Heat Treatment of Steel

Introduction

Heat treatment is the operation of heating and cooling a metal in its solid state to change its physical properties. An important factor is the time the cooling takes.

According to the technique used, steel can be made hard to resist cutting action and abrasion, or it can be softened to permit further machining.

Further, internal stresses may be removed, grain size reduced, toughness increased or a hard surface produced on a ductile interior.

Alloyed steels owe their properties to the presence of one or more elements other than carbon, so I will discuss mainly the techniques of heat treatment of ordinary comercial steels.

Important Words

Ferrite pure iron (e.g. no carbon is present), the shape of the crystal and the ammount of iron atoms that form a crystal depends on the heat

Austenite face-centered iron (g-iron) with one carbon atom in the center

Cementite also called Iron Carbide (Fe₃C), the most hard constituent of steel; occurs naturally at high carbon contents

Pearlite mixture of Ferrite and Cementite; steel has a 100% pearlite composition at the eutektoid point (0,8% carbon)

Martensite very hard content of steel; it is obtained in hardening treatments

Allotropic Changes in Iron and Steel

If a piece of carbon steel is slowly and uniformly heated, and then cooled, and its temperature recorded, an inverse rate curve is obtained.

The three changes in each curve point out that structural changes occur. These points are called critical points. On the heating curve they are designated with c for 'chauffer' (french) which means to heat, on the cooling curve the r stands for 'refroidir' (french) which means to cool.

The changes that occur at these temperatures are a change in the atomic structure, electrical resistance (and loss of magnetism at one point).

Let's heat a piece of steel for example (for cooling the process is reverse).

At Ac₁ the steel becomes non-magnetic, at Ac₂ the body-centered a-iron changes to the face-centered g-iron, at Ac₃ this structure changes back to a body-centered form called d-iron, further heating will melt the steel (at 1536°C iron melts for example).

The temperatures at which these changes happen are different for every steel and should be known, because most heat treatments require heating above this range .

So, for example, steel can not be hardened unless it is heated to a point above the lower, or even the upper, critical point.

The Iron-Carbide-Diagramm

This diagram shows us the temperatures at wich the allontropic changes occur for every carbon content a steel may have.

Three carbon contents are to be mentioned here.

First, at 0,8% carbon the steel consists of a 100% pearlite structure (eutektoid point), at 2,08% the alloy is no longer steel, but cast-iron and at 4,3% the iron-carbon alloy has its lowest melting-point ( Eutektikum).

At 2,08% the cementite content is at about 11%, at 6,67% carbon the steel has a 100% cementite structure.

Time-Temperature-Transformation Diagram

The iron-carbide diagram is useful in selecting temperatures for parts to be heated in various treating operations, and it also shows the type of structure to expect in slowly cooled steels.

Although very useful in all heat-treating operations, it does not give much information concerning effects of cooling rate, grain structure, or structures obtainable with various quenching techniques.

This information is provided by the TTT-diagram or Isothermal transformation diagrams (also called S-curves because of their appearance).

The diagram shows how the structure of an austenitized steel changes when quenched in a given time.

To obtain a (in most cases desired) martensitic structure, the steel has to be cooled with sufficient rapidity, so the cooling curve does not intersect the nose of the transformation curve.

The less carbon the steel contains the more the curve will move to the left side, making the steel more difficult to harden. Most alloying elements move the curve to the right easier to harden.

Fine-grained steels also displace the curve to the left, but on the other hand are coarse-grained steels are more apt to crack or distort during quenching.

Grain Size

Molten steel, upon cooling, starts to solidify at many small centers. The atoms in such groups tend to be positioned similarly. The irregular grain boundaries are the outlines of each group.

The size of these grains depends on a number of factors, but the principal one is the furnace treatment it has received.

Coarse-grained steels are less tough and have a greater tendency for distortion than those having fine grain, but they have better machinability and greater depth-hardening power.

Fine-grained steels, in addition to being tougher, are more ductile and tend less to distort or crack during heat treatment.

The control of grain size is possible through regulation of composition in the initial manufacturing process, but after the steel is made, the control is through proper heat treatment.

Heating the steel until the upper critical point Ac₁ is reached, results in an average grain size of a minimum.

Further heating results in an increase of the size of the grains. So quenching from Ac₁ leads to fine grains, while quenching from a higher temperature would yield a coarser grain.

Hardening 

This is the process of heating a piece of steel to a temperature within or above its critical range and then cooling it rapidly.

