Improved Quality by Electro Slag Re-Melting

Transcription

1 Improved Quality by Electro Slag Re-Melting BY GÜNTER BUSCH* SYNOPSIS Electro Slag Re-Melting is a process performed after the primary melting steps in electro arc, induction or vacuum induction furnaces. Typical steels to be re-melted are tool steel with a share of 20%, stainless steels with 10% share and bearing steel with 5% share. To meet the higher service demands on these steels require increased use of re-melting technologies. The ingots from the previous melting process are used as self-consuming electrodes in the process of electro slag re-melting, which is performed under inert gas atmosphere to form a new refined ingot. The tip of the electrode is just immersed in the top of the slag bath surface and the liquid metal will be cleaned when droplets melt off and pass down through the slag layer on top of the formed ingot. The ingot shape can be round or rectangular in cross section. The main metallurgical tasks for the ESR process are: Increase the ingot cleanliness by removal of non-metallic inclusions Narrow range? Close control of reactive elements like Boron, rear earth elements and/or Titanium and minimum burn off No Oxygen or Hydrogen pick up from the atmosphere Higher material yield due to increased density and reduced top shrinkage cavity Controlled directed solidification resulting in No macro segregation and reduced micro segregation Homogeneous distribution of the alloying elements The result will be steel with consistently higher and more uniform mechanical properties especially fatigue resistance and fracture toughness, which are important for many application especially in bearing, tool and die steels. For special steels and steels used in the aeronautical industry a triple melt will be necessary to achieve the required material properties. The final melt will be performed in a Vacuum Arc Re-melting (VAR) system. Experience in the automotive industry shows that it is possible to significantly increase tool life by using tool steels produced via re-melting technology, leading to reduced cycle (time?) and manufacturing costs. Keywords: Electro Slag Re-Melting, ESR, Vacuum Arc Re-Melting, VAR, Vacuum Metallurgy, Vacuum Induction Melting, Refining, *Head of South East Asian Operation ALD Vacuum Technologies GmbH, Hanau, Germany.

2 Improved Quality by Electro Slag Re-Melting (ESR) Introduction Electro Slag Re-Melting (ESR) has been known for nearly 90 years, but it took another 30 years before it became an acknowledged process for mass production of high-quality ingots. This means electro slag re-melting has been used since the 1960's, first in the manufacturing of Nickel base alloys and now, increasingly, in the production of high grade steel as well. ESR technology is not only applicable for the production of smaller ingots of tool steels and super alloys, but also can be used for refining heavy forging ingots up to ingot weights above 150 tons. [1] The ESR process is used as tertiary metallurgical step after the secondary metallurgy, which is usually done in LF/VD/VOD or AOD or Vacuum Induction furnaces. Main metallurgical objective of electro slag re-melting are the refining, reduction of segregation, homogenization and the increase of the ingot cleanliness. ESR is not used for making alloying additions. In 2016 worldwide high alloyed steel production was roughly 154 million tons distributed as follows: Nickel Alloys Tools Steel and High Speed Steel Bearing Steel Stainless Steel Other alloyed Steel tons tons tons tons tons Fig 1: Annual production for 2016 of high alloyed steel [2] The proportion of re-melted material varies from 65% for the Nickel base alloys to very low percentage for the other alloyed steel grades. In total roughly five million tons of steel are electro slag re-melted. Fig 2: Re-melted share depend on the alloy [2]

3 Applications for Electro Slag Re-melted Special Steel Electro Slag Re-melted steel has consistently higher mechanical properties, such as fatigue resistance & fracture toughness, which are crucial for safety critical components. Some typical applications in different industries are listed below. Aerospace Discs, Rings and Blades in engines Studs, Bolts, Rivets Structural Parts, Bearings and Landing Gear Medical Stents Pacemaker wires Dental Wire Oil & Gas Tools Safety valves, Pipe joints Fittings / Flanges / Seals Power Generation Blades for gas turbines Discs, Rings, Shafts Bearings Nuclear applications Tools Extrusion Tools Die Casting Moulds Precision Moulds Coining Tools Forging Tools Transport Injection Parts Special valves Piston Rings Turbo Charger Bearings Fig 3: Application in different Industries for electro slag re-melted steels [2]

4 Principle of Electro Slag Re-Melting (ESR) During the Electro Slag Re-melting process a self-consuming electrode made by a previous melting process such as EAF, LF/VD or Vacuum Induction will be re-melted to achieve improved alloy homogenization, reduction of voids and refining the material from inclusions and un-desirable elements. electrode liquid pool water cooled mould refined ingot Fig 4: Principle of Electro Slag Re-Melting At the start of the process an electrical alternating current flows from the electrode through an artificial slag to the starting plate at the bottom of the water cooled mould. The refining slag will be superheated and the liquid metal drops from the electrode and sinks though the now liquid slag to the base plate and builds up slowly up as the new and refined ingot. To avoid any oxidation and Hydrogen pick up the ESR process runs under a protective atmosphere of Nitrogen or Argon. For some steels a high pressure Nitrogen atmosphere up to 40 bars is used for enrichment of the steel with Nitrogen in High Nitrohen Steel manufacturing. While the electrode at the top is gradually being consumed the refined ingot grows upwards from the bottom of the water cooled mould. It is important is to keep the gap between the electrode and the water cooled mould constant to avoid any arcing or short circuit to the mould wall. The achievement of constant and continuous melting rate and material flow is essential for the entire process. The ESR process requires a suitable refining slag, the composition and nature of which depend on the re-melted material and application. Typical slag for steel is based on CaF 2, CaO and Al 2O 3 The cross sectional shape of the electrode and ingots can be round or square for flat products.

