Microstructure Of Simulated Heat Affected Zone Research Articles

Microstructural evolution of simulated heat-affected zone in modified 2.25Cr-1Mo steel during high temperature exposure

The effect of heat input with 20, 50 and 80 kJ/cm on the microstructural evolution of the simulated heat affected zone (HAZ) has been studied in a modified 2.25Cr-1Mo steel. The microstructures of simulated coarse-grained HAZ has been examined by optical metallography and transmission electron microscopy. It was found that large amounts of martensite with small quantities of bainite exist in the specimen with 20 kJ/cm. However, significant amount of bainite with a few amounts of allotriomorphic ferrite can be detected in the specimen with 50 kJ/cm heat input. In the case of heat input with 80 kJ/cm, the simulated HAZ microstructures were composed chiefly of bainite with a few amounts of martensite and allotriomorphic ferrite. In order to study the sequences of carbide transformation, the HAZ specimens were tempered at 700°C for different intervals (1, 2, 5, 10, 20, 50, 100, 200 and 500 h). The sequence of carbide transformation in the HAZ zone of this modified 2.25Cr-1Mo steel has been proposed.

Microstructural degeneration of simulated heat-affected zone in 2.25Cr–1Mo steel during high-temperature exposure

The microstructural of simulated heat-affected zone (HAZ) in 2.25Cr–1Mo steels has been investigated. The experiments with different heat inputs were carried out in a high speed dilatometer; the thermal cycles used corresponded to the actual thermal cycles that occurred in the coarse-grained region of the real HAZ. It was found that with a heat input of 20 kJ cm −1, the simulated HAZ microstructure gave larger amounts of lower bainite with significant amounts of martensite. The 20 kJ cm −1 heat input specimens were tempered at 700 °C for different time intervals ranging from 1 to 50 h. A sequence for corresponding microstructural degradation has been proposed.

Microstructural Investigations on Type IV Cracking in a High Cr Steel.

In the present study, microstructural changes that lead to Type IV cracking in a high Cr ferritic steel (11Cr-0.5Mo-2WCuVNb) weld joint has been investigated. Microstructure of the heat affected zone (HAZ) of a weld joint made by gas tungsten arc welding (GTAW) process was systematically studied in the as-welded condition, after different post weld heat treatments (PWHT) and after creep test. In addition, HAZ microstructures were simulated both by heat treatment and by using a weld simulator and creep tests were conducted using specimens with simulated HAZ microstructures. Results showed that, undissolved precipitates present in the fine grained HAZ (FGHAZ) and intercritial HAZ (ICHAZ) accelerate the tempering of these zones during PWHT and microstructural deterioration during creep. Creep tests of HAZ simulated specimens indicated that the creep was minimum for the specimens in which peak temperature of simulation was close to Ac3. Results also suggested that differences in creep properties observed between an actual weld joint and that of a simulated Ac3 microstructure could be due to mechanical constraints present in the weld joint in which a narrow zone of lower creep strength (FGHAZ) is sandwiched between zones of higher creep strength (coarse grained HAZ (CGHAZ) and weld metal on one side and base metal on the other side).

Microstructural dependence of Barkhausen noise and magnetic relaxation in the weld HAZ of an RPV steel

Magnetic Barkhausen noise and permeability spectra have been measured to characterize different microstructure regions such as coarse-grain region, fine-grain region, intercritical structure (composed of tempered martensite and bainite) within the heat-affected zone (HAZ) of SA508-3 steel weldments using simulated HAZ microstructure sample. The intercritical region and coarse-grained region can be distinguished from the BNE and relaxation frequency. The BNE was decreased in the martensite regions and increased in the bainite regions by the post-weld heat treatment (PWHT). The change of relaxation frequency also showed similar trends, but the rate of change was less than that of BNE. The behavior of BNE and permeability spectra on the corresponding microstructure can be explained in terms of carbide morphology and residual stress related with domain wall motion.

Evaluation of microstructures in 2.25Cr-1Mo and 9Cr-1Mo steel weldments using magnetic Barkhausen noise

Magnetic Barkhausen Noise (MBN) measurements have been used to characterise different microstructural regions such as coarse grain region, fine grain region, intercritical structure (composed of retained austenite and tempered ferrite) within the heat affected zone (HAZ) of 2.25Cr-1Mo steel and 9Cr-1Mo steel weldments using simulated HAZ microstructure samples. It is observed that MBN level decreases with increase in hardness corresponding to different microstructures. This behaviour has been confirmed by the measurements on the actual weldments of 2.25Cr-1Mo steel and 9Cr-1Mo steels. It has been observed that the intercritical region and the base metal region can be distinguished from the peak position of MBN which would be useful for measuring the extent of HAZ non-destructively. This study also shows the possibility of measuring the extent of different microstructural regions using miniature MBN probes with prior calibration.

