Chemistry considerations in heat treating operations.

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Kevin R. Cashen

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For the purposes of this discussion I will divide alloying, beyond carbon, into two categories, those elements that contribute to carbide formation and those which reside in, and reinforce, the ferrite (iron). Carbides are extra particles of compounds within the steel that are extremely hard and contribute to abrasion resistance, grain refinement or strength at higher temperatures. Carbides require extra carbon and so carbide forming elements are rarely effective for more than increased hardenability in steels with .8% carbon or less and will rarely be seen in greater ammounts in those steels. As is covered in the introductory writings on building our heat treating tool box, steel itself is an alloy of iron and carbon, within which the carbon can either exist as a dissolved solute in a mixture with the iron or out of solution as a bonded compound of iron carbide (Fe3C) also known as cementite. But if there are other metallic elements present the carbon can combine to form other more complex carbides, these are the carbides discussed in this writing. One reason why the word cementite is used to describe iron carbide, is to easily differentiate it from the other carbides that can also occur within the same steel.

Carbide formers mentioned, in order of strength, are manganese (weak), chromium, molybdenum, tungsten, vanadium. Silicon and nickel do not reside in carbide but act to reinforce the ferrite (the iron portion of steel).

Normalizing:
Carbon- the more below .8%, the greater you can go in temperature to affect your desired changes, and may need to in order to fully distribute the carbon available. At .80% to .84% you have the freedom to go higher, but in the absence of other alloying you are free to stay lower. The more above .84% carbon, the more temperature it will take to get it all dissolved, but cooling rates become much more critical, so do not cool any slower than air cooling.

Chromium, tungsten, vanadium, and other carbide formers will increase the temperatures required to put things into solution so with each increase of these elements you will need an increase in normalizing temperatures.

Manganese and chromium will increase hardenability so do not expect steels with significant amounts of these elements to be softer after normalizing, and with increasing amounts you will see air hardening. When the steel reaches true air hardening levels, air cooling from full solution could result in cracking.

Annealing:
Carbon- the more below .8% the easier it will be to anneal the steel, very simple steels with no alloying and having this range of carbon may even able to go without annealing after a good normalization. At .80% to .84%, cooling from critical will yield total pearlite with no leftover iron or carbon making it the easiest for a traditional full anneal. Extremely slow cooling from high temperatures may result in some cutting resistance in machining. Above .84% carbon an you will need to avoid slow cooling from above critical in order to prevent large carbide formation and grain boundary cementite (iron carbide), do not put this steel in wood ash, vermiculite or a hot gas forge for the night, like other steels. It is better to repeatedly cycle it on the dull red range, without losing magnetism, several times to ball the carbon up and take it out of play.

Chromium, tungsten, vanadium and other carbide formers will help keep grains fine in annealing but will also create coarser and harder carbides in the annealing operation. Avoid extremely high annealing temperatures with these elements present.

High manganese and chromium levels will make annealing in traditional means much more difficult since cooling from above critical can result in hardening. For deeper hardening steels the subcritical annealing methods described above are best.

Hardening:
Carbon- the more below.8% the higher you may have to heat to get full solution. Some steel may be low enough in carbon to require greater heats to reach full hardness. A good soak time is important. At .80% to .84% you have more flexibility in time and temperature. Non-magnetic will be acceptable, as will 1500F if deeper hardening is desired. Soak times, while beneficial, are not as critical with simple carbon steels in this range. Above .84% carbon and it becomes increasingly important to stay below 1500F to avoid embrittlement issues, but more importantly to avoid lack of full hardness due to retained austenite. Lower temperatures for longer times are best for these steels.

Chromium, tungsten, vanadium and other carbide formers will cause resistance to proper solution depending on how thorough the annealing was because coarse carbide structures will take longer to dissolve. With extreme amounts of alloying the carbide bonding of carbon will somewhat cancel out the effects of carbon on heating considerations and greater temperatures or times may be needed to free the carbon for solution but for most basic alloy steels time adjustments are satisfactory .

Manganese, chromium and molybdenum are the primary contributors to deeper hardening. But manganese requires an excess of 1%, while chromium and molybdenum need perhaps half that in order to greatly increase hardenability. Steels containing these elements will not require the same quench speeds as those without them; thus many alloy steels are designated oil hardening. Steels with less than 0.8% carbon can tolerate, and may need, faster quenches and are more forgiving if there is a delay in tempering. Steels with 0.8% carbon or more should be tempered as soon as possible upon reaching hand warm, to room temperature, after the quench.

Tempering:
Carbon – will result in a resistance to hardness loss from tempering, increasing the necessary temperature higher for each increase in carbon over 0.8%. Steels with less than 0.8% carbon will display much greater drops in hardness at lower tempering temperatures.

Chromium, tungsten, vanadium and other carbide formers will result in a resistance to hardness loss from tempering to an even greater extent than that of carbon, with tungsten being particularly resistant to softening.


Recommended reading on this topic: “Alloying Elements In Steel” by E. C. Bain

For information on processing of some of the most common simple steels used by knifemakers I have these pages at my website:
At http://www.cashenblades.com/heattreatment.html you will find a description of the various heat treating processes, but at the bottom of the page you will see a columnar list of common steels, clicking on those steels will give a page with all the compiled information on the various operations and temperatures along with charts and other information. I hope this is helpful to anybody who can use it.



