Tool Steel Size Change in Heat Treatment
Hardened steel comes out of the quench larger than it went in, and how much it moves depends on the family the grade belongs to. The figures below give the expected growth for water-, oil- and air-hardening tool steels, so a die can be left with enough stock for the finish cut after hardening instead of being scrapped for it.
| Family | Typical grades | Expected growth, in. per in. | On a 12 in. length | On a 300 mm length |
|---|---|---|---|---|
| Water hardening | W1, W2 | +0.002 to +0.004 | +0.024 to +0.048 in. | +0.6 to +1.2 mm |
| Oil hardening | O1, O2, O6 | +0.0015 | +0.018 in. | +0.45 mm |
| endurecimiento por aire | A2, A6, S7, H13 | +0.001 | +0.012 in. | +0.30 mm |
| endurecimiento por aire | D2 | ±0.0005 | ±0.006 in. | ±0.15 mm |
Source: Tool and Die Making Troubleshooter (R. M. Leed), Table 8-9, from Bethlehem Steel Corporation distortion data. The 12 in. and 300 mm columns are calculated from the rate in the third column.
| Reaction | Volume change, % | Dimensional change, in. per in. |
|---|---|---|
| Spheroidite to austenite, during heating | -4.64 + 2.21 (%C) | -0.0155 + 0.0074 (%C) |
| Austenite to martensite, during the quench | 4.64 – 0.53 (%C) | 0.0155 – 0.0018 (%C) |
| Spheroidite to martensite, net change after hardening | 1.68 (%C) | 0.0056 (%C) |
Source: ASM Handbook, Volume 11, Failures Related to Heat Treating Operations, Table 2, size changes during hardening of carbon tool steels. The dimensional column is one third of the volume change, the strain relationship given as Eq 2 in the same chapter.
- C is the carbon dissolved in the austenite, not the carbon in the analysis. In a high-chromium grade such as D2 much of the carbon sits in alloy carbides and never enters solution, so the net change for that grade is far smaller than the formula alone suggests. Use the formula to see the direction of each step and the growth table above for the allowance.
- Spheroidite is the microstructure of annealed tool steel as it is delivered.
Reading the table
Growth in at least one dimension is normal. A part that shows none has not transformed completely, and the missing volume will appear later, often after a grinding operation or in service. Steels that hold a lot of retained austenite move the most, which is one reason the air-hardening grades are tempered two and three times.
Geometry decides which dimensions move. A long, thin rectangular part that hardens right through grows in width, thickness and length. A long, thick part that does not harden through grows in width and thickness and shrinks in its length, which is the case that catches out a shop machining a long die block to finished size before heat treatment.
The safe route is to leave stock and finish machine after hardening. Where that is not possible, an air-hardening grade is the closest thing to a guarantee, since D2 and its relatives transform slowly and show the smallest size change of the four families. The stock to remove from as-supplied bar before machining begins is set out in the machining allowance chart.
Four steps reduce the movement before it starts. Stress relieving the part after rough machining releases the stresses left by cutting, so the quench is not the first thing to disturb them. Preheating in one or two stages keeps the temperature difference between thin and thick sections small. Loading the furnace so the part can be suspended vertically removes the bowing that comes from its own weight at temperature. Soaking fully at the austenitizing temperature evens out the transformation across the section, which is the subject of the heat treatment troubleshooting chart.
Where a dimension has to be held tightly, a subzero or cryogenic step completes the transformation that room-temperature cooling leaves unfinished, and the part stops moving afterwards. The order matters. The part is quenched, tempered once to relieve stress, held below zero, returned slowly to room temperature and tempered again. A deep freeze at minus 120 °F is soaked for 120 minutes per inch of thickness, controlled cryogenic cooling goes to about minus 320 °F, and a retemper follows every form of deep freezing. A cycle like this is a heat treater’s decision rather than a shop floor one, and the stages a tool steel cycle has to include are listed in the heat treatment guide.
Machining a tool to finished size before hardening saves grinding time right up to the point where the part comes back oversize, undersize or out of flat, and then it costs a second heat treatment and a remake. For dies that hold a tolerance, the allowance is the cheaper habit. The most common version of the mistake, and the checks that avoid it, are set out in the D2 heat treatment mistakes.
Size is measured on the cold, fully tempered part, never straight out of the quench. A part that reads oversize before tempering will move again through the tempering cycles and through any later subzero step, so check the dimensions at the end of the whole cycle and cross-check the hardness on the same face, using the hardness testing methods when a file is not enough.
Temper twice before measuring
Air-hardening grades are tempered two or three times because each cycle converts more retained austenite and moves the dimensions a little further. A part measured after one temper will not hold the size it shows. The tempering windows and the hardness each one produces are in the tempering chart.
Air hardening wins on stability
The gap between plus or minus 0.0005 in. per in. for D2 and 0.0015 in. per in. for an oil-hardening grade is the difference between one finishing pass and a scrapped block. Where a die is ground after hardening, weight the choice towards the cold work grades that transform in air.
D2 is the reference case
D2 is the grade most often machined close to size before hardening, and the one with the smallest margin for error when the shop assumes no movement. The hardening sequence itself, including the preheat and the tempering cycles that stabilise the size, is in the D2 heat treatment guide.
Keep the heat treat record
The growth figures above are averages over a grade family, and the part in front of you is one heat of steel with one heat treat record. Note what the furnace did and keep the mill certificate with it, so the next part in the same grade is allowed for from data rather than from memory.
Páginas de referencia relacionadas
Tool steel hardenability chart · Tool steel annealing and normalizing chart · Quench cracking in tool steel · Tool steel forging temperature guide · Tool steel delivery conditions · Guía de mecanizado de acero para herramientas D2
Sources: Tool and Die Making Troubleshooter (R. M. Leed, Hanser Gardner), Table 8-9 and the sections on distortion and subzero treatment, drawing on Bethlehem Steel distortion data; ASM Handbook, Volume 11, Failures Related to Heat Treating Operations, Table 2. Reference data for comparison only. Actual movement depends on part design, mass, geometry, grade and hardening practice. Confirm the allowance with your heat treater before the part is machined to size.