Bar at strike point 0 °CColour --Length 0 mmBlows 0FPS 0

Drag to orbit, wheel or pinch to zoom. Tap or click the glowing bar to strike it there with the hammer. The bar's glow follows its real temperature; the hammer face (flat or cross-peen) is chosen in the panel.

Hot Forging: Hammer Blow Energy, Flow Stress and Drawing Out a Bar

One bar of mild steel, one hammer, one anvil. Heat the bar, strike it, and watch how the blow energy, the glowing temperature and the face of the hammer decide how far the steel moves. Reheat before it cools below about 800 degrees C.

Hammer

Hammer face

Bar and fire

Live readouts

Bar temperature through time

Thickness and width along the bar

Flow stress against temperature

Experiment: how many blows to halve the thickness?

Strikes a fresh bar at the strike position, one blow every 1.2 s, until the thickness there is halved. Run once with the real cooling and once with the bar held at its starting temperature, for several starting temperatures. The gap between the two columns is what cooling costs you.

Independent numerical checks

Controls

Strike hits the bar at the chosen position; clicking the bar does the same at the clicked point. Turn bar 90° swaps thickness and width, as a smith turns the work to keep it square.
Put in fire heats the bar in the coal bed (fire temperature slider); take it out again before it burns. Time scale speeds up heating and cooling only; the hammer swing keeps its real speed.
Auto-draw runs a pass of blows from the far end back towards the tongs, turning the bar after every pass.
Keys: Space strikes, T turns, F toggles fire. They only act while this page has focus.

The science

Blow energy. The hammer head carries KE = ½ m v². A fraction η (default 0.5, illustrative) ends up as plastic work in the steel; the rest is rebound and elastic energy in hammer, bar and anvil. With contact area A = b w (bar width b, face length w along the bar) the thickness drop follows from η KE = p A Δh.

Flow stress. Hot steel yields at σ_f = σ₀ (ε̇/10)^m exp[B (1/T − 1/T₀)], T in kelvin, strain rate ε̇ ≈ v/h. Constants: σ₀ = 70 MPa at 1100 °C and 10 s^-1, m = 0.15, B = 5156 K (so 800 °C needs 2.9 times the stress). These are fitted to the ranges typical of hot-working handbooks, not measured on one alloy: spot-check before quoting. Below about 650 °C the steel work-hardens and cracks, shown here as a damage index.

Friction. Friction at the face raises the average pressure (Siebel): p = σ_f (1 + μ w / 3h). A wide flat face on a thin bar needs more energy than a narrow peen, which is why smiths draw out with the peen.

Where the metal goes. Volume is conserved: ε_h + ε_b + ε_L = 0 (true strains). The share spread sideways is modelled as ψ = w/(w + b): a narrow face pushes the steel lengthwise, a face as wide as the bar spreads it evenly. This partition is a simple rule of thumb, not a finite-element result.

Cooling and heating. Each of 40 slices is a lumped body: ρ c V dT/dt = −A[εσ(T&sup4; − T_a&sup4;) + h(T − T_a)] − contact + axial conduction, ε = 0.8, h = 10 W m^-2 K^-1, anvil contact 150 W m^-2 K^-1 (effective, partial contact), steel ρ = 7850 kg m^-3, c = 620 J kg^-1 K^-1, k = 40 W m^-1 K^-1. In the fire the same slices gain heat from coals at the chosen temperature (view factor 0.6, h = 25). Lumping is valid when the Biot number is below about 0.1 (checked below).

Colour. The glow colour is an approximate black-body ramp: first dull red near 525 °C (the Draper point), cherry near 750, orange near 950, yellow near 1100, white above 1300. Real smiths judge colour by eye in changing light, so treat names as guides.

What it leaves out. No grain growth or scale loss, no heat from the plastic work (about 0.1 K per blow here), no anvil temperature rise, no tongs conduction, no elastic recovery, no rounding of the bar corners, no sideways bulging inside a slice, a uniform temperature through the bar thickness. It teaches the energy budget and the heat window, not a process specification. Assets built with Blender; scene by Three.js.