Microstructure and wear behavior of stellite 6 cladding on 17-4 PH stainless steel

doi:10.1016/j.jallcom.2010.09.216

Journal of Alloys and Compounds

Volume 509, Issue 14, 7 April 2011, Pages 4905-4909

https://doi.org/10.1016/j.jallcom.2010.09.216 Get rights and content

Abstract

This paper deals with the investigation of the microstructure and wear behavior of the stellite 6 cladding on precipitation hardening martensitic stainless steel (17-4PH) using gas tungsten arc welding (GTAW) method. 17-4 PH stainless steel is widely used in oil and gas industries. Optical metallography, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were employed to study the microstructure and wear mechanisms. X-ray diffraction analysis was also used to identify phases formed in the coating. The results showed that the microstructure of the surface layer consisted of carbides embedded in a Co-rich solid solution with a dendritic structure. In addition, the dendritic growth in the coating was epitaxial. Primary phases formed during the process were Co (fcc), Co (hcp), lamellar eutectic phases, M₂₃C₆ and Cr₇C₃ type carbides. The results of the wear tests indicated that the delamination was the dominant mechanism. So, it is necessary to apply an inter-layer between the substrate and top coat.

Research highlights

► The microstructure of the surface layer consisted of carbides embedded in a Co-rich solid solution with dendritic structure. Primary phases formed during the process were identified as Co(FCC) and lamellar eutectic phases (M₂₃C₆, M₆C, Cr₇C₃). ► Microhardness profiles showed that hardness increases from interface to the coating surface. This is due to the finer size of the grains at coating surface in comparison to that at interface and also diffusion of Fe adjacent to the interface. ► The delamination was suggested as the dominant mechanism of the wear. In this regard, plate-like wear debris consisted of voids and cracks. In addition, due to increase in surface temperature, Cr₂O₃ oxide phase was formed during wear tests.

Introduction

Stainless steels are widely used in industry since application of such materials leads to increase in service life and reduction of energy consumption [1]. In this regard, precipitation hardened stainless steels with good corrosion resistance, high strength, low distortion, excellent weldability and high hardness (up to 49 HRC), are of considerable interest. Alloy 17-4 PH is the most well known material among the precipitation hardened stainless steels with unique properties which is used in oil, gas and aerospace industries [2], [3], [4]. The inner surface of vapor taps made from this steel are always exposed to sequential impacts, high temperature, erosion due to evolved gases and fast (turbulent) flows. A solution to this problem is the application of a wear and corrosion resistant coating [5].

Co-base alloys consisted of wear and corrosion resistant carbides have been considered as candidate materials for surface hardening of 17-4 PH stainless steel. An acceptable wear resistance can be expected when appropriate type and amount of hard phases are present [5]. Stellite, a type of cobalt based superalloy, is extensively used as a surface hardener under harsh conditions of gas turbines and bearings [6], [7], [8], [9], [10], [11]. Wear resistance in these alloys is obtained by two mechanisms; formation of carbides in the matrix and solution hardening by the presence of Cr, W, Ta, Nb and Mo. Different features of carbides such as distribution, size and shape are affected by the process conditions. Most of the added elements are carbide formers and their effect on the strength of the solid solution is dependent upon the carbon content [6], [12]. Increase in hardness can also be due to formation of intermetallics.

In the related literature, surface alloying is usually done by advanced techniques such as laser cladding due to significant advantages like fast processing speed, relative cleanliness, a very high heating/cooling rate (105 K/s) and high solidiﬁcation velocity (up to a maximum of 30 m/s) [13]. However, there are practically simpler and more cost-effective methods like gas tungsten arc welding in which by discreet controlling of welding parameters, enhanced surface and wear properties can be achieved [14]. These methods are very effective and are techno-economic solutions to wear problem of materials. The cost of the coated material per unit area may be higher than that of uncoated material, but when it is applied to only critical areas of the components, the increase in the cost may be insignificant regarding to the improvement in performance. In this research, gas tungsten arc welding (GTAW) was used to cladding of 17-4 PH stainless steel with stellite 6. Low price equipments, ability of deposition on complicated shapes with large and small dimensions, and availability of a wide range of materials are advantages of this technique. Although precipitation hardening stainless steels are widely used in industries, but, few papers studied cladding of this type of steel. The aim of the present work was to investigate the surface properties of 17-4 PH alloy cladded with stellite 6.

Section snippets

Materials and experimental

17-4 PH stainless steel with the dimensions of 20 mm × 40 mm × 40 mm was used as the substrate material. Stellite 6 alloy in the form of 3 mm diameter wire was selected as coating materials. The chemical compositions of stellite and 17-4 PH stainless steel are shown in Table 1.

Gas tungsten arc welding (GTAW) process was used to clad the samples with stellite 6. The GTAW process parameters are provided in Table 2. After cladding, samples were cooled in air and finally subjected to precipitation

Results and discussion

The dashed line in Co-C phase diagram (Fig. 1) represents the alloy (stellite 6) used in the current study. The first stage of solidification (for stellite 6), by crossing the liquidus, includes solidification of Co solid solution which results in formation of cellular or dentritic. According to Eq. (1), the ratio between G and R is reduced from fusion line (FL) to the center line (CL). Thus, the mode of solidification is changed from planar to cellular, columnar dendrite and equiaxed dendrite

Conclusions

In this study stellite 6 coating was applied to 17-4PH stainless steel using GTAW cladding technique. The following conclusions can be drawn from the results obtained:

•
The microstructure of the surface layer consisted of carbides embedded in a Co-rich solid solution with dendritic structure. Dendritic growth which occurs in coating is epitaxial. Primary phases formed during the process were identified as Co(FCC) and lamellar eutectic phases (M₂₃C₆, M₆C, Cr₇C₃)_.
•
Microhardness profiles showed that

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Microstructure and wear behavior of stellite 6 cladding on 17-4 PH stainless steel

Abstract

Research highlights

Introduction

Section snippets

Materials and experimental

Results and discussion

Conclusions

Surf. Coat. Technol.

J. Mater. Proc. Technol.

Mater. Sci. Eng. A

Mater. Charact.

Corros. Sci.

Surf. Coat. Technol.

Mater. Design

Mater. Sci. Eng. A

Surf. Coat. Technol.

Wear

J. Alloy Compd.