• Jokiaho, Tuomas

Abstract

High hardness, strength, and toughness are the properties required of thick wear-resistant steel plate. To meet these requirements, special care must be taken in the manufacture of the plate. The manufacturing steps for thick plate involve thermal cutting, such as flame cutting, which is the most generally applied cutting method for thick plate in the steel industry. Flame cutting is performed with a heating flame and oxygen jet, which creates a cut edge on the steel plate. It is a suitable method for thick steel plates and high production rates due to the exothermal reaction during the cutting process. However, flame cutting also causes problems. Due to the steep thermal gradient, a heat affected zone (HAZ) is formed at the cut edge. The HAZ includes microstructural changes and hardness variations. In addition, high residual stresses are generated in the cut edge. In the worst case, the flame cutting causes cracking of the plates.<br/>The main purpose of this work is to identify the main contributors behind the <br/>cracking phenomenon of thick plates in flame cutting. In addition, the goal is to give guidelines for a more effective flame cut process and to determine the most suitable microstructural characteristics for thick wear-resistant steel plates and flame cutting. To achieve these goals, a trial batch of thick wear-resistant steel plates was manufactured. The plates were flame cut with different cutting parameters and the residual stress state of the flame cut samples was measured by X-ray diffraction. In addition, both original and flame cut samples were characterized by electron microscopy and mechanical tests.The results of this study showed that residual stress formation during flame cutting can be controlled by choosing the right cutting parameters. Preheating and a <br/>slow cutting speed produced the most beneficial residual stress state: higher <br/>compressive stresses and lower tensile stresses. In addition, it was shown that <br/>cracking increased with increasing segregations in the plate structure. Furthermore, long horizontal prior austenite grain boundaries were found to create beneficial sites for crack formation and propagation. Therefore, in plate manufacturing it is recommended to aim for a small and equiaxed prior austenite grain structure. In addition, it is advantageous to reduce the amount and severity of the segregations in the structure when possible.

Topics

• impedance spectroscopy
• grain
• x-ray diffraction
• Oxygen
• crack
• strength
• steel
• hardness
• electron microscopy