Steel: General information

E

Ed Schempp

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Oct 10, 2003
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I wanted to give a little information about steel; to help you better determine what works for you.

It only takes roughly .8% Carbon to make steel as hard as you can achieve.

Anything over this amount usually develops carbides: chrome, molybdium, vanadium, etc.

These Carbides are harder than steel. The Steel is the supporting Matrix for the carbides.

Not all steels of the same name are the same animal. The Chinese can make any chemistry or alloy they want, but it is not necessarily made by the same method. For example a top pour slag contaminated steel or a bottom pour vaccuum smelt. The performance of a name steel can very widely between manufactures.

Heat treating is an expensive part of the process to bring you a good knife. Some of the stainless steels at .4-.5 %C don't have to be tempered. Another way to cut cost.

Vanadium carbides are the hardest and give the most wear resistance.

CPM steels are at even a higher level of cleanliness and consistency. This process allows for a much higher level of Vanadium without degrading the crystaline structure of the steel. I hope this information helps in your steel evaluations...Ed
 
Thank you for the info, sir. By the way, I love your knives! Happy 4th of July! :cool:

Cheers,
3Guardsmen
 
More more more! Good stuff Ed.
 
Thanx Ed.

I would like to add:

Even a carbon contend of .6% can achieve 60HRC in blades and above is going into ferrum carbides.

I never heard, that you can avoid tempering. Any steel has to be tempered. But hardening .4 carbon is relativly hard work, because you must be so quick to quench it.

Vanadium carbides are the hardest and therefor most wear resistant but tend to form large carbides. A well respected alternativ is to go on tungsten carbides, which are slightly softer but smaller than vanadium.

A mixture of small and large carbides is a good choice for all round blades.

Not any tool steel gives a good knife steel. The higher wear resistant a steel is, the lower is it´s impact toughness. Tool steel can include very crude and large carbides, so you will hardly get an edge on them.

High alloyed steel shows best performance at high hardness. A D2 at 57HRC is wasted money.
 
Thanks Ed and Blop for sharing you knowledge. Always appreciated and treasured. :)
I read somewhere that excess of carbon tends to "steal" the chromium (which gives corrosion resistance), that's why they use nitrogen (for instance INFI or S30V).
 
Blop said:
I never heard, that you can avoid tempering. Any steel has to be tempered.
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Precipitation hardening steels do not need to be tempered.
 
Tempering and hardening are two different things. Tempering is the heating of a hardened steel to reduce hardness and increase toughness. My guess is that the reason why some low carbon stainless don't need to be tempered is because they can not be hardened as high to begin with.
 
Tempering is a process where the steel is heated to weaken bonds in the steel to make it tougher and not brittle. At .4% Carbon full hardness can be in the 46-54 RC range. This material will not get hard enough to have a problem being brittle. It depends on the alloy content of the .4%C material how quick you must quench. The addition of Manganese or Chrome will give the steel deep hardening ability and doesn't present too much problem getting it below the nose of the martinsitic transformation curve. I've made hammers out of 4140 that are full hard, about 46-48 and the RC scale. Sometimes these alloys have additional Carbon added to bring the quenched steel to a higher hardness; like 420 HC and 425 M, this addition of Carbon brings the steel up to a usable hardness say 56-58. This material may still not have to be tempered.

I spent some time at Qualtec with Steve Bailey their heat treating engineer at the time and we reviewed heat treating for various materials. We discussed the aformentioned materials and how Qualtec handled thier heat treating...Take Care...Ed
 
So there are other reasons for manufacturers using the 420 and 425 steels other than just the faster stamping out of the blanks, the fact that these steels are reportedly easier on their equipment and the steel coming to them in a roll form. The lack of need for tempering at times would certainly speed up production then.

Good info here. Thanks Ed.
 
HoB said:
Tempering is the heating of a hardened steel to reduce hardness and increase toughness.
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You temper martensite as it is highly unstable and overstressed. This changes the actual crystal structure from one orientation to another, and can also cause the formation of additional carbides as the primary ones dissolve and the secondary ones like Vanadium form. As the secondary carbides are much harder, this can induce a hardness bump in tempering and the hardness rises, as does the wear resistance and toughness.

There are many heat treatments that do not involve tempering because they don't form martensite and are age and precipitation hardening steels. Bainite heat treatment for example involves holding the steel during the quench at a point above where martensite will form and letting it transform from austenite to bainite.If steels don't have enough carbon to form martensite and instead form pearlite then then don't need to be tempered either.

-Cliff
 
When a high carbon, iron based material is quenched the structure shifts rapidly from one structure (Austenite) to (Martensite). The carbon atoms cannot keep up with the rapid shift in the iron molecules and get "caught" in non-ideal locations. It is the metallurgical equivalent of throwing sand into a gear box. High stress and high hardness is caused by the iron structure being locked up. Tempering allows the carbon to move to less stressful locations in the iron matrix. This reduces the stress in the matrix and in most cases reduces the hardness since the stressed matrix is relieved.

