In this article we will discuss about:- 1. Introduction to Annealing of Steel 2. Objectives of Normalising of Steel 3. Advantages of Normalising over Annealing.
Introduction to Annealing of Steels:
Annealing, in general refers to heating the material to a predetermined temperature for a definite time (i.e. soaking), and then cooling it slowly normally in a furnace by switching it off. The aims of annealing, even in steels, could be varied and that is why, there are a number of types of annealing heat treatments. The nature of processes occurring during each type of annealing depends on the type of steel, temperature of annealing, history and state of the steel before annealing.
Main aims of annealing are:
1. Improvement in mechanical properties of the steels (cast, or hot worked-rolled, forged, etc.) by refining the grain size.
2. Homogenisation of segregated castings and ingots of steels and alloy steels.
3. Restore ductility of cold worked steels, (which also improves electrical and magnetic properties).
4. Improve the machinability and cold formability, particularly of high carbon steels and alloy steels.
5. Relieve the internal stresses of cast, hot worked, or welded structures.
The important aspect of annealing as well as normalising is to obtain (on cooling) pearlite by austenite transformation (and not to bainite, or martensite). In the pearlitic range of transformation (depicted by ‘S’ curve), as the temperature of transformation decreases, pearlite produced becomes finer and finer resulting in increased hardness and strength as illustrated in table 5.1.
Objectives of Normalising Steels:
1. To refine the grain size of the steel castings or of parts whose grain size frequently becomes very coarse during hot working at high temperatures. Fig. 5.2 illustrates that such coarse-grained part on austenitisation for normalising, transforms to uniformly fine grained and more homogeneous austenite. Air cooling of such homogeneous austenite in normalising lowers the temperature range over which proeutectoid ferrite and pearlite form (compared to full-annealing) as illustrated in Fig. 5.15.
(i) Finer proeutectoid ferrite grains,
(ii) Much more finer pearlite (interlamellar spacing is reduced),
(iii) Finer eutectoid grains,
(iv) Amount of proeutectoid ferrite (cementite in case of hypereutectoid steels) is reduced (time of formation decreases as illustrated in Fig. 5.15), and thus, it is more uniformly present in micro- structure.
All these factors make normalised steels to have higher strengths, hardness with slightly decreased ductility as compared to annealed state. Steels are normalised to obtain fine grain size if its original structure is coarse.
2. To improve the mechanical properties of moderately loaded machine parts made of plain carbon steels, particularly forged shafts and rolled stocks, when the mechanical properties thus developed are enough for service conditions. Normalising is then preferred over hardening and tempering, the latter being complicated and expensive process, and as normalising gives rarely warping of parts, and no cracking of parts.
Annealing and normalising are not used as finishing treatment to improve mechanical properties of tool steels, for these are used only in the hardened and tempered state. Annealing is never used to improve mechanical properties of pearlitic class of alloy structural steels, as hardening and tempering, or in some rare case, normalising is used.
3. To eliminate, or reduce micro-structural irregularities, normalising, reduces massive, large sized proeutectoid ferrite (or cementite in hyper eutectoid steels) formation, and also causes along-with grain-refinement, which results in micro-structure of much greater uniformity with resultant good isotropic mechanical properties.
The micro-structural irregularities are better reduced by normalising than by annealing, particularly of steels, which are prone to ferrite-pearlite banding (Fig. 5.17). Banding is most pronounced in steels containing ferrite and pearlite in about equal proportion, for example the steel in Fig. 5.17 has composition- 0.25% C, 0.35% Si, 1.75% Mn; 0.24% Cr. Banding in steel is commonly due to segregation of manganese.
Slow cooling in annealing makes ferrite formation preferentially in the manganese-depleted regions, i.e., produces bands of ferrite where manganese in low, and bands of pearlite where manganese in high.
As in normalising, ferrite forms at greater undercooling (i.e., at low temperatures), it can start forming relatively more uniformly throughout the steel thus minimising the banding. Fig. 5.18 illustrates this. However, this steel if heated to 1250°C for 2 hours, water quenched, reheated to 850°C for 1 hour and then, cooled at 5°C/min. results in complete removal of banding as indicated by Fig. 5.19.
4. To increase the machinability of low carbon steels (instead of annealing). By slightly increasing the hardness, normalising improves the conditions for chip-breaking, which enables it to have better surface- finish on machining.
5. To eliminate, or break coarse cementite network in hypereutectoid steels. Cementite (proeutectoid) network may be often present in hypereutectoid steels in rolled stock, or after carburising the steel.
The cementite network in hardened steel constitutes a very great danger, because in such a state it envelops the britte martensite containing grains, thereby increasing further the brittleness of the tool steel, which is already very brittle due to presence of mainly martensite, i.e., each brittle grain is surrounded by cementite in the form of a brittle envelope.
Tools made from such steels need the presence of hard and wear (as well abrasion) resistant cementite, but not as a network along which fracture can easily propagate. It is necessary to eliminate this cementite network, or remove it if present. During faster air cooling in normalising, the amount of free cementite formed is less. Fig. 5.20 illustrates it. Moreover, cementite- network has insufficient time to develop, and cementite separates in the form of distinct globules.
6. To do general refinement of structure prior to hardening of steel. Hardening is the most complex heat treatment operation. The austenitising temperature and the time at this temperature should be just minimum to avoid decarburisation oxidation and quench-crack formation, and development of non-uniform hardness for this, the main requirement is, that the original micro structure of steel should be homogeneous and fine grained.
