Confused about the various properties of steel...

[bchan77]

Hello all, I keep on hearing various terms used in describing different kinds of steel and I have difficulty ascertaining the exact meaning of each in plain English. I would be grateful if anyone can be so kind as to enlighten me what does each of the following terms mean:

- toughness
- strength
- wear resistance
- corrosion resistance
- hardness
- edge holding

Thanks in advance!

 

[MORIMOTOM]

corrosion resistance: ability to resist rust or pitting.

hardness: based on the rockwell scale, the higher the number, the harder the steel.

edge holding: ability to maintain the same sharpness after use.

 

[mete]

Toughness is what you need for impact use such as chopping with a machete or kukri.

 

[Ravaillac]

- toughness: resistance to impact
- strength: resistance to "static" load (generally bending)
- wear resistance: resistance to abrasion (abrasive type wear)
- corrosion resistance: how easily does the blade rust when in contact with various liquids (water, sea water, food, blood, acids)
- hardness: how hard is the steel (generally determined by Rockwell test -how deep a normalized diamond spike will go in the steel under a normalized static load), not very interesting as such, but higher hardness is ***generally*** correlated with higher wear resistance, higher edge holding and lower toughness...
- edge holding: how well the knife will keep its edge under use. Basically how often will you have to resharpen the knife under "normal" use. Quite subjective since use varies from one person to another, and people have different appreciation of what is blunt and what needs to be resharpened.

 

[HoB]

I think toughness is the somewhat difficult one.

There are two types of toughness:
1.) toughness linked to ductility and impact toughness. They are related somewhat though. Impact toughness is the resistance against braking after a sharp blow is applied. This depends mainly on crack propagation in the material. The unit is energy.
2.) Static toughness. A material is pulled or bend relatively slowly (without application of sharp blows). The maxium force at which the material would actually return to true without any permanent deformation is the strength and the strength times the distance it bending is called resiliance both very much dependent on hardness. After that the ductile region sets in. From the integrated force from the onset of the ductile region to the breaking of the material is the toughness. Units are again energy, but the values of the two methods might be vastly different.

Most often (but not always), materials that exhibit high ductility and therefore high static toughness, will also resist crack propagation very well. A blade with high toughness is not necessarily desirable if the toughness is bought at the expense of stength. Such a blade would not break but it would quickly deform permanently. Meaning the blade would either bend and take a set or the edge would roll severely on hard impacts, but would not chip.

 

[mete]

There is no such thing as 'static toughness'. Tests for toughness such as the Charpy are all dynamic tests !

 

[HoB]

Of course there is:
Integration of the stress-strain curve from the onset of the ductile region to point of failure. That is the basic definition of toughness, as can be found in any material science text book. The stress-strain curve is obtain by slowly pulling/bending/torquing: Slow in comparison to a Charpy impact test. It is of course not completely static, but it is no shock load.

And, yes, of course all test for toughness SUCH AS THE CHARPY are dynamic tests. I never claimed any different. Those are all IMPACT TOUGHNESS tests. This is precisely why I made that distinction. The distinction is necessary, because the failure mode is different (again basic textbook material, even google will find it on the internet). Impact toughness is greatly dependent on crack propagation which is why the test material is usually notched. Also the numbers are completely different. If someone takes the numbers obtained by integration from the stress-strain curve and would compare them with Charpy impact numbers, would probably be greatly confused if he didn't know the difference.

For knives, I think both properties are very important as a blade is subject to impacts as well as side and torque loads. But I am sure there are various different opinions about the importance of impact toughness and stress-stain toughness in the blade world.

Edit: If you are just objecting against the terminology 'static-toughness', mete, I apologize. I have taken my liberty to call it such in order to clearly distinguish from impact toughness, and that was probably not the right thing to do. But I have always been of the opinion, that it is beneficial for the discussions on a knife forum to sacrifice scientific terminology and strictness for the sake of a common understanding and conveyance of fundamental concepts.

 

[mrjed]

If you haven't, read Joe Talmadge's sticky on the maintenance tinkering and embelleshment threads. More than enough good info to get you up to speed on what you need to know and ask.