Primary carbides do NOT dissolve during tempering. Only at high temperatures such as austenitizing are they affected and only the smaller ones. Secondary carbides may form during tempering but it depends on the type of carbide and the temperature. Forming chromium carbide by precipitation is detrimental and should be avoided. This is why stainless steels and high carbon chromium steels are not to be tempered between 800 and 1000 F. Secondary carbides will not effect the overall wear resistance of an alloy since they are microscopic. The increase in wear resistance seen is typically due to the 2-3 point jump in hardness.

Most precipitation hardening steels are martensitic. They are limited in hardness. Most of them run around 40 HRC including the stainless steels.

Tempering is a very important part of the heat treat equation. Undertempering leaves steel very brittle. Too many people look for maximum hardness of alloys and in too many cases use a tempering temperature that is too low. Carbon needs energy to move to lower stress areas in the steel. Temperature and time are very important.
 
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Satrang said:
Primary carbides do NOT dissolve during tempering.
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"It has been shown that the nuclei form on the interfaces between cementite particles and the ferrite. As they grow, carbon is provided by the adjacent cementite, which gradually disappears."

This the primary iron carbide breaks down and its carbon actually is what makes up the secondary carbide. Ref :

http://www.key-to-steel.com/Articles/Art128.htm

That isn't where I read it origionally that was in a textbook, I don't have that reference at hand. The information is near identical however from memory.

Secondary carbides will not effect the overall wear resistance of an alloy since they are microscopic. The increase in wear resistance seen is typically due to the 2-3 point jump in hardness.
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How can you explain the fact that you can get hardness shifts above the martensite maximum if it isn't due directly to the secondary carbides which are much harder than martensite?

-Cliff
 
HoB said:
Tempering and hardening are two different things. Tempering is the heating of a hardened steel to reduce hardness and increase toughness. My guess is that the reason why some low carbon stainless don't need to be tempered is because they can not be hardened as high to begin with.
Click to expand...

In that topic about 440A Roman Landes gave advices how to treat a simple 420 type steel up to 59HRC.

I have to look what Precipitation is. :D
 
Secondary hardening comes from the precipitates putting strain on the matrix of the steel. That is why the secondary hardening range drops after the ideal peak. The precipitates coarsen to a point where they do not strain the matrix. Precipitated carbides are not big enough or in enough volume to directly effect wear resistance other than the hardness peak.

The reference in Key to Steel is for simple iron carbon systems. The formation energy of chromium, molybdenum, tunsten and vanadium carbide is higher so those carbides will always form before iron carbide. The primary carbides found in high alloy materials are stable and do not dissolve unless brought above normal tempering temperatures and for most such as vanadium, tungsten, types well into the austenite range. Primary vanadium carbides will not dissolve until the material is re-melted.
 
Satrang said:
Secondary hardening comes from the precipitates putting strain on the matrix of the steel.
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So what is the maximum hardness of martensite? Do you have a reference for the strain hardening arguement and the volume of secondary carbides?

The reference in Key to Steel is for simple iron carbon systems.
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No it doesn't :

" It would, therefore, be expected that when strong carbide forming elements are present in steel in sufficient concentration, their carbides would be formed in preference to cementite. Nevertheless, during the tempering of all alloy steels, alloy carbides do not form until the temperature range 500-600°C, because below this the metallic alloying elements cannot diffuse sufficiently rapidly to allow alloy carbides to nucleate."

Primary vanadium carbides will not dissolve until the material is re-melted.
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The primary carbides that dissolve are iron carbide. This is one of the formation cites for secondary carbide formation as noted, there are others, and yes there can be alloy carbide still present as it doesn't all have to be dissolved during the soak.

-Cliff
 
I'm glad this thread generated some questions and discussion. I appreciate the information. My original intent was to simplify some stuff about steel, as few are as passionate as myself on this topic. I tend to give people more information than they really want.

I have a challenge for the knowledgeable:

How we can augment my original post to give a non technical synopsis of steel?

Thanks Ed
 
For the user, much of the detals don't matter.

For example tempering causing secondary hardening can increase hardness and wear resistance. Does it matter to the guy holding the knife how the carbides form and how exactly the steel actually gets harder, not really. At a basic level does the user actually gain anything from simply knowing more than tempering just makes the knife "better" or that carbides are actually small hard particles and not magic fairies which keep an edge sharp.

Now you can make an arguement that the more you know the less you can be taken in by hype and this helps at times, but given with the huge amount of conflict among makers and users, this is really difficult and bottom line it doesn't help nearly as much as actually using a bunch of knives and then talking to the makers and manufacturers and see how they react to your questions/comments.

For example you can read about a certain steel or heat treatment offering superior performance. But if you actually use it, and the performance is low, and the maker/manufacturer won't actually support the claims - well what do you believe, what you read or can argue based on materials science or what you have seen. So much of performance can be effected in quality of implementation that you really need to use the knives. I have seen knives of the exact same steel be far too brittle, then far too soft and then perfect.

What should be important to the user is the consistency of heat treatment, what steps is the maker or manufacturer taking to actually insure that their knives were coming out uniformly as they are specified. It makes little difference if you develop a great heat treatment but can't consistently achive the same results. If one blade is at 55 HRC and the next at 59 HRC then your customers may see problems.

So aside from asking about how the steel is hardened, ask how many knives from each run are sample hardness tested and then evaluated for edge retention, impact toughness and flexibility.

-Cliff
 
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