Normalizing Process for Steels
Normalizing Process for Steels
Normalizing is a process in which a steel is heated, to a temperature above the Ac3 temperature or the Acm temperature and then cooled in still air. The purpose of the treatment is to remove the effects of any previous heat treatment (including the coarse-grained microstructure sometimes resulting from high forging temperatures) or cold-working and for ensuring a homogeneous austenite on reheating for hardening or full annealing. The aim of the normalization process is to get a fine-grained, uniformly distributed, ferrite-pearlite microstructure.
Normalizing of steel is a heat-treatment process which is frequently being considered from both thermal and microstructural standpoints. In the thermal sense, normalizing is an austenitizing heating cycle followed by cooling in still or slightly agitated air. Typically, the work-piece is heated to a temperature around 30 deg C to 80 deg C above the upper critical line of the iron-iron carbide phase diagram, i.e., above Ac3 for hypo-eutectoid steels and above Acm for hyper-eutectoid steels. To be properly classed as a normalizing heat-treatment, the heating portion of the process is required to produce a homogeneous austenitic phase (face-centered cubic, or fcc, crystal structure) prior to cooling.
Normalizing is normally done for several reasons such as (i) to modify and / or to refine the grain structure and to eliminate coarse grained microstructure got in previous working operations such as rolling and forging etc., (ii) to modify and improve cast dendritic microstructure and reduce segregation by homogenization of the microstructure, (iii) to produce a homogeneous microstructure and to get desired microstructure and mechanical properties, (iv) to improve machinability of low carbon steels, (v) to improve dimensional stability, (vi) to reduce banding, (vii) to improve ductility and toughness, (viii) to provide a more consistent response when hardening or case hardening, and (ix) to remove macro-structure created by irregular forming or by welding.
Fine grained pearlite is tougher than coarse grained pearlite. Normalizing imparts both hardness and strength to steel work-piece. In addition, normalizing helps reduce internal stresses induced by such operations as forging, casting, machining, forming or welding. Normalizing also improves microstructural homogeneity and response to heat treatment (e.g., annealing or hardening) and improves stability by imparting a ‘thermal memory’ for subsequent lower temperature processes. Work-piece which needs maximum toughness and those subjected to impact are frequently normalized. When large cross-sections are normalized, they are also tempered to further reduce stress and to control mechanical properties more closely.
Normalization eliminates internal stresses, strains, and improves the mechanical properties of the steel, such as improving its toughness and machinability. A better ductility can also be achieved without compromising the hardness and strength.
Normalizing is also frequently being considered in terms of microstructure. The areas of the microstructure which contain around 0.8 % C, are pearlitic (lamellae of ferrite and iron carbide), while the areas which are low in carbon are ferritic (body-centered cubic, or bcc, crystal structure). In hyper-eutectoid steels, pro-eutectoid iron carbide first forms along austenite grain boundaries. This transformation continues until the carbon level in the austenite reaches around 0.8 %, at which time a eutectoid reaction begins as indicated by the formation of pearlite. Air-hardening steels are excluded from the class of normalized steels since they do not show the normal pearlitic microstructure which characterizes normalized steels.
A broad range of steel products can be normalized. All of the standard low-carbon, medium-carbon, and high-carbon wrought steels can be normalized, as well as several castings. Several steel weldments are normalized to refine the microstructure within the weld-affected area. Austenitic steels, stainless steels, and maraging steels either cannot be normalized or are not normally normalized. Tool steels are normally annealed by the steel supplier.
Normalizing is one of the most widely used heat treatment processes applied on almost all castings, over-heated forgings, and very large forgings etc. Normalizing is done to refine the grain structure, improve machinability, relieve internal stresses, and improve the mechanical properties of carbon and low-alloy steels.
Normalizing is a heat treatment which is performed in hot forged or hot-rolled steels to produce more uniform, fine-grained microstructure for subsequent annealing and hardening heat treatments. The normalizing treatment helps to produce a more uniform distribution of precipitates. In case of tool steels with more stable carbides such as chromium carbide and tungsten carbide, the precipitates can be preferentially aligned in the hot-working direction, or present at grain boundaries.
The resultant microstructures are pearlite or pearlite with excess ferrite or cementite, depending upon the composition of the steel. They are different from the microstructures resulting after annealing in that (for steels of the same carbon content in the hypo-eutectoid or hyper-eutectoid ranges) there is less excess ferrite or cementite and the pearlite is finer. These are because of the more rapid cooling.
The mechanical properties achieved after normalizing depend on the rate of cooling in air. A faster rate of cooling can result in a higher strength and higher hardness than otherwise. Hence, when increased strength and hardness are needed, the cooling rate can be improved by using fans.
