Step 7 - Heat Treating The Knife

 

All of the drilling, filing, and basic shaping of the knife was completed with the knife steel in a soft, or annealed state. The blade must now be hardened or heat-treated. Once hardened, it is nearly impossible to drill through it. It can, however, be ground with the appropriate equipment. 

The heat treatment of fine blade steel is absolutely critical to performance. So critical, that we believe the heat treatment process is more crucial to blade performance than the actual type of steel used. Muhlhauser knives are not just meant to look nice, they are designed to be used in the field. They must perform better, longer, and more durably than our competition. To that end, I normally use high performance steels – and make sure they are heat treated to our exacting specifications.

In heat treating steel, we are hardening the steel through an actual change in the composition of the steel itself. Prior to heat treatment, the steel is austenitic - soft and easily workable (well, as far as steel goes anyway). During the heat treatment process we transform the austenite into martensite, making the steel harder, tougher, more durable, and ready to hold an edge.

Now, there are two schools of thought among knifemakers when it comes to heat treating blades. Some chose to send their blades to an outside heat treating company to ensure quality. Others elect to perform their heat treating in-house. We believe that both methods are very acceptable, provided however, that those performing heat treatment processes in-house are highly competent at it.

At M.G. Muhlhauser & Sohne, we heat treat all of our knives at our own shop. We do so to my exacting standards. There are two main reasons we do this: 1- It is our opinion that we, as knifemakers, should know how to properly heat treat our knives, and know how variables within the process affect blade performance both positively and negatively; and, 2- We can control and ensure the quality of the process. As a  reputable knife maker, I do not want to outsource something so vital to the success of our product, or be at the mercy of an outside firm. 

With that said, I have absolutely no issues with those makers who send their blades out for heat treatment. There are plenty of good ones who use this method.

Now on to the actual heat treatment process of our knife blades… First, there is no real "art" in how we treat steel. We use mostly very high tech performance steels, and the manufacturers of these steels have done considerable research in how they should be handled. So, there are specific recipes that must be adhered to for the proper outcomes. Some of the published recipes are for larger, thicker pieces of steel and we have contacted the manufacturer for variables related to knife blade thicknesses. 

The first step is to ensure that the knife is being treated in an atmosphere void of oxygen. There are three main ways to accomplish this, vacuum furnaces, inert gas injection systems, and steel wrapping. I have tried all three and returned to wrapping our knives in steel envelopes. The blade is wrapped in a special steel foil. I place a few strips of wood shavings in each packet along with the knife. I then make an envelope of sorts making sure to double fold every edge so no oxygen can enter.

Naturally, wood will burn before the knife steel. By inserting wood into the envelope, any residual oxygen will be burned as the wood ignites, leaving the knife in an oxygen-free atmosphere as it heats to nearly 2000 degrees Fahrenheit (1093 Celsius). Failure to heat treat a knife without these type of precautions, would cause a great deal of scale to build up on the knife.

Once nicely packaged, the knife goes in the oven. I use Paragon heat treating ovens. With most types of steel, we will normalize the steel, whereby we hold or "soak" the steel at a lower temperature first (say around 1400 degrees F) for a little bit, before ramping the oven to its final temperature. . Each type of steel has a different hold time and temperature sequence. We pre-program these recipes into our ovens to ensure consistency for every type of steel we use. 

Once the steel has achieved the correct target temperature and has soaked for the appropriate time, it is removed from the oven and quickly taken out of the steel envelope. It is then "quenched" in a special quenching oil which has been pre-heated to between 125 and 140 degrees Fahrenheit. This cools the steel extremely rapidly converting most of the austenite to transform into martensite. In some instances, I prefer to use large aluminum quenching plates, rather than the quench oil. 

You noticed I said "most" most of the austenite gets transformed into martensite. Some of the austenite crystals may not transform into martensite at the time of quenching. While it is acceptable to leave the knife steel in this state and then temper it, M.G. Muhlhauser & Sohne performs cryogenic treating to ensure a more complete transformation. Our cryogenic treatment process has the added benefit of increasing toughness and wear resistance. Cryo treatment can be performed with either dry ice (about minus 100 degrees Fahrenheit), or preferably liquid nitrogen (about minus 300 degrees). I use liquid nitrogen as you can see in the pictures. 

At this stage, the steel is a bit too brittle to be used for a knife and is therefore tempered. Tempering the steel involves heating it below its lower critical temperature. For some steels that means heating to 400 F, and others to perhaps 600F. Most of our steels receive a double temper, meaning it goes through two tempering cycles, cooling in between.
To ensure that we got the process right, the steel is then tested for the correct hardness. We perform this test using a Rockwell Hardness tester. Some steels are designed to be used harder than others, and will therefore have different target hardness. For example, we generally treat our 440C stainless steel in a manner which will produce a hardness of approximately 58 on the Rockwell "C" scale. A similar knife made of CPM154CM may have an aim hardness of 60 or 61. To test the steel, the knife steel is placed on a flat surface of the tester, while a diamond tip probe is lowered onto the steel. The diamond probe then has a great deal of force placed upon it by the testing machine, pushing the diamond ever so slightly into the steel – creating a small dimple. The machine measures the movement of the diamond to read the hardness. 

Every single Muhlhauser knife is tested for correct hardness using a calibrated Rockwell hardness tester. Actually, we test every knife three times – just to ensure the quality you have come to expect from us. 

Now its time to clean up the blade and put on the bolsters, handles, and pommels.