Heat treating is the controlled heating and cooling of metal to change its properties, hardness, strength, toughness, or machinability, without changing its shape. The main processes are annealing, hardening (quenching), tempering, and case hardening, and the whole discipline lives or dies on furnace temperature control: the right temperature, held for the right time, verified and recorded.
That is the deceptively simple core. The same piece of steel can be soft and easy to machine or hard and wear-resistant depending entirely on how it was heated and cooled. Because the change happens inside the metal where you cannot see it, heat treaters prove their process with temperature data rather than appearance. This is why the industry is governed by pyrometry standards that dictate how furnaces are calibrated, surveyed, and documented, the record of the temperature is the proof of the part.
This guide explains what each heat-treating process does, why hardening and tempering go together, how case hardening works, and why furnace control standards like AMS 2750 and CQI-9 shape the shop. Heat treating usually sits between forming and finishing; for the surrounding context, see metal fabrication processes.
What is heat treating?
Heat treating is a group of processes that alter the physical, and sometimes chemical, properties of a metal through controlled cycles of heating and cooling. By heating steel to specific temperatures and cooling it at controlled rates, a heat treater changes its internal microstructure, which in turn sets how hard, strong, tough, or ductile the finished part is.
The reason it matters is that most metal parts need conflicting properties at different stages. A gear must be soft enough to machine accurately, then hard enough to survive years of contact loading. A spring must be tough and resilient. A cutting tool must hold a hard edge. Heat treating is how one material is tuned to each of those needs. It is applied to castings, forgings, machined parts, and fabricated assemblies, essentially any metal part whose performance depends on more than its shape.
Why do hardening and tempering go together?
Because hardening alone leaves steel too brittle to use, and tempering fixes that. Hardening heats steel to a high temperature where its structure changes (austenitizing), then quenches it, cools it rapidly in oil, water, polymer, or gas, to trap a very hard but brittle microstructure called martensite. A part quenched and left there is hard, but so brittle it can crack or shatter under load.
Tempering is the corrective second step: the hardened part is reheated to a controlled temperature, typically between 150°C and 700°C, held, and cooled. This relaxes the brittle structure and trades away some hardness in exchange for toughness and ductility. The tempering temperature is the dial: lower temperatures keep more hardness for wear resistance; higher temperatures give more toughness for shock loads. Harden then temper is therefore treated as one linked operation, and getting the tempering temperature right is where a part becomes both hard and usable.
How does case hardening work?
Case hardening gives a part a hard, wear-resistant outer layer while keeping a tough, shock-absorbing core. It works by changing the chemistry of just the surface, most often by carburizing, which diffuses carbon into the surface of low-carbon steel at high temperature, so only that carbon-enriched skin hardens fully when quenched.
The result is the best of both worlds for parts like gears, shafts, and bearings: a hard case that resists wear and surface fatigue, over a core that stays tough enough to absorb impact without cracking. Related methods include nitriding (diffusing nitrogen at lower temperature) and carbonitriding. Case-hardening processes depend heavily on furnace atmosphere and time as well as temperature, because the depth of the hardened case is set by how long carbon or nitrogen is allowed to diffuse, another parameter that has to be controlled and recorded.
| Process | What it does | Typical purpose |
|---|---|---|
| Annealing | Softens, relieves stress, refines structure | Improve machinability, remove stress |
| Hardening (quench) | Rapid cool to form hard martensite | Maximize hardness (before tempering) |
| Tempering | Reheat to trade hardness for toughness | Make hardened parts usable |
| Case hardening | Hardens the surface, keeps a tough core | Wear resistance with impact toughness |
Why do furnace control standards like AMS 2750 and CQI-9 matter?
Because you cannot see the microstructure, so the temperature record is the only proof the part was treated correctly, and that record is only trustworthy if the furnace and its instruments are proven accurate. Two standards dominate. AMS 2750, published by SAE, is the pyrometry standard for aerospace thermal processing; CQI-9, published by AIAG, is the heat-treat system assessment for automotive and is built on the same pyrometry principles.
