Thanks to Jake Smith, Erik Mittag-Leffler, Ashley Wagner, David W. Debora Richards, gspam1, James Straub, Tim Ottawa, Flynn Sharp Knives, David Millington, Joseph Baier, Alex Roy, and Noah for becoming Knife Steel Nerds Patreon supporters! These experiments are very expensive and time consuming so your support is always appreciated.
Podcast Appearances
I appeared on two different podcasts since my last article. On Knife Junkie Podcast we discussed MagnaCut and a range of other topics.
On Mark of the Maker we discussed “Metallurgy Mythbusters” where we went over a bunch of different heat treating myths.
Video Version
I have a YouTube version of the following information in a video below. The content is somewhat different between the video and the article so you may like both.
Thermal Cycling
“Thermal cycling” is very non-specific, it just means heating and cooling steel. It probably isn’t specific enough for our purposes, but it is a common phrase used by knifemakers to refer to the steps they perform after forging but before the final heat treatment. The final heat treatment being the austenitize and quench for full hardness. Some knifemakers are also performing very non-specific cycling, however, with no clear purpose to what they are doing. Instead metallurgists perform specific cycles called normalizing and annealing. This sets up the steel to be soft for machining, drilling, cutting, etc. And for good heat treatment response for hardness, wear resistance, and toughness. Typically no “grain refinement” cycles are added though those are also very popular with bladesmiths so I have shown where they should go in the schematic diagrams below:
Grain Size after Forging
One change that occurs during forging is grain growth. Forging is typically done from very high temperature and with low alloy steels grain growth is very rapid in that temperature range. The amount (and speed) of deformation during forging a knife is not typically great enough to compensate for the effects of temperature. So after forging the grain size is typically relatively large. Here are example micrographs of a low carbon steel after being heated to different temperatures [1]. This steel actually has some micro-alloy added to it to help prevent grain growth so these grains are probably smaller than would be found in steels like 1095 after heating to the same temperatures.
920°C (1685°F) [1]
1000°C (1830°F) [1]
1150°C (2100°F) [1]
1300°C (2370°F) [1]
Another method for estimating grain size is looking at the “fracture grain.” If you break a piece of steel in a brittle condition the appearance of the fracture will correlate with its grain size. The steel must be brittle. Tempered steel and especially soft/annealed/normalized steel will break in a ductile fashion and the fracture will not correlate with the grain size. Below are example fractures from 1084 which were heated to different temperatures for 10 minutes, quenched, and then broken.
1475°F (800°C)
1700°F (925°C)
2000°F (1095°C)
A completely smooth specimen is given a fracture grain size rating of 10 and the coarsest possible appearance is given a rating of 1. This also correlates with ASTM grain sizes. A fracture rating of 10 is approximately equivalent to an ASTM grain size of 10. The ASTM grain size can be converted to an average grain diameter using charts that are available everywhere or by using the conversion equation. Below shows fracture grain size vs temperature for a range of different tool steels. The simple carbon steel W1 sees grain growth at relatively low temperature as you can see. Adding some alloy such as in O7 which has W, V, and Cr leads to less grain growth. A high alloy steel like D2 or T1 sees grain growth at much higher temperatures.
Image from [2]
Microstructure After Forging
The microstructure in terms of what phases are present can vary quite a bit after forging. Mixtures of pearlite, ferrite, austenite, martensite, and carbide can be found with different distributions depending on the forging process, steel, and how fast it was cooled after forging. Below shows the inconsistent microstructure found in O1 and L6 steels after forging.
O1 after forging [3]
L6 after forging [3]
Grain Boundary Carbides
Another issue that can happen with steels during forging is the formation of carbides along the grain boundaries. These are detrimental to toughness and we definitely want to eliminate them.
Carbides on grain boundaries [4]
Heating Up During Normalizing
So in normalizing we want to reduce the grain size from forging, achieve a consistent microstructure, and eliminate grain boundary carbide. While using low temperatures is best for fine grains, the most important thing we need to do is dissolve everything so we can eliminate the inconsistent microstructure and grain boundary carbide. The higher the carbon content, beyond 0.75% carbon, the higher the temperature required for dissolving all of the carbide.
