Microstructure Of Simulated Heat Affected Zone Research Articles

Gleeble Simulation of HAZ in S690QL Steel: Microstructural and Mechanical Properties

When investigating the heterogeneous properties of welded joints, mechanical testing of certain Heat Affected Zone (HAZ) regions, using standard approach test specimens, is very difficult or sometimes even impossible. Inability to precisely position and extract mechanical test specimens, even ones of the subsize dimensions, from the narrow HAZ regions is a limiting factor in the mechanical testing implementation. Detailed investigation of the HAZ is made possible by the use of thermo-mechanical simulations on the Gleeble welding simulator. In scope of this paper several characteristic HAZ microstructures of S690QL grade High Strength Steel (HSS) are being simulated. Multi-pass welding simulations are done on special 10x10 mm square section bar specimens in order to reproduce thermal gradients and characteristic microstructures at any location in a weld. Such simulated HAZ microstructures are of a sufficiently large volume, with homogeneous and repeatable properties, that standard specimen methods for mechanical testing can be readily implemented. Metallographic optical examinations, as well as hardness measurements were done initially. Mechanical properties are focused on determining stress-strain curves for each characteristic weld region. The paper investigates whether the mechanical properties of Gleeble simulated hard-soft combined HAZ regions are better in comparison to exclusively hard or soft HAZ regions. The obtained results can subsequently be used for the material model development.

Microstructural characterization of simulated and actual constituent regions of P91 steel weldment

In the present study simulated and the actual microstructures of the coarse grain, fine grain and intercritical heat-affected zones of P91 steel weldment are characterized using electron backscattered diffraction (EBSD) technique. Both the simulated and actual heat-affected zone (HAZ) microstructures showed profound substructural recovery after post-weld heat treatment/tempering. The subgrain size distributions between the simulated and the actual HAZs are comparable. The actual HAZ microstructures exhibited greater low angle grain boundaries and local strain distributions compared to the simulated HAZ microstructures due to the complex microstructural geometry of the actual weldment. The similarity in the variations of grain size, precipitate size and hardness of the simulated and actual HAZs validates the simulation of HAZ microstructures through furnace heat treatment.

Effect of Nitrate-Reducing-Bacteria on Stress Corrosion Cracking Behavior of Simulated Heat-Affected Zone Microstructures of X80 Steel in a Near-Neutral Ph Solution

Effect of Nitrate-Reducing-Bacteria on Stress Corrosion Cracking Behavior of Simulated Heat-Affected Zone Microstructures of X80 Steel in a Near-Neutral Ph Solution

Microstructurally small fatigue crack initiation behavior of fine and coarse grain simulated heat-affected zone microstructures in low carbon steel

Successive observation of fatigue crack initiation process was conducted on six types of thermally simulated HAZ microstructures to clarify the fundamental reason of fatigue life difference between coarse grain (CG) and fine grain (FG) heat-affected zone (HAZ) microstructures. As a result, regardless of microstructure, fatigue cracks initiated and propagated along shear stress plane when the crack is smaller than prior austenite grain size. In addition, this shear mode fatigue crack growth of CGHAZ were much faster than that of FGHAZ and categorized as microstructurally small fatigue cracks. Although prior austenite grain size of crack initiation life of the FG and CGHAZ were comparable, the remaining fatigue life after the grain size crack initiation of the CGHAZ become shorter than that of the FGHAZ as a function of grain size difference. As a result, total fatigue life of CGHAZ becomes shorter than that of FGHAZ. Therefore, relatively larger part of CGHAZ's fatigue life is consumed by shear mode microstructurally small fatigue crack growth process.

Microstructure and microhardness evolution of thermal simulated HAZ of Q&P980 steel

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. Ac1 and Ac3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450–460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.

Comparative study on corrosion fatigue behaviour of high strength low alloy steel and simulated HAZ microstructures in a simulated marine atmosphere

In this study, corrosion fatigue (CF) behaviour of E690 steel and simulated heat-affected zone (HAZ) microstructures were investigated in a SO2-polluted marine atmosphere. Results revealed that E690 steel and simulated HAZ microstructures were all susceptible to CF cracking in this environment with transgranular fracture. CF cracks were prone to initiate from prior austenite grain boundaries (PAGBs) for parent metal and coarse-grained HAZ (CGHAZ), whereas few microcracks were induced for fine-grained HAZ (FGHAZ). CGHAZ exhibits inferior CF behaviour due to coarse PAGBs and long bainitic lath, whereas FGHAZ exhibits superior CF behaviour because of fine grain size and bainitic lath.

