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.
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