Metallurgy

Introduction to Knife Steels

The steel is without a doubt the heart and sole of any good blade. When considering types of steel for a knife build a knifemaker will always take into consideration the intended purpose of the knife in question. Even in today's high tech world there is no magical steel which will be ideal for all situations. Some steels offer superb edge retention, some a greater degree of toughness, some offer superior resistance to corrosion while other steels offer more flexibility. All of these attributes are brought forth as a result of the engineered chemical composition of the steel. All steels are not created equal!

Finding the "perfect" blade material has been an ongoing quest for centuries. Through modern science and metallurgy, today's steels are engineered to be superior to anything which came before them. From plain carbon steel to high carbon tool steels and even stainless steels, today's modern materials have proven better than their ancient counterparts. In an effort to better understand what makes up a quality blade steel we must first understand the key properties of steel and how the chemical elements which are added to the steel effect those properties.

Key Properties of Steel

Hardness - a measure of a steel's ability to resist permanent deformation under pressure. Imagine pressing your thumb hard against a ball of modeling clay versus a ceramic floor tile. Once you remove your thumb, the tile will have resisted any permanent deformation however, the ball of clay will not have. Hardness is often measured with a Rockwell hardness tester and as a result displayed as a unit of Rockwell. For example 60 RC.

Toughness - the ability to absorb energy without chipping or fracturing. To visualize this, think about striking an egg's outter shell with the edge of a butter knife. Now imagine doing the same to a rubber ball with an equal amount of force. The egg's shell will fracture and chip while the rubber ball will not. In this case, the rubber ball has a much higher degree of toughness than that of the egg shell.

Wear Resistance - the ability to resist abrasion and material erosion during use. Picture yourelf slowly dragging a wax candle across a piece of sandpaper. Now imagine dragging a glass mason jar across that same piece of sandpaper. The candle will be rubbed away where it met the sandpaper while the glass mason jar will show much less wear. In this example, the glass jar has greater wear resistance.

Ductility - the ability of a steel to stretch without fracturing. Picture a wad of chewing gum being stretched from either end in opposing directions. Now picture the same being done to a glass rod. The chewing gum will stretch and become narrower at it's cross section but it can be stretched a long way before it finally breaks. The glass rod however, will not stretch and eventually with enough force it will fracture. In this example the chewing gum is more ductile as it has the ability to stretch much further before breaking.

Strength - the ability to withstand stress without permanent deformation. Think about using enough force to bend a wire coat hanger. Now imagine using that same amount of force while trying to bend a wooden broom stick. The coat hangar will bend and remain bent however, the broom stick remains straight. In this example, the wooden broom stick has a greater degree of strength.

Corrosion Resistance - the ability to resist oxidation (rust) or staining.

Hardenability - the ability of a steel to be hardened through the process of a controlled heat-treatment. The exact process or temperatures required are slightly different for each type of steel. Not all steel is hardenable and that is greatly dependent on it's carbon content.

Machineability - the ease of which a steel can be machined or shaped with tools. Picture using a hacksaw to cut through a carrot. Now picture using the same hacksaw to cut through a steel rod. The carrot cuts with much less effort and therefore has a higher degree of machineability.

Chemical Elements added to Steel

Carbon - is not really an alloying element since it is present in plain carbon steels nonetheless increasing the amount of carbon in a given steel alloy will increase that steel`s hardness.

Chromium - improves hardenability, wear resistance and most of all corrosion resistance. A steel with a chromium content of at least 13% is generally referred to as a stainless steel. Adding this element in high amounts can reduce toughness.

Nickel - improves toughness.

Manganese - improves hardenability, strength and wear resistance while aiding the grain structure.

Molybdenum - improves hardenability and reduces a steel`s brittleness while increasing it`s strength at higher temperatures. Typically, air hardening steels will have at least a 1% molybdenum content.

Vanadium - improves wear resistance and hardenability while it promotes a fine grain structure. Typically fine grain structures are desirable and are an important factor in wear resistance and strength.

Different Types of Steels used by Bladesmiths

Plain Carbon Steels

10XX Steels - These steels are sometimes referred to as "High Carbon" steels. Throughout the 10XX series of steels, 1045-1095 are often used for making knives with 1095 being the most common. In this series of steels, the last two digits of the series designation refer to the amount of carbon in the steel. For example, 1045 contains .45% carbon where 1095 has .95% carbon content. Typically in the 10XX series of steels, the higher the carbon content the lower the manganese. That is to say that 1095 steel has less manganese content than 1045 does. All said and done this equates to 1095 being more wear resistant, it holds a good edge, is easy to sharpen but its less tough while 1045 is less wear resistant and more tough. The major drawback to plain carbon steels is that they rust quite readily. You will often see plain carbon steel blades being coated with something to combat corrosion.

 




Alloy Steels

5160 - A steel which is popular with forgers. This is a plain carbon steel with typically a .56- .64% carbon content (1060 series) and a small amount of chromium added to it. There is enough chromium added to strengthen the steel but not enough to make it a stainless steel. It has good wear resistance but 5160 is known for its outstanding toughness. This steel is well suited for larger blades like swords as it offers great toughness when hardened to the low 50's RC however, 5160 is also good for knives and offers good edge retention when hardened to near 60 RC.

 




Tool Steels

15N20 - Often seen in the making of industrial bandsaw blades.




52100 - Formerly a ball-bearing steel, this is a high carbon tool steel typically containing .98-1.10% carbon. This steel holds an edge well, better than 5160, as it is much harder than many other steels. The biggest drawback to this type of steel is it's lower chromium content which allows it to rust easily.

A2 - An air hardening tool steel. While this steel has less wear resistance than many other tool steels, it is very tough. It is tougher than D2. As a result of being an air hardening steel, it is very difficult to obtain a differential heat treat or edge quench. This steel is often used when making combat knives due to it's high level of toughness. Typically it contains 1.00% carbon & 5.00% chromium.

CPM 10V Steel - CPM is an acronym which stands for Crucible Particle Metallurgy and is a brand name. While having decent toughness this is one of the most wear resistant tool steels.

D2 - This steel contains higher levels of chromium (12%), nearly enough to classify it as a stainless steel and is often referred to as "semi-stainless". It does have a fairly high carbon content of 1.50-1.60%. As a result of the higher levels of chromium it has very good rust resistance. While it is much tougher than most other stainless steels it is typically not as tough as other tool steels. This steel also offers excellent wear resistance and fantastic edge retention but it can be difficult to sharpen. Furthermore this steel can be difficult to polish to a mirror finish.

L6 - Was often used as a band saw material (and still can be) as it's tough and holds an edge well. Though many new bandsaw blades are made with 15N20 many of the older blades were L6. This steel is often used for cutlery however, any knife made from this steel will require constant maintenance to avoid issues with rust. L6 is one of the choices forgers often turn to since much like O1 tool steel it's "forgiving". It is often coupled with O1 or 15N20 for pattern welding damascus billets. Typically it contains 0.70% carbon, 0.70% chromium & 1.40% nickle.

O1 - Popular with forgers as it has a reputation for being "forgiving". This is a hard tool steel which accepts an edge well and offers very excellent edge retention. O1 is also quite tough but not as tough as 5160. Like most all other carbon steels the drawback with this steel is it's susceptibility to corrosion. Typically it contains 0.94% carbon & 0.50% chromium.

 

Heat Treating Practices for Given Steels