Heat Treatment Simulator
Anneal, quench, temper, case harden & age — watch the atoms, the microstructure and the hardness change, in 2D & 3D
Display Controls
Inside the steel
Diagrams
1 What is happening now
2 Recipe
3 Result
Heat-treatment problems
Every value you need is printed in the question. Answers within the stated tolerance are marked correct.
1 Overview
The Heat Treatment Simulator takes one part — a round bar of steel, or of 6061 aluminium — through a real heat-treatment shop and shows what happens to it at every level at once: the part moving between the stations, the thermocouple trace of its surface and core, the atoms, the microstructure under the microscope, the diagrams (Fe–Fe₃C, TTT, CCT, Jominy, tempering, ageing) and, at the end, the hardness and strength of the finished part.
There are twelve processes: harden & temper, austemper, martemper, full anneal, normalise, spheroidise anneal, stress relief, carburise & harden, nitride, induction harden, flame harden and age harden (Al 6061). There are six steels (AISI 1020, 1045, 1080, 4140, 4340 and 8620) and the page has four modes: Simulate, Explore, Practice and Quiz.
2 Getting started
The page opens running Harden & temper on a 100 mm bar of 4140. Press Run to watch it again, drag the timeline to any moment, or choose the material and the process in the two boxes above the canvas. Under the canvas, 1 · What is happening now explains, as the run plays, what the shop is doing and what is happening to the crystal structure (BCC, FCC, BCT, which phase is forming and where), with the steel and the bar size set just below it; 2 · Recipe lists the settings stage by stage in the order the shop runs them (each stage in the colour it has on the timeline; the temper can be switched off), and 3 · Result shows what you made. Any change recalculates the whole treatment at once.
3 The shop (2D and 3D)
In 2D the camera follows the part as the hoist carries it from station to station; the shop map at the top shows where it is. The part glows at the colour hot steel really has at its surface temperature, the optional cut-away (Display Controls) shows the temperature inside it, and the panel at the right is the data logger with the surface and core thermocouple traces. In a quench you see the vapour blanket, then boiling, then plain convection, as the surface cools.
In 3D you can walk round the same shop: drag to turn, right-drag or shift-drag to pan, pinch or Ctrl + scroll to zoom, click a station to read about it. Follow bar rides with the part, Whole shop frames every station, and Cut-away (on at first) slices the furnaces and tanks open so you can see the part inside; the part itself is never cut. Each material has its own colour on the bar, so a change of steel shows at a glance (a code for the lesson: real steels look alike), and choosing a new material starts the run again from the beginning. On a computer or tablet the page opens in 3D, on a phone in 2D; your choice is remembered, and a link with ?view=2d or ?view=3d opens that view.
4 Inside the steel: atoms and microstructure
On a wide screen these sit in the column beside the shop; the Inside the steel | Diagrams switch at the top of that column chooses which one fills it, and Diagrams lists the numbers behind the chosen chart under the graph.
Choose Surface, Half radius or Core. The first circle (left, or top in the desktop column) shows the crystal lattice at that point: body-centred cubic ferrite, face-centred cubic austenite with carbon atoms in its octahedral holes, or body-centred tetragonal martensite with the carbon trapped and the cell stretched (c/a = 1 + 0.045 × %C). As the part heats past Ac1 you see the cell rebuild itself; in a quench you see the diffusionless shear. The second circle is the micrograph at 500×: austenite grains at their real size, ferrite growing at the grain boundaries, pearlite colonies with their lamellae, bainite sheaves, martensite needles, retained austenite, spheroidised carbide and, for aluminium, the precipitates. The bar under them gives the phase fractions.
5 The diagrams
Trace: surface and core temperature against time. Fe–C: the iron–cementite diagram with the alloy as a vertical line, its present temperature as a dot and the lever rule written out. TTT: isothermal start and finish curves of ferrite, pearlite and bainite, with Ms and M90. CCT: the continuous-cooling curves with the part's surface and core cooling curves drawn over them. Profile: hardness from the surface to the centre (and carbon or nitrogen for case hardening). Jominy: the end-quench curve of the steel. Tempering: hardness against tempering temperature for 1 h. Ageing: 6061 strength against ageing time at three temperatures.
