Effect of Zr addition on inclusions, micro-structures and toughness in the heat-affected zone of Mg deoxidised ship steel after high heat input welding

The characteristics of inclusions and microstructure in heat-affected zones (HAZs) of steel plates with Ca deoxidation after high heat input welding of 400 kJ·cm−1 were investigated through simulated welding experiments and inclusions automatic analyzer systems. Typical inclusions in HAZs of steels containing 11 ppm and 27 ppm Ca were recognized as complex inclusions with the size in the range of 1~3 μm. They consisted of central Al2O3 and peripheral CaS + MnS with TiN distributing at the edge (Al2O3 + CaS + MnS + TiN). With increasing Ca content in steel, the average size of inclusions decreased from 2.23 to 1.46 μm, and the number density increased steadily from 33.7 to 45.0 mm−2. Al2O3 + CaS + MnS + TiN complex inclusions were potent to induce the formation of intragranular acicular ferrite (IAF). Therefore, the HAZ toughness of steel plates after high heat input welding was improved significantly by utilizing oxide metallurgy technology with Ca deoxidation.

In the current steel structures of high-rise buildings, high heat input welding techniques are used to improve productivity in the construction industry. Under the high heat input welding, however, the microstructures of the weld metal (WM) and heat-affected zone (HAZ) coarsen, resulting in the deterioration of impact toughness. This study focuses mainly on the effects of fine TiN precipitates dispersed in steel plates and B addition in welding materials on grain refinement of the HAZ microstructure under submerged arc welding (SAW) with a high heat input of 200 kJ/cm. The study reveals that, different from that in conventional steel, the γ grain coarsening is notably retarded in the coarse grain HAZ (CGHAZ) of a newly developed steel with TiN precipitates below 70 nm in size even under the high heat input welding, and the refinement of HAZ microstructure is confirmed to have improved impact toughness. Furthermore, energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS) analyses demonstrate that B is was identified at the interface of TiN in CGHAZ. It is likely that B atoms in the WM are diffused to CGHAZ and are segregated at the outer part of undissolved TiN, which contributes partly to a further grain refinement, and consequently, improved mechanical properties are achieved.

To reduce the construction cost and period of steel structure, high-efficiency welding methods applying high heat input have been widely employed. However, deterioration of the strength and toughness, especially low-temperature impact toughness, due to the coarse grains in the heat-affected zone (HAZ) of weld steels often comes along with high heat input welding. Thick offshore steel has been developed by Ti deoxidization for the use of high heat input welding. The inclusions of the steel were analysed by EPMA and SEM, and the major inclusions were identified to be Al2O3-(MgO) and Al2O3-CaO-(TiO2) types. The steel plates with different thickness over 40 mm were welded with heat input over 100 kJ/cm, and Charpy impact energies of the welding joints at −40 °C were over 75 J. The nano-sized TiN particles in HAZ were observed and were considered to retard the grain coarsening in the HAZ during high heat input welding.

Effect of Mg Content on the Microstructure and Toughness of Heat-Affected Zone of Steel Plate after High Heat Input Welding

The high heat input welding (HHIW) can greatly enhance the welding efficiency of the steel plates. However, coarse grains and brittle microstructures will be formed in the heat‐affected zone (HAZ) during the HHIW process, which deteriorates the toughness of HAZ. In the present study, the effect of the Ti/N ratio on the size and distribution of titanium nitride (TiN) particles, microstructures, and toughness of HAZ after HHIW of 400 kJ cm−1 is investigated for the Ca‐treated steels with different Ti/N ratios of 1.61, 3.79, and 5.00 for TN16, TN38, and TN50 steels. The TN38 steel has the highest number density of the TiN particles with the sizes between 20 and 25 nm and the highest pinning force of the particles in three steels, resulting the smallest grain sizes in its HAZ. In addition, in three steels, the TN38 steel has the main microstructure of intragranular aciculate ferrites with the highest high‐angle grain boundaries density in its HAZ, thereby obtaining the highest value of low‐temperature impact toughness of the HAZ.

