Effect of solution annealing on weldability of aged alloy 800/25Cr–35Ni steel dissimilar welds

Abstract

Dissimilar welding of the aged alloy 800 and the as cast 25Cr–35Ni (wt-%) heat resistant steel was investigated. Microstructures, mechanical properties and weldability of the dissimilar welds were characterised using optical microscopy, scanning electron microscopy and transition electron microscopy equipped by energy dispersive X-ray spectroscopy, and Varestraint test. Since such dissimilar welding was susceptible to crack formation in the heat affected zone of the aged part, the effects of a preweld solution annealing, heat input, interpass temperature and type of filler metal on the weldability of two alloys were investigated. It was found that during the solution treatment, the precipitates produced in the service stage were decomposed and that TiC was formed. In addition, tensile strength and hardness were reduced, but ductility and toughness increased. It was concluded that the most important step to improve weldability and to reduce cracking susceptibility was solution annealing. A suitable annealing treatment was then proposed. The best weldability was found under conditions of solution annealing, low heat input, low interpass temperature and using Inconel 82 or 617 for filler metals.     

Ferrite to austenite reverse transformation process in B containing 9%Cr heat resistant steel HAZ

Abstract

Creep properties of the high Cr heat resistant steel welded joint can be improved by adding B due to prevention of the grain refinement in heat affected zone (HAZ). In the present study, phase transformation behaviour of the B steel HAZ has been investigated to understand suppression mechanism of the grain refinement. During reverse transformation, fine austenite was formed through diffusional transformation at the prior austenite grain boundary in the first stage, and then coarse austenite was formed at the same location of the original austenite. The volume fraction of the fine austenite increased with increasing perk temperature of the weld thermal cycle. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was observed during the reverse transformation by a confocal laser microscope. These results indicate that martensitic or displacive reverse transformation takes place during welding and it prevents the grain refinement in HAZ.     

Use of titanium and zirconium in centrifugally cast heat resistant steel

Abstract

Low carbon 25Cr–35Ni steel (HP type steel) modified with titanium and zirconium has been produced by centrifugal casting. The different phases present in the as cast and aged conditions were described by light optical and scanning electron microscopy with secondary electron imaging and energy dispersive spectroscopy. Results suggest that the use of titanium as a microalloying element reduces secondary precipitation during aging. Moreover, secondary precipitates in the microalloyed steel are much finer and more evenly distributed. On the other hand, zirconium oxides was found to be potential nucleation sites for primary titanium rich carbides contributing to an optimum distribution of these carbides in the tubes. These differences together with the higher stability of the titanium containing primary carbides are responsible for the improvement on ductility and creep resistance found in the present work.     

Suppressing type IV failure via modification of heat affected zone microstructures using high boron content in 9Cr heat resistant steel welded joints

Abstract

Creep rupture strength at 923 K and microstructural evolution of welded joints have been investigated for high boron–low nitrogen–9Cr heat resistant steels developed at the National Institute for Materials Science (Japan). Welded joints were prepared from plates containing 47–180 ppm boron using gas tungsten arc welding and Inconel type filler metal, and showed superior creep properties to those of welded joints of conventional high chromium steels such as P92 and P122. No type IV failure was observed in the boron steel welded joints. A large grained microstructure was observed in the heat affected zone heated to Ac ₃ (Ac ₃ HAZ) during welding, whereas the grains are refined at the same location in conventional steel welded joints. The simulated Ac ₃ HAZ structures of the boron steels have a creep life almost equal to that of the base metal. Large grained HAZ microstructures and stabilisation of M₂₃C₆ precipitates are probable reasons for suppression of type IV failure and improved creep resistance of the boron steel welded joints.     

Deformation and fracture behaviour of P91 steel weldments at high temperatures

Abstract

Microtensile specimen tests are conducted across similar welds of a P91 steel at 873 K. Specimens of different surface conditions are tested at two different loading rates, 0·2 and 0·5 mm min^?1, in order to study the effects of surface finish and loading rate on mechanical properties of P91 steel. Deformation behaviour is studied with an SEM on specimen side surfaces. Correlation between microstructural behaviour and mechanical properties is studied both in microtensile and standard tensile tests.     

