How to Heat Treat 8670

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Podcasts

I appeared on two podcasts this past week, Knife Perspective and Mark of the Maker. Knife Perspective I had been on before so we mostly focused on MagnaCut and other topics. Mark of the Maker was a full interview asking about my background before discussing my book, website, and CPM MagnaCut. So listen to one or the other or both depending on what you’re in the mood for.

8670 Steel

8670 is a nickel-chromium alloyed steel which has high toughness and easy heat treatment. The low alloy content also makes it a good choice for forging. The relatively low wear resistance means great ease in grinding, finishing, and sharpening. It is a great choice for knives requiring high toughness and for beginning knifemakers and bladesmiths.

The Experiment

Pop’s Knife Supply contacted me about doing a set of experiments on their 8670 steel. We talked about different heat treating variables we might test and Joey Berry of Pop’s heat treated and ground the coupons. I measured the hardness and toughness of each condition and I am providing the analysis of the results below. We used my standard 1/4 size unnotched charpy impact specimen you can read about here. Below you can see the different heat treatments that were tested:

Tempering Temperature

As expected, as the tempering temperature increases, the hardness goes down while toughness goes up. However, in certain cases we have seen a steep drop in toughness below certain tempering temperatures, such as with 5160. Some steels also start to see tempered martensite embrittlement at 450°F but this was not seen with 8670. In this case as long as the tempering temperature is between 300 and 450°F, there is a linear trend with hardness and toughness. So choose the tempering temperature according to what the desired combination of toughness and hardness is.

Austenitizing Temperature

We looked at a range of austenitizing temperatures from 1450 to 1625°F. Different datasheets can have a pretty wide range of recommendations on the best hardening temperature for this steel. In this case 1450°F led to reduced hardness and toughness when compared with 1475-1550°F. Using 1600-1625°F led to reduced toughness without an improvement in hardness. The consistent results over the relatively wide range of 1475-1550°F is what makes this steel easy to heat treat. The sweet spot seems to be about 1500°F.

Austenitizing Time and Quench Oil

Some knifemakers are overly afraid of excessive hold time during austenitizing because they think it will lead to grain growth. If the austenitizing temperature is selected appropriately then holding longer than necessary does not lead to deleterious results. However, holding for too short a time can lead to insufficient hardness and even toughness in some cases. Using a time that is a bit longer than strictly necessary also means you can count on consistent results even for different size knives that may need a bit different hold times. In this case we see an improvement in toughness and hardness by austenitizing for 10 minutes rather than only 5 at 1500°F. Using an even longer hold time of 15 minutes results in the same properties.

Another concern that some knifemakers have is quenching too rapidly, worried it will lead to microscopic quench cracks or other bogeyman. And while I would be cautious when quenching anything in water, every steel that we have heat treated with Parks 50 has resulted in excellent toughness regardless of the hardenability of the steel. The benefit of a high hardenability steel is that you can use a slower quench oil and therefore reduce the chance of warping. 8670 has sufficient hardenability to use Parks AAA or even somewhat slower oils as AAA straddles the line between a fast and medium oil. But using Parks 50 results in toughness just as good as AAA. In fact in this case the toughness was better with Parks 50 but that is likely just statistical chance, especially since the 15 minute hold time with Parks 50 resulted in the same toughness as the Parks AAA for 10 minutes.

Hardness vs Toughness and Final Recommendations 

Below I have a chart summarizing the best and worst hardness-toughness combinations from the studied heat treatments. Using 1500°F for 10-15 minutes followed by an oil quench and tempering between 300-450°F led to the best properties. Using either Parks 50 or AAA led to similar results. Select the tempering temperature based on the desired hardness-toughness combination. Underaustenitizing by using 1450°F for 10 minutes or 1500°F for 5 minutes both resulted in reduced hardness and toughness. Overaustenitizing at 1600°F or above led to reduced toughness for a given hardness.

Forging, Normalizing, and Annealing

For stock removal 8670 is ready to go for austenitizing, quenching, and tempering. However, for those that are forging here are some recommendations on temperatures to use. The ASM heat treater’s guide recommends a forging temperature of 2190°F for 8660; the slightly higher carbon content of 8670 likely means that temperature needs to be slightly lower as well. Certainly 2150°F would be safe though you will have less decarb and grain growth from lower temperatures. Normalize from 1550°F for at least 10 minutes. Grain refining cycles can be done from 1450°F. Anneal by heating to 1380°F for 30 minutes, cool at 500°F per hour to below 1200°F and then air cool. That sets up the steel to be drilled, ground, and given the final austenitize, quench, and temper.

Previous 8670 Results and Comparisons with Other Steels

We had a previous specimen of 8670 that tested higher than those above, testing at about 50 ft-lbs at 59.7 Rc, using steel from another supplier. This specimen was austenitized at 1525°F for 10 minutes, quenched in fast oil, and tempered at 400°F. The specimens machined for this study had a rougher surface finish and the grind marks were along an unfavorable orientation for crack initiation. So I heat treated specimens from Pop’s with a different machining method and got somewhat higher results. I used 1525°F for 10 minutes followed by Parks 50 and tempering at 400°F. My specimens were 27.4 ft-lbs, higher than those in the rest of this study using the same steel. So it didn’t quite reach the high of the previous specimens but it explains some of the difference. However, the trends are still good for the specimens we tested and I think these heat treatment recommendations can be used for any available 8670. 27.4 ft-lbs is quite high, better than most high alloy and stainless steels as shown in the charts below.

Summary

8670 is very easy to work with and it is forgiving in heat treatment. It has very good toughness as well. My recommended heat treatment is 1500°F for 10 minutes, quench in fast or medium oil, and temper between 300 and 450°F depending on the desired hardness. This article also has recommended forging, annealing, and normalizing temperatures and some guidance on heat treatments to avoid such as underaustenitizing or overaustenitizing.