Super austenite Alloy 24 is a material that lives up to its name, distinguishing itself from standard stainless steels, 6%Mo steels and all duplex grades thanks to its corrosion resistance, strength and attractive pricing. Developed in the 1990s, production of Alloy 24 ceased following a merger of the manufacturer.
^ Hempel Special Metals Article by Joanne McIntyre
Now, however, Hempel Special Metals has resurrected Alloy 24 and sees a bright future ahead for this material which is ideally suited to demanding applications such as marine scrubbers and the chemical process industry. Stainless Steel World met with CEO Mr. André Hempel and materials specialist Mr. Rolf Kirchheiner (Dipl.-lng.) to learn about the history of Alloy 24 and the exciting future that lies ahead for this remarkable alloy.
The Hempel Special Metals Group is an international sales specialist for high-performance materials in demanding industries such as implants, offshore, aviation and the process industry.
The company has been active in distributing nitrogen-enriched stainless steels since 2004 when it patented grade 316LMoNCu (1.4435NCu). This stainless steel has added nitrogen to create a delta-ferrite free material in demand most notably for the high-end watches.
Another alloy the company is selling in that field is the P2000, a double melted no nickel, high containing nitrogen austenitic stainless steel, which has interesting properties for special bearings, springs and for medical applications. Finally the company offers the 1.4318 which is a low-nickel 304 standard grade. Given the volatility of nickel prices, nitrogen and manganese added, low-nickel stainless steel offer interesting economical alternatives with superior properties as increasing strength.
Hempel’s most recent endeavour is the resurrection of the super austenitic Alloy 24, for which the company see a bright future indeed.
History of Alloys 24 & 31
Alloy 24 was developed together with Alloy 31 in the 1990s by Thyssen Edelstahlwerke and VDM.
“Both Alloy 24 and Alloy 31 are super austenitics developed to fill a niche between nickel alloys and high alloyed stainless steels such as 254MO, the 6 Moly grades, and the lowest nickel alloy grades such as the 400 series, alloy 625, etc.,” begins Mr. Hempel. “Alloy 24 has properties which make it ideal for the construction of submarines and highly-stressed containers. However, when Thyssen took over VDM, they chose to focus on nickel content, so the production of Alloy 24 was halted.”
Twenty years later, Hempel was looking to add a new material to their portfolio, which would differentiate them in the market.
“We were aware of Alloy 24 and that there was still demand from the chemical process industry, although it was no longer obtainable. Research showed this would be an ideal addition to our range, with Alloy 24 being up to 30% cheaper than its closest equivalent, Alloy 31.
Three years ago, we started partnering with Industeel, a specialist plate mill, to introduce Alloy 24 to the market. We had some success with the chemical process industry and in the scrubber market.”
What is Alloy 24?
Alloy 24 is a super austenitic alloy that positively distinguishes itself from classic stainless steels, duplex and other alloyed 6%Mo steels. In a variety of industrial applications, ìt can replace the established lower representatives of nickel-based alloys.
With their highly developed application potential, super austenitics fill the gap between the standard grades of stainless steels and nickel-based alloys. One of the driving forces for the use of super austenites is economy, as they must be able to stand up technically and economically against the higher alloyed nickel-based alloys. The technical superiority of super austenites over standard steels is assumed, and Alloy 24 is an outstanding example of this.
“With the development of Alloy 24, a new austenitic stainless steel has become available – delivering the highest strength and excellent weldability with outstanding corrosion resistance in chloride-containing media,” explains Mr. Kirchheiner. The neutral designation ‘Alloy 24’ is derived from the proportion of the materials average chromium content of 24%. The material number for the original material is 1.4565; the UNS number is S34565.
In Alloy 24, the role of the cheapest alloying element nitrogen (N max 0.6%) is exploited in several ways. Nitrogen has several positive features in the formation of the fully austenitic structure of Alloy 24:
Significant increase in strength
Minimisation of negative carbide precipitation
Increase of corrosion resistance in synergy with chromium and molybdenum.
This technically and economically sophisticated alloy design allows Alloy 24 to be labelled as a Smart Super Austenite. The chemical composition is shown in Table 1.
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