Specialists from Novosibirsk State Technical University (NETI), together with colleagues from the Institute of Nuclear Physics SB RAS named after G.I. Budker (INP SB RAS) improved the wear resistance of chromium-nickel stainless steel.
In everyday life, this material is called stainless steel, but it is used to create not only familiar pots and spoons with forks, but also various parts of the oil refining industry equipment. For oilmen, stainless steel is interesting for its high anti-corrosion characteristics, which is very important in underground operation. Stainless steel would become an even more suitable material for the industry if it were possible to increase its resistance to waterjet wear, that is, to the effects of solid particles moving with the flow of liquid. Novosibirsk scientists applied a layer of a mixture of boron and iron (borides) powders to stainless steel using electron beam surfacing at the ELV-8 industrial accelerator of the INP SB RAS. Subsequent tests for waterjet wear conducted at the M.A. Lavrentiev Institute of Hydrodynamics, SB RAS (IGiL SB RAS) has shown that such improved stainless steel has twice the wear resistance and corrosion resistance than conventional. The results are published in the journal Metallurgy and are part of a large series of studies devoted to the production of steel with improved characteristics for use in extreme operating conditions.
"Specialists in the oil refining industry use equipment whose parts are made of chromium—nickel austenitic steel, because this material has several important properties," said Evdokia Bushueva, Associate Professor of Materials Science in Mechanical Engineering at NSTU-NETI and Candidate of Technical Sciences. — It is corrosion-resistant, and this is very important, since oil refining equipment operates underground in a chemically aggressive environment, for example, it may be affected by groundwater, electrolyte solutions, and related gases. Another important characteristic of stainless steel is its adaptability. The parts of such equipment usually have a complex shape, so the material from which they are created must have plasticity. And the third important point is that stainless steel is relatively inexpensive."
Despite all these advantages, stainless steel has one disadvantage, it has low wear resistance. Wear resistance is the ability of a material to resist the destruction and abrasion of its surface layer under friction conditions. Wear can be different, but when it comes to oil refining, abrasive wear is most often implied.
"Stainless steel is quite plastic, so it is difficult for it to resist the flow of water with particles of a hard abrasive,— added Evdokia Bushueva. — The abrasive acts like a million knives that stick into the surface. First, scuff marks, scratches, and cracks appear, and given that the material is also exposed to an aggressive environment, the corrosion resistance of stainless steel is greatly reduced. As a result, instead of the required thousands of hours, such equipment performs only hundreds. Naturally, industrialists, and therefore scientists, faced the challenge of increasing the operating time of oil—producing equipment."
One of the ways to make classic stainless steel more wear—resistant is to strengthen its surface layer. NSTU-NSTI specialists chose a material based on chromium and iron borides for this purpose, and the method of creating a strengthening layer was electron beam surfacing on the ELV-8 industrial electron accelerator of the INP SB RAS. This accelerator has the status of a unique scientific installation (UNU Stand ELV-6) and is included in the national list of research infrastructure facilities of the Russian Federation.
"An industrial accelerator generates a powerful continuous electron beam, which we use to process the surface of a material, in this case stainless steel, along with a modifying powder placed on its surface," explained Mikhail Golkovsky, a senior researcher at the INP SB RAS. — Among other hardening methods, such as plasma spraying, laser, and electric arc welding, ours has a number of advantages. We can create a thicker surface layer on the material than with laser surfacing, devoid of porosity and weak adhesion to the substrate, characteristic of plasma spraying. If laser surfacing works with a thickness of no more than a dozen microns, then we get a layer of several millimeters devoid of pores. It is important that we ensure its metallurgical adhesion, that is, the adhesion of the deposited layer to the base is not worse than the strength of the base metal itself. In harsh operating conditions, it is important that even a very strong layer itself does not separate from the base. The industrial accelerator has high productivity: the average material processing speed is 2 m2/hour, which is a good indicator. It is also important to note that we work in the atmosphere, not in a vacuum. Methods that involve processing in a vacuum chamber are technologically more complicated and take longer. The power of our electron source is one or two orders of magnitude greater than, for example, lasers. In addition, the absorption coefficient of the beam by the material is 90%, that is, almost all the beam energy goes into the material, unlike lasers, where only 10% is absorbed."
After obtaining samples of stainless steel with a hardened surface layer, the specialists conducted a series of experiments in which they recreated the extreme operating conditions of the oil production equipment.
"Since we focused on the oil industry, one of the tests was for waterjet wear, which we conducted at the Institute of Hydrodynamics facility," commented Evdokia Bushueva. "We exposed the sample to a powerful stream of water with aluminum oxide particles, just sand and air, specifically creating the most extreme conditions. The results were quite good: the waterjet wear of hardened stainless steel is two times less than that of conventional steel. Another result is related to corrosion resistance. We tested our coating in a corrosive environment, recreating the conditions of exposure to emergency solutions. When oil production equipment becomes jammed, for example, when rock gets into it and the mechanism cannot turn, solutions with aggressive acids such as hydrofluoric acid, sulfuric acid, hydrochloric acid, and nitric acid are used. Such a vigorous mixture dissolves the rock very quickly, but the material of the equipment can also corrode. Even here, our sample turned out to be twice as corrosion-resistant as stainless steel."
According to experts, the results obtained are part of a large-scale work on the creation of reinforcing coatings.