If it is coarse-grained, the chemical composition of austenite thus formed (from say coarse pearlite and ferrite) shall not be homogeneously uniform, i.e., some grains may contain more carbon than others (austenite grains at prior cementite lamellae shall have higher carbon content), and the subsequent transformation behaviour shall vary. This leads, not only to non-uniformity of hardness developed (wherever carbon is more, martensite formed has higher hardness than at other places), but greater chances of distortion and even, quench cracks.
Thus, steels having original-coarse-grain structure prior to hardening must be first refined by subjecting to normalising with a sufficient soaking time at austenitisation temperature. Normalising refines the grain structure and produces fine interlamellar spacing of pearlite, which on austenitisation for hardening results in fine austenite grains of uniform chemical composition, because carbon has to diffuse small distances to make austenite homogeneous. The time for this austenitisation is also less.
Hypereutectoid steels are normally tool steels. The original structure of such steels should also conform to certain requirements to develop high and uniform hardness in the as-hardened state without warping and cracking. Such steels are invariably machined to desired shapes and thus have micro- structure consisting of spheroidised pearlite after machining. Such a microstructure is not directly austenitised for hardening purpose, but first should be changed to fine lamellar pearlite and broken network of proeutectoid cementite.
This is because the transformation of globular pearlite into austenite takes place at a considerable slower rate, (as globular state is the most stable state of pearlite) than the transformation of fine lamellar pearlite. Though long period of soaking at the hardening temperature can dissolve all the globules of cementite in austenite but such long periods are undesirable because apart from other draw backs, it may cause oxidation and decarburisation. No machining is done after hardening except some careful grinding to remove such layers.
Also, the steel with spheroidised pearlite has poor hardenability as some undissolved cementite particle accelerate transformation of such inhomogeneous austenite to pearlite, particularly when the steel is to be through-hardened. Normalising, is thus used as a preliminary step prior to hardening to change spheroidised pearlite to lamellar pearlite such as for drills. In such condition, normalising is done at a temperature of 780 to 800°C (above A1 temperature), as there is no network of cementite present here. In case the hyper steel has a network of cementite along lamellar pearlite grains, then normalising is done from the austenitising temperature of above Acm to break this network.
Fig. 5.1. and 5.11 illustrate that normalising of hypoeutectoid steels is done at temperatures somewhat higher than those used for annealing, because the aim is to get austenite of much greater homogeneity of composition which has greater stability when super-cooled by air cooling, i.e., austenite does not transform at higher temperatures when super-cooled by fast air cooling in normalising, but at a lower temperature (Fig. 5.11) to obtain fine lamellar pearlite and ferritic structure with fine grain size.
Lower is the temperature of transformation, finer is the grain size and finer is the inter-lamellar spacing. In full annealing, the slow furnace cooling always makes the austenite to transform at higher temperatures (≈ A1), the small heterogeneity of austenite (if present) making little difference in the temperature of the transformation and the steel ultimately has coarse pearlite and ferrite (as compared to normalised state).
There is, thus no point in heating the steel to higher temperatures (with consequent effects like grain growth, etc.) for annealing purposes. Heating of the steels in both annealing as well as normalising has to be at a higher temperature than A3 (for hypo-eutectoid steels) otherwise refinement of proeutectoid ferrite grains shall not occur, if heated only above A1 temperature.
Normalising temperature for hypereutectoid steels is normally above Acm. so that the brittle cementite network could be dissolved and which is not given time to form again as network due to faster air cooling used in normalising.
One important aspect is to be considered before recommending normalising for a part. During air cooling, the surface and the centre of a part will cool at different rates. More is this difference, larger is the section of the part (rate of cooling is dependent on the surface to volume ratio).
This can effect the final structure obtained and thus, the properties obtained at various sections. This also leads to differential contraction in volume at the surface (more) and at the centre (less) and which induces residual stresses in the part. Complex critical parts may not be given normalising treatment, if these stresses attain dangerous levels.
Advantages of Normalising over Annealing of Steels:
On heating, phase change to austenite occurs in both heat treatments, and grain size of austenite just formed is fine. Although in normalised case, some grain growth may occur, but more uniform and homogeneous austenite is obtained. The faster air cooling in normalising as compared to slow furnace cooling of annealing produces slightly different microstructures and thus, properties due to lower temperature of transformation.
Normalising is preferred over annealing due following advantages:
1. Better mechanical properties are obtained, such as strength and hardness (with slight decreased ductility) in normalising, i.e. if improvement in mechanical properties is the main objective, then normalising is done. Forged plain carbon shafts, or even rolled stocks are normalised, and not annealed.
2. Mild steels have better machinability in normalised state, though steels having carbon 0.3 to 0.4% have better machinability in annealed state.
3. In case the parts are not to have internal stresses at all, then annealing is preferred such as in complicated shapes and critical parts.
4. Normalising has certain advantages from the process point of view:
(i) In annealing, parts cool along with the furnace, whereas in normalising, parts are removed from the furnace to be cooled in air. The empty hot furnace may be employed for heating subsequent batch of parts to be normalised, increasing the productivity of the furnace. Time of heat treatment is less.
(ii) As in annealing, furnace is cooled down to low temperatures and has then to be heated again for the next batch of parts. Here too, fuel, time of heat treatment is more, but more important, the consumption of fuel or power is more.
5. Normalising and annealing have almost similar objectives, but normalising may be preferred because of better mechanical properties and less time and lower cost of operations. However, it cannot substitute annealing for (i) greater softness, (ii) absence of internal stresses, particularly in complicated shapes.
6. Normalising and annealing produce almost similar results in low carbon steels; for example, to obtain softness, normalising may be preferred for lower cost and lesser time. However the difference in properties could be large in high carbon steels.