Tempering of Widmanstätten ferrite in cooling steel castings

The formation of Widmanstätten ferrite is not the goal of a technological process. On the contrary, it should be eliminated in order to avoid a decrease in the ductility of the steel. A detailed study of this structural component, which has no practical use, is often assumed to be unnecessary. At the same time, coarse-grain Widmanstätten ferrite makes it possible to use the range of magnification of a light microscope more efficiently for studying the mechanism of a shift γ→α transformation. The present paper is devoted to the variation of the properties and structure of Widmanstätten ferrite in the process of self-induced tempering that occurs in cooling of cast specimens of steel 20L.     

Effect of microstructure on hydrogen‐induced internal friction in duplex stainless steel and its weld metal

Summary

Microdischarge phenomena are experimentally investigated in vacuum conditions. The experimental set‐up is based on a scanning tunnelling microscope (STM) basically consisting of an electric circuit for microdischarge tests composed of an electrode, the specimen, and a power source as well as a driver ('inchworm') for adjustment of the distance between the electrode and specimen. To make in situ observations of microdischarge phenomena, the experimental set‐up is installed in a scanning electron microscope (SEM) chamber fitted with a photomultiplier. When several hundred volts are applied to the gap between the electrode and specimen at a distance of several hundred nm, dielectric breakdown followed by microdischarge occurs. Light emission is simultaneously detected with the photomultiplier. The microdischarge found falls with an increasing current.     

Theoretical analysis of sensitisation in austenitic stainless steel weld metal.Study of low‐temperature sensitisation in austenitic stainless steel weld metal and its improvement by laser surface melting treatment (5th Report)

The results of modelling the processes of carbide formation in the metal of welded joints in 12Cr18Ni10Ti high-alloy steel, produced by manual arc welding with electrodes with different types of coatings – rutile (OK 61.30) and basic (TsL-11) – are presented. Intercrystalline corrosion can be suppressed by initiating the bonding of carbon in carbides by titanium and niobium transferred into the welded joint from the coating.     

Characteristics of reheat cracking in weld metal. Study of reheat cracking in metal active‐gas welding with flux‐cored wire stainless steel weld metal (1st Report)

The processes taking place during melting and solidification of the deposited powder Sormite alloy and the factors influencing the physical–mechanical properties of the component are analysed and it is attempted to formalize the functional relationships using regression analysis. The transition zone and the deposited layer are studied by metallographic and energy-dispersing analysis. The dependence of the variation of wear resistance on the surfacing speed is determined, and the hardness and the thickness of the deposited layer are analysed. It is shown that to ensure high wear resistance of the deposited layer, it is necessary to produce layers by multilayer surfacing.     

Microstructural changes in weld metal during isothermal process. Study of reheat cracking in flux‐cored arc welding stainless steel weld metal (5th report)

The soiling of the slag, spatter and the fume, etc., which come into contact with the steel sheet surface with welding, is cleaned making use of steel sphere shot material of large particle diameter, high projection pressure with strong peening processing (below, called strong peening cleaning). In this research, the cleaning state of the soiling with welding and improvement of fatigue strength of the hot galvanized welded joint was inspected, when the surface of a SM490A welded joint was cleaned with strong peening cleaning.

The following experimental results were obtained:

1. The fatigue limit of smooth base metal which received strong peening cleaning at about 320 MPa was remarkably high in comparison with smooth base metal at about 245 MPa.

2. The fatigue limit of a welded joint which received strong peening cleaning at about 300 MPa was remarkably high in comparison with a welded joint at about 170 MPa.

3. The strong peening cleaning was highly efficient and the cleaning state was satisfactory.

4. The cause of the remarkable rise of the fatigue limit (300 MPa) of the welded joint which received strong peening cleaning was because the fatigue limit (about 170 MPa, 57%) of the welded joint was improved (about 130 MPa, 43%) with peening cleaning. It was considered that improvement effects were: a rise (about 68 MPa, 23%) of hardness of the weld toe; relief (about 43 MPa, 14%) of stress concentration; increase (about 136 MPa, 45%) of compressive residual stress; and the decrease (about ? 96 MPa, ? 32%) by increase of surface roughness.

5. The fatigue strength of the hot galvanized welded joint decreased remarkably. This was thought to be due to the decrease (about HV40) of hardness of the surface, the decrease (about 188 MPa) of the compressive residual stress and the influence of many factors which accompanied hot galvanizing.

     

Unified rationalization of the Pitsch and T–H orientation relationships between Widmanstätten cementite and austenite

This work demonstrates that understanding the habit planes of cementite plates is an important step to gain an insight into the irrational orientation relationships (ORs) between Widmanstätten cementite and austenite, i.e. the Pitsch and T–H ORs. A reproducible irrational OR in this system is attributed to a unique correspondence between the OR and the habit plane, under the condition that the habit plane is a quasi-invariant plane. The OR is constrained by two parallelism conditions: parallelism of [0 1 0]_C and 〈1 1 0〉_A; parallelism of a group of Δg’s in the zone axis of [0 1 0]_C. The calculated ORs and habit planes are fully consistent with the experimental results from the literature.     

Evaluation of high‐temperature ductility in weld metal. Study of reheat cracking in metal active‐gas welding with flux‐cored wire stainless steel weld metal (2nd Report)

The special features of the weldability of high-strength low-carbon microalloyed steels are investigated.     

