Methods for increasing the cavitation and solid particle erosion resistance of 20GL and 30L steels based on their surface modification

The article presents the results of studies of cavitation and solid particle erosion resistance of samples of 20GL and 30L steels with various surface modification options based on nitriding and boriding processes. Tests for cavitation resistance were carried out according to the ASTM G134-17, and for solid particle erosion resistance - according to the ASTM G76-13. It was revealed that to increase wear resistance, the depth of modification of the considered steels should be at least 80 microns. Based on the totality of cavitation and solid particle erosion studies carried out, the best option for surface modification for 20GL steel is boriding, and for 30L steel nitriding.


Introduction
Water-solid particle erosion and cavitation wear of the impeller and guide vanes is one of the main factors affecting the reliability and performance of hydroelectric turbine elements.
Water-solid particle erosion wear occurs when solid particle erosion particles suspended in water are present in the flow part, while cavitation wear is caused by the mechanical action of a cavitating flow, which manifests itself in the form of high-frequency impacts that occur when cavities collapse on or near the streamlined surface [1,2].The flow part of a hydraulic turbine is subject to these types of wear-mostly the impeller, guide vane blades, and working surfaces of the upper and lower covers.The occurrence and development of wear leads to an increase in hydraulic losses, a decrease in power and efficiency.
In connection with the increasing requirements to increase the operational life of power equipment while maintaining its high reliability, developments to improve the corresponding mechanical characteristics of the structural materials used are relevant.Creating a material surface with the required properties is one of the main tasks when considering the issue of increasing the wear resistance of various products of modern technology, since it is the working surfaces that are most exposed during operation to the combined effects of various damaging factors, and their destruction, as a rule, begins first from the surface.
It is necessary to introduce new radical solutions for the repair of hydraulic machines, based on the use of rapidly developing additive technologies [3] and new solutions for protecting the elements of the flow part using promising methods for protecting their surface.Various coatings applied thermally [4], detonation [5], surfacing [6], and galvanically [7] can be formed on hydraulic turbine components to reduce damage.
At the same time, there is an increase in the number of publications devoted to the modification of the surface properties of structural materials [8][9][10][11][12], because of which structural changes occur in them with an increase, as a rule, in hardness and microhardness, an increase in their wear resistance while the properties and structure of deeper layers remain unchanged.One of the most promising from the point of view of increasing the strength properties of the material, as well as technologically accessible and simple, are the processes of boriding [13][14][15] and nitriding [16][17][18].
Protection of structural materials from cavitation and solid particle erosion effects can be achieved by modifying their surfaces using boriding and nitriding processes (see Figure 1).20GL and 30L structural steels, used for the production of parts of hydraulic turbine operating under conditions of static and dynamic loads, were chosen as the object of research.In this work, the task was to determine the cavitation and solid particle erosion resistance of samples of 20GL and 30L steels with various surface modification options based on nitriding and boriding processes.

Materials and methods
Nitriding of the samples for 2.5, 5 and 10 hours was carried out using an installation for forming coatings in a vacuum.
The nitriding process included the following steps: • preliminary preparation of sample surfaces; • placing samples in the installation chamber; • pumping out the chamber to a vacuum of no more than 8•10 -3 with pre-heating of the samples to a temperature of 150 °C; • ion cleaning of the surface at a pressure in a vacuum chamber of no more than 0.35 Pa and a sample temperature of no more than 350 °C; • modification of the surface of samples in a nitrogen environment for a duration of 2.5, 5 and 10 hours at a pressure in a vacuum chamber of no more than 2.1 Pa and a sample temperature of no more than 380 °C.Boriding of the samples was carried out in a shaft furnace with external heating.The boriding process included the following steps: • heating and holding samples at a temperature of 350°C for 2 hours; • exposure of samples in a boriding bath at a temperature of 880°C for 3 (type I) and 6 hours (type II); • hardening of samples in oil heated to 90°C.After carrying out the modification processes of the surfaces of samples, experimental studies of cavitation and solid particle erosion resistance were carried out.