Those are temperatures shown on top of the “equilibrium” phase diagram, meaning very long hold times. Since we will hold for 60 minutes or less for normalizing the actual temperatures we use will be higher to ensure the carbide is dissolved. While carbon is the most important element for dictating the normalizing temperature, other alloying elements can also affect the required temperature. One common one is chromium, which shifts up the required temperatures. This is why 52100 requires higher normalizing temperatures than other simple steels with similar carbon content.
Below shows schematic diagrams for what occurs during heating for normalizing, where first the ferrite/pearlite is dissolved and replaced with austenite, and then with higher temperature the last lingering carbides are dissolved.
Normalizing Temperatures
Below shows recommended temperatures for normalizing from my book Knife Engineering. For many of these you can actually normalize up to 100°F higher than this temperature to ensure everything is dissolved. Heat for 10-15 minutes and then air cool to normalize.
Cooling Down During Normalizing
After we have a microstructure of austenite we want to cool in air to form pearlite. This will leave us with a consistent microstructure going in to our next steps. Carbide and ferrite forms simultaneously on the austenite grain boundaries. Ferrite can accommodate almost no carbon, so the carbon leaves austenite by forming carbides. This creates carbon-lean regions where the ferrite can form. Therefore alternating bands of ferrite/carbide form which is a structure called pearlite.
Adapted from [5]
This same process occurs throughout the steel, with new pearlite grains nucleating at grain boundaries and growing to consume the austenite and replace it with pearlite.
Adapted from [6]
Different cooling rates lead to different hardness. A fast cooling rate leads to finer pearlite and higher hardness. A slow cooling rate leads to coarser pearlite and lower hardness. When air cooling you don’t necessarily have control over the cooling rate, but with thicker or thinner knives this will affect the cooling rate and the resulting microstructure and hardness.
Hardness of 1095 after normalizing with different cooling rates (generated from CCT curves)
1080 steel air cooled [7]
1080 steel furnace cooled [7]
With different steels the hardness of the pearlitic structure is also different depending on the hardenability of the steel. High hardenability steel like O1 will have a relatively high hardness and very fine pearlite. Low hardenability steel like 1095 is lower in hardness. If the O1 is very thin then some martensite may form. If the hardenability is very high like L6 or an air hardening steel then even more martensite forms. We prefer pearlite if possible with normalizing. Below are hardness measurements I made after air cooling 1/8″ stock knife steels:
Annealing
There are several different ways of annealing steel which I have previously covered in two articles: Part 1 and Part 2. But here I will give a brief summary of the different types. Generally what we are looking for is a “spheroidized” structure rather than pearlite. The round carbides are better for machining than pearlite. And the heat treating response is somewhat different.
Subcritical Anneal
If you heat the normalized steel and heat it to high temperature below austenite transformation and hold, it will spontaneously spheroidize. Below are images of 1080 steel being held at 1200°F (650°C) for different times [4].
Normalized [4]
4 hours [4]
16 hours [4]
64 hours [4]
240 hours [4]
360 hours (double magnification) [4]
Temper Anneal
A temper anneal is similar to a subcritical anneal in that you hold the steel just below the critical temperature to reduce hardness. The difference is that you heat the steel above critical and quench first. So it becomes a very high temperature temper to lead to low hardness steel and a carbide plus ferrite microstructure. Carbon precipitates out of the martensite and coarsens. Coarser carbide and less carbon in the martensite means lower hardness.
1084 tempering curve (ASM heat treater’s guide)
400°C (750°F) [4]
500°C (930°F) [4]
600°C (1100°F) [4]
700°C (1300°F) [4]
Transformation Anneal (Divorced Eutectoid Transformation)
The most common anneal is called a transformation anneal also called a divorced eutectoid transformation (DET) anneal. You heat above critical but not so high that you dissolve all of the carbides. And then during slow cooling the steel transforms to ferrite while growing the carbides by diffusing carbon to the carbides.
