Heat-affected Zone Thermal Cycle Research Articles

Effect of pearlitic and bainitic initial microstructure on cementite spheroidization in rail steels

To improve the performance of the rails and wheels for the railway industry, new microstructures need to be researched. Nevertheless, the soft region observed in the weld heat-affected zone (HAZ), caused by the spheroidization of cementite, leads to localized loss of hardness. A methodology for calculating the percentage of cementite spheroidization is not well defined in the literature. Accordingly, a new calculating method for determining the spheroidized cementite fraction was proposed through image analysis. This calculation method observes the cementite area/morphology, not the spheroidized cementite gross area. Two starting microstructures (pearlite and lower bainite) were compared in their spheroidization tendency by divorced eutectoid transformation (DET) during a simulated HAZ thermal cycle. The phase transformation kinetics through the heat treatments was also analyzed using high-resolution dilatometry tests. The results showed that the lower bainite has a higher tendency for cementite spheroidization than the pearlite due to its initial configuration of carbides. Furthermore, it was confirmed that the higher the cooling rates, the lower the spheroidization tendency.

Influence of simulated heat-affected zone thermal cycle treatment on mechanical performances and microstructural stability of Ni–17Mo–7Cr based superalloy

In the work, a Ni–17Mo–7Cr (wt.%) based superalloy was prepared, and its high-temperature (800 °C) mechanical properties and microstructural stability was evaluated when subjecting to the treatment of simulated heat-affected zone (HAZ) thermal cycle. The results reveal that a lamellar-like structure, which occurs in the alloy, is actually the mixture of Ni matrix and carbides (mainly MoC). When the peak temperature is elevated up to 1350 °C during the HAZ simulation, an equivalent excellent mechanical property is obtained relative to that of the alloy without treatment, due to formation of the typical structures in the alloy. After tensile tests, the hybrid fracture behavior appears on the fracture surfaces, in which the inter-granular fracture with torturous fracture surfaces in company with the ductile fracture are concurrently found. At last, the simulated microstructure can maintain its stability during the tests of isothermal stress relaxation. Based on the results of tensile tests and stress relaxation, we can infer that the simulated microstructure can preserve its excellent mechanical properties and stability at high temperatures.

Effect of high-temperature aging on microstructure and mechanical properties of Ni–Mo–Cr based superalloy subjected to simulated heat-affected zone thermal cycle

In the work, the effect of high-temperature (800–1100 °C) aging treatment on microstructural evolution and mechanical properties of the Ni–17Mo–7Cr based superalloy subjected to heat-affected zone (HAZ) thermal cycle. When the aging temperature is in excess of 800 °C, massive tiny MoC particles and a few chromium carbides are precipitated in the alloy. The lamellar-like structures grow in the alloy and tiny MoC are precipitated around them. In addition, some nano-sized chromium carbides with needle-like morphology are formed in the MoC. With increasing the aging temperature, the tensile and yielding strength decrease slightly due to increased precipitates in the alloy. In comparison with the alloy that does not go through the HAZ simulation, both of their mechanical properties share the close value at each temperature. The retention interval above 1300 °C during the HAZ simulation also influences the alloy's microstructure and mechanical properties. More lamellar-like structures are produced when elevating the retention interval over 5 s. Due to the increase of solidification defect and stress concentration, the tensile and yielding strength decrease sharply when the interval is increased from 5 s to 10 s. Further improving the interval has a limited influence on the alloy's mechanical properties.

Investigation of Strength Recovery in Welds of NUCu-140 Steel Through Multipass Welding and Isothermal Post-Weld Heat Treatments

NUCu-140 is a ferritic copper precipitation-strengthened steel that is a candidate material for use in many naval and structural applications. Previous work has shown that the heat-affected zone (HAZ) and fusion zone (FZ) of NUCu-140 exhibit softening that is due to dissolution of the copper-rich precipitates. This study aims to recover the FZ and HAZ strength by re-precipitation of the copper-rich precipitates through either multiple weld passes or an isothermal post-weld heat treatment (PWHT). The potential use of multiple thermal cycles was investigated with HAZ simulations using a Gleeble thermo-mechanical simulator. The HAZ simulations represented two weld thermal cycles with different combinations of peak temperatures during the initial and secondary weld passes. To investigate the potential for a PWHT for strength recovery, gas tungsten arc weld samples were isothermally heated for various times and temperatures. Microhardness measurements revealed no strength recovery in the multipass HAZ samples. The time-dependent precipitate characteristics were modeled under the HAZ thermal cycle conditions, and the results showed that the lack of strength recovery could be attributed to insufficient time for re-precipitation during the secondary weld pass. Conversely, full strength recovery in the HAZ was observed in the isothermally heat treated samples. Atom probe tomography analysis correlated this strength recovery to re-precipitation of the copper-rich precipitates during the isothermal PWHT.

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.

