Structure Of Heat-affected Zone Research Articles

Effect of heat treatment on microstructures and properties of electron beam welding CLF-1 steel

Vacuum electron beam welding was adopted for the reduced activation ferritic martensitic steel (CLF-1 steel) with a thickness of 32 mm, and a well-formed butt joint without metallurgical defects was obtained under the optimum technological parameters. After welding, the specimens were subjected to high temperature tempering (710 °C/120 min/AC (air cooling), abbreviation HTT) and incomplete annealing (840 °C/120 min/AC, abbreviation IA) heat treatment, and the microstructure and mechanical properties of the as-welded state and two different heat treatment states were compared. The results showed that the as-welded weld metal (WM) was composed of coarse lath martensite, and the heat affected zone (HAZ) was composed of a large amount of massive martensite and dispersed fine MX and M23C6. Compared with the welded state, the martensite structure of HTT state WM and HAZ were refined, nano-scale M23C6 was precipitated along the boundary of tempered martensite in WM and nano-scale MX was precipitated in tempered martensite lath structure, both of which played a role of dispersion strengthening. Compared with the HTT state, the martensite structure of the WM and HAZ in the IA state became coarser, and both M23C6 and MX tend to grow together. The fracture position of the tensile specimens of as-welded state, HTT state and IA state welded joints were all located on the base metal (BM), and the tensile strength decreased successively, while the yield strength and elongation increased successively. The average impact absorbed energy of the BM, as-welded state, HTT state and IA state welds were 236 J, 42 J, 241 J and 51 J, respectively. The impact performance of HTT state welds surpassed the BM.

Effect of induction post-heating temperature on the morphology, microstructure and mechanical performance of the heat affected zone in laser-induction hybrid cladding of full-scale rail

Abstract In the present work, the laser-induction hybrid cladding technology with induction post-heating (post-LIHC) was utilized to deposit coatings on the full-scale rail surface, and the effects of different induction post-heating temperature on the morphology, microstructure and mechanical performance of the heat affected zone (HAZ) were first in-depth analyzed. Results indicate that with the increase of the average induction post-heating temperature (Ta) in post-LIHC, the microstructure of the HAZ first transforms from the fine acicular martensite structure to the composite structure of martensite and pearlite, then changes to the pure pearlite structure, and finally retransforms to the coarse martensite structure. Especially, the morphology of the “Martensite Zone” (MZ) in HAZ undergoes three remarkable change with Ta increasing: “flat-U-shaped” → “flat-W-shaped” → “vanished” → “flat-V-shaped”. Under the induction post-heating window of Ta = 508 °C ~ 565 °C, not only the MZ in HAZ can be inhibited absolutely, but also fine pearlite with much lower interlamellar spacing, smaller grain size and higher grain misorientation forms instead, making the strength and toughness are both enhanced significantly than the other three type HAZs and the rail substrate. In addition, increasing the induction post-heating temperature within the window of Ta = 508 °C ~ 565 °C can increase the interlamellar spacing and reduce the ferrite grain misorientation of the pearlite structure in HAZ, but has no obvious effect on the grain size therein.

Microstructure and Mechanical Performance of Resistance Spot Welded Martensitic Advanced High Strength Steel

Resistance spot welded 1.2 mm (t)-thick 1400 MPa martensitic steel (MS1400) samples are fabricated and their microstructure, mechanical properties are investigated thoroughly. The mechanical performance and failure modes exhibit a strong dependence on weld-nugget size. The pull-out failure mode for MS1400 steel resistance spot welds does not follow the conventional weld-nugget size recommendation criteria of 4t0.5. Significant softening was observed due to dual phase microstructure of ferrite and martensite in the inter-critical heat affected zone (HAZ) and tempered martensite (TM) structure in sub-critical HAZ. However, the upper-critical HAZ exhibits obvious higher hardness than the nugget zone (NZ). In addition, the mechanical properties show that the cross-tension strength (CTS) is about one quarter of the tension-shear strength (TSS) of MS1400 weld joints, whilst the absorbed energy of cross-tension and tension-shear are almost identical.