6 The recipe settings
Every process shows only the settings it uses. The austenitising temperature starts at the textbook value for the steel: 30–50 °C above Ac3 to harden, 15–40 °C above it to anneal and about 55 °C above it to normalise. The soak is timed from the moment the core reaches the furnace temperature. Quenchant and agitation set Grossmann's quench severity H. A warning appears when a setting will not do what the process needs — for example a hardening temperature below Ac1, an austempering bath below Ms, or a 6061 solution temperature above its 582 °C solidus.
7 Results and properties
The result tiles give the surface and core hardness (HV, with HRC and HB from ASTM E140), the yield and tensile strength (Pavlina & Van Tyne 2008), the phase fractions, the austenite grain size, and for case hardening the effective case depth (to 550 HV after carburising, to core + 50 HV after nitriding, to 80 % of the surface hardness after induction or flame hardening). The ductility and toughness bars are indicative only: they show which way a change moves the part, not a value.
8 Explore
Explore has six topics: Basics, Diagrams, Processes, Properties, Surface hardening and Aluminium. Each card explains one idea with a worked example that uses the same equations as the lab.
9 Practice and Quiz
Practice generates problems on the lever rule, Ms, martensite fraction, tempering equivalence, carburising depth, skin depth, grain size and strength from hardness; every number you need is printed in the question. Quiz asks five multiple-choice questions drawn at random from a pool, with the options shuffled, and ends with a star rating.
10 SI and Imperial
The SI / Imperial switch, next to Restart under the canvas, shows temperatures in °C or °F, lengths in mm or inches, strengths in MPa or ksi and cooling rates in °C/s or °F/s. The calculation itself is the same. Your choice is remembered across the site.
11 What the model does not do
The kinetics are the Li et al. (1998) fit for hypoeutectoid low-alloy steels, so the lab stops at 0.9 % C. Phase boundaries of the binary Fe–C diagram are drawn straight between the published invariant points. The latent heat of the transformations, quench cracking, distortion, decarburisation, secondary hardening and the quench sensitivity of 6061 are not modelled. Polymer and molten-salt quench severities, nitrided-case hardness, spheroidising softening and the ductility and toughness bars are indicative.
12 Tips
- Quench a 25 mm bar of 1045 in water and of 4340 in oil, then make both 80 mm: the Profile and CCT tabs show hardenability at work.
- Temper the same quenched bar at 200, 400 and 600 °C and watch the hardness fall while the toughness bar rises — except around 250–375 °C (tempered-martensite embrittlement).
- Raise the austenitising temperature by 150 °C and look at the grain size and the Jominy curve.
Heat Treatment of Steel: How Heating and Cooling Change the Metal
Heat treatment changes the properties of a metal without changing its shape or its composition. The same bar of 4140 steel can be soft enough to machine, tough enough for a crankshaft or hard enough to wear like a tool, depending only on how it was heated and how fast it was cooled. This virtual lab carries a part through a real heat-treatment shop and shows, at the same moment, the temperature inside the part, the atoms, the microstructure and the hardness that result.
What happens inside steel when it is heated
At room temperature iron is ferrite, a body-centred cubic lattice (a = 0.2866 nm) that can dissolve only 0.022 % carbon. Above the lower critical temperature, 727 °C in plain Fe–C, the iron rebuilds itself as austenite, face-centred cubic, whose octahedral holes can hold up to 2.14 % carbon. The carbon from the cementite (Fe₃C, 6.70 % C) dissolves into it. Everything heat treatment does to steel comes from what that carbon is allowed to do on the way back down.
The iron–carbon diagram and the lever rule
The Fe–Fe₃C diagram maps the equilibrium phases. Its eutectoid point is 0.76 % C at 727 °C, where austenite turns into pearlite: lamellae of ferrite and cementite in the proportion (6.70 − 0.76)/(6.70 − 0.022) = 89 % ferrite. A hypoeutectoid steel of 0.40 % C first grows proeutectoid ferrite between A3 and A1; just below 727 °C the lever rule gives (0.76 − 0.40)/(0.76 − 0.022) = 48.8 % proeutectoid ferrite and 51.2 % pearlite. The lab draws the alloy's vertical line, its present temperature and the tie line, and writes the lever rule out as the part cools.