Microstructure evolution in a Ni–Mo–Cr superalloy subjected to simulated heat-affected zone thermal cycle with high peak temperature

Abstract A typical Ni–Mo–Cr superalloy with basic composition of Ni–17Mo–7Cr (wt.%) was fabricated and the relationship between the microstructure and mechanical properties while it underwent simulated heat-affected zone thermal cycle (HAZ) treatment was investigated. The results show that the Ni–Mo–Cr alloy was mainly made up of Ni based solid solution and MoC carbides. The critical peak temperature that a unique lamellar-like structure occurred in the alloy was found to be 1300 °C, and they were firstly determined to be Ni matrix and carbides (MoC and chromium carbides) generated through local melting. Due to the formation of unique structure, the alloy exposed to HAZ thermal cycle with a peak temperature of 1300 °C could still maintain excellent high-temperature mechanical performance. The work carried out here will provide valuable guidelines in designing and applying the Ni–Mo–Cr series superalloys.

Characterization of the Ni–Mo–Cr superalloy subjected to simulated heat-affected zone thermal cycle treatment

A representative Ni–Mo–Cr superalloy with basic composition of Ni–17Mo–7Cr (wt.%) was fabricated in the work and the relationship between the microstructure and mechanical properties while it went through simulated heat-affected zone (HAZ) thermal cycle treatment was investigated. The results reveal that the Ni–Mo–Cr alloy mainly consisted of Ni matrix and MoC carbides. The critical peak temperature that a lamellar-like structure occurred in the alloy was found to be 1300°C. These products were firstly characterized by transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) analysis, and they were essentially Ni matrix and carbides (MoC and chromium carbides) generated through local melting. The equivalent mechanical properties of the alloy relative to that of un-treated alloy were received owing to its unique architecture even the peak temperature during thermal cycle was up to 1350°C. The results obtained suggests these lamellar-like products dispersed near the fusion line in a Ni–Mo–Cr welded joint will not influence the joint’s mechanical strength and stability while the peak temperature in the HAZ was adjusted below 1350°C, providing valuable guideline in designing and applying the Ni–Mo–Cr system superalloys.

Formation Mechanism of Type IV Failure in High Cr Ferritic Heat-Resistant Steel-Welded Joint

The mechanism of type IV failure has been investigated by using a conventional 9Cr ferritic heat-resistant steel Gr.92. In order to clarify the main cause of type IV failure, different heat treatments were performed on the base metal in order to change the prior austenite grain (PAG) size and precipitate distribution after applying the heat-affected zone (HAZ) simulated thermal cycle at the peak temperature of around Ac3 (Ac3 HAZ thermal cycle) and postweld heat treatment (PWHT). The microstructural evolution during the Ac3 HAZ thermal cycle and PWHT was investigated by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA), and transmission electron microscope (TEM). It was found that M23C6 carbides were scarcely precipitated at the newly formed fine PAG, block, and lath boundaries in Ac3 HAZ-simulated Gr.92, because the carbide forming elements such as Cr and C were segregated at the former PAG and block boundaries of the base metal. On the other hand, if all the boundaries were covered by sufficient M23C6 carbides by homogenization of the alloying elements prior to applying the HAZ thermal cycle, the creep strength was much improved even if the fine PAG was formed. From these results, it is concluded that fine-grained microstructure cannot account for the occurrence of type IV failure, and it only has a small effect during long-term creep. The most important factor is the precipitate formation behavior at various boundaries. Without sufficient boundary strengthening by precipitates, the microstructure of Ac3 HAZ undergoes severe changes even during PWHT and causes premature failure during creep.

Influence of Oxide Particles and Residual Elements on Microstructure and Toughness in the Heat-Affected Zone of Low-Carbon Steel Deoxidized with Ti and Zr

The microstructure and toughness in Fe–0.04%C–1.85%Mn–0.03%Si–0.018%Nb steel deoxidized with Ti and Zr have been studied as functions of particle characteristics, austenite grain size and soluble Ti and Zr contents using a simulated HAZ (heat affected zone) thermal cycle (peak temperature, 1400°C; peak holding time, 60 s; time of cooling from 800 to 500°C, 70 s) and submerged arc welding (heat input of 15 kJ/mm), respectively. Microstructures were studied in samples containing 1.0 to 1.5 μm-diameter oxide particles numbering 500 to 2000 mm–2 and with a soluble oxygen content of 10 to 30 ppm (measured before casting) and soluble Ti and Zr contents of 50–150 ppm. The γ grain size after HAZ thermal cycle in the range between 200 and 600 μm is controlled by pinning and solute drag. Small γ grain size below 300 μm was obtained with high soluble Ti and Zr contents of 110–160 ppm, whereas large γ grain size above 300 μm was obtained with low soluble Ti and Zr contents of 60–110 ppm. Two types of microstructures that showed high Charpy absorbed energy (VE(–10°C)= 150–250 J and VE(–50°C)= 50–150 J) were observed independent of γ grain size: One is acicular ferrite and a small amount of grain boundary ferrite (GBF) and ferrite side plate (FSP) and the other is GBF, FSP and granular bainitic ferrite. It was observed that low VE(T) values are attributed to the formation of porosity, large-size particles, carbides (+nitrides) and lathe bainitic ferrite.

