Metallurgical Evaluation Research Articles

A Metallurgical Evaluation of the Powder-Bed Laser Additive Manufactured 4140 Steel Material

Using laser powder bed fusion (PBF-L) additive manufacturing (AM) process for steel or iron powder has been attempted for decades. This work used a medium carbon steel (AISI 4140) powder to explore the feasibility of AM. The high carbon equivalent of 4140 steel (CEIIW ≈ 0.83) has a strong tendency toward cold cracking. As such, the process parameters must be carefully controlled to ensure the AM build quality. Through an orthogonally designed experimental matrix, a laser-welding procedure was successfully developed to produce 4140 steel AM builds with no welding defects. In addition, the microstructure and micro-cleanliness of the as-welded PBF-L AM builds were also examined. The results showed an ultra-fine martensite lath structure and an ultra-clean internal quality with minimal oxide inclusion distribution. After optimizing the PBF-L AM process parameters, including the laser power and scan speed, the as-welded AM builds yielded an average tensile strength higher than 1482 MPa and an average 33 J Charpy V-notch impact toughness at −18°C. The surface quality, tensile strength, and Charpy V-notch impact toughness of AM builds were comparable to the wrought 4140 steel. The excellent mechanical properties of 4140 steel builds created by the PBF-L AM AM process make industrial production more feasible, which shows great potential for application in the aerospace, automobile, and machinery industries.

Process and metallurgical evaluation of outlet pigtails damage in the primary steam reformer of an industrial ammonia plant

In spite of improvements in reformer tube metallurgy and manufacture, outlet pigtail tubes are now seen as a critical and weak link component of primary steam reformer in ammonia plants and often require replacement before the reformer tubes. The present work has been focused to find out causes of damaging 12 outlet pigtails of primary steam reformer in the ammonia plant of Shiraz Petrochemical Complex (SPC) after 7–8.5years of operation from metallurgical and process point of view. A process evaluation based on operating variables and a detailed metallurgical investigation based on microstructural assessment, chemical and reduced thickness analysis, micro hardness measurements, metallography and tensile properties of pigtail samples has been performed. The obtained findings demonstrated that the failure of outlet pigtails was attributed to over-design operating temperatures. Under operation at high temperature, the pigtails undergo the advance stage of irreversible creep and failed before their designing life span.

In Situ Study of Microstructure Evolution in Solidification of Hypereutectic Al-Si Alloys with Application of Thermal Analysis and Neutron Diffraction

Understanding of the kinetics of solid-phase evolution in solidification of hypereutectic aluminum alloys is a key to control their as-cast microstructure and resultant mechanical properties, and in turn, to enhance the service characteristics of actual components. This study was performed to evaluate the solidification kinetics for three P-modified hypereutectic Al-19 pct Si alloys: namely, Al-Si binary alloy and with the subsequent addition of 2.8 pct Cu and 2.8 pct Cu + 0.7 pct Mg. Metallurgical evaluation included thermodynamic calculations of the solidification process using the FactSage™ 6.2 software package, as well as experimental thermal analysis, and in situ neutron diffraction. The study revealed kinetics of solid α-Al, solid Si, Al2Cu, and Mg2Si evolution, as well as the individual effects of Cu and Mg alloying additions on the solidification path of the Al-Si system. Various techniques applied in this study resulted in some discrepancies in the results. For example, the FactSage computations, in general, resulted in 281 K to 286 K (8 °C to 13 °C) higher Al-Si eutectic temperatures than the ones recorded in the thermal analysis, which are also ~278 K (~5 °C) higher than those observed in the in situ neutron diffraction. None of the techniques can provide a definite value for the solidus temperature, as this is affected by the chosen calculation path [283 K to 303 K (10 °C to 30 °C) higher for equilibrium solidification vs non-equilibrium] for the FactSage analysis; and further complicated by evolution of secondary Al-Cu and Mg-Si phases that commenced at the end of solidification. An explanation of the discrepancies observed and complications associated with every technique applied is offered in the paper.

