Effects of B and V on microstructure of FeCrC cladding layers on the surface of Q235B mild steel by TIG welding

Hui Luo; Peng Liu; Chengxin Teng

doi:10.1515/secm-2014-0001

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Effects of B and V on microstructure of FeCrC cladding layers on the surface of Q235B mild steel by TIG welding

Hui Luo , Peng Liu and Chengxin Teng

Published/Copyright: June 7, 2014

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From the journal Science and Engineering of Composite Materials Volume 22 Issue 6

Abstract

The microstructure of iron-based alloy (FeCrC) cladding layers on the surface of Q235B mild steel was analyzed by means of SEM, XRD, and EPMA tests. The results indicated that structure morphology of FeCrC alloy cladding layers by TIG welding is mainly hexagonal and elongated in shape. These structures were composed of some hard phases, such as Cr₂₃ C₆, Cr₃ C_2, and Cr₇ C₃. When element B was added, the structure morphology of iron-based alloy cladding layers could be changed into an irregular clumpy and cracking shape, which appears as a Fe₂₃(C, B)₆ hard phase. When element V was added, the surface morphology was changed into a small clumpy shape, which was composed of (Fe, Cr) C and (Fe, Cr, V) C phase.

Keywords: iron-based alloy; microstructure; phase; surface composite; TIG welding

1 Introduction

TIG welding technology as a material surface treatment technology is widely used, and it is one of the research hotspots for surfacing welding technology in recent times. Recently, domestic and foreign researchers have discussed the application of TIG welding technology used in cladding the iron-based alloy powder on the surface of cast iron [1], mild steel [2, 3], and other materials. And some important alloying elements are added into the surface of materials to generate the hard phase [4–6]. Its purpose is to improve the surface hardness and strength of base material, and further improve the wear resistance [7, 8] and corrosion resistance of the materials.

At present, iron-based alloys have some advantageous characteristics, such as extensive resources, inexpensive price, low manufacturing cost, and other advantages, so they are widely used in cladding materials. And the original characteristic of iron-based alloys is also similar to mild steel and cast iron, which is favorable to obtain a strengthening surface of high quality for metal materials. In this paper, the FeCrC iron-based alloy powder was clad on the surface of Q235B mild steel by TIG welding technology. Then, elements B and V were added into the iron-based alloy powder, and the effect of B and V on microstructure and phase constitution were analyzed. This is helpful to know the effect of a special or single element on the microstructural transition of surface phases in the matrix.

2 Materials and methods

The test plate dimensions of Q235B mild steel as the welding base metal was 200×60×6 mm. The main chemical composition of base metal is shown in Table 1. In this experiment, the cladding layer includes chromium iron (FeCr₅₅ C₃), boron iron (FeB₁₇ C_0.1), vanadium iron (FeV₅₀ A), and reducing iron. All alloy powder particle reached to ≤200 mesh. The chemical composition of all the iron-based alloy power is shown in Table 2. The cladding test on the surface of Q235B mild steel was preformed by means of arc heat processing of TIG welding. The TIG welding process parameters can be seen in Table 3.

Table 1

The chemical composition of Q235B mild steel (wt%).

Elements	C	Mn	Si	S	p-Value	other
Wt%	0.12	0.30–0.70	≤0.03	≤0.045	≤0.045	Bal.

Table 2

The chemical composition of all iron-based alloy powder (wt%).

Iron-based alloy	Chemical composition
Iron-based alloy	Cr	V	B	C	Si	p-Value	S	Impurity
FeCr₅₅ C₃	61.50	–	–	≤0.03	≤1.5	≤0.03	≤0.25	Rest
FeV₅₀ A	–	49.80	–	≤0.40	≤2.0	≤0.06	≤0.04	Rest
FeB₁₇ C_0.1	–	–	14.5	≤0.10	4.0	0.1	0.01	Rest

Table 3

The TIG welding parameters used in iron-based alloy cladding.

