Effects of Austenitization Temperature on the Microstructure of 15 BCr 30 and PL 22 Boron Steels

This paper studies boron precipitation and segregation at austenitic grain boundaries for low carbon boron steels types: PL22 and 15BCr30. The following parameters were evaluated: percentage of martensite/bainite, size and nucleation sites of austenitic grains and precipitates sizes. Three austenitization temperatures were studied (870, 1050 and 1200 °C). The highest martensite percentage occurred for 1050 °C. Iron-borocarbides were detected at grain boundaries for all tested temperatures. At 870 °C the coarse iron-borocarbides are due to non-solubility and coalescence. The highest martensite percentage at 1050 °C is caused by the discrete precipitation of iron-borocarbides at austenitic grains boundaries. The discrete precipitation was due to the low non-equilibrium segregation of boron at grain boundaries. The low non-equilibrium segregation and the small grain size at 1050 °C reduce the total boron concentration at grain boundaries.


Introduction
The austenitization temperature of quenching thermal treatment of boron steel components has large influence on the resulting martensite percentage due to the ironborocarbides precipitation in the matrix and/or at grain boundaries.
Grain boundary boron enrichment may be a result of either equilibrium segregation or non-equilibrium segregation.The equilibrium segregation occurs when the material is maintained at high temperature, allowing an effective diffusion of solute atoms.The free interface energy is then reduced by the absorption of solute atoms.The segregate atoms locate at the atomic layers of grain boundaries 1,2 .
The non-equilibrium segregation is a dynamic process occurring during the cooling, starting from high temperatures, and generates a large solute enriched zone.The zone width is a result of the used thermal treatment.The enrichment is an effect of the pair vacancy/boron diffusion towards the grain boundary 3 .With increasing temperature the vacancy equilibrium concentration raises for steel; also with fast cooling the concentration does not reach equilibrium and the grain boundaries act as sink.
There is an upper limit for boron segregation beyond which iron-borocarbides, Fe 23 (C,B) 6 , precipitate at the austenitic grain boundaries.The grain boundaries turn into the preferential site for ferrite and bainite nucleation.The above mentioned upper limit depends on the alloying elements and process parameters as austenitization temperature and cooling rate.Therefore, boron segregation and iron-borocarbide precipitation must be controlled in order to avoid ferrite and bainite nucleation 2,4 .Some authors 5,6 had shown that the equilibrium segregation of boron prevails for fast cooling starting from 900 °C and the non-equilibrium segregation is the dominant process for higher austenitization temperatures, as 1075 and 1250 °C.Therefore, the purpose of this work was to study the austenitization temperature effects on the microstructure of 15BCr30 and PL22 boron steels.

Material and Methods
The studied materials are 15BCr30 and PL22 steels, industrially produced by rolling and wire drawing to 14.30 mm final diameter.Chemical compositions are shown in Table 1 and the lower (Ac 1 ) and upper (Ac 3 ) transformation temperatures (α → γ) are shown in Table 2 [7][8][9] .
The original microstructure was composed by ferrite with coalesced carbides (Figure 1).The steels were quenched from three austenitization temperatures: 870, 1050 and 1200 °C, with 30 minutes soaking time and oil cooling at 80 °C.
Microstructural characterization of quenched steels was performed by light optical microscopy (LOM) and scanning electron microscopy (SEM -JEOL JSM-6390LV and FEG -JEOL JSM-6701F).The etching reagent used was Nital 2%.The quantification of existing phases was made by the point count method on a grid mask placed over the obtained images, observing a minimum of twenty points per sample, using scanning electron microscopy.
The transmission electron microscopy analyses were performed with carbon replicas and thin films with electron diffraction.The carbon replicas were obtained by carbon deposition on the samples using a metalizer -sputter coater -followed by etching with Nital solution at 5%, and finally assembled on a copper grid.
Thin film technique was used to identify the precipitates and precipitation sites and to analyze the steel matrices.The thin films were obtained by machining billet shapes with 3 mm diameter and 0.20 mm thickness.The thin billets were sanded to 0.08 mm, with sand grit 600, and a central hole was made by electrolytic polishing, with a jet polishing machine -Tenupol -using a 95% acetic acid and 5% perchloric acid solution.After polishing, the samples were washed in distilled water and ethyl alcohol.
The measurement of precipitate sizes was performed using the ImageJ 1.39u software, directly from the images from FEG and TEM.

Results and Discussion
The variation of austenitic grain size with respect to austenitization temperatures is shown in Table 3 and Figure 2. The grain sizes of 15BCr30 and PL22 steels are 15 to 23 µm and 22 to 36 µm, respectively.
The analysis of the grain size shows the growing of austenitic grain with austenitic raising temperatures, for both steels.According to Kapadia (1978) boron does not retard, instead it accelerates grain growing, so carbon is the only growth retarding factor.The smaller grain size of 15BCr30 steel with respect to PL22 steel may be related to the higher carbon content which induces larger diffusion to grain boundaries, avoiding their growing 2 .This same effect justifies the smaller grain sizes difference, 8 µm, for 15BCr30 steel at 1200 °C and 870 °C austenitization temperatures as compared to 14 µm for PL22 steel, at the same two austenitization temperatures.
The variation of martensite and bainite percentages with respect to austenitization temperature for both steels is shown in Table 4, Figures 3 and 4. The proportion of martensite lies between 88 and 95% for 15BCr30 steel and between 74 and 94% for PL22 steel.
For all austenitization conditions the microstructure is a mix of martensite and bainite with lath and butterfly morphology 10 .
The smaller martensite percentage differences of 15BCr30 steel with respect to PL22 steel may be related to the small grain size, as said before.The higher carbon content accelerates its diffusion to austenitic grain boundaries, avoiding its growth 2 .
The characteristic microstructures after quenching, obtained by thin film and observed by TEM are shown in Figure 5.A typical microstructure of low carbon martensite and bainite lath can be identified.The expanded image details show the detected precipitates.) and upper (Ac 3 ) transformation temperatures (α → γ) of studied steels 15BCr30 and PL22 [7][8][9] .The size of precipitates for the studied austenitization conditions are shown in Table 5.

