**2. Computer simulation**

the elements made of such materials. Each modern engineer should be aware of this aspect in one's everyday work during which, when designing a specific constructional solution and selecting a material appropriate for it, one also has to foresee sometimes complex erosion mechanisms the element being constructed will be exposed to and prevent them effectively. For this reason, in parallel to the development of innovative solutions for fabrication and processing of engineering materials, knowledge has been developed in scope of their wear mechanisms, both, on the surface as well as in their core often representing a substrate. The character of the surface often has a direct effect on the product fabrication process, and its functional properties in many cases depend on the surface quality, including shape geometry, roughness, chemical composition, structure morphology or an external appearance. It is worth noting that requirements imposed on the product interior, representing a substrate or core, are usually different from those for the external surface of a given constructional element.

When designing the individual subassemblies of machines or entire devices one has to draw special attention to the resistance of the elements working there, to tribological damages (mechanical, fatigue, adhesion, abrasion, hydrogen and other damages) as well as to non-tribological damages (corrosion, diffusion, cavitation, erosion, ablation and others). Considering the above-mentioned mechanisms, cavitation erosion and wear are often overlooked in engineering design, the dual character of which has an effect on the economics and development of particular fields of economy in the negative and positive sense. Cavitation is generally described as a phenomenon consisting of implosion of gas bubbles in liquid, with such bubbles formed as a result of a rapidly falling pressure causing the creation of shock waves with the length of 0.1–0.2 mm and the speed of several hundred m/s, destroying local surfaces of elements and causing deep cavitation pits and craters. For example, the bodies of machines and devices and constructional elements working directly in a cavitation environment are exposed to long-term wear leading to damages and failures, which are disastrous in the economic sense, of complete assemblies, components or in exceptional cases single parts only. A negative impact of cavitation can also be diagnosed for water supply pumping stations used across the world. The cavitation wear resistance of a flow system of pumps and water supply systems depends on the type of the material applied and its surface treatment, including also its structure and properties. Cavitation pits forming in water supply system elements are typically found on the entire part surface, especially near scratches and surface defects created in a manufacturing process. Such scratches or material defects normally occur by accident in transport, maintenance or due to negligent operation. In processes related to transportation of medium, especially water, threats to a construction's reliability are often encountered due to cavitation wear processes of construction elements of the flow system in rotodynamic single- or multi-stage pumps working in water supply stations and systems, as well as parts of entire water turbine blades. The most effective way to mitigate the effect of cavitation in flow systems is to use materials resistant to cavitation wear for medium transportation constructions. Materials from the group of stainless and acid-resistant steels, i.e. bronze and brass, are thought to be the appropriate engineering materials which can be used in

The discussed issue of cavitation erosion does not apply to selected cases of water distribution or electricity generation only, i.e. to turbines operated in hydropower plants, but most of all to the whole water transportation infrastructure, to different types of power and heating assemblies and complexes working in plants producing electric energy and in combined heat and power

water supply systems and pumps [1–4].

8 Cavitation - Selected Issues

Two steels were selected for detailed examinations in the conditions of cavitation wear. The first one is P265GH steel commonly used for pressure devices working at elevated temperatures, with a ferritic-pearlitic structure, and the other derives from a group of stainless steels, i.e. chromium-nickel X2CrNi18-9 (304L) steel with an austenitic structure. P265GH steel—due to its unlimited availability and attractive, low market prices—is used for constructing heat distribution devices and heating devices, and for less critical constructional parts. X2CrNi18-9 (304L) steel, which is five times more expensive than P265GH steel, is used for production of devices, apparatuses and fittings in the chemical, food, power and petrochemical industry and for constructional elements in the aviation and shipbuilding sector. A chemical composition of the structural steels tested in the conditions of cavitation wear is presented in **Table 1**.

Cavitation generators (**Figure 1**), with the shape and dimensions selected by analysing the results of numerical simulations in ANSYS FLUENT software, described in detail in the earlier publication, were prepared using the above-mentioned P265GH and X2CrNi18-9 (304L) steels [5]. The cavitation generators were tested in the conditions of cavitation wear continuously for 500 PMH (Productive Machine Hour) in a specially designed computer model (**Figure 2**), and then in a constructed author's stream and flow device (**Figure 2**) generating a cavitation environment. The detailed process parameters are presented and described in the publication [5].

It was found based on the computer simulations performed and the obtained numerical results of medium (water) flow at a temperature of around 40°C for the set boundary conditions that


**Table 1.** Chemical composition of the structural steels tested in the conditions of cavitation wear: P265GH by PN-EN 10028:2010; X2CrNi18-9 by PN-EN 10088 [mass fraction, %].

**Figure 1.** Cavitation generator model's dimensions and shape selected based on the analysis of results of numerical simulations in ANSYS FLUENT software; cavitation generator thickness of 5 mm, relative clearance of pp = 11.1 [%] [5].

cavitation implosions (content of steam) simulating the wear are occurring to a high degree. Simulating the wear occurring mainly before a cavitation generator, and especially on the inlet edges and along straight-through openings of a constructional element. A cavitation generator

**Figure 3.** Pressure distribution of flowing medium (water) on the one-twelfth (30°) section of the field area of model of

**Figure 2.** Model of a stream and flow device generating a cavitation environment; (a) isometric diagram of the device, testing and measuring system; (b) simplified computer model of the cavitation generator location with the medium

Effects of Applying WC/C Protective Coating on Structural Elements Working in Cavitation…

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11

(water) flow direction marked red arrow [6].

constructional element [5].

