Tutorial: Designing a Heat Treatment Cycle Using TTT and Jominy Data

This tutorial walks through the complete engineering process for designing a quench-and-temper heat treatment cycle, using AISI 4140 steel as a worked example throughout. You will learn how to read a TTT diagram to understand transformation kinetics, interpret a Jominy hardenability band to predict through-thickness hardness, select an appropriate quench medium and austenitising cycle, and choose a tempering temperature to achieve a specified combination of hardness and toughness. The methodology applies to any hardenable low-alloy steel; only the specific numbers change.

Understanding the TTT and CCT Diagrams

Before designing any heat treatment cycle, you must understand how the steel will transform during cooling. Two complementary diagrams describe this behaviour: the TTT (Time-Temperature-Transformation) diagram and the CCT (Continuous Cooling Transformation) diagram.

The TTT Diagram

A TTT diagram is constructed under fully isothermal conditions. Test specimens are austenitised, then rapidly quenched to a series of temperatures below A1 and held isothermally while the progress of transformation is monitored (by metallographic sectioning, dilatometry, or magnetic measurement) as a function of time. The result is a set of C-shaped (sigmoidal) curves on a temperature-log time plot, defining the start and finish of each transformation product: ferrite, pearlite, bainite, and (on cooling below Ms) martensite.

The minimum time for transformation to begin at any temperature is the nose of the C-curve — the most critical parameter for quench design. For AISI 4140 steel, the pearlite nose lies at approximately 660°C and ~2–3 seconds. The bainite bay spans roughly 250–530°C, with a nose at approximately 450°C and 30–60 seconds. A quench cooling curve that avoids crossing the TTT start lines before reaching Ms will produce a martensitic microstructure. See also the bainite microstructure guide for isothermal transformation detail.

The CCT Diagram and Its Relationship to TTT

In practice, heat treatment involves continuous cooling rather than isothermal holds. The CCT diagram maps transformation start and finish boundaries in the same temperature-log time space, but for constant cooling rate experiments rather than isothermal holds. The key differences from TTT:

• CCT transformation start boundaries are displaced to longer times and lower temperatures compared to the corresponding TTT diagram — by a factor of roughly 1.5–2 on the time axis, depending on the steel and temperature range.
• Isothermal bainite bay features are often less pronounced on the CCT diagram because continuous cooling tends to pass through the bay quickly.
• Specific critical cooling rates for 100% martensite (upper critical cooling rate) and for first transformation start (lower critical cooling rate) can be read directly from the CCT diagram.

For 4140, the critical cooling rate to suppress all pearlite/bainite and obtain fully martensitic microstructure is approximately 8–15°C/s depending on the specific heat composition within the grade band. Both agitated oil (surface cooling rate ~15–25°C/s) and water (surface rate ~200–300°C/s) exceed this requirement — but water is ruled out by quench cracking risk, as discussed later.

1 Define the Property Specification

Heat treatment design begins with the engineering requirement — the target properties the component must achieve. For this worked example, assume the following specification for a 50 mm diameter 4140 steel shaft:

From the target hardness range of 35–42 HRC at surface and 28 HRC minimum at core, you can read the required Jominy positions from the 4140 hardenability band (see Section 4). This converts the engineering requirement into a hardenability requirement.

2 Determine Critical Temperatures (Ac1, Ac3)

The austenitising temperature must be above Ac3 (the temperature at which the last ferrite dissolves on heating for hypoeutectoid steels) to ensure fully austenitic starting microstructure. Below Ac3, undissolved ferrite would remain, and the subsequent quench would produce a mixed martensite-ferrite microstructure with insufficient hardness.

Empirical Formulas for Ac1 and Ac3

For low-alloy steels, Ac1 and Ac3 are estimated from Andrews’ empirical equations (1965), validated against a large database of compositions:

Ac1 (°C) = 723 − 10.7×%Mn − 16.9×%Ni + 29.1×%Si + 16.9×%Cr + 290×%As + 6.38×%W

Ac3 (°C) = 910 − 203×√%C − 15.2×%Ni + 44.7×%Si + 104×%V
             + 31.5×%Mo + 13.1×%W − 30×%Mn − 11×%Cr − 20×%Cu

For AISI 4140 (0.40C, 0.85Mn, 0.25Si, 1.00Cr, 0.20Mo):
  Ac1 ≈ 723 − 10.7×0.85 + 29.1×0.25 + 16.9×1.00
       ≈ 723 − 9.1 + 7.3 + 16.9
       ≈ 738°C

  Ac3 ≈ 910 − 203×√0.40 − 30×0.85 + 44.7×0.25 + 31.5×0.20 − 11×1.00
       ≈ 910 − 128.4 − 25.5 + 11.2 + 6.3 − 11.0
       ≈ 762°C

These are heating-rate dependent — at industrial heating rates of 5–15°C/min, transformation occurs approximately 20–30°C above the equilibrium values. The iron-carbon phase diagram and the Fe-Cr-C ternary system confirm that Cr raises Ac1 and broadens the (α + γ) two-phase field, shifting Ac3 upward relative to binary Fe-C.

