New ways to modify the structure of alloys for nuclear engineering developed by Russian scientists

New construction materials and technologies open up opportunities for producing more compact and lightweight heat exchange equipment for nuclear industry with enhanced reliability and corrosion resistance

The possibilities for creating new equipment for nuclear engineering are largely determined by the properties of structural materials produced by the industry. Industrial production of pipes from titanium alloys in combination with the development of the technology for their welding has made it possible to increase the reliability of heat exchange elements previously manufactured from substantially less corrosion-resistant pipes made of austenitic steels.

This enables a significant increase in the service life of heat-exchange equipment, as well as a significant reduction the weight of heat exchangers, since the density of titanium is noticeably less than that of steel. Currently, heat exchange equipment made of titanium alloys is widely used in nuclear power engineering, chemical, petrochemical and other industries.  As a rule, these heat exchangers must be highly reliable, since inter-circuit leakages of working media are not permissible.

The work of a group of researchers from the Department of Metal Physics of the Research Institute of Physics and Technology of Lobachevsky University (UNN  PTRI) shows that by optimizing the structure, it is possible to significantly improve the characteristics of titanium alloys without additional doping with expensive components like platinum group metals or rare earth elements.

In the framework of the research commissioned by JSC Afrikantov OKBM, a Nizhny Novgorod company of the State Atomic Energy Corporation ROSATOM, since 2010 UNN  PTRI has been working to create nano-modified α- and pseudo-α titanium alloys PT-7M (Ti-2.5Al-2.5Zr) and PT- 3V (Ti-5Al-2V), which are widely used in heat exchanging equipment in  nuclear engineering.

According to Alexei Nokhrin, head of the UNN  PTRI Material Diagnostics Laboratory, this task is very complex, since it is necessary to ensure not only an increase in the resistance of titanium alloys to local types of corrosion, but also to improve their performance properties, such as strength characteristics, corrosion-fatigue strength, resistance to hydrogen embrittlement and  some other parameters.

 “This is a rather nontrivial task, since the volume fraction of β-phase hardening particles in these titanium alloys is very small and some other methods of structure control had to be used to make the alloys harder. To solve this problem, it was proposed to use deformation nanostructuring technologies that make it possible to simultaneously reduce the grain structure to  nano- and submicron levels, and to significantly reduce the local concentration of corrosive impurities at grain boundaries,” says Alexei Nokhrin.

From 2010 to 2015, this work was carried out at the UNN PTRI with the funding and participation of JSC Afrikantov OKBM, and in 2016 it was supported by a grant from the Russian Science Foundation. At the head of this research is Vladimir Kopylov, a visiting leading researcher (Physics and Technology Institute of the National Academy of Sciences of Belarus, Minsk), with whom the UNN PTRI Department of Metal Physics has a long-standing fruitful cooperation. Vladimir Kopylov and Professor Vladimir Segal are the authors of the equal channel angular pressing technology (ECAP), the essence of which is to push a metal billet through two channels of circular or square cross-section, which are connected to each other at a given angle (typically 90°).

 “To obtain samples of titanium alloys, we used modern equipment for complex multi-stage deformation processing, a rotary forging machine R5-4-21 HIP (Germany) and an Italian hydraulic press Ficep HF400L with a rated force of up to 400 tons. This allowed us to first form a homogeneous submicrocrystalline structure in titanium alloys using the ECAP method, and then manufacture titanium rods more than a meter long from these alloys,” explains Yuri Lopatin, head of the UNN PTRI Metal Technology Laboratory.

The proposed approaches have demonstrated a very high efficiency of deformation processing technologies. The bench corrosion tests carried out at JSC Afrikantov OKBM showed that titanium alloys with an optimized structure have unique properties. In particular, submicrocrystalline samples from alloy PT-3V showed 4-6 times higher resistance, and nanostructured samples from the alloy PT-7M,  3-5 times higher resistance to hot-salt corrosion in comparison with standard samples from industrial titanium alloys.

By forming a fine-grained structure, UNN  PTRI researchers working together with their colleagues from JSC Afrikantov OKBM managed to simultaneously increase the hardness and corrosion-fatigue strength of alloys by a factor of 1.5-2 while maintaining their plasticity at a level sufficient for safe operation of heat exchange pipes.

Then the researchers had to solve a new task: to preserve the fine-grained structure in the manufacture of equipment from welded titanium alloys.

 “... We almost immediately faced the task of welding these alloys,” Alexei Nokhrin continues. “It is useless to have a structure with unique strength and corrosion resistance in a titanium alloy, if during its fusion welding you “destroy” this unique structure and get an ordinary weld seam, which will be the “weak” place of the heat exchange tube ... Experiments have shown that the commonly used technologies of argon-arc and electron-beam welding result in a sharp deterioration of the properties of the welded joints of our materials ... ".

 To solve this problem, the UNN  PTRI team used a new technology of high-speed electric pulse heating under pressure, which is a type of spark plasma sintering technology.

This technology was implemented using the high-speed electric pulse heating system “Dr. Sinter model SPS-625 (Japan). These tests were carried out in the Laboratory of Ceramics Technology at UNN  PTRI under the supervision of Head of the Laboratory Maxim Boldin.

Preliminary studies show that the new technology of high-speed solid-phase diffusion welding makes it possible to form a non-porous, fine-grained structure in the weld seam. Due to this, the weld seam has high hardness and corrosion resistance. The width of the weld seam is very small and is visible only in a microscope at high magnification.

“The use of such structural materials and technologies opens up new opportunities for designers: it is possible to make heat exchange equipment that is more compact and lightweight without compromising reliability. It will be less susceptible to a short-term supercritical increase in corrosive aggressiveness of working media during operation,” comments Maxim Boldin.

The results of the work were published in the Journal of Alloys and Compounds (2019, v.785, p.1233-1244; 2019, v.790, p.347-362). The results of testing high-speed diffusion welding of titanium alloys were published as a separate chapter in the collective monograph “Spark Plasma Sintering of Materials” (2019, Chapter 24, pp.703-711, https://doi.org/10.1007/978-3-030 -05327-7_24), published by Springer Nature (Switzerland, Basel).

Experimental Engineering Design Bureau named after Afrikantov (Afrikantov OKBM) is part of the machine-building holding JSC Atomenergomash and belongs to the ROSATOM Corporation. OKBM is the largest Russian developer and manufacturer of various types of reactor plants and equipment for nuclear power plants.

National Research Lobachevsky State University of Nizhny Novgorod (UNN) is a member of the Consortium of core universities of Rosatom State Corporation. In December 2012, a general agreement on strategic cooperation was signed between UNN and OKBM.