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 grade family. The figures below show expected growth for water-, oil-, and air-hardening tool steels, so you can leave a die with enough stock for the finish cut after hardening instead of scrapping it.

Expected growth by tool steel family

Growth of a fully hardened part per inch of dimension. The two worked columns on the right translate the rate to a 12 in. and a 300 mm length, and they are calculated from the rate rather than quoted. A particular block’s growth also depends on its shape, which is why the second half of this page matters as much as the table.

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.

Where the movement comes from

Every size change in hardening is a volume change in the crystal structure, and the table below shows the three steps that add up to it. Volume change is quoted as published, and the third column is one-third of it, which is the change in a single dimension.

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 transformed completely, and the missing volume will appear later, often after a grinding operation or in service. Steels that retain a lot of austenite move the most, which is why 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 length, which is what catches 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. When that isn’t 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 stresses left by cutting, so the quench doesn’t disturb them first. 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 bowing caused by its own weight at temperature. Soaking fully at the austenitizing temperature evens out the transformation across the section, as the heat-treatment troubleshooting chart.

When a dimension must be held tightly, a subzero or cryogenic step completes the transformation that room-temperature cooling leaves unfinished, and the part stops moving afterward. 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 re-temper follows every deep-freeze cycle. A cycle like this is a heat treater’s decision rather than a shop-floor one, and the stages a tool-steel cycle must 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 mistake, and the checks that prevent 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 any later subzero step, so check dimensions at the end of the whole cycle and cross-check hardness on the same face, using 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 D2 heat treatment guide includes the hardening sequence, including the preheat and the tempering cycles that stabilize the size.

Keep the heat treat record

The growth figures above average 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 memory.

Related reference pages

Tool steel hardenability chart · Tool steel annealing and normalizing chart · Quench cracking in tool steel · Tool steel forging temperature guide · Tool steel delivery conditions · D2 tool steel machining guide

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.