This study is concerned with a correlation between the microstructure and the local brittle zone (LBZ) phenomena in high-strength low-alloy (HSLA) steel welds. The influence of the LBZ on toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in microvoid and cleavage microcrack formation were also identified using notched round tensile tests and subsequent scanning electron microscopy (SEM) analyses. The LBZ in the HAZ of a mUltipass welded joint is the intercritically reheated coarse-grained HAZ whose properties are strongly influenced by metallurgical factors such as an effective grain size and high-carbon martensitic islands: The experimental results indicated that Charpy energy was found to decrease monotonically with increasing the amount of martensitic islands, confirming that the martensitic island is the major microstructural factor controlling the HAZ toughness. In addition, microvoids and microcracks were found to initiate at the interface between the martensitic island and the ferrite matrix, thereby causing the reduction in toughness. These findings suggest that the LBZ phenomena in the coarse-grained HAZ can be explained by the morphology and the amount of martensitic islands.

Microstructure and Local Brittle Zone Phenomena in High-Strength Low-Alloy Steel Welds

A new ultrahigh strength niobium-microalloyed pipeline steel of yield strength ∼700 MPa has been processed. The Charpy impact toughness at 0 °C was 27 J and tensile elongation was 16%. The ultrahigh strength is derived from the cumulative combination of fine grain size, solid solution strengthening with additional interstitial hardening, precipitation hardening from carbides, dislocation hardening, and mixed microstructure. The microstructure was characterized by polygonal ferrite, upper bainite, degenerated pearlite, and martensite–austenite (MA) constituents. The microstructure of weld and heat-affected zone (HAZ) was similar to the base metal such that the hardness is retained in the weld region implying insignificant softening in the weld zone. Niobium and titanium precipitates of different morphology and size range evolved during thermomechanical processing and include rectangular (∼500 nm), irregular (∼240–500 nm), cuboidal/spherical (∼125–300 nm), and very fine (

Microstructure and high strength–toughness combination of a new 700 MPa Nb-microalloyed pipeline steel

Stress Corrosion Cracking of Pipelines

The paper describes the microstructural constituents in a industrially processed Nb-microalloyed pipeline steel having yield strength of ∼620 MPa. The microstructure of base, heat affected zone (HAZ), and weld metal of the fabricated steel pipe was examined by optical and transmission electron microscopy. The microstructure of thermomechanically processed pipeline steel primarily consisted of non-equiaxed ferrite of mixed morphologies with small fraction of degenerated pearlite. The microstructure contained high dislocation density, sub-boundaries and dislocation substructures. The HAZ was characterized by a combination of fine and coarse grained polygonal ferrite structure with high density of dislocations and fine cementite particles. In the weld metal, the constituents of complex ferrite were low temperature transformation products formed during continuous cooling such as quasi-polygonal or massive ferrite, acicular ferrite, bainitic ferrite and dispersion of coarse and fine cementite particles in the ferrite matrix. The precipitates in the investigated pipeline steel were of duplex type containing either Nb and Ti or Ti and Mo, even though the steel contained low concentration of titanium. Precipitates of different morphology and size range were observed and include rectangular (∼100–130 nm), cuboidal/spherical (∼20–100 nm), fine (∼10–20 nm) and very fine (

Microstructure of high strength niobium-containing pipeline steel

Porosity variation along multipass underwater wet welds and its influence on mechanical properties