Welding Investigation on GMAW−Cold Metal Transfer of AISI 201LN for Superior Weld Quality

Austenitic stainless steel of AISI 201LN grade has found applications in liquefied natural gas tanks and cryogenic components. They are fabricated using gas tungsten arc welding (GTAW), but weld speed is low due to manual operation. This work aims welding investigation on AISI 201LN Gr. steel with a new hybrid welding approach (i.e., gas metal arc welding [GMAW] combined cold metal transfer [CMT]) for obtaining superior weld quality. Weld experiments were carried out at different welding speed, for example, 300, 400, 600, and 900 mm/min, to study weld quality and its mechanical properties. The microstructural examination of test coupons at higher welding speed shows finer structure in heat-affected zone as well as on weld metal. It was observed that the weld coupon having low heat input (at high weld speed) has maximum tensile strength. Scanning electron microscope analysis shows finer dimples at higher welding speed confirming ductile mode of fracture.

Effect of welding processes and heat input on corrosion behaviour of Ferritic stainless steel 409M

In the present work, Ferritic stainless steel 409M is welded with shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) of welding current 90A, 100A and 110A. Four mm thick plates were used as base material and single V-joint was prepared for welding. Single pass welding was carried out. Microstructures in the transverse section were observed for base metal, Heat Affected Zone (HAZ) and weld joint. Corrosion rate of welded joints have been evaluated by weight loss method in sodium chloride (NaCl) solution. In this investigation results reveal that, corrosion rate of welded joint is more than the base metal. It is also found that corrosion rate increases on increasing welding current. It is observed that corrosion rate is more in SMAW than GMAW and GTAW weld joint. Results are discussed in the light of microstructure. Increased corrosion rate is attributed to welded structure and formation of HAZ due to variable welding heat input.

Influence of low temperature on the heat affected zone structure of welded joint of low-alloyed steel

Influence of low temperature on the heat affected zone structure of welded joint of low-alloyed steel

Development of BMG-B2 nanocomposite structure in HAZ during laser surface processing of ZrCuNiAlTi bulk metallic glasses

In this study, the laser scanning speed used during laser surface processing of a Zr-based bulk metallic glass (BMG) was optimised to obtain amorphous structure in the melt zone and nanocomposite structure in the heat affected zone (HAZ). For this aim, Zr65.7Ti3.3Al3.7Ni11.7Cu15.6 (wt.%) BMG foils with a 100 µm thickness were laser surface treated at the different speeds. The microstructural characteristics of the melt zone and HAZ of the different samples were studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and high-resolution transmission electron microscope (HRTEM). Nanoindentation testing was employed to evaluate the mechanical properties. In addition, molecular dynamic (MD) simulation was used to discover the deformation mechanisms. The results showed that at a laser speed of 22.6 mm/s, the amorphous structure remained, and an improvement in mechanical properties was obtained in the HAZ. The HRTEM images revealed that in the HAZ of the optimum sample, a BMG-B2 nanocomposite structure was formed that improved the mechanical response of the HAZ, considerably. The MD results showed that the existence of B2 phase avoid stress reduction after starting of plastic deformation and even a higher stress during plastic deformation was recorded.

MAG Welding Tests of Modern High Strength Steels with Minimum Yield Strength of 700 MPa

The modern high strength steel plates have an excellent combination of strength and toughness based on micro-alloying and complex microstructure. Retaining this combination of properties in the weld zone is a major challenge for applications in high-demanding structural construction. This work investigates the weldability of three different modern high strength steel plates, with a thickness of 8 mm. Two of the test materials were produced by a thermo-mechanically controlled process (TMCP) and one by a quenching and tempering method (Q&T). Two-passes MAG (metal active gas) welding was used with four different heat inputs. The tests implemented on all the materials included tensile, hardness profiles (HV5), Charpy-V impact toughness tests, and microstructure analysis using scanning electron microscope (SEM). For one of the TMCP steels, some extended tests were conducted to define how the tensile properties change along the weld line. These tests included tensile tests with digital image correlation (DIC), and 3-point bending tests. The most notable differences in mechanical properties of the welds between the materials were observed in Charpy-V impact toughness tests, mostly at the vicinity of the fusion line, with the Q&T steel more prone to embrittlement of the heat affected zone (HAZ) than the TMCP steels. Microstructural analysis revealed carbide concentration combined with coarse bainitic structures in HAZ of Q&T steel, explaining the more severe embrittlement. During the tensile tests, the DIC measurements have shown a strain localization in the softest region of the HAZ. Increasing the heat input resulted in earlier localization of the strain and less maximum strength. The tensile properties along the weld line were investigated in all welding conditions, and the results emphasize relevant and systematic differences of the yield strength at the transient zones near the start and end of the weld compared with the intermediate stationary domain.

