Chemistry and structure of steels at the nano-level 

In RA1 we have used two advanced characterization techniques – atom probe tomography (APT) and transmission electron microscopy (TEM) – to study steels at the nano-level. These two techniques are complementary since APT gives chemical composition at atomic scale while TEM provides information on atomic (crystal) structure and grain boundaries. Correlative experiments (from the same volume) offer a strong, advanced tool for characterization down to the atomic scale. 

In this project we have studied the martensite-austenite (M-A) phase in steels, which is formed near the weld in the base material, in regions reheated into the intercritical region of the iron-carbon phase diagram. These particles form at prior austenite grain boundaries and are brittle, which is bad for toughness.

Needle-shaped TEM/APT specimens were prepared from the micron-sized M-A particles using the focused ion beam lift-out technique. The figure above shows a specimen imaged in the TEM, and the APT volume showing segregation of carbon to grain boundaries and small clusters of phosphorous.
 

Investigating the role of boron in cast iron

Boron may enter cast iron because of boron-containing scrap steel which can be used during the cast iron production. Boron is an undesired element in spheroidal graphite cast iron because it can lead to loss in strength and ductility. The loss in strength occurs because boron promotes the soft iron phase ferrite when copper is used to stabilize the stronger iron microstructure pearlite.

The loss in ductility may happen if the concentration of boron becomes so large that borocarbides are formed towards the end of solidification. The focus of Andreas Bugten`s PhD project is to investigate the mechanisms behind various boron-related phenomenon in spheroidal graphite cast iron using microscopy techniques and modelling.

We closely cooperate with Elkem Silicon Products in Kristiansand, where the cast irons are made and analyzed. At Elkem we study the bulk chemistry, phase fractions, microparticle populations, and local chemical composition using various equipment such as scanning electron microscope (SEM) and optical spark emission spectroscopy (OES).

At NTNU we study the materials using electron back scatter diffraction (EBSD) in the SEM, and also investigating small particles and structures using transmission electron microscopy (TEM). To determine the segregation of boron we have used secondary ion mass spectrometry (SIMS) at UiO.

The SIMS elemental mapping (see Figure 1) has showed that boron segregates to form M23-carbides, as well as towards the spheroidal graphite surface. The M23-carbide chemistry and crystal structure are consistent with that of the phase Fe23(C,B)6 (see Figure 2).

The main results of this work have been submitted in a manuscript to the Journal of Metallurgical and Materials Transactions A at the end of October 2022. 
 

Preheating and preplacing filler wire to improve microstructure and toughness in laser-arc hybrid welding of 45 mm thick steel.

Acicular ferrite (AF) is the most important microstructural constituent to achieve high toughness at low temperatures in weld metal of steels. This is due to the relatively small grain size and large misorientation angles. AF is known to form at non-metallic inclusions (NMIs) originating from alloy elements in the filler metal.

However, in deep and narrow laser-arc hybrid welding (LAHW), the formation of NIMIs is limited due to insufficient transportation of filler wire to the root, and in combination of high cooling rates, bainite-martensite forms which results in high hardness and low ductility. In Research Area 4 we have demonstrated a method for welding a 45 mm thick high strength low alloy steel welded by double-sided LAHW using different groove preparations and techniques to achieve high ductility.

Two external methods were conducted to assist the formation of AF and to increase ductility, i.e., base plate preheating and preplaced metal-cored filler wire into the weld groove prior to welding, see figure 1d. Preheating increased ductility (reduced hardness) and increased the AF fraction due to increased cooling time. In addition, preheating showed to mitigate porosity.

Preplaced filler wire provided an enhanced population of NMIs in the root; thus, significantly increasing the fraction of AF. As seen in figure 3, high impact toughness (> 35 J) was achieved at −50 ℃ by combining preheating and preplaced filler wire, and up to 45% fraction content of AF was reached. 

Figure 1: Bevel for 45 mm thick steel welded by 2 sided laser-arc hybrid process. Figure (d) is with preplaced filler wire.

Figure 2: Hardness in weld (a) No preheating and no filler wire, (b) Preheating, (c) Preheating and preplaced filler wire.

Figure 3: Ductility in weld meal (a) No preheating and no filler wire, (b) Preheating, (c) Preheating and preplaced filler wire.

In 2023 testing of different nano- and micro-particles to include AF under ultra-fast cooling times is planned to be performed since commercial filler wire are poorly designed for such conditions.
 

Simulating the vacancy evolution in atoms by mathematic models

Introduction of high-density nano sized precipitates by heat treatments is an important method to strengthen metallic materials. During the precipitation of such nano precipitates, vacancies (vacant positions in the atom lattices of metals) play important roles in terms of enhancing the diffusion of solute elements and therefore the formation of atom clusters.

The equilibrium concentration of vacancies is usually much higher at high temperatures than at low temperatures. As a result, the excess vacancies will diffuse to and disappear at dislocations and grain boundaries of materials during room temperature storage. This will significantly reduce the total concentration of vacancies in the alloys.

In one of the SFI research areas we have developed a numerical model to simulate the spatial vacancy concentration evolution as functions of temperature and time. In the model, the interactions between solute atoms and vacancies have been well addressed.

The simulation results are in a good agreement with experimental observations. This model is very helpful for a deeper understanding on the precipitation behavior of nano precipitates during room temperature storage and during artificial ageing treatments. It will also be applied for a numerical precipitation model under development. 
 

Web demonstrators

A set of initial web applications demonstrating some possible uses of the platform of the centre have been made available on the project web-site. Please note that we are so far using a trial solution that has a very slow startup (a few minutes), but thereafter works as expected. 
Available apps include the PREMOD precipitation model for 6xxx aluminium alloys, a sample registration app using QR codes, an app for visualisation of ontologies and a search tool for browsing available microstructure models.