Low-temperature Heat-affected Zone Research Articles

Decomposition mechanisms of continuously cooled bainitic rail in the critical heat-affected zone of a flash-butt welded joints

Abstract The joining process of bainitic rails is significant in terms of their industrialization in high-speed and heavy-loaded railways. This paper demonstrates the microstructure changes in the critical zone of the welded joint, which is responsible for the greatest deterioration in mechanical properties. Extensive progress in the decomposition of the retained austenite and bainitic ferrite occurs in the low-temperature heat-affected zone (LTHAZ) of the flash-butt welded joint of low-carbon bainitic rail. The decomposition products of the retained austenite were mainly a mixture of cementite and ferrite. The cementite was mainly precipitated at the boundary of the bainitic ferrite laths, which indicates lower thermal stability of the filmy austenite. Moreover, it was found that a part of the refined blocky retained austenite was decomposed into the ferrite and nanometric cementite, while another remained in the structure. The decomposition mechanisms are rather heterogeneous with varying degrees of decomposition. A relatively high proportion of dislocations and stress fields prove the occurrence of residual stresses formed during the welding process.

Microstructure-based approach to the evaluation of welded joints of bainitic rails designed for high-speed railways

Bainitic rails, due to their wear conditions, favorable mechanical properties and mechanical response under contact fatigue, have been widely investigated in recent years. In this research, the microstructure of a bainitic rail was analyzed after joining using the flash butt welding method. To date, the structural changes of bainitic rails after welding processes that directly affect their mechanical and functional properties have not been analyzed. The microstructure of the base material consisted of degenerated upper bainite, retained austenite and a negligible amount of cementite. No unfavorable martensite was found in the Fusion Zone (FZ), and its microstructure consisted of coarse degenerated bainite and retained austenite with a blocky and film-like morphology. In the High Temperature Heat-Affected Zone (HTHAZ), the bainitic structure was refined in comparison to the FZ, which resulted in a slight increase in hardness. In contrast, a drop in hardness was found in the HTHAZ. The microstructure of this zone consisted of bainite, polygonal ferrite, retained austenite and M/A constituents. The highest decrease in hardness was found in the Low Temperature Heat-Affected Zone (LTHAZ). In this zone, the retained austenite was completely decomposed into a mixture of ferrite and cementite particles.

Microstructure and Properties of Lean Duplex Stainless Steel UNS S32101 Welded Joint by Hot Wire TIG Welding

Lean duplex stainless steel UNS S32101 was welded by hot wire TIG welding and traditional TIG welding, and nice formed welds with no visible defects were obtained. Metallographic microstructure, phase ratio, mechanical properties and pitting corrosion resistance property of the welded joints were tested. Microstructure analysis showed that the hot wire TIG and traditional TIG welded joints had similar microstructures. The welded metal was composed of ferrite, grain boundary austenite (GBA), Widmanstatten austenite (WA), intragranular austenite (IGA). The high temperature heat affected zone (HTHAZ) consisted of ferrite, GBA and IGA. The low temperature heat affected zone (LTHAZ) had semblable microstructures with base metal. The phase ratio of welded joints was measured by manual point count method. The ferrite/austenite ratio of hot wire TIG welded metal was close to 1:1. The welded joints of hot wire TIG and traditional TIG had same hardness distribution. The hardness of hot wire TIG with an average value of 291 HV10 was a little higher than that of traditional TIG with an average value of 280 HV10. Charpy impact test at -40°C showed that the impact values of hot wire TIG and traditional TIG welded joints meet the standard requirements. The results of chemical weight loss method showed that the corrosion rate of hot wire TIG welded joint was less than 10 mdd. Potentiodynamic polarization method results showed that the pitting corrosion resistance of hot wire TIG welded joints was slightly lower than that of base metal. Solid solution treatment significantly increased the pitting corrosion resistance of welded joints and base metal. The hot wire TIG and traditional TIG had similar microstructure and properties under the same arc power, however the welding speed of hot wire TIG was 1.5 times higher than that of traditional TIG and the welding efficiency was greatly improved.

