H13 Steel Microstructure After Quenching

What the quench decides

An H13 die is austenitized in the region of 1010 to 1050 °C and then cooled through the transformation range. What the steel turns into on the way down depends on how fast it passes through that range, and a thick section cannot shed heat at its surface any faster than the core can feed heat out to it.

The report behind this data splits the cooling curve into two windows. The rate between 1750 and 1550 °F governs how much MC carbide precipitates on the austenite grain boundaries. The rate between 1550 and 750 °F governs the matrix, meaning how much pearlite, bainite and martensite the die ends up with.

Both windows are measured below for the two extreme block sizes in the study, a 2 inch cube and a 14 inch cube, quenched in ten media that run from a still vacuum through to a dilute polymer solution.

Matrix microstructure after quenching

The figures are the percentage of each phase at the surface of the block and at its mid-radius. B is bainite, M is martensite and P is pearlite. The gas results are for nitrogen and helium at 2 atm with a gas velocity of 3,400 ft per minute, which is the condition the report used for its gas quenches. The salt baths and the two polymer solutions are the fastest media in the series.

Pearlite, bainite and martensite by quench medium

Source, matrix microstructures produced as a function of quenchant for the two extreme H13 steel block sizes, Table V, printed page 2-16 of the report (PDF page 33).

B is bainite, M is martensite and P is pearlite, and the amounts inside each cell are percentages. The report states that the surface and the mid-radius of the 2 inch block are essentially identical. The gas figures are for nitrogen and helium at 2 atm with a gas velocity of 3,400 ft per minute. A reference table only, it is not an Aobo Steel recommendation.

Carbide precipitation at the austenite grain boundaries

MC carbides form at the austenite grain boundaries while the block sits between 1750 and 1550 °F, and a slow quench holds it inside that window for longer. The report gives the amount as a figure on a 0 to 100 scale, where a higher number means more precipitation at the boundaries. Grain boundary carbides are the feature behind the loss of ductility and impact toughness that follows a slow quench, so this table is read the opposite way round from the one above.

MC grain boundary precipitation by quench medium

Source, MC grain boundary precipitation as a function of quenchant for the two extreme H13 steel block sizes, Table IV, printed page 2-15 of the report (PDF page 32).

Figures are on the 0 to 100 scale the report uses, where a higher number means more MC carbide precipitation at the austenite grain boundaries. The two salt bath entries appear in this table only, and the 575 °F bath is the faster of the two. A reference table only, it is not an Aobo Steel recommendation.

Reading the two tables together

In the 2 inch cube the surface and the mid-radius read the same, because a section that small cannot hold a temperature difference across itself long enough for it to matter. The vacuum quench leaves the most bainite, at 28% against 72% martensite, and the amount drops with every step up in cooling speed, to 2% in high speed oil. Both polymer solutions take that block to fully martensitic.

In the 14 inch cube the same media behave very differently. Mid-radius lands between 25% and 72% bainite whatever the quench, because heat has to travel out through the steel itself and the surface stops controlling the core. At the surface only the 10% UCON A solution reaches fully martensitic, and the vacuum quench is slow enough to leave pearlite in the structure at both positions.

MC precipitation runs the other way. The vacuum quench scores 91 to 98 on the scale, and the two polymer solutions score 0 at the surface of both blocks. At the mid-radius of the 14 inch cube every quench lands between 92 and 98, which is the same message as the matrix table read from the opposite direction, that the middle of a thick H13 block is settled by the steel and hardly at all by the medium around it.

Related reference data

The grade this data belongs to is set out on the H13 tool steel page, and the cooling rates behind these microstructures are on the H13 quenching cooling rates page. The rest of the cycle around the quench is on the H13 heat treatment guide, with the service hardness it has to reach on the H13 hardness chart. What a quench that goes wrong looks like is covered on the quench cracking page, the grades that harden in still air are separated on the air hardening tool steel page, and the die casting side of the same grade is on the die casting die and insert page.

Reference data compiled from Improved Life of Die Casting Dies of H13 Steel by Attaining Improved Mechanical Properties and Distortion Control During Heat Treatment (J. F. Wallace and D. Schwam, US Department of Energy final report DOE/ID/13320-3, October 1998). The MC grain boundary precipitation table is printed on page 2-15 and the matrix microstructure table on page 2-16 of that report. Both were read from the page images and not from an automatic text layer.