When melting high nitrogen steels a range of challenges present themselves, not least the problem of attaining and maintaining high nitrogen content in solution within the melt.
With pressure electroslag remelting (PESR) comes the ability to increase nitrogen content beyond the solubility limit and therefore have a beneficial effect on a range of key material properties.
Nitrogen alloyed stainless steels, both austenitic and martensitic, exhibit attractive properties such as high strength and ductility, good corrosion resistance and reduced tendency to grain boundary sensitization.
The melting of high nitrogen steels (HNSS) poses two problems: how to get high nitrogen contents into the melt and how to keep it in solution during the process of solidification. Gas porosity formation takes place in the solidification stage because of the low solubility limit of nitrogen into the ferrite phase: as a consequence, the liquid is strongly enriched in nitrogen.
There are two possible ways to overcome the low solubility of nitrogen in molten iron and to achieve a high nitrogen content in the steel: alloying with elements which lower the nitrogen activity or melting under pressure.
For this reason the development of pressure electroslag remelting (PESR) is becoming a predominant interest for the production of steels having nitrogen content largely above the solubility limit under atmospheric pressure.
Energietechnik Essen GmbH has been making special forged products and pressure-nitrided alloys for many years. In order to meet the ever increasing customer demands with regard to the physical and chemical properties, such as strength, toughness and corrosion resistance, there have been further enhancements to the pressure electro slag remelting (PESR) process at the company's works in Essen so that they are now able to produce pressure-nitrided steels with outstanding property profiles on an industrial scale with the aid of this technology.
The PESR process makes it possible to increase the nitrogen content beyond the solubility limit. The advantages of an increased nitrogen content are higher hardness and better corrosion resistance. Where austenitic steels are concerned, the austenitic structure is stabilized and the tensile strength increased without reducing the toughness.
The following material qualities are produced at Energietechnik Essen GmbH at the present time:
||X 30 CrMoN 15 1
|P 900 N
|P 900 N Mo
The common advantages of the Pressure/Protective Gas Electroslag Remelting (PESR) process are:
- No hydrogen pick up
- Nitrogen alloying up to approx. 0.8%
- Absence of ingot segregation
- Only minor crystal segregations
- Minimum sulphur contents
- Minimum levels of nonmetallic inclusions
- Excellent material properties
- Low melting loss of elements with high oxygen afinity
As illustration Figure 1 shows the sketch of the Pressure/Protective Gas Electroslag Remelting (PESR) process.
Figure 1: Pressure/Protective Gas Electroslag Remelting (PESR) 1-Pressure sealing 2-Furnace Head 3-Extension Chamber 4-Stinger 5-Stub 6-Electrode 7-Static Mold 8-Slag 9-Liquid Metal 10-Ingot 11-Alloy Feeder; DC-Power Supply