Cooling rate decides the microstructure: TTT and CCT
Slow cooling gives the diagram's equilibrium structure. Faster cooling undercools the austenite, and what forms depends on how long it stays at each temperature. A TTT diagram gives the start and finish times at a constant temperature; a CCT diagram gives them for continuous cooling. The lab computes both from the steel's composition with the kinetic model of Li, Niebuhr, Meekisho and Atteridge (1998) and the rule of additivity, so molybdenum, chromium, nickel and manganese visibly push the curves to the right. Below the martensite-start temperature (for 4140, Andrews' formula gives 326 °C) the austenite left shears into martensite following Koistinen–Marburger: 90 % has formed 209 °C below Ms.
Hardening and tempering
Quenched martensite is hard (its hardness rises with its carbon) but brittle. Tempering reheats it below Ac1 so carbon precipitates as fine carbides. Time and temperature trade against each other through the Hollomon–Jaffe parameter P = T(C + log t), with T in kelvin, t in hours and C = 21.3 − 5.8 × %C: one hour at 550 °C does the same to a 0.40 % C steel as about 5 minutes at 600 °C. Tempering between about 250 and 375 °C is avoided in practice because toughness drops there (tempered-martensite embrittlement).
Hardenability, Jominy and section size
A bar cools fastest at its surface, so a big plain-carbon bar may be martensite only in a thin skin. Hardenability is how slowly a steel can cool and still form martensite; the Jominy end-quench test measures it by spraying one end of a 25.4 mm bar and reading the hardness along it. Grossmann's quench severity H (still oil 0.25–0.30, still water 0.9–1.0, still brine 2.0 per inch) sets how hard the quench pulls heat out. In the lab, quench 50 mm bars of 1045, 4140 and 4340 in oil and compare the core.
| Process | Heat to | Cool | Result |
|---|---|---|---|
| Full anneal | 15–40 °C above Ac3 | in the furnace | coarse pearlite + ferrite, softest |
| Normalise | about 55 °C above the upper critical | still air | fine pearlite, a little stronger |
| Harden | 30–50 °C above Ac3 | water, brine, oil or polymer | martensite, hard and brittle |
| Temper | 150–700 °C, below Ac1 | air | tempered martensite, tougher |
| Austemper | above Ac3, then a salt bath above Ms | hold, then air | bainite |
| Martemper | above Ac3, then a salt bath just above Ms | equalise, then air | martensite with less distortion |
| Carburise | about 925 °C in a carbon-rich gas | quench, low temper | hard high-carbon case on a tough core |
| Nitride | about 520 °C in ammonia | slowly, no quench | hard nitride case, very little distortion |
| Age harden 6061 | 530 °C solution treatment | water quench, age 175 °C × 8 h | T6: 276 MPa yield |
Surface hardening: carburising, nitriding, induction and flame
Carburising diffuses carbon into a low-carbon steel at about 925 °C. The diffusion coefficient of carbon in austenite is D = 2.3 × 10⁻⁵ exp(−148 000/RT) m²/s, the profile follows the error function, and the case grows with the square root of time. Nitriding diffuses nitrogen into ferrite at about 520 °C, below Ac1, so nothing has to be quenched. Induction hardening heats only a skin: the current crowds into a depth δ = 503√(ρ/(μrf)), about 5.5 mm at 10 kHz in steel above its 770 °C Curie point, before a spray quench turns that skin to martensite. Flame hardening does the same with a gas flame.
Precipitation hardening of aluminium
Aluminium alloys such as 6061 are not hardened by martensite. They are solution treated at 530 °C to dissolve magnesium and silicon, quenched to trap them, and aged so they precipitate as fine Mg–Si clusters and needles that block dislocations. 6061 goes from 55 MPa yield annealed (O) and 145 MPa naturally aged (T4) to 276 MPa peak aged (T6) — and over-ageing coarsens the precipitates and loses it again. The lab ages the part at your chosen temperature and draws the curve.