It is generally believed that S phase has lath-morphology. Both lath and rod-shaped S phase were observed in the present study and a closer observation of the selected area diffraction patterns (SADPs) revealed an interesting difference in their orientation relationship with the matrix. A modified orientation relationship has been recently discovered and related to the S phase morpholgy. Although at this time, the effect of the orientation relationship and morphology of S phase on the mechanical properties of Al‐Cu‐Mg alloys is not understood, it is worthwhile to investigate this observation for increasing the understanding of S phase precipitation and growth. Although there have been several investigations relating to the microstructure of friction stir welds in Al‐Cu‐Mg alloys [1‐4], precipitation and growth of S phase in these welds have not been adequately addressed. Two friction stir welds produced on 1-mm thick 2024T3 commercial aluminum alloy (Al‐4.4Cu‐1.5Mg‐0.5Mn) using different heat inputs were studied using transmission electron microscopy (TEM) operating at 200 kV. The process conditions of these welds are shown in Table 1. The ratio of tool rotation speed and the traverse rate gives the heat index N/v. The tool used for friction stir welding consisted of a shoulder with a diameter of 7 mm and a 1 mm long pin having a diameter of 2.5 mm. Based on the process parameters and the tool dimensions, it can be calculated that the heat input per unit length for the low heat input weld and high heat input weld were 99.8 and 269.8 kJ/mm, respectively. This suggests that the high heat input weld experienced significantly higher peak temperature as compared to the low heat input weld. The location in the heat affected zone (HAZ) just outside the thermomechanically affected zone exhibited coarse S phase, which had precipitated due to the heat of friction stir welding. Figure 1 shows the bright field (BF) images and SADPs from these regions for both welds. The majority of S phase in the low heat input weld (1) exhibited a rod morphology, while the S phase in the high heat input weld (2) exhibited a lath morphology. It should be noted that in the [001] SADP for the low heat input weld, both {112}S and {131}S reflections were strong, while in case of the high heat input welds, only the {131}S reflections were strong.

Effect of Calcium and Magnesium Addition on the Cast Microstructure of HSLA Steel

To elucidate the effect of Mn content on the microstructure and mechanical properties of weld metal, flux-cored wires with three different Mn contents were prepared to conduct high heat input welding experiments. Complex inclusions and Mn-depleted zones were observed in the weld metal with heat input of 85 kJ/cm. The study indicated that complex inclusions enabled nucleation of acicular ferrite with interlocking structure, leading to enhanced impact toughness. With decrease in Mn content, the number of complex inclusions with Mn-depleted zone and the volume fraction of acicular ferrite were both decreased. Additionally, the impact toughness of weld metal was significantly degraded with lower Mn content present in martensite-austenite (M-A) constituent and bainite.

Microstructure and mechanical performances of the coarse grain heat‐affected‐zone (CGHAZ) for oil tank steel with different Ti content were investigated through Gleeble‐3500, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectrometer. The results show that the strength and low‐temperature toughness of base material are significantly improved for the high titanium content steel, but the impact toughness of CGHAZ is seriously deteriorated after the high heat input welding and declined sharply with the heat input increasing, while the effects of heat input on impact toughness are very weak for the low titanium content steel, impact toughness of which is gradually larger than that of high titanium content steel with the welding heat input increasing because of the granular bainite increasing, TiN particle coarsening, and (Ti, Nb) N composition evolution during the high input welding for high titanium content steel.

The inclusions, microstructures, and toughness in heat‐affected zone (HAZ) of the shipbuilding steel plates with Mg deoxidation and the different Ti/N ratios are studied after the high heat input welding of 400 kJ cm−1. With increasing the Ti/N ratio from 3.00 to 5.67, the average size of inclusions increases, the mass fraction of total TiC inclusions increase from 2.8% to 5.0%, and the average size of MgO–TiN–MnS increases from 2.82 to 5.49 μm, while the number density that can induce intragranular acicular ferrite (IAF) nucleation decreases from 87 to 62 mm−2. These changes in inclusions are detrimental to induce the IAF nucleation. The HAZ microstructures are composed of the IAFs, polygonal ferrites, and upper bainites (UBs). With increasing the Ti/N ratio, the area fraction of the IAFs decreases from 44% to 14%, and the area fraction of UBs increases from 1.8% to 9.5%. The HAZ fracture mode changes from ductile fracture to brittle fracture after the high heat input welding of 400 kJ cm−1, and the HAZ toughness decreases from 183.3 to 48.7 J at −20 °C.

