Fracture Toughness Of Heat-affected Zone Research Articles

Microstructural Effects on J‐Integral Fracture Toughness of Welded High‐Mn Steels at 298 and 77 K

J‐integral fracture toughness tests are conducted on welded high‐Mn steels at room and cryogenic temperatures. The specimens are categorized into weld metal (WM) and heat‐affected zone (HAZ) to consider the different microstructural effects. The microstructure of WM is composed of dendrites formed during solidification, while the microstructure of HAZ consists of coarsened grains from the welding process. Both specimens show twinning behavior after J‐integral fracture toughness tests at 298 K and the superior K JIc fracture toughness of WM (389 MPa√m) due to the formation of plentiful twins at the crack tip. At 77 K, K JIc fracture toughness of HAZ (221 MPa√m) is lower than that of WM (264 MPa√m) due to different stacking fault energies (SFEs). Twinning still prevails in WM at 77 K, while ε‐martensite is formed in HAZ by relatively low SFE. The interface between γ‐matrix and ε‐martensite can act as a crack initiation site during the fracture toughness test, leading to the deterioration of J‐integral fracture toughness in HAZ.

Statistical Assessment of Fracture Toughness Results from the HAZ of X80 Pipeline FCAW Girth Weld.

Due to the wide application of flux-cored arc welds (FCAW) susceptible to significant scatter in weld and Heat Affected Zone (HAZ) fracture toughness, there is an interest in methods for evaluating the reliability of welds containing defects. The mechanical properties of the FCAW girth weld of an X80 pipeline are tested and then analyzed. By obtaining fracture toughness results from a statistically significant number of SENB specimens, with notches positioned in different HAZ locations, the effect of variation within the results can be evaluated. The results of the fracture toughness tests were analyzed using statistical methods, to compare both the difference in behavior between HAZ microstructures and the variation when a similar microstructure has been sampled. The range of different characteristic toughness values was analyzed using a postulated ECA case to illustrate the sensitivity of the results to how toughness is defined. The analyses supported recommendations to be made on the optimum approach to characterizing HAZ fracture toughness for reliable pipeline assessments in multi-pass girth welds with complex HAZ microstructure distributions.

Experimental study on fracture toughness of quenched and tempered and TMCP high strength steels

In this study, the fracture toughness of five kinds of high strength steel (HSS) with the nominal yield strength of 550 MPa, 690 MPa and 890 MPa were experimentally studied. Two steel delivery conditions, i.e., quenched and tempered HSS and thermo-mechanical control process HSS, were included and compared. The effect of heat input on the fracture toughness of HSS was investigated. A comparative study was organized for the fracture toughness of base material (BM), heat-affected zone (HAZ) and weld metal (WM). Results indict that the fracture toughness of the heat-affected zone of QT steels (QT550 and QT690) first increases and then decreases slightly with the increase of heat input. There is an optimal heat input for quenched and tempered HSS welding to achieve satisfactory fracture toughness. Either too small or large heat input is not beneficial to the resistance to crack propagation. With increase of steel strength, it seems the optimal heat input for quenched and tempered HSS becomes smaller. The effect of heat input on fracture toughness of TMCP HSS is different with quenched and tempered HSS: higher heat input would normally produce higher fracture toughness for HSS TMCP550. With the increase of yield strength, the fracture resistance increases for both quenched and tempered HSS and TMCP HSS.

The Analysis of Low Temperature Toughness on X80 Pipeline Steel Welded Joint

Aiming at the security problems of pipeline steel application, the different positions of the welded joints of circumferentially welding pipeline of X80 steel were investigated by microstructure observation, the hardness, Charpy impact toughness and crack tip opening displacement (CTOD) test at low temperature. The Vickers hardness test results show that there are local softened regions in heat-affected zone (HAZ). Charpy impact test indicate that the ductile–brittle transition temperature of weld is below − 60 °C, the ductile–brittle transition temperature of HAZ is around − 38 °C. CTOD test reveal that the fracture toughness of HAZ shows a large fluctuation since it is in the ductile–brittle transition temperature regime.