Use of kinetic model for thermal properties of steel at high temperatures

Steel thermal properties determination is essential in steel structure temperature prediction for structural fire design or metallurgical evaluation. Thermal properties include specific heat, thermal expansion and heat transfer coefficients. Since the steel thermal properties variation at high temperature may be considered as a chemical process, kinetic theory could be used to explain the variation. To examine this concept, steel specific heat is first considered in this paper. The validity of the kinetic model that successfully applied to steel specific heat can be used to estimate the variation of other steel thermal properties at high temperatures. Complex non-linear equations for steel specific heat computation are provided in design codes. In this paper, it is found that the way steel specific heat varies with temperature during solid state phase transformation is not unique. Instead of using high-order equations, models using simple linear equations and average specific heat for phase transformation are proposed. These models are based on the latent heat principle whereby the latent heat consumed by steel during phase transformation is constant. Furthermore, the proposed simple models can effectively simplify the steel specific heat computation and provide good steel temperature development simulation shown in the case study.

Avaliação metalúrgica da soldagem de revestimento inox austenítico sobre aço SAE 4130

O objetivo deste trabalho foi realizar uma avaliação metalúrgica da soldagem de revestimento inox austenítico sobre a área de selagem de tubos de Riser fabricados em aço SAE 4130. O estudo foi dividido em duas etapas. A primeira etapa teve como objetivo a determinação da taxa de diluição e taxa de deposição para os níveis máximo e mínimo de energia de soldagem conforme procedimentos de soldagem (EPS) com os processos TIG e ER. A seguir, foi feita análise e estudo com o diagrama de Schaeffler para identificar os consumíveis de solda mais adequados, conforme critérios de qualidade estabelecidos. Este estudo identificou o metal de adição 312 para a primeira camada, o 309 e 308 para a segunda. Todos usando o nível máximo de energia de soldagem e o processo de soldagem ER, com benefícios à produtividade. A segunda etapa teve como objetivo a validação dos resultados empíricos encontrados na 1ª etapa, com a execução de soldas de revestimento com duas camadas cada, e a caracterização metalúrgica das juntas soldadas por ensaios mecânicos e análise microestrutural por MO e MEV. Os resultados dos ensaios mecânicos e da microestrutura foram considerados satisfatórios, atendendo os critérios de qualidade adotados. Foi observada uma microestrutura austeno-ferritica com teor de ferrita delta entre 10 a 15 % nas duas camadas do revestimento. O eletrodo 312 assumiu posição de destaque na aplicação da 1ª camada da solda de revestimento sobre o aço SAE 4130, em função do seu maior teor de ferrita delta e cromo, evitando a formação de trincas a quente. Para a segunda camada, tanto o 309 como o 308 podem ser usados. Todas as soldas foram feitas com elevada energia de soldagem, garantindo o aumento da produtividade, sem alteração significativa nas características físicas e mecânicas.

Failure analysis of a bearing in a helicopter turbine engine due to electrical discharge damage

This article documents the metallurgical evaluation of a rolling element bearing that failed due to electrical discharge damage. This rolling element bearing was used in a helicopter turbine engine that failed in-flight, resulting in a hard landing of the helicopter. Optical and electron microscopy as well as energy dispersive spectroscopy were used to evaluate the bearing. Pitting and material transfer on the external bearing races bearing and mating surfaces revealed that the electrical discharge damage occurred while the engine's components were not rotating.

Evaluation of creep damage of INCOLOY 800HT pigtails in a refinery steam reformer unit

Pigtail tubes constitute a critical component of the outlet system of steam reformer units for hydrogen production in refineries. Creep failure of pigtails is a common cause of downtime and potential risk to plant personnel. In this paper, finite element analyses are combined with experimental techniques in order to evaluate creep damage in INCOLOY 800HT pigtails of a reformer unit. A finite element model has been developed in order to investigate the creep behavior of pigtails under various operating conditions. A metallurgical evaluation of creep damage has been performed on pigtail samples removed after 8.5years of operation. The results indicate that the most important parameters affecting the structural behavior of pigtails are the operating temperature, reduction of thickness, magnitude of counterweights and grain size of the alloy. A comparison between the numerical calculations and the metallurgical evaluation is also reported.

