For those who thought they knew everything that could be known about heat treating steel.
Tooling and Production Magazine reported on this new “Flash Processing” technique that raises mechanical properties by about 7% over conventional martensitic HS steels.
This process is also said to improve formability- drawability or rollability- by about 30%. Obviously, that gives designers quite a bit of potential mass savings- key to lower fuel consumption and higher performance- lighter, stronger, more efficient- in transporation applications where these kinds of steels are typically used.
The structure of the steel after processing shows the expected martensite as well as bainite and an abundance of carbides. Traditional heat treatments try to assure a uniformity of the desired resultant microstruture- anomalous structures are considered bad.
The hybrid structure developed by this flash processing technique is a – dare we say- “composite microstructure” which gives the material bulk mechanical properties of improved strength and increased ductility.
We look forward to the continued development of this exciting new process by it’s inventor, Gary Cola, the research team at The Ohio State University, and National Science Foundation (NSF) Center for Integrative Materials Joining for Energy Applications which is leading a consortium of other universities to develop this technology.
1) Martensite is the hardest and most brittle microstructure obtainable in a given steel.
2) Martensite hardness of the steel is a function of the carbon content in that steel.
3) Martensite results from cooling from austenitic temperatures rapidly by pulling the heat out using a liquid quenchant before pearlite can form.
4) As quenched Martensitic structures are too brittle for economic use-they must be tempered.
5) Reheating as quenched Martensite to a temperature just below the AC1 results in the best combinations of strength and toughness.
Because Martensite transformation is almost instantaneous, the Martensite has the identical composition of the parent phase, unlike ferrite and pearlite which result from a slower chemical diffusion process, so each have different chemical compositions than the parent austenite. Formation of Martensite involves a transformation from a body-centered cubic structure to body-centered tetragonal structure. The large increase in volume that results creates a highly stressed structure. This is why Martensite has a higher hardness than Austenite for the exact same chemistry… Photo and Graphs Credit: Cold Finished Steel Bar Handbook
As machinists, we seldom encounter microalloy steels. but what do we need to know?
Microalloy steel is manufactured like any other, but the chemical ingredients added at the initial melt of the steel to make it a microalloy include elements like Vanadium, Columbium (sorry, Niobium for us IUPAC purists), Titanium, and higher amounts of Manganese and perhaps Molybdenum or Nickel.
Vanadium, Columbium Niobium, and Titanium are also grain refiners and aggressive Oxygen scavengers, so these steels tend to also have a very fine austenitic grain size.
In forgings, microalloy steels are able to develop higher mechanical properties (yield strengths greater than say 60,000 psi) and higher toughness as forged by just cooling in air or with a light mist water spray.
Normal alloy steels require a full austenitize, quench and temper heat treatment to develop properties greater than as rolled or cold worked.
Since microalloyed steels are able to get higher properties using forging process heat- rather than an additional heating quenching tempering cycle- they can be less expensive to process to get improved mechanical properties.
The developed microstructure ultimately makes the difference. The microstructure developed in the steel depends on the grade and type.
Normal alloy steels require a transformation to martensite that is then tempered in order to achieve higher properties.
Microalloy steel precipitates out various nitirides or carbides and may result in either a very fine ferrite- pearlite microstructure or may transform to bainite.
For machinists, if the steel is already at its hardest condition, the microalloyed microstructure of either ferrite pearlite or bainite is less abrasive than that of a fully quench and tempered alloy steel.
P.S. The non- martensitic structures also have higher toughness. We don’t tend to machine prehardened steels in the precision machining industry, but if you ever are part of a team developing a process path for machining forgings, or finish cuts after induction hardening, these facts might be good to know. Martensite. Georges Basement Bainite 1000X
Don’t confuse hardness and hardenability. Hardness is a material property. Hardenability is a way to indicate a material’s potential to be hardened by thermal treatment.
Hardness is resistance to penetration. Hardenability describes how deep the steel may be hardened upon quenching from high temperature. The depth of hardening is an important factor in a steel part’s toughness.
The brinell test uses a 10mm hardened steel (sometimes carbide) ball and various levels of force applied over a specified time.
The width of the impressions is measured optically and averaged. (Wider impressions mean the ball penetrated deeper, thus, the material is less hard.) The Brinell hardness number is calculated by dividing the load applied by the surface area of the indentation. Prior to today’s direct reading instruments, the measured indentation diameters could be looked up on a reference chart and the corresponding Brinell hardness number given.
The Rockwell test is similar, but uses different forces and either a smaller ball indenter (Rockwell B scale ) or a diamond indenter (Rockwell C scale).
Hardenability- Jominy Test
In the Jominy test, a standard specimen is heated then water quenched from the end, and a series of rockwell hardness tests are taken in 1/16th inch increments along the length of the specimen.
It is the influence of the steel’s chemical makeup (Carbon and Alloying elements) that determine how a deeply a grade of steel will transform to martensite for a particular quenching treatment. This means that for each grade being heat treated, mechanical properties are a result of cooling rate (quench). An excellent web page on this can be found here.
So what of the difference between hardness and hardenability? Hardness is resistance to penetration under specified conditions of load and indenter. Hardenability is the ability of a steel to acheive a certain hardness at a given depth, upon suitable heat treatment and quench. Hardness can be measured in steels in any condition. Hardenability presumes that the steels will be heat treated to acheive a targeted hardness at a given depth.
One is an actual property, one is a measure of potential.
And now you know.
Web resources: Gordon England Thermal Spray Coatings Farmingdale State College School of Engineering Technologies.