а) b)
To conduct studies of cavitation resistance of samples in the initial state and with modified surfaces, an experimental jet-type stand was used.Tests were conducted according to ASTM G134-17 at a pressure at the nozzle exit of 180 bar, nozzle diameter 850 microns, cavitation number 0.0055.
To conduct studies of the solid particle erosion resistance of samples in the original state and with modified surfaces, an experimental jet-type stand was used.Tests were carried out in accordance with ASTM G76-13 at an air-solid particle erosion flow speed of 170 m/s, flow attack angles of 30º and 90°, and sample surface temperature of 25 ºС.Al2O3 particles (average particle size 250-300 µm) were used as an solid particle erosion material.
To analyze the surface of the samples after testing for cavitation and solid particle erosion resistance, scanning electron microscopy was used in the mode of recording secondary (SE) and backscattered (BSE) electrons on a Tescan Mira 3 LMU installation.Transverse metallographic sections were prepared using a set of equipment for sample preparation.

Results and discussions
A comparison of the cavitation resistance of experimental samples was carried out based on mass loss during the total exposure time on the stand, which was 210 minutes.The test results are presented in the form of histograms of relative cavitation resistance (see Figure 2).Analysis of the results of cavitation tests of steels without and with various surface modification options showed that: • the best boriding option for steels 20GL and 30L is type II (duration 3 hours), an increase in relative cavitation resistance of no less than 1.3 times in terms of mass loss during exposure time on the stand of 210 minutes; • the best nitriding option for steels 20GL and 30L is type III (duration 10 hours), an increase in relative cavitation resistance was recorded by at least 1.9 times in terms of mass loss during exposure time on the stand of 210 minutes.The characteristic appearance of the surface of samples of steels 20GL and 30L without and with various surface modification options after testing for resistance to cavitation wear is presented in Table 1.

Table 1. Type of cavitation traces after testing experimental samples of 20GL and 30L steels without
and with surface modification.A comparison of the solid particle erosion resistance of samples was carried out based on mass loss during the total exposure time on the stand, which was 90 minutes.The test results are presented in the form of histograms of relative solid particle erosion resistance (see Figure 3).The characteristic appearance of the surface of samples of steels 20GL and 30L without and with various options for surface modification based on nitriding after testing for resistance to solid particle erosion wear is presented in Table 2. Images of the surface of the samples with boriding is not given, since the destruction of the surface during testing was much more intense and the experiments were stopped after 30 minutes of exposure on the stand.
Analysis of the obtained results of solid particle erosion tests of the steels under study without and with various surface modification options showed that: • for steel 20GL the best option for surface modification is boriding (type II), an increase in relative solid particle erosion resistance of at least 6 times was recorded by mass loss during exposure on the stand for 90 minutes.Nitriding processes do not improve abrasion resistance.• for steel 30L, the best option for surface modification is nitriding (type III), an increase in relative solid particle erosion resistance of no less than by 15% in terms of mass loss during exposure on the stand for 90 minutes.Analysis of the metallographic studies carried out demonstrates the influence of the duration of the boriding and nitriding processes on the depth of modification of the near-surface layer: increasing the duration of nitriding from 2.5 to 5 and 10 hours leads to a depth of the modified layer of 35±5 μm, 65±5 μm and 85±5 μm, respectively; An increase in the duration of boriding from 3 to 6 hours leads to a depth of the modified layer of 50±5 µm and 100±5 µm, respectively.Thus, it can be argued that the strengthening structure of 20GL and 30L steels should consist of a surface layer of material modified to a depth of 80 to 150 μm, providing increased durability.

Conclusion
Analysis of the results of cavitation and solid particle erosion tests of the studied steels 20GL and 30L without and with various surface modification options showed that, based on the totality of cavitation and solid particle erosion studies carried out: • for steel 20GL, the best option for surface modification is boriding with a depth of the modified surface layer of 85±5 µm; • for steel 30L, the best option for surface modification is with a depth of the modified surface layer of 100±5 microns.The results obtained indicate that carrying out boriding and nitriding processes on hydraulic turbine parts made from steels 20GL and 30L to a depth of at least 80 microns will increase their cavitation and solid particle erosion resistance, and also, in the future, it will extend their time between repairs.

Table 2 .
Appearance of solid particle erosion marks after testing experimental samples of 20GL and 30L steels without and with surface modification.