A comparison of microstructure and mechanical properties of laser cladding and laser-induction hybrid cladding coatings on full-scale rail

With the rapid development of high-speed and heavy-haul trains, the surface damages of rails are becoming more and more severe, and how to promote the surface strength of the rail and prolong its service life with high efficiency are becoming extremely important. Laser cladding (LC), with small heat affected zone (HAZ) and low dilution, is a promising novel way to hardface and repair the rail. However, there are two great barriers for the traditional LC to apply on full-scale rails: one is how to prevent the coating from cracking under the rapid heating and cooling cycle; the other is how to eliminate the martensite structure in HAZ, which may threaten the safety of railway transportation due to its high hardness and low fracture toughness and usually be forbidden in almost all the Railway Standards over the world. In this paper, laser-induction hybrid cladding (LIHC) was innovatively proposed to deposit Ni-based coatings on a full-scale rail. The cracking behaviors, microstructures and mechanical properties of the coatings and HAZs by LC, LIHC with induction pre-heating (pre-LIHC) and LIHC with induction post-heating (post-LIHC) were studied systemically. The results indicate that the cracking and martensite transformation occurred in the HAZ can only be prevented by post-LIHC, where fine pearlite with smaller pearlite block size and lower interlamellar spacing formed instead. Therefore, the abrupt change of microstructure and mechanical properties in the HAZ could be avoided by post-LIHC, and the hardness, strength and toughness of the rails can be improved significantly. The post-LIHC technology shows the potentiality to hardface and repair the full-scale rail.

Uticaj prsline na karakteristike zavarenog spoja pri različitim vrstama opterećenja

The application of high strength steels in design of heavy duty welded structures requires data about properties in different loading conditions. Thanks to high yield stress the wall thickness can be reduced compared to mild structural steels, and accordingly welded joint cross-sections, welding consumables consumption and time for welded joints manufacturing will be minimized. This is of importance for pressurized equipment, but also for other industrial branches (cranes, excavator). Complete characterization of welded joint has to include data for parent metal, but also the properties for weld metal and the heat-affected-zone (HAZ) are necessary, at least in order to compare them with parent metal properties. This is of special importance because of heterogeneity structure in HAZ.

Improved simulation method of automotive spot weld failure with an account of the mechanical properties of spot welds

In this paper, the microstructure and mechanical properties of spot weld were studied, the hardness of nugget and heat affected zone (HAZ) were also tested by metallographic microscope and microhardness tester. The strength of the spot weld with the different parts' area has been characterized. According to the experiments result, CAE model of spot weld with HAZ structure was established, and simulation results of different lap-shear CAE models were analyzed. The results show that the spot weld model which contained the HAZ has good performance and more suitable for engineering application in spot weld simulation.

Thermographic Assessment of the HAZ Properties and Structure of Thermomechanically Treated Steel

Thermomechanically processed steels are materials of great mechanical properties connected with more than good weldability. This mixture makes them interesting for different types of industrial applications. When creating welded joints, a specified amount of heat is introduced into the welding area and a so called heat-affected zone (HAZ) is formed. The key issue is to reduce the width of the HAZ, because properties of the material in the HAZ are worse than in the base material. In the paper, thermographic measurements of HAZ temperatures were presented as a potential tool for quality assuring the welding process in terms of monitoring and control. The main issue solved was the precise temperature measurement in terms of varying emissivity during a welding thermal cycle. A model of emissivity changes was elaborated and successfully applied. Additionally, material in the HAZ was tested to reveal its properties and connect changes of those properties with heating parameters. The obtained results prove that correctly modeled emissivity allows measurement of temperature, which is a valuable tool for welding process monitoring.

Improvement of strength and toughness: The effect on the weldability of high-strength steels used in offshore structures

The effect of strength and toughness on the weldability of high-strength steels is very vital consideration in the offshore oil and gas industries. Improved impact toughness of high-strength steels in offshore structures enables viable exploitation of hydrocarbons in technologically challenging conditions. This article reviews improvements in the weldability and impact toughness of high-strength steels. Steels with high strength are associated with high carbon content and addition of alloying elements as they induce hardness which leads to a higher risk of brittle fracture and hydrogen-induced cracking needs. The combination of high strength with high toughness was studied by examining the toughening mechanism of thermomechanical-controlled processing steels, which have higher strength than conventional steel plates but meet the conflicting requirements of strength, toughness and weldability. The thermomechanical-controlled processing production process entails controlled rolling process combined with accelerated cooling or direct quenching to ensure stable mechanical properties of thermomechanical-controlled processing products in welded constructions. It is concluded that due to their very fine grain size and refined heat-affected zone structure, thermomechanical-controlled processing steels can be an effective cost-saving means for fabrication of offshore structures, particularly in shipbuilding, offshore platforms and pipelines for high-operating pressures.

