Hydrogen Induced Crack Development in Submerged Arc Welded Steel Pipes

The current work examines hydrogen sensitivity in different pipeline steels (X65, X70 and X80 HSLA grades) from four productions. Hydrogen Induced Cracking (HIC) experiments were performed and then the welds were characterized via optical and scanning electron microscopy techniques. The optical micrographs revealed cracks only in one of the four welds. Transverse cracks were found along bainitic-ferrite/carbide islands within the heat affected zone and the base metal of production B. Found inclusions e.g. MnS inside the cracks acted as initiation points for the HIC. However, the weld zones in all productions consisting of acicular ferrite and grain boundary ferrite were found to be resistant in hydrogen embrittlement. Therefore, the presence of bainitic ferrite with carbides at the grain boundaries in the microstructures and the intense presence of MnS inclusions caused HIC in pipeline steel from production B. The manufacturing process, the forming and welding conditions in the examined case seem not to have negatively influenced the pipeline steel in terms of HIC.

The Influence of the Induced Ferrite and Precipitates of Ti-bearing Steel on the Ductility of Continuous Casting Slab

In order to investigate the loss of the ductility of Ti-bearing ship plate steel under 1000 °C, where the ductility begins to reduce rapidly, so the hot ductility of Ti-bearing ship plate steel has been obtained using the Gleeble 1500 thermal-mechanical simulator and also the studies about the effect of grain boundary ferrite films and precipitates containing Ti on the ductility has been carried out. The result showed that the TiN particles precipitating at 950 °C with a larger size and smaller volume fraction cannot effectively suppress the occurrence of recrystallization and the ductility still retains at a high level, although R.A. value presents a certain degree of decline compared with 1000 °C. A large number of smaller Ti(C,N) particles precipitate at 900 °C and can induce the formation of a very small amount of fine grain boundary ferrite, which deteriorates the adhesion strength of the grain boundary, so the R.A. value rapidly reduces to less than 50%. When the temperature falls to close Ae3 (827 °C), the amount of the grain boundary ferrite films increase due to the ferrite phase transformation, but the ferrite film thickness becomes more uneven at the same time, which results in the increase of strain concentration and plays a leading role in causing the decrease of ductility, so the R.A. value has been kept less than 40% as the temperature cooling to 800 °C from 850 °C. When the temperature further decreases, the ductility starts to recover due to the increase of average ferrite film thickness to a greater degree which greatly reduces the strain concentration of the grain boundary.

Effects of Weld Heat Input on Microstructures of Coarse Grain Heat-Affected Zone of Bearing-Nb HF Steel Plate

Effects of weld heat input on microstructures of coarse grain heat-affected zone (CGHAZ) were taken into account for bearing-Nb HF steel plate. For weld heat input from 12 to 20KJ/cm, grain boundary ferrite, degenerate pearlite and acicular ferrite are produced for steel plate with the Si/Mn ratio of 0.26. For higher input, bainite phase is found at the center of specimens. However, for the steel with the Si/Mn ratio of 6.2, granular bainite is dominant at weld heat input from 12 to 20KJ/cm. For further improving the weld heat input to 24KJ/cm, bainite nucleation at prior austenite grain boundaries is suppressed in the considered steel. Nb addition has obvious influences on microstructure.

Simulated Welding Heat Affected Zone Toughness and Microstructure Transformation of a Ti-Zr Microalloyed Low Carbon Steel

Acicular ferrite (AF) can significantly improve the performance of steels and can be promoted through special inclusions. The present experimental steel was a low carbon Ti-Zr deoxidized and microalloyed steel. The welding heat affected zone (HAZ) simulation indicated that under high heat input (100 kJ/cm) and peak temperature (1400°C) conditions, the HAZ exhibited high impact toughness (150 J, -20°C) and desired microstructure. Isothermal heat treatment data suggested that grain boundary ferrite formed at ~600°C and AF formed at ~500°C. Intragranular plates sheaves formed at ~450°C. MnS coherent precipitation on ZrO2TiOx inclusions promoted AF nucleation.

Intragranular ferrite morphologies in medium carbon vanadium-microalloyed steel

The aim of this work was to determine TTT diagram of medium carbon V-N micro-alloyed steel with emphasis on the development of intragranular ferrite morphologies. The isothermal treatment was carried out at 350, 400, 450, 500, 550 and 600?C. These treatments were interrupted at different times in order to analyze the evolution of the microstructure. Metallographic evaluation was done using optical and scanning electron microscopy (SEM). The results show that at high temperatures (? 500?C) polygonal intragranulary nucleated ferrite idiomorphs, combined with grain boundary ferrite and pearlite were produced and followed by an incomplete transformation phenomenon. At intermediate temperatures (450, 500?C) an interloced acicular ferrite (AF) microstructure is produced, and at low temperatures (400, 350?C) the sheave of parallel acicular ferrite plates, similar to bainitic sheaves but intragranularly nucleated were observed. In addition to sheaf type acicular ferrite, the grain boundary nucleated bainitic sheaves are observed.

Effect of Nb on Microstructure Evolution of Coarse-Grained Heat-Affected Zone with Large Heat Input Welding

The effect of Nb microalloying on microstructure transformation of coarse-grained heat-affected zone of high strength low alloy steels were investigated utilizing different heat input welding simulation. For the low-Nb steel, the microstructures of coarse-grained heat-affected zone mainly consisted of acicular ferrite, bainite and grain boundary ferrite for small heat input welding; the amount of acicular ferrite decreased whereas grain boundary ferrite, polygonal ferrite and pearlite increased with increasing heat input. In constrast, for the high-Nb steel, granular bainite was the dominant microstructure. The formation of granular bainitic microstructure was associated with the solid solution of Nb, which suppressed ferrite transformation and promoted the formation of granular bainite. The hardness of coarse-grained heat-affected zone increased with increasing Nb content, and decreased with decreasing heat input, which was attributed to the microstructural change.