All About Heat Treat

Our Heat Treats Are Head and Shoulders Above the Competition. In Fact, Its Not Even Close. Here’s Why

The heat treat on most production knives, even very high end ones, suck.

There I said it. The industry standard heat treats that big manufacturers use are absolute trash and leave an unimaginable amount of performance on the table. In fact, there’s no point in even using half of these super steels when the heat treat you are running leaves them with a soft, mushy matrix, low toughness, and low stain resistance. If you know about knifemaking, you will of course immediately recognize that I am referencing the use of high tempering temperatures (1000F) vs. low tempering temperatures (~400F) and the importance of the other steps needed to optimize the steel before tempering. To summarize, the use of rapid quenching techniques, cryo, and low tempering temps make the absolute best knives in the world, but it comes with tremendous cost and difficulty that most manufacturers can’t tolerate. These techniques are only practiced by custom makers and a tiny handful of small to mid sized producers who have the skillset and patience to deal with the difficulties of low tempering.

To understand why all the major players in the knife industry are using crappy heat treats, we need to first take a deep dive into all the steps involved and why they are important. And remember, when it comes to HT, rockwell hardness does not tell the full story.

Heat treating knife steel is composed of roughly 5 main steps. They are as follows, annealing/stress relief, auestenitizing, quenching, cryo/ cold treatment, and finally tempering.

Annealing/ Stress Relief

The state that knife steel comes in from the rolling mill is very important for creating optimal microstructures during heat treat. Milling and grinding steel before heat treat creates stress and changes the microstructure. In order to create consistent microstructures and get rid of residual stress, the steel must be run through an expensive and time consuming annealing cycle. To do this, we have to heat the steel to just below critical temps and then let the steel very slowly cool back down to room temperature. This process can take a whole day and occupies the oven the entire time, making it a very costly process. However, its needed to make great knives. Most manufacturers skip this step completely.

Austenitization/ Soak

This is the meat of heat treating, where steel is transformed from the soft, consistent pearlite structures we obtain through annealing, and is transformed into a phase known as austenite. Austenite is a soft, ductile structure (its created when steel glows red hot)Picking the right austenitization temperature will dictate the amount of carbide that is formed. Generally, higher temps= more carbide production and higher hardness. Super steels like 3V and Z Tuff require much higher temps than simple steels like 52100. We run Z Tuff and 3V at 1950F+.

Quench

You’ve probably seen videos online of guys taking red hot swords out of a forge and plunging them into oil while the blade catches fire. This is quenching, but for most steels we use, the process is not as dramatic. However, getting the quench speeds right is one of the most important factors for creating a great blade, and not everyone does this step correctly. When it comes to quench, speed is everything. We are taking the steel from a critical temperature of 1,550- 2,000+F and within a matter of seconds, cooling it to room temperature. This rapid cooling creates the microstructure that makes steel hard. This structure is called Martensite. To get the most out of super steels like Z Tuff and CPM 3V, a rapid quench is needed. The problem is, these steels are typically not quenched in oil, but instead, are quenched in AIR, and very few heat treaters can truly get this step right. We have all of our blades heat treated at Kowalski, where they use specialized ultra high pressure ovens that release inert nitrogen at up to 10BAR of pressure. This rapidly quenches the blade and creates excellent Martensite.

However, in order to have consistency across all the blades, this step must be done in small batches, which increases costs. Most production companies run large batches where the blades may not all cool at the same rate, leaving inconsistent Martensite formation. And when Martensite doesn’t form fully, it leaves behind the bane of all knife making, RETAINED AUSTENITE.

Retained austenite is left over austenite that doesn’t fully convert to Martensite. As we discussed earlier, austenite is soft and ductile, not what we are looking for to create a thin, strong knife edge. Left over austenite makes the edge soft and mushy. While we can prevent retained austenite with a fast quench, some will always remain, which leads us to the next step, Cryo.

Cryo

Cryogenic treatment is how we get rid of any remaining retained austenite. It seems counter intuitive, soaking a blade for several hours in liquid nitrogen helps the remaining austenite convert fully to martensite. Cryo is a necessary step for all super steels and take steels like Z tuff and 3V to the next level of performance. We will often see a 2-3 point increase in rockwell hardness after cryo. However, this is a costly and time intensive step, which means many production manufacturers skip it entirely.

As you can imagine, a knife straight out of quench and cryo is going to be very hard, and also very brittle. At this point in the process, the steel is more like a ceramic, extremely hard but with no durability. We need to take the knife from this glass like state, and transition it into tough, durable, resilient knife steel. that’s where temping comes in.