If this post, and the resulting thread, has information that you feel is useful enough to be linked to in the sticky index at the top of the page you may indicate that by voting in the pole. If you feel the information, or resulting thread, is not helpful, productive, or positive enough for all visitors to benefit from, do feel free to indicate that with a “no” vote in the pole. Both opinions will be regarded with equal value in ensuring only the best information is highlighted in this forum. To avoid a pole majority that does not reflect the forum majority (e.g. all “yes” votes, but only two people voted), and to encourage other input, only threads capable of generating at least two pages of interest should be considered for linking in the sticky. If you feel there is a better way to qualify a thread please please feel free to share your suggestions with me.

If I can help with any questions feel free to e-mail me at kevin@cashenblades.com
 
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Kevin, This is excellent, I really appreciate the format you've chosen. I've read this post a couple times this morning and the heat treat information on your personal site several times regarding my 2 chosen steels; O1 and Aldo's 1084. I'm blessed to be receiving a digital Sugar Creek Kiln for Christmas this year. I had thought I'd deciphered that 1500* for a 20 minute soak would be a great time and temperature for both steel types; that way I only have to use one program. The particular brand of O1 I am using lists carbon content at .95 so this statement "Above .84% carbon and it becomes increasingly important to stay below 1500F to avoid embrittlement issues, but more importantly to avoid lack of full hardness due to retained austenite. Lower temperatures for longer times are best for these steels." has me thinking 1475* may be a better choice.

I also have a question relating to tempering. You said “Steels with 0.8% carbon or more should be tempered as soon as possible after the quench.” I’m confused by this relating to Mf. Should I wait for Mf? Is it OK to wait to temper until my kiln cools so I can use it for the temper also? I’m not even positive of the Mf temp for O1 or 1084 as the charts aren’t exact. Room temp in my shop is about 25* right now and will be well over 100* in the summer. Should I get a mini fridge and put the summer room temp blades in it to get more consistent results with the winter results?

Any direction you could provide would be greatly appreciated. Thanks again for doing this!
 
Couple of small points:

Carbon – will result in a resistance to hardness loss from tempering, increasing the necessary temperature higher for every percentage greater than 0.8%. Steels with less than 0.8% carbon will display much greater drops in hardness at lower tempering temperatures.

By "every percentage" do you mean every .01%, every .1%, etc. Intuitively, the meaning is pretty clear but the wording could use some cleanup.

Chromium, tungsten, vanadium and other carbide formers will result in a resistance to hardness loss from tempering to any even greater extent than that of carbon, with tungsten being particularly resistant to softening.[/i]

Typo: "any even greater extent"

As a general question, are there limits on the amount of total alloying you consider this presentation useful for? Such as,
forging steels only, up to air hardening tool steels, stainless steels, etc. Do CPM steels and their ilk need to be treated differently?
 
Kevin, This is excellent, I really appreciate the format you've chosen. I've read this post a couple times this morning and the heat treat information on your personal site several times regarding my 2 chosen steels; O1 and Aldo's 1084. I'm blessed to be receiving a digital Sugar Creek Kiln for Christmas this year. I had thought I'd deciphered that 1500* for a 20 minute soak would be a great time and temperature for both steel types; that way I only have to use one program. The particular brand of O1 I am using lists carbon content at .95 so this statement "Above .84% carbon and it becomes increasingly important to stay below 1500F to avoid embrittlement issues, but more importantly to avoid lack of full hardness due to retained austenite. Lower temperatures for longer times are best for these steels." has me thinking 1475* may be a better choice.

I also have a question relating to tempering. You said “Steels with 0.8% carbon or more should be tempered as soon as possible after the quench.” I’m confused by this relating to Mf. Should I wait for Mf? Is it OK to wait to temper until my kiln cools so I can use it for the temper also? I’m not even positive of the Mf temp for O1 or 1084 as the charts aren’t exact. Room temp in my shop is about 25* right now and will be well over 100* in the summer. Should I get a mini fridge and put the summer room temp blades in it to get more consistent results with the winter results?

Any direction you could provide would be greatly appreciated. Thanks again for doing this!

TASelf, if you would like to e-mail me (kevin@cashenblades.com) I would be happy to discuss your questions.
 
Great stuff Kevin!

I certainly voted to have the thread indexed! Very well worded. Although there might be many questions from those with limited experience, Once a person goes through the process(es) several times, the information you laid out will become more and more understandable/evident.

Kudos!
 
I definitely voted for the thread to be indexed. I don't understand all of it completely but it will cause me to further research the parts I don't understand. Thanks, Kevin, the road to knowledge is always under construction.
 
A vote for indexing also. For myself I find a well written general overview before getting into further detail helps me learn faster. I'm still a beginner at this and I think your explanations are very clear
 
Kevin,
Thank YOU!!!! I'm going to print this off, so I can study it and add it to my testing, and personal research pieces. Thats just how I learn these days, by doing it and taking note. With brain damage, memory is a bit of an issue and not to be relied on!!!
Thanks for taking the time to help all of us knuckle heads, Jar Heads included!
Thanks Bro`, Rex
 
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