The Influence of Temperature and Soaking time on the Microstructure and Mechanical Properties of annealed AISI 1045 Steel

In this present work AISI 1045 medium carbon steel was subjected to different conventional heat treatments mainly a full annealing, normalizing and hardening. The annealing processes were first carried out at annealing temperatures 850,900 and 950°C for one-hour soaking time. Later the periods of annealing were changed from five minutes up to two hours all of which at 850°C austentization temperature. In addition, normalizing and hardening processes were performed at an intermediate austenitization temperature of (850°C) and the soaking time mainly one hour. Therefore, the main objective of the current study is to investigate the effect of both annealing temperatures and soaking times on the microstructure, macrohardness and impact toughness of 0.45%C. Meanwhile, the results of 0.0.45%C steel annealed at 850°C for one hour were compared to that of normalized and hardened steels.It was concluded that, the change in the macrohardness and absorbed energy of 0.0.45%C annealed steel was mainly attributed to the morphology, grain size and the amount of formed austenite through its influence in defining the final microstructure of the products at room temperature.In contrast, the proportion of ferrite and pearlite micro-constituents in the structure of normalized steel was found to have a predominant role in such behavior whereas the macrostructure of hardened steel reveals almost martensite phase that made this steel possess the maximum hardness and the lowest impact toughness among all tested samples.


Introduction
The AISI 1045 medium carbon steel is the most common form of steels as it provides material properties that are acceptable for many applications. It is neither extremely brittle nor ductile; therefore, AISI 1045 is widely used in various structural and elements of machines.
Today heat treatment process is widely used to achieve high mechanical properties of a given steel. Annealing, normalizing, hardening and tempering are the most important heat treatments often used to modify the microstructure and mechanical properties of engineering materials particularly steels. Meanwhile such heat treatments involve heating the steel to a temperature that is high enough to promote the transformation of steel microstructure to onephase constituent called austenite. This stage is known as austenitization or homogenization [1].
After the structure of steel becomes 100% austenite, it is cooled to room temperature with various cooling rates ranged from very slow cooling rate (furnace cooling) to rapid cooling rate (water quenching) encountered in full annealing and hardening processes respectively.From metallurgical point of view, the slow or moderate cooling rates enable the carbon atom to diffuse out of the austenite structure. Therefore, there will be a sufficient time to the reaction of austenite to ferrite to take place as predicted in the Fe -Fe3C phase diagram. On the other hand, when the cooling rate is further increased the time becomes insufficient to decompose the austenite to ferrite as mentioned before, rather than the carbon is trapped in the solid solution. The resultant structure is called martensite [2].
Because of the austenite structure is the only source of either ferrite-pearlite microstructure formed during slow cooling or near equilibrium or the martensite phase formed during rapid cooling both the microstructure and the mechanical properties of a given steel will related to its thermal history [3,4]. Due to this, although austenitization implies heating a steel to the onephase austenite field irrespective of the type of heat treatment, however, the morphology, the grain size and the homogeneity of the formed austenite seem to have an important role in determining the final structure of the heat treated steel which in turn alters its mechanical properties.
It can be emphasized that not only the cooling rate is an important part of the heat treatment, but the temperature and the time of austenitization are also the vital part of the process. Accordingly, the main objectives of this study are to investigate the effect of various austenitization temperatures and soaking times of the full annealing process on microstructure, hardness and impact toughness of medium carbon steel grade 45. In addition, the microstructure, hardness and impact energy of AISI 1045 carbon steel are also investigated after being annealed, normalized and hardened at a fixed austenitization temperature and time for comparison viewpoint.

Materials utilized
In this study a set of medium carbon steel 0.0.45%C are used. The nominal chemical composition of the AISI 1045 as obtained by emission spectroscopy at the high vocational center of casting is presented in Table (1).

Annealing process
The annealing cycle consists of heating the steel samples in a digitally controlled electrical furnace capable of 1200°C maximum temperature (SIB 13940). The steel samples were heated to a temperature enables the microstructures of the steel to transform to purely austenite depending upon the A3 line of Fe -Fe3C phase diagram.
The actual austenitization temperature as anticipated from the Fe -Fe3C phase diagram of 0.0.45%C steel is approximately 770°C. So that the austenitization temperature is about 820 °C (50°C above the A3 line as recommended. Nevertheless, three austenitization temperatures were applied in this study namely 850, 900 and 950°C. The steel is then maintained at the chosen temperature for various periods of time, which is known as holding or soaking time. The soaking times were randomly selected ranging from a few of minutes (5 minutes) to a relatively longer period (120 minutes). The samples were let to cool to room temperature inside the furnace after being shutdown. The annealing conditions are summarized in Table (2).

