High Temperature Heat-affected Zone Research Articles

Formation of heterogeneous microstructure and its effect on corrosion properties of high-silicon stainless steel S38815 weldment

This study aims to clarify the formation of heterogeneous microstructure and its effect on corrosion properties of high-silicon stainless steel (high-Si SS) S38815 weldment. A series of tests about microstructure and electrochemical characteristics were performed, including the electron backscatter diffraction (EBSD) analysis, Mott-Schottky tests, electrochemical impedance spectroscopy (EIS) tests, potentiodynamic and potentiostatic polarization tests. The formation mechanism of heterogeneous microstructure related to high-silicon and its correlation on corrosion was discussed extensively. The results show that the significantly deformed and recrystallised grains were observed at the low temperature heat affected zone (LT-HAZ) and high temperature heat affected zone (HT-HAZ) respectively. The metastable pitting and low impedance values were formed in the above region, despite their low donor density. The deterioration in corrosion resistance of S38815 weldment as a result of high-angle grain boundaries (HAGBs) and carbides is more pronounced than the beneficial impact of Cr and Si oxidation. This study provides valuable references to the manufacturing and enhancing the anti-corrosion properties of this high-Si SS.

Evaluation of methods used for simulation of heat-affected zones in duplex stainless steels

The weldability of duplex stainless steels partly depends on the ferritization of the high-temperature heat-affected zone (HT-HAZ). This area is rather narrow, and it can be challenging to visualize and determine its actual impact on the properties. To address this, various methods were applied to study the grain growth and austenite reformation in the HT-HAZ of the lean duplex grade UNS S32101. Thermo-mechanical Gleeble® simulations were conducted at 1360 °C with different holding times and cooling rates. Subsequently, the grain size and ferrite content were measured on polished and etched cross-sections. Bead-on-plate welds were performed on the same heat of 6-mm plate thickness using the gas tungsten arc welding (GTAW) process. The shielding gas was Ar + 0–8% N2 to illustrate the effect of nitrogen additions on the HT-HAZ morphology. The arc was either stationary, welding at one spot for 0.5–120 s, or travelling at different speeds to generate varying heat inputs and temperature gradients. The thermo-mechanical simulations approximated the results obtained by travelling arc welding and allowed for a more comprehensive investigation. Stationary arc welding was not suitable for HT-HAZ studies as it quickly caused nitrogen depletion and resulted in significantly higher ferrite contents compared to the travelling arc welds.

Characteristics of high-temperature heat-affected zones in duplex stainless steels

AbstractThe level of ferritization in the high temperature heat-affected zone (HT-HAZ) affects the weldability of duplex stainless steels. The modern grades show different sensitivity and especially the nitrogen content has been proposed to control the grain growth and austenite formation. In this work, 3-, 4- and 5-mm thick UNS S32101, UNS S32304, UNS S32205 and UNS S32750 with a nitrogen range of 0.09–0.28 wt.-% were considered. Gas tungsten arc welding (GTAW) was conducted bead-on-plate with pure argon (Ar) and Ar + 2% N2 as shielding gas. Measurements of the HT-HAZ width and ferrite content were compared with values reported for welds and thermo-mechanical Gleeble® simulations in existing literature. The HT-HAZ width decreased with the material thickness and the nitrogen content in the base metal, while only the nitrogen alloying had a clear effect on the phase balance. Nitrogen additions to the shielding gas efficiently counteracted weld metal nitrogen loss and improved the austenite formation as compared to 100% Ar but had no clear effect on the HT-HAZ width nor ferrite content. A positive influence of the base material nitrogen content on the phase balance could be validated with thermodynamic simulations using Thermo-Calc and DICTRA, but the negative effect of high cooling rates was underestimated.

Study on Fatigue Performance of Pulsed Tungsten Inert Gas Welding Joint of Duplex Stainless Steel Thin Tube.

To solve the shortage of austenite phase precipitation caused by nitrogen loss in the welding process of UNS S2205 duplex stainless steel (DSS), shielding gas nitriding was investigated by adding different N2 contents in Ar shielding gas during the welding process. A good thin-walled pipe butt joint was formed using the pulsed tungsten inert gas (P-TIG) welding method with Ar-N2 shielding gas. High cycle fatigue tests of the weld joints were conducted to study the effect of shielding gas nitriding on the fatigue properties. Fatigue tests at three stress levels of 225 MPa, 270 MPa, and 360 MPa were carried out on the weld joints with different N2 contents, and the fatigue samples were all fractured in the high temperature heat-affected zone (H-HAZ). Within the current process parameters, the fatigue life of the 4 vol.% N2 welded joints was optimal. Fatigue striations appeared in the fatigue crack propagation zone, and the transient fracture zone was similar to the tensile fracture. Under the low-stress level, the area of the crack propagation zone under 4 vol.% N2 was the highest, the tear ridges all expanded around the crack source area, and the fatigue crack propagation zone presented a radial distribution. The proliferation and expansion of dislocations were mainly carried out in the austenite grains, and the dislocation density of the fatigue specimens under 4 vol.% N2 was smaller than that of the Ar specimens. Shielding gas nitriding effectively improved the balance of the two-phase ratio and the hardness of austenite phase, optimized the internal slip system, inhibited the proliferation of dislocations in the austenite phase, and improved the fatigue life of weld joints.