Image from [8]
The temperature of annealing is very important because if you heat to too high a temperature then too much carbide is dissolved. Without carbide it is more favorable for the steel to form pearlite. Below shows 1080 steel annealed from different temperatures.
1080 steel annealed from 1385°F (a), 1450°F (b), 1600°F (c), 1750°F (d). Image from [9].
Here are recommended annealing temperatures from Knife Engineering for a good DET anneal.
The cooling rate of the anneal is important to the final hardness and microstructure. Datasheets and steel manufacturers use very slow cooling rates that lead to a relatively coarse structure. This is the lowest hardness and most machinable structure. However, with a faster cooling rate the structure is finer which can have certain benefits. Below shows 52100 as-received compared with 52100 that I annealed by placing in vermiculite. You can use cooling rates at about 600°F/hr (300°C/hr) for most low alloy steels.
52100 steel as-received from the steel manufacturer (“slow” DET anneal)
52100 with “fast” DET anneal
Forge Heat Treating
One of the things I wanted to see is how much the prior microstructure affects the response to heat treatment. This is particularly important when heat treating without temperature control where you can’t easily do a soak at the appropriate temperature. The general recommendation to new knifemakers heat treating with a forge is to heat 100-150°F (50-80°C) higher than non-magnetic before quenching. Trying to heat some amount over non-magnetic leaves a lot to skill (and chance) and so I don’t like recommending to people that they perform forge heat treatments. Furthermore, one knifemaker once contacted me about testing 52100 that he had heat treated in a forge to look at different parameters and the toughness was horrible. The fracture grain was very coarse confirming they were overheated.
On the left is overaustenitized steel from the knifemaker heat treating with a forge. On the right is furnace heat treated 52100 with an appropriate austenitizing temperature.
The Experiment
But if the steel could be heated to non-magnetic and quenched for the appropriate microstructure without some unknown degree of heating beyond that, then we could remove a lot of the guesswork. The steel becomes non-magnetic when it has transformed to austenite. Read this article for more on checking steel with a magnet and what happens in the steel. I performed experiments with 52100 and 1084 from Alpha Knife Supply.
With both steels I overheated them at 2100°F (1150°C) for one hour and air cooled. Both were normalized, the 52100 at 1700°F (925°C) and the 1084 at 1550°F (845°C) for 15 minutes and air cooled. That is the normalized condition that I tested. I also tested them with a “fast DET” anneal where they were cooled at 600°F/hr (300°C/hr) after holding at 1450°F for 52100 and 1385°F for 1084, both for 15 minutes. Those temperatures are right about at non-magnetic. These were also compared to the as-received condition from the steel company which has a coarser microstructure as I originally stated.
1084 steel as-received (slow DET anneal). Coarse spheroidized carbides.
1084 normalized. Pearlite microstructure. The diagonal streaks are scratches because I struggled to get a good polish on these soft specimens.
1084 with fast DET anneal. There is some evidence of pearlite so it is probably good that the recommended annealing chart from Knife Engineering is at a slightly lower temperature of 1365°F.
Annealed 1084 showing the decarb layer where grains are visible. This isn’t exactly the same grain size as the rest of the structure because the transformations between carbide/pearlite and austenite also affect the grain size. However, it can provide some indication of the grain size of the bulk material. This is around a 9 ASTM grain size which is very fine. So the normalize and anneal was effective in refining the grain size after the high temperature grain growth treatment.
Quenching and Hardness Testing
I heated each steel to non-magnetic, which was about 1385°F for 1084 and 1445°F for 52100. I went up in 20°F increments until each became non-magnetic. I quenched each in Parks 50 and measured the hardness.
1084 had only 1 Rc difference in as-quenched hardness whether it was normalized for pearlite or heat treated from the as-received condition. It is a simple carbon steel so it is primarily carbon with some Mn and Si added. Carbon diffusion is very fast so even with a coarse spheroidized microstructure the final hardness is comparable.