Gigacycle Fatigue Fracture of Low Strength Carbon Steel, Tested using a Simulated Heat Affected Zone Microstructure

Gigacycle Fatigue Fracture of Low Strength Carbon Steel, Tested using a Simulated Heat Affected Zone Microstructure

Effect of Ti–Mg–Ca treatment on properties of heat-affected zone after high heat input welding

The combined influence of Mg and Ca treatment on the properties of heat-affected zone (HAZ) of low-carbon steel after high heat input welding was systematically studied. Experimental steels deoxidized with different elements were prepared, i.e., C–Mn steel with Al, Ti–Ca steel with Ti and Ca, Ti–Mg–Ca steel with Ti, Mg and Ca. Results showed that the inclusions in C–Mn steel were mainly Al2O3 and MnS with low density and large size. However, the average size was refined to only ~ 0.34 μm in Ti–Mg–Ca steel and the amount increased remarkably. Microstructure of simulated HAZ for 200 kJ/cm changed from ferrite side plates or upper bainite to acicular ferrite after treatment with Ti, Mg and Ca. Ca addition decreased the strain field around inclusions and enhanced the ability of acicular ferrite nucleation. In situ observation of Ti–Mg–Ca steel showed that the movement of austenite grain boundaries was retarded and nucleation sites of acicular ferrite were greater than Ti–Ca steel because of Mg addition. Impact energy of HAZ at − 40 °C was increased from 7 to 232 J and showed excellent stability because of Ti–Mg–Ca treatment. High volume fraction of acicular ferrite acted as obstacles toward cleavage cracks.

Microstructure and Corrosion Behavior of Simulated Welding HAZ of Q315NS Steel in Sulfuric Acid Solution

The microstructure evolution and the corrosion behavior of welding heat affected zone (HAZ) of Q315NS steel in 50 wt % H2SO4 at 20 °C was investigated with thermal simulation technique, surface analysis and electrochemical tests. The microstructure of ferrite and pearlite was observed in base metal (BM), fine grained region (FGHAZ) and inter critical region (ICHAZ) while coarse grained region (CGHAZ) consisted of granular bainite. The CGHAZ exhibited the highest microhardness and the largest average grain size. The passivation process occurred on the surface of all specimens. Different microstructure give birth to different corrosion behaviors between CGHAZ and BM, FGHAZ, ICHAZ. The dense oxide film were formed on the surface of ferrite while oxide film with micro voids were formed on the surface of pearlite in BM, FGHAZ and ICHAZ after immersion in 50 wt % H2SO4 solution for 12 h. The rod-shaped corrosion product was formed on the surface of CGHAZ while the porous-structured corrosion product was formed on the surface of BM, FGHAZ and ICHAZ after immersion in 50 wt % H2SO4 solution for 72 h. The corrosion resistance of BM, CGHAZ, FGHAZ and ICHAZ increased during the first 12 h and then declined slowly with increasing immersion time. The BM had the best corrosion resistance while the CGHAZ had the lowest corrosion resistance throughout the corrosion process.

Effect of Welding Thermal Cycles on Microstructure and Mechanical Properties of Simulated Heat Affected Zone for a Weldox 1300 Ultra-High Strength Alloy Steel

In the present study, the investigation of weldability of ultra-high strength steel has been presented. The thermal simulated samples were used to investigate the effect of welding cooling time t8/5 on microstructure and mechanical properties of heat affected zone (HAZ) for a Weldox 1300 ultra-high strength steel. In the frame of these investigation the microstructure was studied by light and transmission electron microscopies. Mechanical properties of parent material were analysed by tensile, impact and hardness tests. In details the influence of cooling time in the range of 2,5 ÷ 300 sec. on hardness, impact toughness and microstructure of simulated HAZ was studied by using welding thermal simulation test. The microstructure of ultra-high strength steel is mainly composed of tempered martensite. The results show that the impact toughness and hardness decrease with increase of t8/5 under condition of a single thermal cycle in simulated HAZ. The increase of cooling time to 300 s causes that the microstructure consists of ferrite and bainite mixture. Lower hardness, for t8/5 ≥ 60 s indicated that low risk of cold cracking in HAZ for longer cooling time, exists.

