Precipitation and control of BN inclusions in 42CrMo ... - Springer Link

International Journal of Minerals, Metallurgy and Materials V olume 20 , Number 9 , September 2013 , P age 842 DOI: 10.1007/s12613-013-0805-5

Precipitation and control of BN inclusions in 42CrMo steel and their effect on machinability Yu-nan Wang1,2) , Yan-ping Bao1,2) , Min Wang3) , and Le-chen Zhang1,2) 1) State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China 2) School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China 3) National Engineering Research Center of Flat Rolling Equipment, University of Science and Technology Beijing, Beijing 100083, China (Received: 6 November 2012; revised: 7 December 2012; accepted: 6 January 2013)

Abstract: The precipitation and control of boron nitrogen (BN) inclusions in 42CrMo steel were investigated and their effect on machinability was analyzed. First, the precipitation regularity of BN in 42CrMo steel was studied by theoretical calculation. Then, the machinability of the steel was investigated through contrast cutting experiments, and the composition and cooling rate of the steel were controlled to analyze the variation laws of the size, distribution, and area ratio of BN inclusions. Finally, the results were combined with the machinability of the steel to analyze the relationship among them. It is found that the machinability of the steel is mainly influenced by the diameter and quantity of BN inclusions. Fine and dispersedly distributed BN inclusions are more beneficial for the improvement in machinability of 42CrMo steel than coarse and sparse BN inclusions. Keywords: steel; inclusions; boron nitrogen; precipitation; machinability

1. Introduction In recent years, with the development of automobile industry, the machinability requirements of steel for automobile industry are increasing. How to satisfy the demands for the hammering process of automobile steel and obtain excellent mechanical properties and good processing formation performance is a “bottleneck” of the development process of automobile steel at present. Generally speaking, cutting materials, which have been studied at present, such as lead and sulfur free-cutting steel, are easily leading to brittleness problems and environmental pollution [1-7]. Therefore, it is urging to develop new freecutting steel that can both satisfy the demands of automotive free-cutting steel and overcome the existing problems of free-cutting steel. To solve these problems, an exploratory research on new environment-friendly free-cutting steel, boron nitrogen (BN) free-cutting steel, which uses BN type inclusions to improve the machinability of the steel, was performed in this article. BN is called white graphite. It has the Corresponding author: Yan-ping Bao

same hexagonal crystal structure as graphite and lubrication action when it is added into the steel. Therefore, based on these properties, it is feasible to add BN into the steel to improve its machinability. Moreover, there exists no harmful effect on the steel quality and environment [8]. The lubrication action of BN inclusions had been investigated in 1990s [9] by adding BN inclusions to the steel to improve its machinability. However, the cutting effect was not stable and had not fulfilled the expectation, but a nonoxide adhesive layer whose main component was AlN had been found on the tool surface. In recent years, Tanaka et al. [10-15] explored the possibility of adding BN to improve the machinability of the steel. The influences of steel composition and tool material on the machinability have been investigated and confirmed that BN type inclusions could improve the machinability of the steel significantly. However, in previous researches, the precipitation and control of BN inclusions and the influence of the existing form of BN inclusions in the steel on its machinability have not been investigated. The purpose of this article is to clarify

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c University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2013

Y.N. W ang et al., Precipitation and control of BN inclusions in 42CrMo steel and their ...

the influence of the existing form of BN inclusions in the steel on its machinability and is looking forward to making certain contribution to the optimization of the steel’s machinability.

2. Experimental A 42CrMo round billet (φ36 × 650 mm) was used in the experiment. Its chemical composition is shown in Table 1. Table 1.

