Understanding Pan and Knife Metals
Austenite and Martensite 

by Byron Bitar, Ph.D.
Owner, A Cook's Wares
Professor of Philosophy
Geneva College 

Part I

         The stainless steel used in pans and cutlery is quite different.  In making informed decisions about cookware, it helps to understand the basic differences.

         Steel is a mixture of iron and carbon.  Iron is a soft metal composed of fairly large iron crystals.  By mixing in carbon, it becomes hard.  The more carbon, the harder the metal.  Steel has up to 2% carbon; cast iron over 2%.

         Chromium is an essential ingredient in stainless steel.  It was first isolated as an element in the 17th century and from that time used as an alloy in iron.  Chromium is valuable because it protects iron from corrosion, particularly rust.  It bonds with oxygen to form a layer on the surface of the metal.  Oxygen cannot penetrate the layer to oxidize, that is to say, "rust" the metal.  However, it was not until 1915 that chromium stainless steel began to be used commercially.  The first use was for metal to make knives.  In the 1930's, it began to be used to make pans.

         There are three basic kinds of stainless steel:  ferritic, austenitic, and martensitic.  All have carbon and chromium.  Ferritic stainless steel is the softest and least corrosion resistant, and not of direct interest to us.  It is austenitic and martensitic stainless steels that are used in kitchen equipment.  Steel in an austenite form is used in pans and utensils; steel in a martensite form is used in cutlery. 

         Austenitic stainless steel:  used for pans and utensils
         Martensitic stainless steel:  used for knives

While "austenite" and "martensite" technically refer to phases of steel, in the interest of simplicity, I am going to use the terms to refer to the two different kinds of steel as well.   After all, the two kinds are composed of primarily steel in an austenite or martensite form.

         Austenite has a low carbon content (less than .12%) and a high nickel content (usually 8% - 10%).  Nickel is harder than iron, and also resists oxidation and corrosion.  Martensite has a higher carbon content (usually .45% - 1%) and usually no nickel.  Austenite is softer and can be pressed into shape to make pans and utensils.  It is also very corrosion resistant; it can hold hot and cold liquid and acidic foods for long periods of times and not corrode.  In addition, it is non-magnetic.  That is why a magnetic core has to be engineered into pans so they can work on an induction range.

         Martensite is very hard, about 10 times as hard as austenite, and has to be cut and ground to make a knife.  Its hardness enables it to take a micron thin edge and hold the edge through repeated slices and chops involving food, bones, and cutting boards.  Martensite is less corrosion resistant than austenite, and is magnetic.  Because it is magnetic, knives can be stored on a magnetic wall holder, such as a magnabar, and magnetic honing steels can be used to realign a dull edge.
     

     
         The austenite used in pans and utensils is usually called "18/8" or "18/10."  The first figure gives the chromium content - namely 18%; the second figure gives the nickel content - namely 8% or 10%.  18/10 stainless steel is more expensive; it is also more corrosion resistant and more silver in appearance.  However, either steel can be (a) highly polished, (b) polished, or given (c) a mildly polished satin finish or (d) a non-polished brushed look.  The degree of polishing has nothing to do with the amount of nickel.  For instance, the 18/10 stainless steel of All-Clad's Stainless Steel line of pans is highly polished; the same steel in the Sitram Professional and Catering lines is polished, and in the Bourgeat Excellence and Tradition Plus lines is satin.  Also, the 18/10 stainless steel in Mauviel's Pro-inox saucepans, frypans, and sauté pans is highly polished, but, in the same line, the buffet casseroles, stew pots, and stock pots have a satin finish.

                        Austenite 18/10 Stainless Steel:  18% chromium, 10% nickel
                        Different Pan Finishes
                        Highly polished:  All-Clad Stainless Steel
                        Polished:  Sitram Professional
                        Satin:  Bourgeat Excellence

         Almost all of the pan and utensil companies A Cook’s Wares® deals with use 18/10 stainless steel; Endurance and Update use 18/8.

         The martensite used in cutlery today usually has chromium.  In which case it is called "stainless steel" or "no-stain steel."  However, martensite need not have chromium.  "All carbon steel" knives - such as Sabatier au Carbone - lack the chromium.  As a result, the metal is softer than their stainless steel cousins, and the knives both dull and sharpen easier.  All carbon knives also rust and stain readily.  Nevertheless, some cooks prefer them because of the ease of honing and sharpening them.  That is why A Cook’s Wares® carries them.   

