In this article we will discuss about:- 1. Introduction of Austenitic Grain-Size of Steels 2. Actual Austenitic Grain Size of Steels 3. Importance 3. Effects 4. Austenitic Grain-Size Growth of Heating 5. Methods of Revealing.
- Introduction of Austenitic Grain-Size of Steels
- Actual Austenitic Grain Size of Steels
- Importance of Austenitic Grain-Size of Steels
- Effects of Austenitic Grain-Size of Steels
- Austenitic Grain Growth of Heating of Steels
- Methods of Revealing Austenite Grain Size of Steels
1. Introduction of Austenitic Grain-Size of Steels:
In recent years, more after the advent of micro-alloyed steels, the term ferritic grain-size of steel has become common, but the term grain-size of steel always means the austenitic grain-size, unless specifically mentioned as ferritic grain-size. Austenitic grain-size or just the grain-size of steel means, the grain-size of austenite that existed prior to its transformation to ferrite and carbide mixture or martensite, that is, the size of prior austenite-grains (that existed at a higher temperature) before the steel is cooled, and before the austenite is transformed to other structural constituents.
The austenitic grains-size is an important specification of steels as this size determines many properties which arc of significant importance specially in steels, which are hardened.
Ferritic-grain size is important sometimes, such as in very low carbon steels, or in ferritic types of alloy steels, or now HSLA steels because the properties of these steels arc strongly effected by the ferritic grain size and is thus measured. Hall- Petch equation (2.40) can be applied to obtain the yield strength of these steels. As the phase ferrite occurs at room temperature in steels, ferritic grain size is the grain size of ferrite grains present in ferritic steels and can be measured easily.
However, in those steels, in which ferrite forms on cooling from austenite, then fine austenite grains produce fine ferrite grain size. Hence, finer the austenite grain size, finer is the resulting ferrite grain size, and better are the mechanical properties of the steel. Thus, it is important to have fine austenite grains.
Austenite normally does not occur at room temperature, except in some alloys steels. It is a high temperature phase, stable only above Ae1 temperature-for example above 727°C in plain carbon steels. The difficulties encountered in measuring austenite grain size, at room temperature, and the methods used to measure it. Let us first see why there is so much stress in knowing the austenitic grain size of steel.
2. Actual Austenitic Grain Size of Steels:
A steel heated just through A1 temperature invariably has fine grains of austenite (depending slightly on the ferrite-cementite dispersion) and this is its original austenite grain size. Inherently coarse grained steel starts coarsening as the temperature is raised above A1 (or time, at the temperature, is increased) and the process continues.
Inherently fine grained steel resists coarsening up to its grain coarsening temperature and then, abruptly increases rapidly its austenite grain size, so that at sufficiently high temperature, its austenite grains may be even larger than that of inherently coarse grained steel.
These coarse grains do not get refined in subsequent slow cooling to A1. This led to the concept of actual grain size of steel that is it is the size of austenite grains under a given particular condition and is of great practical importance.
The actual austenite grain size of a steel depends on (a) temperature to which the steel is heated, (b) holding time at the temperature, (c) tendency of the steel to grain growth, i.e. the inherent nature of the steel. An inherently fine grained steel, if not heated above its grain coarsening temperature, retains the same actual austenite grain size, whereas an inherently coarse grained steel keeps continuously getting coarsened.
The properties of a steel are affected only by the actual grain size and not by the inherent grain size. For example, if two steels of the same grade but one is inherently coarse grained and the other is inherently fine grained, have the same actual austenite grain size due to heat treatment at different temperatures. Their properties will be the same. These steels otherwise would have many properties to be different.
The actual grain size affects the properties, but the inherent grain size is a deciding factor for hot working, or case-carburising. Inherently fine grained steels have a wider temperature interval for austenitising for hardening (as grain coarsening does not take place) as compared to inherently coarse grained steels. Inherently fine grained steels can be rolled or forged at a high temperature and the finishing temperature could be high without getting grains coarsened. So is true of case-carburising.
