Equilibrium microstructural transformations in steel

Ordinary steels

Despite, that a large number of complex alloy steels have been developed and used in the world, common steels (steel of ordinary quality, ordinary steel) remain, apparently, the most important iron-carbon alloys, which are used in technology.

Ordinary steels are commonly referred to as those iron-carbon alloys, which contain up to 2,0 % carbon. In practice, most common steels also contain a fairly large amount of manganese., which remained after the deoxidation process, which was carried out before casting. Below we will neglect the influence of this manganese and consider steels as simple iron-carbon alloys..

What makes steel thermally hardened?

Known, what pure iron has at temperatures below 910 ºС body-centered cubic atomic structure (OCK structure). If you heat iron above this temperature, then the structure will change to that, which is called the face-centered cubic atomic structure (Fcc structure). Upon cooling, the reverse transition occurs and the bcc structure is formed again (picture 1).

Picture 1 - Two types of atomic structure of iron in solid state:
a - body-centered cubic structure (OCK structure)
b - face-centered cubic structure (Fcc structure)

The peculiarity of these reversible transformations is that, what:

  • Carbon up to 2,0 % can completely dissolve in the fcc structure with the formation of, what is called "solid solution"
  • In the structure of the bcc, no more than 0,02 % carbon.

When a steel sample with an atomic structure fcc is slowly cooled, its atomic structure changes to bcc and all excess carbon dissolved in it (more 0,02 %) will be released from the atomic bcc structure. If the cooling is fast enough, then this carbon release does not occur. The ability of steel to receive thermal hardening is based on this phenomenon..

reference: Crystal structure of other metallic elements

Figure 1a – The crystal structure of some of the most important metallic elements [2]

Austenite, ferrite and cementite

  • Austenite - solid solution, which is formed by the dissolution of carbon atoms in the atomic fcc structure of iron (to 2,0 % carbon). Indicated by the symbol "γ" ("gamma"). The atomic fcc structure of pure iron is also denoted.
  • Ferrite - solid solution, which is formed by the dissolution of carbon atoms in the atomic bcc structure of iron (0,02 % carbon). Indicated by the symbol "α" ("alpha"). The atomic bcc structure of pure iron is also denoted.
  • Cementite. When carbon is released from austenite, it doesn't come in the form of elemental carbon (graphite), and as a compound of iron carbide Fe3C, which is called cementite. This connection, like most other metal carbides, is very hard. Therefore, with an increase in the carbon content (and, hence, cementite) the hardness of the slowly cooled steel also increases.

Equilibrium state diagram of steel

Equilibrium state diagram (status diagram, phase diagram) steel in the picture 2 shows temperatures, at which transformations begin and end for any solid solution of carbon (austenite) with bcc atomic structure and iron with fcc atomic structure. On the picture 2 only part of the overall iron-carbon phase diagram is shown. It is she who is very useful when considering the heat treatment of steels..


Picture 2 - The left side of the equilibrium state diagram of iron-carbon [1]

On the very left edge of this diagram there is an area, which is labeled "ferrite" (ferrite). It indicates the ranges of temperature and chemical composition, inside which carbon can dissolve in the bcc structure of α-iron:

  • To the left of the inclined line AB, all "available" carbon is dissolved in the bcc structure of iron with the formation of ferrite in the state of a solid solution.
  • Any point to the right of the AB line, representing the chemical composition (carbon content) and temperature, indicates, that the solid solution α is saturated, therefore the excess carbon (unable to dissolve) will be present as cementite.
  • The slope of the AB line indicates, that the solubility of carbon in the bcc structure of iron increases from 0,006 % at room temperature to 0,02 % at a temperature 723 ºS.

Microstructural transformations in steel

We will consider transformations, which occur in the structure of three characteristic types of steel. All these steels were heated to a temperature, which is sufficient, so that they become austenitic and then slowly cooled.

Conversions to carbon steels 0,40 %

  • If steel, containing 0,40 % carbon, heated to some temperature above U1 it goes into a completely austenitic state (picture 2 (i)). Temperature U1 called the "upper critical temperature" of steel.
  • When cooled just below the temperature U1 atomic structure starts to change from fcc to bcc.
  • Consequently, small crystals start to precipitate from austenite (grains) iron with bcc structure. These crystals with a bcc structure (picture 2 (ii)) contain a small amount of carbon (less 0,02 %) Are ferrite crystals.
  • With a further drop in temperature, ferrite grains grow in size due to austenite (picture 2 (iii)). Since ferrite is practically pure iron, then it follows, that the vast majority of carbon is collected in these shrinking austenite grains.
  • When the steel sample reaches the temperature L1, it consists of about half of ferrite (containing 0,02 % carbon) and half austenite (containing 0,8 % carbon). Temperature L1 called the lower critical point.
  • The composition of austenite at this stage corresponds to point E on the state diagram. At this temperature (723 ºS) austenite can contain in solid solution no more than 0,8 % carbon. therefore, when the temperature keeps dropping, carbon begins to precipitate as cementite. At the same time, individual sections of ferrite remain separated from each other, and in place of the remaining austenite, a lamellar structure is formed from alternating plates of cementite ferrite (picture 2 (iv)).

