Gobboon Lothongkum

Effect of 12 Heat Treatment Conditions After HIP Process on Microstructural Refurbishment in Cast Nickel-Based Superalloy, GTD-111

The present work is an attempt to obtain the most suitable and practicable repair condition, which could provide the desired microstructural characteristics by rejuvenation method of hot isostatic pressing (HIP), followed by 12 heat treatment conditions for long-term serviced gas turbine blades, cast nickel based superalloy grade GTD-111 operated by Electricity Generating Authority of Thailand (EGAT). The hot isostatic pressing could mostly heal any internal structural voids and cracks (by means of sintering), which were generated during service. It was found that no microcrack was observed after a long time of HIP process such as for 18 ks HIP time. However, microvoids were still found even after 18 ks-HIP process at 1473 Κ but in a very small amount. Furthermore, during solution treatment, coarse carbides and over-exposed coarse gamma prime precipitates, which had formed previously during service, would dissolve into the matrix. Then specimens were processed through a series of precipitation aging, which re-precipitated the strengthening phase to form the proper morphology in size and shape as well as distribution that is almost similar to the new one. * Corresponding author: P. Wangyao. Tel.: +662-218-4233; E-mail: panyawat@hotmail.com Metallography examination were performed by using scanning electron microscope and image analyzer after hot isostatic pressing and heat treatments to evaluate the rejuvenated microstructures.

Numerical Modelling of Cold Cracking Initiation and Propagation in S 1100 QL Steel Root Welds

Although the phenomenon of hydrogen assisted cold cracking (HACC) and respective avoidance procedures have extensively been investigated in the seventies and eighties, the reasons for recent failures are still a lack of knowledge about the basic hydrogen effects on steel microstructures and, in particular, a lack of welding procedure specifications and standards accounting directly and consistently for cold cracking avoidance in modern high strength structural steels with yield strengths of up to 1 100 MPa. In previous several contributions the consequences of various heat treatment procedures targeted at HACC avoidance have been shown, as for instance their effects on stress-strain build up and on hydrogen diffusion in high strength steel welds. But, a principal interaction of three local influences on hydrogen assisted cold cracking, i.e. local microstructure; local mechanical load and local hydrogen content have not yet been studied in detail for these materials. For this, a numerical model for HACC has been developed, accounting particularly for crack-initiation and crack-propagation criteria, like the hydrogen redistribution during the process of cracking. The numerical model has been used to investigate HACC in such materials, i.e. in the weld microstructures of an S 1100 QL steel, under severe restraint and various hydrogen levels. The results were achieved by in depth thermal and structural finite element simulations combined with numerical hydrogen diffusion modelling. By such procedure, HACC in single-layer welded plates with thickness of 20.0 mm at realistic restraints has been studied. As a particular result, it turned out that the crack-initiation location is typically in the centre of the weld metal (WM), where only a single crack is initiated at hydrogen contents of up to 10.0 Nml/100 g Fe. But, it was evidently shown by such analyses that the crack-initiation location is shifted into the HAZ and that multiple cracking occurs at higher hydrogen contents of up to 15.0 Nml/100 g Fe.

Effects of Preheating and Interpass Temperature on Stresses in S 1100 QL Multi-Pass Butt-Welds

Most of the research on Hydrogen Assisted Cold Cracking (HACC) in high strength steel welds conducted over the last several decades has focused on single-pass welds, especially considering materials with yield strengths about 700 MPa. The guidelines for avoiding cracking that have been developed from such work are therefore useful only where a root pass is the critical event. The well-known guideline is using preheating temperature. Such guideline is very limited when applied to multi-pass welds. In order to support this need, this paper presents the influence of inhomogeneous Hydrogen Removal Heat Treatment (HRHT) procedures, i.e. sole preheating, controlled interpass temperature and combined preheating and controlled interpass temperature, on the residual stresses in multi-pass welds of S 1100 QL. Thereafter, these results are used to identify HACC problems in S 1100 QL and are not reported here. The results were achieved by decent thermal and structural finite element simulations of a five-layer welded 12 mm thick plate at a realistic restraint provided by respective Instrumented Restraint Cracking (IRC) test. The simulations show that the inhomogeneous heat treatment procedures significantly increase the residual stresses as compared to welding without any heat treatment. In contrast to more general anticipations, an increasing controlled interpass temperature does not necessarily lead to a stress reduction, but can even increase the stresses dependent on the location in the multi-pass welds. Maximum residual stresses generally appear in the upper third part of the weld and are not located beneath the top surface where is a typical location used to detect residual stresses in real welded components. If the restraint intensity given to the welded component is not proper, such heat treatment procedures with various temperatures seem to be useful to reduce residual stresses in multi-pass welds.

