hardness-toughness again

wnease

wnease

Joined
Apr 22, 2004
Messages
878
This never ends...

Roughly, higher hardness = lower toughness, but

You know those steels that have secondary hardening at high temper temperatures... like gaining a couple points at 1000F

Are they losing toughness? The 3V shows increased toughness with the higher temper. Is that the result of some dreaded R.A. being transformed to tempered martensite?

I want what every maker wants... higher toughness at higher hardness!

I initially thought it might have something to do with the tempered martensite embrittlement but that doesn't seem to apply.

Speaking of toughness, anyone know the impact velocity of the striker in a charpy test?
 
Yes, generally those steels are loosing toughness. What is happening will be steel type dependent. For the high speed steels, some other tool steels, and the more highly alloyed stainless steels like ATS-34 and 154CM, the hardness goes up due to carbides forming (precipitating) at the higher temperatures. 3V specifically I dont know that much about and havent seen its tempering curve.

The hardness vs toughness relationship can get complicated pretty quickly. At higher tempering temperatures, the "lower hardness/increase toughness" rule holds pretty consistently. "Higher temperatures" is relative to the type of steel. The tool steels and stainless steels have all kinds of funny things going on up to 1000 F in some cases. Many steels have peaks and valleys in the toughness vs. tempering temperature (hardness) relationship. To make it more complicated, increasing tempering temperature can increase hardness, as you already noted. Some dont have valleys so much as level areas. Some have neither. 4340 steel illustrates this pretty well. There is a peak toughness at about 480 F, then it drops dramatically, and then climbs again, but doesnt reach the same toughness level as before until 800 F. After this point, toughness increases pretty quickly as tempering temperature increases and hardness goes down.

Also, at the hardnesses we as knife nuts are interested in, the Charpy test is notoriously scatter prone. Comparisons can still be made, but data from different sources cannot always be directly compared.
 
Secondary hardness is the formation of different carbides of V, Mo, or W at the higher temperatures [~1000 F].This is beyond the temperature that you would find RA.
Those who have worked with CPM 3V a lot such a J Hossom have found that the 1000 F temper is tougher.
I agree that the toughness tests such as Charpy give you a rough comparison, never exact.
 
Broos said:
These guys machines are designed for max energies of 400kJ & 540 kJ ASTM E23, EN10045-2 and ISO 148 tests, and can reach max velocities of 17 f/s.

http://www.idspackaging.com/packaging/us/tinius_olsen/testing_machines/12342_0/g_supplier.html

The practical difficulty & theoretical complexity of the characterization of toughness is emphasized by looking at the technical literature.
Click to expand...


AH!

thanks all

17 fps is pretty slow, nothing like the impact velocities achieved by hand, i chronographed my strikes at over 160fps, higher velocity shock test might help us
 
Just to clarify some of the issues touched on here. Standard pendulum Charpy/Izod are measured in foot pounds which is the results of distance traveled by the pendulum times the weight of the impact head, not a matter of velocity or speed of the head. Yes the arm of a tester moves much slower than a human arm can swing a knife, but due to the mounting and anvil setup and the localization of force the amount of energy delivered is much more than a human arm could deliver. My unit maxes out at 240 Ft Lbs. and if a piece of steel manages to absorb it you are not going to be able to break it with your bare hands. I have likened a piece of L6 maxing a tester as imagining a two hundred pound man wearing ice skates jumping with one foot on a 1cm X 1cm piece of steel supported only on the ends.
 
thanks Kevin,

i was curious about the velocity though, because i break stuff a lot (work granite as my day job) and you can pile energy and momentum into the stuff with an excavator hoe but the higher velocity impact of a hammer shatters it. Cf. tempered glass, also ballistics, not sure if any of this is applicable, also watch a lot of dry pasta break lol
 
I like the pasta analogy :Dand have used it myself:thumbup:. Still perhaps the best example of impact toughness vs. gradual loading is vinyl house siding, in gradual tension the stuff will stretch like taffy but under sudden load it will shatter. Although I wouldn’t do it these days, years ago my fragile flesh and bone hand was able to break concrete in Tae Kwan Do exercises as long as the speed was fast enough to exceed the brittle mode of the patio block. The concept in steel rests upon the load exceeding the rate at which slip based deformation can compensate for it. Apply 60 lbs. of weight to steel gradually and it can slowly yield and deform without fracture to handle it, but drop it from 4 feet for sudden loading and the steel cannot absorb all that energy at once with slip based deformation so you get fracture instead. But one cannot just apply greater speeds and get a direct correlation to standard impact tests.

When dealing with simple iron/carbon systems the lower the hardness is the greater toughness can be, and this is why you got elaborate developments in differential hardness in blades such as on the Japanese swords. With simple steels such as bloomery and tamahagane all you had to work with is varying levels of hardness to control brittleness. Most modern sword makers and enthusiast for this reason believe that the low 50’s HRC is all one wants in a sword, which is the case if you are working with ancient type steels. But alloying changed all of that. With the intentional additions of alloying elements one can have much higher hardness without the brittleness. In fact if modern swords makers do not take this into account while working with alloy steels they can rob themselves of much of the steels hidden potential in toughness since many modern alloys will reach a peak in the hardness vs. toughness curve at a higher hardness than many of the simple carbon steels and drawing them back more can lead to loss of both hardness and toughness.
 
When inertia and impulse get involved with the small contact area of a chisel or hammer face, things will happen that can't be done with tons of force spread out over a few sq. inches.

That makes me think of one of the funniest things I saw on a TV Home Video show. This large yacht was docking and the pilot cut the engines before it was fully stopped moving forward. The dock man with the forward line saw that it was slowly moving toward the dock at around an inch a second. As it came within a few feet of the dock, he leaned out ,putting his hands on the bow and pushing on it to stop the boat. He was, of course, wasting his time and the bow slowly cut about two feet into the dock, all the time while this guy was turning red pushing as hard as he could ( fortunately, he was not injured). Even though it was barely moving, he had no chance of reversing the inertia of the ships' many tons ( P=mv).
Stacy
 
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