this from the start.
This is caused by fine surface pitting from the solution treatment quench — if the quench water is too hot (> 40 °C) or has poor circulation, steam pockets form on the surface creating local soft spots. Upon aging, these areas have different grain boundary precipitate morphology, leading to uneven etching response. Control the quench water temperature to 20–30 °C and ensure proper agitation.
For H13, 1.2344, and similar hot work tool steels — yes, absolutely. Vacuum eliminates decarburization (no heating scale), gives better surface quality, and allows high-pressure gas quenching for less distortion than salt bath. The added cost (~2x normal) pays for itself through fewer rejected dies. For cold work steels (D2, O1) in general mechanical parts, atmosphere-controlled box furnaces may be sufficient.