Effect of Single and Multiple Thermal Cycles on Microstructure and Mechanical Properties of Simulated HAZ in Low Carbon Bainitic Steel

The correlation of microstructure and mechanical properties in simulated heat affected zone (HAZ) of a low carbon bainitic steel plate of thickness 28 mm using single and multiple thermal cycles was investigated in the Gleeble 3800 Thermo Mechanical Simulator. Optical microscopy, field emission scanning electron microscopy, hardness, and impact strength measurements were done to characterize the weld HAZ simulated steel samples. Peak temperatures (Tp) of 1300, 1150, 1000, 900, 800, 700, and 600°C with heat input 50 kJ/cm were used for single pass weld. In two pass weld cycles, peak temperature (Tp1) 1300°C, and (Tp2) of 1100, 1000, 900, 800, and 700°C were used. This heat input is in line with the submerged arc welding (SAW) process extensively used in the fabrication of ship building industries. The impact toughness of weld HAZ simulated specimens was determined at −50°C by using a Charpy impact tester. Best impact toughness values (minimum 75 J) in weld HAZ simulated specimen were observed at the peak temperatures from 600 to 1150°C in single pass weld and from 1000 to 1100°C in two pass weld HAZ thermal cycles.

In-situ Corrosion Characterization of API X80 Steel and Its Corresponding HAZ Microstructures in an Acidic Environment

During heat treatment processing, microstructures of heat affected zone (HAZ) were formed in X80 pipeline steel. After observation by optical microscopy, scanning electron microscopy and transmission electron microscopy, microstructure of the as-received X80 steel was confirmed to be acicular ferrite, while the microstructures of quenched, normalized and annealed X80 steels were lath bainite, granular ferrite and quasi-polygonal ferrite, respectively. After immersion in the simulated acidic soil solution for 48 h, corrosion rates of these four steels were determined by mass loss measurements and corrosion products were examined by Raman spectroscopy and X-ray photoelectron spectroscopy. Scanning vibrating electrode technique was used to characterize the micro-galvanic corrosion behaviors of the synthetic bimetallic electrodes which were formed by coupling each of the simulated HAZ microstructures with the as-received steel in direct physical and electric contact. It is demonstrated that the as-received steel acts as cathode in the as-received/quenched and as-received/normalized couples, while the annealed steel acts as cathode when coupling with the as-received steel. The distinction of current density between the galvanic couples reduces with prolonging the immersion time.

Physical Simulation of Weldability of Weldox 1300 Steel

In the present study, the investigation of weldability of new ultra-high strength - Weldox 1300 steel has been presented. The thermal simulated samples were used to investigate the effect of welding cooling time t8/5on the microstructure and mechanical properties of the heat affected zone (HAZ). In the frame of these investigation the microstructure was studied by the light (LM) and transmission electron microscopies (TEM). It has been shown that the microstructure of the Weldox 1300 steel is composed of tempered martensite, and inside the laths the minor precipitations mainly V(CN) and molybdenum carbide Mo2C were observed. Mechanical properties of parent material were analysed by the tensile, impact and hardness tests. In details the influence of cooling time in the range of 2,5 - 300 s. on hardness, impact toughness and microstructure of simulated HAZ was studied by using welding thermal simulation test. The results show that the impact toughness and hardness decrease with the increase of t8/5under the condition of a single thermal cycle in simulated HAZ. The continuous cooling transformation diagrams (CCT-W for welding conditions) of Weldox 1300 steel for welding purposes was also elaborated. The steel Weldox 1300 for cooling time in the range of 2,5 - 4 s showed martensite microstructure, for time from 4 s to 60 s mixture of martensite and bainite, and for longer cooling time mixture of ferrite, bainite and martensite. The results indicated that the weldability of Weldox 1300 steel is limited and to avoid the cold cracking the preheating procedure or medium net linear heat input should be used.