3 Select Austenitising Temperature and Soak Time

Austenitising Temperature

The standard austenitising range for 4140 is 845–870°C (Ac3 + 80–110°C above the calculated Ac3 of 762°C, accounting for heating rate effects). This provides:

• Complete dissolution of Cr₇C₃ and Mo₂C carbides, putting all carbon and alloying elements into solution in austenite
• A margin above Ac3 that ensures fully austenitic microstructure even given compositional gradients from segregation
• Moderate austenite grain size (ASTM grain size 6–8) — excessive grain growth above ~950°C would reduce toughness by increasing the martensite packet size

Soak Time Calculation

Rule of thumb soak time:  t = 1 h per 25 mm of section thickness (minimum)

For 50 mm diameter bar:
  Equivalent section = diameter / 2 = 25 mm (for round bar, use radius for soaking)
  → t = 1 h (surface to core equilibration)

  After furnace equilibration add:
  + 15 min for charge loading compensation
  → Total furnace time at 845°C: 1 h 15 min

Note: For alloy steels with stable carbides (e.g., tool steels with MC/M6C),
longer soak times (2–4 h) may be required to dissolve carbides fully.

The soak time must be sufficient for the following to occur: temperature equilibration from surface to core (thermal homogenisation), dissolution of alloy carbides into the austenite matrix, and homogenisation of carbon and alloying element concentration gradients (compositional diffusion). Insufficient soak time leaves undissolved carbides and composition gradients, producing lower hardness and non-uniform microstructure after quenching.

4 Interpret the Jominy Hardenability Band

The Jominy end-quench test (ASTM A255 / ISO 642) provides the hardenability data needed to predict what hardness your component will achieve after quenching. The test produces a curve of hardness versus distance from the quenched end (J-distance, measured in 1/16 inch increments in ASTM convention, or mm in ISO).

4140 Hardenability Band (ASTM A304)

Mapping Jominy Position to Component Cross-Section

Once you have the Jominy band, you need to map it onto your specific component geometry. Standard Jominy-to-component cooling rate correlation charts (published in ASM Handbook Vol. 4 and from Grossmann) relate the Jominy distance to the equivalent position in a round bar of given diameter, quenched in a medium of known severity (H-factor).

For a 50 mm (2 in) diameter bar, oil quench (H ≈ 0.35):
  Surface equivalent → J ≈ 4–5 (6.4–8 mm Jominy distance)
  3/4 radius equiv.  → J ≈ 8–10
  1/2 radius equiv.  → J ≈ 12–14
  Centre equivalent  → J ≈ 16–20

Reading from the 4140H band:
  Surface hardness: 52–57 HRC (as-quenched)  → meets specification
  Core hardness:    36–48 HRC (as-quenched)  → meets specification
  
After tempering at 550°C (see Step 6):
  Surface: ~35–40 HRC (tempered)             → within 35–42 HRC target
  Core:    ~28–33 HRC (tempered)             → meets ≥28 HRC requirement

The quenching and tempering guide provides detailed Grossmann correlation charts for round, square, and plate geometries in various quench media.

5 Select the Quench Medium

Quench medium selection balances two competing requirements: sufficient cooling severity to suppress pearlite/bainite transformation (achieving the required hardness) while minimising thermal gradients that cause distortion and quench cracking. For AISI 4140, this balance strongly favours oil over water.

Grossmann Quench Severity (H-factor)

Why Oil Quench for 4140?

AISI 4140 contains ~1% Cr and ~0.20% Mo which provide sufficient hardenability (DI ≈ 90–130 mm) that full martensite to centre is achievable in sections up to ~70 mm in agitated oil. Water quenching produces a much higher surface cooling rate but creates severe thermal gradients — the surface contracts rapidly while the core is still hot and austenitic, generating large tensile stresses on the surface as the core finally cools and transforms. These stresses frequently cause quench cracking, particularly at section changes, keyways, splines, and bore edges.

Alternative: Martempering for Complex Shapes

For components with significant section changes (gears, flanged shafts), martempering offers an alternative. The component is quenched into a salt bath or hot oil held just above Ms (~320°C for 4140), held until temperature equilibration occurs (without transformation, since 4140 bainite nose is well above Ms), then air-cooled to room temperature. The much smaller thermal gradient at the transformation temperature dramatically reduces distortion and quench cracking risk. See the quenching and tempering guide for process parameters.

