Controlling the Austenite Grain-Size of Steels | Metallurgy

The control of ferrite grain size in ferrite-pearlitic steels, or of pearlite colony size in eutectoid steels, is obtained by controlling the austenite grain size.

This can be achieved during following stages of processing steels:

1. During solidification

2. During mechanical working

3. During heat treatment.

1. During Solidification:

Types of steels based on deoxidation practice- Plain carbon steel ingots can be classified into four types based on the deoxidation practice employed- Rimmed, Capped, Semi-killed, and killed.

Commercially pure aluminium is for the most commonly used inoculating element (though ferro-silicon, ferro-manganese, etc. are also commonly used) to control the grain size in cast steels, because of- (i) Its great effectiveness, (ii) Produces inherently fine grained steels, (iii) Its capacity to be used in combination with other metals like silicon, manganese, titanium, zirconium, etc. and (iv) Its low cost.

Though the efficiency of aluminium addition is highest (50-75%) when added in ingot mould, but is commonly added in ladle (30-40% efficiency), while making basic steels as it gives greater homogeneity and cleaner steel. The undesirable non-metallic inclusions formed due to oxidation of aluminium which further react with slag, then go to slag. Acid steel practice necessitates aluminium addition in ingot mould to avoid its heavy losses to slag otherwise.

Nearly all inoculating additions are readily oxidised by the oxygen dissolved in molten steel, and aluminium forms alumina, silicon (added as ferro-silicon) forms silica, etc. It is believed that alumina (also silica, etc.) provides the surface for heterogeneous nucleation of solidification of steel. In order to be effective, it has to be present in a highly dispersed state, so that more are the alumina particles, more are the nucleation sites, and smaller is the grain size of the solidified steel.

Alumina has high melting point and the very mechanism of its formation through the reaction of aluminium with oxygen of molten steel, increases its fine dispersion. The state of deoxidation of steel, hence, is important in the production of grain size controlled steels and only a minimum amount of reactive oxygen should be left in the bath so as to prevent excessive formation of alumina and its entrapment in solid steel (as non-metallic inclusions).

If excessive oxygen is present in the steel, it should be removed through addition of ferro-silicon and ferro-manganese as their products of oxidation form fusible slag. That is why now, the normal practice is that oxygen concentration is first reduced to a suitable level and then, aluminium is added in amount corresponding to the nitrogen content of the steel, so that as the steel cools, a dispersion of Al-N particles takes place, which is responsible for the production of inherently fine grained steels.

Inherently Fine Grained and Coarse Grained Steels:

Based on the tendency of austenite grains to grow when heated above upper critical temperature, the steels could be classified as:

(i) Inherently fine grained Steels, or fine grained Steels

(ii) Inherently Coarse grained Steels or coarse grained Steels,

This classification is based primarily on how the steel was deoxidised. Inherently fine grained steels had been deoxidised with aluminium. Aluminium, that does not combine with oxygen during deoxidation, combines with nitrogen in steel and forms a dispersion of fine aluminium nitride particles.

The uniformly dispersed fine nitride particles in turn pin the austenite grain boundaries and thereby inhibit grain growth. Inhibition of austenite grain growth by aluminium nitride is the classic example of grain boundary pinning by small particles of second-phase.

Steels deoxidised with silicon are coarse grained steels. These and even, semi-killed steels do not have particle dispersions effective in inhibiting austenite grain growth, that is, when these steels are heated above Ac₃ temperature, austenite grains grow continuously with the rise of temperature (Fig. 2.24 and 2.25) as well as with time.

Whereas when an Al-killed steel is heated above Ac₃ temperature, austenite grains grow very little due to second-phase inclusions of AlN during the time normally used and up to a temperature of about 1000°C, but if this temperature is exceeded, the grain growth is very rapid, and the final grain size at a given temperature can be greater than in the silicon-killed steels in which austenite grains had been continuously growing.

