Help with a messed up Heat Treat

[RobertMorman]

So as I understand the heat treat process it is a sequential process of normalization, followed by the quench, then ultimately the temper. Here is my dilemma:

I forged a piece of 5160 with intentions of making a filet knife for a fisherman buddy. I made sure to only strike while the steel was a dull orange color or brighter.  After forging I profiled and did my preliminary grind to make the final grind easier. I normalized twice ( I have recently read on this forum the preferred number is 3 and will implement this next time), quenched in preheated oil and tempered at 400 for 2 hours and allowed it to cool inside the oven down to room temp. I got that straw color I have heard is a pretty good signal that you didn't overheat it. Final grind completed and as I was getting ready to sharpen it, I decided to test the flex on the blade to see if I needed to grind it a little thinner or not. I felt crunching (like a twig right before it snaps) and figured it was the glue on the handle separating and made a note to address that. I flexed it the other way and it snapped revealing a pretty large grain structure which leads me to believe that the crunching i felt was the blade getting ready to give way. My question is, what does everyone here do differently to ensure a tight grain structure. I'll be a little flabbergasted if 1 extra normalization cycle was all I needed to tighten up that pile of sugar crystals I saw.  Also, when I normalized I heated to nonmagnetic and allowed it to cool back to black. It was still hot enough to likely melt a hole in my palm if I handled it. Should I be allowing it to cool further before starting the next normalization cycle? If so, how cool should it get? Room temp? 

[NoGoodWithUsernames]

From what I've read grain growth comes from being at heat without being worked. Heat soaking basically. I don't know how that plays with the quenching and tempering but my first though would be it was sitting in the forge too long while you were doing the normalizing cycles, or possibly while forging if you were doing other things at the same time.

[Buzzkill]

There are a few possibilities.  If you overheated the steel to the point where it was nearly burning, you may not be able to reduce the grain size afterwards.  When broken the gain somewhat resembles cottage cheese at that point. If you burn the steel it is no longer suitable to use for a blade.

If you normalized too hot you may not have been able to reduce the grain size much.  For 5160 you might start your first cycle in the forging range, but your last cycle should be just barely above critical temperature.  The magnet is your friend here. You want it to be non-magnetic, but just barely, on the last one.  It is not necessary to cool down to room temperature.  You should be doing this in low light and you only need to wait until the blade has completely stopped glowing before starting another cycle.

Beyond that, when you were heating for the quench if you got the steel too hot that would again promote large grains in the structure.  We have to get above critical temperature to get the transformation, but going hotter than needed weakens the steel.  Those are the things I can think of that may have contributed to the excessive grain size you observed.

[George N. M.]

Welcome aboard from 7500' in SE Wyoming.  Glad to have you.

The only thing that I can add is that straw color seems too hard for something flexible like a fillet knife.  I think I would have looked for a blue temper color.

"By hammer and hand all all arts do stand."

[Latticino]

I have had good success with the following heat treatment process:

1. After forging is complete and before any grinding I first normalize the grain by heating to above the austentizing temperature (after the phase change that is visible by decalescence) and holding it there for a few minutes.  Typically this is a bright orange, almost yellow color.  Grain will grow, but it will even out in size.  Note that for all cycles the goal is to have your entire blade be at relatively equal temperature.
2. Let cool down to black slowly in air (should be magnetic again), don't leave on a heat sink.
3. Heat up to just below austentizing temperature (around 1400 deg. F), and not above that to reduce the grain size.  This is a red-orange color.  There should be no phase change and the blade should not go non-magnetic.  Hold at this temperature only long enough to equalize the temperature throughout.
4. Let cool down to black slowly in air
5. Heat to around 1200 deg. F for stress relief (sub-critical anneal).  This is just barely glowing in a dark room.  For thin stock I sometimes just use the dragons breath from my forge for this.
6. Let cool slowly to room temperature.
7. Grind surfaces to 120 grit (unless very thin kitchen knives, then I grind after hardening and tempering).  I haven't made a fillet knife, but flexibility is more of a function of blade thickness than it is of hardness.  Ability to spring back to the original straight form is a function of correct heat treatment and the balance between toughness and hardness that you achieve with proper cycles and material selection, particularly tempering (as George mentioned).  For blades that get a lot of stock removal I may do a second stress relief cycle after this to help minimize warping.
8. Heat to austentizing temperature and quickly quench in proper quenchant.  5160 is a deep hardening steel and can be quenched in a relatively slow oil like Parks AAA, McMaster 11 second, or even canola at no more than 120 deg. F.
9. Temper for 2 separate one hour cycles at temperature selected for your steel to achieve the desired edge hardness and toughness.

[swedefiddle]

Good Morning,

This is the Lesson of Hard Knocks!! Write down in a book, what you did, it gives you something to look back to. The parent material obviously had some stress still in it, next time if you are doing everything the same, draw the Temper to a different shade of Blue. Which Blue, you will have to experiment with. If you use new 5160, it won't have a flaw from when it was a Spring. You have to do your own experimenting, using your own Tools/Equipment. Not everyone will have the same equipment and eye-sight!!

If at first, you don't succeed.............................Enjoy the Journey.

Neil

[JHCC]

The large grain size is definitely a function of overheating during heat treatment. Latticino's method will definitely help, although keep in mind that the highest temperature the steel reaches before quenching will determine the final grain size.

[Latticino]

John,

I agree, with the clarification that I believe it is a function of both temperature and time that determines grain growth.  As I understand it once you exceed the phase change temperature for the steel involved (different for each type of steel, per Cashen for 5160 blades this temperature should be approximately 1525 deg. F) austinite begins to form.  Initially these austenite grains (sections of austenite aligned in a matrix with each other with defined boundaries separating other similar austenite matrixes aligned in different directions) should be fairly small, but with time and temperature will start to align with each other and combine into larger grains.  At least that is how I picture what is happening.  Higher temperatures make this alignment happen faster, longer soak times make more grains align and combine.