Electrochemical corrosion behavior of LDX 2101® duplex stainless steel in a fluoride-containing environment

The effect of fluoride on the electrochemical corrosion behavior of an LDX 2101® duplex stainless steel (DSS) was studied. Open-circuit potential (E_OC) and electrochemical impedance spectroscopy (EIS) measurements were carried out in artificial saliva and with the addition of fluoride (1 wt% NaF). The electrochemical corrosion behavior of the AISI 316L austenitic stainless steel (SS) was also evaluated for comparison. Both open-circuit potential and EIS results indicate that DSS and austenitic SS undergo spontaneous passivation due to spontaneously formed oxide film passivating the metallic surface, in the simulated aggressive environments. However, LDX 2101® exhibits superior corrosion resistance as compared with AISI 316L, and this improvement is ascribed to the formation of a passive film which shows a higher protective effect than the one formed on AISI 316L.     

Ductile—brittle transition of steel electron beam weld metal in small punch test

Abstract

Toughness evaluation of narrow high energy beam welds via the conventional Charpy V notch (CVN) impact test is not always successful because cracks often deviate from the fusion zone into the heat affected or unaffected base metal zones. The small punch (SP) test was applied to electron beam welds of a 490 MPa strength steel to evaluate the toughness by measuring the ductile-brittle transition temperature (DBTT). The steel was electron beam welded using various heat inputs and the welds were examined via CVN and SP tests at 40-298 K. The SP DBTTs obtained from the SP energy showed a linear correlation with the CVN DBTTs. The load-deflection curves during the SP tests and the fractography of SP tested specimens indicated the existence of ductile-brittle transitions. The results suggest that the SP test can be used to predict the DBTTs of electron beam welds, which cannot be accomplished via the conventional CVN impact test.     

Microcracking in multipass weld metal of alloy 690 Part 1 – Microcracking susceptibility in reheated weld metal

Abstract

Microcracking behaviour in the gas tungsten arc multipass weld metal of alloy 690 was investigated. The majority of microcracks occurred within about 300 μm from the fusion line of the subsequent weld bead and propagated along the solidification boundaries in the multipass weld metal. The morphology of the crack surface indicated the characteristic texture of ductility dip cracking. The microcracking susceptibility of the reheated weld metal was evaluated via the spot Varestraint test using three different filler metals having varying contents of impurity elements such as P and S. Microcracking occurring in the spot Varestraint tests consisted predominantly of ductility dip cracking, with a small amount of liquation cracking. The ductility dip cracking temperature range was about 1350–1600 K in the weld metal FF1, and narrowed in the order of weld metals FF1>FF3>FF5. The ductility dip cracking susceptibility was reduced with decreasing contents of impurity elements in the filler metal. It was concluded that the amount of (P + S) in the filler metal should be reduced as much as possible (to about 30 ppm in total) to suppress microcracking in the multipass weldment.     

Microcracking in multipass weld metal of alloy 690 Part 2 – Microcracking mechanism in reheated weld metal

Abstract

To elucidate the microcracking (ductility dip cracking) mechanism in the multipass weld metal of alloy 690, the hot ductility of the reheated weld metal was evaluated using three different filler metals with varying contents of impurity elements such as P and S. Hot ductility of the weld metal decreased at temperatures over 1400 K, and the weld metal containing a low quantity of impurity elements showed much higher ductility than that containing a high quantity of impurity elements. Local deformability at high temperature of the alloy 690 reheated weld metal was compared with that of Invar alloy. Grain boundary sliding in alloy 690 occurred not in the intermediate temperature range (800–1000 K), where grain boundary sliding was activated in Invar alloy, but at high temperatures just below the melting temperature of alloy 690. The computer simulation of microsegregation suggested that the deterioration of hot ductility is caused by the grain boundary segregation of impurity elements during the multiple thermal cycling. The ductility dip cracking in the reheated weld metal resulted predominantly from the embrittlement of grain boundaries due to the imbalance between intergranular strength and intragranular strength at high temperature.     

Correlation between the crystallography and morphology of proeutectoid Widmanstätten cementite precipitates

Electron backscatter diffraction was used in conjunction with deep etching to examine the relationship between the crystallography and three-dimensional morphology of cementite precipitates in an Fe–1.34 wt% C–13.1 wt% Mn steel. Orientation relationships (ORs) between more than 200 proeutectoid Widmanstätten cementite precipitates and the surrounding austenite were determined to be on or very near either the well-known Pitsch OR or the Farooque–Edmonds OR. Scanning electron microscopy of the same specimens after deep etching was used to determine the three-dimensional morphology of each of the precipitates for which an OR was determined. These precipitates could be identified as either monolithic plates or conglomerates of laths. Results show that monolithic plates consistently exhibit the Pitsch OR and conglomerates of laths have the Farooque–Edmonds OR, indicating that the precipitate morphologies are dictated by their orientation relationships.     

Electrolytically descaling behavior of duplex stainless steel in neutral solution under dynamic conditions北大核心CSCD

Electrolyticall descaling behavior of 2205 duplex stainless steel in a neutral solution under dynamic conditions was investigated with rotating disk electrode. The results showed that, with the increasing movement speed of the test specimen from 0 to 30 m/min, the weight loss value of the specimen and the descaling efficiency of pulse electrolysis decreased gradually, while pits formed on the specimen surface enlarged. The descaling efficiency and pit size did not change obviously when the movement speed over 30 m/min. The descaling rate decreased rapidly with the increase of electrolysis time and then became stable gradually. Based on the characteristics of nucleation, growth and escape of gas bubbles at the pores and cracks in the oxide scale on the specimen surface during electrolysis, the mechanism related with the effect of bubbles was discussed in the electrolytic descaling process under different movement speed conditions. © 2016, Corrosion Science and Protection Technology. All rights reserved.     

New methods of predicting dissimilar steel weld metal microstructures by Schaeffler Diagram

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