Effects of Applying WC/C Protective Coating on Structural Elements Working in Cavitation… http://dx.doi.org/10.5772/intechopen.80719 11

i.e. chromium-nickel X2CrNi18-9 (304L) steel with an austenitic structure. P265GH steel—due to its unlimited availability and attractive, low market prices—is used for constructing heat distribution devices and heating devices, and for less critical constructional parts. X2CrNi18-9 (304L) steel, which is five times more expensive than P265GH steel, is used for production of devices, apparatuses and fittings in the chemical, food, power and petrochemical industry and for constructional elements in the aviation and shipbuilding sector. A chemical composition of the structural steels tested in the conditions of cavitation wear is presented in **Table 1**. Cavitation generators (**Figure 1**), with the shape and dimensions selected by analysing the results of numerical simulations in ANSYS FLUENT software, described in detail in the earlier publication, were prepared using the above-mentioned P265GH and X2CrNi18-9 (304L) steels [5]. The cavitation generators were tested in the conditions of cavitation wear continuously for 500 PMH (Productive Machine Hour) in a specially designed computer model (**Figure 2**), and then in a constructed author's stream and flow device (**Figure 2**) generating a cavitation environment. The detailed process parameters are presented and described in the publication [5]. It was found based on the computer simulations performed and the obtained numerical results of medium (water) flow at a temperature of around 40°C for the set boundary conditions that

**C [%] Mn [%] Si [%] Al [%] Cr [%] Ni [%] Cu [%] Ti [%] N [%] S [%] P [%]**

0.16 0.99 0.23 0.047 0.027 0.013 0.026 0.001 0.003 0.008 0.019

max <0.03 <2.0 <1.0 — 19.50 10.50 — — <0.11 <0.045 <0.015

P265GH max — — 0.4 — 0.3 0.3 0.3 0.03 0.012 — —

**Table 1.** Chemical composition of the structural steels tested in the conditions of cavitation wear: P265GH by PN-EN

min. 17.50 8.00

**Steel Chemical composition**

10028:2010; X2CrNi18-9 by PN-EN 10088 [mass fraction, %].

X2CrNi18-9 (304L)

10 Cavitation - Selected Issues

**Figure 1.** Cavitation generator model's dimensions and shape selected based on the analysis of results of numerical simulations in ANSYS FLUENT software; cavitation generator thickness of 5 mm, relative clearance of pp = 11.1 [%] [5].

**Figure 2.** Model of a stream and flow device generating a cavitation environment; (a) isometric diagram of the device, testing and measuring system; (b) simplified computer model of the cavitation generator location with the medium (water) flow direction marked red arrow [6].

**Figure 3.** Pressure distribution of flowing medium (water) on the one-twelfth (30°) section of the field area of model of constructional element [5].

cavitation implosions (content of steam) simulating the wear are occurring to a high degree. Simulating the wear occurring mainly before a cavitation generator, and especially on the inlet edges and along straight-through openings of a constructional element. A cavitation generator

500 PMHs of continuous work in such device, the cavitation generator was cleaned in an ultrasonic cleaner, and then weighed on an analytical scale, AS/X by RADWAG. Surface roughness was determined with a Surtronic 25 contact profilometer by Taylor Hobson. At least 4 measurements, along the length of 16 mm in different areas of the constructional element, were performed to determine surface roughness of each generator. Detailed macroscopic examinations with a scanning electron microscope, SUPRA 35, at the accelerating voltage of 5–20 kV using secondary electrons (SE) detection, with the magnification of 100–2000×, were

Effects of Applying WC/C Protective Coating on Structural Elements Working in Cavitation…

The results of mass loss and surface roughness measurement examinations before and after

The results of macroscopic examinations of the applied P265GH and X2CrNi18-9 (304L) steels in conditions of cavitation wear made with a SUPRA 35 electron scanning microscope using

A constructional element such as a cavitation generator made of ferritic-pearlitic steel, designated as 200-P265GH, wet sanded with sandpaper with the grain size of 200, weighed

surface roughness class according to PN-EN ISO 1302:2004. A negligible mass loss of approx.

0.627 to 0.41, which is further classified as the 8th surface roughness class. A constructional element made of ferritic-pearlitic steel designated as 1000-P265GH, i.e. wet sanded with sandpaper with the grain size of 1000, had the weight of 57.1835 g and a surface roughness factor

 of 0.15, thus falling to the 10th surface roughness. The highest mass loss, of as much as 0.1752 g in relation to all the operated generators, and the growth of the roughness factor R<sup>a</sup> from 0.15 to about 0.5, was found after operating a generator marked as 1000-P265GH, which

**Figure 5.** Loss of mass, i.e. the mass of cavitation generator made of P265GH and X2CrNi18-9 (304L) steel after use in

cavitation wear conditions in a continuous flow blast machine in a closed cycle [6].

0.03 g was also found as a result of generator operation and a roughness factor R<sup>a</sup>

of 0.627, thus falling to the 8th

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undertaken for preliminary identification of the cavitation wear results [6].

use are shown, respectively, in **Figures 5** and **6**.

Ra

secondary electrons (SE) detection are shown in **Figures 7**–**10**.

57.6271 g before use and featured a surface roughness factor R<sup>a</sup>

**Figure 4.** Implosion distribution in form of content of steam on the one-twelfth (30°) section of the field area of model of constructional element [5].

was selected characterised by a relative clearance Pp of 11.1 [%], which has reached an inlet pressure of the medium of 244,000 [Pa], for the number of cavitations (content of steam) of 0.98 [%], as shown in **Figures 3** and **4**. The areas most susceptible to cavitation wear were also defined by analysing numerical results, with such areas being most of all the areas of straight-through openings, and a central opening with the diameter of 4 mm was found to be most susceptible, in particular.
