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.     

Effect of carbon content on creep rupture strength and microstructure in heat affected zone of heat resistant ferritic steel: alleviation of decrease in creep rupture strength in heat affected zone of heat resistant ferritic steel

Abstract

The effect of the carbon content on the creep rupture strength and the microstructural change in weld heat affected zone (HAZ) during high temperature services was investigated in order to alleviate the decrease in the creep rupture strength in HAZ of heat resistant ferritic steels. The test specimens were prepared from 9Cr–3Co–3–W–V, Nb ferritic steel plates with carbon content ranging from 0˙005 to 0˙1%. After the simulated HAZ thermal cycle treatments at the peak temperature of 1273 K was applied on these specimens, creep rupture tests, aging tests and microstructural examinations were conducted. As a result, it was clarified that the creep rupture time of simulated HAZ became longer and the decrease in the creep rupture strength in HAZ was alleviated by decreasing the carbon content. Then the mechanism to explain the effect of the carbon content was discussed from a viewpoint of the growth of precipitates, such as M₂₃C₆ and MX, during long term heating.     

Observation of twinned martensite in weld heat affected zone of Grade 100 low carbon microalloyed steel

Abstract

The microstructural changes caused by a low nominal heat input of 0.5 kJ mm^-1 in the coarse grained heat affected zone (CGHAZ) of Grade 100 microalloyed steel were investigated. Microhardness measurements suggested that the CGHAZ was martensite of maximum theoretical hardness for the carbon content of the steel. The bulk of the CGHAZ was lath martensite containing none of the small and few of the intermediate sized Nb precipitates responsible for strength and grain size control in the steel plate. Twinned martensite was unexpectedly observed in local areas of the CGHAZ. The formation of twins, which are normally seen in steels with a higher level of carbon, is explained by a combination of the rapid heating rates, high peak temperatures, precipitate dissolution and dispersion, and rapid cooling rates.     

Softening characteristics in ultra-narrow gap GMA welded joints of ultra-fine grained steel

Abstract

When a 800 MPa grade ultra-fine grained steel with ferrite grains less than 1 μm and dispersed fine cementite is welded, fine ferrite grains are coarsened resulting in remarkable softening in the heat affected zone (HAZ). The peak temperature at an arbitrary location in HAZ during welding was calculated by heat conduction analysis and the effect of welding thermal history on the microstructure of the UFG steel HAZ was examined by microscopic observation. Softening as a result of ferrite grain coarsening was observed in the region where the peak temperature reaches between 920 and 1300 K for the ultra-fine grained steel with an Ac ₁ temperature of 980 K and Ac ₃ of 1150 K. The formation of martensite–austenite constituents started as a second phase above the Ac ₁ temperature and they curbed HAZ softening in the peak temperature range between 1000 and 1250 K.     

3D numerical study of effects of temperature field on sensitisation of Alloy 690 butt welds fabricated by gas tungsten arc welding and laser beam welding

Abstract

This study examines the effects of the temperature field on the sensitisation of Alloy 690 butt welds fabricated using the gas tungsten arc welding (GTAW) method and the laser beam welding (LBW) method respectively. The welding thermal cycles of the two welding methods are simulated using ANSYS software based upon a moving heat source model. The validity of the numerical model is confirmed by comparing the simulation results with the corresponding experimental findings. Agreement is found between the numerical results for the temperature field and the experimental temperature measurements. In addition, it is shown that the LBW weldment experiences a more rapid heating and cooling effect than the GTAW weldment, and therefore has both a smaller heat affected zone and a narrower sensitisation region. Thus, the validity and general applicability of the thermal welding model are confirmed.     

Hardness prediction models based on HAZ simulation for in-service welded pipeline steels

Abstract

Welding onto pressurised pipelines that contain flammable fluid to facilitate repairs or branch connections, is a critical procedure with considerable risk to personnel and infrastructure. Limiting the heat input is obviously an important consideration to prevent 'burn-through', but the potential for rapid cooling of the weldment increases its susceptibility to hydrogen assisted cold cracking (HACC). Therefore, one of the most important factors for in-service welding procedure development relates to the increased risk of formation of hard, brittle microstructures in the grain coarsened heat affected zone (GCHAZ) of the weld, microstructures that increase the risk of HACC. The present work has been successful in utilising heat treatment simulations to derive two new hardness prediction models that more accurately predict hardness in the GCHAZ for typical in-service welding applications than a commercially adopted and widely used hardness prediction algorithm. Although it is acknowledged that further work is required to validate the models for a wider range of in-service welding conditions, the performance of the models demonstrates their potential for developing improved in-service welding procedures.     