Unlocking the influence of microstructural evolution on hardness and pitting corrosion in duplex stainless welded joints

In this study, the evolution of microstructure and its correlation with hardness and pitting corrosion resistance in duplex stainless steel (DSS) welded joints were investigated. The results showed that the precipitation of chromium nitride, refined grain size and large residual deformations contributed to the increase of microhardness in the heat affected zone (HAZ) and weld metal (WM). Poor pitting corrosion resistance was found in the HAZ, especially in the high temperature heat affected zone (HT-HAZ). The chromium nitride precipitates and secondary austenite were the main pitting nucleation sites.

Initiation mechanism of pitting corrosion in weld heat affected zone of duplex stainless steel

The pitting corrosion resistance of duplex stainless steel welds was investigated. Pitting corrosion mainly occurs in high-temperature heat-affected zone (HT-HAZ), where the peak temperature reaches ferrite single-phase region. Most pitting corrosion in the HT-HAZ initiates from (Ti,Cr)N in the ferrite. Even the titanium content is less than 50 ppm, titanium-containing nitrides precipitate in the HT-HAZ. While no clear Cr-depleted zone develops around (Ti,Cr)N, almost no passive film forms on (Ti,Cr)N surface. Therefore, pitting corrosion is initiated not only by Cr-depleted zone around Cr nitride but also by the lack or vulnerability of the passive film on the (Ti,Cr)N surface.

Finite element modelling and experimental validation of microstructural changes and hardness variation during gas metal arc welding of AISI 441 ferritic stainless steel

Grain growth and hardness variation occurring in high-temperature heat affected zone (HAZ) during the welding processes are two thermal dependant aspects of great interest for both academic and industrial research activities. This paper presents an innovative finite element (FE) model capable to describe the grain growth and the hardness decrease that occur during the gas metal arc welding (GMAW) of commercial AISI 441 steel. The commercial FE software SFTC DEFORM-3D™ was used to simulate the GMAW process, and a user subroutine was developed including a physical based model and the Hall–Petch (H-P) equation to predict grain size variation and hardness change. The model was validated by comparison with the experimental results showing its reliability in predicting important welding characteristics temperature dependant. The study provides an accurate numerical model (i.e. user subroutine, heat source fitting, geometry) able to successfully predict the thermal phenomena (i.e. coarsening of the grains and hardening decrease) that occur in the HAZ during welding process of ferritic stainless steel.

Microstructural Investigation of a Friction-Welded 316L Stainless Steel with Ultrafine-Grained Structure Obtained by Hydrostatic Extrusion.

The paper presents the microstructural investigation of a friction-welded joint made of 316L stainless steel with an ultrafine-grained structure obtained by hydrostatic extrusion (HE). Such a plastically deformed material is characterized by a metastable state of energy equilibrium, increasing, among others, its sensitivity to high temperatures. This feature makes it difficult to weld ultra-fine-grained metals without losing their high mechanical properties. The use of high-speed friction welding and a friction time of <1 s reduced the scale of the weakening of the friction joint in relation to result obtained in conventional rotary friction welding. The study of changes in the microstructure of individual zones of the friction joint was carried out on an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and electron backscattered diffraction (EBSD) analysis system. The correlation between the microstructure and hardness of the friction joint is also presented. The heat released during the high-speed friction welding initiated the process of dynamic recrystallization (DRX) of single grains in the heat-affected zone (HAZ). The additional occurrence of strong plastic deformations (in HAZ) during flash formation and internal friction (in the friction weld and high-temperature HAZ) contributed to the formation of a highly deformed microstructure with numerous sub-grains. The zones with a microstructure other than the base material were characterized by lower hardness. Due to the complexity of the microstructure and its multifactorial impact on the properties of the friction-welded joint, strength should be the criterion for assessing the properties of the joint.

