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

The work presented in this paper focuses on the effect of the presence of non-equilibrium δ-ferrite on the impact properties of a supermartensitic stainless steel. To generate homogeneous δ-ferrite...

Effect of δ-ferrite on impact properties of supermartensitic stainless steel heat affected zones

https://spiral.imperial.ac.uk/bitstream/10044/1/61871/7/1-s2.0-S0749641917305132-main.pdf

Is stored energy density the primary meso-scale mechanistic driver for fatigue crack nucleation?

Microalloying with various elements, including titanium, coupled with thermomechanically controlled processing, has become a major technology for the manufacture of high-quality steel plate. In this research, the influence of TiN inclusions on the impact toughness of low-carbon plate steels microalloyed with titanium, vanadium, and boron was investigated. The three experimental steels had Ti/N ratios of 2.44, 3.5, and 4.2, and all three had a granular bainite microstructure. However, Charpy V-notch testing showed that steel A had very high toughness at both room temperature and −20 °C, whereas steels B and C showed very low toughness at −20 °C and moderate toughness at room temperature. Scanning electron microscope fractography revealed that coarse TiN inclusions had acted as cleavage fracture initiation sites in steels B and C. The effect of Ti and N levels on TiN formation and growth is analyzed using alloy thermodynamics. It is shown that not only is the Ti/N ratio important, but also the product of total Ti and N plays a most important role in TiN formation and growth. It is concluded that the product of the total Ti and N contents should not be greater than the solubility product of TiN at the solidus temperature to prevent the precipitation of TiN particles before solidification. Furthermore, the ratio of Ti to N should also be maintained lower than the stoichiometric ratio of 3.42 to ensure a low coarsening rate for the TiN inclusions during soaking before rolling.

Effect of TiN inclusions on the impact toughness of low-carbon microalloyed steels

TiN particles in steel can limit austenite grain growth during reheating or welding. However, cuboidal TiN particles of large size (>1 μm) can act as cleavage fracture initiation sites and affect the low temperature toughness. Based on the assumption that the local principle stress acts uniformly everywhere within a grain, for a fixed particle size, a critical value of ferrite grain size can be obtained below which the grain boundary effectively retards the propagation of micro-cracks, which can be nucleated from TiN particles. In the present study, the relative effect of ferrite grain size and TiN particles on cleavage crack propagation has been assessed. Ti-containing high strength low alloy (HSLA) steels were processed using different thermo-mechanical schedules and their microstructures and Charpy impact properties were investigated. It has been found that in spite of the presence of large TiN particles, refinement in ‘effective ferrite grain size’ can significantly improve the impact toughness, provided the matrix strength and meso-texture (i.e. the intensity of low-angle boundaries) are restricted to low values.

Cleavage initiation in steel: Competition between large grains and large particles

The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-to-brittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notch-tip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusions in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents.

The Mechanism of Brittle Fracture in a Microalloyed Steel: Part I. Inclusion-Induced Cleavage

In Part I of this study, cuboidally shaped inclusions were found to be responsible for cleavage initiation in a low-carbon, microalloyed steel. In Part II, electron microdiffraction was used to identify these inclusions as the fcc phase (NaCl prototype) in the titanium-nitrogen system. A model for cleavage as induced by these inclusions is proposed. A microcrack begins on one side of the TiN inclusion, propagates to the other side, and then transfers into the matrix. Initiation at a particular location in the particle is believed to be caused by dislocation pileup impingement and stress concentrations such as crystal defects and surface irregularities within the TiN. Dislocations in the TiN inclusions were imaged by transmission electron microscopy (TEM). After the TiN microcrack transfers into the matrix, propagation spreads radially. From the area of crack transfer, two simultaneous propagation paths reverse directions and rotate around the particle. The particle separates these cracks for a short distance, they travel on different cleavage planes, and upon rejoining, a ridge of torn matrix is created. The location of this ridge can be used to infer where cleavage began in the TiN and where the microcrack transferred into the matrix. Tessellated residual stresses arising from differential thermal contraction between the TiN and the matrix are suggested to increase the cleavage-initiating potency of TiN inclusions.

D. P. Fairchild

Papers

The Mechanism of Brittle Fracture in a Microalloyed Steel: Part I. Inclusion-Induced Cleavage

The mechanism of brittle fracture in a microalloyed steel: Part II. Mechanistic modeling