Effect of Reheat Treatment on Microstructural Refurbishment and Hardness of the As-cast Inconel 738

Abstract This work investigates the effect of rejuvenation heat treatment conditions for refurbishment of the long-term serviced gas turbine blades, which were made of as-cast nickel base superalloy grade, Inconel 738. The reheat treatment conditions consist of solutionizing treatments at temperatures of 1,438, 1,458 and 1,478 K for 14.4 ks and aging treatments at temperatures of 1,133, 1,148 and 1,163 K for 43.2, 86.4, 129.6 and 172.8 ks. The results show that increase in aging times results in continuous increase of size and area fraction of gamma prime (γ′) particles. The higher solutionizing temperature leads to the lower area fraction and smaller size of gamma prime particles. Regarding the microstructure characteristics, the most proper reheat treatment condition should be solutionizing at temperature of 1,438 K for 14.4 ks and aging at temperature of 1,133 K for 172.8 ks, which provides the highest area fraction of gamma prime particles in proper size.

Experimental determination of E-pH diagrams for 316L stainless steel in air-saturated aqueous solutions containing 0-5,000 ppm of chloride using a potentiodynamic method

Purpose – Aims to investigate the effect of chlorine on corrosion behaviours of stainless steels.Design/methodology/approach – Very complicated thermodynamic calculations are needed to establish the E‐pH diagrams of commercial alloys, because they comprise of many elements. To avoid these complex calculations and facilitate corrosion prevention of AISI 316L stainless steel, the potentiodynamic method was used to construct the E‐pH diagram. The polarization curves were carefully experimented at the scan rate of 0.1 mV/s. The experimental conditions were aqueous solutions saturated with air (oxygen concentration 7.8‐8.5 ppm) containing chloride 0, 50, 500 and 5,000 ppm, pH 2, 4, 6, 8, 10 and 12, and at 25°C. The transpassive or pitting potential, the protection potential, the primary passive potential and the corrosion potential were determined from the polarization curves and plotted with respect to the pH of the solution. The ions in solution were investigated by qualitative chemical analysis and stated i...

An Improvement in Tarnish and Corrosion Resistance of 94Ag-4Zn-Cu Alloys with Sn Addition

The tarnish and corrosion resistance of 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy compared with Ag-5.95Cu alloy were investigated. The tarnish results show that the DE* value of Ag-5.95Cu alloy is higher than those of 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy due to the sulfide formations such as Ag2S, Cu2S and CuS. The DE* value significantly decreases with increasing tin content. This is attributed to the matrix enrichment of tin which protects the sulfur reaction on surface. From corrosion test results, 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy provide the noble shift in the corrosion potentials and pitting potentials but the negative shift in the corrosion current density compared with Ag-5.95Cu alloy. Corrosion rate of 94Ag-4Zn-Cu-Sn alloys and Ag-5.89Sn alloy decrease markedly compared with Ag-5.95Cu alloy because it depends on the alloying elements and the microstructural changes. Due to high solubility of zinc and tin, the microstructures of tin-rich alloys consist of the higher portion of single phase and less eutectic structure than that of Ag-5.95Cu alloy.

