Austempering Section Size and Bainite Hardness Chart

What austempering does

Austempering quenches the steel into a bath held above the temperature where martensite starts to form, holds it there until the austenite has become bainite, and then lets it cool to room temperature. The bath is normally a hot salt bath. Since the steel never passes through the martensite range during the hold, no fresh martensite forms, and the part comes out with a bainitic structure together with the low distortion that a uniform, moderate cool gives. Hardness is set by the bath temperature itself rather than by a separate tempering step. A bath close to the martensite start temperature grows lower bainite, which is the harder of the two forms, and a bath well above it grows upper bainite, which is softer.

The limit on the process is hardenability. The section has to cool from the austenitizing temperature past the nose of the transformation diagram and reach the bath before any pearlite forms, so a steel with weak hardenability can only be austempered in thin sections. Manganese is the cheapest lever on that, which is why the section size limits in the table below are grouped by carbon and manganese together. The treatment also asks for a quench fast enough to miss the nose and a cool afterwards that is slow, and those two demands tighten against each other as the section thickens.

Maximum section size that can be austempered

Section size limit by carbon and manganese

Source, Table 12.2 on printed page 116 of the source, which is PDF page 120 of the copy consulted. The table is reproduced with the source’s own figure for the largest diameter and sheet thickness that can be fully austempered.

The source prints the first carbon range as .95-1.05 and the sixth manganese range as 0.0.90-1.20. Both are printed above in their corrected forms and no value was changed. Diameters and sheet thicknesses are in inches. The maximum HRC column is the hardness the bainite itself reaches, not the hardness of a tempered part.

Hardness that bainite can reach

Fresh martensite against maximum bainite

Source, Table 12.1 on printed page 116 of the source, which is PDF page 120 of the copy consulted.

The first designation is printed as 10113 in the source and does not match a standard AISI number. The row is reproduced as published. Rc is the Rockwell C hardness of the structure as formed, before any tempering.

The second table is the reason austempering is not a route to maximum hardness. For the same analysis the hardest bainite comes out between two and seven points below fresh martensite, and the gap widens as the carbon falls. Above roughly 40 HRC an austempered part is generally reported to be tougher than a quench and tempered part at the same hardness, and below that hardness the quench and tempered condition is as good or better, so the process is normally aimed at the upper band. Mixed bainite and martensite structures push hardness past the bainite ceiling, and they are made by interrupting the hold rather than by moving the bath temperature alone.

Related reference data

The hardening and tempering sequence for the grades we ship is on the tool steel heat treatment guide, and the temperatures the process starts from are on the tool steel hardening and tempering chart. The clear case for austempering in a through hardening steel is the 52100 bearing steel used for bearing races, where the treatment is chosen to hold distortion down, and the cold work grades we hold and ship such as D2 tool steel are usually taken through a different route. What happens when a bainitic or martensitic structure comes out wrong is covered on the tool steel heat treatment troubleshooting chart.

Compiled from Metallurgy of Steel for Bladesmiths and Others Who Heat Treat and Forge Steel (John D. Verhoeven). The figures are reproduced from the published tables named above and are a reference summary rather than an Aobo Steel specification.