For Maximum Toughness Without Sacrificing Edge Performance, Consider A Soft-Backed Blade.
For centuries, smiths from various cultures recognized the necessity of blades that do the job without critical failure. Some Eastern cultures specialized in differential hardening, particularly the Japanese. European cultures tended to forge weld a hardenable edge to a softer spine. In each case, the end result is an edge hard enough to cut well and a spine tough and resistant to breaking.
One of the measures of a modern knifemaker is the willingness and ability to successfully navigate the certifications processes of the world of knives, such as the American Bladesmith Society’s (ABS) journeyman and master smith certifications. Similarly, one measure of a knife blade is its ability to perform various tasks that put its edge geometry and heat treatment to the test.
If you’ve been around the knife industry any time in the past 40 years or so, you’ve no doubt seen stories of blades that demonstrate edge holding ability that also have the strength and flexibility to bend without breaking. The key to this kind of performance is for the blade to have a hard edge and a soft back. Pardon the pun, but let’s break that down a bit.
First things first—a knife must cut. One key to premier cutting performance is a well-hardened edge. A properly hardened edge comes from the use of steel with adequate carbon, brought to the proper hardening temperature, quenched in the proper quench medium, and tempered to the proper hardness.
Each of these variables has an impact, but at the end of the day if the maker does things right, the edge of a well-performing blade will be hardened and tempered martensite in the 58 to 60 HRC Rockwell hardness range, plus or minus a point or two depending on geometry. If the edge is left too soft, the blade will not cut as it should.
But what about the blade’s spine? Does it need to be as hard as the edge? The answer is “it depends.” Hardness as a metallurgical property is directly opposed to toughness, in that the harder the blade the more brittle it is. At the same time, an unhardened blade of annealed steel is very difficult to break but won’t hold an edge. If the maker hardens the blade fully from edge to spine, the knife will be more likely to break under chopping or bending loads.
Let me talk about “more likely” in this case. Are you more likely to be struck by lightning or bitten by a shark? As it turns out, you’re about 7.5 times more likely to be struck by lightning than bitten by a shark, but the odds of being struck are only 1 in 500,000. Either way, neither is very likely. I state that because while it is empirically true that a fully hardened blade is “more likely” to break than a blade with a soft back and hardened edge, if the maker does the heat treatment properly and the user uses the blade properly, either will perform very well with a very low risk of catastrophic failure.
Soft-Backed Bennies
Even if the overall risk of breaking in normal use is small, it still makes sense that some makers want to chase that last bit of performance by taking advantage of the strength of the soft-backed blade. So what does a soft-backed blade do? What are the benefits? By hardening the edge but intentionally softening the spine, the blade is more resistant to breaking when subjected to extreme force. This remains true whether the force comes to the blade parallel as in chopping or perpendicular as in bending.
ABS journeyman smith and past Forged in Fire champion Shawn Ellis is a proponent of the soft-backed blade. He originally learned the edge hardening process from ABS master smith Jim Crowell in the early 1990s when Jim demonstrated edge hardening using an acetylene torch.
He heated the edge only, quenched the blade, and then used the torch to draw the temper back. To temper, he started with the torch at the spine and used a wet rag to keep the edge cool. When he was finished, the edge was a light bronze and the spine was blue.
Even though that was quite a few years ago, the method Crowell taught Ellis remains viable. Heating only the edge leaves the spine soft and annealed but transforms the edge into the tempered martensite required for cutting performance. Using the torch afterward to “blue back” the blade relieves any internal stresses that may result from heating only the edge to the critical temperature, that is, to a non-magnetic state.
Ellis found that heating the blade at least a third of the way from edge to spine resulted in better performance. Subsequently, he switched to heat treating in a forge, and then later in an oven.
While it’s possible to edge harden with a forge or an oven, the methodology changes. Edge hardening in this way results in a martensitic edge and a spine that retains the microstructure it previously had. If the blade was forged and normalized, the spine would be soft pearlite. If the blade was ground from bar stock, the spine would be spheroid carbides.
Another way to achieve a hard edge with a soft back is to quench the edge only. You can bring the whole blade up to temperature in a forge or oven, and then place only the edge of the blade in the oil for hardening. This is the method currently taught at the ABS schools.
In this case, you need enough oil volume to cool the steel properly, so you can’t just fill your quench tank with oil to the half-inch level. Instead, use a limiter plate to keep the edge at the right depth while allowing the tank to have an increased volume of oil. You rock the blade in the oil in such a way that the edge is quenched properly while the spine cools slowly.
The slow cooling of the spine results in a pearlite microstructure, while the quenching of the edge results in martensite. The result is again a hard edge and soft back, and a “blue back” draw is again recommended. Blades that are edge hardened are resistant to breaking, though the soft spine may bend and stay bent.
The final way to achieve a soft-backed knife is the most difficult but results in the highest performance. In this method, the blade is hardened completely, with the whole blade up to critical temperature and the whole blade quenched fully. After regular tempering in the oven, the blade is then torch tempered on the spine.
In this scenario, the spine is pushed past blue all the way to gray, but not to the point where it starts to show heat color. Essentially in this case, you’re applying differential tempering rather than differential hardening, adding heat in the 650-800°F range to the spine only, resulting in highly tempered martensite. Since you’re working with higher temperatures, extra care must be taken to keep the edge cool by keeping it immersed in water or wet sand.
ABS master smith Scott McGhee teaches the Introduction to Bladesmithing class at Haywood Community College in Haywood, North Carolina. Students learn from McGhee how to make a knife that will pass the ABS performance test using a full quench and a torch draw. He prefers 1075 carbon steel and uses two propane torches to draw the spine evenly, keeping the edge cool in wet sand.
STRUCTURAL RESULTS
I asked metallurgist Larrin Thomas for insight on the structural results of the different edge hardening approaches. According to Larrin, “When it comes to Rockwell hardness, torch tempering a fully hardened blade to 650-800°F results in hardness in the Rockwell hardness range of 45 to 55 HRC, depending on the steel and the time spent at the tempering temperature. Slow cooling the spine by quenching the edge will result in soft pearlite similar to air cooling during normalizing and grain refining. This puts the spine in the range of 20 to 40 HRC, depending on the steel. Some steels have such high hardenability that they will partially harden during air cooling, such as oil quenching steels O1 or L6, and may even reach 60 HRC if the cross section is thin enough.
“The softer the spine, the more resistant the blade is to breaking but the more likely it is to take a “set” during bending. In other words, less bending of a softer spine would be required for the blade to stay permanently bent. The ideal condition would be one where the steel has sufficient hardness to avoid staying bent but sufficient toughness where it will not fracture. This combination of properties is best achieved by a full quench and a spine torch temper.”
For many years now the certified smiths of the ABS have demonstrated competence at these processes. If you want to climb the ladder of knifemaking skills and certifications, or if you want to climb the ladder of high performance hard-use knives, the heat treatment of a soft-backed knife is a skill you’ll want to master. If you’re the kind of knifemaker or the kind of knife buyer who wants 100 percent of the toughness a blade can offer without sacrificing edge performance, consider a soft-backed blade.
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