Martensitic stainless steel is formed by the creation of martensite. Martensite has been a key element of quenched steel for hundreds of years, but was officially named in the 20th century after the metallurgist Adolf Martens (1850 - 1914).

Martensitic stainless steels can be heat treated and hardened, but have reduced chemical resistance when compared to austenitic stainless steels. Martensitic stainless steel is often used when hardness is critical, such is in knives, where surface hardness creates a sharper blade.

What ismartensitestructure

Chip Splitters – Notches in the circumference of a corn-cob style Endmill cutter resulting in narrow chips. Suitable for rough machining.

Austenitic stainless steels contain austenite, a form of iron which can absorb more carbon than ferrite. Austenite is created by heating ferrite to 912 degrees C, at which point it transitions from a body centred cubic crystal structure to a face centred cubic crystal structure. Face centred cubic structures can absorb up to 2% carbon.

How ismartensiteformed

Concave – Small hollow required on the end face of an Endmill. This feature is produced by a dish angle produced on the cutter.

Ferritic steels are made up of ferrite crystals, a form of iron which contains only a very small amount (up to 0.025%) of carbon. Ferrite absorbs such a small amount of carbon because of its body centred cubic crystal structure - one iron atom at each corner, and one in the middle. This central iron atom is what gives ferritic stainless steels their magnetic properties.

Dish Angle – The angle formed by the end cutting edge and a plane perpendicular to the cutter axis. Dish ensures that a flat surface is produced by the cutter.

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Angular Edge – The cutting edge that is a straight line and forms an angle with the cutter axis. The cut that an Angular edge tool produces will not be as flat as a helical cutting edge.

Core Diameter – The diameter of a cylinder (or cone shape with tapered Endmills) tangent to the flutes at the deepest point.

Define martensitein steel

Define martensiteand austenite

Relief-Space – Provided by removing material immediately behind the cutting edge. Done to eliminate the possibility of heeling or rubbing.

Martensite is a body centred cubic form of crystallised iron which is created when heated austenite is rapidly cooled by quenching. The increased rate at which Martensite crystals are created prevents cementite from being formed, and causes carbon atoms to become unnaturally trapped in crystals which would ordinarily expel excess carbon during gradual cooling.

Entrance Angle – The angle formed by a line through the center of the cutter at 90 to the direction of feed and a radial line through the initial point of contact. As this angle approaches 90 degrees the shock loading is increased.

Cementite is a form of iron which contains even more carbon than ferrite and austenite. Cementite contains up to 6.67% carbon. Because of its increased carbon content, cementite is hard and brittle, and its presence is usually a byproduct, rather than by design. Cementite commonly occurs in steels when excess carbon, such as left-over carbon which cannot be absorbed into ferrite, must be used for the formation of crystals.

When austenite cools, it generally reverts back to its ferrite form, which makes austenite difficult to utilise at anything below the extreme temperatures of a smelting furnace. Austenite can be forced to retain its crystal structure at low temperatures with the inclusion of chemical additives, such as the nickel and manganese found in many austenitic stainless steels.Austenitic stainless steels cannot be significantly hardened by heat treatment, but can be hardened by cold working. Austenitic stainless steels are widely used, particularly in stainless steel screws, due to their excellent resistance to corrosion.

Hook – A term used to refer to a concave condition of a tooth face. This term implies a curved surface rather than a straight surface.  Hook must be measured at the cutting edge, making measurement difficult.

Chamfer – A short relieved flat installed where the periphery and face of a cutter meet. It is used to strengthen the otherwise weak corner.

Martensitemicrostructure

Gash width – The width of the gash feature. The space between cutting edges, which provides chip space and resharpening capabilities. Sometimes called the flute.

Axial Runout – The difference between the highest and lowest indicator reading that is taken at the face of a cutter near the outer diameter.

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Martensitic pronunciation

Flute – Space between cutting teeth providing chip space and regrinding capabilities.  The number of cutting edges.  Sometimes referred to as “teeth” or “gullet”.  The number on an end mill will determine the feed rate.

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What ismartensiteand austenite

Gash (Notch) – The secondary cuts on a tool to provide chip space at corners and ends. The space forming the end cutting edge, which is used when feeding axially.

As iron cools, austenite crystals transition back into ferrite crystals, losing excess carbon which cannot be properly absorbed by the newly formed ferrite. The excess carbon creates patches of crystals with a mixture of low-carbon ferrite and leftover high-carbon cementite, and these mixed crystals are known as pearlite.

Differential pitch cutters – A specifically designed variation in the radial spacing of the cutter teeth. This provides a variation in tooth spacing and can be beneficial in reducing chatter. This concept is based on reducing the harmonic effect of the tool contacting the part in an exact moment of vibration.

Ferritic stainless steels are less widely-used due to their limited corrosion resistance and average strength and hardness.

Tangential rake angle – The angle made by a line tangent to a hooked tooth at the cutting edge & a radius passing through the same point in a plane normal to the axis.

The vast majority of metals have a crystalline structure in their solid state, meaning that they are made up of crystallised lattice structures of atoms. By definition, all steels, including stainless steels, are primarily made up of crystallised iron atoms with the addition of carbon. The iron in steel can exist in several different crystalline structures, dependent on the conditions of its creation. Ferrite, austenite, and martensite are all examples of iron’s crystal structures, and all are found within different types of steel. One of the defining differences between these crystal structures is the amount of carbon they can absorb - a greater carbon content generally, though not always, makes a steel harder, but more brittle.

Temperedmartensite

As a liquid, molten iron is not crystalline, and crystals are only formed when the material cools. When the material cools, steel solidifies as individual crystals forming gradually, which can mean that any one type of steel is actually made up of several crystal types as the metal slowly forms crystals through multiple temperature stages. This means that regardless of their defining crystal structure, it is not uncommon for steels to contain small mixed amounts of ferrite, austenite, and cementite.

Helix Angle – The angle formed by a line tangent to the helix and a plane through the axis of the cutter or the cutting edge angle which a helical cutting edge makes with a plane containing the axis of a cylindrical cutter.

Cutting Edge – The leading edge of the cutter tooth.  The intersection of two finely finished surfaces, generally of an included angle of less than 90 degrees.

Back taper – A slight taper resulting in the shank end of the cutting diameter being smaller than the cutting end. This condition aids not only the plunging or drilling condition but also tends to compensate for deflection.

While the information below covers ferritic, austenitic, and martensitic steels, almost all of Accu's stainless steel components have an austenitic crystalline structure. For more specific information on exact austenitic steel grades, please see our article on the many different grades of austenitic stainless steel.

Helical – A cutting edge or flute which progresses uniformly around a cylindrical surface in an axial direction. The normal helical direction is a right direction spiral.

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Rake – The angular relationship between the tooth face or a tangent to the tooth face at a given point and a reference plane or line.  An angular feature ground onto the surface of an end mill.

This video from Real Engineering does a great job of explaining how the crystalline structure of steel affects the physical properties of the material - the key points are discussed at 2:55: