I have seen hundreds of 304 SS Forgings which have machining as the final step finishing process before shipment to the end users where toughness is also required. Hence toughness and normal machining cannot be opposing qualities. May be use of 303 SS with intentional sulfur is added is for manufacturing or service demand high/deep machining operations.

Dear Switznner,

I think Flank wear has a direct correlation with cutting speed, and similarly tool life has direct correlation with cyclic plasticity parameters derivable from material toughness data, although it is difficult to get material removal rate of machining process from some model by directly feeding these data. But as you mentioned tough materials always oppose the hertzian pressure built on to them. Average chip length and shear plane angle are also two parameters can be correlated to material toughness.

can refer this https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5129156

Difficult to model but some multi parametric studies always can be done.

I think there is no relation between toughness and machinability. The long chips during machining is related to the ductility not toughness. And toughness can be evaluated by standard testing methods.

Hi N.T. Switzner

For accuracy of description, there is some relationship between toughness and machinability, albeit a very loose correlation. Machinability can definitely be better described by the material microstructure and shear properties.

Toughness, better described as fracture toughness is the work required to create two new surfaces in a material. Relationship wise, mode II and III are expected to give a better correlation to machinability compared to mode I, despite a loose one.

To complex problems further by considering cutting speed, dynamic fracture toughness will need to be applied. Now we are digging down a rabbit hole.

Regards,

Wee

Thank you very much for the responses. One definition of toughness is “resistance against a crack propagation”. Isn’t chip breaking a form of crack propagation? So without considering optimizing machining settings, couldn’t there be an inverse correlation between toughness and machinability for metals?

Hi N.T. Switzner

Yes, as described above, there will be a correlation, but with a high standard deviation.

Dear Dr. Kaustav Barat

I really appreciate your answer with suggestions of machining parameters that could relate to toughness including "average chip length and shear plane angle". I really appreciate you sharing the article: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5129156

I will have to take a closer look at this article. Some of the plots are unclear, are there any updated versions or clearer versions of this article?

Thank you!

Nathan

Thanks Nathan, here is the best print I am having

Dear Sirs, machinability and toughness are technical properties of the materials.

To make a rational comparison or state some correlations we need numbers.

To get numbers we must define repeteable tests for machinability (e.g. Quantity of produced chips per hour in turning with a defined tool, at defined machining conditions) and for toughness (e.g. Charpy V or DWTT tests at defined conditions of temperature, etc.)

Dear Eliseo,

Great suggestion! I am not a machinist, but I believe we would need to carefully design some test parameters like type of machining (lathe turning), speed, feed, coolant type and amount, tool shape, rake angle, etc. It would be nice to have a load cell measuring tool force. Would be nice to measure tool temperature, if possible. Maybe high speed video as well and/or infrared high speed video. :)

Then we would need to choose a figure of merit (FOM), like you mentioned and how to measure that FOM. You suggested rate of chip production, which would be good and I also thought of chip size/shape. We might be able to automate these evaluations using a light microscope or SEM with automated particle size, shape and quantity evaluation software.

We would have to decide on some materials that will give us a nice spectrum of toughness values (like 1018 steel, which should be tough vs 1050 steel, which should be low toughness, and some in between). We would have to specify the heat treatment (maybe normalized) so that we get ferrite/pearlite microstructures (to simplify the experiment and minimize the complicating factor of different microstructural constituents for the initial tests).

Then, I completely agree with your suggestion of Charpy or DWTT. I might prefer Charpy at this point just because DWTT samples require so much material. We would have to decide how many Charpy tests and what temperatures to produce a full transition curve (ten temperatures, 3 at each temperature(?)).

Sounds like a cool experimental design. If anybody reading this makes plans to carry out these experiments, please let me know. I would love to stay involved and collaborate!!

Thanks!

Nathan