Normalizing and hardening
The increase in cooling rate due to air cooling or quenching compared with furnace cooling affects the transformation of austenite and resultant microstructure.
The cycle of the normalizing heat treatment employed in this study consists of heating the steel samples to a temperature of 850°C, held at this temperature for 60 minutes, then the specimens were removed quickly from the furnace and allowed to cool in the still air to room temperature. As for the hardening process, the same austenitization temperature and soaking time used in the normalizing treatment mainly 850°C and 60 minutes respectively were used. Meanwhile, the samples were rapidly cooled, by quenching them in tape water at room temperature.
It is worth noting that the aim of employing normalizing and hardening heat treatments is to have a rough comparison between it and full annealing process. Accordingly, the normalizing treatment were not carried out at different austenitization temperatures and times as being done in the full annealing process. As a result, to limit the operating variables that may led to contradictory result, so that the interpretation of the obtained data will be much more difficult.

Hardness measurement
The hardness test measures the resistance to penetration of the surface of a material by a hard object.In this study the macro-hardness of different heat treated samples (annealed, normalized and hardened) were determined using a Vickers's hardness tester (Ernest NR3D). Prior testing the surface of the pieces were grinded to remove any scales, oxide layers, etc.
That formed during the heat treatments to ensure a full contact between the surface of the specimen and the indenter as recommended. The presence of such layers may have a contribution in the obtained results.About ten measurements were made at various locations on the surface of each specimen and out of which the average value was reported. Moreover, the hardness of the as received (supplied) is also measured for comparison viewpoint.

Impact test
Impact test is designed to measure the resistance to failure of material to a suddenly load,the test measures the energy absorbed prior to fracture.Bar test, pieces of 10x10x55mm in dimensions were tested using a Charpy impact machine of ZWICK type. The test is carried out in accordance with ASTM E23 standard specifications [5]. Three specimens of each heat treatments as well as from as received steel were tested and from which the average value was calculated. It is to be mentioned, that the Charpy specimens of 10x10x55 mm were not sectioned from the treated steel strips and then subjected to impact loading later rather than all heat treatments were done on the actual Charpy bars prior impact loading.

Optical microscopy
Small specimens of one cm 2 in surface area were sectioned from each regime and mounted in epoxy resin. Silicon carbide emery paper with successive grade up to 1200 grit were used in grinding the surface of the samples to be microscopically analyzed. Using a polishing wheel, the specimens were polished with alumina mixture followed by 1µm diamond paste until their surfaces were mirror-like. The polished specimens were then etched with 2% Nital and microstructurally observed using optical microscope (Nikon OPTIPHOT with Amscop 3.7).

Effect of annealing temperature and time on microstructure
The microstructure of as received steel as shown in Figure (1) consists of visible grains of pearlite (dark) in ferrite (white) matrix. The pearlite has irregular shape and randomly distributed over the ferrite base. In addition, some grains seem to have being under gone deformation process in such that they were enlarged in the direction of deformation during the final manufacturing process. The main constituents of the microstructure of all annealed specimens are pearlite and ferrite. Because the samples were cooled by very slow cooling rate (furnace cooling), the transformation of austenite to pearlite and ferrite through the eutectoid reaction (below A1 line in Fe-Fe3C phase diagram) can be considered to take place under equilibrium condition. Consequently, the relative amounts of either pearlite or pro-eutectoid ferrite can be anticipated applying the lever role in (Fe-Fe3C) phase diagram.
The amounts of pearlite and pro-eutectoid ferrite were calculated using the lever rule to be equal 57% and 43% respectively. These amounts of pearlite and ferrite are assumed to be essentially ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬ : ‫والتطبيقية‬ ‫األساسية‬ ‫العلوم‬ Journal of Alasmarya University: Basic and Applied Sciences unchanged regardless the change in the austenitization temperature or time as long as the cooling rate remains very slow. Consequently, the main influence of increasing temperature or time of austenitization is to enlarge the grain size (coarsening) of both pearlite and ferrite. This effect can be well observed by comparing the two Figures (2-3). It is worth mentioning that such influence of both annealing temperature and time on the grain size of the annealed steel, were reported before by many authors [6,7].