1084 hardness hardness after quenching from just above non-magnetic:
52100, however, showed relatively significant differences between different prior microstructures. From the normalized condition it was a full 67 Rc after quenching. With the as-received microstructure it was only 60.6 Rc. The fast DET was in the middle though closer to the as-received than to the normalized condition. The chromium addition to 52100 delays the “kinetics” (speed) of the transformation.
52100 hardness after quenching from just above non-magnetic:
The coarse spheroidized structure leads to a relatively long distance that diffusion has to occur. Pearlite is a finer structure where transformation can occur more rapidly. Martensite (quenched steel) essentially has perfectly evenly distributed carbon. If you temper high enough and long enough you do get a structure that starts to look more like a fast DET anneal, however.
Spheroidized carbide schematic
Schematic of pearlite
Schematic of quenched martensite
Effect of Prior Microstructure on 52100 Toughness
In a previous experiment with 52100 steel we found a significant difference in toughness between steel heat treated from the as-received condition and the “fast DET” annealed condition. The finer resulting microstructure apparently increased toughness. However, the hardness of the steel from the as-received condition is 1-2 Rc lower than I would expect. Perhaps the steel was particularly coarse from the manufacturer. Read more about these experiments in this article on heat treating 52100.
Effect of Prior Microstructure on CruForgeV Toughness
We also did a series of experiments on a few years ago with CruForgeV which you can read about in this article. To that set of experiments I added a toughness test from the as-received condition to go with it in the past couple weeks for this article. In the original experiment the steel was forged between 1550°F/845°C and 2000°F/1100°C (the effect of forging temperature was one of the tests performed). Each was normalized from 1600°F/870°C. Then they were given three different anneals:
Subcritical anneal: 1250°F/675°C for 2.5 hours. I don’t think this was long enough for significant spheroidization so it was probably mostly pearlite.
Temper anneal: 1450°F (785°C) for 10 minutes, quench, then 1250°F for 2.5 hours.
Fast DET anneal: 1460°F for 30 minutes followed by 670°F/hr (375°C/hr).
The hardness after 1500°F for 10 minutes, Parks 50 quench, and temper at 400°F is shown below:
Then below is the resulting toughness for the different conditions. This also includes other heat treatments with the fast DET condition to show the overall trends of hardness vs toughness:
So the overall toughness-hardness balance was not really affected by the different prior processing but the finer microstructure anneals did lead to higher hardness as expected. However, the prior microstructure does change the optimal austenitizing temperature range. The datasheet for CruForgeV recommends 1500-1550°F. However, using 1450°F with the fast DET led to similar hardness-toughness to using 1500°F with the as-received condition. And using 1550°F with the fast DET prior microstructure led to terrible toughness. So having the finer microstructure of the fast DET led to a drop of about 50°F in the optimal austenitizing temperature range.
Choosing Between Normalized and Annealed Steel for Final Heat Treatment
One reason that I prefer the fast DET rather than a subcritical anneal, temper anneal, or normalized structure is because of the difference in heat treatment response. When you have control over temperature with a furnace there is a range of hardness and microstructure that you can choose between. But with a pearlitic or martensitic starting microstructure you are basically limited to high hardness. Tempering higher to bring the hardness down would lead to issues with tempered martensite embrittlement. However, as I said before when heat treating in a forge the benefits of pearlite not requiring a soak is probably more useful. See the chart below for 52100:
Adapted from [10]
Cycling for Grain Refinement
As discussed previously in this article, grain refining cycles can be performed in between normalizing and annealing. This is like a normalizing cycle but is done from a lower temperature, such as 1400-1450°F. Cycling from low temperature has been shown to lead to finer grain size such as in the 1060 steel below which was cycled from 1490°F (810°C) [11]:
Overheated at 2000°F/1100°C (a), 1-cycle (b), 3-cycles (c), 5 cycles (d), 8 cycles (e). Image from [11].
Effect of Cycling on Carbide Structure
However, grain refinement is not the only change to the microstructure. This cycling treatment with the 1060 also led to spheroidization of the carbide. After several cycles the microstructure looked pretty close to a fast DET structure.