Perlite

This lamellar structure of ferrite and cementite contains exactly 0,8 % carbon and occupies about half of the volume for steel containing 0,4 %. This is an example of, what is called "eutectoid" in metallurgy. In steel, this eutectoid is called perlite. The thing is, that on the etched steel surface it acts as a "diffraction grating", which breaks down white light into its constituent components of its spectrum, which gives this surface a pearlescent look. To see these alternating plates of ferrite and cementite, of which perlite is composed, need an increase of at least 500.

Transformations in hypoeutectoid steels

Steel, which contains less 0,8 % carbon, called hypoeutectoid steel. Such steels are transformed from austenite into a mixture of ferrite and pearlite upon cooling from the austenitic state, similarly to the steel with carbon content discussed above. 0,4 %:

  • Transformations begin at the corresponding upper critical temperature at a point on the CE line of the phase diagram. The position of this point depends on the composition of the steel (carbon content).
  • The transformations end at the lower critical temperature 723 ºS.
  • The relative amount of ferrite and pearlite depends on the carbon content of the steel (picture 2), but in any case, ferrite will be almost pure iron, and perlite will contain exactly 0,8 % carbon.

Transformations into eutectoid steel

Steel containing 0,8 % carbon called eutectoid steel.

  • When this steel is cooled, transformations from austenite do not begin., until steel reaches point E on the equilibrium diagram (picture 2).
  • Then the transformation begins and ends at the same temperature 723 ºS.
  • Since this steel contains 0,8 % carbon, it means, that the final structure will be completely pearlite (picture 2 (we)).

Transformations in hypereutectoid steels

Become, which contain more 0,8 % carbon, are called hypereutectoid:

  • Steel, which contains, let's say, 1,2 % carbon will begin to transform from austenite, when the temperature drops to the lower critical at point U2.
  • Since carbon at this time is present in steel in excess relative to the eutectoid composition, then it will be highlighted first. It won't be pure carbon, and acicular in shape cementite crystals along the boundaries of austenite grains (picture 2 (viii)).
  • When cooling steel from point U2 to temperature 723 ºС austenite gradually loses carbon and at a temperature 723 ºC the remaining austent contains only 0,8 % carbon. This remaining austenite is then all converted to pearlite (picture 2 (x), as in the case of hypoeutectoid and eutectoid steels.

Primary ferrite and primary cementite

Any steel, which contains more 0,8 % carbon, if allowed to slowly cool from austenitic state, will have a microstructure, consisting of cementite and perlite. The pearlite part of the microstructure always contains alternating layers of ferrite and cementite in "correct" proportions, which provide the total carbon content for pearlite 0,8 %. this implies, that any change in the total carbon content in steel above 0,8 % leads to a corresponding change in content primary cementite. The terms "primary cementite" and "primary ferrite" are used to refer to cementite or ferrite, who stand out first before, how retained austenite turns into pearlite.

Nonequilibrium martensitic transformation

Above, we dealt only with those types of structures in ordinary steels, which are formed during their slow cooling from the austenitic state. Such conditions are typical for such industrial steel heat treatment processes as normalization and annealing.. With very rapid cooling of steel from the austenitic state, which happens, eg, about water quenching, another structure is formed, which is called martensite. This structure is absent in the equilibrium diagram, since it is not an equilibrium structure. Rapid cooling eliminates equilibrium.

Martensite is very hard. Unfortunately, martensite is also quite brittle. Therefore, steel in this state is used only, if very high hardness is required. To increase the toughness of the steel after hardening (by reducing the hardness) steel is tempered. The degree of modification of the martensite structure depends on the tempering temperature. The tempering temperature can vary from 250 to 650 ºС depending on the required combination of mechanical properties in the finished product. At any temperature applied, tempering helps the microstructure to a certain extent return to equilibrium by precipitating microscopic particles of cementite.

Source:

  1. Engineering Metallurgy – Part I – Applied Physical Metallurgy / R. A. Higgins – 6thed. – 1999
  2. Physical Metallurgy Principles, Fourth Edition / Reza Abbaschian, Lara Abbaschian, Robert E. Reed-Hill, 2009