Effect of the Heat Input in the Mechanical and Metallurgical Properties of Welds on AHSS Transformed Induced Plasticity Steel Joined with GMAW Processes in the Automotive Industry

ABSTRACTThe effect of the heat input on the mechanical and metallurgical properties of the welds has been investigated in the heat affected zone (HAZ) of welds joined with gas metal arc welding (GMAW), using normal production welding parameters. The thermal effect in the HAZ of the welds is important for the optimization of the welding parameters used when weld transformed induced plasticity (TRIP) steels, because this will have a great influence in the mechanical and metallurgical properties of the weld. In this work 3 samples was welded a high, average and low heat input, with the variation of welding parameters to obtain different thermal affectation to investigate the variations in different parts of weld joint: weld, HAZ and base metal, due the heat applied for the welding process used. Mechanical properties were evaluated by tension test, microhardness and fatigue testing and metallurgical evaluation with optical metallograpy, scanning electron microscopy (SEM), fractograpy and X-Ray diffraction (XRD).The results obtained shows that the mechanical properties of the tension test decrease when the heat input increase and the microhardness exhibit a softening zone in the HAZ with lower hardness and the fatigue life were similar for all heat inputs for the high stress levels, but only in low stress there is a difference. For metallurgical properties the metallographic evaluation shows ferrite, bainite - martensite and retained austenite, and the fractography analysis exhibit a ductile fracture in all cases and the content in volume fraction of retained austenite increases in the HAZ of welds when increasing heat input in to the base metal due the thermal effect.

Metallurgical evaluation of flaw indications in tie chord transition butt welds from the I-64 Sherman Minton Bridge

In September 2011, the I-64 Sherman Minton Bridge over the Ohio River between New Albany, IN and Louisville, KY was closed due to the detection of cracks in fracture-critical tie chord members. The bridge remained closed for over five months while non destructive testing and repairs were performed. Metallurgical evaluations were conducted by Applied Technical Services, Inc. (ATS) on 21 cores extracted from fracture critical members of the bridge. The cores were reported to contain rejectable indications as identified by visual inspection (VT), magnetic particle inspection (MT), ultrasonic testing (UT) and/or radiographic testing (RT). The purpose of this investigation was to determine the nature and cause of the flaws and if there was any evidence of active propagating cracks. Of the 21 cores examined, 11 contained surface flaws within the vertical butt welds or heat-affected zones. One core contained a major subsurface planar discontinuity. Five cores contained minor subsurface indications. The base metal properties were consistent with high yield strength, quenched-and-tempered low alloy steel, i.e. “T1 ® ”/ASTM A514. The weld metal properties were also consistent with low alloy steel. All flaws and other weld discontinuities observed both at and below the surface of the web members were associated with the original tie chord welds or subsequent early weld repairs. These flaws were formed during or shortly after welding while in the fabrication shop. Fracture mechanics calculations determined that at a critical flaw size, temperatures below 30° F (-1.1°C) could possibly result in failure of the bridge. Metallurgical analysis of the flaws revealed no evidence of active crack growth due to fatigue, corrosion fatigue or stress corrosion. The flaw surfaces exhibited intergranular separation along with mixed mode brittle and ductile features characteristic of hydrogen-induced cracking. The fracture critical members of the tie chords box girders were reinforced by bolted steel plates and the bridge placed back in service in February, 2012.

Metallurgical Evaluation of Farmer’s Steelmaking in Finland

Metallurgical Evaluation of Farmer’s Steelmaking in Finland

Studying the Effect of Different Combinations of Salt Modifier on the Mechanical Properties and Microstructure of A356 Al-Si Alloy

The present study aims to develop A356 Al-Si alloy using high purity aluminium and various master alloy in gas fired pit furnace. Three different ratio of salt modifier (1:1, 1:2 & 1:3) was used to prepare the casting. Sand casting and permanent mould casting techniques were used to prepare the alloys. Optical microscope and universal tensile testing machine were used for the metallurgical evaluation of the prepared alloy. It was observed that the addition of modifier improved the mechanical properties and microstructure of the alloy. It was also observed that modifying agent NaF with CaCl2in 1:1 ratio has shown the best results in terms of microstructure and the mechanical properties.