One cycle (top), 5 cycles (middle), 8 cycles (bottom). Images from [11].
Effect of “Grain Refining” Cycles on Heat Treatment Response
So while I think a pearlite structure is best for a forge heat treatment perhaps we would want to add a grain refining cycle or two to help with the grain size. In my previous 52100 samples I also tried a grain refining cycle from 1445°F after normalizing and compared the hardness to see if there was a difference:
The hardness was somewhat reduced when compared with the original normalized steel. To determine why I looked at the microstructure and found that the steel had partially spheroidized even after only one “grain refining” cycle:
Effect of “Grain Refining” Cycles on 1084 Toughness
I did a set of experiments on 1084 with different prior processing to see if grain refining cycles can improve toughness. For each I heated them to 1475°F for 10 minutes, quenched in Parks 50, and tempered at 400°F/205°C. For one condition I used the steel “as-received” from the steel company. In the other two I overheated both at 2100°F for an hour to simulate grain growth from forging. In one I annealed from 1380°F with no normalizing or grain refining. In the other I normalized from 1550°F, then did two grain refining cycles from 1450°F, and finally ended with the same anneal as the other specimen. The hardness was a point higher on the two specimens that I annealed because of the finer microstructure from the fast anneal. However, the toughness was no different whether I did the grain refining cycles or not. The fracture grain of all of the specimens was fine so it could be that there was no difference as long as the final austenitize was done correctly. Or perhaps the anneal leading to a fine distribution of carbides in combination with a fine grain size meant that the prior normalizing and grain refining had no benefit.
1084 steel overheated and annealed prior to austenitizing
1084 steel overheated, normalized, “grain refined” and austenitized
I still recommend normalizing as that is for the purpose of dissolving everything and having a consistent pearlite microstructure. However, it appears that adding extra grain refining cycles is not necessary for a fine grain size, and led to no improvement in toughness.
Stress Relieving
Another type of heat treatment that can be performed prior to the final austenitize is a “stress relief” treatment. This one is commonly done because of stresses induced in the steel during grinding the annealed steel. If the steel is significantly heated in different portions from grinding this can lead to increased chances of warping or cracking during austenitizing and quenching. A stress relief is typically done from 1200°F/650°C for 2 hours.
Thermal Cycling for Stock Removal?
It is relatively common for knifemakers to do various cycling treatments with low alloy steels even for knives that have not been forged (ie stock removal knives). They typically do this with the goal of grain refinement. However, as shown in the above experiments, the changes to the carbide structure and heat treatment response are much more significant than any changes to grain size. If you are forging some knives and doing stock removal with others, I recommend doing the same set of normalizing and annealing treatments to keep the heat treatments consistent. Otherwise some knives will have different hardness, toughness, etc. If you have a furnace where you can control austenitizing temperature and soak time then the steel is probably fine from the manufacturer. If you are heat treating without temperature control in a forge then it may be better to start with a pearlitic structure from normalizing.
Thermal Cycling Stainless?
Stainless steels and high alloy tool steels cannot be normalized. Most of the steels used in knives still have significant carbide all the way up to melting temperature. So the carbides cannot be dissolved as with low alloy steels. And during air cooling you will get martensite rather than pearlite because of the high hardenability. And as shown in charts earlier in this article, grain size stays fine during forging much better in high alloy steels because all of the carbides “pin” the boundaries preventing grain growth.
If you are forging stainless or high alloy tool steels you should simply anneal instead. You can do a standard datasheet anneal which is a slow transformation anneal. I have not seen research on faster anneals on these types of steels and the rates are likely significantly slower than in low alloy steels so I would probably stick with the datasheet rather than attempting a “fast DET” like I recommend with the low alloy steels. However, one alternative is a temper anneal. The required temperatures and times are significantly higher than low alloy steels but experiments on T1, M1, and M2 high speed steels found an improvement in grain size after the final austenitize and quench if done in a specific way. Read more in this article or in Knife Engineering.
How to “Thermal Cycle” Low Alloy Steels in a Forge
1) Normalize by heating to the approximate recommended temperature and air cool.
