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

1993 ◽  
Vol 115 (1) ◽  
pp. 66-75 ◽  
Author(s):  
J. P. Tronskar
Keyword(s):  
Fracture Toughness ◽  
Chemical Composition ◽  
Brittle Fracture ◽  
Yield Strength ◽  
Heat Affected Zone ◽  
Welding Process ◽  
Offshore Structures ◽  
Structural Steels ◽  
Coarse Grained ◽  
Experimental Investigations

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.

FE Simulation of Cold Cracking Susceptibility in X70 Structural Steel Welded Joints

2011 ◽  
Author(s):  
Vigdis Olden ◽  
Odd Magne Akselsen
Keyword(s):  
Hydrogen Embrittlement ◽  
Yield Strength ◽  
Structural Steel ◽  
Base Metal ◽  
Heat Affected Zone ◽  
Welded Joints ◽  
Fe Simulation ◽  
Coarse Grained ◽  
Best Fitting ◽  
Low Sensitivity

Fracture mechanics SENT testing and FE simulation to establish hydrogen influenced cohesive parameters for X70 structural steel welded joints have been performed. Base metal and weld simulated coarse grained heat affected zone have been included in the study. The base metal did not fail at net section stresses lower than 1.29 times the yield strength and reveals low sensitivity to hydrogen embrittlement. The weld simulated coarse grained heat affected zone was prone to fracture at stresses above 64% of the yield strength, which indicates hydrogen embrittlement susceptibility. The cohesive parameters best fitting the experiments are δc = 0.3 mm and σc = 1700 MPa (3.5·σy) for the base metal and δc = 0.3 mm and σc = 2100 MPa (2.6·σy) for the coarse grained heat affected zone.

Mechanical Behaviour of Intercritically Reheated Coarse-Grained Heat Affected Zone in High Strength Line Pipe Steels

2018 ◽  
Author(s):  
Madhumanti Mandal ◽  
Warren J. Poole ◽  
Thomas Garcin ◽  
Matthias Militzer ◽  
Laurie Collins
Keyword(s):  
Impact Toughness ◽  
Heat Affected Zone ◽  
Welding Process ◽  
Charpy Impact ◽  
High Strength ◽  
Coarse Grained ◽  
Fracture Mechanisms ◽  
Line Pipe ◽  
Secondary Cracks ◽  
The Impact

Multipass welding of high strength steels used for fabrication and joining of transmission pipelines presents a number of metallurgical challenges. A key concern is both the strength and toughness of the heat affected zone (HAZ) adjacent to both seam and girth welds. In this work, a systematic study has been conducted on regions of the heat affected zone in the base metal where the first welding pass produces a thermal excursion which results in a coarse-grained heat affected zone (CGHAZ). The subsequent weld pass involves intercritical annealing of this region, i.e. a microstructure associated with intercritically reheated coarse grain heat affected zone (ICCGHAZ). The small ICCGHAZ region is often identified as being particularly susceptible to crack initiation. This work was undertaken to understand microstructure development in this zone and how the ICCGHAZ may affect the overall performance of the HAZ. Gleeble thermomechanical simulations have been conducted to produce bulk samples representative of different welding scenarios. Charpy impact tests and tensile tests have been performed over a range of temperatures. It was found that when a continuous necklace of martensite-austenite islands form on the prior austenite grain boundaries (i.e. for a M/A fraction of ≈10%), the Charpy impact toughness energy is dramatically decreased and the ductile brittle transition temperature is significantly raised. Detailed studies on the secondary cracks have been conducted to examine the fracture mechanisms in the different microstructures. The results show that the lower bainite microstructures obtained after the 1st thermal treatment, representative of CGHAZ have excellent impact properties. The impact toughness of the microstructures typical of ICCGHAZ is strongly dependent on the composition as well as morphology and spatial distribution of the resulting martensite-austenite (M/A) islands transformed from inter-critically formed austenite. This zone can play a significant role in fracture initiation and thus needs to be considered in alloy and welding process designs.