Structure Analyses of Experimental Welds Designed for Repair Welding of VVER 1000 Presssure Vessel

Microstructural analysis and microhardness measurement of experimental repair weld joints were carried out in heat affected zone (HAZ) of the base material (BM) – steel 15Ch2NMFA after weld deposition of several layers by welding metal of Inconel FM 52. A temper bead welding (TBW) technique was used. Required microhardness up to 350 HV0.1 in heat affected zone of base material in distance 0.5 mm from fusion line was exceeded in all three experimental blocks with various welding energy inputs during deposition of three or five weld layers. Metallography showed reaustenitization and structure transformation in HAZ after the deposit of first layer in all three blocks. After deposit of the last layer the structure of HAZ in two blocks was tempered only and one variant of them have been recommended for futher technological tests. Structure of HAZ of the third block was not full tempered even after the last layer deposit.

Modeling Heating Effects in the Heat-Affected Zone of High-Strength Pipe Steel K70 with Double-Pass Submerged Arc Welding

Simulation modeling is used in order to study metal structure in a heat-affected zone (HAZ) during submerged-arc welding. A correlation is established between low values of impact energy observed and features of HAZ structure formation during submerged-arc welding of low-alloy steel of strength category K70. It is shown that the appearance of brittle precipitates with an increase in content of the main alloying elements at large grain boundaries is a reason for low impact energy in the area of large HAZ grains during steel K70 welding. Development of these areas is connected with repeated heating (simulating a second pass during rolling) in a temperature range somewhat lower than the Ac 1 point. A reduction in cooling rate in the first heating cycle (which corresponds to an increase in linear energy during welding) facilitates precipitate enlargement over grain boundaries. Multipass welding and a reduction in pass linear energy are recommended in order to exclude the formation of these areas and to improve and stabilize impact energy in a welded joint HAZ.

Prediction of type IV cracking behaviour of 2.25Cr-1Mo steel weld joint based on finite element analysis

Creep tests were carried out on 2.25Cr-1Mo ferritic steel base metal and its fusion welded joint at 823 K over a stress range of 100–240 MPa. The weld joint possessed lower creep rupture strength than the base metal and the reduction was more at lower applied stresses. The failure occurred in the intercritical region of heat-affected zone (HAZ) of the joint, commonly known as Type IV cracking. Type IV cracking in the joint was manifested as pronounced localization of creep deformation in the soft intercritical region of HAZ coupled with preferential creep cavitation. The creep cavitation in intercritical HAZ was found to initiate at the central region of the creep specimen and propagate outwards to the surface. To explain the above observations, the stress and strain distributions across the weld joint during creep exposure were estimated by using finite element analysis. For this purpose creep tests were also carried out on the deposited weld metal and simulated HAZ structures (viz. coarse-grain structure, fine-grain structure, and intercritically annealed structure) of the joint. Creep rupture strength of different constituents of joint were in the increasing order of intercritical HAZ, fine-grain HAZ, base metal, weld metal and coarse-grain HAZ. Localized preferential creep straining in the intercritical HAZ of weld joint as observed experimentally was supported by the finite element analysis. Estimated higher principal stress at the interior regions of intercritical HAZ explained the pronounced creep cavitation at these regions leading to Type IV failure of the joint.

Characterization of microstructural effects in a percussion laser-drilled powder metallurgy Ni-based superalloy