Effects of Copper (Cu), Indium (In), Tin (Sn), Antimony (Sb) and Zinc (Zn) in Sterling Silver Alloys on Mechanical Properties Improvement

Abstract The jewelry industry has been searching for sterling silver with high hardness values as it is believed by the industry to have direct effect on scratch resistance as well as improved mechanical properties such as “spring” property. Even though the direct relationship between hardness and how springy the jewelry articles are could not be linked. However it is a conventional wisdom of goldsmith to observe the usual trend for alloys with higher hardness to be more elastic. Moreover it is a common and cheap test to apply to a jewelry article. Therefore it is common find jewelry manufacturers to discuss and compare hardness values. The current commercial alloys could reach 140–170 HV after careful heat treatments and/or mechanical treatments. In our research, we focused on a systematic alloy development study of ultra-hard silver alloy systems. These systems could reach high hardness values in as-cast state without subsequent post-processing treatments. The complex system comprises of silver content in the range of 80.0–92.5 wt.- % with supersaturated matrix. Twenty-two alloying element candidates and fifteen supersaturated ternary and quaternary compositions were investigated. Vickers hardness tests were performed on prototyped alloy ingots. X-ray diffraction analysis was used to measure the lattice distortion as well as to identify second or third phases in the alloys, if they existed. Metallography and electron microscopes were used to identify the microstructures. Seven developed alloy systems were presented in this paper as the investigation led to the discovery of a quaternary ductile alloy system with the hardness value of 142 HV as cast. The microstructure of the quaternary alloy was mostly single phase and these preliminary results suggested the alloy could be a good candidate for jewelry application.

Effect of Co Addition to Heat- Treated P/M 316L Stainless Steel on α′-Martensite Formation and Mechanical Properties

Abstract The effect of various Co additions to P/M 316L stainless steel on α ’-martensite formation and mechanical properties (bending strength and hardness) were investigated. Powder mixtures were compacted using a single action press at 498 MPa and sintered at 1,300°C for 30 min under hydrogen atmosphere, followed by heat-treating in air at 800°C and 900°C for 25 h, 50 h, 75 h, and 100 h, respectively. It is found that oxide formation in closed pores at high temperatures can induce the α ’-martensite formation and reduce the area fraction of porosities, resulting in higher hardness of the heat-treated specimens than that of the sintered specimens. The Co additions can also reduce the amount of α ’-martensite. Long-term heating results in a slight decrease of bending strength due to high connected oxide formation in the internal pores.

Characterization of Eutectic Sn-Cu Solder Alloy Properties Improved by Additions of Ni, Co and In

Abstract Recently, the cost factor of raw materials has emerged continuous improvement of tin-copper solder alloys. This work aims to improve the microstructural and mechanical properties of the eutectic tin-copper solder alloys by addition of nickel, cobalt and indium. The temperature profile of reflow soldering in surface-mount assembly was simulated to produce realistic joint microstructures. Key properties for electronic soldering were characterized, i.e. melting behaviour, solidification beha-viour, wettability, electrical resistivity, microstructures and mechanical properties. The results showed that addition of Ni, Co and In can effectively improve the mechanical properties of Sn-0.7Cu solder alloy i.e. microhardness and shear strength by providing preferred interfacial morphology and formation of intermetallic compound precipitates. The new solder alloys also showed reduced liquidus and solidus temperatures whereas no significant degradation of other key properties were seen.

Effect of Temperature Dropping During Reheat Treatments on GTD-111 Microstructure

Abstract The general standard reheat treatment condition to refurbishment long-term serviced turbine blades, which are made of cast nickel based superalloy, GTD-111, is usually following by solution treatment at 1438 K, 1458 K and 1478 K for 10.8 to 14.4 ks, combination with primary aging at 1328 K for 3.6 ks, and secondary aging at 1118 K for 86.4 ks. However, in practical reheat treatment process, the change of temperature during any heat treating could occur accidentally any time. To simulate this effect, the droppings of temperatures during solution treatment were chosen and carried out to temperature level of 1118 K then heating again to the solution temperature levels. The temperature droppings (according to various programs) were performed during solution treatment. From the results, it was found that effect of temperature dropping during solution treatment greatly influenced the final rejuvenated microstructures.