Normalizing is mainly adopted on plain carbon and low-alloy steels. The hardness resulting from the normalizing treatment depends on the dimensions and composition of the steel and the rate of cooling. Normalizing is mainly applied for unalloyed and low-alloy hypo-eutectoid steels. For hyper-eutectoid steels normalizing is performed only in special cases, and for these steels the austenitizing temperature is 30 deg C to 80 deg C above the Ac1 transformation temperature.
The normalizing process consists of heating the steel to certain temperature region followed by air-cooling. The heating the steel above the critical temperature Ac3 or Acm and holding at this temperature for a short time depends on the type of steel. Fig 1a shows the range of austenitizing temperatures for normalizing unalloyed steels depending on their carbon content and Fig 1b shows the thermal cycle of a normalizing process.
Fig 1 Austenitizing temperatures for normalizing and time-temperature regime
For achieving homogenization of austenite, hypo-eutectoid steels are heated to 30 deg C to 80 deg C above the critical temperature Ac3, and held at this temperature for 20 minutes to 40 minutes depending on the chemical composition. Exceeding the indicated temperature range can result in excessive austenite grain growth. Holding at the normalizing temperature for a longer time can also result in excessive grain growth or the exceeding of the tolerated limit.
After the desired holding time has elapsed, the components are cooled in air. The resultant microstructures are composed of fine pearlite with ferrite in hypo-eutectoid steels. It is normally noticed that the newly formed grain boundaries do not correspond to the old ones. If the initial microstructure is coarse-grained or irregular, normalizing leads to considerable improvement of the microstructure, together with improvement in mechanical properties.
In the same way, normalizing of hyper-eutectoid steel is done by heating the steel to 30 deg C to 80 deg C above the critical temperature, Acm, holding at this temperature for a short period, until the complete phase transformation has taken place, and subsequently cooling in air. This operation is done not only to refine the grain size but also to dissolve the networks of carbides which have developed during forging, rolling, or in some cases, during case-carburizing. At room temperature, the newly formed microstructure consists of fine-grained pearlite with cementite. The normalized microstructure is more suitable for spheroidizing treatment to get good machinability.
Cooling in air after austenitizing the alloy steels can result in higher hardness. In such cases, they can be tempered at 600 deg C to 650 deg C to render them machinable. Instead of adopting a lengthy annealing treatment, certain alloy steels can be subjected to normalizing, followed by tempering, so that the treatment time is reduced.
During heating and holding at the normalizing temperature, the initial ferrite-carbide microstructure which is stable at low temperatures transforms to austenite. The dissolution of carbides in case of alloy steels during heating depends on the alloy content of the steels.
During cooling, austenite transforms into ferrite and cementite. In case of the low-alloy tool steels, cementite and pearlite are formed during air-cooling. The carbides in this microstructure are spheroidized in subsequent annealing treatments. In high-alloy tool steels, because of their high hardenability, martensite can form during air-cooling, which can cause cracking, and hence, a normalizing treatment is to be avoided for the high alloy steel grades.
Since the type of microstructure, and, hence, the mechanical properties, are affected by the rate of cooling, considerable variations can occur in normalized steels because of differences in section thickness of the shapes being normalized. The effect of section thickness on the microstructure of a normalized 0.5 % carbon steel is shown in Fig 2a and Fig 2b.
Fig 2 Representative microstructure of normalized plain carbon steel
The parameters of a normalizing process are the heating rate, the austenitizing temperature, the holding time at austenitizing temperature, and the cooling rate. Normalizing treatment refines the grains of a steel which has become coarse-grained as a result of heating to a high temperature, e.g., for forging or welding. Fig 3 shows the effect of grain refining by normalizing a carbon steel having 0.5 % C. Such grain refinement and homogenization of the microstructure by normalizing is normally performed either to improve the mechanical properties of the work-piece or (previous to hardening) to get better and more uniform results after hardening. In some cases, normalizing is applied to get better machinability of low-carbon steels.
Fig 3 Effect of grain refining by normalizing a carbon steel of 0.5 % carbon
A special need for normalizing exists with steel castings since, because of slow cooling after casting, a coarse-grained microstructure develops which normally contains needle-like ferrite (Widmanstatten structure), as shown in Fig 4. A normalizing treatment at 780 deg C to 950 deg C (depending on chemical composition) removes this undesirable microstructure of unalloyed and alloyed steel castings having carbon in the range of 0.3 % to 0.6 %.
Fig 4 Microstructure of a steel casting
After hot rolling, the microstructure of steel is normally oriented in the rolling direction, as shown in Fig 5. In such a case, of course, mechanical properties differ between the rolling direction and the direction perpendicular to it. To remove the oriented microstructure and to get the same mechanical properties in all directions, a normalizing treatment is given.
Fig 5 Microstructure of case hardening 20MnCr5 steel
After forging at high temperatures, especially with work-pieces which vary widely in cross-sectional size, because of the different rates of cooling from the forging temperature, a heterogeneous microstructure is achieved which can be made uniform by normalizing.