3. Importance of Austenitic Grain-Size of Steels:
The size of austenitic grains is the most important structural characteristic of heated steel. The austenitic grain size strongly effects its own transformation behaviour and the mechanical properties of the micro-structures formed from austenite. Austenitic grain boundaries are preferred sites for the nucleation of pro-eutectoid phases (ferrite in case of hypo-eutectoid steels, and cementite in hyper-eutectoid steels) and pearlite, which are diffusion controlled transformation products.
Coarse austenite grains having less grain boundary area, have fewer nucleation sites, thus diffusion-controlled transformation of austenite is retarded paving way for easy transformation to martensite (diffusion-less transformation product). Fig 2.17 illustrates TTT curves of same steel having different austenitic grain sizes. The curve is more towards right in case of coarse grained steel. Same rate of cooling (see Fig. 2.17) transforms austenite to martensite in coarse grained steel (Fig. b) but pearlite in fine grained steel.
Thus, it has two main effects:
1. As martensite forms easily in coarse austenite grained steel, the hardenability is increased of this steel, though this method to improve hardenability is not resorted to as the coarse grained austenite results in coarse martensite, which makes the steel little less hard and more brittle, that is, even in hardened steels, martensite obtained from fine austenite grains is preferred, due to better mechanical properties of this martensite.
Even in low carbon martensite, the yield, or flow strength is more as austenitic grain size decreases. Lath martensite which occurs as packets, where each packet is effectively a grain. It has been proved that packet size increases with increase in parent austenitic grain size. Thus, strength of lath martensite may be related to either, prior austenitic grain size, or lath-martensite packet size.
2. As the preferred nucleation and growth of pro-eutectoid phases and pearlite at the austenite grain boundaries increases with the decrease in austenite grain size, a direct relationship is obtained between the austenitic grain size and the grain size of the transformation products, that is, finer the austenitic grain size, the finer the grain size of ferrite-cementite product and vice-versa. Low carbon steels in annealed or normalised state, being predominantly ferritic, show increase of strength as well as toughness with the decrease in grain size of austenite and thus, ferrite. This has been a contributing principle in the development of HSLA steels.
The impact toughness of steel is most sensitive to the size of the austenite grains in the hardened and tempered state. The charpy impact value improves with decreasing austenitic grain size to the extent that its value for a fine-grained steel can exceed several times that of a coarse grained steel of the same grade (Fig. 2.15).
The reason, partly is due to segregation of impurity atoms to the austenitic grain boundaries during austenitisation (more segregation takes place if grain boundary area is less as is the case in coarse-grained steels), and thus, the fracture frequently takes place along prior austenitic grain boundaries.
The co-segregation of impurities like Sb, P, Sn, As along the large angle grain boundaries of austenite grains weakens the adhesion at these boundaries to cause fracture along them. Such intergranular fracture is quite brittle.
The impact resistance is low, even in slowly cooled steels having coarse austenitic grain size. If say, a hypo-eutectoid steel is heated to different austenitising temperatures, which result in attaining different austenite grain sizes in different samples, if is now cooled at the same rate, then free ferrite forms a continuous network in coarse-grained steels, but not in fine austenite grained steels, because of small and large grain boundaries areas respectively.
A thick connected ferrite (soft and weak phase) network leads to lower impact value, lower elastic ratio, and lower elongation. As a steel of lower impact value has better machinability but poor surface finish, steels are often given double heat treatment (it is a different term than used for heat treatment after carburising) to produce coarse and fine grains to bestow good machinability and good surface finish successively.
In alloys of Fe-Ni and Fe-Ni-C, the Ms temperature is lowered significantly by decreasing the austenitic grain size, probably due to higher strength of fine grained austenite, which in turn increases the shear resistance of austenite to transform to martensite. Carbon steels may also show similar effects on Ms temperature. Austenite grain size is thus the most important structural characteristic of the steels.
4. Effects of Austenitic Grain-Size of Steels:
Generally, Tine grained steels (in cast, or wrought form) are preferred over coarse grained steels, because fine grained steels have better toughness for corresponding hardness and strength (Fig. 2.15) at normal temperatures (definitely below the equi-cohesive temperatures). Coarse grains raise the ductile to brittle transition temperature. One of the most significant effects of fine grains is that fine grains improve the strength considerably as compared to coarse grains.
This enhancement in strength by grain refinement has led to large scale use of HSLA (high strength low alloy) steels, which in the hot rolled condition have yield strengths in the range of 290-550 MPa, and ultimate tensile strengths in the range of 415-700 MPa.
