Sub-zones Of Heat Affected Zone Research Articles

Effect of grain size and segregation on the cryogenic toughness mechanism in heat-affected zone of high manganese steel

High manganese steels have been nominated as desirable materials used for various cryogenic applications due to their excellent cryogenic toughness caused by twinning. High manganese steel welded joints were obtained through gas metal arc welding, and the microstructural differences at three subzones of heat-affected zone (HAZ) were utilized. The aim was to investigate the effect of grain size and Mn/C/Cu segregation on the cryogenic toughness mechanism of high manganese steel welded joints. The results indicate that the average impact absorbed energies at −196 °C for 1.0, 3.0, and 5.0 mm from the fusion line (denoted by FL + 1, FL + 3, and FL + 5) is 118.6 J, 102.4 J, and 117.2 J, respectively. Compared with base metal, the embrittlement occurred in HAZ. Large grains improve toughness, while small grain boundaries, segregation, and precipitation in HAZ deteriorate toughness. Larger grain size facilitates the primary twins with thinner inter-twin spacing and twin thickness, and the high density of these primary twins inhibits the activation of secondary twins. This, in turn, enhances cryogenic impact toughness. Although the stacking fault energy (SFE) throughout the welding joint lays within the range for the twinning-induced plasticity effect, the enrichment of Mn/C/Cu slightly increases the local SFE and critical resolved shear stress for twining within the segregation zone. This phenomenon makes the activation of primary twins more challenging. Furthermore, the C segregation at grain boundaries leads to the precipitation of fine Cr23C6 carbides. These carbides serve as crack initiation points during impact tests, contributing to degradation in cryogenic impact toughness.

Microstructure and Thermal Mechanical Behavior of Arc-Welded Aluminum Alloy 6061-T6

In this study, the welding thermal cycle, as well as the microstructural and mechanical properties of welded AA6061-T6 plates, were studied. The plates were prepared and bead-on-plate welded using gas metal arc welding (GMAW). Numerical simulations using SYSWELD® were performed to obtain the thermal distribution in the welded plates. The numerical heat source was calibrated using the temperatures obtained from the experimental work and the geometry of the melting pool. The mechanical properties were obtained through microhardness tests and were correlated with the welding thermal cycle. Moreover, the mechanical behavior and local deformation in the heat-affected zone (HAZ) were investigated using micro-flat tensile (MFT) tests with digital image correlation (DIC). The mechanical properties of the subzones in the HAZ were then correlated with the welding thermal cycle and with the microstructure of the HAZ. It was observed that the welding thermal cycle produced microstructural variations across the HAZ, which significantly affected the mechanical behavior of the HAZ subzones. The results revealed that MFT tests with the DIC technique are an excellent tool for studying the local mechanical behavior change in AA6061-T6 welded parts due to the welding heat.

Comparative study about the results of HAZ physical simulations on different high-strength steel grades

With continuous improvements, structural steels are available in even higher strength grades above 1000 MPa yield strength. As the great majority of these steels are used in welded structures, their weldability needs to be taken into account. Several factors can cause difficulties during welding of these steels, but in this paper the softening behavior and the toughness characteristics of the heat-affected zone (HAZ) are examined. As the critical parts of the HAZ in a real welded joint are relatively small, their investigating ability is limited. However, the physical simulation provides a way of evaluating specimens made from a given material to produce the specified HAZ areas in a suitable size range for subsequent testing. In this research work, three different strength categories of high-strength structural steels (with yield strength of 960 MPa, 1100 MPa, and 1300 MPa) are investigated by physical simulation. In the case of different technological variants of gas metal arc welding (GMAW) process, the effect of the cooling time t8/5 is investigated in different HAZ subzones considered to be critical. The thermal cycles were determined according to the Rykalin 3D model. The investigated cooling times were t8/5 = 5 s, 15 s, and 30 s. The properties of the selected coarse-grained, intercritical and intercritically reheated coarse-grained zones are analyzed by laser scanning microscope, scanning electron microscope, hardness test, and instrumented Charpy V-notch impact toughness test. Furthermore, additional investigation like JMatPro calculations, electron backscatter diffraction measurements, and prior austenite grain size calculation were carried out. As a result of the tests, the investigated heat-affected subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Overall, the results of the tests show that the application of shorter t8/5 cooling time can be beneficial for the investigated high-strength steel grades, since significant toughness reduction and the risk of softening occur in the whole cooling time range.