From the metallurgical aspect, the grain refinement and the uniform distribution of the newly formed ferrite-pearlite microstructure during normalizing treatment can be explained with the following mechanism. At normalizing, the steel is subjected first to the alpha to gamma (ferrite-pearlite to austenite) transformation, and after the holding time at austenitizing temperature during cooling, to a recurring gamma to alpha (austenite to ferrite-pearlite) transformation. The effect of normalizing depends on both austenitization and cooling from the austenitizing temperature.
During austenitizing a far-reaching dissolution of carbides is aimed at, but this process competes with the growth of austenite grains after complete carbide dissolution, which is not desirable. Besides the carbide dissolution, the degree of homogenization within the austenite matrix is important for getting a new arrangement of ferrite and pearlite constituents in the microstructure after normalizing. Both dissolution and homogenizing are time-dependent and temperature-dependent diffusion processes which are slower when the diffusion paths are longer (higher local differences in carbon concentration) and the diffusion rates are smaller (e.g., increasing quantities of alloying elements). Hence, especially with alloyed steels, lower austenitizing temperatures and longer holding times for normalizing give advantages taking into account the austenite grain growth. As shown in Fig 6a, high austenitizing temperatures result in a coarse-grained austenite structure, which yields a coarse structure after normalizing.
Fig 6 Influence of austenitizing temperature and microstructure of C35 steel during normalizing
Holding time at austenitizing temperature can be calculated using the empirical formula t = 60 +D where ‘t’ is the holding time in minutes and ‘D’ is the maximum diameter of the work-piece in millimetres.
When normalizing hypo-eutectoid steels (i.e., steels with less than 0.8 % C), during cooling from the austenitizing temperature, first a pre-eutectoid precipitation of ferrite takes place. With a lower cooling rate, the precipitation of ferrite increases along the austenite grain boundaries. For the desired uniform distribution of ferrite and pearlite after normalizing, however, a possibly simultaneous formation of ferrite and pearlite is necessary. Steels having carbon contents between 0.35 % and 0.55 % C especially tend to develop non-uniform ferrite distributions as shown in Fig 6b. The microstructure in this figure indicates excessively slow cooling in the temperature range of pre-eutectoid ferrite precipitation between Ar3 and Ar1 temperatures. On the other hand, if the cooling through this temperature region takes place too fast, with steels having carbon contents between 0.2 % and 0.5 %, formation of an undesirable needle-like ferrite (oriented at austenite grain boundaries), the so-called Widmanstatten structure, can result as shown in Fig 6c.
Formation of pearlite follows only after complete precipitation of ferrite by transformation of the remaining austenite structure at Ar1 temperature. It starts first at the boundaries of ferrite and austenite and spreads to the interior of the austenite grains. The higher the number of the pearlitic regions formed, the more mutually hindered the pearlite grains are in their growth, and hence the finer the grains of the normalized microstructure. The influence of alloying elements on the austenite to ferrite and pearlite transformation can be read off from the relevant CCT (continuous cooling transformation) diagram.
Care is to be taken for ensuring that the cooling rate within the work-piece is in a range corresponding to the transformation behaviour of the steel in question which results in a pure ferrite-pearlite microstructure. If, for round bars of different diameters cooled in air, the cooling curves in the core have been experimentally measured and recorded, then by using the appropriate CCT diagram for the steel grade in question, it is possible to predict the microstructure and hardness after normalizing. To superimpose the recorded cooling curves onto the CCT diagram, the time-temperature scales are to be equal to those of the CCT diagram.
Fig 7a shows, for example, that the unalloyed Ck45 steel (carbon 0.45 %) attains the desired ferrite-pearlite microstructure in the core of all investigated bars of different diameters cooled in air. On the other hand, as shown in Fig 7b, the alloyed 55NiCrMoV6 steel cooled in the same way in air transforms to martensite and bainite. In this case, to get a desired microstructure and hardness after normalizing, a much slower cooling rate of around 10 deg C per hour, i.e., furnace cooling, is to be applied from the austenitizing temperature to the temperature at which the formation of pearlite is finished (around 600 deg C).
Fig 7 CCT diagrams for unalloyed and alloyed steels
The purpose of normalizing varies considerably. Normalization can increase or decrease the strength and hardness of a given steel in a given product form, depending on the thermal and mechanical history of the product. Actually, the functions of normalizing can overlap with or be confused with those of annealing, hardening, and stress relieving. Improved machinability, grain-structure refinement, homogenization, and modification of residual stresses are among the reasons for which normalizing is done.