Al-N particles may coalesce and/or dissolve. All the Al-N particles need not be dissolved before the grains can grow rapidly. The temperature of abrupt coarsening is referred to as “grain coarsening temperature’. This temperature also depends on the amount of Al added in steel and time at the temperature. For example, a steel containing, say 0.007% N and 0.03% Al should remain fine grained up to a temperature of about 1000°C.

Inherently coarse grained and inherently fine grained, or just coarse grained and fine grained steels do not mean that the given steel always has a coarse grained, or fine grained austenite. It only means that coarse grained type acquires a coarse grain structure at a lower temperature than inherently fine grained steel. Normally, the austenitising temperature of many finishing heat treatment does not exceed 980°C.

The fine grained steels are thus, able to retain a fine austenite grain size even in long carburising cycles, where the coarse grained steels might coarsen considerably. That is why it is an almost universal use of steel making practice that produces inherently fine grained steels for critical heat treated parts and for alloy steels used for carburising.

In hypereutectoid steels, austenitic grain growth is retarded when heated in temperature interval from A₁ to A_cm by the network of cementite along the austenitic grain boundaries (just as AlN particles). A_cm line in Fe-Fe₃ C diagram is a very steep line and if one wants to completely austenites a hyper-eutectoid steel, very high temperature of heating is required, and with the dissolution of proeutectoid network of cementite, that is, by dissolving the grain boundary pinning phase, by heating it above A_cm temperature, very intensive austenite grain growth takes place.

Thus, most of the heat treatments (except normalising) of hyper-eutectoid steels are carried out by austenitisation at just above A₁ temperature, where the austenite grain size is minimum though complete austenitisation does not take place there. In hypo-eutectoid steels, proeutectoid ferrite itself is a soft and weak phase, and its grains may be in large size as compared to austenite just formed at just above A₁ temperature.

Thus, it is essential, first to get a uniform and fine grain size of austenite. In most of the heat treatments of hypo- eutectoid steels, complete austenitisation is desirable and thus, is attained by heating the steels to temperatures just above A₃. It does result in slight coarsening of austenite grains as compared to the size at just above A₁ temperature, but it is tolerated in order to completely eliminate the coarse ferrite, and to get a uniform grain size of single phase homogeneous austenite.

Apart from deoxidation by Al additions, the control of austenite grain size can be done by having in steels small amount of elements, around 0.1%, in particular the transition metals such as Ti, V or Nb, which are strong carbide and nitride forming elements and behave like Al in producing particle dispersions of their carbides and nitrides that inhibit the austenite grain growth, even after long soaking periods at relatively high austenitising temperatures. These steels show improved toughness, even in heat-affected zones of welds. The critical-grain coarsening temperature also depends on the element added.

2. During Mechanical Deformation:

As the initial austenite grain size (just above A₁) depends also on the initial structure of the steel, and thus, also on the amount of cold work to some extent, as it provides greater nucleation sites for austenite formation. The energy stored during cold working promotes more nucleation sites of austenite to obtain finer austenite grains.

The recrystallisation temperature of cold worked steels is lower than the temperature at which austenite forms. Thus during hot rolling, simultaneous recrystallisation can give small austenite grains if during this controlled rolling the last pass of rolling, if made at the lowest temperature and still in the austenite region to avoid coarsening of austenite grains. This method can be used to control the grain size in sheets, plates or bars.

Very low carbon steels can be worked up to 600°C and steels with carbon 0.3% to 0.8% can be worked up to A, whereas hypereutectoid steels up to A_cm temperature. But this method cannot be applied to rolling of sections because of different finishing temperature associated with different parts of the same section.

Grain Refinement in ‘HSLA’ Steels:

In recent years, ultra-fine grains have been obtained in micro-alloyed steels having small amounts of strong carbide-forming elements such as Nb, V or Ti, though Nb in amount about 0.05% is preferred over others, because of its low solubility in steels. These elements show decrease of solid-solubility in austenite with the fall of temperature.