Steel
The presence of iron-borocarbides Fe 23 (C,B) 6 and the average size variations for the three austenitization conditions indicate that iron-borocarbides coalescence occurred at 870 °C austenitization temperature.For the remaining two austenitization temperatures, iron-borocarbides solubilization and reprecipitation were observed.
The austenitization temperature of 870 °C lies bellow the Fe 23 (C,B) 6 solubilization (965 °C) 11 , therefore avoiding boron segregation toward the grain boundaries 12 .However the precipitate Fe 23 (C,B) 6 coalesces and raises the grain boundaries interface energy, reducing the boron effect on steel hardenability.
Comparing the produced iron-borocarbides at 1050 and 1200 °C, it can be seen that there is a larger precipitation/   reprecipitation at grain boundaries for 1200 °C.This fact can be explained by the larger non-equilibrium segregation, which spurs the grain boundaries iron-borocarbide precipitation at bainite production temperature, during quenching cooling, reducing the steel hardenability.Another important issue is the growth of grain size with increasing austenitization temperature.The grain size growth reduces the grain boundary area, and the reduction of the grain boundary area increases the total boron concentration at the boundary, also stimulating the higher precipitation of iron-borocarbides at 1200 °C austenitization temperature [13][14][15] .
The total boron concentration level needed to precipitate Fe 23 (C,B) 6 must to be lower at high austenitization temperature (1200 °C), because the boundary area is smaller due to the austenitic grain growth.Therefore, the high boron concentration at grain boundary may accelerate the reprecipitation of iron-borocarbides (Figure 5c).This reprecipitation results from the reduction of boundary area, due to grain size growth and to boron concentration raise by non-equilibrium segregation.This high concentration of iron-borocarbides at boundaries reduces the boron effect, lowering the steel hardenability.
The precipitates planes and angles, identified for all the conditions of austenitization, are shown in Table 6.The cementite particles are observed at the highest and lowest temperatures, but were not listed in Table 6.The electron diffraction analysis indicated the presence of iron-borocarbide Fe 23 (C,B) 6 and cementite Fe 3 C for all austenitization temperatures.The maximum angle variation was 1.50° and the maximum interplanar distance variation was 0.05 Å.
The bright field, the dark field and the electron diffraction from the precipitates are shown in Figure 6.The images show several diffracted points because the diffraction opening width was larger than the precipitates area; also because several precipitates were located in the same area as the matrix itself, all diffracting.However, only a few diffraction points (planes) were identified, according the indication in each diffraction image.
For instance, Figure 6 (a 1 e 2 ), show the precipitate bright field and dark field for 15BCr30 steel and Figure 6(a 3 ) shows a precipitate diffraction obtained from the sample austenitized at 870 °C, with interplanar distances of 1.200 and 1.460 Å, as  the angle between planes of 112.80°.In this case identified by diffraction standards as iron-borocarbide Fe 23 (C,B) 6 planes (840) and (640), with interplanar distances of 1.183 and 1.466 Å, respectively, and angle between planes of 111.30º.
The other samples also were analyzed, measured and compared with interplanar distances and angles between planes, in order to identify the present precipitates in the studied steel, according to Table 6.iron-borocarbides precipitation at austenite grain boundaries; 6.The presence of iron-borocarbides was observed for all austenitization conditions.However, the amounts of precipitates varied with respect to austenitization temperatures tested; and 7.Among the studied temperatures, the 1050 °C austenitization temperature is the one that promotes: the precipitates dissolution, the smallest ironborocarbide reprecipitation during cooling and the highest production of martensite microstructure.

Figure 5 .
Figure 5.Typical microstructures of austenitized and oil cooled samples, observed by TEM -The arrows indicate the precipitates -a) 870 °C -spherical precipitates at some lath boundaries (black arrow) and at aligned plates inside the laths (white arrow); b) 1050 °C -elongated precipitates at grain boundaries; c) 1200 °C -precipitates with spherical and plate morphologies (red arrows).

Table 4 .
Martensite and bainite percentage variation with respect to austenitization temperature for 15BCr30 steel.

Table 5 .
Sizes of precipitates identified in the two steels.

Table 6 .
Planes and angles of some identified precipitates in steels.
Microstructure analysis showed that the highest martensite percentage was obtained for austenitization temperature of 1050 °C.The lower percentage of martensite at austenitization temperature of 870 °C was attributed to the low percentage of solute boron; also for 1200 °C, attributed to the higher nonequilibrium segregation of boron at grain boundaries, indicated by iron-borocarbides precipitation; 5.The growth of grain size with increasing austenitization temperature reduces the grain boundary area.The decreasing area raises the total boron concentration at grain boundary, facilitating the Fe 23 (C,B) 6