If the carbon content is known, the prober austenitization temperature may be obtained by refering to the iron-carbide diagram.

In any heat-treating operation, the rate of heating is important. Heat flows from the exterior to the interior of steel at a defined rate.

If the steel is heated too fast, the outside becomes hotter than the inside and thus a uniform structure cannot be obtained.

If the piece is irregular in shape slow heating is even more important due to the danger of warping and cracking.

The hardness obtained by a given treatment depends on the quenching rate, the  carbon content, and the work size of the piece.

As I stated before, steel cannot be heated instantly throughout, because of this it is logical that the other way round steel cannot be cooled down in a moment, thus the hardening depth is limited because the inner parts of the workpiece take longer to cool down.

In large workpieces even the surface hardness will decrease slightly, because a lot of heat flows from the interior to the exterior, and so the surface cannot be cooled that fast.

Steels with a carbon content up to around 0,6% increase their hardenability with the ammount of carbon. Above this point hardness can only be increased slightly, because the steels are now made up entirely of pearlite and cementite in the annealed state. Pearlite responds best to hardening treatments.

Different Quenching Media

The main difference between th media in use is the speed in which they cool a piece to the desired temperature.

Air is the slowest media, only high alloy steels can be hardened by air-cooling.

Oil is faster than air but by far slower than water, the advantage of oil is that the piece is less likely to crack or warp during quenching.

Constituents of Hardened Steel

It has been stated that above the upper critical point all carbon steels are composed of austenite, a solid solution of carbon in g-iron.

If such a steel is cooled down slowly it will obtain a structure as laid out in the iron-carbide diagram. But if it is quenched fast then a martensite structure will form itself.

Martensite is a body-centered g-iron with an additional carbon atom in it's middle - this causes internal stresses that make this structure very hard but brittle.

Through control of the cooling time other structures may be produced that are not as hard, but more ductile.

Sorbit, Trostit and Bainite are very similar to Martensite with the exception that they are a little bit softer an more ductile.

Tempering

I have already mentioned the brittleness of martensite. By tempering or 'drawing', the hardness and brittleness may be reduced to the desired point for service conditions.

As these properties are reduced, there is also a decrease in tensile strength and an increase in the ductility and toughness of the steel.

The operation consists of the reheating of the quench-hardened steel to some temperature below the critical range (aprox. 150-300°C), followed by any rate of cooling. Although this process is similar to annealing it softens the steel not to such an extent. The final structure obtained from tempering is called tempered martensite.

Annealing

The primary purpose of annealing is to soften hard steel so that it may be machined or cold-worked.

This is usually acomplished by heating the steel slightly above the Ac₃ Point, holding the temperature until the piece is uniformly heated, and then cooling it slowly and controlled so that the temperature in the interion and on the surface are approximately the same.

This is called full anealing, because it completely refines the crystalline structure and so softens the metal. Annealing also relieves internal stresses set up in the metal.

Surface Hardening

Carburizing

This is the oldest known method of producing a hard surface. This process is merely heating iron or steel above Ac₁ while in contact with some carbonaceous material, which may be solid (coke or charcoal), liquid (cyanide salt bath with less nitrogen) or gaseous (propane or any natural hydrocarbon gases).

Iron, at temperatures close around its critical temperature, tends to absorb carbon. This carbon forms a solid solution with the iron and converts the outer layers into a high-carbon steel. How deep the carbon is absorbed depends on the time and the temperature.

Nitriding    

In this process the metal is heated to the temperature of around 500°C and held there for a period of time while in contact with ammonia gas or liquid cyanide salt.

Nitrogen from the gas is introduced into the steel, forming a very hard, but thin layer (for example from 0,02 to 0,6mm if liquid cyanide salt (in this case with less carbon) is used, the depth for ammonia gas is lower).

The surface effectively resists even corrosive action of water, salt-water spray, alkalies, crude oil and natural gas.

Induction Hardening

Here a high-frequency alternating current is used to heat the metal. For low hardening depths frequencies of about 500.000 Hz will be used, the frequency is lower if the depth is higher.

Since the hardening is accomplished by an extremely rapid heating and  quenching of the surface, the interior is not affected. The hardness obtained is the same as that obtained in conventional treatments and depends on the carbon content.

The equipment for induction hardening is expensive but this is offset by the advantages of the process, like fast, clean operation, little distortion, freedom of scaling and low treating cost.

Vocabulary

Appendix