Microstructure and Fatigue Behavior of 2205/316L Stainless Steel Dissimilar Welded Joints

The relation among microstructure and fatigue behavior of 2205/316L stainless steel dissimilar welded joints was investigated. Plates of 6.35 mm in thickness with a single-V joint configuration were gas metal arc welded (GMAW) in a single pass by feeding at 6 m/min an ER2209 filler wire with a heat input of 1.2 kJ/mm. Grain growth in the high temperature-heat affected zone (HT-HAZ) occurred mostly at the mid-height of the plates, delimiting the width of this region up to ~1.28 and ~0.73 mm of the 2205 and 316L plates, respectively. Dilution of the 316L plate with the ER2209 filler altered the solidification mode in this side of the weld and led to a significant content of austenite along the fusion line. Fatigue tests were performed using sinusoidal waveform at room temperature applying uniaxial cyclic loading, between constant stress limits within the elastic deformation of tension and compression (Δσ) with stress ratio R = −0.3. With stress ranges of 98% and 95% the fatigue specimens rapidly failed in much less than 106 cycles. The failure crack initiated at the surface of the 316L in the HT-HAZ near the weld toe. Surface analyses of unbroken specimens before and after fatigue testing revealed a significant increment in roughness of the 316L base material owing to the formation of intrusions and extrusions.

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.

Effect of Laser Power on Tensile Performance of TA15 Laser-Welded Lock Bottom Joint

Taking TA15 titanium alloy as the research object, laser welding experiments of are performed on the lock-bottom joints, and the microstructure and tensile performance of the joints are analyzed. The laser welding speed is constant and the laser power is 1000 w, 1300 kw, 1600 kw respectively. The bottom weld width and the width of high temperature heat-affected zone (HT-HAZ) increase significantly as laser power increase. The microstructure of the heat-affected zone (HAZ) is relatively complicated, mainly composed of secondary α, acicular α structure and β. The microstructure at the weld metal is comprised of the martensite α′ and acicular α. The tensile strength, fracture morphology and chemical composition of the welded joint are studied by tensile test, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analysis techniques. The results show that all the specimens fractured near the heat-affected zone, and the fracture is a ductile fracture in the shape of a dimple. The tensile strength of welded joint decreases with the increase of laser power. When the laser power is 1.0 kw, 1.3 kw and 1.6 kw, the tensile strength is 723 MPa, 705 MPa and 695 MPa respectively, which is 64.55%, 62.95% and 62.05% of the base material strength. EDS results show that Mo and V elements are the strengthening elements, while the burning loss of Mo and V reduce the strength of the joint.

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.

Determination of Grain Growth Kinetics and Assessment of Welding Effect on Properties of S700MC Steel in the HAZ of Welded Joints

The welding of fine-grained steels is a very specific technology because of the requirement for the heat input limit value. Applying temperature cycles results in an intense grain growth in a high-temperature heat-affected zone (HAZ). This has a significant effect on the changing of strength properties and impact values. The intensity of grain coarsening in the HAZ can be predicted based on the experimentally determined activation energy and material constant, both of which define grain growth kinetics. These quantities, together with real measured welding cycles, can be subsequently used during experiments to determine mechanical properties in a high-temperature HAZ. This paper shows a methodical procedure leading to the obtainment of the material quantities mentioned above that define the grain growth, both at fast and slow temperature cycles. These data were used to define the exposure temperature and the soaking time in a vacuum furnace to prepare test samples with grain sizes corresponding to the high-temperature HAZ of welded joints for the testing procedures. Simultaneously, by means of the thermo-mechanical simulator Gleeble 3500, testing samples were prepared which, due to a temperature gradient, created conditions comparable to those in the HAZ. The experiments were both carried out with the possibility of free sample dilatation and under a condition of zero dilation, which happens when the thermal expansion of a material is compensated by plastic deformation. It has been found that shape of the temperature cycle, maximal achieved cycle temperature, cooling rate, and, particularly, the time in which the sample is in the austenite region have significant effects on the resulting change of properties.

Physical and kinetic simulation of nitrogen loss in high temperature heat affected zone of duplex stainless steels

