New casting steel for very low temperature strength

Cryogens like liquid oxygen and nitrogen embrittle standard steel grades, complicating the part design process for very cold applications.

Steels used in the low temperature range — down to –196 °C, face a major performance challenge. To date, austenitic steels have been used. However because of their low yield strength, these are subject  to the risk of early deformation and must therefore be cast with very thick walls.

This was the starting point for the development of the new low-temperature material DUX CRYO, which displays significantly higher strength values with good toughness. The advantage is the ability to cast parts with thinner walls, which saves not only on weight but also on costs.

Instead of austenitic steel, SCHMOLZ + BICKENBACH GUSS has recently started using martensitic steels for jobs in the lowtemperature range – this is the result of an extensive research project funded by the German federal ministry of economics and technology. These materials are excellent for tempering and also display significantly higher yield strength (Rp0.2 ≥ 490 N/mm²) than austenitic steel alternatives — a property particularly advantageous in such extreme temperatures.

However, demands are placed not only on the strength but also on the toughness (KV (–196°C) ≥ 40 J). The prerequisite for high strength at low temperatures is primarily a low content of selected trace elements.

Otherwise, the segregations caused can result in embrittlement of the casting. “For us, the challenge lay in achieving reliable manufacture of the castings with a focus on optimised structure and therefore adequate strength — without cracks appearing in the casting volume,” says Dr. Petra Becker, head of research & development at SCHMOLZ + BICKENBACH GUSS.

Extensive development work:  

The starting point for the research project was the low temperature material X8Ni9, which is used as a standard sheet and forging material for applications down to –196°C. However, due to the high cracking sensitivity of the coarse-grained primary structure, no casting alternative to the material existed previously. The aim was to present the material as a casting modification by combining findings from analytics, metallurgy and heat treatment. In addition to extensive materials testing and comprehensive literature research, this also involved cooperation with external experts.

The latest technologies were used here, e.g. a casting technology simulation, thermodynamic calculations of the material and heat treatment, and the latest methods of analysis for the investigation results. One of the findings was that the requirements in terms of purity of the input substances and in terms of melting and shaping technology are particularly important. Additionally, the heat treatment parameters must also be set extremely precisely.

Based on the findings, experimental production was then undertaken — from melting and casting through heat treatment to mechanical testing. After casting, the castings were subjected to extensive checks. In addition to visual and colour penetration checks, these included ultrasound and x-ray examinations.

Large bulk carriers for liquefied natural gas store the fuel at temperatures as cold as –164°C but must survive the high stress and strain environment of large oceangoing ships.

It was possible to demonstrate that the alloying concept together with the selected cooling conditions really does produce crack-free castings. Furthermore, a series of heat treatment tests took place in order to optimise the mechanical values.

The result of this series of tests is the new low-temperature material DUX CRYO. This stands out for its increased yield strength and outstanding low-temperature strength.

The casting can therefore be designed and constructed with significantly thinner walls. This allows greater design freedom, saves on weight and costs, and conserves resources.

“Because of the chemical composition, the new material is more advantageous than austenite — with a similar nickel content, it contains no chromium,” Becker states.

The material can be mechanically processed with no problems.

Diverse applications:

DUX CRYO is suitable for all areas in which work is carried out at temperatures between –100°C and –196°C for example, wherever cryogens such as dry ice or liquid oxygen and nitrogen are used. This applies including for air liquefaction and separation systems, in which air components are separated using thermal separation processes to extract nitrogen, oxygen, argon and other noble gases in high-purity concentration as well as in liquid and gaseous form.

Another application with a promising future is the liquefaction of natural gas: here, the natural gas is cooled to as low as –164°C in so-called LNG terminals — the demands on the components used are accordingly also high. Similar demands apply for cold grinding and cryogenic recycling.

These processes are used in the food industry and in the area of composite materials. The aim here is the grinding of materials with a low softening point.

“This material could also have interesting potential in the areas of soil freezing, industrial refrigeration technology and oil sands extraction,” Dr Becker expounded.

The same applies for all components that are used at low external temperatures: whether pumps in Alaska or deep-sea offshore applications.