Plasma Transferred Arc Process Research Articles

Thermal fatigue testing of Stellite 6-coated hot work tool steel

The performance of Stellite 6 coating deposited on X32CrMoV33 hot work tool steel via Plasma Transfer Arc (PTA) process was investigated under steel thixoforming conditions. The Stellite 6 coating made a very favorable impact on the thermal fatigue performance of the hot work tool steel. The coated hot work tool steel survived much longer, for nearly 5000 cycles before the first thermal fatigue crack was detected on the coating. This marked improvement is attributed to the higher oxidation resistance of Stellite 6 alloy and its ability to retain its mechanical strength at elevated temperatures. The Cr content of the Stellite 6 alloy facilitated the formation of stable Cr-rich oxides which sustained the thermal stresses generated at the surface without spalling and thereby retarded crack initiation. The peak compressive and tensile stresses acting on the coating were estimated to be 500 MPa and 170 MPa, respectively, and eventually led to thermal fatigue cracking. Once the crack initiated, the impact of microstructural features was only minor.

Numerical Modeling of Residual Stress and Prediction of Heat Affected Zone Induced by PTA Process

The objective of this study is to investigate the effects of Heat Affected Zone produced by Plasma Transferred Arc process and residual stress in substrate steel 55NiCrMoV7. In this study, based on ABAQUS code, a sequentially coupled thermal, metallurgical, mechanical 2-D finite element model is developed. In the numerical simulations, the coating and Heat Affected Zone are used to predict the residual stresses. The simulation results revealed that the final residual stress in substrate steel do seem to be influenced by the HAZ.

Neutron Diffraction Residual Stress Evaluation and Numerical Modeling of Coating Obtained by PTA Process

The Stellite 6 hardfacing alloys have been deposited on steel substrate using a Plasma Transferred Arc (PTA) with complex geometry. The residual strain of the PTA technology at the surface of coating layer and the interface were determined by neutron diffraction method. In the present work, a numerical model for the residual stresses formed during the PTA process with physical conditions and mechanical properties using the Abaqus code is analysed. The result reveals that the residual stresses obtained by the numerical simulation are in very good agreement with experimental results by the neutron diffraction.

Effect of Boron Carbide Addition on Wear Behaviour of Cobalt Based Hardfacings by Plasma Transferred Arc Process

Cobalt based alloy with the addition of 5 and 10 wt% boron carbide have been deposited by PTA process on stainless substrates effectively. The dry sliding wear performances of the coated alloys have been studied at room as well as at elevated temperatures. Addition of boron carbide to satellite 6 resulted in refinement of grains and formation of complex carbides and borides, which has lead to increase in hardness and wear resistance of the hardfaced layers. Superior wear resistance has been found to be at the composition of Stellite 6 with 5 wt% addition of boron carbide.

Improving the ballistic immunity of armour steel weldments by plasma transferred arc (PTA) hardfacing

This investigation describes about improving the ballistic immunity of armour steel joints which are fabricated by sandwiching of plasma transferred arc (PTA) hardfaced interlayers in between soft austenitic stainless steel (ASS) welds. From the results, the welds with sandwiched interlayer stopped all the projectiles successfully, irrespective of processes used, whereas welds without sandwiched interlayer were failed. In order to know the cause of failure, a detailed metallographic examination was carried out. The variation in microstructure and hardness at various zones of the weld are discussed. For the first time, it was found that the armour steel could be hardfaced by the PTA process with tungsten carbide powder.

Selection of covering materials for synthesising fabrication of TiB2 based coating with PTA process

A technique for the synthesising fabrication of TiB2 based coating using plasma transferred arc (PTA) process on the surface of plain steel was explored. For the selection of the precursors, various covering materials contained both of B contained powders and Ti contained powders were tested using PTA process. By the analysis of the coatings surface quality and the interface to the substrate, the possibility of some powder combinations between B contained powder materials and Ti contained powder materials used as precursors for TiB2 based coating fabrication were evaluated. The experiment results show that the covering materials contained TiO2 and/or B2O3 powders are not suitable for TiB2 based coating fabrication by PTA process, whereas the powder mixtures just as Fe–B + Ti, Fe–Ti + Fe–B or Fe–Ti (or Ti) + B4C are suitable for TiB2 based coating fabrication by PTA process.