Avoiding Local Brittle Zones in Offshore Pipeline Girth Welds for Reeling Installation Involving Large Plastic Strain

2013 ◽  
Author(s):  
Jens P. Tronskar ◽  
Vebjørn Andresen
Keyword(s):  
Fracture Toughness ◽  
Plastic Strain ◽  
Strain Curve ◽  
Structural Steels ◽  
Coarse Grained ◽  
Unstable Fracture ◽  
Large Plastic Strain ◽  
Acceptance Criteria ◽  
Offshore Pipeline ◽  
Girth Welds

Pipelines for reeling are designed to tolerate the large plastic strain associated with the reeling installation process based on widely accepted strain based design principles for subsea pipelines as described in Det Norske Veritas (DNV) Offshore Pipeline Code OS-F101: 2012 [1]. Engineering Critical Assessment (ECA) to develop flaw acceptance criteria for automatic ultrasonic testing (AUT) for girth welds subject to large plastic strain shall according to DNV-OS-F101: 2012 [1] and DNV RP-F108 [2] be carried out in accordance with BS 7910 [3], at assessment Level 3B, with amendments and adjustments described in Appendix A of DNV-OS-F101 for strain-based loading. This is a tearing analysis using the material specific failure assessment diagram (FAD), the material stress-strain curve and the fracture resistance J-R curve (or CTOD-R curve) for the HAZ or WM. It is therefore essential that the pipeline girth welds exhibit maximum load behavior and large tearing capacity to enable development of workable and practical flaw acceptance criteria for the girth welds on the stalks. Welds in offshore structural steels are known from the early 80s introduction of low carbon-manganese micro-alloyed steels, to occasionally exhibit low fracture toughness associated with so-called local brittle zones (LBZ) in the HAZ. Similarly, in the 90s LBZs were found in pipeline seam welds welded at high arc energies. Presence of such microstructures may have a dramatic effect on the coarse grained HAZ CTOD fracture toughness properties causing unstable fracture in the CTOD tests and CTOD values below 0.1 mm at test temperatures of 0°C and below. Recently low CTOD critical fracture toughness values due to pop-ins and unstable fracture initiation in the HAZ have been experienced for pipeline girth welds for reeling and investigation confirmed these were caused by LBZs. This paper makes a comparison with the situation experienced earlier for welds in structural steels and pipeline seam welds, to understand the factors influencing the LBZ formation, and to show how such problems can be avoided. To avoid LBZs formation in the girth welds is imperative for reeling installation, where the large plastic strain associated with reeling installation affects every girth weld.

Notch-Ductility Transition of Structural Steels of Various Yield Strengths

1972 ◽  
Vol 94 (1) ◽  
pp. 299-305 ◽  
Author(s):  
A. K. Shoemaker
Keyword(s):  
Fracture Toughness ◽  
Brittle Fracture ◽  
Fracture Behavior ◽  
Shear Fracture ◽  
Dynamic Fracture Toughness ◽  
Structural Steels ◽  
Published Data ◽  
Specimen Thickness ◽  
Plastic Energy ◽  
Lateral Contraction

