Some studies on nickel based Inconel 625 hard overlays on AISI 316L plate by gas metal arc welding based hardfacing process

Highlights

• Gas metal arc welding (GMAW) based hardfacing process of IN625 on AISI 316L plate substrate was characterized.

• SEM-EDS and elemental composition confirmed the uniform dispersion of elements and rich precipitates of Mo and Nb.

• Enrichment of alloying elements like Ni, Cr, Mo and Nb on the overlays were responsible for the improved wear resistance.

• Reciprocating and Abrasive wear tests on the IN625 hardfaced samples revealed superior wear resistance than the substrate.

Abstract

The aim of the study was to examine the characteristics of Inconel 625 hard overlays over AISI 316L used for oil and gas industry applications. Weaving technique was adopted using robotic gas metal arc welding (GMAW) process in this study to reduce the dilution % and contact angle. Multiple overlapped beads were fabricated with a welding current of 120 Amps, welding speed of 250 mm/min with sinewave weaving technique. The hard overlays were then examined to expose the microstructural variation and mechanical integrity. In addition, reciprocating wear test and three-body abrasion tests were performed to study the wear characteristics. Microstructural results showed the existence of precipitates rich in Mo and Nb dispersed in the interdendritic regions with a dendritic structure. Hardfaced overlay and interface samples showed an increase of 17.8% and 3.7% respectively in ultimate tensile strength than the substrate material. Microhardness test exhibited intended results which follow an increasing trend from substrate to the overlay with an average of 246.02 HV_0.5 at the overlay region and 184.94 HV_0.5 on the substrate zone. Wear test results revealed excellent wear resistance of the hardfaced samples in comparison with substrate specimens. Co-efficient of friction (CoF) of hardfaced samples were 0.16 and 0.12 while the CoF of substrate specimen were 0.25 and 0.53 at room temperature and 180 °C respectively in ball on flat reciprocating wear test. Specific wear rate of hardfaced samples during the three-body abrasion test at 50 N and 130 N load decreased by ⁓39% in comparison with substrate. The surfaces of the worn samples from the wear test were characterized by SEM wear tracks and Energy Dispersive Spectroscopy analysis.

Introduction

Inconel 625 (IN625) is a solid solution strengthened nickel superalloy which is predominantly used for several overlay coating applications in aviation, chemical industry, heat exchangers, marine, power plants, and industrial boilers because of excellent corrosion resistance with higher strength particularly at elevated temperatures [[1], [2], [3]]. IN625 has higher fatigue and creep strength than that of stainless steels [4] and it is heavily utilized in the surface repairing industries [5]. Hardfacing (surface repairing) is widely employed in manufacturing industries, particularly for engineering components in the aircraft industry and intermediate heat exchangers in nuclear power plants [6,7]. IN625 provides better properties on the substrate surface by improving hardness, toughness and increases the wear resistance with excellent bonding [8]. Studies revealed that Ni-based hardfacing offers better wear resistance at elevated temperatures in relation to Fe-based alloys [9,10]. Comparing with other repairing techniques such as thermal splashing galvanizing, cladding, etc., hardfacing has a major advantage of excellent bonding and suitable thickness (more than 2 mm in a single pass) to avoid wear and corrosion [11,12]. AISI 316L stainless steel finds its applications in chemical, power plants, oil and gas industries, etc., being a less expensive structural material due to cost-effective corrosion resistance [13,14]. It is specially used in corrosive environments due to better mechanical properties, good weldability, excellent corrosion resistance, and economic considerations. While using the material for a prolonged period in this environment will weaken the properties which further promotes deterioration of the components [14]. Despite better mechanical properties, wear resistance of AISI 316L was weak in sliding conditions [[15], [16], [17]] and fatigue strength [18]. In spite of increasing the mechanical properties of the substrate, weld overlaying of harder material over weaker substrate enhances the tribological properties such as wear and corrosion resistance. Thus the component is made to combat heavy service conditions [19]. The findings by the researcher [20] revealed better wear resistance using Ni–Mo–Cr–Si coating over the 316L substrate with the aid of laser cladding process. Hardness of austenitic stainless steels cannot be increased significantly by heat treatment process in order to improve wear resistance and hardness, hardfacing is and superior process [21]. Austenitic stainless steels do not show resistance to galling and seizing. They undergo adhesive wear which can be avoided by hardfacing with nickel-based alloys [22]. Hardfacing is an important functional coating to protect service elements from friction or impact loads [23]. Hardfacing can be done by different welding techniques such as gas metal arc welding (GMAW), oxyacetylene gas welding (OAW), submerged arc welding (SAW) and shielded metal arc welding (SMAW). Dilution of the substrate, weld efficiency, bonding strength and cost of consumables are the factors that govern the selection of a suitable process for weld hardfacing [24]. Laser surface modified Inconel 625 alloy shows better performance in seawater corrosive environment when compared with surface injected tungsten carbide and titanium carbide particles [25]. Nickel-based clads are better compared to stellite claddings for nuclear applications, where cladded stellite left after service act as a radioactive source but in Ni-based clads it can be avoided [26]. The comparative studies on Nickel-based superalloy over AISI 304 substrate by Ref. [27] demonstrated better quality powder coatings with higher hardness compared to wire deposited coatings. In addition to that, laser deposition being a cost-effective and low heat input process enhances the life of stainless steel substrate. While comparing Nickel-based superalloy such as Inconel 625 with stainless steels, the cost is not considered for particular applications where safety or purity plays a crucial role [28]. The investigations from Ref. [5], reported defect-free Inconel 625 coating and the clad is distinctly divided into different regions such as cladded region, unmixed region, and substrate. The bonding interface and the cladded region displayed higher tensile strength than that of the substrate at both room temperature and elevated temperature. Bishal Silwal et al. [29] examined hot wire addition of IN625 on AISI 347 substrate reported the effect of primary current and torch angle on continuous bead formation and at higher current range liquation cracking on HAZ of substrate was recorded. However, few research works reported Inconel 625 weld overlaying on AISI 316L mainly concentrated on Cold metal transfer (CMT), Plasma Transferred Arc (PTA) and Laser clads due to lesser dilution rate than GMAW. The present work emphasizes overlaying of Inconel 625 on AISI 316L substrate with weaving using GMAW process which curtails the dilution of substrate metal than stringer bead overlays. In this work, multiple (overlapped) weld overlays of Inconel 625 was deposited on the substrate metal without any defects, accruing minimum dilution and durable bonding between the substrate and overlay. The present study investigated the overlay bead characterization, metallurgical aspects, mechanical properties and wear mechanisms of the hardfaced sample.