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Tempering: Inconsistent information


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I have been reading various literature on the topics of heat treating and tempering and appeared to have come across inconsistent -even contradictory- information regarding tempering.

 

On the one hand there is the school of thought that says hardness and toughness are DIRECTLY inversely proportional to one another. That is, the harder you get a steel, the less tough it is and this can be controlled via hardening and quenching techniques and/or tempering.

 

On the other hand there appears to be the line of thought that tempering actually ADDS performance to a steel in the sense that you can reduce the brittleness greatly but not lose out on much hardness.. which goes againt the first school of thought.

 

Now does anyone actually know what is going on with the martensitic structure during tempering and how a steel might possibly be less brittle yet still hard, and more importantly what is everyone experience in the matter? Which school of thought is true in practice?

 

Also, would triple tempering contribute to this effect that tempering seems to add to steel?

 

 

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Toughness can also be a property of the steel, I don't have any examples, I'm sure much wiser people here will be able to give those. Unfortunately I doubt it is as black and white as you suggest. Many alloying elements can impact the toughness of a steel, regardless (but probably aided by) heat treatment.

 

JH

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First of all be sure you are comparing apples to apples. What I mean is, there are a large number of alloys out there and they do not all respond to tempering exactly the same. Some steels actually have a secondary hardening range, in other words a range of tempering temperature where they gain hardness. D2 for example.

 

Personally, I do not find the idea that hardness and toughness are inversely proportional contradictory to the idea of adding performance by tempering. Like most other things in life, it is a compromise. Also, it is possible to manipulate the structure of the steel to be tougher or more brittle, at the same hardness, by decreasing or increasing grain size.

 

When steel is heated up it absorbs carbon into the crystalline lattice of the metal. If you imagine the crystals of the steel as a three dimensional grid, the steel holds one carbon atom in the center of each "box." Heat the steel up and add energy to the atomic bonds and the "box" gets slightly bigger - just enough to hold one carbon atom in the center of each face of the "box." Cool the steel slowly enough and the carbon migrates back to the grain boundaries and the "box" once again contains one carbon in the center. However, cool the steel fast enough (quench it) and the carbon cannot migrate and is stuck in the crystalline lattice at a lower energy state. In other words, the atomic bonds have lost energy and the grid has shrunk a bit but, the carbon is still stuck. This causes strain and distorts the shape of the "box" or the "grid" and that is what makes Martensite and also the reason it is hard. Tempering heats the steel up and allows carbon to move back to the grain boundaries and releases some of the strain on the crystalline structure. With simple steels (low alloy) the relationship is pretty direct, more heat, or more time, allows more carbon to move. Temperature has the greatest affect but, it is important to not forget that time has an affect as well.

 

Triple tempering, is an effort, on the part of bladesmiths, to combat something called temper embrittlement. Many times steels with alloying elements (other than carbon) do not convert to 100% martensite when quenched but, instead, have some percentage of what is called "retained austenite." The tempering heat can precipitate formation of fresh, untempered, martensite from this retained austenite. Tools that will see heavy use could possibly break if this condition is present. Obviously this is a concern for individuals whose reputations depend on the perceived superior performance of what they are selling. The solution? Simply temper again. Want to really make sure there are no problems? Give it a third temper, just to be sure.

 

~Bruce~

“All work is empty save when there is love, for work is love made visible.” Kahlil Gibran

"It is easier to fight for one's principles than to live up to them." - Alfred Adler

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I highly recommend the Free Book to download - Metallurgy of
Steel for Bladesmiths and Others Who Heat Treat and Forge Steel

by Professor Verhoeven


http://www.feine-klingen.de/PDFs/verhoeven.pdf

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That book is not longer (legally) free as far as I am aware: http://www.bladesmithsforum.com/index.php?showtopic=8949

 

Also, Bruce - sounds like you are a bit mixed up on the body center cubic and face center cubic structures. The center and face (respectively) of the cube have another iron atom. The carbon atom fits in between those atoms. My quick and dirty example is post #11 here: http://www.bladesmithsforum.com/index.php?showtopic=23966. Everything else looks pretty good.

 

A fun thing about describing the properties of steel (or most any material) is the incorrect usage of terminology. This drove me crazy reading Wayne Goddard's book (The Wonder of Knifemaking, 2nd ed).

Hardness: Resistance to plastic (permanent) deformation.

Ductility: Ability to plastically deform without failure.

Strength: Typically refers to tensile strength. Measured as a load per area (psi/MPa).

Toughness: Ability to absorb energy in the plastic range. The area under a stress-strain curve indicates a material's toughness. Below is a chart showing the stress-strain curves for general ferrous materials.

Impact toughness: Sometimes incorrectly referred to as impact strength. The amount of energy absorbed during failure when abruptly stressed. The test for this is the Charpy impact test - a calibrated hammer on a pendulum.

 

stress_strain_ferrous.jpg

 

Generally speaking: The harder something is, the higher the tensile strength, but the lower the ductility and impact toughness. If you see a chart that lists hardness versus strength it is an approximation/rough guideline. As Bruce mentioned, that can be manipulated via grain size, pre-stressing (work hardening), and such. Depending on application your desired properties will vary extensively. Nobody wants a camp knife that breaks when chopping kindling, but equally nobody uses a razor as a pry-bar.

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No one will dispute Verhoeven's knowledge of metallurgy. I have spent many hours reading his thoughts and trying to relate them to my work. He can be a bit over most people's head though. He is so advanced that most of us who are not PHD's can easily get a little lost.