Diameter tungsten (Φ/mm)	Welding current (I /A)	Arc voltage (U/V)	Argon gas flow (Q/l·min^-1)	Welding speed (V/mm·min^-1)
3.2	120–140	15–20	6–8	50–60

The 4∼5-mm thick cladding layer was completed successfully on the surface of Q235B mild steel. Then, the test plate of the processed region was cut into 10 mm×10 mm×8-mm samples by the line cutting method. After the samples were prepared, the cross-section of cladding layer was polished and etched. Twenty percent HNO₃ solution was used to corrode the cross-section of the cladding. The microstructure morphology of cladding layer was analyzed by SEM of JSM 6380LA type. In addition, the non-cladding surface was polished to remove about 2-mm thickness, and the phase constitutions on the cladding surface were analyzed by Rigaku D8 X-ray diffraction (XRD).

3 Results and analysis

3.1 Microstructure of the FeCrC cladding layer

The FeCrC cladding layer is composed of Cr (40 wt%), C (2.5 wt%), and Fe (rest). The microstructure characteristic and element content of phase (EDS) are shown in Figure 1. According to Figure 1, the microstructure of the edge and center region of cladding layer is obviously different. However, the substrate structure appears as a lamellar pearlite. The second phase shows a dark structure with uniformly distributed long strips and hexagonal structures on the substrate. The element composition of both structures was analyzed by EDS. The test point was separately point 001 and point 002. The EDS results indicated that the long strip structure is composed of Cr (42.02 wt%) and Fe (57.98 wt%), and the Cr (41.38 wt%) and Fe (58.62 wt%) are hexagonal in structure. So both structures are the same compounds and they could be compounds of the Fe-Cr system. The different shape for both structures should be related to thermo-field distribution by TIG welding.

Figure 1

Microstructure and chemical composition (EDS) analysis of FeCrC alloy cladding: (A) the edge in the cladding layer, (B) the center in the cladding layer.

The EDS analysis indicated that new structures may be Fe-Cr compounds. Therefore, the XRD analysis is helpful to determine the phase constitutions of new structures. The XRD diffraction pattern is shown in Figure 2. According to the analysis of the cladding layer XRD diffraction pattern, the cladding structure appears as some new phases, such as Cr₂₃ C₆, Cr₃ C₂, and Cr₇ C₃. The Literature [9] points out that these hexagonal structures are primary carbides. There are some internal voids, which are typical M₇ C₃ hard particles. However, these single or multiple voids are attributed to the effect of primary carbides which grow into a nest and produce stress defects under the higher stress.

Figure 2

The XRD pattern for FeCrC alloy cladding.

3.2 Effect of B on the FeCrC cladding layer

The B element is an important element for refining the grain of metal materials. Therefore, in this study the iron-based alloy powder FeCrC was the added B element. The element ratio of the alloy powder is Cr (40 wt%), B (3 wt%), C (2.5 wt%), and the rest is Fe. The SEM image for the center region of the cladding layer, microstructure, and chemical composition by means of EDS analysis are shown in Figure 3. As can be seen from Figure 3A, the structure morphology is significantly different from the one of FeCrC cladding layer. The structure of the cladding layer changes the lath and hexagonal structure into an irregular clumpy and cracking structure, which was mainly composed of elements Fe, Cr, C, and Si (see Table 4). And the irregular granular structure is made up of two alloys of Fe and Cr elements. However, the cracking shape structure (point 008) includes less Cr element, more Fe and C elements, and a small amount of Si element that could be the matrix structure. The decrease of Cr content should be an effect of Cr solution strengthening.

Figure 3

Microstructure and chemical composition (EDS) analysis of FeCrBC alloy cladding: (A) microstructure of FeCrBC alloy cladding, (B) EDS analysis of point 007, (C) EDS analysis of point 008.

Table 4

EDS analysis results of cladding layer of test point.