These standards require regular proof that a furnace holds temperature where it says it does. A Temperature Uniformity Survey (TUS) checks that every point in the working zone stays within tolerance; a System Accuracy Test (SAT) checks the control instruments against a reference. AMS 2750 classifies furnaces into classes by how tight their uniformity must be, from about ±3°C for the tightest class to ±28°C for the loosest, and instruments into types by accuracy. For aerospace work, Nadcap accreditation (administered by PRI) audits all of it. The practical effect on the shop is constant: calibration, surveys, and records are not paperwork bolted on, they are how the process is proven. This is the same logic as automotive's IATF 16949 quality discipline, applied to temperature.
How do you run a heat-treating operation well?
The goal is repeatable properties on every part, proven with data, from furnaces that are demonstrably in control. Here is a practical operating sequence.
- Keep furnaces provably in control. Stay current on Temperature Uniformity Surveys and System Accuracy Tests so every load runs in a furnace known to hold temperature; a drifted furnace produces bad parts that look fine.
- Lock the recipe to the part. Tie each part number to its exact cycle, temperature, time, quench, atmosphere, and temper, so the right recipe runs every time and cannot be guessed from memory.
- Capture the temperature record with the load. Record the actual thermal profile for each load and tie it to the parts in it, so proof of correct treatment is automatic, not reconstructed.
- Verify results, not just the cycle. Confirm hardness and, where required, case depth on samples, and track results with statistical process control so drift shows up before it becomes scrap.
- Trace every load back to its parts and material. Maintain the link from load to part to material lot so a questioned part's full thermal history is quick to retrieve; see traceability in manufacturing.
- Attack furnace downtime deliberately. Furnaces are the constraint; an unplanned outage backs up every job. Track machine downtime so the real losses get fixed first.
None of this requires replacing the furnaces or the pyrometry equipment. It requires connecting them so recipes, thermal records, hardness results, and traceability live in one place instead of scattered across chart recorders and binders (how Harmony connects the floor). Lean thinking still applies, cut waste, standardize the work, inside the guardrails of AMS 2750 and CQI-9; see lean manufacturing. Parts often arrive from a foundry or machining and leave for finishing, carrying that record forward.
What do the standards and numbers say?
- AMS 2750 published by SAE International is the pyrometry standard governing temperature sensors, instrumentation, System Accuracy Tests, and Temperature Uniformity Surveys for aerospace thermal processing (SAE International).
- AMS 2750 classifies furnaces into classes 1–6 by required temperature uniformity, roughly ±3°C for the tightest class to ±28°C for the loosest, and instruments into types A–D by accuracy (SAE International).
- CQI-9 the Special Process: Heat Treat System Assessment is published by the Automotive Industry Action Group (AIAG) and is built on the same pyrometry principles as AMS 2750 (AIAG).
- For aerospace, Nadcap accreditation of heat-treating special processes is administered by the Performance Review Institute (PRI) (PRI Nadcap).
Where does connecting the floor fit in heat treating?
Right at the seam between the furnace and the mountain of required proof. Heat-treat shops rarely lack furnace capacity or metallurgical skill; they lose time proving what they did, assembling thermal records, tying loads to parts, and preparing for Nadcap or CQI-9 audits from chart recorders and binders. Connecting recipes, thermal profiles, hardness results, and traceability so they are captured as the load runs turns that proof from an after-the-fact scramble into a byproduct of doing the job.
That is the honest value: not replacing pyrometry discipline or metallurgical judgment, but making recipe control, thermal records, results, and traceability one connected record instead of scattered charts. It is the same pattern behind any real-time operational platform, connect what exists, capture at the source, and make the record instantly available, as one manufacturer did when it replaced paper logging with real-time capture (the CLS case study). For the systems view, see what is a manufacturing operating system.