The combined influence of Mg and Ca treatment on the properties of heat-affected zone (HAZ) of low-carbon steel after high heat input welding was systematically studied. Experimental steels deoxidized with different elements were prepared, i.e., C–Mn steel with Al, Ti–Ca steel with Ti and Ca, Ti–Mg–Ca steel with Ti, Mg and Ca. Results showed that the inclusions in C–Mn steel were mainly Al2O3 and MnS with low density and large size. However, the average size was refined to only ~ 0.34 μm in Ti–Mg–Ca steel and the amount increased remarkably. Microstructure of simulated HAZ for 200 kJ/cm changed from ferrite side plates or upper bainite to acicular ferrite after treatment with Ti, Mg and Ca. Ca addition decreased the strain field around inclusions and enhanced the ability of acicular ferrite nucleation. In situ observation of Ti–Mg–Ca steel showed that the movement of austenite grain boundaries was retarded and nucleation sites of acicular ferrite were greater than Ti–Ca steel because of Mg addition. Impact energy of HAZ at − 40 °C was increased from 7 to 232 J and showed excellent stability because of Ti–Mg–Ca treatment. High volume fraction of acicular ferrite acted as obstacles toward cleavage cracks.

High heat input welding is one of the alternatives adopted by the world’s major shipyards for increasing productivity in operations for joining materials in shipbuilding. However, the thermal cycles generated during welding may cause microstructural transformations that are detrimental to the mechanical properties, mainly toughness in the heat-affected zone (HAZ). The main aim of this study was characterization of the microstructure and assessment of the mechanical properties of the HAZ of EH36 shipbuilding steel produced by controlled rolling followed by accelerated cooling (Thermo Mechanical Control Process (TMCP) – thermo-mechanically controlled process) compared to a steel of the same grade produced by conventional rolling, both welded by the submerged arc process with two levels of heat input: 76 and 130 kJ/cm. It was observed that the presence of a more refined microstructure in the different regions of the HAZ, associated with the smaller grain size of the base metal and the lower carbon equivalent, were the main factors contributing to the excellent toughness of the HAZ of TMCP steel compared to conventional steel. The results achieved show that it is possible to obtain welded joints with excellent mechanical properties and toughness when using TMCP steel for high heat input welding, and its use is a possible strategy for optimizing fabrication times and costs in the shipbuilding industry.

The heavy plate used for offshore structure is one of the important strategic products. In recent years, there is an increasing demand for heavy shipbuilding steel plate with excellent weldability in high heat input welding. During the thermal cycle, the microstructure of the heat affected zone (HAZ) of plates was damaged, and this markedly reduced toughness of HAZ. So, how to improve the toughness of HAZ has been a key subject in the fields of steel research. Oxide metallurgy is considered as an effective way to improve toughness of HAZ, because it could be used to retard grain growth by fine particles, which are stable at the high temperature.The high strength steel plate, which satisfies the low temperature specification, has been applied to offshore structure. Excellent properties of the plates and welded joints were obtained by oxide metallurgy technology, latest controlled rolling and accelerated cooling technology using Ultra-Fast Cooling (an on-line accelerated cooling system). The 355MPa-grade high strength steel plates with normalizing condition were obtained, and the steels have excellent weldability with heat input energy of 79~287kJ/cm, and the nil ductility transition (NDT) temperature was -70°C, which can satisfy the construction of offshore structure in cold regions.

In this paper, the HAZ of pipeline steel in high heat input welding is researched, including the microstructure and properties of HAZ, the embrittlement of HAZ and the existing problems and solutions of HAZ. The results show that the welding normalized zone is the best part of the HAZ. The main embrittlement mode of HAZ in the pipeline steel is the coarse grained embrittlement and the microstructure embrittlement. The theoretical research system of adding nano-oxide induced the intra-granular acicular ferrit nucleation in HAZ will be a new research direction of solving the appearing problems of pipeline steel in high heat input welding.

Corrosion behavior in high heat input welded heat-affected zone of Ni-free high-nitrogen Fe–18Cr–10Mn–N austenitic stainless steel