Galvanic corrosion behaviors of the low-carbon ferritic stainless steel ERW (electrical resistance welding) joint in the simulated seawater

PurposeThe purpose of this paper is to focus on the galvanic corrosion behaviors of the low-carbon ferritic stainless steel electrical resistance welding (ERW) joint in the simulated seawater.Design/methodology/approachThe electrochemical methods such as electrochemical noise, galvanic current and TOEFL polarization curve tests were used to study the galvanic corrosion behaviors of ERW joints of low-carbon ferritic stainless steel in simulated seawater. On this basis, a reliable accelerated corrosion method was developed.FindingsThe corrosion type of the base metal and joint is the typical local corrosion. The order of corrosion resistance from strong to weak is: weld zone > base metal > low-temperature heat-affected zone (HAZ) > high-temperature HAZ. The results of constant current-constant potential accelerated corrosion test show that after constant current-constant potential accelerated corrosion, the joints present a typical groove corrosion pattern. The groove initiating area is located in the HAZ, and the corrosion degree in the weld zone is relatively light, which is consistent with the electrochemical test results.Originality/valueThis paper has clarified the galvanic corrosion behaviors of low-carbon ferritic stainless steel ERW joints. Moreover, a reliable accelerated corrosion method for the low-carbon ferritic stainless steel ERW joint has been developed.

Welding Capabilities of Nanostructured Carbide-Free Bainite: Review of Welding Methods, Materials, Problems, and Perspectives

This article presents state-of-the-art welding methods and the weldability aspect of steels, particularly high-carbon nanobainitic (NB) steels, without carbide precipitates (CFB—carbide-free bainite). On the basis of research conducted to date, all welding methods with parameters and weld metals for NB CFB are presented. It was found that the process parameters significantly affected the mechanical properties of the welds, which were determined primarily by the properties of the low-temperature heat-affected zone. The microstructures of welded joints in the heat-affected and fusion zones are also described. The general requirements for welding processes, as well as problems and perspectives for further research, are presented.

Effect of Rolling Conditions on Microstructure and Mechanical Properties of Thick Steel Plates for Offshore Platforms

In this study, three kinds of steels are manufactured by varying the rolling conditions, and their microstructures are analyzed. Tensile and Charpy impact tests are performed at room temperature to investigate the correlation between microstructure and mechanical properties. In addition, heat affected zone(HAZ) specimens are fabricated through the simulation of the welding process, and the HAZ microstructure is analyzed. The Charpy impact test of the HAZ specimens is performed at -40 °C to investigate the low temperature HAZ toughness. The main microstructures of steels are quasi-polygonal ferrite and pearlite with fine grains. Because coarse granular bainite forms with an increasing finish rolling temperature, the strength decreases and elongation increases. In the steel with the lowest reduction ratio, coarse granular bainite forms. In the HAZ specimens, fine acicular ferrites are the main features of the microstructure. The volume fraction of coarse bainitic ferrite and granular bainite increases with an increasing finish rolling temperature. The Charpy impact energy at -40 °C decreases with an increasing volume fraction of bainitic ferrite and granular bainite. In the HAZ specimen with the lowest reduction ratio, coarse bainitic ferrite and granular bainite forms and the Charpy impact energy at -40 °C is the lowest.

Effect of Welding Heat Input on Microstructure and Toughness of Heated‐Affected Zone in Steel Plate with Mg Deoxidation

Effect of heat input on the microstructure and toughness of heat‐affected zone (HAZ) in steel plate with Mg deoxidation has been investigated through welding thermal cycle simulation method. With the heat input increasing from 100–400 kJ cm−1, the average prior‐austenite grain (PAG) size is increased from 43 to 56 and then to 173 µm, and the HAZ microstructure is changed from fine bainite lath to relative larger bainite lath and then to grain boundary ferrite (GBF) and intragranular acicular ferrite (IAF). With the heat input increasing up to 400 kJ cm−1, a great amount of IAF is nucleated by the MgO–MnS inclusions with the size about 2 µm. The Charpy impact values at −40 °C of HAZ are larger than 140 J, and excellent low temperature HAZ toughness is obtained for the 68 mm thickness steel plate with Mg deoxidation with the heat input in the range from 100–400 kJ cm−1.