The production of American Petroleum Institute (API) class steels using the traditional controlled rolling route rather than the process involving accelerated cooling necessitates a careful adjustment of steel composition associated with the optimization of the rolling schedule for the deformation and phase transformation characteristics of these modified alloys. The current work presents a study of two, NbCr and NbCrMo, steel systems. The microstructure obtained is correlated not only with the resulting mechanical properties, but also with the weldability and resistance to damage in the aggressive environments to which the materials are exposed. The evaluation of the steels was undertaken at two stages along the production route, sampling the material as plate and as tubular product, according to the API 5L 2000 standard. Tensile testing, Charpy-V impact testing, and hardness measurements were used to determine the mechanical properties, and microstructural characterization was performed by optical and scanning electron microscopy. The results showed that it was possible to obtain good impact properties, for both steels, in plate and tube formats. The Charpy-V impact energy, measured at −20 °C from 100 to 250 J corresponds to a toughness level above that required by the API 5L 2000 standard, which specifies 68 to 101 J at 0 °C. The yield strength (YS) to ultimate tensile strength (UTS) ratio was determined to be 0.8, the API standard establishing a maximum limit of 0.93. Both of the alloys investigated exhibited a bainitic microstructure and were successfully processed to fabricate tubular products by the “UOE” (bending in “U”, closing in “O,” and expanding “E”) route. with regard to weldability, the two experimental steels exhibited a heat-affected zone (HAZ) for which toughness levels (using the temperature associated with a 100 J impact energy as a base for comparison) were higher than those for both the base metal (BM) and the weld metal (WM) itself. In order to perform the evaluation of the behavior of the steels in an aggressive environment, more specifically their resistance to the deleterious effects of H2S, slow strain rate tests (SSRTs) were carried out, immersing the samples in a sodium thiosulfate solution during the tests. Though no secondary cracking was observed in the test samples, the ductility levels measured were lower than those for the same materials tested in air. Constant load tests were also conducted according to the standard NACE conditions. Despite the more aggressive nature of the test solution in these cases, no samples of either steel suffered failure.

High-strength steel development for pipelines: A brazilian perspective

The Effect of Nickel on the Mechanical Properties of High-Oxygen Underwater Wet Welds

This study was part of a program to investigate the influence of multiple post weld heat treatment (PWHT) on the fracture toughness and defect tolerance of a welded joint. The present work reports base metal data obtained for a quenched and tempered BS7191 Grade 450EM steel (0.10wt%C-1.08wt%Mn), weld metal data for a ferritic multipass weld obtained by shielded metal arc welding using an AWS E-9018M type electrode, and heat affected zone (HAZ) data obtained using a modified bead on groove technique for different PWHT conditions. The effect of the repeated heat treatment cycles on the mechanical properties was evaluated by hardness tests and toughness testing assessed by Charpy V-notch and crack tip opening displacement (CTOD) techniques. The characterization of the microstructure was undertaken utilizing optical and electron microscopy. As fabrication codes for new equipment do not allow more than three PWHT cycles, the application of more cycles is only justifiable for old equipment when a fitness for purpose criterion is applied and these restrictions are not applicable. The results obtained are currently applied in repair work and revamps of pressure vessels and gas storage tanks.

Toughness evaluation of a shielded metal arc carbon-manganese steel welded joint subjected to multiple post weld heat treatment

The use of oxidizing electrodes for wet welding of offshore structural steels, in spite of their low susceptibility to hydrogen HAZ cracking, is limited, in part by the poor mechanical properties of their weld deposits. Low levels of carbon, manganese and other deoxidizers, together with high oxygen contents seems to be one of the reasons for this low performance. This work investigated the influence of nickel additions on the tensile strength and impact resistance of wet welds deposited at 1.1 m of water depth. It was found that maximum values of toughness and tensile strength occur for nickel contents between 2 and 3 weight percent. Nickel additions also had a strong effect in reducing the grain size of equiaxed ferrite in the reheated region of underwater wet welds thereby improving their mechanical properties. The deterioration of mechanical properties for nickel contents higher than 3 weight percent was attributed to weld metal solidification cracking.

José C. G. Teixeira

Papers

High-strength steel development for pipelines: A brazilian perspective

The Effect of Nickel on the Mechanical Properties of High-Oxygen Underwater Wet Welds

Toughness evaluation of a shielded metal arc carbon-manganese steel welded joint subjected to multiple post weld heat treatment

Use of nickel to improve the mechanical properties of high oxygen underwater wet welds

Comparison between real and simulated degradation in a 1.25% Cr–0.5% Mo steel for high temperature service