In the combustion zone of gas turbine engines for aircraft, it is essential to maintain a large temperature differential between the combustion gases and the Ni superalloy components so that fuel is used efficiently while avoiding degradation of the turbine blades, rotors, casings, etc. This is typically achieved using a combination of thermal barrier coatings (TBCs) and cooling holes [1–3]. In the latter case, air from the compressor section is forced through internal channels in the components and this emerges from arrays of fine (\500 lm in diameter) cooling holes, establishing a thin air layer between the combustion gases and the TBC. Current generation engines can contain in excess of 10 cooling holes and most of these are produced by electrodischarge machining (EDM) or laser drilling. The use of laser drilling is increasing because it enables holes to be drilled through components with TBCs (e.g. [4, 5]); this is a significant advantage over EDM where the holes must be formed in uncoated components and the subsequent application of a TBC can cause cooling-hole blockage. Laser drilling of aerospace components is normally performed in percussion mode whereby the laser and component are stationary and a few short (m each pulse melts and partially vaporizes the alloy, leading to ejection of the molten material and vapor through the irradiated surface [4–6]. Percussion laser drilling is usually preferred over both laser trepanning with a moving beam, which produces very high quality holes but with a longer cycle time, and single-shot drilling, which produces more debris and other artifacts (e.g. [5, 6]). The main metallurgical effects that can occur during percussion laser drilling are the formation of a recast layer and/or a heat-affected zone (HAZ) [5–8]. The recast layer is comprised of molten/vaporized metal that is not ejected from the hole by the vapor generated during the laser pulse but instead resolidifies on the sidewall of the hole [5, 6]. The HAZ consists of a region of the base metal surrounding the hole that has not melted during the drilling but has undergone microstructural changes due to the thermal history in a manner akin to the HAZs formed during laser welding and cladding (e.g. [9–12]). It is known that the recast and/or HAZ could lead to significant fatigue debits for laser-drilled components, and indeed this has been the motivation behind recent work on femtosecond ablative laser drilling in which recast and/or HAZ effects can be avoided [13, 14]. While there have been several parametric studies in which optical and scanning electron microscopy have been used to measure the extent of the recast and HAZ layers for such laser-drilled components (e.g. [15–18]), there has been little high-resolution characterization work to reveal the structures of these layers. In our work, we have used a variety of high-resolution microstructural characterization techniques to investigate the recast and HAZ structures for a variety of different Ni-based superalloys and preliminary data have been presented elsewhere [19]. The present paper describes a more detailed electron microscopy study of these structures for percussion laser-drilled samples of the powder metallurgy (P/M) superalloy IN100, which has a nominal composition of Ni–18.5Co–12.4Cr–5.0Al–4.3Ti– 3.2Mo–0.8V–0.07C–0.06Zr–0.02B (all in wt%). This particular alloy was selected because in the P/M form it has a very uniform chemistry and a well-defined microstructure comprising a hierarchy of L12 c0 precipitates on different J. K. M. Garofano H. L. Marcus M. Aindow (&) Department of Chemical Materials and Biomolecular Engineering, Materials Science and Engineering Program, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA e-mail: m.aindow@uconn.edu

Microstructure of Al–Mg–Si Weld Joints Produced by Pulse TIG Welding

Al–Mg–Si alloys (6000 series) are heat-treatable aluminum alloys and are extensively used in various sectors such as aerospace, automobiles, and general construction. Fabrication of these alloys often requires welding. Pulse TIG offers many advantages over conventional TIG welding as far as control of microstructure is concerned. This research investigated the effect of pulsed TIG welding parameters such as pulse duration, peak current, pulse frequency on the microstructure of heat-affected zone, and fusion line and weld metal of Al-0.5%Mg-0.5%Si weld joints. It was observed that a significant change in microstructure takes place in moving from the base metal to weld centerline. Dissolution of phases present along the grain boundary occurs as the fusion line is approached. Grain structure of heat-affected zone, fusion line, and weld metal appears to have tangible relationship with pulse parameters.

Characterization of Creep Resistance for Local Structure of Power Plant Weldment Using SP-Creep Technique

Power plant weldments are composed of various microstructures. Due to welding and PWHT processes, the microstructure of the base metal adjacent to fusion line is transformed into entirely different microstructures, collectively known as heat affected zone (HAZ). Creep, on the other hand, is considered as the most important deterioration mechanism of heat resistant components found in power plants. Therefore it is essential to evaluate creep properties of HAZ, which is considered to be very hazardous in weldment. Recently, most of the creep tests for HAZ are conducted using cross weld type specimen. However there are some problems with this type of creep test due to the results being largely varied according to the volume fraction of HAZ. In this paper, SP-Creep test, which has confirmed the availability for creep properties evaluation, has been conducted on each of the weldment microstructures. The results showed that each microstructure has a different creep behavior. The overall creep properties of HAZ are worse than those of the weld metal. Among the HAZ structures, fine grained HAZ has the worst creep properties.

Effect of weld heat input on toughness and structure of HAZ of a new super-high strength steel

Fracture morphology and fine structure in the heat-affected zone (HAZ) of HQ130 super-high strength steel are studied by means of SEM, TEM and electron diffraction technique. Test results indicated that the structure of HAZ of HQ130 steel was mainly lath martensite (ML), in which there were a lot of dislocations in the sub-structure inside ML lath, the dislocation density was about (0.3∼ 0.9) x 1012/cm2. No obvious twin was observed in the HAZ under the condition of normal weld heat input. By controlling weld heat input (E ≤20 kJ/cm), the impact toughness in the HAZ can be assured.