The invention relates to a blade made of steel alloy and more specifically, to a blade of a refiner or a disperser.
Stainless steels are steels having a chromium concentration higher that 12 wt-%. The corrosion resistance of stainless steel is good, which is based on the chromium oxide layer forming onto the steel surface and protecting the underlying steel from corrosion. By changing the composition of steel, i.e. alloy elements and their quantities, the crystal structure of stainless steel can be adjusted. Different crystal structures produce different properties in the steel.
One crystal structure of stainless steel is martensitic crystal structure. The martensitic crystal structure is achieved when the steel alloy is rapidly cooled and carbon does not have time to leave the interstitial sites of austenitic steel and the crystal structure turns into martensitic in the phase transition. Martensitic steel is one of the hardest and strongest steel types. In addition, it has the lowest ductility, i.e. steel having martensitic crystal structure is one of the most brittle steel types. However, this type of steel has good abrasion resistance which is mainly based on hard carbides formed by chromium and carbon, as well as strong martensitic matrix.
The abrasion resistance of martensitic stainless steel can be improved by increasing the carbon content of the steel alloy in which case the amount of chromium carbides in the structure increases. However, the carbon content cannot be increased infinitely because when the carbon content of the alloy increases, its impact ductility decreases. This is because the chromium carbides separate as the steel solidifies from the final melt wherein a carbide lattice is formed in the structure. Fractures developed in a steel product progress along the hard and brittle carbide lattice easily all the way through the whole structure. The greater the chromium carbide content in the structure, the more easily the fracture develops and progresses.
Applications of the invention include the blades of mechanical pulp refiners, low consistency refiners, fibreboard refiners and dispersers. These blades can be formed of two or more rotationally symmetrical cast pieces with the shape of a plate, cylinder or cone or combinations of these blade shapes placed against each other. Said blades can be alternatively formed of smaller parts such as segments of a circle, a cone or a cylinder which are combined to form a rotationally symmetric blade surface.
The surfaces of the blades of refiners and dispersers to be fitted against each other consist of blade bars and grooves. During refining the pulp suspension or wood chips fed between the refiner blades are guided between the blades over the refiner blades to the opposite side in respect of the feeding edge and from there onward in the process.
The refiner blades are under constant abrasion during the grinding. The lifetime of the blades is also decreased by foreign particles such as sand, glass and metals or paper fillers that end up between the refiner blades.
At present, refiner blades are manufactured from steel alloys with low, medium and high carbon content. Steel alloys with high carbon content have been presented in, for example, WO patent publication 01/68260 and EP patent application 1507023. A disadvantage of these martensitic stainless steels with medium and, in particular, high carbon content is that they have a high content of chromium carbide resulting in a uniform and thick carbide lattice and thereby low impact ductility and brittle structure. The problem with high carbon steel is essentially greater than low carbon steel having a smaller chromium carbide content, wherein a uniform carbide lattice does not form or it remains very thin.