Research has shown that the acicular ferrite microstructure in steel weld metal, which provides an optimum combination of strength and toughness, is indeed intragranularly nucleated bainite. It is possible to maximize the content of acicular ferrite by increasing the intragranular nucleation sites while maintaining a critical weld metal cooling rate and the steel hardenability. This paper highlights recent research related to nucleation and growth of acicular ferrite during decomposition of austenite.

The mechanism of acicular ferrite in weld deposits

https://estudogeral.sib.uc.pt/bitstream/10316/27386/1/Effect%20of%20explosive%20characteristics%20on%20the%20explosive%20welding.pdf

Effect of explosive characteristics on the explosive welding of stainless steel to carbon steel in cylindrical configuration

https://iopscience.iop.org/article/10.1088/1468-6996/13/2/023001/pdf

Progress in thermomechanical control of steel plates and their commercialization

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion de...

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Stress Corrosion Cracking of Pipelines

The concept of hardenability used in the welding field differs from that used in the heat treatment. It represents the hardened depth after quenching in the heat treatment while it is related with the welding condition for fully hardened heat-affected-zone (HAZ) in the welding. The effect of steel chemical composition on hardenability is expressed by a multiplying factor for the former and by carbon equivalency for the latter. The metallurgical relation between these two factors can be clarified by a heat conduction analysis of a quenched round bar. Steel weldability means susceptibility to hydrogen-assisted cold cracking which mostly occurs in the welding of high strength steels. Many carbon equivalents with different coefficients have been proposed to assess the cold cracking susceptibility which is affected greatly by the hardness at HAZ. Since the HAZ hardness is interactively determined by the carbon content and hardenability, carbon equivalency for assessing susceptibility to cold cracking must consider this interactive effect. Toward the international standardization of the guideline for the avoidance of cold cracking, the methods based on different carbon equivalency are discussed.

Physical Metallurgy of Steel Weldability

The formation and stability of inclusions in low alloy steel welds were investigated using equilibrium calculations. Based on the results, the origin of inclusions effective in acicular ferrite production is discussed. Particular emphasis is placed on the effect of the Al/O ratio and titanium addition on inclusion formation since these two factors are experimentally critical to acicular ferrite production. Mullite (2SiO2.3Al2O3) is found to be formed in addition to Ti3O5 in the steel melt at 1800 K when the melt has a wt-%Al/wt-%O ratio of ∼0·6 optimum for acicular ferrite production. However, the mullite is unstable in the austenite and galaxite (MnO.Al2O3), having the spinel structure, becomes stable at the lower temperatures. Therefore, it is proposed that galaxite is responsible for the nucleation of acicular ferrite. The formation of the galaxite should be in the solid state after weld solidification, being associated with pre-existing mullite. Titanium additions are found to be beneficial to...

Thermodynamic study of inclusion formation in low alloy steel weld metals

Preheating is carried out to avoid cold cracking in steel welding. The occurrence of cold cracking is considered to be governed by accumulating diffusible hydrogen, welding residual stresses and hardness at the crack initiation site. The hydrogen accumulating at the crack site depends on the initial diffusible hydrogen content in weld metal, the weld heat input and the wall thickness. The local residual stress is governed by the weld metal yield strength, the joint restraint and the notch stress concentration factor. The HAZ hardness is influenced by the steel chemical composition, the weld heat input and the plate (wall) thickness.These influential factors affect cold cracking independenly or in an interacted manner. It must be, thus, difficult to predict the necessary preheat temperature by a theoretical method or simple formulae. The method presently proposed is completely based on the empirical results by y-groove weld cracking tests. The present method determines the necessary preheat temperature through the charts describing the following respective effects : 1) steel composition ; 2) diffusible hydrogen content of weld metal ; 3) weld heat input ; 4) wall thickness ; 5) weld metal yield strength ; 6) joint restraint. As to the steel composition, this method uses CEN carbon equivalent that preferably assesses weldability of a wide variety of steels. Also, this method considers a logarithmic dependence of the weld metal hydrogen and the analysis of hydrogen diffusion in a weld has proved that the hydrogen effect on cold cracking must be logarithmic.

/pdf/a-chart-method-to-determine-necessary-preheat-temperature-in-15531vlbom.pdf

A Chart Method to Determine Necessary Preheat Temperature in Steel Welding

Prediction of HAZ hardness of transformable steels

High toughness are requested in the weld metals for offshore steel structures and steel line-pipes used at low ambient temperatures. Micro-alloying of titanium and boron effectively improves the toughness of low-alloyed weld metals with tensile strength ranging between 490 and 590 N/mm2. It is well known that refined intragranular ferrite or acicular ferrite nucleates on titanium containing oxides. However, there have been few reports on the chemical composition at local positions of these effective inclusions and their crystal structures. Two types of submerged arc weld metals were used ; one is a silicon-and-manganese type weld metal with a ferrite with aligned second phase and the other is a titanium type one with the acicular ferrite. The mechanical and metallurgical examination included the microscopic observation, Charpy impact tests of the welds and the characterization of oxides in weld metals with X-ray diffractions and analytical electron microscope. The following facts were clarified from the above investigations. The oxides in the titanium bearing weld metal are crystallized in a form of (Mn, Ti) (Al, Ti)2O4 with angularly rugged surfaces, while the oxides of a Si-Mn type are amorphous with smooth spherical shape. Titanium as low as 0.005 wt% in a weld metal satisfactorily crystallized oxides if titanium is included in oxides with aluminum and manganese.

Nobutaka Yurioka

Papers

Physical Metallurgy of Steel Weldability

Thermodynamic study of inclusion formation in low alloy steel weld metals

A Chart Method to Determine Necessary Preheat Temperature in Steel Welding

Prediction of HAZ hardness of transformable steels

Chemical Composition and Crystal Structure of Oxide Inclusions Promoting Acicular Ferrite Transformation in Low Alloy Submerged Arc Weld Metal.