Test point	Chemical composition (wt%)
Test point	Fe	Cr	C	Si	Total
Point 007	69.75	24.97	5.28	0	100
Point 008	87.00	6.53	4.40	2.07	100

The XRD analysis results of FeCrBC cladding layer are shown in Figure 4. The test result showed that the cladding layer may have Cr₇ C₃, Fe₂₃(C, B)₆ and FeB compounds. Under the TIG welding, element B and Cr diffused in the surrounding liquid and formed Fe-B and Cr-C compounds with the decreasing of the temperature. However, during the cooling process of welding, the different cooling rate and the coagulation unbalanced freezing results in diffusion of B and Cr at different speeds. It reduced a rapidly decrease of B content in the uncrystallized metal melt. As a result, relatively high content of element C promoted the Fe₂₃(C, B)₆ compound produced [10].

Figure 4

The XRD pattern for FeCrBC alloy cladding.

3.3 Effect of V on the FeCrBC cladding layer

When the element B in the iron-based alloy was added, the microstructure produced an obvious change. Element V was also an important element for enhancing the properties of cladding layer. Therefore, element V was also added into the FeCrBC iron-based alloy, and the new iron-based alloy was composed of Cr (40 wt%), B (3 wt%), V (10 wt%), and C (2.5 wt%), the rest is reductive iron. The cladding layer added element V was analyzed by SEM. The test results, microstructure, and chemical composition of phases (EDS analysis) are shown in Figure 5.

Figure 5

Microstructure and chemical composition (EDS) analysis of FeCrBVC alloy cladding: (A) microstructure of FeCrBVC alloy cladding, (B) EDS analysis of point 017.

According to Figure 5A, when the element V was added into the cladding layer, an obvious microstructural change was produced. The morphology of the structure was changed from the irregular clumpy and cracking shape to being small clumpy and uniformly dispersed in the matrix. This reason is that element V plays an important role for graining refinement and inhibiting austenite grain growth [11].

Moreover, according to the EDS analysis (point 017), the small clumpy structure includes Cr (44.14 wt%), V (11.89 wt%), and Fe (43.97 wt%). The phase constitution of FeCrBVC is mainly composed of Cr₇ C₃ (primary carbide), Cr₃ C₂ (eutectic carbides), Fe₂ B and Fe (Cr, V) C compounds by XRD (see Figure 6).

Figure 6

The XRD pattern for FeCrBVC alloy cladding.

4 Conclusions

The iron-based alloy power with element B and V was used on cladding on Q235B mild steel by TIG welding heat. When the welding current is 120∼140 A, arc voltage is 15∼20 V, welding speed is 50∼60 mm/min, a good cladding layer without holes, cracks and slag can be obtained. The matrix showed a flake pearlite structure when the FeCrC cladding alloy was used, and some phases existed in the matrix, such as Cr₂₃ C₆, Cr₃ C₂, and Cr₇ C₃ that show the hexagonal and lath morphology. However, when element B was added, the hexagonal and lath structure in the FeCrC cladding layer changed into an irregular clumpy and cracking shape, which is mainly composed of the Fe₂₃(C, B)₆ phase. And when element V was added, irregular clumpy and cracking shape structure morphology in FeCrBC cladding layer changed into the small clumpy, uniformly dispersed in the matrix, which is mainly composed of (Fe, Cr)C and (Fe, Cr, V)C.

Corresponding author: Peng Liu, School of Materials Science and Engineering, Shandong Jianzhu University, Jinan 250101, P. R. China, e-mail: liupeng1286@sina.com

Acknowledgments

This work was financially supported through the National Natural Science Foundation of China (Grant No. 51305240).

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Received: 2014-1-1

Accepted: 2014-4-20

Published Online: 2014-6-7

Published in Print: 2015-11-1

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Articles in the same Issue

https://doi.org/10.1515/secm-2014-0001

Keywords for this article

iron-based alloy; microstructure; phase; surface composite; TIG welding

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