Determining the exact temperature can be done by color or maybe with a laser thermometer. My cheap laser thermometer is always way off so I don’t know if more expensive ones can do better. Fortunately normalizing can be done from a relatively wide range of temperature. When the steel is fully magnetic again during air cooling it is done transforming. If heat treating in a forge this is the best microstructure going in to austenitizing because no soak is required.
2) Heat to non-magnetic (and no higher) and then place in vermiculite. This step is recommended if your final austenitize and quench will be done in a furnace.
While it might sound funny to do normalizing and annealing with a forge if you have a furnace, the cycles can be done very rapidly in the forge so it can be more convenient.
How to “Thermal Cycle” in a Furnace
1) Normalize by heating to the recommended temperature for 10-15 minutes and air cool.
2) Anneal by heating to the recommended temperature for 15-30 minutes, cool at 600°F/hr (300°C/hr) to 1100°F. After 1100°F the steel can be furnace cooled or air cooled it doesn’t matter.
Summary and Conclusions
There are a lot of different recommendations for different cycles to perform after forging but those recipes sometimes miss what it is we are trying to achieve with normalizing and annealing. Normalizing is to dissolve everything and have a consistent microstructure. Annealing is to give a spheroidized structure which is easily machinable and easily cut and drilled, and to have consistent final heat treatment response. Grain refining cycles don’t seem to provide any benefits beyond a normalize and anneal in the experiments that I performed. When performing a final heat treatment in the forge there are benefits to having a normalized structure rather than annealing because the heat treatment response is so rapid. That means you can heat to non-magnetic without going hotter and quench from there. I will next be doing experiments with forge heat treatments with a normalized structure in a range of steels to see how well this works for a forge heat treating beginner like me.
[1] Souza, Samuel da Silva de, Paulo Sérgio Moreira, and Geraldo Lúcio de Faria. “Austenitizing Temperature and Cooling Rate Effects on the Martensitic Transformation in a Microalloyed-Steel.” Materials Research 23 (2020).
[2] Roberts, George Adam, Richard Kennedy, and George Krauss. Tool steels. ASM international, 1998.
[3] http://www.georgevandervoort.com/images/met_papers/IronandSteel/ToolSteels_Longest.pdf
[4] Samuels, Leonard Ernest. Light microscopy of carbon steels. Asm International, 1999.
[5] Porter, David A., and Kenneth E. Easterling. Phase transformations in metals and alloys (revised reprint). CRC press, 2009.
[6] Brooks, Charlie R. Principles of the heat treatment of plain carbon and low alloy steels. ASM international, 1996.
[7] Toribio, Jesús, Beatriz González, Juan-Carlos Matos, and Francisco-Javier Ayaso. “Influence of microstructure on strength and ductility in fully pearlitic steels.” Metals 6, no. 12 (2016): 318.
[8] Verhoeven, J. D., and E. D. Gibson. “The divorced eutectoid transformation in steel.” Metallurgical and Materials Transactions A 29, no. 4 (1998): 1181-1189.
[9] Payson, Peter. The annealing of steel. Crucible steel company of America, 1943.
[10] Stickels, C. A. “Carbide refining heat treatments for 52100 bearing steel.” Metallurgical Transactions 5, no. 4 (1974): 865-874.
[11] Saha, Atanu, Dipak Kumar Mondal, and Joydeep Maity. “Effect of cyclic heat treatment on microstructure and mechanical properties of 0.6 wt% carbon steel.” Materials Science and Engineering: A 527, no. 16-17 (2010): 4001-4007.
also posted this on patron I am wondering how you’re running the forge, as I understand and have observed over my five years of forging knives. The more propane you run through a forge, the hotter it is (depending on the design of the forge). With my Atlus knife company Graham Forge, you can adjust the temperature by not running the regulator at full bore, and I use a psi gauge to try to conserve my propane. Full bore on my forger has it run at to forge weld temperature, say 8 psi, significantly less if I heat it up using a lower PSI rating as 8psi will maintain a relatively high heat once the forge has heated; however, I did just get a newer burner from the one I have been using so I need to take the time to play with it. I have in the past tested it with a thermal couple and could get the thing to hold 1500, but a thermal couple and propane environment don’t mix well. So you may want to experiment with a psi gauge to help control the temp. of the forge (and sacrifice a few thermal couple probes) to get a good reading.
