Microstructure change caused by (Cr,Fe)23C6 carbides in high chromium Fe–Cr–C hardfacing alloys

https://doi.org/10.1016/j.surfcoat.2006.01.010Get rights and content

Abstract

A series of high chromium Fe–Cr–C hardfacing alloys were produced by gas tungsten arc welding (GTAW). Chromium and graphite alloy fillers were used to deposit coatings on ASTM A36 steel substrates. These coatings were especially designed to vary the size and proportion of the (Cr,Fe)23C6 carbides that are present in the microstructure at room temperature. Depending on the three different graphite additions of those alloy fillers, hypoeutectic, eutectic and hypereutectic microstructures were obtained on coated surfaces. No crack formation was found on the coatings. According to the X-ray diffraction analysis and microstructure characteristics, the hypereutectic composites consist of three phases, i.e. Cr–Fe solid solution (α), (Cr,Fe)23C6 and trace amounts of (Cr,Fe)7C3. Massive (Cr,Fe)23C6 contain (Cr,Fe)7C3 in the center, and cause its high hardness value up to HRC 70. And the massive (Cr,Fe)23C6 are surrounded by a eutectic structure to restrain cracks from appearing. All the microstructure constituents were rationalized in terms of a ternary diagram.

Introduction

Fe–Cr–C alloys are used in severe conditions where there is extreme erosion and therefore abrasion resistance is necessary. Their exceptional abrasive and erosive wear resistance results primarily from their high volume fraction of hard carbides, though the toughness of the matrix also contributes to the wear resistance.

The investigations of Fe–Cr–C alloy microstructures have shown that these types of materials have hypoeutectic, eutectic, and hypereutectic structures [1]. M7C3 primary carbides form in large amounts at higher carbon concentrations.

These types of microstructures have good wear resistance properties [2], [3].

This kind of hard material can be represented by high chromium white cast iron, by extremely high hardness value of M7C3 (about 1600 HV) [4], [5], [6], [7]. M7C3 is surrounded by austenite; which is relatively soft compared to M7C3, so crack will spread along the interface between austenite and M7C3. This problem causes a serious harmful effect for crack produced in the hardfacing materials.

High chromium content Fe–Cr–C hard facing alloys also can be used commercially for components that are designed to endure harsh abrasive conditions. The massive carbides present in the microstructure are M23C6. These can be described as composites with large and hard carbides in a softer body center cubic Cr–Fe alloy matrix.

If high chromium content (Fe–Cr–C) hard facing alloys are in hypereutectic structure, i.e. primary M23C6 is surrounded by the Cr–Fe and M23C6 eutectic structure, they will reduce the occurrence of the crack. Because the lamellar eutectic structure will resist crack spreading along the grain boundary.

The hard facing alloys obtained using high-energy density sources such as electron beam welding, plasma arc and laser have been widely applied to enhance the wear and corrosion resistance of material surface [8], [9], [10].

The gas tungsten arc welding (GTAW) process (also called TIG welding) is used when a good weld appearance and a high quality of the weld are required. In this process, an electric arc is formed between a tungsten electrode and the base metal. The arc region is protected by a kind of inert gas or a mixture of inert gases. The tungsten electrode is heated to temperatures high enough for the emission of the necessary electrons for the operation of the arc.

The criteria used in the selection of weld surface for wear applications and hardness are connected to their microstructure features.

In this investigation, GTAW process is used as a high energy density beam to form a high chromium Fe–Cr–C hard surface above the ASME A36 steel with chromium and graphite alloy filler. The microstructure changes on the coated surfaces were systematically studied by using optical microscope (OM), field emission scanning electron microscopy (FE-SEM), electron probe micro analyzer (EPMA) and X-ray diffraction (XRD).

Section snippets

Experimental procedures

The base metals (40 × 40 × 10 mm) for the welding surface were prepared from ASTM A36 steel plates. Before welding, these specimens were ground and cleaned with acetone.

In order to get Cr–Fe solid solution, hypoeutectic, eutectic, and hypereutectic high chromium Fe–Cr–C structures, different amounts of graphite and chromium powders are mixed together. Exert a constant high pressure to the powders to form alloy filler (30 × 25 × 3 mm). Then put the alloy filler above the base metal. The components of

Results and discussion

GTAW surface modification by means of alloying is a process in which chromium and graphite alloy filler of desirable compositions and a thin surface layer of the substrate material were simultaneously melted and then rapidly solidified to form a dense coating bonded to the base material. Because the substrate material is carbon steel besides chromium and carbon, the hardfacing layer also have iron to form Fe–Cr–C alloys. The thickness of the coatings with different carbon additions ranged from

Conclusions

A new type of Cr–Fe–C eutectic composite hardfacing alloy was prepared from industrial grade materials using GTAW process. With the aid of XRD and optical microscopy, the hypoeutectic and eutectic composites were found to consist of Cr–Fe solid solution (α) and (Cr,Fe)23C6 carbide. In hypoeutectic structure, (Cr,Fe)23C6 is fine lamellar structure. In eutectic structure, (Cr,Fe)23C6 is equi-axed dendrite. Hypereutectic composites were found to consist of three phases: α, (Cr,Fe)23C6 and trace

Acknowledgements

The authors would like to thank the National Science Council of Taiwan for its financial support under projects numbered NSC93-2216-E-005-002.

References (15)

  • L. Lu et al.

    Mater. Sci. Eng. A

    (2003)
  • Y. Matsubara et al.

    Wear

    (2001)
  • A.F. Zhang et al.

    Wear

    (2004)
  • H. Berns

    Wear

    (2003)
  • J.D. Xing et al.

    Wear

    (2002)
  • S. Aso et al.

    Wear

    (2001)
  • Q.Y. Hou et al.

    Surf. Coat. Technol.

    (2005)
There are more references available in the full text version of this article.

Cited by (0)

View full text