Investigation of the Tensile Shear fracture of Advanced High Strength Steel spot welds

Damage of Advanced High Strength Steel resistance spot welds is investigated in Tensile Shear by means of coupled microtomography, metallography and fractography. Two main failure mechanisms and failure zones are identified: (i) strain localization in the base metal/subcritical Heat Affected Zone (HAZ), (ii) ductile shear at the interface in the weld nugget. A finite element model is developed in order to illustrate these competing failure mechanisms leading to distinct macroscopic failure types. The local constitutive behavior is obtained from tensile tests on simulated HAZ microstructures and an estimation of the Rice integral (J) at the notch tip is provided for the assessment of interfacial failure. The model enables capturing the main trends in failure type transition as a function of weld geometry as well as a reliable estimation of the load bearing capacity.

Experimental and modeling investigation of the failure resistance of Advanced High Strength Steels spot welds

Damage of Advanced High Strength Steel resistance spot welds is investigated in Cross Tension by means of coupled microtomography, metallography and fractography. Three main failure mechanisms and failure zones are identified: (i) strain localization in the base metal/sub-critical Heat Affected Zone (HAZ), (ii) ductile shear around the weld and (iii) semi-brittle fracture in the weld nugget. A finite element model is developed in order to illustrate how the mechanisms compete and lead to a given failure type. The local constitutive behavior is obtained from tensile tests on simulated HAZ microstructures. The model enables capturing the main trends in the transition between failure types as a function of weld geometry as well as a reliable estimation of the load bearing capacity.

Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy

The effect of preweld overaging heat treatment on the microstructural response in the heat-affected zone (HAZ) of a precipitation-hardened nickel-base superalloy INCONEL 738LC subjected to the welding thermal cycle (i.e., rapid) was investigated. The overaging heat treatment resulted in the formation of an interfacial microconstituent containing M23X6 particles and coarsening of primary and secondary γ′ precipitates. The HAZ microstructures around welds in the overaged alloy were simulated using the Gleeble thermomechanical simulation system. Microstructural examination of simulated HAZs and those present in tungsten inert gas (TIG) welded specimens showed the occurrence of extensive grain boundary liquation involving liquation reaction of the interfacial microconstituents containing M23X6 particles and MC-type carbides. In addition, the coarsened γ′ precipitate particles present in the overaged alloy persisted well above their solvus temperature to temperatures where they constitutionally liquated and contributed to considerable liquation of grain boundaries, during continuous rapid heating. Intergranular HAZ microfissuring, with resolidified product formed mostly on one side of the microfissures, was observed in welded specimens. This suggested that the HAZ microfissuring generally occurred by decohesion across one of the solid-liquid interfaces during the grain boundary liquation stage of the weld thermal cycle. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that the temperature at which the alloy exhibited zero ductility during heating was within the temperature range at which grain boundary liquation was observed. The on-cooling ductility of the alloy was significantly damaged by the on-heating liquation reaction, as reflected by the considerably low ductility recovery temperature (DRT). Important characteristics of the intergranular liquid that could influence HAZ microfissuring of the alloy in overaged condition are also discussed.

On the role of liquated γ′ precipitates in weld heat affected zone microfissuring of a nickel-based superalloy

The role of γ′ precipitate particles, which is the principal strengthening phase of most of the precipitation hardened nickel base superalloys, on weld heat affected zone (HAZ) cracking was investigated. The HAZ microstructures around welds in a commercial nickel-based superalloy IN 738LC were simulated using Gleeble thermomechanical simulation system. Microstructural examination of the simulated HAZs and those present in actual tungsten inert gas (TIG) welded specimens showed that γ′ particles persisted during heating to the welding simulation temperatures, where they reacted with the surrounding γ matrix producing liquid film by a eutectic-type reaction, which subsequently infiltrated the grain boundary regions. The on-cooling ductility of the alloy was significantly reduced as shown by the comparatively low ductility recovery temperature. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that HAZ cracking resistance was damaged by liquation reaction involving the γ′ precipitate particles. The results support previously reported observations which indicated that γ′ precipitate particles could contribute to HAZ microfissuring in superalloys through liquation reaction besides the generally reported rapid solid-state re-precipitation of γ′ phase.

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

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

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.