Application of Response Surface Methodolody to Prediction of Dilution in Plasma Transferred Arc Hardfacing of Stainless Steel on Carbon Steel

The application of response surface methodology was highlighted to predict and optimize the percentage of dilution of iron-based hardfaced surface produced by the PTA (plasma transferred arc welding) process. The experiments were conducted based on five-factor five-level central composite rotatable design with full replication technique and a mathematical model was developed using response surface methodology. Furthermore, the response surface methodology was also used to optimize the process parameters that yielded the lowest percentage of dilution.

Predicting the dilution of plasma transferred arc hardfacing of stellite on carbon steel using response surface methodology

Control of dilution is important in hardfacing, where low dilution is typically desirable. At present, most fabrication industries use shielded metal are welding, gas metal arc welding, gas tungsten arc welding and submerged are welding processes for hardfacing purposes. In these processes, the percentage of the dilution level is higher, ranging between 10% and 30%. In Plasma Transferred Arc (PTA) hardfacing, a solidified metallurgical bond between the deposit and the substrate is obtained with minimum dilution (less than 10%). This paper highlights the application of response surface methodology to predict and optimize the percentage of the dilution of a cobalt-based hardfaced surface produced by the PTA process. Experiments were conducted based on a fully replicable five-factor, five-level central composite rotatable design and a mathematical model was developed using response surface methodology. Furthermore, the response surface methodology was used to optimize the process parameters that yield the lowest percentage of dilution.

Understanding the Parameters Controlling Plasma Transferred Arc Hardfacing Using Response Surface Methodology

Control of dilution is important in hardfacing, where typically low dilution is desirable. At present, majority of the fabrication industries use shielded metal arc welding (SMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and submerged arc welding (SAW) processes for hardfacing purposes. In these processes, the percentage of dilution level is higher, ranging between 10–30%. In plasma transferred arc (PTA) hardfacing, a solidified metallurgical bond between deposit and substrate is obtained with minimum dilution (less than 10%). This article highlights the application of response surface methodology (RSM) to predict and optimize the percentage of dilution of nickel based hardfaced surface produced by PTA process. The experiments were conducted based on five-factor, five levels central composite rotatable design with full replications technique and mathematical model was developed using RSM. Further, the RSM is used to optimize the process parameters that yield the lowest percentage of dilution.

S56 Finite Element Analysis and Neutron Diffraction Evaluation of Residual Stress in Stellite Coating by PTA Process

A grade 6 cobalt-based superalloy was coated on steel substrate by the Plasma Transferred Arc (PTA) method. The macrostructure and microstructure of the interface and of the co-based superalloy layers are studied. The superalloy grains are deformed during the cladding process. At the interface, the superalloy grains elongate and tend to follow the geometry of the interface. Observation with a scanning microscope reveals zones of localised Affected Zone Thermal at the interface. A numerical analysis has been carried out to simulate the PTA process deposition with physical conditions and mechanical properties using the Abaqus code. A simplified model based on energy conservation and wall functions is used to predict the process, solid shield and shroud on the temperature and the residual stress of the PTA technology at the surface of coating layer and the interface. The results reveal that the residual stress obtained by the numerical simulation are in very good agreement with experimental results performed by the neutron diffraction.

Microstructural characteristic of NiTi coating on stainless steel by plasma transferred arc process

In this study, NiTi powder mixture was coated on the surface of an austenitic stainless steel (AISI 304) by plasma transferred arc (PTA). Coating was carried out by using 80, 90, 100 A current density under Ar atmosphere. Coating layer and interface were examined with SEM, EDX, XRD analysis and micro hardness test. Thickness of coating increased with current density. No cracks or pores were detected in the interface. Higher arc current densities caused a coating layer poor in NiTi alloy. Secondary phases such as Cr 2Fe 7Ni, Fe 7Cr 2Ni and Ni 3Ti also formed in the coating layer.

Microstructure and properties of Cr 3C 2-modified nickel-based alloy coating deposited by plasma transferred arc process

