Jae-il Jang

Assessing welding residual stress in A335 P12 steel welds before and after stress-relaxation annealing through instrumented indentation technique

Abstract Conventional nondestructive techniques for welding residual stress measurement have many disadvantages in the field because of poor repeatability, large scatter in data, complex procedures, inaccurate results, etc. To overcome these difficulties, an instrumented indentation technique was applied to evaluate the welding residual stress in A335 P12 steel welds in electric power-plant facilities before and after stress-relaxation annealing. Comparison of our results with stress values obtained from a destructive saw-cutting test showed that the instrumented indentation technique is very useful for quantitative/nondestructive evaluation of welding residual stresses in industrial facilities such as power-plants.

Effects of microstructural change on fracture characteristics in coarse-grained heat-affected zones of QLT-processed 9% Ni steel

Abstract This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite–austenite (M–A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed.

Determination of welding residual stress distribution in API X65 pipeline using a modified magnetic Barkhausen noise method

A modified magnetic Barkhausen noise (MBN) method was applied to obtain the residual stress distribution in an API X65 pipeline weldment. Different weldment microstructures affected the magnetic response and yielded different MBN values. In order to reflect the microstructural variations in the heat-affected zone (HAZ), calibration samples were extracted from four different regions: weld metal, coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and base metal. This approach yielded that compressive residual stresses existed in the CGHAZ contrary to the tensile results using the base-metal-based calibration method. Compared with the results from the mechanical saw-cutting method, it can be concluded that the data obtained with the HAZ-based calibration method were more reliable.

Evaluation of fracture toughness using small notched specimens

Abstract As a part of small specimen testing techniques, the notched specimen technique is studied to overcome the difficulties in introducing fatigue precrack to small fracture toughness specimens. It was noted that stress triaxiality and the plastic constraint ahead of a notch root decreased with increasing notch root radius ( ρ ). Considering that the applied stress at the notch tip is redistributed and relaxed due to the increased plasticity, the fracture toughness obtained from notched specimens was corrected. Fracture toughness testing was conducted using an instrumented impact and a static three-point bending tester. The specimens had notch root radii which ranged from a fatigue precrack to a Charpy notch root radius of 250 μm. Fracture toughness values corrected from the results of notched specimens were very consistent with the plane strain fracture toughness obtained from precracked specimens. In addition, a limiting notch root radius ( ρ 0 ), below which the fracture toughness was independent of ρ , was observed. To investigate this phenomenon, the fracture surface was observed with a scanning electron microscope (SEM).

Determination of microstructural criterion for cryogenic toughness variation in actual HAZs using microstructure-distribution maps

It is well known that heat-affected zone (HAZ)-notched fracture mechanics tests, such as crack tip opening displacement (CTOD) test, are very sensitive to the microstructural gradient in the HAZ. This study aimed to investigate the distribution of microstructures, including local brittle zones (LBZs), in the actual HAZ of advanced 9% Ni cryogenic steel, and to determine a microstructure criterion for the change in cryogenic toughness of the steel HAZs using microstructure-distribution maps. To estimate the microstructural influence more systematically, K-grooved HAZ specimens were prepared. Analysis using these microstructure-distribution maps constructed from the specimens tested at 111 K revealed that the weakest-link-type fracture occurs mainly in regions having a large fraction of LBZs (over 17%), whereas the fracture toughness of the other regions was found to be governed by a mixture-rule of the microstructures existing at the precrack tip front of CTOD specimen.

Nondestructive observation on tensile property change of hydrogen-exposed Cr-Mo-V steel HAZ using an instrumented indentation technique

Long-term service of materials in environments of high hydrogen pressure can cause various sorts of damage such as temper embrittlement, hydrogen-assisted degradation and hydrogen attack [1‐3]. The demand in the petrochemical refinery industry for superior resistance to this damage has promoted the use of V-modified Cr-Mo steels for petrochemical reactor vessels. Like other structural steels, the Cr-Mo-V steels undergo welding during vessel construction. The superior properties of this steel must be carefully reconsidered because, as is well accepted, welding can seriously alter metallurgical and mechanical properties, generally for the worse. In particular, the coarse-grained heataffected zone (CGHAZ) adjacent to the fusion line is known as one of the weakest regions in the welded joints [4]. In addition, it has been reported that the CGHAZs of Cr-Mo steels are much more sensitive to hydrogeninduced damage than the base material [2]. Therefore, proper evaluation of the time-dependent change in mechanical properties of Cr-Mo-V steel joint CGHAZs exposed to hydrogen is very important in assessing the safety performance of the petrochemical reactor vessels under high hydrogen pressure. The present work, the first of a series of studies of the time-dependent degradation of CGHAZs in Cr-Mo-V steels according to increasing hydrogen exposure time, uses a nondestructive instrumented indentation technique to clarify the change in tensile properties.

