Heat Treatment Basics

[Willie71]

As I mentioned in another post, going through the very well written and informative stickies gave me a lot of great information, but it took several reads to get the basic concepts sorted out in my head. Here is what I narrowed it down to in the most basic form possible:

Metallurgy 101:

Steel loses its magnetism at 1414f.

Normalizing is the process of restoring the steel to an even, regular grain structure. Grain growth is a byproduct of too much heat. It is common during forging and welding. The specific normalizing temps and rates of temperature change is specific to each alloy. The normalizing processes is achieved by getting the metal to a specific temperature above the nonmagnetic temperature and cooling at a controlled rate. Time at that temp and rate of cooling is known for each steel. Normalizing is similar to annealing, but results in a harder and stronger steel than annealing. See chart here: http://info.lu.farmingdale.edu/depts/met/met205/normalizing.html

Annealing: (Quoted directly from the stickies) Full annealing is the process by which the distorted cold worked lattice structure is changed back to one which is strain free through the application of heat. This process is carried out entirely in the solid state and is usually followed by slow cooling in the furnace from the desired temperature. If I am understanding this correctly, annealing will repair the structure of the steel after cold working, whereas normalizing the steel is repairing the problem with grain growth. Also quoted from the stickies: Stress-Relief Annealing is sometimes called subcritical annealing, is useful in removing residual stresses due to heavy machining or other cold-working processes. It is usually carried out at temperatures below the LCT, which is usually selected around 1000oF. LCT is "lower critical temperature."

Normalizing and Annealing are basically the same process when dealing with low carbon steels, heating to about 100f above upper critical temperature and slowly cooling. Normalizing happens when cooling in air, and annealing is slow cooling in the furnace. When dealing with high carbon steels, there is a a difference, and spheroidizing annealing happens below the critical temperature for prolonged time. It is used to increase the machinability of the steel. Full annealing happens above LCT, but as the carbon content increases, the UCT increases, so the annealing temp is below the UCT with higher carbon steels. See chart here: http://info.lu.farmingdale.edu/depts/met/met205/ANNEALING.html Note the difference between the temperature range in high carbon steels from normalizing compared to annealing.

Hardening or heat treating is going a certain amount past 1414f, usually about 100f or so depending on the steel, then as quickly as possible quenched in the correct medium such as water, slow oil or fast oil, or air depending on the steel. The temp of the quench solution is a controlled variable. This is the process that gives blades and other hardened tools such as files their hardness. The metal can be too brittle at the hardness that is reached.

Tempering is the process of reducing the hardness by basically baking the steel at lower temps such as 300f or 450f depending on the desired hardness (often referred to as Rockwell rating.) It is different from normalizing as the grain isn't put back to its pre grown state, just a reduction in hardness.

The sequence of these processes was provided by Stacy in a post below:



"The things that need to be learned and understood are:
Choosing the right steel - Sounds simple, but picking the right steel for the task the knife will do, and using a steel that fits the equipment and procedures you are capable of using is often not done.
Pre-conditioning the steel for working it - This covers the normalizing after forging and grinding, spheroidization before grinding, and any annealing steps needed for forming and shaping.
Preparing the steel for HT - These steps are really part of the HT. This includes normalizing the steel, and refining the grain. At the end of these steps, the steel should have a fine grain structure ready to be austenitized and then converted to martensite.
Hardening the steel - As said, this is just the continuation of the preceding step. In hardening the steel is taken above the critical point and converted to austenite, and then cooled at a rate to yield the maximum amount of martensite while incurring the minimal amount of stress. The temperatures, cooling media and rates are very specific to the steel type used.
Tempering the steel - After hardening the steel needs to be converted from brittle martensite to tempered martensite. This increases the toughness and decreases the brittleness. It also slightly decreases the hardness, but the increase in toughness makes up for that. Another thing that may be happening in the temper is conversion of retained austenite. This converts to new martensite, and will be tempered in the second temper cycle."

There is a lot more to this, but this is the basic idea.