Influence of GMAW-P current waveforms on heat input and weld bead shape

Abstract

Manufacturers use different, proprietary pulse current programming algorithms in their synergic 'one knob control' pulsed gas metal arc welding (GMAW-P) power supplies. Since the different pulse waveforms produced by these supplies can produce somewhat different welding characteristics, inconsistent results can be obtained when transferring welding procedures between different GMAW-P power supplies. A primary objective of this work was to characterise the differences in welding heat input and weld bead shape that could be produced by the pulsed current waveforms from four different commercial supplies. To eliminate the affects of subtle differences in electrical characteristics and to ensure that the exact shape of all waveforms was fully known, the comparison was also done by simulating the waveforms generated by three of the four power supplies on the fourth supply, which was equipped with waveform programming capability. Then, the four waveforms were used to create 'bead on plate' welds over a range of wire feed speed settings, and corresponding heat inputs were calculated from current and voltage samples recorded by a computer data acquisition system during welding. Welds were also done at the same wire feed speed setting using a constant voltage supply. All welds were then cross-sectioned for penetration and dilution measurements. In general, all of the waveforms produced good metal transfer and weld quality. However, the heat input and beads shapes varied noticeably. The heat inputs for the four pulse waveforms and constant voltage welds differed by as much as 150 J mm^?1 (17% of the maximum heat input) at the highest wire feed speed of 212 mm s^?1. The weld penetration differed by 1 mm (22% of the maximum penetration) at a wire feed speed of 169 mm s^?1 and the dilution differed a maximum value of 6.5% dilution (22% of the maximum dilution) at a wire feed speed of 169 mm s^?1 .     

Atmospheric corrosion of zinc Part 2: Marine atmospheres

Abstract

This paper summarises the results obtained in the MICAT (Mapa Ibero-Americano de Corrosão Atmosférica (Latin American Map of Atmospheric Corrosion)) atmospheric corrosion project (an Iberoamerican project on atmospheric corrosion, involving 14 countries and 75 atmospheric test sites) for zinc specimens exposed in the Latin American region for 1–4 years at 23 pure marine and 19 mixed marine (i.e. SO₂ polluted) sites. The atmospheres at these sites were characterised climatologically and in terms of their pollution levels so that their corrosivity could be expressed in accordance with ISO standards. Morphological and chemical characterisation of the zinc corrosion product layers (ZCPLs) was performed using scanning electron microscopy coupled with energy dispersive spectrometry (SEM–EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and soluble salt evaluation techniques. The results obtained show that the corrosion rate of zinc is a function of both the chloride pollution level and the time of wetness. Some synergetic effects of Cl^- and SO₂ were demonstrated, although some special types of behaviour were also observed.     

Application of discrete distribution point heat source model to simulate multipass weld thermal cycles in medium thick plates

Abstract

An analytical solution to the heat flow differential equation has been proposed, which allows an adequate and simplified approach to multipass welding conditions in the heat affected zone. The solution is based on the medium thick plate temperature distribution initially proposed by Rosenthal. The model applies a discrete distribution of point heat sources localised on any point of the plate that is being welded. This approach allows changing the position of the heat sources in the groove from one pass to another, reproducing multipass welding conditions. Actual thermal cycles of multipass welding of AISI 304 and 2304 stainless steels were recorded and compared with simulated thermal cycles, which verified the agreement between the simulated and the actual HAZ thermal cycles. Microstructures of alloys UNS S32304 and UNS S32205 were reproduced in the Gleeble system using the calculated thermal cycles and their comparison with actual weld microstructures confirmed the utility of the proposed heat flow model for metallurgical weldability studies involving multipass welding.     

A two stage approach for the validation of welding and heat treatment models used in product development

Abstract

Many components used in the aerospace industry have complex shape and are manufactured from high strength materials. Performing large scale tests is costly and time consuming, therefore, simulation tools are needed to support an effective product development process. Using manufacturing simulations during product development requires a validated model of the material and manufacturing process. In this paper, a validation scheme is proposed for thermomechanical models of welding and post-weld heat treatment. The scheme was investigated by comparing simulations using shell elements with experimental results, which showed good agreement when predicting residual stresses after welding, but an overestimation of the out-of-plane deformations when simulating both welding and heat treatment. However, the simulations showed that the outof-plane deformation is strongly influenced by the initial geometry. It can be concluded that the simulation model is adequately accurate to be used in concept evaluation.     