Characterization of microstructural strengthening in the heat-affected zone of a blast-resistant naval steel

The influence of simulated heat-affected zone thermal cycles on the microstructural evolution in a blast-resistant naval steel was investigated by dilatometry, microhardness testing, optical microscopy, electron backscatter diffraction and atom-probe tomography (APT) techniques. Coarsening of Cu precipitates were observed in the subcritical and intercritical heat-affected zones, with partial dissolution in the latter. A small number density of Cu precipitates and high Cu concentration in the matrix of the fine-grained heat-affected zone indicates the onset of Cu precipitate dissolution. Cu clustering in the coarse-grained heat-affected zone indicated the potential initiation of Cu reprecipitation during cooling. Segregation of Cu was also characterized by APT. The hardening and softening observed in the heat-affected zone regions was rationalized using available strengthening models.

Effect of carbon content on creep rupture strength and microstructure in heat affected zone of heat resistant ferritic steel: alleviation of decrease in creep rupture strength in heat affected zone of heat resistant ferritic steel

The effect of the carbon content on the creep rupture strength and the microstructural change in weld heat affected zone (HAZ) during high temperature services was investigated in order to alleviate the decrease in the creep rupture strength in HAZ of heat resistant ferritic steels. The test specimens were prepared from 9Cr–3Co–3–W–V, Nb ferritic steel plates with carbon content ranging from 0˙005 to 0˙1%. After the simulated HAZ thermal cycle treatments at the peak temperature of 1273 K was applied on these specimens, creep rupture tests, aging tests and microstructural examinations were conducted. As a result, it was clarified that the creep rupture time of simulated HAZ became longer and the decrease in the creep rupture strength in HAZ was alleviated by decreasing the carbon content. Then the mechanism to explain the effect of the carbon content was discussed from a viewpoint of the growth of precipitates, such as M23C6 and MX, during long term heating.

Nanosize boride particles in heat-treated nickel base superalloys

Grain boundary microconstituents in aged nickel-based superalloys were studied by transmission electron microscopy techniques. A nanosized M 5 B 3 boride phase, possibly formed by intergranular solute desegregation-induced precipitation, was positively identified. The presence of these intergranular nanoborides provides reasonable clarification of a previously reported reduction of grain boundary liquation temperature during the weld heat affected zone thermal cycle.

Effects of heat-affected zone peak temperatures on the microstructure and properties of 2090 Al alloy

The influence of the heat-affected zone (HAZ) thermal cycles on the microstructure and mechanical properties of 2090 Al alloy has been investigated. Emphasis is placed on the effects of the HAZ peak temperatures on the T3 and T8 tempered 2090. The HAZ thermal cycles were simulated using a Gleeble, and tensile tests were conducted at 293 and 77 K. The difference in properties between the HAZ and the base metal is primarily caused by the dissolution of strengthening phases. At 578 K, degradation in strength occurs due to dissolution of δ’ phase and, at 773 K, due to dissolution of T1 phase. The thermal cycle to 648 K leads to an observable growth of T1 phase, but the volume fraction is too low to be an effective strengthener. As a result, at the 293 K test temperature, the base metal (BM) tempers become irrelevant with the peak temperatures above 578 K, and the ultimate tensile strength (UTS) curves of both the BM-T3 and BM-T8 coincide and follow the same trend up to 853 K. At the 77 K temperature, the strength and elongation of the BM and the thermally cycled specimens in the T3 and T8 tempered conditions follow the trend seen at 293 K and exhibit higher strengths and elongations.

An analytical electron microscope investigation of the phase transformations in a simulated heat-affected zone in alloy 800

The heat-affected zone (HAZ) hot-cracking behavior of Alloy 800 was investigated with hot ductility (Gleeble) testing during a simulated HAZ thermal cycle. Microstructural analyses were performed by optical metallography, fractography, electron microprobe analysis, and analytical electron microscopy on specimens that were water quenched from selected temperatures during this thermal cycle. Analysis of analytical electron microscopy (AEM) and Auger electron spectroscopy (AES) data suggests that incipient melting of the grain boundaries occurs at temperatures of 1300 °C and above. The HAZ hot-cracking mechanism was consistent with aspects of a constitutional liquation phenomenon involving a nonequilibrium eutectic-type reaction between grain boundary Ti(C, N) and the austenitic matrix. The extent of expected HAZ cracking would be low as the liquation of Ti(C, N) was localized and no lower melting intermetallic solidification products(e.g., Laves) were observed. The mechanistic observations were consistent with classical thermodynamic and solid state diffusion models.