 

[Bo T]

"Annealing is the process of reducing the hardness by basically baking the steel at lower temps such as 300f or 450f depending on the desired hardness (often referred to as Rockwell rating.) It is different from normalizing as the grain isn't put back to its pre grown state, just a reduction in hardness."

This process is called tempering. Annealing is where you soften the blade by time controlled cooling from the critical temperature.

 

[redd1981]

Your definition of annealing sounds like my definition of tempering.

From what I have read,annealing is raising a steel to critical and cooling as slowly as possible to make it as soft as it can be while cold.

I have also heard people use the term "annealing" when they are normalizing.

Im sure someone who knows more than we do will be along shortly to straighten us both out.


EDIT: hehe looks like BO beat me to it.

 

[Willie71]

Corrected via edit. Thank you. What is the difference between annealing and normalizing? As information is clarified, the original post will be edited to reflect the corrections.

 

[redd1981]

Im pretty sure your definition of normalizing is pretty much correct.... stress reduction and grain refinement.

annealing makes the steel in a softer state for drilling/grinding.

 

[Bo T]

The crystalline structure that one wants upon annealing is a type of pearlite that is more easily shaped when cold. I think it is called spherezoid?? It is easier to cut and grind and file. After the blade is shaped, it often is normalized which, hopefully, lowers the grain size. It is often referred to as grain refinement. The goal is to get the austenitic grain as small and uniform as possible before quenching which will lead, again hopefully, to a small and uniform martensitic grain structure. This in turn is said to lead to a tougher knife at a given hardness.

 

[Don Nguyen]

Properly annealed steel will result in Spheroidite, which has small and evenly distributed carbides.

 

[Willie71]

Directly quoted from the stickies:

The following is the list of the reasons for normalizing the steel :

To produce a harder and stronger steel than full annealing

To improve the machinability

To modify and refine the grain structure

To obtain a relatively good ductility without reducing the hardness and strength

 

[Don Nguyen]

I think the point about normalizing for strength compared to annealing is a bit weird. Yes, it results in pearlite, which has greater strength than that of spheroidite, but pearlite is kind of considered the "default" of steel. So instead of looking at it in terms of normalizing for "strengthening annealed steel", it's more in the thought process of annealing for "softening (other) steel".

I could be wrong about this, but I'm writing it to find out if I'm wrong.

 

[Bo T]

This is a great resource for information. I downloaded it off the internet. It is free for personal use. There seems to be a lot going on in the ferrite + cementite matrix. And ending up with spheroidite imparts all of the characteristics that you refer to.

Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel
John D. Verhoeven
Emeritus Professor
Iowa State University

 

[Willie71]

From what I am reading, full annealing looks similar to normalizing, where subcritical annealing looks similar to tempering in terms of process, but temperature and rated of temperature change, and the effect on the metal differ. I suspect this is where some of the confusion in the terms comes from. I originally read about annealing in terms of subcritical annealing.

 

[me2]

When reading the general definitions, keep in mind that they apply to all steels, while we are concerned with only blade steels here. Full annealing of 5160 vs 1095 results in quite different structures. Annealing also applies to softening of any metal after cold working. There is a little industry specific terminology as well. Bladesmith's normalizing is not the same as industrial normalizing, as industrial normalizing involves dissolving all the carbides (if present) into solution and then air cooling to get a fine and evenly distributed carbide structure (again if present), while also ending up with even grain size. Not necessarily smaller, because of the relatively higher temperatures, but even. While it's true that high temperatures contribute to grain growth, very few people talk about the bad mojo from an uneven grain size. The larger grains gobble up the smaller ones and get even bigger at an alarming rate. You'll see directions "Do not normalize" on some steels, particularly air hardening stainless and tool steels. They can't be normalized in the industrial sense because dissolving all the carbides will require temperatures high enough to damage the steel, or even start to melt it.

 

[Stacy E. Apelt - Bladesmith]

From what I am reading, full annealing is similar to normalizing, where subcritical annealing is similar to tempering in terms of process. temperature and rated of temperature change differ. I suspect this is where some of the confusion in the terms comes from. I originally read about annealing in terms of subcritical annealing.