Boiler Stack Economizer Tube Failure

A metallurgical evaluation was performed to investigate the failure of a type 304 stainless steel tube from a boiler stack economizer. The tube had three distinct degradation mechanisms: pitting corrosion, chloride stress corrosion cracking, and fatigue fracture. The primary failure mechanism for the tube was fatigue fracture, but the other mechanisms may have eventually caused a tube failure in the absence of fatigue. This paper details the visual, SEM/EDS, and metallographic examinations used to determine that these failure mechanisms were each present in the same tube.

CNT-reinforced aluminum composites: processing and mechanical properties

This work focus the development of aluminum silicon composites reinforced with carbon nanotubes.Several composites were obtained by a powder metallurgy route through hot pressing technique. The influence of different volume fraction of carbon nanotubes in metallurgical and mechanical properties of the composite was studied. Metallurgical evaluation was made by means of SEM/EDS for interface reaction between matrix and reinforcement and also reinforcement distribution in the matrix. Mechanical evaluation was made by tensile tests.

Failure analysis of a high pressure natural gas pipe under split tee by computer simulations and metallurgical assessment

Crack failure of a 36inch high pressure gas pipe observed during regular inspection of a station has been investigated and the results are presented in this paper. The crack, approximately one meter long, was initiated from a notch inside the hot tapped hole in a pipeline installed about 30years ago. The study was conducted by reviewing the design history and construction data, visual inspection, pipe material characterization, stress and modal analysis by using finite element method. Investigations revealed that the valve, directly connected to the split tee, faced large dynamic periodic forces due to a pressure drop between two pipelines. Metallurgical evaluation of the pipe material by optical microscope and fractography of the crack surface by scanning electron microscopy indicated the presence of elongated inclusions in the steel microstructure together with some indications of fatigue fracture as a poorly formed sawtooth profile. Based on dynamic analysis, it was found that the first mode shape, the maximum displacement and, therefore, the maximum stress were exactly situated within the crack initiation zone. It was concluded that the notch effect in the hot tapped hole, the position of the supports under the split tee and the presence of a large periodic stress were responsible for the initiation and fatigue propagation of the crack in the gas pipe.

Mechanical and Metallurgical Evaluation of Carburized, Conventionally and Intensively Quenched Steels

Steels subjected to carburizing, quenching, and tempering are widely used for components that require hardness and superficial mechanical resistance together with good core toughness. Intensive quenching is a method that includes advantages including crack prevention, increased mechanical resistance, and improvement in fatigue performance when subjected to very fast (intensive) cooling. However, achieving these advantages requires the formation of sufficiently high surface compressive residual stresses and fine grains at the core of steel components. If the cooling rate is sufficiently high after intensive quenching, then low-hardenability, killed plain carbon steels may be used instead of higher-cost, low alloy steels because compressive residual stresses are formed at the surface of steel parts. The objective of this study was to compare between carburized non-killed AISI 1020 steel samples, which were not modified by Al that were subsequently conventionally and also intensively quenched to determine the effect of quenching on achieving the necessary formation of fine grain size. For comparison, carburized AISI 8620 steel test specimens were conventionally quenched. After quenching, all test specimens were characterized by metallurgical and mechanical analyses. The results of this study showed that when the two quenching methods were compared for carburized non-killed AISI 1020 steel, intensive quenching method was found to be superior with respect to mechanical and metallurgical properties. When comparing the different steels, it was found that intensively quenched, non-killed, AISI 1020 steel yielded grain sizes which were three times greater than those obtained with conventionally quenched, carburized AISI 8620 steel. Therefore, the benefits of intensive quenching were negated. These results show that plain carbon steels must be modified by Al to make fine grains if intensively quenched plain-carbon steel is to replace alloyed AISI 8620 steel.