Microstructural characterisation of stir zone containing residual ferrite in friction stir welded 304 austenitic stainless steel

Abstract

Friction stir welding was applied to a 2 mm thick 304 austenitic stainless steel plate. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively, which are typically observed in friction stir welds in aluminium alloys. The hardness of the SZ was higher than that of the base material and the maximum hardness was observed at the TMAZ. The higher hardness at the TMAZ was attributed to high densities of dislocations and subboundaries. Microstructural observations revealed that the ferrite was formed along grain boundaries of the austenite matrix in the advancing side of the SZ. It is suggested that the frictional heat due to stirring resulted in the phase transformation of austenite to ferrite and that upon rapid cooling the ferrite was retained in the SZ.     

Numerical modelling of 3D plastic flow and heat transfer during friction stir welding of stainless steel

Abstract

Three-dimensional (3D) viscoplastic flow and temperature field during friction stir welding (FSW) of 304 austenitic stainless steel were mathematically modelled. The equations of conservation of mass, momentum and energy were solved in three dimensions using spatially variable thermophysical properties using a methodology adapted from well established previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate and temperature dependent flow stress. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing of alloys. The computed results showed significant viscoplastic flow near the tool surface, and convective transport of heat was found to be an important mechanism of heat transfer. The computed temperature and velocity fields demonstrated strongly 3D nature of the transport of heat and mass indicating the need for 3D calculations. The computed temperature profiles agreed well with the corresponding experimentally measured values. The non-Newtonian viscosity for FSW of stainless steel was found to be of the same order of magnitude as that for the FSW of aluminium. Like FSW of aluminium, the viscosity was found to be a strong function of both strain rate and temperature, while strain rate was found to be the most dominant factor. A small region of recirculating plasticised material was found to be present near the tool pin. The size of this region was larger near the shoulder and smaller further away from it. Streamlines around the pin were influenced by the presence of the rotating shoulder, especially at higher elevations. Stream lines indicated that material was transported mainly around the pin in the retreating side.     

Effect of post-weld heat treatment on joint properties of friction welded joint between brass and low carbon steel

Abstract

This paper describes the effect of post-weld heat treatment (PWHT) on joint properties of copper–zinc alloy (brass) and low carbon steel friction welded joints. The as-welded joint obtained 100% joint efficiency and the brass base metal fracture without cracking at the weld interface, and had no intermetallic compound layer. The joint efficiency with PWHT decreased with increasing heating temperature and its holding time, and its scatter increased with those increasing parameters. When the joint was heat treated at 823 K for 360 ks, it did not achieve 100% joint efficiency and fractured between the weld interface and the brass base metal although it had no intermetallic compound. The cracking at the peripheral portion of the weld interface was generated through PWHT. The cracking was due to the dezincification and the embrittlement of the brass side during PWHT.     

Electrolyte and temperature effects on pitting corrosion of type 316LN stainless steels

Abstract

The influence of electrolyte composition and temperature on the pitting corrosion resistance of nitrogen bearing (0·015, 0·198, and 0·56%N) type 316L stainless steels has been investi gated. Anodic polarisation curves were determined in neutral chloride solution at room temperature and at elevated temperatures of 308, 318, and 333 K. Similar polarisation studies were also conducted at room temperature in 1N H₂ SO₄ and in an acidic chloride solution containing 1N H₂ SO₄ and 0·5M NaCl. The results show that the critical pitting potential E_pp in neutral chloride and acidic chloride media increases as the nitrogen content of the alloy increases, indicating that resistance to pitting increases with the addition of nitrogen. However, in 1N H₂ SO₄ solution, the transpassive potential was almost independent of the nitrogen content. As the temperature of the neutral chloride medium was increased, the E_pp values decreased, irrespective of nitrogen content. The decrease in E_pp is attributed not only to the temperature induced modification of the passive oxide film but also to chloride induced activity at the passive film/solution interface. Nevertheless, the alloy con taining 0·56% nitrogen showed better resistance to pitting at 333 K than did the alloy with 0·015% nitrogen at room temperature. SEM examination of the pitted specimens showed clear evidence of pitting for the alloy with 0·015% nitrogen, but insignificant pitting attack for the alloy with 0·56%N. However, the alloy with 0·56%N displayed some pitting attack when the experiments were conducted at 333 K.     

Effect of post-weld heat treatments on microstructure and mechanical properties of electron beam welded flow formed maraging steel weldment

Abstract

Seamless tubing of C-250 maraging steel manufactured by the flow forming technique was joined by the electron beam welding process. Various post-welding heat treatments were conducted to improve the overall mechanical properties of the welded tubing. For the 480°C/6 h/air cooling post-weld aging treated maraging steel, a significant increment of 11% reversion austenite was present in the weld metal. Only the tensile strength of this aging treated metal met the required specification while its percentage elongation reached only 50% of the specification, attaining only 35% of the strength of the parent metal. For the post-welded solution + aging treated maraging steel, only the yield strength met the specification. Moreover, a significant amount of reversion austenite pools was also present at the grain boundaries of the material located at the weld metal. Although the homogenisation treatment could improve the hardness of the weld metal, it failed to have the tensile strength of the steel met the specification.     