BN type inclusions formed in high Cr ferritic heat resistant steel

Abstract

Scanning electron microscopy observations of fractured steel rod samples were made to clarify the formation and dissolution behaviour of boron nitrides in high Cr ferritic heat resistant steels during heat treatment at high temperature. A large number of coarse size BN type inclusions of 2 to 5 μm were distributed at the bottom of the dimples. They did not dissolve during annealing at 1150°C. However, they had a tendency to begin dissolving and reducing its particle size with time at 1200°C and dissolved completely in a short time at 1250°C. From the chemical analyses of boron and nitrogen in many high Cr heat resistant steels and SEM observations of their fractured samples, critical boron and nitrogen concentrations for the formation of coarse size BN type inclusions of 0·001%B and 0·015%N were found, and the solubility limit of BN was represented as log [%B]=?2·45log [%N]?6·81 at 1150°C.     

Effects of adding overspray powder on microstructures and mechanical properties of spray deposited 8009 aluminium alloy

Abstract

Under normal condition the cooling rate in a spray deposition process is too low to form 8009 aluminium alloy. In order to increase the cooling rate, overspray 8009 aluminium alloy powders were added. The effects of adding overspray powder on microstructures and mechanical properties of the spray deposited 8009, with and without addition of the overspray powders, were studied. It is shown that adding proper content of overspray powders significantly improves the mechanical properties of the spray deposited 8009 alloy. The mechanism involved in modification of the microstructures and the mechanical properties of the spray deposited 8009 alloy produced with the overspray powder addition approach was discussed.     

Development of boron white cast iron

Abstract

With boron substituting for carbon in cast iron composition and eutectic borides substituting for eutectic carbides in microstructure as the hard wear resistant phase, a new kind of wear resistant white cast iron has been developed. The microstructure and mechanical properties of this new white cast iron both in the as cast state and after appropriate heat treatments were studied. The results show that the as cast microstructure of the boron white cast iron comprises a dendritic matrix and interdendritic eutectics, and the eutectic compound is that of M₂B or M′_0˙9Cr_1˙1B_0˙9 type, where M represents Fe, Cr or Mn and M′ represents Fe or Mn. The morphology of the eutectic borides is much like that of carbide in high chromium white cast iron, but the hardness of boride is higher than that of carbide. The matrix in as cast microstructure comprises martensite and pearlite. After austenitising and quenching, the matrix mostly changes to lath type martensite and the eutectic borides remain unchanged. In addition, two different sizes of particles, with different forming processes during heat treatment, appear in the matrix. The boron white cast iron possesses higher hardness and toughness than conventional white cast iron and nickel hard white cast iron, and has a better balance between hardness and toughness than high chromium white cast iron.     

Coking of HP tubes in ethylene steam cracking plant and its mitigation

Abstract

Coke deposited on the inner wall of industrial HP cracker tubing during the cracking of ethane has been investigated in detail to elucidate the mechanism of the coking process. The outer wall of the deposit, adjacent to the metal tube, consists of filamentary coke. Lamellar coke occupies the central zone of the deposit, while the inner zone, adjacent to the reaction gases, is comprised of coke particles. The formation of the filamentary coke at the tube wall is closely related to composition of the tube material, but both the lamellar coke and the coke particles are derived from the gas phase reactions so bear no relation to the tube material. The difference in morphology between the lamellar and particulate types of coke is attributed to changes in the coking process. Possible ways to alleviate or prevent coking of the tube include reduction of the catalytic activity of the material at the inner wall of the tube and improvement of the ethylene steam cracking process.     

Direct reduction process using fines and with reduced CO2 emission

Abstract

A direct reduction process for refractory oxides and a life cycle analysis comparison with conventional pelletising and sintering process for chargechrome are presented. The new route has potential to reduce the environmental impact of the ferroalloy industry and to extend the use of nonrenewable resources by utilising waste that is not economically recoverable by conventional technologies; increasing metal recoveries; significantly reducing CO₂ emissions; reducing or eliminating the use of coke; and reducing electricity consumption during smelting.