Flow and hardening behavior in the heat-affected zone of welded ultra-high strength steels

The applications of thermomechanically processed ultra-high strength steels (UHSS) are rapidly increasing, and welding these UHSSs seems inevitable in steel structures. However, welding heat causes unwanted microstructural transformations in the heat-affected zone (HAZ). Due to the localized nature of these changes throughout the HAZ, evaluating the true stress–strain values of these localized HAZ subzones is essential to improve the accuracy of analytical or numerical models. Hence, this study utilized experimental thermal simulations to replicate HAZ subzones of two types of UHSSs, i.e., direct-quenched S960 and quenched-and-tempered S1100, and employed tensile test in conjunction with digital image correlation to plot the true stress–strain and hardening curves of the subzones. Both UHSSs manifested similar trends but with various fluctuations in their hardening capacities throughout their HAZ subzones. Next, hardening parameters from Hollomon, Voce, and Kocks-Mecking approaches were extracted by fitting the experimental results with the semi-empirical equations. For both UHSS types, the Voce approach, on average, was more accurate in modeling the plastic deformation. Also, hardening parameters achieved via the Voce approach’s fittings agreed with the parameters from Kocks-Mecking plots; this consistency pointed to the predictability of the plastic flow and hardening behavior of both UHSS types. According to the microstructural investigations, the hardening behavior of the investigated HAZ subzones depended on two types of microstructure constituents: ferritic and lath-like features. Ferritic features dominantly governed the plastic flow and hardening near the fusion line, while by getting distant from the fusion line, the lath-like features became more dominant.

The influence of CMT-MAG and MAG welding-processes on microstructure and mechanical behaviour of C-Mn E410 structural-steels

PurposeAn experimental investigation for developing structure-property correlations of hot-rolled E410 steels with different carbon contents, i.e. 0.04wt.%C and 0.17wt.%C metal active gas (MAG) and cold metal transfer (CMT)-MAG weldments was undertaken.Design/methodology/approachMechanical properties and microstructure of MAG and CMT-MAG weldments of two E410 steels with varying content of carbon were compared using standardized mechanical testing procedures, and conventional microscopy.Findings0.04wt.%C steel had strained ferritic and cementite sub-structures in blocky shape and large dislocation density, while 0.17wt.%C steel consisted of pearlite and polygonal ductile ferrite. This effected yield strength (YS), and microhardness being larger in 0.04wt.%C steel, %elongation being larger in 0.17wt.%C steel. Weldments of both E410 steels obtained with CMT-MAG performed better than MAG in terms of YS, ultimate tensile strength (UTS), %elongation, and toughness. It was due to low heat input of CMT-MAG that resulted in refinement of weld metal, and subzones of heat affected zone (HAZ).Originality/valueA substantial improvement in YS (∼9%), %elongation (∼38%), and room temperature impact toughness (∼29%) of 0.04wt.%C E410 steel is achieved with CMT-MAG over MAG welding. Almost ∼10, ∼12.5, and ∼16% increment in YS, %elongation, and toughness of 0.17wt.%C E410 steel is observed with CMT-MAG. Relatively low heat input of CMT-MAG leads to development of fine Widmanstätten and acicular ferrite in weld metal and microstructural refinement in HAZ subzones with nearly similar characteristics of base metal.