Homogenization of castings by normalizing can be done in order to break up or refine the dendritic microstructure and facilitate a more even response to subsequent hardening. Similarly, for wrought steel products, normalization can help to reduce the banded grain structure because of hot rolling, as well as large grain size or mixed large and small grain size because of forging practice. The details of normalizing treatments applied to three typical production parts are given in Tab 1, which also lists the reasons for normalizing and gives some of the mechanical properties obtained in the normalized and tempered condition.
|Tab 1 Typical applications of normalizing and tempering of steel components|
|Component||Steel||Steel grade||Properties after treatment||Reason for normalizing|
|Cast 50 mm valve body, 19 mm to 25 mm in section thickness||Ni-Cr-Mo||Full annealed at 955 deg C, normalized at 870 deg C, tempered at 665 deg C||Tensile strength – 620 MPa, yield strength – 415 MPa, elongation in 50 mm – 20 %, reduction in area – 40 %||For meeting mechanical properties requirements|
|Forged flange||AISI 4137 steel grade||Normalized at 870 deg C, and tempered at 570 deg C||Hardness – 200 HB to 240 HB||For refining of grain size and for achieving the needed hardness|
|Valve bonnet forging||AISI 4140 steel grade||Normalized at 870 deg C and tempered||Hardness – 200 HB to 240 HB||For achieving uniform microstructure, improved machinability, and needed hardness|
|AISI – American Iron and Steel Institute, HB – Brinell hardness|
Fig 8a shows that high-carbon steels with large quantities of pearlite have high transition temperatures and hence fail in a brittle manner even well above room temperature. On the other hand, low-carbon steels have sub-zero transition temperatures and are quite tough at room temperature.
Fig 8 Impact transition curves and comparison of normalization and full annealing
Depending on the mechanical properties needed, normalizing can be substituted for conventional hardening when the size or shape of the component is such that liquid quenching can result in cracking, distortion, or excessive dimensional changes. Hence, components which are of complex shape or which incorporate sharp changes in section can be normalized and tempered, provided that the properties achieved are acceptable. The rate of heating is normally not critical for normalizing, on an atomic scale, it is immaterial. In components having high variations in section size, however, thermal stress can cause distortion.
Time at temperature is critical only in that it is to be sufficient to cause homogenization. Sufficient time is to be allowed for solution of thermo-dynamically stable carbides, or for diffusion of constituent atoms. Normally, time sufficient for complete austenitization is all what is needed. One hour at temperature, after the furnace recovers, per 25 millimetres of component thickness, is considered to be standard. Components are frequently austenitized adequately in much less time (with a saving of energy). In cases where normalizing is done to homogenize segregated structures, longer times are needed.
The rate of cooling influences considerably both the quantity of pearlite and the size and spacing of the pearlite lamellae. At higher cooling rates, more pearlite is formed, and the lamellae are finer and more closely spaced. Both the increased quantity of pearlite and the higher fineness of the pearlite result in higher strength and higher hardness. On the other hand, lower cooling rates result in softer components. The mass has also an effect on the hardness (through its effect on cooling rate). In any component having both thick and thin sections, the potential exists for variations in cooling rate, and hence for variations in strength and hardness as well. This can also increase the probability of distortion or even cracking. Cooling rate sometimes is improved with fans to increase strength and hardness of the components or to decrease the time needed, following the furnace operation, for sufficient cooling of the components to permit convenient handling.
After the parts have cooled uniformly through their cross-section to black heat below Ar1 temperature (the parts are no longer red, as when they are removed from the furnace), they are to be water quenched or oil quenched for decreasing the total cooling time. In heavy sections, cooling of the centre material to black heat can need considerable time. Thermal shock, residual thermally induced stress, and resultant distortions are factors to be considered. The microstructure remains essentially unaffected by the increased cooling rate, provided that the entire mass is below the lower critical temperature, Ar1, although changes involving precipitates can occur.
Carbon steels – Steels containing 0.2 % C or less normally receive no treatment subsequent to normalizing. However, medium-carbon or high-carbon steels are frequently tempered after normalizing to get specific properties such as a lower hardness for straightening, cold working, or machining. Whether tempering is desirable depends on specific property requirements and not on carbon content and section size requirements. Because of pearlite lamellae and spacing, a low-carbon or medium-carbon steel of thin section can be harder after normalizing than a high-carbon steel of large section size subjected to the same treatment.
Alloy steels – For alloy steel forgings, rolled products, and castings, normalizing is normally used as a conditioning treatment before final heat treatment. Normalizing also refines the structures of forgings, rolled products, and castings which have cooled non-uniformly from high temperatures.
Alloy carburizing steels such as AISI 3310 and AISI 4320 steel grades are normally normalized at temperatures higher than the carburizing temperature for minimizing the distortion in carburizing and to improve machining characteristics. Carburizing steels of the AISI 3300 steel grade series sometimes are double normalized with the expectation of minimizing distortion. These steels are tempered at around 650 deg C for intervals of up to 15 hours for reducing the hardness to below 223 HB (Brinell hardness) for machinability. Carburizing steels of the AISI 4300 and AISI 4600 steel grades series can be normally normalized to a hardness not exceeding 207 HB and hence need not be tempered for machinability.
