Pasi Peura

Iterative Determination of the Orientation Relationship Between Austenite and Martensite from a Large Amount of Grain Pair Misorientations

An automatic, iterative method to determine the orientation relationship between parent austenite and martensite is described. The algorithm generates the orientation relationship from grain boundary misorientations through an iterative procedure based on correct symmetry operator assignment. The automatic method is demonstrated to work on both martensitic and bainitic steels and to provide comparable results to a manual grain selection method.

ZnFe Coated 22MnB5 Steels in Direct Press Hardening: The Relationships between Coating Structure and Process Parameters

Two types of press hardening experiments were carried out to investigate the behavior of ZnFe coated 22MnB5 steel in direct press hardening process. The coating properties were studied using variable process temperatures and times with a flat-die and a forming tool. Coatings were analyzed with optical and scanning electron microscopes. The results indicated that steels that have low coating weights may be processed successfully with short dwell times. For high coating weights a significantly longer dwell time is needed. The behavior of ZnFe coating in hot press forming experiments was in line with literature and the findings of the flat-die experiments. Thus, the feasibility of the experimental press hardening equipment was confirmed.

The Characterization of Flame Cut Heavy Steel – The Residual Stress Profiling of Heat Affected Surface Layer

Flame cutting is commonly used thermal cutting method in metal industry when processing thick steel plates. Cutting is performed with controlled flame and oxygen jet, which burns steel and forms cutting edge. Flame cutting process is based on controlled chemical reaction between steel and oxygen at elevated temperature. Flame cutting of thick wear-resistant steels is challenging while it can result in cracks on and under the cut edge. Flame cutting causes uneven temperature distribution in the plate, which can introduce residual stresses. In addition, heat affected zone (HAZ) is formed and there both volume and microstructural changes as well as hardness variations are taking place. Therefore flame cutting always causes thermal stress, shape changes and consequently residual stresses to the material. Material behaviour under thermal and mechanical loading depends on the residual stress state of the material. Due to this, it is important to be able to measure the residual stresses. The aim of this study was to examine residual stresses on the cutting edge as a function of different flame cutting parameters. Also resulting microstructures and hardness values were verified. Varying parameters were the cutting speed, preheating and post heating procedures. Flame cut samples were investigated with X-ray diffraction method to produce residual stress profiles of the heat affected surface layer. Results indicated that different cutting parameters provide different residual stress profiles and that these profiles can be modified by changing the cutting speed and pre-or post-treatment procedures. Cutting parameters also affect the depth of the reaustenized region in the surface. The results correlate well with the actual industrial flame cutting and thus they provide an effective tool for optimizing the flame cutting process parameters.

Characterization of Flame Cut Heavy Steel: Modeling of Temperature History and Residual Stress Formation

Heavy steel plates are used in demanding applications that require both high strength and hardness. An important step in the production of such components is cutting the plates with a cost-effective thermal cutting method such as flame cutting. Flame cutting is performed with a controlled flame and oxygen jet, which burns the steel and forms a cutting edge. However, the thermal cutting of heavy steel plates causes several problems. A heat-affected zone (HAZ) is generated at the cut edge due to the steep temperature gradient. Consequently, volume changes, hardness variations, and microstructural changes occur in the HAZ. In addition, residual stresses are formed at the cut edge during the process. In the worst case, unsuitable flame cutting practices generate cracks at the cut edge. The flame cutting of thick steel plate was modeled using the commercial finite element software ABAQUS. The results of modeling were verified by X-ray diffraction-based residual stress measurements and microstructural analysis. The model provides several outcomes, such as obtaining more information related to the formation of residual stresses and the temperature history during the flame cutting process. In addition, an extensive series of flame cut samples was designed with the assistance of the model.

Microstructural Response of High Aluminum Steels to Quenching and Partitioning Treatment

Quenching and Partitioning (Q&P) is a novel heat treatment process that is able to produce steels with a microstructure consisting of controlled amounts of finely divided retained austenite in a martensitic or ferritic-martensitic matrix. Following quenching to a temperature in the Ms-Mf range, the steel is subjected to partitioning treatment that concerns the diffusion of carbon atoms from supersaturated martensite phase to the austenite phase, resulting in the possibility of stabilizing it down to room temperature. Competing reactions such as cementite and/or transition carbide precipitation must be suppressed by suitably alloying with certain elements, conventionally silicon. In this study, aluminum was used as the main precipitation suppressor. Three Nb-microalloyed experimental steels with aluminum at different levels in the range 2–3%, with or without additions of Si, Ni and Cu were subjected to Q&P treatments. The microstructures and phase compositions of the quenched and partitioned steels were characterized with optical and scanning electron microscopy combined with EBSD phase mapping and X-ray diffraction measurements. The mechanical properties of the steels were studied with microhardness and tensile testing. The preliminary experiments suggest that aluminum-bearing steels can retain significant amounts of austenite after partitioning in a ferritic-martensitic microstructure. Promising strength-ductility ratios were also found in tensile testing. Based on the encouraging results, future work will be directed to produce microstructures with higher martensite fractions to impart higher strengths in steels combined with good ductility and formability.Copyright