What are the limitations of 'cold engineering' and are they being overcome?

While still considered specialised, cryogenics is being put to use in more mainstream applications such as in hospital magnetic resonance imaging (MRI) scanners, forward looking infrared (FLIR) thermal imaging cameras as well as in the mass shipments of liquefied natural gas (LNG). LNG for example has seen significant investment to stop dependency on any one natural resource from any one country. The result of all of this is that cryogenic engineering and the associated materials have been the focus of much development. Operating at such low temperatures places a very particular set of demands on the materials used. Experience with brittle fracture of steel ships operating in the Arctic during World War II demonstrated that, while many metals have good properties and characteristics at room-temperature, they sometimes do not perform adequately at much lower temperatures. For example, ferritic, martensitic and duplex stainless steels tend to become brittle as temperature is reduced and can fracture. These failures can occur without any visual warning of deformation of stretching or bulging. This has meant that austenitic stainless steels have predominately been used, as these retain most of their properties at cryogenic temperatures. These have commonly been used in Arctic locations and in the handling and storage of liquid gases like nitrogen at temperatures as low as -196°C. As a rule of thumb, the lower the operating temperature, the higher the alloying contents needed – particularly nickel. Austenitic steels have been most commonly applied to extremely cold environments, but are not without compromise. The metals have low yield strength that is frequently subject to the risk of early deformation. To overcome the issue, parts and components made from castings are usually designed with much thicker walls than would normally be required. This was the starting point for a low-temperature steel developed by German specialist steel and steel casting manufacturer, Schmolz + Bickenbach Guss. It wanted to use a martensitic based steel that could be developed to offer high strength and toughness at cryogenic temperatures to allow design engineers to specify thinner walls of cast components and save both weight and cost. Martensitic steel materials are excellent for tempering and also display significantly higher yield strength than austenitic steels. However, particular demands are placed not only on the strength, but also on the toughness. The prerequisite for high strength at low temperatures is primarily a low content of selected trace elements. Otherwise, the segregations caused can result in embrittment within the casting. Dr Petra Becker, head of research and development at Schmolz + Bickenbach Guss, says: "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." The result was the production of the martensitic low temperature stainless steel, Dux Cryo, which Schmolz + Bickenbach Guss has recently started using in replacement of the more traditional austenitic steels. Extensive development work The starting point for the research was the nickel alloy steel X8Ni9, which is commonly used as a standard sheet or forging material for cryogenic applications. However, due to the high cracking sensitivity of its coarse-grained primary structure, no casting version of the material existed. The aim was to modify a X8Ni9 alloy to make it suitable for casting processes by combining findings from analytics, metallurgy and heat treatment. Extensive materials research, development and testing was combined with external expertise to see if it was possible. One of the early key findings identified a number of key requirements, including the importance of purity to any input substance, its melting point and the shaping technology applied. Additionally, it found that heat treatment parameters had to be precisely controlled. Based on these findings, the team set about experimental production to cast the modified nickel alloy from melting and casting through to heat treatment and thorough mechanical testing. The cast test pieces were subjected to extensive visual and colour penetration checks that included ultrasound and X-ray examinations. This analysis made it possible to demonstrate that the alloying concept together with the selected cooling conditions produced a crack-free casting. Furthermore, a series of heat treatment tests took place that further optimised the new steels mechanical values. Becker explains: "Because of its chemical composition, the new material is more advantageous than austenite steel [and although it has] a similar nickel content, [Dux Cryo] contains no chromium. The further advantage is it can be mechanically processed with no problem." Dux Cryo has already found a number of applications, with more being identified. It is generally considered suitable for applications carried out in temperatures between -100°C and -196°C. This temperature range makes it particularly suitable for cryogens such as dry ice, liquid oxygen and liquid nitrogen. Another area that has been identified is for air liquefaction and separation systems, where air is separated using a thermal separation processes that extracts nitrogen, oxygen, argon and other noble gases in high-purity concentration in both liquid and gaseous forms. Another application with a promising future is the liquefaction of natural gas (LNG). Here, the natural gas is cooled to as low as -164°C. The rise of LNG terminals in the UK has seen the demand for cyrogenic components substantially rise in recent years. "This material could also have interesting potential in the areas of soil freezing, industrial refrigeration technology and oil sands extraction," says Dr Becker. "The same applies for all components that are used at low external temperatures such as whether pumps in Alaska or deep-sea offshore applications."