also, what do you mean by way off with the temperatures of your digital thermometer? is it 50-100 degrees or 5-10 degrees off? as way off is not a scientifically accurate statement and is prone to different interpretations depending on the individual. i use a relatively cheap one at a high enough temprtature range that also allows me to set the refractive index of the material i am testing.
By way off I mean way off, 200 degrees F off. Adjusting emissivity does not help. Since I don’t have a better one to recommend I have to state that mine isn’t sufficiently accurate enough to recommend.
Thank you for answering. What brand is it as it sounds like a good idea to avoid it?…or you could have a defective one.
PS: Its my current understanding that you want to use the shorter soak times to HT in a forge compared to HT in an oven or salt bath. NJSB has altered heat-treating charts for this on their website created by Kevin Cashin that may help with your experiments in forge heat treating. Keep up the great work.
Kevin Cashen didn’t make the NJSB datasheets he is name dropped. Those datasheets don’t exactly fit the recommendations I am giving in this article.
An excellent article Dr. Thomas and as always quite in depth and kind of makes me feel foolish. I say that because I’ve told some folks cycling ‘as it comes’ steel that they were wasting their time and electricity for nothing. Well now it seems, I’m the one who is foolish and wrong. I guess I should apologize… Nahh. LOL
I see I’ve done some of my forge knives the right way though by following some common procedures after forging. I’ve used three forges in my life, but my brother’s shop made the best forge I’ve used and the most. It is quite controllable with regulators for all inputs including the venturi air intake, he even put a thermocouple in it and could control the heat to within 50F degrees, low to high. Means if the TC said 1450 it was close to 1500. I’m going to be heading his way in October and I think we’ll try some new steels. We can even melt steel with the infusion of some O2, but it’s kind of scary.
If you wonder about some of the work he has done just look at the lamps in Pirates of the Caribbean movies. I learned blacksmithing in my brother’s shop before I ever forged a knife. He has a 50 lb Little Giant trip hammer that saves my old arm some work. I just wished he had a Rockwell hardness tester.
curiously low temps for some steels re DET anneal. the idea is to use 5min soak + lower temp on steels that would be normally no-soak? is DET temp always = quench temp, like with 52100?
I’m having a difficult time understanding your question. Are you asking why DET annealing temperatures are as low as they are? Or asking why austenitizing temperatures are low?
I’m stating:
the DET temps are lower than expected. For example aust temp of 5160 in an earlier article of yours was ~1500F, not 1380F.
I’m asking:
Considering how 52100 is DET-annealed and quenched at the same temp here (1445F, 14:17 in the video), is the same true of other steels?
AND
is that representative of a shift towards using soak times and lower temps IN GENERAL in steels that are considered no-soak usually?
5160 was not given a DET anneal in the article you are referencing. You are correct that 1500-1525F had the peak properties in that study.
As I said in the video and the article, the low austenitizing temperatures were chosen to compare the potential for a forge heat treatment from non-magnetic and no higher. The temperatures used were approximately where the steel became non-magnetic with short soak times. But to do such treatments requires a prior microstructure that transforms very rapidly.
I am not recommending a shift to short soak times or lower temperatures. I wrote about this in the recent 8670 article where we found that temperatures that are too low or soak times that are too short result in both reduced hardness and toughness.
I see now, the temp was not chosen because it is, by chance, the same as from the DET table, but simply because the 52100 becomes non-magnetic at it. I should have listened/read more carefully.
Thinking about it, if the method dictates lower than ideal aust temp, why not use brine instead of oil to compensate?
Table salt melts at 800C/1470F so it could also be used as a reference point.