5 Enhanced Properties through Electro Slag Re-Melting Electro Slag Re-melted ingots have advanced mechanical and chemical properties, such as homogenous elemental distribution, which are required by many applications in different industries. The following metallurgical advantages will be gained when using the ESR process Refining from non-metallic inclusions Higher material yield with reduced top shrinkage cavity Close control of reactive elements like Boron, rear earth elements and/or Titanium and minimum burn off Maintained Sulphur level and avoidance of de-sulphurisation??? (you may need to No Hydrogen or Nitrogen pick up from the atmosphere due to protective Argon atmosphere No macro segregation and reduced micro segregation Homogenized alloying element distribution Significant reduction of voids Due to the smaller dendrite arm spacing compared to the original cast ingots the specific density is increased. This has also a positive effect in reduction on the efforts for forging. Fig 5: Increased density by electro slag re-melting [3], [6] Fig. 5 shows clearly that even with lower forging ration higher ingot densities can be achieved compared to a conventional cast ingot. This effect can be used to significantly reduce the amount of deformation needed in forging or hot rolling.

6 Via the smaller and controllable liquid metal pool a directional solidification is encouraged. This is beneficial for the macro and micro structure of the re-melted steel. Fig 6: Structure of an electro slag re-melted ingot [3] [4] Figure 6 shows the macro-structure of an ESR ingot with a clear dendritic structure, globular grain in the top section. The ingot has no voids and the shrinkage at the top is reduced to an absolute minimum. The homogenization of the alloying material is another benefit of this re-melting technology. Due to the smaller melt pool and the directional solidification there will be no macro segregation in an ESR ingot and micro segregation is also significantly reduced. Figs. 7 & 8 show the segregation of Carbon and Chromium in samples from a 26NiCrMoV11-5 steel before and after re-melting.

7 Fig. 7: Carbon segregation before (blue colour) and after ESR (red colour) processing measured over the ingot height [3] Fig. 8: Chromium segregation before (blue colour) and after ESR (red colour) processing measured over the ingot cross section [3] Both samples show significant improvement in the homogenisation of alloying elements in the electro slag re-melted ingot. This has a positive influence to the mechanical properties of the steel. ESR ingots do not contain so called white spots by keeping the sulphur level in steel constant and improving the carbon distribution in the refined ingot.

8 Fig. 9: Carbon distribution over the ingot cross section with 460mm diameter at a High Speed Steel The cleanliness value describes the number of non-metallic inclusions in a material. The lower the K1 value the lower are the inclusion levels. Fig. 10:Cleanliness values of different melt processes measured on a 17-4 (SUS630) stainless steel [4] Figure 10 shows a significant improvement (reduction) of the inclusion count by using ESR technology compared to the standard melt processes LF/VD or Vacuum Induction Melting (VIM). An additional improvement is achievable by using Vacuum Arc Re-melting (VAR), which is mostly required by aircraft and nuclear applications.

9 Economical View Certainly the additional re-melting process costs additional investments, consumes energy, needs additional man power and of course time. As described in this paper re-melting technology improves the characteristics of the steel, which leads into product with higher and more consistent mechanical properties. For example inclusions or voids at the finished surface of a tool, recognized after machining means that the entire tools must be scrapped. Quite often such defects can be only detected after the machining. The entire tool must be scrapped and the already spent machining effort is wasted. Voids and inclusions in tools will reduce service lives. This means the lifetime of tools made of re-melted steels is much higher. The following example of specific costs shown in Fig.11 relates to a tool used in the automotive industry manufactured from SKD61 grade steel in the non - re-melted, ESR re-melted and triple melted steel by ESR and VAR (vacuum arc remelting) conditions. Primary melted LF/VD VIM Re-melted ESR Twice re-melted ESR - VAR Material Costs Tool manufacturing costs Total tool costs Life time (number of cycles) Specific costs 0,54 0,37 0,22 Fig. 11: Specific costs of a tool in automotive industry made from SKD61 steel [4] The steel price of this tool increases due to additional manufacturing steps by more than double. This increase is only from 2,7% to 5,5% of the entire costs (material and machining) of the tool. The lifetime, the number of cycles, increases from to This means that the costs per cycle reduces from 0,54 uro down to 0,22 uro Hence improved technical efforts in steel making by use of re-melting technology will lead to lower costs in the final end product. Conclusion Electro Slag Re-melting technology is a standard state of the art process in today s steel making. Major steel manufacturers worldwide are using this technology. New high demanding requests for mechanical and chemical properties of steel require more and more re-melting technology. It is proven that these characteristics will improve by this technology. Modern systems of Electro Slag Re-melting operate under protective atmospheres to avoid any pick up of Oxygen and Hydrogen. Driving and demanding industries like automotive, aerospace and medical industry require remelted steels for different applications.

10 ESR processes are beneficial also for so called normal application in bearing, tool and die steel. The enhanced characteristics of re-melted steel leads to higher life time and/or cycles and to reduced costs of the final products With the combination of ESR and VAR (vacuum arc re-melting) optimum properties can be achieved, which are requested especially by aerospace industry and applications in nuclear power plants as well as in medical applications. References and Sources: [1] ALD Vacuum Technologies, ESR Electro Slag Re-melting [2] SMR Premium GmbH [3] Choudhury and co-workers [4] Influence of the Melting Procedure for the Properties for High Pressure Die Casting Steel, Böhler International GmbH, Bangkok November 2013 [5] Economical Production of Superalloy Using Advanced ESR Process Harald Scholz, Rainer Schumann, ALD Vacuum Technologies [6] Saarschmiede, Germany You need to correct these refs as 1-3