Novel method for quantitative assessment of slag detachability in austenitic stainless steels welds made by SMAW

Abstract

It is essential and important that slag which protects the molten metal from atmospheric contamination during shielded metal arc welding (SMAW) process gets removed as the weld metal gets solidified. During multipass welding in particular it is desirable that slag gets removed easily and totally before the next pass of welding. Otherwise it would lead to slag inclusions in the weld. Hence, the ease with which the slag gets detached from the weld termed as slag detachability is a vital parameter in deciding the weld quality. Till date, slag detachability has been assessed on qualitative terms only. Hence, it is necessary to develop test procedures and methodology for assessing the slag detachability in quantitative terms. In this work, the authors propose a novel slag detachability testing technique called slag detachability tester and a methodology for quantitative assessment of slag detachability. The test procedure and the methodology have been tested first on austenitic stainless steels. The slag on the weldment was removed by dropping varieties of hammers with different end shapes from a standard height with a custom designed drop weight tester. After the test, the adherent slag on the weldment was inspected and measured using infrared (IR) thermography. The acquired IR images were suitably processed using image processing software to identify the area of slag particles sticking on the weld. Testing parameters were standardised and using these data, an expression is proposed for assessing the slag detachability in quantitative terms.     

Long fatigue crack growth behaviour of ferritic steel weld metal

Abstract

Single edge notched weld joint specimens were tested at different stress levels to study the long fatigue crack growth behaviour of a ferritic steel (nuclear grade SA333 Grade 6 steel) weld metal at two stress ratios R of 0·1 and – 1. A two slope behaviour was noticed in the crack growth rate versus stress intensity factor range plots at both stress ratios. Different parameters were employed to generalise the load ratio effect on fatigue crack growth rate.     

Effect of aging on fatigue crack growth behaviour of Cu bearing HSLA-100 steel

Abstract

The effect of aging on fatigue crack growth rate (FCGR) of Cu bearing HSLA-100 steel has been studied. The steel was solution treated, water quenched and aged at various temperatures in the range of 350–700°C. The fatigue crack growth resistance of the steel decreased for the initial stages of aging from 350–500°C. Further aging up to 650°C resulted in an improvement in the crack growth resistance. Beyond 650°C, once again an inferior crack growth resistance was observed. This nature of variation of FCGR behaviour was similar to the trend portrayed by the strength properties with aging treatment. The results are related to the changes in the microstructural constituents owing to the aging treatment.     

Influence of Si,Ca and Ag addition on corrosion behaviour of new wrought Mg–Zn alloys

Abstract

The development of new wrought magnesium alloys for automotive industry has increased in recent years owing to their high potential as structural materials for low density and high strength/weight ratio demands. However, the poor mechanical properties and low corrosion resistance of the magnesium alloys have led to searching a new kind of magnesium alloys for better strength, ductility and high corrosion resistance. The main objective of the present research is to investigate the mechanical properties and the corrosion behaviour of new magnesium alloys, Mg–Zn–Ag (ZQ) and Mg–Zn–Si–Ca (ZS) alloys. The ZQ6X and ZS6X–YCa alloys were prepared by using hot extrusion method. Hardness AC and DC polarisation tests were carried out on the extruded rods, which contain different amounts of silver or silicon and calcium. The potential difference in air between different phases and the matrix was examined using scanning Kelvin probe force microscopy. The microstructure was examined using optical and electron microscopy (TEM and SEM), X-ray analysis and EDS. The results showed that the silver addition improved the mechanical properties but decreased the corrosion resistance. The addition of silicon and calcium also affected both mechanical properties and corrosion behaviour. These results can be explained by the effects of alloying elements on microstructure of Mg–Zn alloys such as grain size and precipitates caused by the change in precipitation and recrystallisation behaviour.     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Hot strength of creep resistant ferritic steels and relationship to creep rupture data

Abstract

Experimental data on the tensile strength of ferritic steels designed for prolonged service at elevated temperatures have been assessed as a function of many variables, including the testing temperature. The resulting model has been combined with other data on the intrinsic strength of pure ferritic iron and substitutional solute strengthening to show that there is a regime in the temperature range 780–845 K beyond which there is a rapid decline in the microstructural contribution to strength. This decline cannot be attributed to changes in microstructure, but possibly to the ability of dislocations to overcome obstacles with the help of thermal activation. There is evidence of an approximate relationship between the temperature dependence of hot tensile strength and creep rupture stress.     

Structural refinement of cast magnesium alloys

Abstract

The structure of cast magnesium alloys (grain size and precipitate morphology and size) affects the properties of the products and the scope for use of the alloys. The structure can be controlled by minor additions of inoculants, which are largely determined on the basis of the composition of the alloy concerned. The present paper reviews the scientific background of structural refinement by inoculation and its application to Mg–Zn, Mg–Al, and Mg–Al–Si alloys.     

New method for preparing composite containing nanometre diamond particles

Abstract

Different powder mixing methods were used to prepare Al-5Cu and Cu-10Sn wt- based composites containing nanometre diamond. A new method for obtaining ultrafine Cu, Sn particles and a good distribution of nanometre diamond aggregates was developed. The nanometre diamond aggregates were well dispersed in the Cu-10Sn matrix. However, the distribution of nanometre diamond in the Al-5Cu based composite was unacceptable. Owing to the dispersion of nanometre diamond, Cu-10Sn based composite possessed higher hardness and lower bend strength than Cu-10Sn alloy.     

Analysis of ridging in three ferritic stainless steel sheets

Abstract

In the present work, three types of ferritic stainless steel sheets AISI 430A (16%Cr), AISI 430E (16%Cr, 0·36%Nb) and AISI 434 (16%Cr, 1%Mo), known to display different ridging behaviours, were characterised in both microstructure and texture using optical metallography and electron backscatter diffraction techniques. It was concluded that surface microridging is a result of the differential plastic behaviour of {111}〈uvw〉 and {001}〈uvw〉 grain colonies existing in the sheet; severe ridging (undulations) is a consequence of through thickness texture and microstructure inhomogeneities that lead to grain buckling under internal compressive stresses.     

Investigation of cracks in original material of Cleuson-Dixence shaft

Abstract

After the big accident in the Cleuson-Dixence Hydropower Plant in December, 2000, several investigations were performed. This contribution summarises the efforts undertaken to clarify the mechanism of the catastrophic failure by the investigation of original material taken from the shaft near the zone of the accident. By extensive non-destructive testing, four plates were removed by flame cutting and tested in the laboratory again. From these results some specimens were taken for further investigations using light microscope and scanning electron microscope. It was found that the investigated cracks are caused by the hydrogen induced cold cracking mechanism after welding, which is also called delayed cracking. By applying a proper pre- and post-weld heat treatment, these cracks can be avoided and by a diligent use of non-destructive testing methods, the required quality of the welding can be assured.