 

What's most important is to test your blades on a regular basis. If not satisfied, then do the research (or experimentation) to improve your H/T'ing for the steel of your preference. When satisfied remember the old adage, "If it ain't broke, don't fix it.".

 

Gary

Gary

 

ABS,CKCA,ABKA,KGA

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According to Verhoeven"Several things are going on in the steel during the tempering
process that result in the loss of
strength and the gain in ductility
and toughness. The first thing that
happens is the relief of the
high degree of volume strain
in the steel produced by the
formation of the higher volume per atom marten
site phase. Then a series of internal
structure changes occurs which is generally pa
rtitioned into the 3 stages of tempering.
Stage 1
: This first stage consists of the formati
on of very small carbide
s in the martensite,
so small (around 10 nm) they can only be seen
in an electron microscope. These first
formed carbides are metastable carbides
(do not appear on the equilibrium phase
diagram), epsilon carbide (Fe
2.4
C) in hypoeutectoid steels and Hagg (Fe
2.2
C) and eta
(Fe
2
C) carbides in hypereut
ectoid steels. Stage 2
: This stage is simply the
decomposition of any retained austenite into carb
ides and ferrite. It is only important in
high carbon steels where % retained au
stenite is significant. Stage 3
: This stage occurs at
the highest tempering temperatures and here
the metastable carbide
s are replaced with

small particle of cementite, the stable carbide of steels"

 

Okay, so it appears the sudden formation of martensite makes everything much more compressed because of the larger atomic structure, so tempering relieves this strain by what I am assuming to be an expansion of the actual atomic distances and does not affect grain size.

 

If I'm reading correctly, Tiny carbides also form which increase the hardness as well as the wear.. so I can see how this would offset the loss of hardness via the %reduction of martensite that the tempering also induces, in a sense reducing brittleness while also maintaining hardness (though in a different way).

 

Thanks all for your thoughts

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Okay, so it appears the sudden formation of martensite makes everything much more compressed because of the larger atomic structure, so tempering relieves this strain by what I am assuming to be an expansion of the actual atomic distances and does not affect grain size.

 

If I'm reading correctly, Tiny carbides also form which increase the hardness as well as the wear.. so I can see how this would offset the loss of hardness via the %reduction of martensite that the tempering also induces, in a sense reducing brittleness while also maintaining hardness (though in a different way).

 

Not quite. In tempering grain size shouldn't really change, except when you are changing phases. That means that when precipitating carbides (stage 1), changing carbide (stage 3), and decomposing austenite to ferrite and carbides (stage 2) the steel will be getting new grains which will start small and (potentially) grow. The existing stable grains won't change much (10nm carbide precipitations aren't really big worth mentioning in terms of grain size changes). Also the tiny carbides (the 10nm ones) will not do anything noticeable for your hardness and wear resistance; they are just too small to do much. Martensite is body center tetragonal. The steel wants to be in body center cubic, but the carbon is pinned in a position that stretches the atomic lattice. Tempering allows the lattice to settle a little, thereby reducing the stress on the bonds. This is the "relief of the high degree of volume strain".

 

The biggest factors to consider in tempering are the relief of stresses due to martensitic transformation and the decomposition/transformation of retained austenite. This is why 1060-1084 steels are so great to start with. There should be no retained austenite, so you are only worried about the relief of martensitic stresses, which are simple and straight forward (more heat means more stress relief). When we go to higher carbon (1095) and low alloy steels (52100 and such) we get the opportunity for retained austenite. Depending on the alloy the austenite may form carbide and ferrite, but it may also form bainite or martensite too! This is certainly the case with D2 and 440C where chrome carbides precipitate at grain boundaries and the resulting local change in chemistry produces instantaneous formation of untempered martensite around a ferrite cored grain. I have pictures if interested.

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So when you say "Tempering allows the lattice to settle a little, thereby reducing the stress on the bonds.", does this infer that it is merely the lattice of the martensite settling but it still fundamentally remains martensite? In other words, tempering does not actually revert martensite back?

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Absolutely! With the exception of the TINY carbide precipitations the martensite just relaxes, and gets less "sharp". When you look at martensite the grains are very "needle-like". Upon tempering they start to round, the more tempered the rounder they get. a 1350F temper REALLY changes things, but that is really out of the scope for what smiths are really trying to accomplish. The only thing that will change, or revert back, is austenite, which generally does not want to be stable at room temperature. It will "revert" to ferrite, bainite, carbide, and/or martensite.

 

It is often difficult to imagine where the carbon is in the lattice until it clicks for you. Essentially there are two different cube configurations and the carbon sits in a different place within the cube for each configuration. When the configuration changes from one to the other slowly the carbon moves to where it really wants to be. When it is changed rapidly the carbon gets stuck in between, creating a third configuration. When tempering we are imparting enough energy into the lattice to let the carbon get a little more comfortable, but not all the way to where it would really like to be.

 

This is what BCC (ferrite) looks like in 3D with "relatively accurate" sizes. The ping pong balls are Fe, or Mn, or something similar. The marble is carbon. Note how it can fit between things. The carbon wouldn't actually be right there, but it kind of gets the idea across.

WIN_20140327_205641.JPG

(Not my model. I shared the idea of using ping pong balls when I was giving an Intro to Metallurgy class to a machinists group and the instructor ran with it and created this.)

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  • 4 weeks later...

hardness and toughness are DIRECTLY inversely proportional to one another

It is a OK rule of thumb but isn't accurate in all situations. My own rule of thumb: I'd suggest some tempering even if you want ultimate hardness. It depends on the steel composition.
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