[velegski]

If I'm reading Dr. Larrins book right, grain growth is a combination the steels carbides and heat.  The higher the carbide content in given steel,  the smaller the grain size.  The higher the forging temps the larger the grain size.  The time spent at the higher forging temps allows the grain growth to increase. Too high a temperature and we cause carbon and alloy segregation.

Resetting the grain size by reheating a steel to its recommended normalizing temp, in this case 1525-1550 degrees F and soaking/holding at that temp allows the alloys and carbides to reset. Soak times for knifes are generally under 5/6 minutes.

[JHCC]

Dr. Larrin Thomas has an excellent discussion of thermal cycling and grain growth both in his blog:

https://knifesteelnerds.com/2021/08/28/how-to-thermal-cycle-knife-steel/

And on his YouTube channel:

 

(This is largely the same information, but presented in slightly different ways.)

3 hours ago, Latticino said:

it is a function of both temperature and time that determines grain growth. 

I'm not sure about this; according to the graph in blog post linked above, grain size is a function of temperature only, not of time. I suspect that time is an issue when one is heat-treating in an uncontrolled environment, such as a gas forge that is actually burning hotter than your target temperature. For example, if your forge is burning at 1,900°F and you are aiming for 1,550°F, leaving the blade in longer increases its temperature and therefore its grain size. However, if you're working with a furnace or a salt bath that can be set to 1,550°F, then the grain will only grow to a certain size and no larger. 

[anvil]

I suggest you download "the heat treaters guide companion". It is an android apk and its free. It lists the manufacture's specs for many steels. Then follow the specs for the steel you are using.

The steps for nearly all steels is:

1:normalize

2:anneal:

3:cold work

4:harden

5:temper. 

Many people dont anneal, yet it is listed as a step for nearly all steels. I suggest you follow the manufactures specs until your successes outnumber your failures, then experiment with other processes. 

Cashen is a good hands on source and his process is as above.

The vid above is good and tends to debunk the internet myth concerning multiple normalizing steps. There are a few steels that the specs do suggest more than one normalizing cycle.

[ThomasPowers]

And there are a few steels that the ASM HT Handbook suggest NOT to normalize. (IIRC S-1 is one of them.)

[anvil]

Correct, there are a few like that, and a few that require 2 normalizing steps. RTB( read the book) is usually the best course to get this data. 

[Steve Sells]

I feel I need to add that I can agree that when the grain is kept small, there is no need for thermal cycling, but in many cases we allow the grains to grow so that is why I always advocate cycling to reduce grain,, JHCC photos didn't show how large or small the grain was before the normalizing , so that makes it hard to decide if cycling is always a waste of time or a benefit

[Buzzkill]

On 1/6/2023 at 1:31 PM, JHCC said:

grain size is a function of temperature only, not of time.

This may be true generally, but getting the elements of the alloy all dissolved and evenly distributed throughout the piece can certainly be affected by time.  Some of the elements can affect grain size.

Regardless, it's not a guessing game here.  The manufacturers tell us how to properly heat treat their products.  Professional knife makers tailor some of these processes to match the dimensions of blades rather than large parts, but in either case there are recipes that, if followed correctly, *will* yield consistently good results.

[JHCC]

26 minutes ago, Buzzkill said:

getting the elements of the alloy all dissolved and evenly distributed throughout the piece can certainly be affected by time

As Dr. Thomas clarifies in the article linked above, the assumption is that the workpiece is at equilibrium -- in other words, that the piece has been held at the specific temperature long enough for everything to dissolve that's going to dissolve and whatever phase changes are going to happen have happened. Yes, that does take time, and yes, alloying elements can affect grain size, but once a piece has reached equilibrium, it can be held at that temperature without additional grain growth. (He also notes that the normalizing temperatures he gives for various alloys account for the higher temperatures necessary to dissolve other alloying elements besides carbon, such as chromium.)

27 minutes ago, Buzzkill said:

Professional knife makers tailor some of these processes to match the dimensions of blades rather than large parts

Basically having to do with the fact that a thinner blade will respond to changes in temperature faster than a larger block; e.g., a Bowie knife will reach 1,550°F faster than a 175 pound anvil (especially if you are heating in a furnace or molten salt pot set to the target temperature).

1 hour ago, Steve Sells said:

JHCC photos didn't show how large or small the grain was before the normalizing , so that makes it hard to decide if cycling is always a waste of time or a benefit

Dr. Thomas addresses this question farther down in the article, specifically regarding whether or not grain refinement is beneficial:

Quote

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. [emphasis added] 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.

[chart: hardness and toughness results for 1084 steel treated as above]

[photo: 1084 steel overheated and annealed prior to austenitizing]

[photo: 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.

 

Once again, I am neither a metallurgist nor a bladesmith, so my own experience in this field is quite limited. However, as Buzzkill says, it's really not a guessing game; steel gonna do what steel gonna do.

[anvil]

JHCC, I agree with this 100%. When you first posted this vid and I watched it I immediately understood what he was saying. For what it's worth, years ago I bought steel from Carpenter Tech due to a Turley suggestion. He said there support crew loved spending time with smiths who heat treated traditionally. Basically they supported Turley and what they presented to me was what Dr. Thomas made clear in his vid. Can't thank you enough!

 

 

 

 

 

 

 

[JHCC]

Support crew makes a world of difference. The manager of my local Harbison Walker outlet LOVES working with glassblowers and blacksmiths on smaller applications of his products -- I think it makes a refreshing change from the massive quantities needed in the local steel industries.