W1 Tool Steel Heat Treatment Guide
The heat treatment cycle for W1 water hardening tool steel, set out as the source publishes it, with the preheat and austenitizing steps, the brine quench and the hardness each tempering temperature leaves. A reference summary of published practice and not an Aobo Steel specification.
The W1 water hardening cycle
W1 is the simplest of the tool steels listed in the source and the oldest of the grades still sold. The cycle below is the one the source sets out for it. Every temperature and time in the table is printed there, including the soak rate and the quench medium, so a shop can run a part straight from this page.
Preheat, austenitize, quench and temper for W1
| Step | Temperature | Time or rate |
|---|---|---|
| Preheat | 1200 °F (650 °C) | 10 to 15 minutes, or until the part is uniformly heated |
| Austenitize and soak | 1425 °F (775 °C) | 5 minutes per inch of smallest cross section once the part reaches temperature, which is 15 minutes on the 3 in (75 mm) cube in the source |
| Quench | To 125 to 150 °F (52 to 65 °C) | Water with 10% salt, agitated for the whole quench, flat work entered on edge |
| First temper | 350 °F (175 °C) | 2 hours per inch of cross section, started as soon as the part is hand warm |
| Second temper, when used | 325 °F (160 °C) | 2 hours per inch of cross section, for parts with fine detail or hard service |
Source, Heat Treating W1 Tool Steel, printed pages 93 to 97.
The W1 chapter gives the austenitizing temperature as 1425 °F (775 °C). The heat treating reference information for the same grade on printed page 199 gives 1475 °F (800 °C). Both figures are printed in the source and both are shown here as published.
Why the quench is water with salt
W1 is quenched in water, and the source adds 10% salt to that water. The salt coats the part as it cools, which cuts scale and evens the hardness across the surface. Plain water boils against hot steel, forms a layer of vapour that drives the water away from the face, and leaves the soft spots where the austenite never reached martensite. The part is cooled to 125 to 150 °F (52 to 65 °C) and tempered at once.
Two practices go with the quench. The water has to be agitated for the whole of it, and a flat section has to enter the tank on edge rather than flat so that both faces cool at the same moment. A few milliseconds of difference between the two faces is enough to distort the part. The water hardening grades are the ones most exposed to cracking and to growth at the quench, and this is where it happens.
Tempering W1
The source gives one temper at 350 °F (175 °C) for 2 hours per inch of cross section, and a second temper 25 °F (14 °C) lower for parts carrying fine detail or going into hard service. W1 does not normally need the second temper. Hardness is bought with toughness along the whole of the table, so the temper is chosen from what the tool has to do rather than from the last column alone.
Hardness left by each tempering temperature
| Temper, °F | °C | Hardness, HRC |
|---|---|---|
| As quenched | No temper | 67 |
| 300 | 150 | 65 |
| 400 | 205 | 62 |
| 500 | 260 | 59 |
| 600 | 315 | 55 |
Source, heat treating reference information for W1, printed page 199, read against the water hardening group table on printed page 142.
The two tables in the source differ on two lines. The reference table on page 199 prints 300 °F (150 °C) at 65 HRC and 400 °F (205 °C) at 62 HRC. The selection table on page 142 prints the same two tempers at 64 HRC and 61 HRC, and marks 300 °F (150 °C) as the optimum temper for the grade.
Chemistry of the water hardening grades
Both water hardening grades are printed in one block in the source. W1 carries 1.05% carbon, 0.25% manganese and 0.20% silicon, and W2 carries 0.95% carbon with 0.20% vanadium. The chromium, molybdenum, tungsten, cobalt and sulfur lines are left empty for both. These are plain carbon tool steels, which is why they machine easily in the annealed condition and why they are the least forgiving of the quench. The source notes that W1 has the same basic chemistry as AISI 1095 and that the difference between the two lies in the manufacturing rules, above all the control of grain size in the annealed bar.
The condition the steel arrives in decides how much of this cycle matters. W1 is supplied annealed, machined oversize, and the quench pulls the part back to the hardness in the tempering table and no further.
What the grade is used for
The water hardening grades harden to a shallow case rather than through the section. The source puts the case at 63 to 65 HRC about 0.050 in. (1.27 mm) deep, with a softer, tougher core under it. That combination gives the sharpest edge of any steel in the marketplace, which is why the finest kitchen knives are made from it, and it also makes a good coining die. Drawn back to 48 to 50 HRC the same steel gives a spring temper.
Two limits belong with the table. Decarburization and scale are most prominent on the water hardening grades, so the surface of the finished part is a machining allowance rather than the hardness the tempering table promises, and that matter moves with the furnace atmosphere of the shop doing the work. The quench is also severe enough that the source describes it as violent. A part is straightened while it is still hot, above 400 °F (205 °C).
Related reference data
The tempering window for the other grades is on the tool steel tempering chart, and the same grades read on the impact test are on the tool steel impact toughness chart. The two groups that quench more slowly are described on the oil hardening tool steel and air hardening tool steel pages, and soak rates by section across the whole range are collected on the tool steel soak time chart. The condition the steel is bought in is on the annealed condition of tool steel page, the chemistry of the grades we stock is on the tool steel composition chart, and the schedules that cover the whole range are on the tool steel hardening and tempering chart.
Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, second edition, Hanser). The cycle comes from the W1 chapter on printed pages 93 to 97, the tempering table and chemistry from the heat treating reference information for W1 on printed page 199, and the selection notes for the water hardening group from printed pages 142 and 143.