Hypereutectoid alloy steels such as AISI 52100 alloy steel are normalized for partial or complete elimination of carbide networks, hence producing a structure which is more susceptible to 100 % spheroidization in the subsequent spheroidize annealing treatment. The spheroidized structure provides improved machinability and a more uniform response to hardening. Some alloy steel grades need more care in heating to prevent cracking from thermal shock. They also need longer soaking times because of lower austenitizing and solution rates. For several alloy steels, rates of cooling in air to room temperature are to be carefully controlled. Certain alloy steels are forced-air cooled from the normalizing temperature in order to develop specific mechanical properties. This is a normalizing treatment only in the microstructural sense.
Forgings – When forgings are normalized before carburizing or before hardening and tempering, the upper range of normalizing temperatures is used. However, when normalizing is the final heat treatment, use is made of the lower range of temperatures.
Furnaces – Any appropriately sized furnace can be utilized for normalizing. Furnace type and size depend upon the specific need. In a continuous furnace, forgings to be normalized are normally placed in shallow pans, and a pusher mechanism at the loading end of the furnace transports the pans through the furnace. Furnace burners located on both sides of the furnace fire below the hearth, and combustion products rise along the walls of the work-zone muffle and exhaust into the roof of the furnace. No atmosphere control is used. Combustion products enter the work zone through ports lining both sides of the entire hearth. A typical furnace is 9 metre long and has 18 gas burners (or 9 oil burners) on each side. For the purpose of the temperature control, such a furnace is divided into three number 3 metre zones, each having a vertical thermocouple extending into it through the roof of the furnace.
Processing – Small forgings are normally normalized as received from the forge shop. A typical furnace has five pans in each of the three furnace zones. Heating is adjusted so that the work-piece reaches normalizing temperature in the last zone. After passing through the last zone, the pans are discharged onto a cooling conveyor. The work-piece, while still in the pans, is cooled in still air to below 480 deg C. It is then discharged into tote boxes (box or tray for storing, handling, and transporting materials in industrial operations), where it cools to room temperature. Total furnace time is around 3.5 hours, but during this period the work-piece is held at the normalizing temperature for only 1 hour.
Normalizing of large open-die forgings is normally done in batch-type furnaces which are pyrometrically controlled for narrowing the temperature ranges. Forgings are held at the normalizing temperature long enough to allow complete austenitizing and carbide solution to occur (normally one hour per 25 millimetres of section thickness), and then are cooled in still air.
Axle shaft forging – In forging of an axle shaft made of fine-grained AISI 1049 grade steel, only one end of the forging bar is heated to upset the wheel-flange section. When the part is examined in cross-section from the flanged end to the cold end, the metallurgical conditions described below are revealed.
The hot-worked flanged area of the axle shaft shows a fine-grained structure as a result of the hot working at the forging temperature (around 1,100 deg C). However, a section adjacent to the flange, which also have been heated to the forging temperature but which have not been hot worked, shows a coarse-grained structure. Nearer the cool end of the shaft, a zone which reached a temperature of around 700 deg C shows a spheroidized structure. The cold end of the axle shaft retained its initial fine grain size throughout the forging operation.
In subsequent operations, this axle shaft is to be mechanically straightened, machined, and induction hardened. Because of the mixed grain structure, these operations posed several problems. The coarse-grained area adjacent to the flange is extremely weak in the transverse direction, and there is a possibility that fracture can occur if this section is subjected to a severe straightening operation. The spheroidized area does not respond adequately to induction hardening since the solution rate of this type of carbide formation is too sluggish for the relatively rapid rate of induction heating. Also, the mixed metallurgical structure presents difficulties in machining. Hence, normalizing is needed in order to produce a uniform fine-grained microstructure throughout the axle shaft prior to straightening, machining, and induction hardening.
Low-carbon steel forgings – In contrast to the medium-carbon axle shaft discussed in the preceding paragraphs, forgings made of carbon steels containing 0.25 % C or less are seldom normalized. Only severe quenching from above the austenitizing temperature has any significant effect on their microstructure or hardness.
Structural stability – Normalizing and tempering is also a preferred treatment for promoting the structural stability of low alloy heat-resistant alloys, such as AMS 6304 (0.45 % C, 1 % Cr, 0.5 % Mo, and 0.3 % V), at temperatures up to 540 deg C. Wheels and spacer rings used in the cold ends of aircraft gas-turbine engine compressors are typical of components subjected to such treatment to promote structural stability.
Multiple normalizing treatments – These are used to get complete solution of all lower-temperature constituents in austenite by the use of high initial normalizing temperatures (e.g., 925 deg C), and to refine final pearlite grain size by the use of a second normalizing treatment at a temperature closer to the Ac3 temperature (e.g., 815 deg C) without destroying the beneficial effects of the initial normalizing treatment.
