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Understanding Heat treatment: Example 5160 - Help needed

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Hi guys,

this post is going to be a tad longer so i hope you bear with me. The basic reasoning behind it is, that i want to validate my understanding of heat treatment and the processes that happen in the steel.

I am looking to have a knive made. 8 mm thick 60cm overall length. A very large chopper. The steel of choice is 5160. I really do want to write my own HT protocoll. This is not because i don't trust a potential maker, but i want to feel like i am part of the process and put my theoretical knowledge to use. Plus that way i know what happened to the knife and i can adjust as necessary with the next order and if it fails i have noone but myself to blame.

When i send the following out per mail to a smith i got that i was wrong with my concept. But unfortunately not very detailed. So i will repost my suggestion here together with my reasoning and thoughts behind it. Detailing the WHY.

I do appreciate any and all feedback. Here it goes:

Assumption: blade is fully heat treated, no differential hardening used.

The outline below describes the process after the forging itself, meaning the blade will already have the form it needs.

1) Tripple normalizing.
The grain of the steel can have grown during the forging process. Normalizing helps resetting the structure and ensures you have a fine grain in the steel matrix to work from. Basically it resets any errors that can have happened during the forming process.

2) Stress relieving
Theoretically a piece of steel laying on a work bench has no inert stress. Actually it means the sum of the various stresses inside the steel are 0. By working in a stress relieve cycle you can release them. This means the blade can warp and might need to be straightened out before the actual heat treatment.

3) The austenizing temperature is approx 830-840°C (between bright red and orange red in the color spectrum). The hold time is 8 mins, which equalls 1 minute per mm thickness of the blade. The temperature is a tad higher then necessary and carries the risk of grain growth but the time frame is so short that it should not occur. The advantage is that you gain a little bit of increased hardness in the finished product. Since the hardening is done in the forge i did not include a heat ramp.

4) quenching is in warm oil (room temperature is just fine). insert the blade spine first. This prevents the thick part of the blade from radiating heat into the thinner part, hence prolonging its cooling cycle. After the blade is submerged it can be moved around for an increased cooling rate.

alternative suggestion: broken quench. Starting with water and moving over to oil. But that would be entirely optional, as the cooling rate of water is very high it would run the risk of cracking the blade if the time window isnt exactly right.

5) A steel with a carbon content of 0.6% has a Martensite finish temperature below ambient. That means a cryo treatment is useful as it allows to reach close to 100% martensite with next to 0% remaining austenite.
As cooling medium i suggested acetone and dry ice or ethanol and dry ice. Both should be sufficient to reach Mf.
Hold time about 3 hours. While the "switch" of the structure from austenite to martensite happens at the speed of sound, it doesnt happen in every cell at the same time. The long time period ensures that the entire still matrix has reacted.
Cryo needs to be done immediately after quench to precent the remaining austenite from stabilizing. The blade can be stored in boiling water if an immediate treatment is not possible.

6) following the cryo treatment the blade needs to be tempered. I suggested a kitchen oven at 200°C. (going too high risks the chance of blue-brittleness) Preheated with an oven thermometer as the dials are not accurate enough, and every electric oven has immense temperature swings during its heating phase (thats why you pre-heat when you do a cake). Hold time is 2 hours. While the function of the temper is not linear (the main effect happens in the first few minutes) 2 hours gives ample time for the entire blade to even out in temperature and for the steel structure to react.
Directly after the first temper the blade is to be quenched in water (as cold as possible). This shifts the temperature range of the physical attributes of the steel to lower temperatures (not sure of the english term, in german its "Kerbschlagbiegezähigkeit"). This has an advantage if the knife is used during winter and temperatures below 50°C . So.. pretty much always.

7a) Cryo treatment if available same as above

7b) There is a chance that there is still some left over austenite at this point. Called secondary austenite. It will have changed to secondary martensite after the 2nd cryo treatment. That needs to be tempered as well with the same process as detailed in Step 6. In other words: a 2nd Temper cycle.

8) after the cold water quench following the 2nd temper the blade benefits from another cycle of stress relief. this is to be done at 50°C below the last temper, so 150°C. This does not lead to a matrix change but releases build up stress in the steel . Any form of matrix change carries the risk of inducting stress into the blade. Hold time 2 hours. Afterwards let it air cool.

Expected results:
approx. 58-59HRc.
a very fine grain
close to 100% tempered martensite in the structure.

That is my thinking behind my suggestions. Please correct me where i am wrong and why this wouldnt work.

Thanks a lot in advance.

yours sincerely,


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Not going to review or go into the detail you possibly want but there are couple procedural issues that you might want to address:

  1. Don't quench spine first.  You want the edge to get cooled immediately and quickly (you have only a matter of seconds before the thin edge cools too slowly past the nose).
  2. Room temperature is only acceptable if you are using quench oil formulated for that temperature.  If you are using vegetable oil you should preheat the oil to at least 130 deg. F
  3. If not differentially heat treating get the whole blade into the quench as quickly as possible.
  4. Grind blade with consistent thickness and tapers to minimize warping
  5. Agitation in the quench should be longitudinally with the blade axis (not side to side).  I believe that this is the root for the superstition that quenching in a north/south orientation prevents warpage.  You agitate in a north south direction based on the "compass" direction the blade enters the quench and don't change that orientation by moving it from side to side..
  6. Not sure what you mean by a stress relief cycle.  As far as I know the normalization addresses this.

off site link removed because  that information there is already posted here

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I'll just add a my $0.02. Don't do the water then oil thing. Also, normalizing doesn't fix all of your mistakes. Overheating can still ruin a blade even with normalizing. You are on the right track, though. Show us pictures when they are finished!

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I would venture you're overthinking this ;) 5160 is described in most heat treatment guidance I've read as a very forgiving steel to heat treat. I've yet to read heat treating guidance that calls for added complications of things like cryo on  simple 5160. That type of treatment shows up in the instructions for high alloy and stainless steels. If you don't trust the smith to do the heat treat on his own, I might recommend you offer to pay to have it sent to a professional heat treating company who will have a specific recipe for the given steel to get the most out of it and all of the equipment to do so precisely. 

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