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AStudyonanEnergy-savingandHigh-efficientPackBoridingTechnologyforToolandDieSteels

ARTICLEinPHYSICSPROCEDIA·DECEMBER2013

DOI:10.1016/j.phpro.2013.11.014

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FeiXie

ChangzhouUniversity

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1875-3892 © 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Chinese Heat Treatment Society doi: 10.1016/j.phpro.2013.11.014

Physics Procedia 50 ( 2013 ) 76 – 81

Available online at www.sciencedirect.com

ScienceDirect

International Federation for Heat Treatment and Surface Engineering 20th Congress Beijing, China, 23-25 October 2012

A study on an energy-saving and high-efficient pack boriding technology for tool and die steels

Fei Xiea, b, *, Xuemei Yea, Jian Chenga, Li Suna a Materials Science and Engineering School, Changzhou University, Changzhou 213164, China

b Key Laboratory of Advanced Metallic Materials of Changzhou City, Changzhou University, Changzhou 213164, China

Abstract

Alternating current field (ACF) enhanced pack boriding was carried out on three kinds of tool and die steels of T7, T12 and W18Cr4V at low and moderate temperatures. A column-shaped electrode was positioned in the center of a pack container, the wall of the container was taken as another electrode. Samples and the boriding agent were set between the electrodes. The test results revealed that the ACF with a current of 4 A and a voltage of 30~50V enhanced the boriding of those tool steels. Compared with the conventional pack boriding, the case thickness of the sample was increased by more than 70% by ACF enhanced pack boriding. The case microstructure was improved to some extent. With the new boriding technology, the treating temperature can be lowered, the soaking time can be shortened, and the utilization ratio of agents can be increased. Preliminary analysis has been made on mechanisms of ACF’s effects to the pack boriding. © 2013 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of the Chinese Heat Treatment Society.

Keywords: pack boriding; tool and mould steels; alternating current field

1. Introduction

Properties of tools and dies in modern industry have direct influences to production efficiency, products quality and cost. It has been shown that the diffusion of boron into tool steels by proper boriding results the formation of boride case whose adhesion is generally much better than the cases prepared by various physical vapor deposition methods. The boride case endues surfaces of tools and dies with high hardness and high wear resistance, by which

* Corresponding author. Tel.: +86 13401316979 (mobile).

E-mail address: xiefei@cczu.edu.cn

© 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Chinese Heat Treatment Society

77 Fei Xie et al. / Physics Procedia 50 ( 2013 ) 76 – 81

the service life of the tools and moulds can be prolonged (Oliveira et al., 2010; Campos et al., 2008; Uslu et al., 2007; Jurci et al., 2011; Taktak, 2007). Pack boriding is the most adopted boriding process for its fewer requirements on equipment, relative ease of handling and safety. However, pack boriding has drawbacks of high processing temperature (850~1050 ), long process duration (3~16h) for getting a boride case with an effective thickness (Jurci et al., 2011; Taktak, 2007; Wang, 1980; Sinha, 1990). It is a process with much consuming in energy and time. Studies were made on a direct current field enhanced pack boriding (DCFPB) which could greatly accelerate pack boriding with energy-saving (Xie et al., 2006; Xie et al., 2012; Xie et al., 2012).

An alternating current field enhanced pack boriding (ACFPB) process is introduced in the present paper. As a 50Hz ACF is used instead of a DCF and the treated part is not taken as an electrode, cathode as in DCFPB, the device for ACFPB is simpler than that for DCFPB. Characteristics of the ACFPB were investigated by taking some tool and die steels as target treated at low and moderate temperatures.

2. Experimental details

Three kinds of quenched and low temperature tempered tool and die steels T7, T12 and W18Cr4V were taken for the research. Samples had been ground and cleaned to remove surface contaminations before boriding. Main compositions of the steels are presented in table 1. The boriding media were constituted of 10 wt.% masteralloy of Fe-B, 5 wt.% KBF4, 5 wt.% charcoal with the balance of SiC.