      
         The martensite used in Solingen cutlery has a formula that specifies the elements in the metal.  You will sometimes see the formula stamped on the blade of the knife.  The Solingen international formula, true of Schaaf First Class and Goya knives, is as follows:

X 45 Cr MoV 15

X means the metal is stainless.
45 gives the carbon content:  .45%.  (More accurately, it is between .42% and .5%.)
Cr MoV 15 gives the total percentage of chromium, molybdenum, and vanadium together:  15%.
The chromium is about 14%, and the molybdenum and vanadium together less than 1%.

         To understand more about austenite and martensite, the nature of iron crystals needs to be explained.  This is a bit technical, but will help a lot in comprehending the differences.  That is the subject of Part II.
        
                                                                Part II

         Austenite is the stainless steel used for pans and utensils, and martensite the stainless steel used for cutlery.  In brief, their differences are as follows:

         I want to discuss the nature of iron crystals to help you better understand the differences, and also understand the process of making the steel used in cutlery.

         Iron crystals have two different forms.  One is a body-centric cubic arrangement known as "bcc," and the other is a face-centric cubic arrangement known as "fcc."  Sounds complicated, but it is not.  Let's start with the body-centric arrangement.  Imagine a cube with an iron atom at each corner.  Then imagine an additional iron atom in the center of the cube.  That is the body-centric arrangement.  Its crystal consists of a total of 9 iron atoms.  The body-centric arrangement is the arrangement pure iron atoms have at room temperature.

         The face-centric cubic arrangement is different.  Imagine a cube again with one iron atom at each corner.  That is eight atoms.  Now imagine another iron atom in the middle of the face of each cube, but none in the very center of the cube.  There are six faces, so that is six more iron atoms.  That is the face-centric arrangement.  The face-centric iron crystal has a total of 14 iron atoms.  The faces of the face-centric crystal are 25% larger than the faces of a body-centric iron crystal.  That means there is more space for other atoms, such as carbon, to lodge in the face of the crystal forming an iron alloy.  When this happens, it is called a "solid solution."

         Three more points and the technical material will be covered.

         First, iron crystals link together to form crystal structures called lattices.  The corner atoms of one crystal are shared with the next.  So two bcc crystals have four shared atoms.  Two fcc atoms have five shared atoms.  The individual crystals are fairly small.  It takes millions of these tiny crystals to form a single grain of iron.

         Second, at room temperature iron crystals have a body-centric arrangement.  When the iron is heated above 1674º F, the crystals change into the face-centric arrangement.     When iron is further heated to get above 2541º F, the crystals go back to the body-centric arrangement.

         When iron is in a body-centric arrangement, it is called “ferrite.”  When it is in a face-centric arrangement, it is called austenite.

Iron's crystal arrangements

         Note:  at 2,800º F, iron becomes liquid.

         2,541º F and above - body-centric arrangement (bcc); ferrite ("delta iron")
         1,674º to 2,541º F - face-centric arrangement (fcc); austenite ("gamma iron")
         room temp to 1,674º F - body-centric arrangement (bcc); ferrite ("alpha iron")

         Third, there are three ways elements, such as carbon, form alloys with iron.
(1) Solid solution:  The atoms of the element can get in between two crystals, embedded                    in the common face of the crystals. This is called a “solid solution.”

(2) Carbides:  The second way is for the element to form a single molecule with iron          atoms.  The molecule is known as a carbide.  For instance, three iron atoms and one carbon atom form a single molecule (Fe₃C) called cementite.  The carbides exist as little islands of particles in and among the crystal lattices, breaking up the crystal lattices.
(3) Substitutional solid solution:  The third way is for the element to take the place of    an iron atom in an iron crystal.  This is what nickel does. A nickel atom takes the place of    an iron atom in the iron crystal.  This is called a “substitutional solid solution.”

         That's all the technical material.  I am sorry about having to go into these details, but it is necessary to really understand austenite and martensite.

Austenite       

         Austenite used in pans is unusual.  Why?  Because at room temperature the stainless steel has the austenitic crystal arrangement, that is to say, the face-centric arrangement. How is that possible?  It is due to the nickel.  Nickel takes the place of iron atoms in the iron crystal and holds the crystal in a face-centric arrangement.  Nickel, for that reason, is called an “austenizer.”  Chromium is also held in solid solution.  In general, austenite consists of very clean fcc crystals in which all the alloying elements are held in solid solution.  The resulting austenite stainless steel is soft enough so it can be pressed at room temperature into the shape of pans and utensils, and, because of the nickel and chromium, is very corrosion and stain resistant.  Also the nickel is held securely in the iron crystals so it will not leech into the food.  Same with the chromium.  So it is a very hygienic steel for cooking and storing food.