The increased mechanical properties of fine grained steels as compared to coarse-grained steels, is due to the effect of grain boundaries Grain boundaries act as obstacles to the dislocation motion. As the orientation of the atoms on either side of a grain boundary is different and random, a dislocation moving on a common slip plain in one grain, can rarely move on a similar slip plane in the adjacent grain.
Also, the grains are separated by a thin non-crystalline region, which is the characteristic structure of a common or, large angle grain boundary. Thus, the dislocations are stopped by a grain boundary.
The dislocations pile-up against the grain boundary (Fig. 2.16). The smaller is the grain size, more frequent is the pile up of dislocations, and more difficult it is to move a dislocation to cause yielding, or higher stresses are needed to move dislocations in a fine grained material than in a coarse grained material. This can be explained in another way.
In a large grain, pile-ups contain larger number of dislocations (Fig. 2.16), which in turn cause higher stress concentrations in neighbouring grains. The shear stress, τ, at the head of a dislocation pile-up is equal to n τ, where n is the number of dislocations in pile-up, and τ is the shear stress on the slip plane. Thus, larger the grain size, easier it will be to propagate the yielding process in the next grain.
Hall and Petch gave a general relationship between yield stress (and other mechanical properties) and grain-size in a polycrystalline material:
where, σi is the yield stress for a single crystal of the same material having no grain boundaries; d is the average grain diameter; k is the “locking parameter”, which measures relative-hardening-contribution of the grain boundaries; σy is the yield strength. The yield strength increases with decreasing grain diameter. Table 2.5 gives Hall-Petch constants for some polycrystalline materials. Hall-Petch relationship has been found to be applicable to even HSLA steels, having ferrite grains in steel of about 2-3 µm (ASTM 14-15).
Table 2.6 compares effects of grain size on the properties of the materials. Now-a-days, steels are supplied to grain size specifications. As fine-grained materials have better room temperature strength, ductility, toughness and other properties, thus, many heat treatments are carried out to obtain fine-grain structures. As most of the heat treatment performed on steels alter the grain-size, it is essential to control the grain-size of steel to achieve better properties.
Effect of Solute on Grain Growth:
The solute atoms (impurities, or alloy additions) when present in solid solution state in austenite, interact with dislocations at grain boundaries forming grain boundary atmospheres just like the Cottrell atmospheres inside the grains, which hinder the motion of grain boundaries required for grain growth and thereby restrict the grain growth.
However, it may not prove to be very effective method, because the steel is at quite high temperatures and thermal energy is able to break the grain boundary atmospheres, that is, the solute atoms diffuse away from the dislocations at the grain boundaries, and grain growth occurs.
When the solute atoms are present in the form of small sized uniformly dispersed second phase inclusions, or particles as oxides, carbides, nitrides, sulphides, etc., then these effectively stop, or decrease grain growth. Vanadium (apart from Al, Ti, Nb etc.) even in small amounts around 0.1% when present as finely and widely dispersed carbide and nitride plays an important role in limiting grain growth in steels heated for hardening.
At ordinary hardening temperatures, these inclusions do not dissolve and act as inhibitor to grain growth. If the temperatures are raised high so that -dispersed inclusions dissolve, then a large increase in austenitic grain size of the steel takes place. Aluminium nitride behaves in the same way when it is present in steels, or alloy steels used for carburising.
Zener and Paranjpe gave an explanation for the pinning of grain boundaries by the second-phase inclusions, or particles. Suppose a second phase inclusion is located in a grain boundary, which is shown as a vertical straight line in Fig. 2.23 (a), and in this position, is in mechanical equilibrium state under the action of two equal and opposing surface tension forces. If the grain boundary breaks away from the inclusion, then the missing segment of boundary (= 2 r) shown as dotted line in Fig. 2.23 (a) must be created.
Energy is required to create this boundary segment, which makes this movement of grain boundary difficult and thus, grain growth is inhibited.