Chemical composition of the experimental

wt%

material C 0.42

Si 0.24

Mn 0.80

P 0.014

Cr 1.09

Mo 0.19

Al 0.031

S 0.0067

BN samples were produced in cold crucible induction melting experiments. The experiments occurred in a tubular resistance furnace (SK2-4-16). A crucible with a capacity of 1 kg was used, and its Al2 O3 purity is 99.99%. Argon was used to guarantee the inert atmosphere during the whole experimental process (2 L ·min−1 ), and the experimental temperature was controlled at 1650◦ C. The experimental process went as follows. The crucible contained 1 kg of 42CrMo steel was placed in a resistance furnace under arc heating conditions until the steel was melted completely and the required temperature was achieved. Ferro boron and nitrogen were added into the steel to obtain different contents of B and N. The cooling methods of the samples were divided into water quenching, air cooling, and furnace cooling. The composition and morphology of BN inclusions in the samples were observed by optical microscopy (OM, 52XA) and scanning electron microscopy (SEM, Zeiss Ultra 55) with energy-dispersive spectroscopy (EDS, Oxford Instruments Inca x-Max 50) after the samples had been polished and lapped. To observe the three-dimensional morphology of BN inclusions, the samples were electrolyzed by the electrolyte solution composed of hydrochloride, bromine water, and acetone (volume ratio, 45:45:10). The size, distribution, and area ratio of BN inclusions in different samples were analyzed by OM and the image analysis system (Image-Pro Plus 6.0), which counted 50 fields of view in each sample. The precipitation behaviors of main inclusions in the steel which contained different amounts of B and N have been investigated by theoretical calculation. It is used to provide a verification and theoretical basis for the experiments. Cutting experiments were performed to investigate the machinability of different samples. For inspecting the influence of the existing form of BN inclusions on the machinability of the steel, comparative cutting experiments of several samples were carried out. The experimental materials were BN free-cutting steels and comparison samples. The cutting materials have not been through any heat treatment. Experimental conditions are shown in Table 2.

Table 2.

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Conditions of cutting experiments

Tool Rotational speed Feed method Coolant Cutting mode

Bench drilling machine (LT − 13), optical microscope, high-speed drill 560 r·min−1 Manual feed Dry Drill

There are two evaluation methods of cutting performances: (1) the curve of the average width of the flank wear with the change of cutting length and (2) the chip shapes of the experimental materials. The experimental processes are shown as follows: high-speed drills were used to cut the experimental materials, and cutting was interrupted to measure the average width of the flank wear by OM. To obtain a statistically valid conclusion, each sample undertook three tests, and the average value was adopted. In addition, the chip shapes of the experimental materials were compared to reflect their chip breaking performance.

3. Results and discussion 3.1. Theoretical calculation of precipitation To investigate the precipitation behaviors of main inclusions in the samples, the software FactSage (CRCTThermFact, Inc., Montréal, Canada) was applied to calculate the precipitation temperature and the content of main inclusions in the samples. (Fig. 1). The chemical composition of the samples obtained in cold crucible induction melting is shown in Table 3. The precipitation temperature and the final content of main inclusions are shown in Table 4 in detail. As shown in Fig. 1, the precipitation order of inclusions in samples BN1-BN4 is Al2 O3 > TiN > AlN > BN > MnS. The liquidus temperature and the solidus temperature of samples BN1-BN4 are 1501◦ C and 1432◦ C, respectively [16]. As shown in Table 4, Al2 O3 precipitates in liquid phase, whereas TiN, AlN, BN, and MnS precipitate during solidification. Therefore, it is possible to control the existing form of BN by controlling the solidification cooling rate. The precipitation temperature and final precipitation content of TiN, Al2 O3 , and MnS are hardly affected by changing B and N contents. The competitive precipitation concentrates on BN and AlN. As the reaction of Al and N could be inhibited by B, the precipitation of AlN is subdued with the decrease of the N/B ratio in the sample, and AlN would not precipitate when there is enough B reacting with N in the steel, as shown in Figs. 1(a) and 1(b). The content of Al in the steel hardly affects the precipitation of BN, which has been proven in previous researches [17-18]. The precipitation temperature of BN is determined by B and N contents in the samples. When the N content is constant, the precipitation temperature of BN increases with the increase in B content, and vice versa. The pre-

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is enough B reacting with N in the steel. Thus, it is difficult for AlN and TiN to precipitate in samples BN1-BN4. There is no AlN and just a few TiN inclusions in samples BN1-BN4. Since the diffusion coefficient of N is smaller, the growth rate of BN inclusions is determined by the diffusion coefficient of N during solidification.