Willie,
Sorry, but they are very different things. Similar is way too broad a term for these comparisons. In the same way one could say boys and girls are similar ( one head, two arms, etc.....with only minor other differences )
The subject is pretty well covered in the metallurgy stickies, and a good book on basic metallurgy will also help.

While I understand the desire to find a shorter learning curve, trying to sum up blade metallurgy into a few short paragraphs is like summing up brain surgery as:
Cut skull open
Remove tumor
Close up skull

I have written some pretty lengthy treatises on the subject, most of which can be found in the stickies or by search.
The things that need to be learned and understood are:
Choosing the right steel - Sounds simple, but picking the right steel for the task the knife will do, and using a steel that fits the equipment and procedures you are capable of using is often not done.
Pre-conditioning the steel for working it - This covers the normalizing after forging and grinding, spheroidization before grinding, and any annealing steps needed for forming and shaping.
Preparing the steel for HT - These steps are really part of the HT. This includes normalizing the steel, and refining the grain. At the end of these steps, the steel should have a fine grain structure ready to be austenitized and then converted to martensite.
Hardening the steel - As said, this is just the continuation of the preceding step. In hardening the steel is taken above the critical point and converted to austenite, and then cooled at a rate to yield the maximum amount of martensite while incurring the minimal amount of stress. The temperatures, cooling media and rates are very specific to the steel type used.
Tempering the steel - After hardening the steel needs to be converted from brittle martensite to tempered martensite. This increases the toughness and decreases the brittleness. It also slightly decreases the hardness, but the increase in toughness makes up for that. Another thing that may be happening in the temper is conversion of retained austenite. This converts to new martensite, and will be tempered in the second temper cycle.

 

[Willie71]

I will edit that sentence. I meant to draw attention to the similarities in process, but they are different processes with different intentions. I certainly do not mean to suggest what is being written here is all that one needs to know, but as an overview as what one would find at the beginning or end of a chapter in a text book. Seeing how the processes relate to each other and how they differ is what I am trying to help new people like myself get a handle on.

 

[Stacy E. Apelt - Bladesmith]

Seeing how the processes relate to each other and how they differ is what I am trying to help new people like myself get a handle on. .
That is what I was doing here:

The things that need to be learned and understood are:
Choosing the right steel - Sounds simple, but picking the right steel for the task the knife will do, and using a steel that fits the equipment and procedures you are capable of using is often not done.
Pre-conditioning the steel for working it - This covers the normalizing after forging and grinding, spheroidization before grinding, and any annealing steps needed for forming and shaping.
Preparing the steel for HT - These steps are really part of the HT. This includes normalizing the steel, and refining the grain. At the end of these steps, the steel should have a fine grain structure ready to be austenitized and then converted to martensite.
Hardening the steel - As said, this is just the continuation of the preceding step. In hardening the steel is taken above the critical point and converted to austenite, and then cooled at a rate to yield the maximum amount of martensite while incurring the minimal amount of stress. The temperatures, cooling media and rates are very specific to the steel type used.
Tempering the steel - After hardening the steel needs to be converted from brittle martensite to tempered martensite. This increases the toughness and decreases the brittleness. It also slightly decreases the hardness, but the increase in toughness makes up for that. Another thing that may be happening in the temper is conversion of retained austenite. This converts to new martensite, and will be tempered in the second temper cycle.

 

[Willie71]

Stacy, the sequence you provided is very helpful. Unfortunately, it does not explain in basic terms what needs to happen to the steel in terms of temperature and time to carry out those processes. As someone who is new, and learning, figuring out what is happening, along with how to make it happen is part of the curve. Getting a sense of the temperatures involves, the margin of error, and the time temperatures need to be sustained greatly affects equipment choice and what steels one can successfully work with. With a non PID forge, with an accurate pyrometer, I would only feel confident with heat treatment and normalizing. I would think I would need better sustained control to do a full anneal. A simple post like the one at the top cannot replace the full knowledge needed to be successful working with steel, but it can give people enough information to know what the steps are, what they need to consider at each step, and what type of equipment will allow them to successfully carry out each process, so they can further research those steps specific to the steel they are working with.