The notch-ductility transition of six structural steels, A36, ABS-Class C, A302-Grade B, HY-80, A517-Grade F, and HY-130, ranging in yield strength from 36 to 137 ksi, was studied with the use of 5/8 and 1 in. dynamic-tear (DT) test specimens. The results were compared with previously published data for V-notch and fatigue-cracked Charpy tests and dynamic fracture-toughness (KID) tests. Energy, lateral-contraction, and fracture-toughness values were compared. The results of this study showed that the full-shear upper energy shelves in the Charpy V-notch and DT specimens are the products of constant average plastic energy densities for each steel and the plastic volume estimates for the fracture of the different specimens. The transition from ductile to brittle fracture behavior is essentially the same in the fatigue-cracked Charpy and DT specimens since, for each steel, the same lateral contraction was measured in each specimen broken at a given temperature. This lateral contraction increased exponentially with temperature until a full-thickness shear fracture developed. However, the maximum lateral contraction increased with increased test-specimen thickness, suggesting that the Kc values corresponding to full-shear fracture should also increase with thickness. Using the proportionality found between the lateral contraction and the values of KID2/σYDE for the brittle-fracture behavior of these steels, the Kc values are estimated to be as much as 4.5 times greater than the KIc values at the same temperatures. In general, the notch-ductility transition can best be quantitatively characterized by the lateral contraction through KID and Kc values, whereas upper shelf energies are related by constant plastic energy densities and plastic volumes which develop during fracture.

An investigation into the effect of welding parameters on fatigue crack growth rate and fracture toughness in friction stir welded copper sheets

2015 ◽  
Vol 232 (3) ◽  
pp. 191-203 ◽  
Author(s):  
Ali Alavi Nia ◽  
Ali Shirazi
Keyword(s):  
Fracture Toughness ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Heat Affected Zone ◽  
Base Material ◽  
Welding Process ◽  
Stir Zone ◽  
Welding Parameters ◽  
Friction Stir

In the present study, the effect of various factors of friction stir welding including rotational and traverse speeds of tool and in fact, the amount of the heat transferred within welding was evaluated on the resistance to fatigue crack growth and fracture toughness in different zones of welding copper sheets. In order to better assess these two properties, other mechanical properties such as tensile strength and hardness were also studied and the microstructure of different zones of welds was investigated using optic and electron microscopies. By doing this study, it became clear that the less the heat transferred to the plunging during the welding process, the better properties the resulting welds will have which well justifies the use of cooling in this study. Transferring heat to plunging causes the growth of grains in various zones and can cause the fatigue crack growth in heat-affected zone to increase averagely about 4.2 times the base material for different Δ K. In contrast, the occurrence of dynamic recrystallization in the stir zone as well as fragmentizing and alignment of grains in this zone can increase the resistance to fatigue crack growth up to 9-fold the resistance of the base material. The other interesting result of this study was that although the properties of stir zone improve by increasing the number of welding passes, the properties of its weakest zone i.e. the heat-affected zone will decline.

CTOD Fracture Toughness Tests and Numerical Simulation for Welded Joints of Q370qE

2009 ◽  
Author(s):  
Qiming Yu ◽  
Weiguo Wu ◽  
Jin Gan
Keyword(s):  
Fracture Toughness ◽  
Finite Element Analysis ◽  
Finite Element ◽  
Brittle Fracture ◽  
Yield Strength ◽  
Ocean Engineering ◽  
Plastic Part ◽  
Element Analysis ◽  
The Finite Element Analysis ◽  
Base Steel

In this investigation, Based on BS7448 fracture toughness test experiment standard and DNV-OS-C401, three-point crack tip open displacement (CTOD) bending tests were made using Q370qE base steel, weld seam and heat-affected zone (HAZ) samples at different sample thicknesses and at different temperature to study the elasto-plastic fracture toughness of the materials frequently used in ocean engineering structure, and the results were calculated using the p-V curves. The results show that different CTOD values corresponding to the occurrence of brittle fracture before yield strength, brittle fracture after yield strength, and ductile fracture are related to different material types, thicknesses, temperature; and the CTOD value is made up of elastic part and plastic part, the value of plastic part greatly influence the CTOD value. Finally, using ANSYS commercial software, bending test models all of base steel, welded joint, HAZ with various center through crack sizes were computed by three-dimensional finite element method, from the results of the finite element analysis, the simulated p-V curve was obtained. It could be found that the simulated curve was close with the experimental curves, so the finite element analysis was accurate. And these works also have important project practical value to the ocean engineering structure designer.

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