Retained Austenite for Quenched Tool Steels

Because, during the quenching procedure, the temperature is quickly reduced when the steel is under this phase, not allowing the time enough to the microstructure to transform back. They have a high content of alloy elements, increasing its tendency to phase transformation. On some publications, you'll find the term "equivalent carbon content", i.e., because the ally elements affect temperability, you must treat the steel as it had more carbon than it really has. It just behaves tis way.

This is a problem, because you may have the phase transformation after the part finalization. It can induce stress, and can even lead to phase transformation to what is called tempered martensite. This is a very fragile structure, that can originate a crack in the part under no load. This condition is also observed when one uses an incorrect tempering temperature, causing the so called "secondary temper hardening".

Action required: Use the correct tempering procedure after quenching your steel. The data can be found in the good references of the matter.

The large amounts of alloying additions in tool steel, relative to plain-carbon steels, lowers the temperature of completion of the transformation of austenite (the high temperature phase) to martensite (the metastable phase-mixture that is the "equilibrium" microstructure of tool steels at and around room temperature) to well below room temperature. Hence, quenching a tool steel from the appropriate elevated temperature to room temperature does not drive the austenite-to-martensite reaction to completion. The result is a mixture of martensite and austenite, the latter being the "retained austenite". This microstructure is metastable over time, and the gradual isothermal transformation of the retained austenite to martensite produces dimensional instabilitry in the part. Also, failure to temper the martensite may result in loss of ductility and toughness in service.

EDALDER is correct when they state that the alloying elements lower the Ms temperature (martensite start). An important point is that the lower the Ms temperature reduces the amount of martensite which forms on cooling to room temperature. WHat has remained unstated so far is that the lower Ms temperature is also driven by the high carbon contents of tool steels, not just the alloying additions. the key point is that because of the lowered Ms temperature, there is insufficient undercooling of the Ms temperature (difference between Ms temp and normal room temperature), It is precisely this undercooling which is what drives the volume fraction of martensite formed.

So high carbon and high alloys mean lower MS temp which means less MArtensite conversion and greater percentage of retained austenite. This is problematic because The conversion of austenite to martensite is independent of time, it is driven by undercooling of the Ms! Thus, upon achieving a new low temperature 9Say first winter in alaska on an oil rig, the lower temperatures would cause new martensite to be formed and this martensite would NOT be tempered. In technical terms we would describe this as an athermal kinetic. an equation for estimating the volume fraction of martensite formed can be found in Acta Metallurgica Vol 7, 1959 p 59-60. it was prepared by D.P.Koistinen and R.E.Marburger.

This explains why cryogenic treatments are important to fully convert all of the retained austenite into martensite. It extends the degree of undercooling of Ms that "develops" the martensite.

Tempering Martensite is important.

EDALDER again has it correct that the martensite is brittle and the cause of failure if it is left in the untempered condition. When martensite is formed, it happens so fast that the carbon atoms are trapped, and cannot diffuse. The resultant supersaturated ferrite actually has a body centered tetragonal crystal structure. (Higher carbon = higher tetragonality.) There is a lot of energy tied up in this structure associated with the fine plates of martensite and its relatively high number of dislocations per unit of volume. These condiitions mean that martensite stores much Strain energy. This retained strain energy makes the martensitic microstructures unstable, so that they are quite amenable to decomposing upon application of heat. This decomposition results in increased toughness. The application of this tempering heat may not be noticeable in the microstructure some cases, but in reality altering tempering temperatures changes both the martensite matrix, as well as the dispersion of carbides in the steel.

Lessons you must heed:

1 high amounts of retained austenite are a function primarily of high carbon content in tool steels,

2 High alloying additions and High Carbon serve to lower the Ms temperature.

3 Lower Ms temp means less austenite gets converted to martensite.

4 Conversion of austenite to martensite is athermal- independent of time, so that the only way to increase Martensitic conversion is to severely undercool the Ms temperature. (this means cryogenics)

5) The martensite so formed MUST BE TEMPERED to gain toughness and not create potential problems due to the high retained strain energy in the microstructure.

Milo

PS. I believe that some of the terms in the rescobar posting may be misconstrued, particularly those regarding tempering.