Physical simulation of the heat-affected zone in nickel-free high-nitrogen stainless steel weld at various peak temperatures and cooling rates

Nickel-free high-nitrogen stainless steel (NFHNSS) stands out for its remarkable strength, ductility and toughness making it a preferred choice for applications demanding robust structural integrity. Physical simulations of NFHNSS heat-affected zone under different peak temperatures and cooling rates were studied. Based on the phase diagram simulated from Thermo Calc, physical simulations were carried out on 16 specimens with varying peak temperatures ( 800 °C, 900 °C, 1000 °C and 1250 °C) and various cooling rates (CR) as (25 °C/s, 15 °C/s, 5 °C/s and 2 °C/s). The specimens were grouped and characterised based on peak temperature under varying cooling rates. The microstructural changes, precipitates, hardness and tensile properties of the base metal and heat-affected zone (HAZ), were investigated. HAZ subzones are very sensitive to peak temperature and cooling rate lead to variation in grain size. The test results showed significant refinement in HAZ grain size and significant improvement in strength with a fast cooling rate. Furthermore, transmission electron microscopy analysis showed Cr2N formation with respect to the cooling rate. Yield and tensile strength increased with an increase in CR and ductility reduced slightly. Whereas, with an increase in peak temperature yield strength, tensile strength reduced and ductility increased due to grain coarsening

Microstructural evolutions and impact toughness in simulated welding heat affected zones for a high-strength carbide-free bainitic rail steel

Systematic experimental investigations were conducted to study the microstructures and impact toughness of each heat affected zone (HAZ) formed during rail flash-butt welding. A high-strength carbide-free bainitic rail steel was subjected to different thermal simulation cycles to separately reproduce each HAZ subzone by tailoring the peak temperature (PT) with respect to 700, 850, 920, 1000 and 1350 °C, and hence to generate the corresponding microstructures by using Gleeble-3500 simulator. Results show that the HAZ subzones exhibit complicated microstructures depending on the PTs, and with increasing PT the dominant bainitic microstructure type evolves from polygonal bainitic ferrite (700 °C) to a mixture of fine bainitic ferrite and granular bainite (850–1000 °C), and finally to coarse bainitic ferrite and granular bainite (1350 °C). Impact tests demonstrate that the impact toughness initially increases significantly as the PT reaches 920 °C (i.e., fine-grained HAZ), beyond which the impact toughness starts to decrease. The fine-grained HAZ displays optimal impact toughness in HAZs, yet which is lower than the base metal. Moreover, the morphology and distribution of martensite-austenite (M-A) constituents is strongly dependent on the welding PT, and the high fraction blocky and coarse slender M-A constituents is considered to be detrimental for the impact toughness.

Characterization of the heat-affected zones in the laser welded joints of quenched and thermo-mechanically processed high-strength steels

The objective of this research is to compare the heat-affected zone (HAZ) characteristics of quenched and thermo-mechanically processed high-strength steels. Laser-welded butt-welded joints of S690QL, S1100QL, and S700MC steels were used for the analysis. The aim was to describe the effect of microstructure, chemical composition, and manufacturing process on the HAZ´s properties. The considered parameters included the grain size, the width of individual HAZ subzones, and the distribution of hardness throughout the HAZ. The results showed that the content of C, Mo, and microalloying elements Nb and Ti has the greatest influence on the HAZ´s properties. This was observed mainly through the average grain size in the CGHAZ and the hardenability of the CGHAZ and FGHAZ. Based on measured data, the mean grain size of quenched steels is up to 15% larger and the width of CGHAZ is 20% wider than that of thermo-mechanically processed steels. At the same time, it is possible to observe a martensitic structure in the CGHAZ and FGHAZ of QL steels that is absent in S700MC steel.

Microstructural characterization of the physical simulated and welded heat-affected zone of 9% Ni steel pipe

The microstructure, Vickers microhardness, and prior austenite grain size (PAGS) of the heat-affected zone (HAZ) in a circumferential welded joint of a 9% Ni steel pipe were compared with those of a physically simulated (Gleeble machine) HAZ to validate a developed methodology for evaluating the HAZ via finite element method simulation. The welding of the cap pass was computationally simulated; subsequently, the thermal cycles of the HAZ subzones were reproduced (thermomechanical simulator), and the HAZ (physically simulated and of the welded joint) microstructures were compared. The HAZs in these conditions showed similar microstructures, microhardnesses, and PAGSs, thereby validating the developed methodology. The subcritical (SCHAZ), intercritical (ICHAZ), fine-grain (FGHAZ), and coarse-grain HAZs (CGHAZ) exhibited base metal unaltered microstructure, dual-phase microstructures (martensite and ferrite), refined martensite, and coarse martensite with coalesced bainite, respectively. The physically simulated HAZ was investigated using X-ray diffraction spectroscopy and transmission electron microscopy, and it exhibited globular and thin-film austenite morphologies, with minimum and maximum contents in the CGHAZ and ICHAZ, respectively. The developed computational and physical simulation methodology can be reliably reproduced and can evaluate the HAZ microstructure of the 9% Ni steel welded joint.

