Heat Treatment Services

A forging leaves the press with a structure shaped by how it cooled rather than by what the design needs. Thin sections cool faster than thick ones, so a single part can contain different grain sizes and hardnesses. It may also carry residual stresses, and it may be too hard to machine easily or too soft to carry its design load.

Heat treatment replaces that accidental structure with a deliberate one. By heating steel to controlled temperatures and cooling it at controlled rates, its microstructure — and therefore its hardness, strength, toughness and machinability — can be set within a specified range. See does forging need heat treatment.

What changes inside the steel

Above its critical temperature, steel's structure transforms to austenite. How it cools from there decides what it becomes: slow cooling gives soft ferrite and pearlite; fast cooling (quenching) traps carbon in hard, brittle martensite; controlled reheating (tempering) then trades some of that hardness for toughness. Nearly every heat treatment cycle is a variation on this sequence.

For the difference between the two most commonly confused processes, see normalising versus annealing.

The table gives commonly used temperature ranges for grades we forge. Actual cycles depend on section size, the specified hardness band, the furnace and the governing specification, so these are guidance rather than a procedure.

Temperatures to avoid when tempering

Quenched and tempered alloy steels can lose impact toughness if tempered in the range of roughly 250–400°C, a phenomenon known as tempered martensite embrittlement. Tempering for structural components is therefore normally carried out above this range. Martensitic stainless SS 410 has its own reduced-toughness range, roughly 400–580°C, which is avoided where impact toughness matters.

Stress relieving below the tempering temperature

Stress relieving is carried out below the part's previous tempering temperature — commonly in the region of 550–650°C for quenched and tempered alloy steels, with a margin below the original tempering temperature — so that the hardness achieved by quenching and tempering is not lowered.

Quenching and tempering is the process that gives loaded forgings — crankshafts, axle shafts, steering knuckles, high-tensile bolts — their combination of strength and toughness.

Choosing the quench medium

A steel with high hardenability does not need a severe quench, and giving it one only increases distortion and the risk of quench cracking. Alloy steels are therefore normally oil or polymer quenched. Plain carbon steels need faster cooling to harden, especially in larger sections — and even then, their limited hardenability means thick sections will not harden through.

Section size limits what any cycle can achieve

Heat treatment cannot overcome a grade's hardenability. A thick section of plain carbon steel will have a softer core however carefully it is quenched. Matching grade to section size is therefore part of heat treatment planning, not just material selection. See EN8 versus EN19.

Setting the hardness band

Tempering temperature controls final hardness: the higher the tempering temperature, the lower the hardness and the higher the toughness. For each grade, section size and required band, the tempering temperature is chosen to land inside the specification, and results are confirmed by hardness testing on every batch.

Many forged components need a hard, wear-resistant surface over a tough core. Two families of process achieve this.

Induction hardening heats only where it is needed, with low distortion and short cycle time. Carburising hardens the whole contour of a gear tooth uniformly but takes hours and causes more distortion. See induction hardening and carburising versus induction hardening. Carburising for customer parts is arranged through qualified processors, with records supplied.

Heat treatment is a process whose result is invisible. A part that has been correctly hardened and one that has not look identical. Control and verification are therefore the whole discipline.

Process control

• Recorded furnace charts for every charge, showing temperature against time
• Charge identification linking each chart to the batch and heat number
• Loading practice that allows uniform heating and quenching, and supports long parts to limit distortion
• Quench medium condition monitored, because degraded quenchants change cooling rates
• Prompt tempering after hardening, reducing the risk of cracking in parts left standing

Verification

Hardness readings can be converted approximately between scales with our hardness conversion calculator, but a part specified on one scale should be tested on that scale.

Reference standards for heat treating systems

Automotive heat treatment is commonly assessed against AIAG CQI-9, and temperature measurement practice in demanding sectors is often based on AMS 2750 pyrometry requirements. These frameworks set expectations for furnace temperature uniformity surveys, thermocouple calibration and record keeping.

See forging distortion problems, decarburisation and why forged parts crack.

1. 01Name the grade and standard, for example 42CrMo4 to EN 10083-3.
2. 02State the condition, such as quenched and tempered, normalised or annealed.
3. 03Give a hardness or strength band, not a single value, on a stated scale.
4. 04Define where hardness is measured, or require a survey across heavy sections.
5. 05For surface hardening, state surface hardness, effective case depth and the hardness at which case depth is measured, and the zones to be hardened.
6. 06State mechanical tests required, such as tensile or Charpy impact, and at what temperature.
7. 07Specify documentation, such as furnace charts and material certificates to EN 10204 3.1.

A drawing that says only "hardened" leaves the most important properties to whoever does the work. Our forging drawing checklist covers the rest of a complete specification.

A heat treatment cycle is not only a pair of temperatures. The time at temperature, the way parts are loaded and the cooling that follows all depend on how thick the parts are.

Soak time

A part must be held at temperature long enough for its core to reach temperature and transform. Thick sections take longer. A widely used rule of thumb for through-heating steel is roughly one hour per 25 mm of section thickness, but actual soak times depend on the furnace, load size, part geometry and specification, and are set in the process instructions rather than by rule of thumb alone.

Cooling rate at the core

During quenching, the surface of a thick part cools much faster than its core. That is what limits core hardness, and why the grade's hardenability must match the section. It is also why hardness surveys across heavy sections are more informative than a single surface reading.

Mixed sections in one part

A part with thin and thick sections cools unevenly during quenching, generating internal stress that drives distortion and, in extreme cases, cracking. Designs with uniform sections and generous transitions heat treat more predictably. See forging distortion problems.

Furnace loading

How parts are stacked, spaced and supported matters. Parts packed too tightly heat unevenly and shield each other from the quenchant; long shafts laid unsupported can sag at temperature. Loading practice is part of the controlled process, not an afterthought.

The right heat treatment follows from what the part must do. The table maps common requirements to the treatments that usually meet them.

Because heat treatment cannot be seen on the finished part, its records are the evidence. For parts where properties matter, these are reasonable to request:

• Furnace chart showing temperature against time for the charge, identified to the batch
• Process parameters — austenitising temperature and time, quench medium, tempering temperature and time
• Hardness results with test locations and scale
• Case depth results for induction hardened, carburised or nitrided parts, with the hardness at which effective case depth was measured
• Mechanical test results where tensile or impact testing is specified
• Traceability linking the charge to the heat number on the material certificate

A supplier who cannot produce the chart for a specific batch is relying on habit rather than control. See forging quality documents.

Heat treatment is carried out on forgings made at our plant, integrated with forging and machining so a hardness or distortion question is answered from one set of records. Related services: metallurgical testing, closed die forging and precision CNC machining. Request a quote.