Austenite – caused by nickel atoms
Austenite – an unusual steel because at room temperature it has a face-centric crystal structure.  This is due to the nickel mixed into the steel.

Martensite    

         Martensite is also unusual, but quite different.  It is formed (1) by alloying iron with carbon and usually chromium, and (2) by rapid cooling, called “quenching.”  The process begins with a piece of ferritic steel.  It has a bcc crystal structure with a little bit of the carbon held in solid solution, but most of the carbon contained in the form of carbides, that is to say, individual molecules formed with iron as explained above.  The metal is heated above 1674º F.  The ferritic structure is erased.  The hot steel forms ordinary austenite with a fcc crystal structure.  The carbon in the carbides is released and flows in between the iron crystals to form an iron and carbon solution.  When a forged knife is made, red-hot austenite steel is placed in a forge and pounded into shape.  The pounding also (1) breaks up the size of the crystal lattices and (2) randomizes the direction of the crystal lattices to form a fine grain in the metal.

         At this point, if the typical steel alloy is allowed to gradually cool to room temperature, it will reform into ferritic steel.  Carbon will flow out of solution back into carbides.  But that is not allowed.  The metal is plunged into a liquid, usually liquid nitrogen, to rapidly cool it.  This is called “ice-quenching.”  More generally, it is called “hardening by heat treatment.”  It causes most of the carbon to be trapped in solid solution in a bcc crystal formation.  This "unnatural" solid solution with a distorted crystal structure - that is to say, too much carbon in solution - is super hard, 10 times harder than the metal that was first heated.  It has a diamond pyramid hardness of 1000!

Martensite – caused by quenching, the rapid cooling of steel
Martensite – an unusual steel because at room temperature it has a lot of carbon trapped in its crystals.  This is due to the quick cooling of the steel, called “quenching.”  The quick cooling in Germany is called “ice-hardening.”
The hardening of the metal by trapping carbon particles in its crystals is called “solution hardening” or “hardening by heat treatment.”

         The resulting martensite is hard, but also brittle.  To get flexibility and tensile strength back into the metal, it is heated to between 600º F and 1,500º F, never above, and then cooled again.  This enables some of the carbon atoms to come out of solution and form carbides - a more natural, relaxed state for a bcc crystal formation.  The process of getting flexibility and tensile strength back into the metal is called “tempering.”  The carbides also contribute to the metal's hardness; this is called “precipitation hardening.”

                Two kinds of hardening used to produce martensite:

Solution hardening - trapping carbon in the face of iron crystals.  This is achieved by rapid cooling or quenching.
Precipitation hardening - permitting carbon to form molecules with iron, called “carbides.”  This is achieved by alloying the metal and tempering it.

         In summary, martensite is formed by quenching, creating an unusual steel which is super-saturated with carbon in solid solution, even though the carbon is usually only .5%.  It is then tempered to gain flexibility and tensile strength.

                    Two procedures used to produce martensite:

Quenching – rapid cooling which creates martensite, a steel that is super-hard because super-saturated with carbon in solid solution.
Tempering – heating to moderate temperature (600°-1,500° F) which relaxes martensite by permitting some of the carbon to move out of solution to form carbide particles.  The martensite has greater tensile strength and flexibility.

         An important moral to our story is this:  when sharpening a knife, do not use an electric sharpener that will significantly heat up the knife-edge.  If sparks are being produced, the heat is too high.  It will erase the crystal structure resulting in ordinary soft ferrite.  Chef'sChoice electric knife sharpeners are engineered so that cannot happen.

         Last point:  The austenite used for pans and utensils, because of the nickel, cannot be hardened by quenching; i.e., by heat treatment.  More generally, it can handle a wide range of heat and cold without affecting its crystal structure.  That is important since pans and utensils, such as bowls, get heated and cooled regularly, and often quickly, even put in the freezer.

Summary

Austenite (austenitic stainless steel) - a very stable corrosion resistant stainless steel with a face-centric crystal structure at room temperature, due to the nickel in substitutional solid solution in the iron crystals, and due to the chromium in ordinary solid solution.