Suppose the grain boundary has moved as illustrated in Fig. 2.23 (b). The boundary assumes a curved shape but, strives to maintain itself normal to the surface of inclusion, which is also the direction (and magnitude) of surface tension force at the circular line (in three dimensions) of contact between the grain boundary and the surface of the inclusion. If r is taken as the radius of the inclusion, then the total line of contact is 2 π r cos θ, where θ is the angle between the surface of the grain boundary and the surface where it meets the inclusion.
If σ is the surface tension of grain boundary, then the component of the surface tension normal to the grain boundary surface is σ sin θ. The product of this component and the length of contact between the inclusion and the grain boundary gives the pull of the boundary on the particle. But by Newton’s second law, this is also the restraining, or drags force exerted by the inclusion on the grain boundary, and is,
f = 2 π r σ cos θ sin θ … (2.42)
This force is maximum, when θ = 45°, then sin θ cos θ = 1/2, and thus,
fmax = π r σ … (2.43)
The equation (2.43) shows that the restraining, or drag force of a single inclusion varies directly as the radius of the inclusion. But the volume of each inclusion varies as the cube of its radius, thus, the effect of second-phase inclusions, or particles in hindering the grain boundary motion, (that is, the grain growth), is greater, if smaller and more abundant are the second-phase inclusion, instead of just a few of them. Thus, to stop the grain growth on heating the steel, uniform but finely dispersed second- phase inclusions are made to be present.
Normally, the second-phase inclusions dissolve if steel is heated to higher temperatures, or these inclusions tend to coalesce at higher temperatures and thus, are then left with fewer inclusions. Both these factors, that is, the dissolution of the inclusions or increasing the size of the inclusions by coalescence reduce, or remove the retarding effect of the inclusions and rate of grain growth increases tremendously and suddenly.
5. Austenitic Grain Growth of Heating of Steels:
Invariably the first step in heat treatment cycles is to heat the steel to austenitise the steel. Above the lower critical temperature, austenite nuclei form at the interfaces between ferrite and cementite depending on the original structure of steel as shown in Fig. 2.13 (a) & (b). Fig. 2.14. The number of nuclei is always sufficiently large.
At temperatures just above the upper critical temperature, when the structure is fully austenitic, that is, just after the transformation is complete, the initial grain size of austenite is fine (though more disperse is the initial structure, the finer is the austenite grain formed).
As the temperature is raised further, or holding time at a given temperature is increased, then grain coarsening occurs. This behaviour is schematically illustrated in Fig. 2.18. The grain coarsening characteristic is effected by the deoxidation practice used for the steel.
Austenitic grain growth is a natural spontaneous process and is caused by the tendency to reduce the surface energy by reducing the total surface area of the grain boundaries. A high temperature accelerates the rate of this process. Driving force is the surface energy stored as grain boundary energy. Certain grains grow at the expense of smaller grains, which due to their size, are less stable.
Fig. 2.19 illustrates diffusion of atoms across the grain boundary because the atom is more stable on the concave grain where it has more neighbours, that means, stronger bonding than would normally be expected in the convex grain. Thus the boundaries move towards its centre of curvature of the grain as illustrated in Fig. 2.19 and Fig. 2.20.
In a two dimensional model of austenite grains, at the junction of three grains, the surface tension forces must be balanced assuming that surface tension force, γ, along the grain boundaries of single phase austenite is same, and thus (Fig. 2.21),
that is, under equilibrium condition, the three grains must meet at 120° at the triple points, that is, in a two dimensional model of austenite grains, straight-side hexagons forming a hexagonal network of straight boundaries is stable (Fig. 2.21).
In actual steels, when austenite grains nucleate and grow, they meet one another at different moment of time and at different points of their surfaces, so that austenitisation produces grains of different sizes and with different number of sides such as 3, 4, 5, etc. as illustrated in Fig. 2.20. On heating, these grains grow.
At a triple point, the angle between the boundaries may not be the equilibrium angle, 120°. Fig. 2.22 illustrates as to how a triple point moves. Due to surface tension forces at the triple point, grains with less than six sides should have their grain boundaries convex outwards and those, with more than six sides have convex inwards. The grain boundaries migrate towards the centre of curvature (Fig. 2.20).