Experimental analysis of the practical LBZ effects on the brittle fracture performance of cryogenic steel HAZs with respect to crack arrest toughness near fusion line

Abstract Focusing on crack arrest behavior, this study investigates the practical influence of local brittle zones (LBZs) on the brittle fracture resistance of heat-affected zones (HAZs) in advanced 9% Ni cryogenic steel welds, and discusses whether the LBZs of this steel in practice have potentially deleterious effects as previously thought, or not. By analyzing the variations in brittle crack arrest toughness (Ka) and brittle crack initiation toughness (Kc) within actual HAZ, it is found that LBZs of this steel may not be harmful in consideration of crack arrest toughness near fusion line.

Evaluation of cryogenic fracture toughness in SMA-welded 9% Ni steels through modified CTOD test

As the first step of the study for the safety performance of LNG storage tank based on the concept of fitness-for-purpose, the change of cryogenic toughness within the X-grooved weld HAZ (heat-affected zone) of SMA (shielded metal arc)-welded QLT (quenching, lamellarizing, and tempering)-processed 9% Ni steels, was investigated qualitatively and quantitatively. In general, CTOD (crack tip opening displacement) test is widely used to determine the fracture toughness of steel weldments. But there is no standard or draft for evaluating the toughness of thick weldment with X-groove such as in this case. Therefore, in this study, modified CTOD testing method for fatigue precracking. calculation of CTOD, examination of fractured specimen was proposed and used. And the results of modified test were compared with those of conventional CTOD test and Charpy V-notch impact test. In addition, the relationship between the fracture toughness and microstructure was analyzed by OM, SEM and XRD. The cryogenic toughness in HAZ decreased as the evaluated region approached the fusion line from base metal. The decrease in toughness was apparently caused by the reduction of the retained austenite content and the absence of grain refinement effect in the coarse-grained zone in HAZ. The austenite reduction resulted from the decrease in nucleation sites for α’γ reverse transformation due to the increase in fraction of coarse-grained zone within HAZ. More complex thermal cycles in the mixed zone of weld metal and base metal caused the poor stability of retained austenite in the zone by the redistribution of alloying element in retained austenite. Due to this reason, the toughness drop with decreasing test temperature in F.L. (fusion line)-F.L.+3 mm was larger than that in F.L.+5 mm and F.L.+7 mm.

Instrumented Indentation Technique to Measure Flow Properties: A Novel Way to Enhance the Accuracy of Integrity Assessment

While most in-field technologies for structural integrity diagnosis focus on precise crack detection, the instrumented indentation technique has emerged as one of the most practically useful technologies for non-destructive and quantitative in-field measurement of mechanical properties. In a similar vein, here an advanced indentation technique for determining tensile properties and its application to structural integrity assessment are introduced and discussed. This novel indentation technique can enhance the accuracy of fitness-for-service (FFS) assessment by application to failure assessment diagram (FAD) construction.Copyright

Fitness-for-Service Assessment for Weldments of the Natural Gas Pipeline by Using Failure Assessment Diagram

There are buried natural gas pipelines of which total length amounts to about 2.1×106 m in Korea, and it is very important issue to evaluate FFS (Fitness-for-service) when a crack-like flaw was found in operating pipelines. But, the research about this had not yet been performed in Korea. So, this study constructed a FFS code appropriate to Korean natural gas pipeline through comparing and analyzing API 579 and BS 7910 that are lately. In addition, we developed the user-friendly software based on FFS code, so that field service workers who have little idea about fracture mechanics can use easily. The best merit of this code is that it is possible to evaluate FFS for welding HAZ in Korea natural gas pipeline.Copyright