Double normalizing is normally applied to carbon and low-alloy steels of large dimension where extremely high forging temperatures have been used. Locomotive-axle forgings made of carbon steel, containing 0.45 % C to 0.59 % C and 0.6 % Mn to 0.9 % Mn, are double normalized to get a uniformly fine grain microstructure along with other exacting mechanical-property requirements. Forgings made of a low-carbon steel (0.18 % C) with 1 % Mn intended for low-temperature service are double normalized to meet sub-zero impact requirements.
Bar and tubular products
Frequently, the finishing stages of hot-rolling mill operations used in making steel bar and tube produce properties which closely approximate those achieved by normalizing. When this occurs, normalizing is unnecessary and can even be inadvisable. However, the reasons for normalizing bar and tube products are normally the same as those applicable to other forms of steel.
The machinability of steel bars and tubular products depends on a combination of hardness properties and microstructure. For a low-carbon alloy steel, a coarse pearlitic microstructure achieved by normalizing or annealing maximizes machinability. In the case of medium-carbon alloy steel, a lamellar pearlitic microstructure achieved by annealing is desirable in order to optimize machinability. For a high-carbon alloy steel, a spheroidized microstructure lowers the hardness and increases the machinability of the alloy steel. Prior processing, component configuration, and processing following machining are to be taken into consideration when determining the need for annealing or normalization.
In general, annealing improves machinability more than normalization does. Normalizing is used to correct the effects of spheroidization, but the steel bar or tube still needs to be annealed. Multiple annealing and tempering are normally used on only small-diameter parts such as wire gauge products. AISI 4340 grade of steel is one of the few steels which is typically supplied to the customer with a normalized heat treatment because of machining specifications standard in the aircraft industry.
Tubes are easier to normalize than bars of equivalent diameter since the lighter section thickness of tubes permits more rapid heating and cooling. These advantages help minimize decarburization and promote nearly more uniform microstructures in tube products.
Furnaces requirements – Continuous furnaces of the roller-hearth type are widely used for normalizing tube and bar products, especially in long lengths. Batch-type furnaces or other types of continuous furnaces are satisfactory if they provide some means for rapid discharge and separation of the load to permit free circulation of air around each tube as it cools. Continuous furnaces need to have at least two zones, one for heating and one for soaking. Cooling facilities are to be sufficient so that uniform cooling can proceed until complete transformation has occurred. If tubes are packed or bundled during cooling from a high temperature, the purpose of normalizing is defeated, and a semi-annealed or a tempered product result.
Normally, protective atmospheres are not used in roller-hearth continuous furnaces for normalizing bar or tube products. The scale which forms during normalizing is removed by acid pickling or abrasive blast cleaning.
In industrial practice, steel castings can be normalized in car-bottom, box, pit, or continuous furnaces. The same heat-treating principles apply to each type of furnace.
Furnace loading – Furnaces are loaded with castings in such a manner that each casting receives an adequate and uniform heat supply. This can be done by stacking castings in regular order or by scattering large and small castings so that load concentration in any one area is not excessive. At normalizing temperatures, the tensile strength of steel is greatly reduced, and heavy unequal sections can become distorted unless bracing and support are provided. Hence, small and large castings are arranged so that they support each other.
Loading temperature – When castings are charged, the temperature of the furnace is to be such that the thermal shock does not cause metal failure. For the higher-alloy grades of steel castings, a safe furnace temperature for charging is 315 deg C to 425 deg C. For low-alloy steel grades, furnace temperature can be as high as 650 deg C. For cast carbon steels and low-alloy steels with low carbon contents (low hardenability), castings can be charged into a furnace operating at the normalizing temperature.
Heating – After the furnace has been charged, the temperature is increased at a rate of around 225 deg C per hour until the normalizing temperature is reached. Depending on steel composition and casting configuration, a reduction in the rate of heating to around 28 deg C per hour to 55 deg C per hour is necessary to avoid cracking. Extremely large castings are to be heated more slowly to prevent development of extreme temperature gradients.
Soaking – After the normalizing temperature has been reached, castings are soaked at this temperature for a period which ensures complete austenitization and carbide solution. The duration of the soaking period can be pre-determined by microscopic examination of samples held for different times at the normalizing temperature.
Cooling – After the soaking period, the castings are unloaded and allowed to cool in still air. Use of fans, air blasts, or other means of accelerating the cooling process is to be avoided.
Sheet and strip
Hot-rolled steel sheet and strip (around 0.1 % C) are normalized mainly to refine grain size, to minimize directional properties, and to develop desirable mechanical properties. Uniformly fine equiaxed ferrite grains are normally achieved in hot-rolled sheet and strip by finishing the final hot-rolling operation above the upper transformation temperature. However, if part of the hot-rolling operation is performed on steel which has transformed partially to ferrite, the deformed ferrite grains normally recrystallize and form abnormally coarse-grained patches during the self-anneal induced by coiling or piling at temperatures of 650 deg C to 730 deg C. Also, relatively thin hot-rolled material, if it is inadvertently finished well below the upper transformation temperature and coiled or piled while it is too cold to self-anneal, can possess directional properties. These conditions are unsuitable for some types of severe press-drawing applications and can be corrected by normalizing.