The nickel-based alloy with 30 wt.% chromic carbide (Cr 3C 2) particles has been deposited on Q235-carbon steel (including 0.12 wt.% C) using plasma transferred arc (PTA) welding machine. The microstructure and properties of the deposited coatings were investigated using optical microscope, scanning electron microscope (SEM) equiped with X-ray energy spectrometer (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), microhardness testes, and sliding wear test. It was found that the γ(Ni, Fe), M 7(C,B) 3, Ni 4B 3, and (Cr,Fe) 2B phases existed in the Cr 3C 2-free nickel-based alloy coating obtained by PTA process. The typical hypoeutectic structure and composition segregation in the solid solution could be found clearly. The addition of 30 wt.% Cr 3C 2 particles led to the existing of Cr 3C 2 phase and the microstructure changing from hypoeutectic structure into hypereutectic structure. The composition segregation in the solid solution could not be found clearly. The average microhardness of the Cr 3C 2-free nickel-based alloy coating increased by 450–500 HV after the addition of 30 wt.% Cr 3C 2 particles. The partial dissolution of Cr 3C 2 particles led to the enrichment of carbon and chromium in the melten pool, and hence caused the formation of more chromium-rich carbides after the solidification process. The undissolved Cr 3C 2 particles and the increasing of chromium-rich carbides was beneficial to enhance the hardness and wear resistance of the Cr 3C 2-modified nickel-based alloy coating deposited by PTA process.

Microstructure and Bonding Strength in Overlaying of Stellite 6 Alloy on Spheroidal Graphite Cast Iron by Plasma Transferred Arc Process

In this study, stellite 6 alloy was overlaid on spheroidal graphite (SG) cast irons with various carbon contents (1.5wt%~3.8wt%) and a fixed silicon content (approximately 2.5wt%) using the plasma transferred arc (PTA) process at different overlaying current (140A~220A) while the travel speed of the PTA torch was maintained constant. Results indicated that the solidification structures of the stellite 6 overlayers were dendritic and had a large amount of interdendritic precipitates (M7C3 and M23C6 carbide) after the satellite 6 alloy had been overlaid on SG cast irons with different carbon contents (1.5wt%~3.8wt%) at a low overlaying current (I=140A). The partially melted zones of the substrates below the carbide-containing interfacial layers consisted of eutectic carbides (ledeburites) and fine pearlites. The amount of the carbide-containing interfacial layers and ledeburites increased following the increase in the overlaying current. The results of the tearing test reported that the occurrence of the carbide-containing interfacial layers was detrimental to the bonding strength between the overlayers and the substrates. The metallography of the fractured area of the tearing specimens after the bonding strength test revealed that fractures always occurred at the carbide-containing interfacial areas. On the other hand, the overlayers were rich in iron content when the overlaying currents were high (I=190A and 220A). Owing to the dilution effect, the matrices of the overlayers were α-Fe with lamellar M7C3 carbides. The results of the tearing test indicated that the bonding strength of the overlaid specimens was relatively low, and fractures always occurred in these highly diluted overlayers.

Effects of Y 2O 3 on the microstructure and wear resistance of cobalt-based alloy coatings deposited by plasma transferred arc process

Abstract Cobalt-based alloys with different Y 2O 3 contents were deposited on Q235A-carbon steel using plasma transferred arc (PTA) welding machine. The effect of Y 2O 3 on the microstructure and wear resistance properties of the cobalt-based alloys were investigated using an optical microscope, a scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that a cobalt-based solid solution with a face-centered cubic crystal structure was presented accompanied by the secondary phase M 7C 3 with a hexagonal crystal structure in the Y 2O 3-free cobalt-based alloy coating. Several stacking faults exist in the cobalt-based solid solution. The addition of Y 2O 3 leads to the existence of the Y 2O 3 phase in the Y 2O 3-modified coatings. Though stacking fault exists in the Y 2O 3-modified coatings, its density increases. The addition of Y 2O 3 can refine the microstructure and can increase the wear resistance properties when its contents are less than or equal to 0.8 wt.%. However, further increase of its contents will lead to the agglomeration of undissolved Y 2O 3 particles at the γ-Co grain boundary, and will lead to a coarse microstructure and lower wear resistance properties.

Microstructure and properties of Fe–C–Cr–Cu coating deposited by plasma transferred arc process

The iron-based alloy with 3 wt.% copper has been deposited on Q235-carbon steel using plasma transferred arc (PTA) welding machine. Then the deposited coating was aged at 773 K for 35 h. The microstructure and properties of the as-deposited and the as-aged coatings were investigated using optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), and sliding wear test. More detailed information about the crystallographic structure and phase compositions of the as-deposited and as-aged coatings was obtained with the application of Rietveld refinement method to analyze X-ray diffraction spectra. It was found that the as-deposited coating consisted of α phase (martensite microstructure) with body-centered cubic (b.c.c) crystal structure, M23C6 phase with face-centered cubic (f.c.c) crystal structure, and M7C3 phase with orthorhombic crystal structure. The preferred direction at the (110) crystal plane and the high density dislocation existed in martensite matrix. The ageing treatment at 773 K for 35 h not only led to the precipitation of ε-Cu phase from M7C3 grain boundary but also increased the relative contents of the total Cr-rich carbides from 22.8 wt.% in the as-deposited coating to 28.7 wt.% in the as-aged coating. The enhancing of microhardness and sliding wear resistance was closely related to the precipitation of ε-Cu phase and the increase of total Cr-rich carbides when the deposited coating was aged at 773 K for 35 h.