3.2. Influence of cooling rate and chemical composition on the machinability of samples The chemical composition, hardness, and cooling method of cutting materials are shown in Table 5. It could be obtained through field measurement and simulation [2122] that the cooling rates of water cooling, air cooling, and furnace cooling are 3600, 25, and 3.5 K·min−1 , respectively. In practical production, the central cooling rate of 42CrMo round billet is ∼26 K·min−1 , which is similar to the air cooling rate. Because the cooling rate of water quenching is too fast to precipitate BN, it is not discussed in this article. In previous research [8], the influence of free B on the hardenability of the steel could be eliminated with the N/B ratio higher than 1.7 in the steel. In samples BN5BN8, all the N/B ratios are higher than 1.7; it means that free B hardly influences the hardenability. The relationship between the average width of flank wear and cutting length is shown in Fig. 3. As shown in Fig. 3, the machinability of sample BN8 is the best and the machinability of the comparison sample is the worst. Comparing samples BN6-BN8, it is known that the machinability is determined by B content in the samples at the same cooling rate. The sample with the B content of 0.02wt% possesses a better machinability than the sample with the B content of 0.015wt%. Comparing Table 5. Sample BN5 BN6 BN7 BN8 Comparison

C 0.34 0.34 0.33 0.35 0.35

Si 0.23 0.22 0.24 0.23 0.23

Fig. 2.

Change of diffusion coefficient with tempera-

ture.

Fig. 3.

Relationship between the average width of

flank wear and cutting length in samples.

Chemical composition, hardness, and cooling method of samples Mn 0.69 0.67 0.64 0.60 0.60

P 0.013 0.012 0.011 0.010 0.010

Composition / Cr Mo 0.96 0.20 0.97 0.17 0.95 0.19 0.94 0.18 0.94 0.18

wt% Al 0.018 0.015 0.021 0.028 0.028

sample BN5 with BN8, it shows that when the B and N contents are in common, the sample by air cooling possesses a better machinability than the sample by furnace cooling. Therefore, cooling rate and the contents of B and N both have significant influence on the machinability. The chip breaking performance of the steel is represented by the chip shape, and the typical chip shapes of samples are shown in Fig. 4. As shown in Fig. 4(a), the typical chip shape of BN8 is short spiral with the length less than 50 mm, and it is ideal. The terrible chip breaking performance of the steel is expressed by long spiral chips and extrusion shape chips as shown in Fig. 4(b), which are the typical chip shapes of the comparison sample. In sum-

S 0.0043 0.0030 0.004 0.0050 0.0050

B 0.020 0.011 0.015 0.020 —

N 0.040 0.040 0.040 0.040 0.039

Hardness, HB

Cooling method

226 231 230 229 235

Furnace cooling Air cooling Air cooling Air cooling Nature cooling

mary, BN free-cutting steel possesses a better chip breaking performance than the comparison sample.

3.3. Influence of the existing form of BN inclusions in the steel on the machinability To clarify the effect of the existing form of BN inclusions in samples on the machinability, it is necessary to analyze it deeply. Typical BN inclusions in samples are shown in Fig. 5. It shows that there are mainly single BN inclusions in samples, and the morphology of BN inclusions is mainly nearly spherical and massive. The distribution of BN inclusions in sample BN8 is shown in Fig. 6. It is obvious that BN inclusions account

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Int. J. Miner. Metall. Mater., V ol. 20 , No. 9 , Sep. 2013

Fig. 4.

Fig. 5.

Typical chip shapes of samples: (a) BN8; (b) 42CrMo.

SEM images of typical BN inclusions in samples: (a-c) two-dimensional shape; (d-f ) three-dimensional

shape.

for a large proportion of the inclusions in the sample. With 100 inclusions in each sample randomly selected, SEM and EDS were adopted to confirm their compositions. The ratios of single BN, composite BN, and other inclusions are shown in Fig. 7. Comparing sample BN5 with BN8, it can be seen that the results are similar with the increase in cooling rate. Comparing sample BN6 with BN8, the proportion of single BN inclusions increases when the contents of B and N increasing, and the proportion of composite BN inclusions is similar. According to the theoretical calculation and SEM analysis, the main inclusions in samples are BN inclusions. Furthermore, the effects of B and N contents and solidification cooling rate on the existing forms of inclusions concentrate on BN inclusions. Therefore, us-

ing the statistical results of all inclusions to replace those of BN inclusions are credible. As mentioned above, the results of average diameter, average quantity, and the area ratio of BN inclusions in different samples are shown in Table 6. To investigate the influence of B and N contents in the steel on the existing form of BN inclusions, samples BN6BN8 were analyzed. The existing forms of BN inclusions with different B contents are shown in Fig. 8. It shows that when the B content increases, the average diameter of BN inclusions increases slightly, and the average quantity and the area ratio of BN inclusions increase as well. The precipitation temperature of BN inclusions is determined by B and N contents in the steel. With the increase of B

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content in the steel, the precipitation temperature of BN

Fig. 6.