Normalizing is also used to develop high strength in alloy steel sheet and strip if the products are sufficiently high in carbon and alloy contents to enable them to transform to fine pearlite or martensite when cooled in air from the normalizing temperature. In general, the hardened material is tempered to attain an optimum combination of strength and ductility.
Processing – The normalizing operation consists of passing the sheet or strip through an open, continuous furnace where the material is heated to a temperature around 55 deg C to 85 deg C above its upper transformation temperature of 845 deg C to 900 deg C, hence achieving complete solution of the original structure with the formation of austenite and then air cooling the material to room temperature.
Furnace equipment – Normalizing furnaces are designed to heat and cool sheets singly or two in a pile. They are built in the form of long, low chambers and normally comprise three sections namely (i) a pre-heating zone (12 % to 20 % of the total length), (ii) a heating, or soaking zone (around 40 % of the total length), and (iii) a cooling zone, which occupies the remaining 40 % to 50 % of the length.
Heating arrangements – Normalizing furnaces are normally heated with gas or oil and do not use protective atmospheres. Hence, sheets are scaled during heat treatment. Burners are arranged along each side of the heating zone. They are normally above the conveyor, but occasionally are both above and below it. The furnace roof, which is higher in the preheating and soaking zones than in the cooling zone, is normally built in sections. In the majority of the furnaces, both the preheating zone and the cooling zone are heated by the hot gases from the heating zone. However, both of these zones can be equipped with burners for more accurate temperature control. Air is excluded by regulating the draft to maintain a slight pressure within all zones.
Conveyor-type furnaces – In modern furnaces of the conveyor type (the only type suitable for treating short lengths), sheets are carried through each of the three zones on rotating disks made of heat-resistant alloys. These disks have polished surfaces, which prevent them from scratching the sheets, and are staggered for ensuring uniform heating. The disks are mounted on water-cooled shafts, which are driven by variable-speed motors through chains and sprockets or shafts and gears. These furnaces can be up to 2.5 metres wide and from 25 metres to 60 metres long. Fuel consumption is 2,300 mega-joules to 5,200 mega-joules per ton of steel treated, and production rates vary from 3 tons per hour to 12 tons per hour.
Normalizing in a three-zone conveyor-type furnace equipped with pyrometric controls is a relatively simple operation. If scratching of sheets is to be avoided, the sheets are brought to the charging table and hand laid, one or more at a time, on a rider or conveyor sheet. Heavy sheets are normalized singly, but lighter sheets can be stacked two in a pile. For controlling heating and for retarding scaling, single sheets can be laid on a rider sheet and covered with a cover sheet. Sheets are carried by disk-rollers into the preheating zone, where they absorb heat rapidly because of the large temperature differential between the sheets and the interior of the furnace and because of the large surface-to-volume ratio.
As the sheets become heated and the temperature differential is reduced, the rate of heat absorption slackens. After traveling 4.5 metres to 6 metres, the sheets enter the soaking zone at a temperature several degrees below the normalizing temperature. Heating is completed in the soaking zone, which is maintained at a constant temperature, and sheets are held at the required temperature for a time sufficient to convert the microstructure to austenite before they are passed into the cooling zone. The sheets emerge from the cooling zone at a temperature which can be varied between 150 deg C and 540 deg C, and are conveyed for a short distance on the runout table, where, after being cooled rapidly in air, they are carefully removed from the rider sheet. The trip through such a furnace is carried out at a uniform speed of 0.03 metre per second to 0.1 metre per second and needs 5 minutes to 20 minutes to complete.
Catenary furnaces – The catenary, or free-loop, type of furnace is designed for continuous normalizing of cold-reduced steel unwound from coils. It does not have rolls or any other type of conveyor for supporting the material passing through the heating zone. The heating zones of catenary furnaces range in length from 6 metres to 15 metres. The preheating and cooling zones are normally shorter than those in conveyor-type furnaces, and for some kinds of work-piece can be omitted entirely. At their exit ends, catenary furnaces can incorporate pickling or other descaling equipment for removing surface oxides formed on the steel during normalizing.
Comparison with annealing
Normalizing process of steel differ from the annealing process of steel with respect to heating temperature and cooling rate (Fig 8b). In case of normalizing, the steel is heated to a higher temperature and then removed from the furnace for air cooling. In comparison, in case of annealing the heating temperatures are lower and the cooling take place in furnace at a much lower rate. Because of the faster cooling rate in case of normalizing, the steel possesses higher strength and hardness when compared with the steel which has undergone annealing treatment.
