Pressurized electoslag process
Pressurized electoslag process
Keywords Alloys,Nitrogen, Steel
At the Essen plant of Vereinigte Schmiedewerke GmbH (VSG), a company jointly owned by Krupp Stahl and Thyssen Stahl, a generation of high-performance steels has been developed to meet customers' increasing demands for enhanced mechanical (strength, toughness)and chemical (corrosion resistance) properties.
Nitrogen, as an alloying element, is the key to the research conducted in cooperation with the universities in Aachen, Bochum (Germany), and Zurich. Low carbon steel can absorb only 0.04 per cent nitrogen under atmospheric pressure. By alloying with elements that increase nitrogen solubility (chromium and manganese, for instance), Essen developed an austenitic stainless steel containing between 0.5 to 0.6 per cent (by weight) nitrogen.
If even higher levels of nitrogen content are desired, melting under high pressure becomes a necessity. The so-called pressurized electroslag remelting process was chosen.
An electrode having the desired chemical composition is dripmelted through a bath of molten slag into water-cooled copper mould. (Alternating current flows, via the electrode,through the slag to the mould, causing electrical resistance heating.)Non-metallic inclusions remain in the slag, and oxides and sulphides are largely removed by chemical reaction, thereby creating clean steel.
The pressurized electroslag remelting process unit has locks through which alloying elements such as nitrogen-bearing Si3N4 can be added continuously. During remelting, Si3N4 dissociates into silicon and nitrogen in the molten slag, and both elements enter the metal pool. As remelting proceeds, a dense ingot is formed that exhibits little segregation.
Alloying with nitrogen increases a steel's resistance to deformation without substantial loss of toughness. Nitrogen also enhances corrosion resistance, opening up many applications where high mechanical stresses at elevated temperatures have to be withstood. The interstitially dissolved nitrogen exerts a strong austenite-forming influence.
The pearlite-like Cr2N precipitations in the matrix structure have been found to enhance creep resistance in the 500-700°C temperature range. The outstanding effect of nitrogen, beyond its solubility limit in austenitic steels, however, is the increased strength.
Defect-free welds can be obtained if the following practices are followed routinely:
(1)use electrodes capable of adequate nitrogen absorption;
(2)the welding puddle should have no leading or trailing edges ( low heat input);
(3)the weld must be optimally prepared.
High-nitrogen steels are particularly useful for the power generation and shipbuilding industries,cryogenic engineering, chemical processing equipment, and plant and sheet fabrications.