6 Design the Tempering Cycle

As-quenched martensite in 4140 will be approximately 55–60 HRC — extremely hard and brittle, with residual tensile stresses from the quench. Tempering is mandatory. The tempering temperature controls the final hardness-toughness balance.

Tempering Stages in Carbon and Low-Alloy Steels

Tempering proceeds in distinct metallurgical stages as temperature increases:

Tempering Chart for AISI 4140

Secondary Hardening in Mo-Containing Steels

AISI 4140’s molybdenum content (0.15–0.25%) produces a secondary hardening response at approximately 500–550°C — a slight hardness plateau or even increase relative to the general softening trend, caused by the precipitation of fine Mo₂C carbides within the tempered martensite matrix. This is beneficial: it allows higher strength to be retained at the tempering temperatures needed for adequate toughness, and it explains why 4140 retains useful strength at 500°C, which is not the case for plain carbon steels of equivalent carbon content. Compare martensite formation and bainite microstructure for context on transformation products and their tempering response.

7 Complete Heat Treatment Cycle Specification

The full heat treatment process sheet for the 50 mm diameter 4140 shaft, ready for workshop implementation:

Calculating Ideal Critical Diameter (DI) from Composition

The ideal critical diameter DI is a single-number summary of a steel’s hardenability. It allows rapid comparison of different grades and heats, and feeds directly into Grossmann’s correlation for predicting the actual critical diameter in a real quench medium. The Grossmann multiplying factor method computes DI as:

DI (inch) = f(C) × f(Mn) × f(Si) × f(Cr) × f(Mo) × f(Ni) × f(Cu) × f(B)

where f(C) is base DI from carbon content alone (grain size dependent),
and f(X) are Grossmann multiplying factors for each element.

For ASTM grain size 7, base DI from carbon:
  C = 0.40% → f(C) = 0.204 in (5.18 mm)
  Mn = 0.85% → f(Mn) = 3.42
  Si = 0.25% → f(Si) = 1.18
  Cr = 1.00% → f(Cr) = 3.50
  Mo = 0.20% → f(Mo) = 1.75
  Ni = 0.00% → f(Ni) = 1.00

DI = 0.204 × 3.42 × 1.18 × 3.50 × 1.75 × 1.00
   = 0.204 × 3.42 × 1.18 × 6.125
   = 5.04 inch ≈ 128 mm

Cross-check: typical published DI for 4140 = 90–130 mm (good agreement)

Note that the Grossmann multiplying factors are available in tabular form in ASM Handbook Vol. 4 and in Bhadeshia & Honeycombe Chapter 13. Composition within a grade specification significantly affects DI — a 4140 heat at the lower end of composition ranges will have DI ≈ 90 mm, while a heat at the upper end may reach 140 mm. For critical applications, use the actual certified heat analysis to calculate DI rather than nominal composition.

Quality Verification and Applicable Standards

Every heat treatment cycle for structural components must be verified against the specified mechanical properties. The following inspection steps are standard for 4140 quench-and-temper:

• Hardness testing: Rockwell HRC at three surface locations minimum. Core hardness by sectioning a representative piece from the batch (destruc tive, per heat or per lot). ASTM E18 governs Rockwell testing procedure.
• Microstructural verification: Metallographic section, Nital (2%) etch. Confirm tempered martensite morphology, absence of free ferrite, grain size, and decarburisation depth (<0.05 mm for most shaft applications). See the martensite formation guide for microstructural identification.
• Tensile testing: Per ASTM A370, test to confirm YS, UTS, and elongation within specification. Typically one tensile specimen per heat per section size range.
• Impact testing: Charpy V-notch per ASTM E23, at specified temperature. For the shaft application above: minimum 60 J at room temperature, longitudinal.
• Applicable product specifications: ASTM A322 (bar stock chemistry), ASTM A434 (quenched and tempered mechanical property requirements by grade), SAE J404 (chemistry), SAE J1268 (hardenability requirements for 4140H).

Key References

• Bhadeshia, H.K.D.H. and Honeycombe, R., Steels: Microstructure and Properties, 4th ed. Butterworth-Heinemann, 2017. (Chapters 3, 5, 12, 13)
• ASM Handbook Vol. 4A — Steel Heat Treating Fundamentals and Processes. ASM International, 2013.
• Grossmann, M.A. and Bain, E.C., Principles of Heat Treatment, 5th ed. ASM International, 1964.
• ASTM A255 — Standard Test Methods for Determining Hardenability of Steel.
• ASTM A304 — Standard Specification for Carbon and Alloy Steel Bars Subject to End-Quench Hardenability Requirements.
• ASTM A434 — Standard Specification for Steel Bars, Alloy, Hot-Wrought or Cold-Finished, Quenched and Tempered.

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