Distribution of BN inclusions in sample BN8.

Fig. 7.

Ratios of different inclusions in samples.

Table 6.

Results of average diameter, average quantity, and the area ratio of BN inclusions in samples BN5-BN8

Sample BN5 BN6 BN7 BN8

Fig. 8.

inclusions increases slightly and the growth time of BN inclusions gets a little longer. However, since the cooling rate of air cooling is fast and the diffusion coefficient of N in austenite is slow, the increase in average diameter is not obvious. To investigate the influence of cooling rate on the existing form of BN inclusions, samples BN5 and BN8 were analyzed. According to previous researches [23-24], the average size of BN inclusions increases and the average quantity of BN inclusions decreases with decreasing the cooling rate. The existing forms of BN inclusions at different cooling rates are shown in Fig. 9. As shown in Fig. 9, the average diameter and the area ratio of BN inclusions decrease with the increase in cooling rate, whereas the average quantity of BN inclusions increases. As shown in theoretical calculation, the temperature range in which BN precipitates rapidly is from 1200 to 1400◦ C. When the cooling rate decreases, there is more time for BN inclusions to precipitate and grow in the temperature range. Therefore, the average diameter and the area ratio of BN inclusions increase significantly. In summary, with the same cooling rate, the average quantity and the area ratio of BN inclusions increase along with the increase of B content, whereas the change of average diameter is not significant. With the same contents of B and N, the average diameter and the area ratio of BN inclusions decrease along with the increase of cooling rate. On the contrary, the average quantity of BN inclusions increases when the cooling rate is increased.

Average diameter / μm 4.7 2.9 3.0 3.0

Average quantity / mm2 103.61 381.69 435.13 480.65

Area ratio / % 0.603 0.529 0.562 0.598

Effect of B contents on the existing forms of BN inclusions in samples BN6-BN8: (a) average diameter

and average quantity; (b) area ratio.

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Fig. 9.

Int. J. Miner. Metall. Mater., V ol. 20 , No. 9 , Sep. 2013

Effect of cooling rates on the existing forms of BN inclusions in samples BN6-BN8: (a) average diameter

and average quantity; (b) area ratio.

The results of cutting experiments are as follows: when the diameters of BN inclusions are similar, with the increase of the quantity and area ratio of BN inclusions in samples, the machinability of the samples becomes better; when the contents of B and N are the same, the machinability of the samples becomes better with a decrease in diameter of BN inclusions and an increase in quantity of BN inclusions. In a word, the effect of fine and dispersedly distributed BN inclusions on the machinability of the steel is outstanding. On the contrary, the effect of coarse and sparse BN inclusions on the machinability of the steel is indistinctive. Since the cooling rates of air cooling and practical production of 42CrMo steel are similar, the existing form of BN inclusions in samples by air cooling has reference significance for practical production.

4. Conclusions (1) According to the theoretical calculation, the precipitation order of inclusions in the steel is Al2 O3 > TiN >AlN > BN >MnS. Al2 O3 precipitates in liquid phase, while TiN, AlN, BN, and MnS precipitate during solidification. The precipitation temperature and the amount of BN precipitation increase with the increase of B and N contents. B has effective inhibitory effects on the precipitation of AlN and TiN. The growth rate of BN is determined by the diffusion coefficient of N in austenite. (2) At the same cooling rate, the average quantity and the area ratio of BN inclusions increase with an increase in B content, whereas the change of average diameter is not obvious. With the same contents of B and N, the average quantity of BN inclusions increases with an increase in cooling rate. On the contrary, the average diameter and the area ratio of BN inclusions decrease. (3) The machinability and chip breaking performance of the steel could be improved obviously with BN added into the steel. Fine and dispersedly distributed BN inclusions have much more good influence on the machinability

than coarse and sparse BN inclusions in the steel.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51274029) and the China Postdoctoral Science Foundation of China (No. 2012M510319).

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