Effect of annealing temperature on hardness and impact energy
The influence of austenitization temperature on hardness and impact toughness of 0.45% C steel is shown in Figures (4). The decrease in the hardness with increasing temperature is indicated clearly by the trend of the curve. The abrupt decreasing percentage occurred at 850C being 39%. That may be partially attributed to a considerably high hardness of as supplied steel (302 HV). In contrast, the decreasing percentage at 900°C and 950°C equal 23% and 12% respectively. This finding is in contradictor with the results of [7] who found that raising annealing temperature from 900°C to 940°C has led to an extensive improvement in the hardness of 0.45%C. In discussing such effect, one should assume that the structure of 0.0.45%C steel after austenitized is to be purely homogenous austenite. With this assumption, all data excludes the possibility of improper structure formation due to impure austenitic sample.As a result, the type of constituents exhibited by the final microstructure of annealed steel did not appear to have a significant influence in such behavior. Instead, the grain size of the obtained steel microstructure seems to have a decisive role in reducing the hardness. Moreover, some equations were suggested to explain such behavior. In this aspect, the decreasing in hardness value with increasing grain size could be explained in the light of the well-known equation derived by Hall and Patch [8]: σy: is the yield strength. σ0: is the fraction stress which opposes dislocation movements. ky: is the material constant. d: is the mean grain size.
The hardness of a given steel is commonly well-related to its strength in such that a steel of high strength normally exhibits high hardness. From the above equation one can realize that the strength and thus the hardness of a given metallic material with known σ0 decrease and K decreases with increasing the value of d.
It is believed that the thermal movements of atoms in the austenite range are rapid enough to cause grain growth so that, higher temperatures in the austenite range is capable to greatly increasing the size of the initial austenite grains. The size of austenite grains that is attained ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬ : ‫والتطبيقية‬ ‫األساسية‬ ‫العلوم‬ Journal of Alasmarya University: Basic and Applied Sciences before a metal is cooled back to room temperature is important in determining a number of physical properties of the final structure including the hardening response of the steel [9].
Consequently, as the annealing temperature is raised, the austenite grain size increases, hence the ferrite will be equated and relatively coarse grains and the pearlite will have a coarse interlamellar spacing. This type of structure is characterized by lower hardness, strength and increase ductility. This is consistent with [10] who has found that the grain size of ferrite and pearlite of as received 0.45C steel were increased from 43 µm -64 µm to 116 µm and 158µm respectively by homogenizing annealing.
It is worth pointing that, although impact energy is more sensitive to the bulk composition of the material and not as the property of hardness which is almost sensitive to the surface condition, there is commonly good correlation between them among many materials particularly carbon steels. Generally, as hardness increases, impact energy drops and vice versa. Based on the above-mentioned principle one can realize that the impact energy was shown to gradually increase with temperature Figure (4). In spite of the lower impact toughness of the as received steel, the relationship could be considered almost linear. The low impact energy exhibited by the as supplied material is again attributed to its microstructure as noticed elsewhere. The impact energy of the tested steel is almost doubled when the steel is firstly annealed at 850°C. In contrast, increasing the annealing temperature by 100°C (850-950°C) has caused a relatively small increment in the absorbed energy (20J).

Effect of soaking time on hardness and impact energy
The curve of Figure (5) that represents the change of hardness with soaking time can be divided into three regions. The first region (I) includes the soaking time up to 30 minutes through which the percentage of decrement was observed to be relatively small, about 7% and 9% after annealing periods of 5 and 30 minutes respectively. When soaking time was extended to one-hour region (II), a sharp decrease in the hardness was noticed. In this aspect, the percentage of decrease being approximately three times as that of region (I). In region (III), the hardness is still decreased but with a relatively small percentage (16%) compared to part (II). During 5 minutes of annealing, the transformation of proeutectoid ferrite to austenite is to be expected.However, the dissolution of cementite (Fe3C) in austenite is a diffusion controlled slow process as reported by [10,11]. Therefore, the pearlite phase in the original structure is mostly remained and do not decompose to austenite after 5 minutes of homogenization and when cooled the microstructure would be similar to that of as received one. Because of thepearlite, possess a higher hardness than that of ferrite and the relative amount of pearlite in the original structure is greater than that of ferrite as mentioned before, a high decrease in hardness would not be anticipated after 5 minutes of austenitization. By increasing the time to 30 minutes, as many as austenite do form. However, complete transformation of austenite may not be attained, so that the hardness values were not significantly affected as mentioned earlier (9%).
Technically, it has been recommended one hour of annealing is more than enough to cause a full transformation to austenite phase. Besides this effect, one hour of annealing is capable to cause a substantial grain coarsening, as shown in the microstructure of this sample Figure (3). Therefore, the lower hardness exhibited by such steel could basically be relevant to the grain coarsening as far as the finer the grain size of austenite the finer of the grain size of ferritecementite products.
It is to be expected that, increasing the time of annealing after grain coarsening begin to cause further substantial decrease in the hardness property. Nevertheless, the curve of Figure ( As for the change of impact energy with soaking time Figure (5), it can be seen that the impact energy increases with soaking time by different rates. The overall rate of increasing equal approximately (3J/ min.), i.e. two hours of annealing were capable to enhance the impact toughness of the as received steel by more than two times. This behavior is considered to be reasonable in view of the obtained results of hardness. It is well known if hardness of a given steel drops, its ductility will generally increase which in turn reflects the increase in the metal toughness. However, the net amount of decrease in hardness and the net amount of increase in impact resistance are not comparable. Whereas the hardness value of the as received steel was reduced by a factor of half, the impact energy was found to remarkably improved by a factor of 2.4 (71/29).