Mechanical and Microstructural Characterization of Heat-Affected Zone Materials of Reactor Pressure Vessel

In order to provide the technical basis for the judgment of the needs of surveillance specimens of heat-affected zone (HAZ) in reactor pressure vessel (RPV) steels, we performed a research on the characterization of metallurgical and mechanical properties of the HAZ in RPV steels. The distributions of grain size and phases were drawn as a map based on temperature histories around HAZ obtained from welding simulation. Referring to the HAZ map, typical HAZ materials were made by simulating temperature histories including postweld heat treatment (PWHT) within the HAZ. Metallurgical and mechanical characteristics for those HAZ materials were compared with those of base metal. From the results, it is clear that mechanical properties of HAZ materials depend not on the prior austenitic grain size but mostly on the phases. Concerning on the fracture toughness in HAZ, HAZ materials close to weld metal indicated higher toughness than that of base metal due to mixed structure of martensite and lower-bainite, while HAZ materials close to base metal were equivalent or slightly lower toughness than that of base metal due to the same phase as base metal of upper-bainite.

An experimental and theoretical consideration of the effect of prior creep damage on the heat affected zone fracture toughness of CrMoV steel

Material encompassing the heat affected zone has been removed from a region of a manual metal arc weldment in a CrMoV steel. This zone contained creep cavities on grain boundaries oriented normal to the direction of the maximum principal stress arising from the stress relief heat treatment. Fracture toughness tests using reconstituted Charpy geometry specimens with side grooves were undertaken at temperatures of 150 and 350 °C to assess the effect of the creep cavitation on the measured fracture toughness. The results are compared and discussed with respect to the predictions of a theoretical two-dimensional model.

Effects of weld strength undermatch on fracture toughness of HAZ notched weldments in a HSLA steel

In the present work the effects of weld strength undermatch on fracture toughness of heat affected zone (HAZ) have been studied. In the investigation a high strength low alloyed steel (HSLA) with 800 MPa strength class was used, and the undermatched welded joints were made with two weld strength mismatch levels. Three-point bending test specimens with crack depth to specimen width ratio a/W ranging from 0.05 to 0.5 were extracted from the welded joints. The test results show that strength mismatching gives an obvious influence on the fracture toughness of coarse grained HAZ for the undermatched joints. The lower the weld strength mismatching, the higher the fracture toughness of the HAZ. In addition the tendency of fracture toughness change with crack depths is much the same as in previous studies on base metals or weld metals, that is, fracture toughness of the HAZ is increased with reduction of crack depths. From the measured results it shows that the macroscopically mechanical heterogeneity of the welds may have more important influence on the fracture toughness of the HAZ than the meso-heterogeneity in the reheated coarse grained HAZ. Furthermore, numerical verification indicates that the stress triaxiality at crack tip may be the essential reason for the change of fracture toughness of HAZ. It is also shown that the yield strength of HAZ determined by the limit load in the three-point bend test represents the combinative effects of HAZ and its surrounding materials.

Heat-Affected Zone Fracture Toughness of 420–500-MPa Yield Strength Steels: Effects of Chemical Composition and Welding Conditions

During the last five years, high-strength steels with yield strengths in the range 420 to 500 MPa have attracted considerable interest within the offshore industry, primarily due to the potential for weight saving and reduction in volume of weld metal through the use of reduced section thicknesses. With respect to chemical composition these steels are developed following much the same philosophoy as for the modern normalized structural steels. Due to the increased stress level in these higher strength steels, it is anticipated that brittle fracture initiation occurring in the coarse-gained HAZ will be more critical for these steels than for the lower strength normalized grades. The objective of this paper is to present the results from several experimental investigations carried out at VERITEC during the last five years to study the factors affecting the crack tip opening displacement (CTOD) fracture toughness of the heat-affected zone (HAZ) in structural steels in the yield strength range 420–500 MPa. Typical CTOD fracture toughnesses of the HAZ in normalized 350-MPa yield strength steels used in offshore structures are also presented for comparison. The results of the investigations confirm that the same chemical compositional factors which are known to influence the HAZ fracture toughness of normalized steels are also important for the 420–500-MPa yield strength steels. It is demonstrated that the width of the HAZ is important for the initiation of brittle fracture of pop-in and that this width must exceed a certain minimum value for such events to occur. For different welding conditions, i.e., welding currents/travel speeds, the width of the coarse-grained HAZ changes considerably along the periphery of the weld bead. Thus, one may obtain widely differing results depending on the exact fatigue pre-crack placement. In addition to the weld cooling time between 800 and 500° C, Δt8/5 and the retention time above 1000° C during the weld thermal cycle, RT1000, the welding process and consumables also exert a strong influence on the CTOD values of the coarse-grained HAZ.

HAZ fracture toughness of titanium containing steels

AbstractSteels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm−1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.MST/887

溶接熱影響部の応力除去焼鈍と破壊靱性

溶接熱影響部の応力除去焼鈍と破壊靱性