High temperature heat affected zone (HTHAZ) of duplex stainless steels is prone to local corrosion attack due to a high ferrite fraction and nitride formation. Literature commonly attributes formation of this undesirable microstructure to rapid cooling from high peak temperatures. However, this study investigated the possible role of nitrogen loss in HTHAZ using a combination of physical and kinetics simulation. Applying a stationary gas-tungsten arc (GTA) on a water-cooled plate, a technique known as arc heat treatment, showed that considerable nitrogen loss occurred already after 0.5 min up to 150 µm from the fusion boundary. This zone was extended to 1300 µm after 600 min arc heat treatment. The results of bead-on-plate GTA welding and Gleeble testing replicating the thermal cycle in HTHAZ showed that the ferrite fraction of the real HTHAZ was 7% higher than that for Gleeble samples. This agrees with results from arc heat treatment, where ferrite fraction was found to increase due to nitrogen loss. Numerical and Dictra approaches were developed to simulate the kinetics of nitrogen loss in HTHAZ considering ferrite as the nitrogen rapid diffusion path towards the weld pool. Simulation showed good agreement with both welding and physical simulation. A combination of thermodynamic and kinetics simulations properly predicted the ferrite fraction at 1100 °C for different arc heat treatment times. In conclusion, the experiments (physical simulations and GTA welding) and kinetics simulation showed that nitrogen was lost from HTHAZ to the weld pool.

Initial weldability study on Al0.5CrCoCu0.1FeNi high-entropy alloy

The weld solidification behavior, segregation tendency, phase formation, and cracking susceptibility of Al0.5CoCrCu0.1FeNi high-entropy alloy (HEA) were investigated. The AlCoCrCuFeNi alloy system has received wide attention due to its high temperature stability and strength retention. Characterization of the as-melted, heat-treated, and hot-rolled and heat-treated alloy conditions was performed and compared to thermodynamic CALPHAD-based calculations. Autogenous gas tungsten arc (GTA) and laser beam welding was done on the different alloy conditions. Microstructural changes in the fusion zone and heat-affected zone (HAZ) were investigated by optical microscopy and micro-hardness. Hot cracking susceptibility was evaluated using the cast pin tear test (CPTT). The fusion zone solidification microstructure of Al0.5CoCrCu0.1FeNi alloy consists of primary face centered cubic dendrites and interdendritic body centered cubic phase that forms at the end of solidification. GTA welding results in the partial dissolution of the interdendritic phase in the high-temperature HAZ, adjacent to the fusion boundary. Welding on the heat-treated alloy leads to HAZ softening due to the dissolution of strengthening nano-sized precipitates. No solidification cracking or HAZ liquation cracking was observed. Laser welding did not result in a drop in hardness or significant microstructural changes in the HAZ. The CPTT results correspond well with the predicted solidification temperature range (141 K) for Al0.5CoCrCu0.1FeNi alloy and its overall good weldability that was observed in the present work.

Evaluation of microstructural and mechanical properties of Fe-16Cr-1Ni-9Mn-0.12N austenitic stainless steel welded joints

This study has been aimed to find out the proper welding procedure/conditions for a newly developed Fe-16Cr-1Ni-9Mn-0.12N austenitic stainless steel (ASS), which is uniquely modified from 200 series alloy, in order to control the microstructural metastability of the weld metals. The welded joints of 4mm thick sheet were prepared in a square butt configuration by using three modes of metal transfer, i.e., short-circuit (SC), spray (S), and pulse (P) mode of metal transfer and three austenitic filler metals (304L, 308L and 316L) under two shielding gas environments using gas metal arc welding (GMAW) process. It is observed that the variation in modes of metal transfer and filler metals can effectively manipulate the metastability of γ-phase, formation of δ-phase and grain size of the weld metals. Among the modes of metal transfer, pulse mode of metal transfer produces more metastable γ-phase and higher δ-phase fraction responsible for finer grain structure in weld metals irrespective of filler metals used. Again, higher microstructural metastability was obtained with the 304L filler metal irrespective of modes of metal transfer used. It is evident that the weld metals having higher microstructural metastability also improved mechanical properties (i.e. hardness, strength and toughness). However, weak fusion boundary (FB) region of welded joint limits the beneficial effect of metastability and creates a weakest link in the joint. On the other hand, high temperature heat affected zone (HTHAZ) having a coarse austenite grain surrounded by grain boundary precipitates is governed only by the modes of metal transfer.

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.

Influences of Energy Input and Metal Powder Addition on Carbide Precipitation in AISI 430 Ferritic Stainless Steel Welds

In the present work, 16% chromium AISI 430 ferritic stainless steel weld produced under different conditions of energy input and metal powder addition using TIG torch melting is investigated for carbide precipitation. A Box Behnken experimental design paradigm is invoked with current, speed and metal powder concentration as the primary variables while type of powder is the categorical variable. The result indicates that for a given concentration of metal powder in the melt pool, the sensitized geometry increases with energy input. Titanium as well as the mixture of aluminum and titanium powders reduces the width of the sensitized zone while aluminum powder increases the width of the zone. Titanium powder prevents the development of completely ditched structure in the high temperature heat affected zone (HTHAZ) whereas aluminum powder accelerates the formation of ditched structure in the HAZ.