Study of the welding procedure in nanostructured super-hard Fe- (Cr, Mo, W) - (C, B) hardfacing
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
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The deposited material presented a high concentration of alloy elements, within the Fe- (Nb, Cr, Mo, W) - (C, B) system.
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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.
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