Microstructure and mechanical properties of a friction stir processed Ti–6Al–4V alloy

Friction stir processing (FSP) trials were performed on 2mm-thick Ti–6Al–4V sheets using various processing parameters including tool rotational speed (800–1000RPM) and tool traverse speed (1–4IPM). A processing window was established with the W–1% La2O3 tool to produce defect-free friction stir processed (FSP'd) materials. The stir zone (SZ) material of the FSP'd samples showed a fully β transformed microstructure characterized by typical basket-weave lamellar α/β structure. The prior β grains in the SZs contained multiple α variants, and the average sizes ranged from 12μm to 38μm influenced by the processing parameters. Based on metallurgical evaluation, the microstructural evolution in different regions from base material (BM) through heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ) to SZ was established. The mechanical properties of the SZs were evaluated by tensile tests. Compared to the base material, the processed samples exhibit higher tensile strength and comparable ductility. The tensile strength was affected by the microstructure of the prior β grain size and α colony size, which are controlled by processing parameters. The lower tool rotational rate and/or higher traverse speed produced a lower peak temperature and a shorter dwell time above the β-transus temperature in the SZ, which resulted in finer prior β grains and smaller α colonies, leading to higher tensile strength.

Automotive power window mechanism failure initiated by overload

Due to its nature of usage, typically automotive power window mechanisms fail due to fatigue. However, many of them could have been prevented because its initial crack is caused by overload. This paper outlines simplified manufacturing process and common failures of the mechanism. It also discusses about representative after market failures. Of the hundreds of after market returned products, representative ones were selected for metallurgical evaluation. Combined with comparison with newly manufactured product, a case study was performed. It was concluded that the failure was due to overload although the crack propagation was through fatigue loading as the mechanism was installed and used.

Indigenous development and airworthiness certification of 15–5 PH precipitation hardenable stainless steel for aircraft applications

In this paper, we discuss the optimization of chemical composition, processing (forging and rolling) and heat treatment parameters to obtain the best combination of mechanical properties in case of a Fe–15Cr–5Ni–4Cu precipitation hardenable stainless steel. The ε-copper precipitates that form during aging are spherical in shape and coherent with the matrix and principally provide strengthening in this alloy. The orientation relationship is found to be Kurdjumov–Sachs (K–S), which is common in fcc–bcc systems. Results obtained from metallurgical evaluation (mechanical property and metallography) on 15–5 PH alloy during type certification on 3 different melts were used for the optimization, attempted in this study. The mechanical properties following strain deformation has been carried out using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). In the aged conditions, the 15–5 PH alloy exhibited brittle failure with extensive cleavage and/or quasicleavage fracture. This paper reports all results and also factually shows that indigenously developed and produced 15–5 PH stainless steel matches in its properties with the equivalent aeronautical grade precipitation hardening stainless steels globally produced by internationally renowned manufactures.

Root Cause Analysis of Failed Diesel Engine Aluminum Pistons

Root cause analysis was performed to determine the cause of failure of numerous cast aluminum pistons used in high speed diesel engines over ~7 months. The analysis consisted of metallurgical and engine systems evaluations. The metallurgical evaluation of two of the initial failed pistons showed failure by exposure to incompletely atomized and combusted fuel droplets that melted and eroded the piston crown outer diameters. The initial suspected root cause of the uncombusted fuel droplets was the presence of fuel-tank biological growth. The tanks were cleaned and the engines restored to service, but there were additional piston failures after only a short time back on-line, including piston seizures that fractured the pistons and cylinder liners. A subsequent, much more detailed engine system evaluation showed that the true root cause explaining all the failures was incorrect fuel injection timing. Two key points to be taken from this analysis are: (1) determination of the true root cause in this case required continuous and close interaction among metallurgical and engine systems personnel throughout an extended analysis process; (2) getting to the true root cause may require tenacious ‘detective’ work to track down and eliminate all other potential causes.

High Strength Steel as a Solution for the Lean Design of Industrial Buildings

This paper presents the actual case of an industrial building at CBMM's plant in Araxá, Brazil as an example of lean design using microalloyed steels. The structure was made mostly with microalloyed ASTM A572 steel instead of the traditional carbon manganese ASTM A36 steel. There is a metallurgical evaluation of microstructures for both steel types and their respective mechanical properties. The effect of niobium increasing strength and toughness simultaneously allowed a 22% savings in total steel consumption of this project.