Microstructural characterization and electron backscatter diffraction analysis across the welded interface of duplex stainless steel

The microstructural evolution, orientation relationships, boundary characteristics, grain type, local deformation, and microhardness across the welded interface of duplex stainless steel (DSS) were investigated. The DSS welded joint consisted of four typical zones: base metal (BM), low-temperature heat-affected zone (LTHAZ), high-temperature heat-affected zone (HTHAZ), and weld metal (WM). The apparent microstructural changes in the HTHAZ and LTHAZ were secondary austenite and Cr2N precipitation. A modified cooperative precipitation mechanism of secondary austenite and Cr2N at the interface was proposed. Furthermore, the ferrite in both the HTHAZ and LTHAZ maintained the same distribution as the ferrite texture in the BM, while this ferrite texture disappeared completely in the WM. Different austenite grains in the different zones exhibited different orientation relationships with the ferrite matrix. Special grain boundaries were mainly distributed between the austenite grains, while the ferrite grains primarily contained random grain boundaries. Austenite twins constituted the largest proportion of the special boundaries. The special austenite grain boundaries in the BM and LTHAZ were higher in relative frequency than those in the HTHAZ and WM. The ferrite grains in the HTHAZ and WM mainly consisted of substructured grains. In the BM, the recrystallization degree of ferrite was significantly lower than that of austenite grains. The local deformations were mainly generated in the grain boundaries and within the deformed grains. The HTHAZ exhibited the highest hardness, while the BM had the lowest hardness. The LTHAZ had a lower hardness than the HTHAZ and higher hardness than the BM.

Effect of beam offset on microstructure and mechanical properties of dissimilar electron beam welded high temperature titanium alloys

Electron beam welding was used to join dissimilar Ti55 to TA15 titanium alloys. The aim of this study was to investigate the effect of the beam offset on microstructure and mechanical properties of the welded joints. Microstructural characterization was investigated by optical microscopy, scanning electron microscopy and X-ray diffraction. Tensile test was conducted at room temperature and 550°C. Results indicated that the fusion zone consisted of martensite α′ and acicular α. Heat affected zone (HAZ) in both Ti55 and TA15 sides consisted of two regions; Low temperature HAZ (LT-HAZ) was composed of primary α, secondary α and prior β and high temperature HAZ (HT-HAZ) was composed of martensite α′ and acicular α. The hardness of fusion zone was lower than HAZ of both Ti55 and TA15 alloy. Energy-dispersive spectroscopy was applied to display microstructure evolution inside the fusion zone. Tensile results showed that offsetting the beam position towards Ti55 alloy led to higher values of joint strength. Beam offsetting towards TA15 alloy lowered the hardness and strength of welded joints. Increasing the beam offset to the values >0.3mm towards Ti55 or TA15 resulted in premature fracture due to increasing the possibility of porosity formation inside the weld metal.

Investigation on Microstructure and Impact Toughness of Different Zones in Duplex Stainless Steel Welding Joint

This paper investigated on microstructure and impact toughness of different zones in duplex stainless steel welding joint. High-temperature heat-affected zone (HTHAZ) contained coarse ferrite grains and secondary precipitates such as secondary austenite, Cr2N, and sigma. Intergranular secondary austenite was prone to precipitation in low-temperature heat-affected zone (LTHAZ). Both in weld metal (WM) and in HTHAZ, the austenite consisted of different primary and secondary austenite. The ferrite grains in base metal (BM) presented typical rolling texture, while the austenite grains showed random orientation. Both in the HTHAZ and in the LTHAZ, the ferrite grains maintained same texture as the ferrite in the BM. The secondary austenite had higher Ni but lower Cr and Mo than the primary austenite. Furthermore, the WM exhibited the highest toughness because of sufficient ductile austenite and unapparent ferrite texture. The HTHAZ had the lowest toughness because of insufficient austenite formation in addition to brittle sigma and Cr2N precipitation. The LTHAZ toughness was higher than the BM due to secondary austenite precipitation. In addition, the WM fracture was dominated by the dimple, while the cleavage was main fracture mode of the HTHAZ. Both BM and LTHAZ exhibited a mixed fracture mode of the dimple and quasi-cleavage.