In a similar vein, I do inservices on how to do therapy for new therapists. A one hour discussion cannot give anyone the needed information to do therapy, (similar, but not the same as your example on brain surgery) but the basics of deciding what type of therapy is warranted for the presenting client is possible. From there, the therapist can followup with the needed training and supervision.

 

[Bo T]

Stacy, the sequence you provided is very helpful. Unfortunately, it does not explain in basic terms what needs to happen to the steel in terms of temperature and time to carry out those processes. As someone who is new, and learning, figuring out what is happening, along with how to make it happen is part of the curve. Getting a sense of the temperatures involves, the margin of error, and the time temperatures need to be sustained greatly affects equipment choice and what steels one can successfully work with. With a non PID forge, with an accurate pyrometer, I would only feel confident with heat treatment and normalizing. I would think I would need better sustained control to do a full anneal. A simple post like the one at the top cannot replace the full knowledge needed to be successful working with steel, but it can give people enough information to know what the steps are, what they need to consider at each step, and what type of equipment will allow them to successfully carry out each process, so they can further research those steps specific to the steel they are working with.

This is an issue that I struggle with. If you check out the acceptable parameters for a steel like W1 you will find a large acceptable range for the amount of carbon in the steel. Steel on the outside of the billet will be different that steel on the inside of the billet. The larger the billet the greater the possible variance. So a protocol that someone established (experimentally) for the W1 that they were using might not work as well for the W1 that someone else is using. This applies to an extent to all steels. The margin of error can be very large before you start working the steel. An excellent exercise (that I do not have the patience for) would be to examine the normalization and annealing protocol for W1 steel at the low acceptable value for carbon and at the high acceptable value for carbon. One suggestion that I see over and over again is test it.

 

[Willie71]

From my very limited understanding, even steels that are named the same may respond differently than the same named steel from a different batch. Ultimately, a Rockwell tester would be ideal, but is a big investment early on in the process. I was discussing this with my wife, and she agreed that in future, to make sure you are providing what you say you are, having an actual test to prove your process resulted in what you want it to is necessary. I would think that entry level makers, who sell for less than $200.00 per knife would not need that specificity. If selling a $400,00 knife, I would think the buyer would want to know exactly what they were getting. When I build bicycle frames, I use slightly different heat settings from one brand or batch of brazing rod to the next. This is where experience comes in. I wouldn't see any "how to" as 100% repeatable, but as a starting point to refine your own process.

 

[Stacy E. Apelt - Bladesmith]

All I can say is read the fuller versions that you said are confusing, or get a book or two on HT and read them several times. HT and metallurgy are a complex subject and there is a lot to absorb. Some folks never seem to be able to wrap their heads around it. That is why I have posts like Metallurgy for Dummies.

Whenever I get my book finished ,( I know, I know), this subject will be dealt with on two levels. The "How to do it" instructions, and the "What is happening" explanations.

 

[Willie71]

The issue isn't so much that the information is confusing, its the sheer volume of it that takes a while to organize. As mentioned before, adult learners organize information in a visual spatial way moreso than an auditory sequential way. When the volume of information goes up, the need for a categorical framework goes up as well. Its the difference between deductive and inductive reasoning. Without the framework, there is no where for the adult learner to store information in working memory while comparing and contrasting ideas. If you notice in my posts, there is white space, clear headings, and comparisons of ideas. I have stated several times that the attempt here is to be helpful in providing a framework for learning, not to replace the need for the full knowledge. Australia is the world leader in educational reform, with Canada a bit behind, but still in the top 10. I suspect you are taking offense to my attempt to help with my knowledge of the psychology of learning. I do not intend that or mean this as a criticism. I am aware that what I am talking about isn't well known in many countries. I can guarantee the psychology of learning is every bit as complicated as metallurgy, and is a specialty field. The traditional lecture based education that is prevalent in most post secondary institutions has been shown to be one of the least effective methods of teaching, yet most educational facilities and personnel become very defensive when presented with this information. Most students learn the material well enough for their papers or exams, but the learning does not turn into functional knowledge and stay retained in students' heads.