Even in Fig. 2.22, grains B and C gradually grow by the movement of boundaries at the expense of grain A. Austenite grains with more than six sides should grow and become larger, while those with less than six sides are consumed by the neighbours. Fig. 2.18 illustrates continuous increase of austenitic grain size as the temperature of the steel is raised above upper critical temperature/Acm, the thermodynamic stimulus is the energy of the grain boundaries. The size of a grain obtained on heating to a given temperature does not change naturally on subsequent cooling.
6. Methods of Revealing Austenite Grain Size of Steels:
One difficulty in estimating the grain size of austenite is that, it is a high temperature phase (except in some alloy steels), and is no longer present when metallographic examination is made, but has decomposed to aggregate of ferrite and carbide, or martensite.
Fortunately, however, austenite can be made to transform in such a manner that former austenite grains are delineated by a network of proeutectoid cementite, or free proeutectoid ferrite, or pearlite (depending on the composition of steel), which form preferentially at the grain boundaries of austenite during the decomposition of austenite. These phases are not allowed to form beyond the extent necessary to just delineate the former austenite grain boundaries.
The following are some of the methods of revealing grain size:
1. Delineation by Pro-Eutectoid Cementite, or Proeutectoid Ferrite:
In slowly cooled hyper-eutectoid steels, the austenite grain boundaries are revealed by the formation of network of cementite (Fig. 2.27 a). In hypo-eutectoid steels having carbon from 0.6% to 0.77% (so that large amount of free ferrite does not form), the austenite grain boundaries are revealed by the formation of proeutectoid ferrite there (Fig. 2.27 b).
2. McQuaid-Ehn Test:
This test was originally used for detecting the “abnormal structure, or abnormal steels.” In this method, a steel is carburised at 925°C for 8 hrs, followed by slow cooling, while it is still packed in the carburised medium. The hyper-eutectoid case developed during carburising shows on cooling, network of cementite at the austenite grain boundaries and looks like as illustrated in Fig. 2.27 (a), where the estimation of the grain size can be made.
This test may not reflect the true grain size of austenite due to long carburising period at 925°C, even of Al-killed steels, as some grain growth may take place. For the same reasons, this is an overly severe test for steels that usually might be austenitised at only 850°C. Thus, this test should be limited lo steels for carburised parts.
3. A more reliable test consists of austenitising the steel at its recommended temperature, quenching and then tempering for 16 hrs at 510°C, or (3 hrs at 525°C) near the optimum temperature for temper-embrittlement (thus, applicable to alloy steels).
The polished specimens are etched for one minute in a boiling, saturated aqueous solution of picric acid, and finally lightly polished to reveal a dark network of prior austenitite grain boundaries on a light back-ground. It is good method for Cr-Ni, or Cr-Ni-Mo steels in the constructional steels.
4. The austenite grain-size of eutectoid steel, if is not deep-hardening, can be estimated by austenitising (at its normal temperature) and quenching, when at some intermediate position of sample, ‘split transformation’ takes place, that is, very fine dark etching pearlite delineates the grain boundaries with light-etching martensite (Fig. 2.28 a).
5. When 100% martensitic structure, or slightly tempered martensite, is etched with a solution of 80 ml H2O, 28 ml oxalic acid (10%) and 4 ml H2O2, or etched with hydro-chloric-picric acid solution in alcohol, the difference in orientation of austenite grains may be sufficiently rellected in martensitic structure enabling to determine the size of the austenite grains. (Fig. 2.28 b).
6. The austenitic grain-size of hardened high speed steel is best measured, if the micro-structure is examined on quenching and in untempered state (as the tempering tends to obscure the austenite grain boundaries), or when these have been tempered above 600°C, and a 5% nitric acid etches the grain boundaries very sharply. More distinct contours are obtained, if etchant used is picric acid.
7. The practical rough estimation of coarseness, or fineness, of prior austenite grain-size is obtained by the texture of fully hardened martensitic steel.
8. Vacuum Grooving:
This method is based on preferential evaporation of austenite grain boundaries. Heat treatment is done for 1 hr., or less at 900°C. Though it is used for a wide range of steels, it may not reflect the true grain size as some grain coarsening might take place due to high temperatures used.
This method is based on preferential oxidation of steels. Heat treatment is for 1 hr. at 855°C. Though it is used mainly for hypo-eutectoid steels, it may not reflect the true grain size of steels because of high temperature used.