Sulfide stress cracking susceptibility of the heat-affected zone of an 9% Ni steel welded joint

A 9% Ni steel (ASTM A333 Gr. 8) was selected for the first time for use in CO2 injection units (CO2-IUs) in the Brazilian pre-salt; however, because of the presence of H2S, it is susceptible to sulfide stress cracking (SSC), especially in welded joints. In this regard, the susceptibility to SSC of the subzones of the heat-affected zone (HAZ) of a 9% Ni steel (BM) welded pipe joint was evaluated. The thermal cycles of the HAZ were simulated numerically and physically. HAZ subzones were evaluated through optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers microhardness, and slow strain rate testing. The BM and subcritical HAZ showed similar microstructures (fine ferrite and carbide/retained austenite (γret) aggregates) and behaviors in aggressive environment, with plastic elongation ratios (REs) of 0.15 and 0.17, respectively. The intercritical HAZ (ICHAZ; fine ferrite and martensite) had the highest microhardness and γret content, with a decrease in RE (0.04); fine-grain HAZ (FGHAZ; martensite and bainite) showed the smallest RE (0.01) and brittle fracture. The coarse-grain HAZ (CGHAZ; coarse martensite and coalesced bainite) showed the lowest plastic region and brittle fracture, with RE of 0.05 and the highest time-to-failure ratio. The high microhardness of the ICHAZ, FGHAZ, and CGHAZ (>250 HV, NACE 0175 / ISO 15156) made them more susceptible to SSC than the BM. These results lead to the conclusion that both the 9% Ni steel and its welded joints are prone to SSC, and the use of this steel in CO2-IUs presents an operational failure risk.

Optimization of PWHT of Simulated HAZ Subzones in P91 Steel with Respect to Hardness and Impact Toughness

Appropriate post weld heat treatment (PWHT) is usually obligatory when creep resistant steels are welded for thermal power plant components that operate at elevated temperatures for 30-40 years. The influence of different PWHTs on the microstructure, hardness, and impact toughness of simulated heat affected zone (HAZ) subzones was studied. Thereby, coarse grained HAZ, two different fine grained HAZ areas, and intercritical HAZ were subjected to 20 different PWHTs at temperatures 740–800 °C and durations 0.5–8 h. It was found that the most commonly recommended PWHT, of 3 h or less at 760 °C, is insufficient with respect to the hardness and impact toughness of coarse grained HAZ. To obtain a Vickers hardness ≤ 265 HV and impact toughness at least equal to the impact toughness of the base metal (192 J) in the coarse grained HAZ, it took 8 h at 740 °C, 4 h at 760 °C, more than 1 h at 780 °C, and 0.5 h and 800 °C. Even after 8 h at 800 °C, mechanical properties were still within the target range. The most recommendable post weld heat treatments at 780 °C for 1.2–2 h or at 760 °C for 3–4 h were identified. All specimens subjected to these treatments exhibited appropriate hardness, impact toughness, and microstructure.