What this heat treatment simulator models
This is a free heat treatment simulator that runs in the browser with no sign-up. Each part of the process is computed from a published model, not drawn by hand:
| Part of the process | What the simulator computes | Model or source |
|---|---|---|
| Heating and quenching the bar | Temperature at every radius, at every station | Implicit radial heat conduction; steel k and cp from EN 1993-1-2; quench severity from Grossmann H |
| Critical temperatures | Ac1, Ac3, Ms and Bs of each steel | Andrews (1965); Li et al. (1998) for Bs |
| Transformations on cooling | Ferrite, pearlite, bainite and martensite fractions | Li et al. (1998) kinetics with additivity; Koistinen–Marburger for martensite |
| Austenite grain growth | Grain size and ASTM grain number | Sellars–Whiteman; ASTM E112 |
| Hardness and strength | HV, HRC or HB, tensile and yield strength | Maynier et al.; ASTM E140; Pavlina & Van Tyne (2008) |
| Tempering | Softening with time and temperature | Hollomon–Jaffe parameter |
| Carburising and nitriding | Carbon and nitrogen profiles, case depth | Fick’s second law; carbon in austenite from Callister, nitrogen in ferrite from da Silva & McLellan (1976) |
| Induction hardening | Heated skin depth | δ = 503√(ρ/(μrf)) |
| Ageing 6061 aluminium | Yield and tensile strength against ageing time | Shercliff–Ashby form, calibrated to ASM O, T4 and T6 |
| Jominy end-quench | Hardness along the bar | The same model on a 1D bar under a water jet (H = 4 in⁻¹) |
Stated limits: the Fe–Fe₃C lines are straight between the published invariant points, the micrograph is drawn from the computed phase fractions rather than simulated grain by grain, and distortion and quench cracking are flagged as a risk, not computed.
Frequently Asked Questions
What is the difference between annealing, normalising, hardening and tempering?
All four start by heating steel, but they cool it differently. Annealing heats about 15 to 40 °C above the upper critical temperature and cools slowly in the furnace, giving soft coarse pearlite. Normalising heats about 55 °C above the upper critical temperature and cools in still air, giving finer pearlite that is a little harder and stronger. Hardening quenches from the austenite range in water, oil or polymer so the austenite becomes hard, brittle martensite. Tempering then reheats the martensite below the lower critical temperature to trade some hardness for toughness.
Why does quenching make steel hard?
Above the critical temperature steel is face-centred cubic austenite, which can dissolve up to 2.14 % carbon. Cooled slowly, the carbon has time to diffuse out and the iron becomes soft body-centred cubic ferrite plus cementite. Quenched fast, there is no time for diffusion: the iron lattice shears into body-centred tetragonal martensite with the carbon trapped inside, stretching the cell (c/a about 1 + 0.045 × %C). The trapped carbon and the strained lattice block dislocations, so martensite is very hard.
What is the difference between a TTT and a CCT diagram?
A TTT (time–temperature–transformation) diagram shows how long austenite takes to transform when it is held at one constant temperature. A CCT (continuous cooling transformation) diagram shows where the transformations start and finish when the steel is cooled continuously, which is what happens in a real quench. CCT curves lie lower and to the right of the TTT curves because the steel spends only part of its time at each temperature.
What does the Jominy end-quench test measure?
The Jominy test measures hardenability: how deep a steel can be hardened. A 25.4 mm diameter, 102 mm long bar is austenitised and one end is sprayed with water. The cooling rate falls along the bar, so the hardness measured every 1.5 mm from the quenched end shows how slowly the steel can cool and still form martensite. Alloy steels such as 4140 and 4340 stay hard much further along the bar than plain carbon steels such as 1045.
How does carburising harden only the surface?
A low-carbon steel such as 8620 is held at about 925 °C in a gas with a high carbon potential. Carbon diffuses into the austenite following Fick's second law, so the profile follows an error function and the case grows with the square root of time. When the part is quenched, the high-carbon case becomes hard martensite while the low-carbon core stays tough. The effective case depth is usually taken as the depth to 550 HV.
Who uses this simulator?
Students of mechanical engineering, materials science and manufacturing technology meeting the Fe–C diagram, TTT and CCT diagrams for the first time; technical and vocational trainees learning heat-treatment practice; teachers who need to show what happens inside the steel, not just the hardness that results; and engineers checking how section size, quenchant and steel grade change the outcome of a hardening job.
Explore Related Simulators
Measure the result in the Hardness Testing Simulator and pull a tensile specimen in the UTM Virtual Lab or the Stress-Strain Curve simulator. Build the BCC and FCC lattices yourself in Build Your Crystal, see why a quenched part fails by cracking in the Fatigue Testing simulator, and follow the heat out of the bar in Heat Transfer Modes.
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