Influence of molybdenum on the microstructure and wear resistance of nickel-based alloy coating obtained by plasma transferred arc process

The influence of 6 wt% Mo on the microstructures and wear resistance properties of nickel-based alloy coating obtained by plasma transferred arc process were investigated using optical microscopy, scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS) and X-ray diffraction. More detailed information about the crystallographic structure and phase compositions of the coatings was obtained with the application of Rietveld refinement method to analyze X-ray diffraction spectra. The results show that the morphology of Cr-rich carbides observed in the interdendritic region of the nickel-based alloy coating changed from plate-like to net-like, as well as the refinement of Ni-rich dendrites because of the addition of 6 wt% Mo. The weight fraction of Cr-rich carbides increased from 36 wt% to 45 wt%. The wear resistance for 6 wt% Mo-modified nickel-based alloy coating increased by 47.2%. The microstructural change and phase contents variation were responsible for the improvement of the wear resistance of the 6 wt% Mo-modified nickel-based alloy coatings.

Boriding a plain carbon steel with the plasma transferred arc process using boron and chromium diboride powders: microstructure and wear properties

Iron–boron and iron–chromium–boron coatings have been produced by alloying and superficial melting of steel using the plasma transferred arc process (PTA). The microstructures of the alloyed surfaces consisted of needlelike borides and eutectic regions. Both coatings presented high hardness and excellent wear resistance.

Synthesis of boride coatings on steel using plasma transferred arc (PTA) process and its wear performance

Boriding of the surface of a tool steel using boron powder and the plasma transferred arc process was investigated. It was shown that this method is an easy and effective technique in producing uniform alloyed layers with a thickness of about 1.5 mm and a hardness between 1000 and 1300 HV. The microstructure of the borided surfaces consists of primary Fe 2B-type borides and a eutectic mixture of borides and martensite. Some cracks are observed in the eutectic regions but they do not seem to critically affect the behaviour of the coatings in sliding wear. The wear rate of pin on disc tests is primarily affected by the applied load and it lies between 10 −5 mm 3/m for low loads and 10 −2 mm 3/m for high loads. Two distinct regimes of mild and severe wear are obtained separated by a critical load. Mild wear is due to the load supporting effect of borides and severe wear is due to their breakage above a critical load. The wear rate is not significantly affected by the sliding velocity and is consistent with the friction coefficient. The friction coefficient varies from 0.13 to 0.23 and depends strongly on the oxidation status of the wear track. The sliding velocity affects the sliding distance where the coefficient of friction reaches equilibrium.

SiO 2 particle erosion of spheroidal graphite cast iron after surface remelting by the plasma transferred arc process

Two spheroidal graphite cast irons of the same composition (Fe 3.54 wt.%-C 2.48 wt.%-Si) and different graphite nodule counts were surface hardened by plasma transferred arc surface remelting. They were then eroded by angular SiO 2 particles (∼295 μm grit size) under two average particle velocities including ∼36 and ∼66 m s −1. According to the results, the presence of eutectic Fe 3C in the remelted layer can improve the erosion resistance at oblique impact by increasing the flow resistance; but the eutectic Fe 3C particles are also subjected to brittle cracking under large-angle impact. Hence, while oblique impact shows larger improvement of the erosion resistance with increasing content of eutectic Fe 3C, large-angle impact indicates that the improvement can be achieved only when the content of eutectic Fe 3C and the particle velocity are properly restricted. Brittleness effect of eutectic Fe 3C also causes the maximum erosion rate to occur at larger impact angle for the remelted layer of more eutectic Fe 3C.

Development of functional overlay alloys by plasma transferred ARC process: T.Tomita. J. Jpn Thermal Spraying Soc. vol 35, no 3, 1998, 238–246. (In Japanese.)

Iron–boron and iron–chromium–boron coatings have been produced by alloying and superficial melting of steel using the plasma transferred arc process (PTA). The microstructures of the alloyed surfaces consisted of needlelike borides and eutectic regions. Both coatings presented high hardness and excellent wear resistance.