Jang-Bog Ju

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.

Evaluation of fracture toughness by small-punch testing techniques using sharp notched specimens

The small punch (SP) testing method has been used to assess the reliability of industrial facilities such as fusion reactor structures and power-generation systems. Conventional SP tests have evaluated flow properties, transition temperature, fracture strain, and other mechanical properties by analyzing load-deflection curves. However, previous research has not used a fracture-mechanics approach that considers flaws, stress analysis and fracture toughness. In this study, in order to obtain the fracture toughness based on fracture mechanics by SP tests, a sharp notch was machined into the central part of the SP specimens. A stress-intensity factor for sharp-notched SP specimens is proposed from analysis of stress fields near the crack tip. The crack initiation point was determined by analysis of the load-deflection curves combined with acoustic emission signals. The fracture toughness of SA 508 Class 3 steel was successfully evaluated by using the load at the crack initiation point.

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.

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.

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.

Metallurgical and Mechanical Features of API 5L X65 Pipeline Steel Weldment

Substantial differences amongst metallurgical and mechanical properties of base metal (BM), weld metal (WM) and heat-affected zone (HAZ) occur in general in welded steel structures It is common practice in various engineering structures to evaluate the fracture performance of welded structures by mechanical testing. Especially, the HAZ of steel welded joints shows a gradient of microstructure and mechanical properties from the fusion line to the unaffected base metal. This study is concerned with the effects of metallurgical and mechanical factors on the fracture performance of API 5L X65 pipeline steel weldments, as they are generally used for main natural gas transmission pipelines in Korea. First of all, we investigated the microscopic and macroscopic fracture behavior of the various micro-zones within the HAZ from the viewpoint of metallurgical factors. The effects of mechanical factors such as welding residual stress in steel weldment and strength mismatch between BM and WM, particularly in high strength steel weldments, are also analyzed. Therefore, the fracture performance of API 5L X65 pipeline steel weldment was mainly dependent on the change of macrostructure and its distribution in the welded joints.Copyright

Assessment of fracture characteristics from revised small punch test using pre-cracked specimen

Small punch (SP) test has been utilized to analyze the neutron irradiation damage of nuclear vessels. Since this technique is easy, simple, and nondestructive, it can be applied to evaluate the mechanical properties and material degradation of in-service components. Conventional SP test has evaluated the ductile-brittle transition temperature and the equivalent fracture strain by the interpretations of load-deflection curve and the change of specimen thickness, respectively. The assumption that fracture occurs at maximum load is, however, not reasonable because the crack initiates at smaller load. In this study, in order to evaluate quantitatively fracture characteristics based on fracture mechanics, the pre-crack is introduced to SP specimen and acoustic emission is used to determine the crack initiation point. Using the load at crack initiation point, the fracture toughness of thin plate is calculated through bending theory. Therefore, the fracture characteristics of thin plate can be evalualed more reliably by using revised SP test.

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

Micromechanism of local brittle zone phenomenon in weld heat-affected zones of advanced 9% Ni steel

9% Ni steel has been widely used around the world as a material for the inner walls of liquefied natural gas (LNG) storage tanks because of its excellent cryogenic fracture toughness at or below 111 K, the boiling temperature of LNG. A new quenching, larmellarizing and tempering (QLT)-processed 9% Ni steel was developed and recently has been used as a material for super-sized LNG tanks because it enhances cryogenic toughness considerably over the conventionally heat treated 9% Ni steels such as quenching and tempering (QT) and double normalizing and tempering (NNT)-processed ones [1]. Like other structural steels, the QLT-9% Ni steels experience welding processes during the construction of the tanks. It is now widely recognized that a small zone of abnormally low fracture toughness, referred to as a local brittle zone (LBZ), can exist in the heat-affected zones (HAZs) of multi-pass welded structural steel. In general, the coarse-grained HAZ (CGHAZ) adjacent to the fusion line has the lowest toughness among various regions within HAZ because of unfavorable microstructure such as large prior austenite grain size [2–4]. Metallographic analyses reveal that CGHAZ can be roughly categorized into four characteristic zones according to the peak temperature of subsequent thermal cycle in a multi-pass welding procedure: (i) unaltered CGHAZ (UA CGHAZ), the region reheated above specific temperature of grain growth or not reheated at all, (ii) supercritically reheated CGHAZ (SCR CGHAZ), the region reheated above AC3, (iii) inter-critically reheated CGHAZ (IC CGHAZ), the region reheated between AC1 and AC3, and (iv) sub-critically reheated CGHAZ (SC CGHAZ), the region reheated below AC1. Many studies have been conducted on the LBZ phenomena observed in these CGHAZs for the various structural steels, used mainly for offshore structures [2–4]. However, for cryogenic steel such as 9% Ni steel, there have been few studies on the existence and behavior of LBZs at cryogenic temperature, and far fewer researches are available for QLT-9% Ni steel HAZ. The present work was undertaken to clarify the LBZ phenomenon of QLTtreated 9% Ni steel using simulated HAZ specimens. The 9% Ni steel used in this study is a commercial grade for LNG storage tanks in Korea, whose chemical composition is 0.066C-0.24Si-0.65Mn-0.005P0.005S-9.28Ni. The steel plates are normally processed by the QLT (Q; 1093 K for 60 min, quench, L; 963 K for 80 min, quench, and T; 853 K for 60 min, quench) heat treatment. For weld simulations, oversized Charpy specimen blanks (11 × 11 × 60 mm) were thermally cycled in metal thermal cycle simulator (MTCS). After reaching the first peak temperature (TP1) concerning CGHAZ of 1623 K, the specimens were cooled down with the constant cooling time from 1073 K to 773 K ( t8/5) of 13.5 and 19.4 s. The cooling rates were approximately equivalent to those of a submerged arc welding (SAW) and shielded metal arc welding (SMAW) processes with heat inputs of 23 kJ/cm and 28 kJ/cm, respectively, in a 22-mm-thick plate [5]. These simulated welding conditions were based on the actual welding conditions for the tanks in Korea [1]. The peak temperature of the second weld thermal cycle (TP2) was varied between 1473 K and 823 K. The Charpy V-notch impact specimens were machined from the simulated specimen blanks and then tested at 77 K. The results of the Charpy impact tests at 77 K are shown in Fig. 1, as a function of the peak temperature of the second thermal cycle. In the figure, the second thermal cycles with peak temperatures between 1473 K and 1373 K simulate UA CGHAZ while the cycles between