TEMPERING

The final step, and the one that nearly all manufacturers get wrong. Tempering is the step where we heat the blade to a moderate temperature and take the ultra hard martensite and transform it into a more durable and resilient structure named, you guessed it, tempered Martensite.

This is where things get tricky and most manufacturers cheap out and take the easy route. Super steels like Z Tuff and 3V have a large tempering window, anywhere from 300F to 1,000F will temper the blade, BUT the microstructures that form are completely different. The fundamental issue here is that tempering in certain temperature ranges has the side effect of changing the dimensions of the steel. Steel that is treated with a low tempering temperature (400F) will GROW as it tempers (think of ice expanding as it freezes), while steel that is tempered at higher (1000F) temperatures, will largely remain stable.

For as long as super steels have been around, the industry standard temper has always been 1000F. This high temper has several advantages for uses in tool and dies where the parts are in large cross sections, need high dimensional stability, and need to be able to work in high heat environments without losing hardness. Basically, when you make a tool like a stamping die, you want it to come out of heat treat the same size it went in and maintain its hardness during use which will likely involve very high temps.

If we translate this concept to knife making, this means your knife has a very high probability of coming out the same size, and critically, will not warp. It also means that your knife can undergo high temperature grinding operations post heat treatment (like CNC Berger grinding) without losing rockwell hardness. this means easy knife making.

However, the 1000F has one MAJOR disadvantage that makes it not optimal for performance in cutting tools. The high temper alters the matrix of the steel and leads to the formation of secondary carbides. What are secondary carbides you ask? They are additional carbide structures that form during tempering. While adding more carbides might sound good, when these carbides precipitate, they make the matrix of the steel soft and mushy, which is exactly what we want to avoid in an ultra thin cutting edge. This mushy matrix is so detrimental that fundamentally alters the performance of the edge in measurable ways. The high temper means a weak, soft matrix that is extremely prone to chipping, breaking, and edge damage. Additionally, steels with a high temper cannot support an ultra thin, low angle edge which is what creates cutting performance. Finally, the high 1000F temper also lowers stain resistance as free chromium is converted into chromium carbide.

Lower edge stability, lower toughness, lower stain resistance. All things we don’t want in a knife

The difference between a steel like Z WEAR, CPM 3V, or CPM10V, or Z TUFF run with an optimal low temper HT vs. the industry standard HT is so great that they might as well be different materials altogether.

So why is the low (400F) temper so good, and why isn’t it more widely used?

Tempering a material like CPM 3V or Z TUFF at 400F truly brings the material to life. The lower temps create a more optimal microstructure and avoid the matrix degradation that occurs from secondary carbide formation. Super steels with the low 400F temper have massively better toughness, edge stability, and stain resistance than their high temper 1000F counterparts.

BUT this absolutely massive increase in performance has several trade offs.

• WARPAGE- As we mentioned, the 400F temper means lower dimensional stability. This manifests in a bad way for knife makers. When tempering a blade a 400F, the blade will actually GROW. This growth will often create warpage, which can be very difficult to correct and typically needs to be corrected by hand. If you recall the picture at the top of the page, you can see the process that most of us makers use to straighten blades. We use a small peening hammer with a carbide insert to gently tap the blade straight. This process can take a very long time and must be done by hand over hundreds of small taps. It also has the side effect of leaving not so pretty marks on the blade that have to be removed later on. If you are a big manufacturer making hundreds of knives per day, aint nobody got time to sit there for 20min hand straightening a blade and then have to go back and grind out the peening marks. While we have some proprietary methods we use to prevent warping, the reality is, a significant portion of our blades still need time consuming hand straightening.

• GRINDING- When a steel is tempered at 400F, the highly sought after microstructures that we have spent so much time creating by optimizing every HT step can be totally destroyed if the blade is subjected to even a split second of heat over 400F. Keeping temperatures below 400F is easily done when the blades are ground by hand with sharp, fresh belts, low speeds, and quenched every pass to prevent heat build up or ground wet with coolant. This is the complete opposite of what large scale CNC grinding operations look like. Even with coolant, unless special precautions are taken, industrial grinding processes can very easily ruin the temper on a knife. Because of this, the low 400F temper and its benefits are only really practical for small scale makers who are grinding and finishing knives by hand in highly controlled environments.

To sum it all up, the blades from Huntsman Knife Co. and other small scale makers are significantly better than any mainstream mass produced knife because the optimal low temper heat treats we utilize simply do not scale to high volume manufacturing. Like our motto says, our knives are Made the Old Way.