Both annealing and normalizing do not present considerable difference in the ductility of low carbon steels. The tensile strength and the yield point of the normalized steels are higher than those of the annealed steels except in the case of low carbon steels.
As in the case of annealing, normalizing also results into the formation of ferrite, cementite and lamellar pearlite. But in normalizing, since the cooling rates are higher, transformation of austenite takes place at much lower temperatures when compared with annealing. Because of it, the transformation product, pearlite is finer with lower interlamellar distance between the two neighboring cementite plates.
The main difference between full annealing and normalizing is that fully annealed work-pieces are uniform in softness (and machinability) throughout the entire component, since the entire component is exposed to the controlled furnace cooling. In the case of the normalized component, depending on the component geometry, the cooling is non-uniform resulting in non-uniform material properties across the component.
Normalizing relieves internal stresses caused by cold working, while grain growth is limited by the relatively high cooling rate and hence the mechanical properties (strength, and hardness) of a normalized steel are better than those of an annealed steel.
Quality of surface after machining of a normalized component is also better than those of an annealed component. This effect is caused by increased ductility of annealed steel favouring formation of tearing on the machined surface.
Properties after normalizing
Since the cooling rate in the normalizing heat treatment is not controlled, the resulting microstructure is dependent on the thickness of the steel work-piece. Hence, the effect of increased mechanical properties is higher in thin work pieces.
Normalized steel has higher hardness and strength than annealed steel because of (i) the quantity of pearlite in the normalized steel is more than that in the annealed steel having the same carbon content, because of the shifting of the eutectoid composition to a lower value, (ii) the dispersion of pearlite and ferrite phases is finer, and (iii) the pearlite of normalized steel is finer and has a lower interlamellar spacing than that of annealed steel.
Application of normalizing
Normalizing is the most extensively used industrial process since it is more economical to normalize the steel as against annealing. In normalizing since the cooling takes place in air, the furnace is ready for next cycle as soon as heating and soaking is over as compared to annealing where furnace cooling after heating and soaking needs 8 hours to 20 hours depending upon the quantity of charge. Hence in several cases annealing is replaced by normalizing to reduce the cost of heat treatment. Normalizing is adopted if the properties requirements are not very critical.
Some typical examples of normalizing in commercial practice are (i) normalizing of gear blanks prior to machining so that during subsequent hardening or case hardening dimensional changes such as growth, shrinkage, or warpage can be better controlled, (ii) homogenization of cast and wrought microstructures, (iii) improvement of machinability and grain size refinement of cast microstructures of castings, (iv) stress relieving of castings, (v) homogenization of the microstructure of cast steels and alloy steels which are characterized by segregated, cored and dendritic microstructures as well as non-uniform properties, and (vi) homogenization of wrought steels and alloy steels after mechanical working such as forging, rolling, and extrusion etc. which have non-uniform microstructure and properties.
In some few cases, when the steel is hot or cold worked, it is necessary to perform a normalizing heat treatment in order to recover its original mechanical properties. In case of normalizing heat treatment on weld metal the original as welded metal fine grained microstructure is changed to a coarse-equiaxed ferrite with ferrite-carbide aggregates and the yield strength and tensile strength properties are considerably reduced.
It is very rare for a forging to be used without some sort of thermal treatment because of the heavy mechanical stresses impressed on the component and the variations in the microstructure. Normalizing is one of the simplest heat-treatments which can address refining (or normalizing) the microstructure and equalizing the effects of the range of temperatures, the material has been subjected to during the forging operations. Normalizing of forgings is very beneficial to any subsequent hardening operations.
Steels which have undergone plastic deformation consist of pearlite which is irregularly shaped and relatively large, but varying in size. Normalizing is a heat treatment used on steel so as to refine its crystal structure and produces a more uniform and desired grain size distribution.
Advantages of normalizing
Normalizing is used to eliminate the coarse grain structure got in the previous working operations such as rolling, and forging etc. The temperature used for forged components can be in the upper range of the normalizing temperature. Normalizing also avoids the coarse microstructure which results from the slow cooling of regular annealing.
Carburizing steels and carbon tool steels can be normalized to break up the continuity of the cementite network surrounding pearlite, since slow cooling re-establishes the network. Refinement of the size of ferrite and pearlite and modifying and refining of cast dendritic microstructure also take place during normalizing.
Normalizing minimizes the distortion and improves the machinability of alloyed carburizing steels. Normalization is at a higher temperature than the carburizing temperature (950 deg C to 970 deg C). Normalized medium carbon, high carbon, and alloyed steels are then tempered to lower the hardness (200 HB) for the purposes of machining.
Compared with annealing, normalizing improves the mechanical properties. In several cases, annealing with subsequent tempering can be substituted for normalizing. Furnaces which are used for normalizing are of both batch type and continuous type.