Table 1 Main chemical composition of the steels for samples, wt-%

Steel code C Si Mn Cr W V T7 0.17~0.24 0.17~0.37 0.35~0.65 T12 1.15~1.24 0.35 0.40 W18Cr4V 0.70~0.80 0.20~0.40 0.10~0.40 3.80~4.40 17.5~19.0 1.00~1.40

Fig.1 Schematic of the apparatus for the ACFPB ( 1: lid; 2, 10: samples; 3: container wall (electrode); 4: heat-resistant insulating tube; 5, 12: heat-resistant insulating plate; 6: current-controllable AC supplier; 7: column-shaped electrode; 8: conducting line; 9: boriding agent; 11: clay sealing )

Main components of the experimental ACFPB apparatus are schematically shown in Fig.1. A column-shaped

electrode, component 7, was positioned in the center of the pack container, component 3, the wall of which was taken as another electrode. Samples, components 2 and 10, and the boriding agent, components 9, were set between the electrodes. ACF was applied on the electrodes with a constant current of 4A and a voltage of ~50V by an adjustable AC supplier, components 6, when the soaking temperature was reached. CPB was carried out for comparison in a separate container heated in the same furnace for ACFPB. Soaking time was 4 h for all boriding

78 Fei Xie et al. / Physics Procedia 50 ( 2013 ) 76 – 81

processes. When the soaking was finished, the containers were cooled in the furnace to room temperature. Detailed process parameters are given in table 2. Table 2 Experimental process parameters

Process code

Furnace temperature,

Soaking time, h

ACF Current, A Voltage, V

ACFPB1 800 4 4 ~30 ACFPB2 750 4 4 ~35 ACFPB3 550 4 4 ~45 CPB1 800 4 CPB2 750 4 CPB3 550 4

An optical microscope (OM) was used to investigate microstructures of the borided samples. The boriding case

thickness was obtained by averaging the distance from the surface to the tip of the sawtooth shaped boride at a middle position of the sample. Information on the phases of the case was obtained by both OM investigation and X-ray diffraction (XRD, Cu K radiation, 100mA, 40KV) analysis. The case hardness was obtained by Vickers’ hardness testing with a load of 0.1kg.

3. Results and discussions

3.1. Boride cases’ thickness and hardness

Table 3 presents the borides case thickness of the samples treated differently. When borided at 800 or 700 , the ACFPB samples’ case is thicker than that of corresponding CPB samples by at least 70%. When borided at 550 , almost no borides case formed in samples treated by CPB. However, boride cases with a thickness varied from 8 to 30 m were produced by ACFPB. These results indicate that the ACF enhanced boriding process on tool and die steels greatly. Table 3 Thickness of the boriding cases treated with different process

Table 4 gives Vickers’ hardness of the borides case at positions 10 m from the surface of the differently treated

T7 and T12 steels. It can be seen that the ACFPB case hardness was similar to that of CPB, which indicates that the ACF had no negative influence to the case hardness of the ACFPB treated T7 and T12 steels. Table 4 Micro-hardness of the borided cases treated with different process

Process code ACFPB1 ACFPB2 CPB1 CPB2 Steel T7 T12 T12 T7 T12 T7 T12 HV0.1 1642 1654 1693 1652 1672 1586 1679

Process code

Thickness, m T7 T12 W18Cr4V

ACFPB1 ~110 ~92 ACFPB2 ~82 ~20 ACFPB3 ~30 ~25 ~8

CPB1 ~61 ~51 ~9 CPB2 ~30 ~28 ~6 CPB3 ~0 ~0 ~0

79 Fei Xie et al. / Physics Procedia 50 ( 2013 ) 76 – 81

3.2. Microstructures and phases

Investigations through OM revealed basic features of the differently borided cases. The boriding cases were comprised of two phases of outer darker FeB and inner brighter Fe2B for all CPB samples treated either at 800 or 750 .

The microstructures and phases in ACFPB cases had close relations with the soaking temperature and compositions of the samples. The boriding cases of T7 and T12 steels by ACFPB at 800 were comprised almost completely of Fe2B phase. The amount of FeB phase increased with the lowering of the boriding temperature for the two steels. The boriding cases of W18Cr4V by ACFPB at 800 , 750 and 550 ,respectively, were all comprised of two phases of FeB and Fe2B.