Hi Larrin – So does all machining and shaping happen in-between the thermal cycling and heat treatment? Does the final heat treatment cause any dimensional or shape changes to the workpiece? I’m wondering about knife edges and also steel coins. Does all sharpening happen before the heat treat or after?
In coining they like to strike the blanks after annealing, and I wonder if heat treating the coins after striking would cause any problems with their final shape or with the electroplated nickel and copper layers. (Or with electroless nickel plating.) The nickel plating can achieve high hardness with heat treatment.
Rough grinding and machining is done on annealed steel or sometimes normalized steel if it won’t be annealed. Final grinding and sharpening is done after the quench and temper. There are small dimension changes to the knife with heat treatment as martensite is slightly bigger than annealed steel. The most common shape change is a warp when it comes to knives.
Thank you for your article. I would like to ask whether laser cutting will lead to annealing?
Laser cutting means a “heat affected zone” which is usually hardened, especially for a high alloy steel.
What will happen if the cooling rate down to 1100F for the DET is a little faster than 600F/H, lets say around 800F/H? I’m not so concerned with the machinability of the annealed steel, but mostly the affect upon hardening.
Hello Larrin, I have a question regarding the Stress relieving: Sometimes I read it is very important to rise the temperature slowly (100-300°C/h) hold for 1-2h and even slower cool (some source say 30°C/h some other 100°C/h). Another sources don’t mention how slow the temperature change should be. Do you have any insight to this? Thank you
Heating rates like that are typically to ensure that the piece heats evenly, without the core being significantly colder than the surface. But knives are thin enough where this isn’t typically an issue.
hey larrin i’m a bit new to smithing and struggling a bit to understand why cant stainless steel be normalized could you explain why when there is chromium or any alloy metal carbides cant dissolve but with low alloy steel it can?
During normalizing one of the goals is to dissolve all of the carbide. A high carbon and chromium content means the carbides don’t dissolve without melting the steel so that goal can’t be accomplished.
So the high chromium prevents carbide from dissolving does it just body block it like it’s in the way when the carbide is on the side of the crystal structure so it can’t get in it or the chromium is in the crystal structure but it breaks up the stratus because it’s larger then the ferrite or carbide
More carbon and more chromium means the carbide is more difficult to dissolve, it will form at a higher temperature, etc.
i dont know why i didnt get the notification guess i just didnt push the the box. ok i think i need to research i bit more or just settle for low alloy. thanks again appreciate the help
I can understand that normalizing would be required after forge welding or any forging at all , but when I work with high alloy steels and form them with stock removal, would normalizing be required before final heat treatment after a bit of welding with a TIG torch or oxy fuel torch – say to add a bolster or guard?
After those types of processing you need a “stress relief” which is sort of like a very high temperature temper. Something like 1200F for 1-2 hours. Should say in the datasheet for the steel.
Ah, that makes sense. often I also use a plasma table to cut the blade out of a flat bar of high alloy steels and tool steels. In that case the edges receive high temperature, probably even higher than laser cutting; so with plasma cutting and arc welding being used before final heat treatment, would stress relief be enough, or would you recommend normalizing the stock?
You can’t normalize high alloy steel. You either have to anneal it or grind away the HAZ (heat affected zone).
Hmm, I suppose grinding the bevel and edge of the blade, and as much sanding/grinding is done to make the blade uniform, would remove the HAZ after cutting. If welding is done as well, stress relief would still be used before I start austenitizing.
Thank you for writing this awesome article! There are quite some legends about the ways thermal cycling can influence mechanical properties of steel, so it’s helpful to clarify how it works and how it’s done.
You mentioned grain refinement and carbide structure as well as carbide size. I assume that if the final austenitizing temperature is not too high, those factors can contribute to dislocation density. This topic is covered in multiple of your articles. In one of them, named “What Happens During Tempering of Steel?”, you stated that there is an optimal carbide size for hindering dislocation movement. I just wondered what size that might be approximately – is it more on the side of transition carbides that are precipitated during tempering (a few nanometers), or on the side of secondary and tertiary cementite that is precipitated during recristalization (a few micrometers)?
Nanometers
Thanks!