Effects of torch configuration and welding current on weld bead formation in high speed tandem pulsed gas metal arc welding of steel sheets

Abstract

Undercut and humping bead are the common defects that limit the maximum welding speed of tandem pulsed gas metal arc (GMA) welding. In order to increase the maximum welding speed, effects of the inclination angle, interwire distance and welding current ratio between the leading wire and trailing wire on bead formation in high speed welding are investigated. The undercut and humping bead is attributed to the irregular flow of molten metal towards the rear part of the weld pool. This irregular flow can be prevented by the trailing wire with a push angle from 5° to 13° , which provides an appropriate component of arc force in the welding direction. The irregular flow is also related to the distance between the leading wire and the trailing wire, and the flow becomes regular when the distance is in the range 9–12 mm. Moreover, the stabilisation of the bulge of the weld pool between the two wires, the presence of enough molten metal below the trailing arc, and the reduced velocity of molten metal flow towards the rear part of the weld pool, are essential to increase the maximum welding speed. These conditions can be obtained by adjusting the ratio of the leading arc current to the trailing arc current. A maximum welding speed as high as 4–4·5 m min^?1 is achieved by setting the current ratio to a value ranging from 0·31 to 0·5.     

Effects of latent heat of fusion on thermal processes in laser welding of aluminium alloys

Abstract

Based on the Green's function method, a mathematical model allowing for the latent heat of fusion and solidification is developed to describe the steady state, two-dimensional heat flow during welding of thin plates. It is demonstrated that the latent heat has a pronounced effect on shape and size of the weld pool and mushy zone. The thermal efficiency of base metal fusion by a line heat source η _t can exceed 0·4839 considerably if the latent heat is taken into account. It is shown that the known simplified approaches for considering the latent heat can introduce large errors into the estimation of η _t. The calculated and experimental weld pool shapes are compared.     

Influence of initial gap on weld expulsion in resistance spot welding of dual phase steel

Abstract

The weld expulsion is prone to occur and severely affects the nugget quality when the initial gap between dual phase (DP) steel sheets exist in resistance spot welding (RSW). To investigate the effect of initial gap on weld expulsion, a finite element model was developed to analyse the weld nugget formation process with different initial gaps for DP steels. An estimation method of expulsion occurrence based on the ratio of the nugget radius R_n and the contact radius R_c between sheets was proposed to get the critical initial gap without expulsion. The simulation and experimental results showed that the weld expulsion would not happen until the gap spacing reaches the critical value. The critical initial gap of DP steel is much smaller than that of low carbon steel. For both DP steel and low carbon steel, the critical initial gap would increase with the thickening of the steel sheet.     

Fracture resistance of simulated heat affected zone areas in HSLA structural steel

Abstract

The fracture toughness of some areas in the multi-pass heat affected zone (HAZ) of a high strength low alloy (HSLA) structural steel was analysed in a straightforward way using precracked, cylindrical specimens tested on a conventional tensile machine. The specimens were made from samples with a simulated HAZ microstructure; however, the size of the samples was restricted by the limitations of the Gleeble machine. The brittleness of the samples was an indication of the detrimental effect of welding on their toughness. The specimens were not large enough for a direct K_Ic measurement over a wide testing temperature range; it was necessary to modify the results. The low fracture toughness and the substantial shift of fracture transition temperatures suggest that welding of the investigated steel could be a delicate procedure.     

Influences of post-weld heat treatment on fatigue crack growth behaviour of strength mismatched HSLA steel welds

Abstract

Welding of high strength low alloy steels (HSLA) involves usage of low, even and high strength filler materials (electrodes) compared with the parent material depending on the application of the welded structures and the availability of the filler material. In the present investigation, the influences of post-weld heat treatment (PWHT) on fatigue crack growth behaviour of under matched (UM), equal matched (EM) and over matched (OM) weld metals has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. The Shielded Metal Arc Welding (SMAW) process has been used to fabricate the single 'V' butt joints. Centre Cracked Tension (CCT) specimens have been used to evaluate the fatigue crack growth behaviour of the welded joints. Fatigue crack growth experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading (R = 0). From this investigation, it has been found that the fatigue performance of over matched joints is superior compared to under matched and equal matched joints. Moreover, PWHT reduced the magnitude of the tensile residual stress field in the weld region and subsequently enhanced the fatigue performance of the joints irrespective of weld metal strength mismatch.