Study of the welding procedure in nanostructured super-hard Fe- (Cr, Mo, W) - (C, B) hardfacing

https://doi.org/10.1016/j.ijrmhm.2020.105178Get rights and content

Highlights

  • Fe-based ultra - hardfacing was obtained from a Flux cored arc welding process.

  • The percentage of carbides and carboborides were greater than 65%.

  • NbC and (Mo/W)C carbide clusters were found.

  • The size of the carbides changed with the number of layers.

  • The hardness of the deposit was between 900 and 1100 HV.

Abstract

The optimization of the tribological properties of the surfaces by means of hardfacing techniques has made great progress in the metallurgical field in recent times. Alloys of the Fe-Cr-C and Fe-C-B type present excellent wear performance under severe conditions, where the incorporation of Nb, Mo and W improves the performance of severe abrasive wear. In this context, new semi-automatic welding consumables have been designed that deposit iron base material of high alloy, with complex carboborides of W, Mo and Cr which present very high hardness and resistant to abrasive wear. The purpose of this work was to compare microstructural variations coupons welded with one or two layers, with or without shielding gas. The chemical composition was measured on each coupon, the microstructure was characterized by X-ray diffraction and scanning electron microscopy. Dilution percentage was determined and Vickers microhardness profiles (HV2) were made on the different phases (HV0.025). It was found that the dilution with and without shielding gas were 26% and 19%. The hardness was 960 and 1100 HV2. An increase in hardness was observed in the recrystallized areas, as well as a higher percentage of carboborides in the last bead.

Introduction

The improvement of the wear resistance of machine components can be achieved with proper deposition of hard layers of abrasion resistant materials [1]. The addition of alloy elements and rapid solidification generate fine microstructures that homogeneously distribute the hard phases producing an excellent combination of hardness and toughness [2]. The hard and thick phases of high hardness are important to achieve high abrasion resistance. The hardness of the hard phases and/or the hardness of the matrix must be greater than the hardness of the abrasive element [2,3].

Iron based alloys containing niobium (Nb), chromium (Cr) and molybdenum (Mo) in combination with boron (B) and carbon have been designed for hardfacing applications due to their high hardness and wear resistance [4]. Iron alloys with high chromium are widely used for the hardfacing of industrial components of mining, cement, thermal power plants and iron and steel industries, due to their high hardness and excellent abrasion resistance, which is attributed to the formation of chromium carbides [5,6]. The wear properties are affected by microstructures and by the amount of carbide phases. The thicker microstructures and with a small amount of carbides present a great loss of weight during abrasion [7]. In this regard, Fe-Cr-Nb-C-B consumables were manufactured to improve wear resistance performance with the addition of W and Mo that produce block-type carbides that improve the wear resistance and fracture of lengthened eutectic phases or carboborides. However, the control of carbide sizes and their distribution has become an important challenge for recharge alloys under study due to the fragility of primary lengthened carbides. Therefore, the wear resistance of hardfacing depends on many factors such as the type, shape and distribution of hard phases, as well as the toughness and behavior of the matrix [8]. In the semi-automatic welding process, all these factors aforementioned are strongly influenced by the welding procedure. The purpose of the following work is to study the influence of the number of layers and the shielding gas on the microstructure and microhardness of Fe-base- (Cr, Mo, W) - (C, B) alloy.

Section snippets

Materials and methods

The consumable used was a 1.6 mm diameter tubular wire, deposited by means of the semi-automatic welding process under gaseous protection and without it, with a free wire length of 20 mm.

Four coupons with 1 and 2 layers were welded, with Ar-20% CO2 and without gaseous protection. The parameters were chosen based on previous work [9,10]. The welding sequence was 4 beads for the first layer and 3 beads for the second layer, as shown schematically in Fig. 1. The stick out was 18 mm with gas, and

Visual inspection

In Fig. 2 show the top view of the coupons.

It was observed that the A1 and A2 samples showed higher levels of spatters and little slag. This could be associated with the change of transfer mode the spray to globular repelled [14]. Most of the beads contained cracks, produced by the stress relief, which is normal for this type of deposits. The samples welded with two layers presented the higher number of cracks.

Macrography

Fig. 3 show cross section of the welded specimen where the base metal, weld deposit

Conclusions

  • -

    The deposited material presented a high concentration of alloy elements, within the Fe- (Nb, Cr, Mo, W) - (C, B) system.

  • -

    The microstructure was formed by α-Fe and γ-Fe, detecting the presence of metal carboborides ((Cr,Fe)7 (B,C)3, (Cr,Fe)23 (B,C)6). Nb carbides were also identified for all samples. The percentage of carbides and carboborides was greater than 65%. A slight increase in γ-Fe and carboborides (Cr, Fe)7(B, C)3 was observed in the welded samples under gas protection and with two

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors are grateful to EUTECTIC-CONARCO Argentina, to AIR LIQUIDE Argentina, to EUTECTIC-USA, to the INTI - MECHANICAL ELECTRONIC MICROSCOPY LABORATORY for the realization of scanning electron microscopy and APUEMFI for financial support for this project.

References (26)

  • Mohtasham Bahoosh et al.

    Macro-indentation fracture mechanisms in a super-hard hardfacing Fe-based electrode

    Eng. Fail. Anal.

    (October 2018)
  • S. Pawar et al.

    Effect of different carbides on the wear resistance of Fe-based hardfacing alloys

    Int. J. Refract. Met. Hard Mater.

    (January 2019)
  • G. Azimi et al.

    Effect of silicon content on the microstructure and properties of Fe–Cr–C hardfacing alloys

    J. Mater. Sci.

    (2010)
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