Quenching Chart for Water, Oil and Air Hardening Steels

What a quench has to do

A quench has one job inside the section being hardened. The steel has to pass the range where pearlite and carbide form, quickly enough that those transformations do not get started and slowly enough that the part does not tear itself apart on the way down. A quench that is too slow leaves soft spots in a thick section, and a quench that is faster than the grade needs adds distortion and cracking risk for nothing. The ranking below sets a minimum speed, not a target.

The first table gives the speed of the quenchants themselves and the second places each grade in one of four quench speed groups. Read together they answer the practical question of what to quench a given grade in.

Quenchant cooling speed

The nickel ball test times a 22 mm nickel ball heated to 1625 °F (835 °C), where nickel is nonmagnetic, into 200 ml of quenchant held inside a magnetic field. The ball is pulled to the magnet as it cools past roughly 670 °F (355 °C) and becomes magnetic again, and the seconds between the start and that moment are the number quoted. A shorter time means a faster quenchant, so a 7-10 second oil pulls heat out of the part faster than a 15-20 second one.

Deionized water sits at 2 seconds and a 9 per cent saltwater brine at 1.5, both well below the fastest oil in the table. Salt destabilises the vapour blanket that insulates a part at the start of a quench, and the brine therefore cools faster than plain water. Among the oils the speed is set by the base oil and the additives, and makers sell fast, medium and slow grades. Parks 50, Duratherm 48 and Houghto-Quench K are fast oils, Houghto-Quench G and Duratherm G are medium oils, and Parks AAA is sold as a medium-fast oil. Agitating the bath or moving the part breaks up the vapour blanket and raises the effective cooling rate of any of them.

Nickel ball cooling time of five common quenchants

Source, nickel ball cooling time for different quenchants, Table 21.1 on printed page 258 of the source, which is PDF page 268 of the copy consulted.

The nickel ball time ranks quenchants against each other and is not a hardness specification. The values are printed as the source gives them. A reference table only, it is not an Aobo Steel specification.

Quench speed each steel needs

The four groups run from the fastest quench at the top to the slowest at the bottom. Water hardening grades have low hardenability and need water, brine or a very fast oil to reach full hardness in a normal section. Fast oil and medium oil grades harden in those oils, and air hardening grades reach full hardness in still air, in a forced air or gas quench, or between two aluminium plates.

The boundaries are approximate, as the source states, and a grade near the bottom of a group can usually be quenched in the next slower medium when the section is thin. 52100 sits in the fast oil group and will still harden in a medium oil when the section is not too thick. The reverse direction is not safe. A grade quenched in a medium slower than its group needs will not reach full hardness.

Many shops run a fast oil rather than water on the water hardening grades, because water quenches crack and warp thin sections easily and a fast oil can carry a thin or partly ground part. That group is also the one used for differential hardening by the clay method, since only a low hardenability grade will hold a soft spine once clay slows the cooling there, and the hamon line comes out of that treatment.

28 steel grades grouped by the quench speed they need

Source, ranking knife steels by hardenability, Table 21.2 on printed page 260 of the source, which is PDF page 270 of the copy consulted.

The group boundaries are approximate, as the source states, so a grade at the bottom of a group can often be quenched in the next slower medium when the section is thin. The last row stands for steels of 4 per cent chromium and above. A reference table only, it is not an Aobo Steel specification.

Reading the chart

Two points are worth reading before the chart is used as a process sheet. The oils in the table are ranked by the nickel ball time, while a real bath is also judged on the shape of its cooling curve, and the same oil class from two suppliers can differ inside the class. Water is the fastest quenchant here, and its cooling rate falls as the bath warms, because a hotter bath makes the vapour blanket easier to form and longer lived. Air hardening only means that the grade will harden without a liquid quench, and a forced air or plate quench is still needed on heavier sections.

Related reference data

The three groups behind this ranking are compared on the air hardening tool steel page and the oil hardening tool steel page, and the section size each group hardens through is on the hardenability chart. Measured cooling rates for one hot work grade are on the H13 quenching cooling rates page. Full hardening and tempering cycles are on the tool steel heat treatment guide, and two grades from the table have their own schedules on the O1 heat treatment page and the 52100 heat treatment page.

Reference data compiled from Knife Engineering, Steel, Heat Treating, and Geometry (Larrin Thomas). Table 21.1 is printed on page 258 and Table 21.2 on page 260 of the printed book, which are PDF pages 268 and 270 of the copy consulted. Both tables on this page were read from the page image rather than from an automatic text layer.