For both CPB and ACFPB, the frontier of the boriding layer became smoother with the increase of contents of carbon and alloying elements in the treated steel. Typical microstructures of the boriding layers on T7, T12 and W18Cr4V steels are provided in Fig. 2.

Fig.2. Typical microstructures in cross-sections of boriding cases a T7 process CPB1; b W18Cr4V process CPB2; c T12 process ACFPB2; d W18Cr4V process ACFPB2

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Fig. 3. XRD spectra of samples treated by ACFPB Fig. 4. XRD spectra of samples treated by CPB

Figure 3 gives XRD patterns for T12 steel samples treated by ACFPB at different temperatures. According to the relative intensity of diffraction peaks for FeB and Fe2B, it could be confirmed that very few FeB phases were present in the boriding case of the sample treated by ACFPB at 800 .The case was mainly comprised of Fe2B. And the Fe2B had a strong (002) preferred orientation. The amount of FeB phase increased with the decrease of the boriding temperature. When the boriding was carried out at 550 C, the FeB had a strong (002) preferred orientation. The FeB layer was so thick that the inner Fe2B almost could not be detected by the X ray and the diffraction intensity of the Fe2B became very weak.

The XRD results for CPB cases of T12 steels are shown in Fig.4. The diffraction peaks in the XRD patterns were all for FeB. And the FeB had a strong (002) preferred orientation. No peak for Fe2B was detected, which was an indication that the outer FeB was very thick.

4. Discussions

The test results indicated that the ACF not only significantly enhanced the boriding process to produce a much thicker boriding case in the tool and die steels, but also had effects on the case’s microstructure in reducing the amount of brittle FeB phase, even leading to a single Fe2B phase case. On the contrary, the CPB cases were all comprised of two phases of FeB and Fe2B, which are more brittle than the case with single Fe2B phase. The properties of the ACFPB case were therefore modified.

Active boron / boron-containing species in CPB are produced by media’s decomposition and chemical reactions with the heat from the furnace. Furnace temperature is the main controlling factor for the production of the active boron / boron-containing species. Active boron / boron-containing species moves randomly to the treated samples by thermal diffusion in CPB. The species’ moving speed depends on the treating temperature and the porosity between the powder media.

ACF supplied extra energy to the media molecules by enhancing the vibration of the molecules’ atoms, expedited chemical reactions in the media. ACF’s electro-magnetic stirring effect increased diffusion in the media. Therefore, more fresh and active boron-containing species moved to the sample’s surface.

ACF’s electro-magnetic function also influenced the thermal vibration of atoms of the treated sample and the movement of electrons surrounding the atoms by induction. This should be helpful in producing vacancy in the treated steels. The defect would promote the diffusion of boron in the treated sample, which will not only increase the boriding case thickness, but also reduce the content of boron in the sample’s surface.

The energy consumed by applying ACF was much less than the energy saved by lowering furnace temperature in ACFPB. Therefore, compared with the CPB applying ACF in pack boriding had an effect in saving energy for the treatment.

0

10000

20000

30000

40000

50000

60000

70000

30 40 50 60 70 80 90 100

CP

S

550

750

T12 steel FeB

Fe2B

2

(002)

(002)

800

0

4000

8000

12000

16000

20000

30 40 50 60 70 80 90 100

CP

S

800

750

2

FeBFe2B

T12 steel

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5. Conclusions

(1) Pack boriding on the tool and die steels could be enhanced by applying an ACF with 4A current and 30~50 voltages at low and moderate temperatures. The boriding case thickness was thicker than that of CPB by more than 70%. The boriding case structure was also modified.

(2) The studied ACFPB process could significantly lower the treating temperature, reduce soaking time, save energy and cut down the cost for boriding the tool and die steels.

Acknowledgements

Financial support from the National Science Foundation of China (Grant No. 51171032) is kindly acknowledged.

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