Effect of laser beam offset on microstructure and mechanical properties of pulsed laser welded BTi-6431S/TA15 dissimilar titanium alloys

Laser beam welding was used to weld dissimilar joints in BTi-6431S/TA15 titanium alloys. The effect of laser beam offset on microstructural characterizations and mechanical properties of the joints were investigated. Microstructural evolution of the joints was characterized by optical microscopy (OM) and X-ray diffraction (XRD). Tensile testing was conducted at room temperature and at 550 °C. The results demonstrated that with the exception of some porosity, a good quality joint could be achieved. Martensite α’ and acicular α structures were present in the fusion zone (FZ). The amount of martensite α’ present with the −0.2 mm beam offset was less than that with the 0.2 mm beam offset. Acicular α and martensite α’ transformations occurred in the high temperature heat-affected zone (HT-HAZ) of both the BTi-6431S and TA15 alloys. In the low-temperature heat-affected zone (LT-HAZ), the BTi-6431S and TA15 alloy microstructures exhibited a mixture of secondary α, primary α, and prior β phases. The microhardness values in the FZ followed the order: −0.2 mm> 0 mm> 0.2 mm. Tensile testing at room temperature and at 550 °C resulted in fracture of the TA15 alloy base metal. The fracture morphology exhibited a ductile dimple feature.

Characterization of Intergranular Corrosion in Heat-Affected Zone of Low Carbon 12Cr-Ni Ferritic Stainless Steel

Intergranular corrosion in heat-affected zone of low carbon 12Cr-Ni ferritic stainless steel was investigated by the method of practice Z in ASTM A 763-93. The microstructure of welded joint was characterized using an optical microscope and a scanning electronic microscope. Severe intergranular corrosion (IGC) was only observed in low temperature heat-affected zone (LTHAZ) adjacent to base metals at the back of the welded joints in unstabilized steel. On the other hand, IGC was not observed in the welded joints of stabilized steels. According to the analysis of microstructure, the severe IGC is attributed to both the precipitated phase along the grain boundaries inducing the presence of chromium-depleted zone and the welding residual tensile stress promoting the corrosion. Therefore, stabilized elements addition such as Ti and Nb is required in order to avoid the IGC in welded joint of these steels. Relief residual tensile stress by post weld heat treatment would also be an effective method to avoid the IGC in the welded joints of these steels.

Plasma Arc Welding of Modified 12%Cr Stainless Steel

This article deals with the plasma arc welding properties of 6 mm thick modified X2CrNi12 stainless steel conforming to the grades EN 1.4003 and UNS S 41003 with carbon content below 0.015% to improve the weldability. The butt welds produced without filler metal and with AISI 316L austenitic type of consumable were subjected to tensile and bend tests as well as Charpy impact toughness testing. Examinations including fractography, metallography, chemical analysis of the weld metal, ferrite content, grain size, and hardness analyses were carried out. Sound plasma arc welded joints of modified 12 Cr revealed the microstructure-property relationship, such as high ferrite content (≥ 70%) resulting in ferrite grain coarsening mainly at the high temperature heat-affected zone (HTHAZ) has no adverse effect on tensile or bend properties, but has negative effects on low temperature toughness. Enhanced toughness was provided in case of the low temperature heat affected zone (LTHAZ) with finer grained microstructure. The increase at the weld metal hardness in Weld 2 is related to the austenitic type of filler used.

Residual stress and microstructure in welds of 13%Cr–4%Ni martensitic stainless steel

The objectives of the present study were to characterize the distribution of residual stress and the microstructure changes induced by welding in the heat-affected zone (HAZ) of 13%Cr–4%Ni used in hydraulic turbine fabrication to deduce best practices. To characterize residual stress after welding, hole-drilling, X-ray diffraction (XRD) and the contour method were used on a FCAW 6 passes UNS W41036 (410NiMo) weld deposited on UNS S41500 (13%Cr–4%Ni) stainless steel; the results were put side to side with the weld microstructure and hardness to assess their criticality. Transverse compression was found in and around the last bead of the weld by XRD and hole-drilling. Longitudinal compression stress was found in and around the last bead while longitudinal tension was found near the low-temperature HAZ. The contour method showed that despite high compression in the last bead, high longitudinal tension exists in the rest of the weld and just below the weld. The superposition of both residual stress distribution and results from microstructural characterization shows that in multipass welding of 13%Cr–4%Ni martensitic stainless steel, cracking susceptibility is higher in the weld than in the HAZ, let it be fatigue cracking, environment-assisted cracking or cold cracking during welding. Post-weld heat treatment proved to be very efficient in lowering residual tension found in the first bead and in lowering the hardness of the weld. These results underline the importance of following proper procedures when welding these steels; this being even more true when the assembly is loaded in fatigue.