Microstructural evolution of simulated heat affected zone in ASTM A533 type B steel plates

In order to investigate the weldability of ASTM A533 type B steel plates, simulated heat affected zone (HAZ) experiments with heat inputs of 20, 50, and 80 kJ cm−1 were carried out. The thermal cycles used corresponded to the actual thermal cycles that occur in the coarse grained region of the real HAZ. The microstructural development and the toughness of the simulated HAZ were studied. It was found that with a heat input of 20 kJ cm−1 the simulated HAZ microstructure gives larger amounts of lower bainite with significant amounts of auto tempered martensite. In the cases of heat inputs of 50 and 80 kJ cm−1 both the simulated HAZ microstructures were composed chiefly of upper bainite. The specimen obtained from the simulation of a heat input of 20 kJ cm−1 possesses higher toughness than those obtained from the simulations of heat inputs of 50 and 80 kJ cm−1 Furthermore, the results also indicate that the tempering response is more sensitive in autotempered martensite than in bainite.

Hot tensile properties of simulated heat-affected zone microstructures of 9Cr1Mo weldment

The heat-affected zones (HAZ) of 9Cr1Mo steel weldments consist of coarse-grain martensite with δ-ferrite, coarse-grain martensite, fine-grain martensite and intercritical structure. These HAZ microstructures have been simulated with respect to microstructure, hardness and grain size by isothermal heat treatment cycles in the temperature range 973–1573 K. Tensile tests at a strain rate of 3 × 10 −4s −1 over the range 298–873 K showed that 0·2% YS and UTS values for the various microstructural conditions were in the descending order of coarse-grain martensite, coarse-grain martensite with δ-ferrite, fine-grain martensite, base metal and intercritical structure. The presence of δ-ferrite in the coarse-grain region of HAZ restricted the grain growth, the increase in strength and the decrease in ductility. This minimises the problem of cracking in the coarse-grain region of HAZ in this steel which is often encountered in low-alloy CrMo ferritic steels. The regions with intercritical structure showed the lowest strength and fracture energy. Fine-grain martensite exhibited a good combination of strength and ductility in the HAZ.

Tempering effects on high temperature brittle intergranular fracture in Mn–Mo–Ni steel

AbstractThe influence of tempering on high temperature brittle intergranular fracture has been examined in the simulated heat affected zone microstructures of three Mn–Mo–Ni steel alloys, namely, the base alloy and phosphorus doped or phosphorus–sulphur doped base alloy. Crack growth behaviour was determined as a function of stress intensity at 550 and 450°C in high vacuum conditionsfor material as quenched and after tempering at 615°C. The resulting fracture surfaces were examined in detail and analysis of crack tip and general grain boundary chemistry was performed using scanning Auger electron spectroscopy. Microstructural and rheological changes induced by tempering were evaluated via electron microscopy and hardness tests. At 550°C, tempering largely eliminated high temperature brittle intergranular fracture, leaving intergranular microvoid coalescence as the dominant fracture mode. Crack growth rates were attenuated by tempering in alloys susceptible to high temperature brittle intergranular fractur...

The prediction of LCF properties of HAZ obtained during the welding process of SA 508 Class 2 and DIN 22NiMoCr37 structural materials

The main objective of this investigation has been to study the influence of the heat affected zone (HAZ) on room temperature low cycle fatigue (LCF) properties of two structural steels, type SA 508 Class 2 and DIN 22NiMoCr37. The predicted LCF properties of the HAZ have been related to the corresponding material microstructure obtained under quenched and tempered conditions. The HAZ microstructure has been reproduced by a weld cladding simulation heat treatment and any subsequent heat treatment on the simulated HAZ microstructure has been omitted. Although the thermal cycle introduced by the simulated welding process modifies the structure of the affected region and, as a result, diminishes the monotonic and cyclic ductile properties, it has been found that the cyclic strength properties of the simulated HAZ microstructure have been increased. The Feltner and Landgraf prediction criterion has been applied in order to obtain a reliable estimation of the LCF resistance of the materials tested in this investigation.

Caustic cracking of 9Cr 1Mo steel at 300°C

The stress corrosion cracking of correctly heat-treated 9Cr-1Mo in caustic solutions at 300°C has been examined using a bursting tube version of the slow strain rate technique. Some susceptibility was detected at all concentrations examined (1–8 M), the more concentrated solutions (3, 4 and 8 M) which favoured less protective oxide formation, producing the more severe cracking. The cracking in these strong solutions proceeded at velocities up to 0.1 mm h −1 and produced wide oxide filled cracks. This cracking is not strongly dependent on potential, although more anodic potentials promote thicker oxide formation while cathodic potentials favour general dissolution. Normalized and tempered, annealed, and a simulated heat-affected zone microstructure were all equally susceptible to cracking. Some constant pressure tests were performed which suggested cracking could occur at stresses below the yield stress. In more dilute solutions susceptibility was much less, and the crack velocity was about 0.01 mm h −1, i.e. ten times slower than in 8 M NaOH. The relationship between slow strain rate tests, conventional tests and practical experience are discussed. The behaviour of 9Cr 1Mo in caustic below 2 1 2 M places it in the low susceptibility category and failures under realistic conditions are unlikely.