Effect of Welding Heat Input on Simulated HAZ Areas in S960QL High Strength Steel

When the weldability of high strength steels is analyzed, it is the softening in the heat-affected zone (HAZ) that is mostly investigated, and the reduction of toughness properties is generally less considered. The outstanding toughness properties of quenched and tempered high strength steels cannot be adequately preserved during the welding due to the unfavorable microstructural changes in the HAZ. Relevant technological variants (t8/5 = 2.5–100 s) for arc welding technologies were applied during the HAZ simulation of S960QL steel (EN 10025-6) in a Gleeble 3500 physical simulator, and the effect of cooling time on the critical HAZ areas of single and multipass welded joints was analyzed. Thermal cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones were investigated by scanning electron microscope, macro and micro hardness tests and instrumented Charpy V-notch pendulum impact tests. The examined HAZ subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Due to the observed brittle behavior of all subzones in the whole t8/5 range, the possible lowest welding heat input should be applied in order to minimize the volume of HAZ that does not put fulfillment of the allowed maximal (450 HV10) hardness at risk and does not lead to the formation of cold cracks.

Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel

Modern weldable high strength steel grades are typically based on low-carbon alloy concepts using microalloying for obtaining a good strength-toughness balance. Such steel grades having a yield strength in the range of 420 to 690 MPa are very commonly used in pipelines, heavy vehicles, shipbuilding and general structural applications. Thermomechanical processing during hot rolling combined with accelerated cooling is an established means of producing such steel grades. Considering the alloying concepts, the use of niobium and molybdenum, and in selected cases boron, is very efficient to achieve high strength and good toughness. However, all targeted applications of such high strength steels involve extensive welding. Thus, heat affected zone properties are of particular importance. The present paper investigates the effects of Nb, Mo and Ti on the heat affected zone properties. Variations of the Mn and Si contents are considered as well. Additionally, the influence of post-weld heat treatment in the coarse-grained heat-affected zone (HAZ) is considered. In this approach, HAZ subzones were generated using laboratory weld cycle simulations in combination with systematic variation of alloying elements to scrutinize and interpret their specific effects. The results indicate that Mo and Nb, when alloyed in the typical range, provide excellent HAZ toughness and guarantee sufficiently low ductile-to-brittle transition temperature. An alloy combination of Nb, Mo and Ti improves performance under hot deformation conditions and toughness after post-weld heat treatment.

Characterizing Creep Behaviour of Modified 9Cr1Mo Steel by using Small Punch Impression Technique for Thermal Powerplants

Modified 9Cr1Mo (P91) having ferritic/martensitic structure is a candidate material for thermal powerplant industry due to its enhanced creep resistance. Fusion welding of P91 steel nurtures heat affected zone (HAZ) which gives rise to infamous type IV failure/cracking. The origination of type IV cracking still remains ambiguous mainly due to sampling constraints from P91 - HAZ subzones owing to thin width of HAZ. In present work, this issue is investigated with the help of small punch impression creep technique. Thermo – mechanical simulator Gleeble®-3800 is used to physically simulate HAZ subzones of P91 steel representing HAZ – CG, HAZ – FG and HAZ – IC. Impression creep tests done on parent metal P91 and its simulated HAZ subzones at a stress level of 270 MPa and 600℃ reveal HAZ – IC as the weakest subzone followed by HAZ – FG. Coarsening of chromium rich carbides on prior austenite grain boundaries (PAGBs) associated with blended phase transformations in HAZ – IC render it as the most degraded HAZ subzone in P91 steel thus becoming an initiating location for type IV cracking. Furthermore, presence of relatively fine carbides combined with less inter-lath spacing in P91 steel and its HAZ – CG contribute towards retarding creep rate leading to an increase in heterogeneous creep behaviour among HAZ subzones and parent metal.

Mechanical Behaviour of MAG Pulsed Welded Joints from 6082 T6 Aluminium Alloy

One of the difficulties encountered by aluminum alloys fusion welding consist in reducing of the mechanical resistance characteristics of the weld seam and of the heat-affected zone (HAZ) compared to those of the base metal. The paper investigates the microstructural and mechanical properties changes of pulsed MAG welded joints made from the 6082-T6 alloy delivered in the form of 5 mm thick sheets. The dissolving phenomena of the hardenable phases in the sub-zones near the molten metal bath and the overaging of alloy in HAZ sub-zones at temperatures lower than those of precipitates redissolving, justifies the decrease of the mechanical resistance in the HAZ zone.