Effect of cooling rate on hardness and impact energy
The influence of various heat treatments on the macrohardness and impact energy is shown in Figure (6). First, it has to be recognized that normalizing and hardening treatments were carried ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬ : ‫والتطبيقية‬ ‫األساسية‬ ‫العلوم‬ Journal of Alasmarya University: Basic and Applied Sciences out at a fixed austenitization temperature and time in particular 850°C and one hour respectively. Thus, the comparison between the results obtained from these two treatments and that of annealed one would be restricted only at this condition. Perhaps the aspect of particular interest is the increase in the hardness of normalized and hardened steels compared to the annealed one. The quenched (hardened) steel reveals the maximum hardness, which is more than two orders of magnitude higher than that of annealed steel. The normalized steel also shows a considerable increase with respect to annealed one.
From metallurgical viewpoint, the relative volume fraction of the constituents of the normalized steel microstructure cannot be evaluated in terms of Fe-Fe3C equilibrium phase diagram since air cooling retards the equilibrium condition necessary for utilizing the Fe-Fe3C diagram. In this regard, there is less time for the formation of proeutectoid ferrite. Consequently there will be less proeutectoid in the hypo eutectoid steel compared with annealed one. In view of the above, the proportions of ferrite and pearlite that will exist at room temperature are no longer equal to 43% and 57% respectively for all annealed steels as being stated before, rather than the relative amount of ferrite would be much less than 43% and that of pearlite would be much greater than 57%. For example, the microstructure of 0.50% C steel in the annealed condition as reported by [2] composes of 38% proeutectoid ferrite and 62% pearlite whereas that of normalized steel is 10% proectectoid ferrite and 90% pearlite.
The presence of high amount of pearlite phase in the final structure of normalized steel has the predominate role in enhancing its hardness Figure (7) as long as the ferrite is extremely softer than pearlite. The reported Brinell hardness number of ferrite is 50-100 BHN and pearlite is 180 BHN [12]. Alternatively, when AISI 1045 steel quenched rapidly in water, its structure is expected to have almost a martensitic feature Figure (8). Martensite is a hard micro-constituent in steel, its Vicker's hardness is 870. The existence of martensite in quenched steel thus accounts for drastically increase in the steel's macro hardness. Furthermore, according to the T.T.T diagram of this type of steel, the probability of formation of ferrite is high in quenched steel as well as retained austenite. It is reported that, the presence of pro-eutectoid ferrite and paralytic ferrite hinder the dislocation movement, and may have contributed to higher hardness value of the water quenched sample [13]. Regarding to the impact energy of the differently heated steel shown in Figure (6 ), it can be observed that as hardness increases, impact energy drops as being noticed in previous sections (3.1 and 3.2). As can be seen from Figure (6), doubling the hardness of hardened steel (388/184), was accompanied by abrupt reduction in its energy by about four orders of magnitude (58/14). This behavior is to be accepted since both ferrite and pearlite are much ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬ : ‫والتطبيقية‬ ‫األساسية‬ ‫العلوم‬ Journal of Alasmarya University: Basic and Applied Sciences more capable of absorbing impact load than the martensitic does. Similar comparison could be made with respect to the annealed and normalized steels.

Conclusions
From the current investigation, the following remarks could be withdrawn: 1-It was confirmed that, the macro hardness and the impact toughness of AISI 1045 steel are strongly dependent on its microstructure that can be controlled via heat treatment process.
2-The decrease in Vickers's hardness is always combined with an increase in the absorbed energy.
3-The change in the macro hardness and impact toughness of various annealed steels was found to be entirely dependent on the morphology, grain size and the amount of transformed austenite irrespective of the relative fraction of the final products at room temperature.
4-Varying the cooling rate appears to provide a dramatic influence on the hardness and impact resistance of 0.45%C steel. In this aspect, the change in such properties were mainly attributed to the proportions of ferrite and pearlite micro-constituents in the structure of normalized steel. Admittedly the formation of martensite phase in the water quenched (hardened) samples has a unique vital role in imparting the highest hardness and lowest toughness of the hardened steel among all tested specimens.

Recommendations
Based on the present study, it is advisable that further work is required. In particular, annealing, normalizing and hardening heat treatments need to be carried out at different austenitization temperatures and soaking times that is in order to have a satisfactorily comparison between the microstructures and mechanical properties obtained by such heat treatments. Moreover, the study may also extend to include other classes of steel.