Ferrite grain refinement in ferrite-pearlite steels is obtained by restricting the growth of freshly recrystallised austenite grains during hot rolling and/or by inhibiting the recrystallisation of austenite during hot rolling so that γ—> α transformation occurs in unrecrystallised austenite. The situation demands heavy deformations and low finishing temperatures in the austenite range below about 950°C. Mo and Cr are weakly effective. Mn promotes grain-growth.

At the start of hot-rolling at around 1300°C, Nb, C and N are in solid solution state and steel has coarse grains of austenite. When the steel is subsequently hot rolled, its temperature falls gradually (Fig. 2.26 a) and the solubility of carbonitride of Nb also decreases. Fine precipitates of the carbonitride form from austenite.

These precipitates effectively ‘pin’ down the migrating grain-boundaries during repeated recrystallisation of deformed-austenite between passes at successive stands of the rolling mill.

Initially the precipitate particles hinder growth of austenite grains and at lower temperatures of rolling, still finer precipitate form. All of them inhibit even recrystallisation of deformed and elongated austenite grains (4th and 5th steps in Fig. 2.26 d).

The closely spaced grain boundaries of deformed and in extreme cases elongated austenite provides more potential sites for the nucleation of very fine ferrite as the temperature drops from A₃ to below A₁. The end result is, thus a very fine grain-size of ferrite of about 2-3 µm, that is ASTM NO 14-15. Mild steels with normal yield strength of order 207 MPa can be made to have by micro-alloying and controlled-rolling a yield strength of the order of 345 MPa to 550 MPa. The other main advantage is that these steels have excellent weld ability of mild steels and the cost of heat treatment step has been avoided.

3. During Heat Treatment:

Steel castings invariably have coarse grains and widmanstatten structure with its associated brittleness. Annealing helps in removing them and produces fine grains. Annealing and normalising, the two very common methods of refining the grains, apart from other beneficial changes. A few words are important here to mention about normalising.

Normalising is the process of heating the steel to a low temperature in the austenite region, holding for a period long enough to attain temperature uniformly, that is, a uniform homogeneous austenite and then, air cooling.

This process, on heating, forms relatively fine austenite grains, which then transform to fine ferrite grains, or fine pearlite colonies. In as-hot rolled steels, the ferrite grain size is generally determined by the temperature at last pass of rolling (hot), which is subject to wide variation. Normalising also reduces carbide size in alloy steels.

The process, Short-cycle repetitive normalising produces ultra- fine ferrite grains of ASTM 15 (1.7 m) in carbon steels without depending on the deoxidation practice, i.e. Si-killed, or Al-killed steels could be used.

The important factors are:

(i) Initial Micro-Structure:

Best results are obtained if initial micro-structure is fine, preferably tempered martensite.

(ii) Austenitising Temperature:

This temperature should be barely above Ac₃ temperature of the steel, so that no grain coarsening occurs. For low-carbon, unalloyed steels the temperature is 845°C, or lower depending on the carbon content of the steel.

(iii) Holding Time:

The holding time at the austenitising temperature should be just a few seconds and then it is air cooled. In each of the four or five cycles, the ferrite grains become smaller but beyond that there is little further refinement.

In another technique, ultra-fine grains can be obtained, even in plain carbon steels, but has been used for expensive alloy steels due to high cost involved. With increasing austenitisation temperature, the rate of nucleation of austenitic grains increases more intensively than the linear rate of their growth, resulting in a very fine austenite grains, if the holding time there, is very short to avoid the growth. The cooling may be rapid, even by water sprays and the cycle may be repeated.

In practice, an alloy steel plate is first heated in the three phase region of austenite + ferrite + carbide in a normal gas-fired furnace (preheating). Each phase restricts the growth of others.

As this plate comes out of furnace, it is passed through an induction coil which very quickly raises the temperature above Ac₃. The followed rapid cooling by water helps in transformation at the lower temperature. Ultra-fine grained ferrite is obtained.