The use of the small punch test to solve practical engineering problems

Two examples of the use of the Small Punch Test (SPT) to solve practical engineering problems are studied in this paper. The first is the mechanical characterization of powder metallurgical samples. In this case, the complex geometry of the samples does not allow the use of any other type of mechanical test, while knowledge of the mechanical properties of the sintered product before its use allows the modification of process parameters aimed at optimizing the product easily and economically. The second example is the mechanical characterization of the heat affected zone (HAZ) of a weld joint, which is currently difficult to characterize, except using hardness profiles. Application of the SPT to samples machined from the different HAZ subzones allows a precise description of the characteristic mechanical behavior of the different microstructures produced in the welding thermal cycle.

An analysis of the microstructure of the heat-affected zone of an ultra-low carbon and niobium-bearing acicular ferrite steel using EBSD and its relationship to mechanical properties

We describe here the microstructure of simulated thermal cycles with the different peak temperatures associated with the heat-affected zone (HAZ) in a 600 MPa acicular ferrite steel. The microstructure, crystallographic features, and the effective grain size of different regions in the HAZ were examined by transmission electron microscopy (TEM) and electron back scattering diffraction (EBSD) analysis. The study indicated that the effective grain size decreases with decrease in peak temperature and attains the smallest value at a peak temperature of 900 °C. However, EBSD analysis indicated that the effective grain size of the sub-zones is small, which contributes to the excellent low temperature toughness in the sub-zones of HAZ. Two hardness peaks were observed at temperatures of 800 °C and 1200 °C because of grain refinement. The study convincingly demonstrates that EBSD can accurately identify the effective grain size contribution on complex and fine-grained microstructures, and is helpful in predicting the grain refinement mechanism.

Influence of microstructural variations in the weldment on the high-temperature corrosion of 2.25Cr-1Mo steel

In order to study the influence of microstructural variation on the oxidation of the weldment of 2.25Cr-1Mo steel, regions with different microstructures were identified by optical microscopy. The weld metal, the base metal, and the heat-affected zone (HAZ), as well as the subzones within the HAZ, i.e., the intercritical (ICR), the fine-grain bainite (FGB), and the coarse-grain bainite (CGB) regions were separated from the weldment by precise steps of metallography. Transmission electron microscopic examinations for the identification of the secondary phases in microstructurally different regions and subzones have suggested that M23C6 and M7C3 pre-cipitates form predominantly in the subzones of HAZ, whereas the Mo2C type of carbide forms exclusively in the weld-metal and base-metal regions of the weldment. However, population and distribution of the secondary phases were different in the three subzones of the HAZ. In order to understand the influence of these microstructural variations on the oxidation behavior, the various regions and subzones were oxidized at 773 and 873 K. The HAZ and its constituents were found to oxidize at much higher rates than the weld metal and the base metal. Relative compositions and morphologies of the scales were compared by scanning electron microscopy with energy-dispersive analyses of X-rays (SEM/EDX), and secondary ion mass spectrometry (SIMS). Scale formed over the weld metal shows a greater tendency for spallation, as suggested by tests monitoring acoustic emission. X-ray diffraction (XRD) patterns of the scales over these specimens were taken. Results of the SEM/EDX, SIMS, and XRD investigations suggest for-mation of inner scales with less Cr(i.e., less protective) over the HAZ than over the weld-metal and the base-metal regions. Variation in the Cr contents of the scales formed over the various regions is proposed to arise from the difference in microstructural features in different regions of the weldments.

Determining the constitutive properties of the heat-affected zone in a resistance spot weld

A methodology is described for determining the mechanical properties of the heat-affected zone (HAZ) in a resistance spot weld. To perform detailed finite-element analyses of spot weld overload failures, it is necessary to have the mechanical properties of the various HAZ subzones. To this end, simulated HAZ material samples were fabricated in a process that utilized a resistance spot welding machine fitted with specially modified electrodes. Tensile tests at several loading rates were performed on miniature notched-bar specimens machined from the simulated HAZ material. These tests provided the empirical data from which the strain hardening capacities and strain rate sensitivities of the weld HAZ were determined through detailed finite-element analyses of the notched-bar tensile tests.