Evaluation of Internal Stresses in Modified Near-Surface Layer of 20Kh13 Steel during Incubation Period of Water Droplet Impact Erosion

Abstract

This study investigates the change in internal stresses of ion nitrided and Cr–CrC ion-plasma-coated 20Kh13 steel samples during the incubation period of the droplet impact erosion. To obtain internal stress values, a method based on measuring the deformation (curvature) of the sample along the surface profiles before and after its modification/coating, as well as during the subsequent droplet impact has been applied. The results suggest that the initial droplet impact on the uncoated, nitrided and coated material leads to a sharp increase in the initial internal stress values. The rate of increase in internal stress decreases with the surface damage development and accumulation. The proposed method of stress evaluation can be used in selecting a criterion for comparing the erosion resistance of a material and a method for modifying its surface.

1. Introduction

Internal stress significantly affects the strength, adhesion and other performance properties of coatings and modified layers of metallic materials. Stresses occur both at the stage of surface modification and during use of products under droplet impact, leading to erosive wear of the protected material [1,2,3,4,5,6,7]. One of the important criteria of erosion resistance of coatings is their resistance to destruction at the initial stage, i.e., during the incubation period, when damage accumulates without visible signs of destruction of the protected surface [8] and without mass removal.

The origin and development of surface damage in metallic materials during the incubation period is a highly complex multivariate process. This process is particularly affected by surface damage caused by internal stresses cyclically generated by droplet impact. This is due to the interaction, propagation and reflection of stress waves. These interactions can lead to stresses that exceed the strength limit of the modified layer/applied coating/substrate interface [9].

The origin and development of damage over time is predetermined by the structure and properties of the structural materials as well as the effect of the stress state of the modified layer/applied coating-substrate system [10,11,12,13,14,15,16,17,18,19,20,21,22,23].

The properties of the interfacial surfaces play an important role, as the main failure mechanism is related to the change in stress state caused by high velocity droplet impacts. In this sense, thinner coatings with greater adhesion may be the best option for protection against droplet impact erosion. A possible solution to increase the strength and adhesion properties of the near-surface material layer could be the use of diffusion coatings based on nitrogen saturation to a certain depth to form an erosion-resistant modified layer in the material itself.

This study is aimed at evaluating the change in internal stresses in the modified near-surface layer of ion nitrided and Cr–CrC ion-plasma-coated 20Kh13 steel samples before and after the droplet impact. The goal of the study is to determine the relationship between the stress state in the near-surface layer of the material with different types of protection (nitriding, coating) and the end of the incubation period of the wear process.

The method used to achieve this was based on the determination of stresses in the near-surface modified layer/coating by measuring the deformation (curvature) of the sample.

2. Materials and Methods

The Stoney formula [24,25,26,27], written as [28], can be used to determine the stresses due to surface modification/coating by measuring the curvature of the sample resulting from stress in the surface modified layer/coating:

$$\sigma =\frac{4\delta }{3l^2}\frac{E}{\left(1-\nu \right)}\frac{t_s^2}{t_f},$$

(1)

where σ is the stress in the modified layer, δ is the normal (in the Z axis direction, see Figure 1) curvature of the sample due to surface modification/coating, l is the length of the surface profiles used to measure the curvature δ (see Figure 1c), E is the Young’s modulus of the sample material, ν is the Poisson’s ratio of the sample material, t_s is the sample thickness, t_f is the thickness of the modified layer/coating.

The normal curvature of the sample δ(x) (see Figure 1c) due to surface modification is determined after subtracting the original profile measured before the surface modification (curve 1) from the profile measured after the modification (curve 2).

The use of Formula (1) for evaluating stresses after droplet impact can be difficult due to problems in correct determination of the thickness t_f. Nevertheless, the change in the normal curvature value can be used to represent the change in stresses in the surface layer of the material during droplet impact during erosion tests.

As samples, substrates made of 20Kh13 blade steel sheet in the form of plates of size 6 × 20 × 1 mm were used. In the lower part of the sample (see Figure 2a) a fixing hole was made to fix it to the holder during erosion tests. The holder was installed and fixed on the rod (see Figure 2a) of erosion test rig “Erosion M”, Moscow Power Engineering Institute. The surface of the plates was subjected to electrolyte-plasma pre-polishing and then the original surface profiles were measured. The profiles were measured using surface profiler as shown in Figure 2b. The length of the profiles was 11 mm due to the fixing hole.

The original profiles were smoothed (see Figure 3) using surface profiler software by removing the roughness component and, in part, the waviness that could complicate the overall curvature determination.

After the preliminary surface profiles were determined, the substrate surface was modified. The following surface modifications were carried out on 20Kh13 steel:

-

ion nitriding (type I for 2.5 h, type II for 5 h and type III for 10 h);

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application of ion plasma coating based on Cr–CrC.

Ion nitriding of 20Kh13 steel samples was carried out using the pilot facility “Gefest+” [29], Moscow Power Engineering Institute. The process of ion nitriding of sample surfaces included several stages: preparation of sample surfaces; loading samples into a vacuum chamber; pumping vacuum chamber to high vacuum with preheating to 150 °C; ion surface cleaning; and modification of sample surfaces in nitrogen medium.

Cr–CrC based coating of 20Kh13 steel samples was carried out by ion plasma method also using “Gefest+” facility. Chromium target cathodes (99.9%) were used for this coating formation process. Process of coating formation included the following main stages: preliminary preparation of samples surfaces (removal of surface impurities and degreasing) and loading samples into the chamber; pumping chamber to high vacuum with preheating of samples; ionic cleaning and proper formation of coating on the surface of samples. The chromium sputtering was carried out in the reaction gas medium. High purity methane was used for the formation of sub-layers of chromium carbide (CrC). When pure chromium was applied (as adhesion sublayer and intermediate layers) no reaction gas was supplied.

Representative views of surface and near-surface layer of 20Kh13 steel with different types of modification before testing are shown in Figure 4.

After surface modification, the samples were subjected to erosion tests using the UNU “Erosion-M Hydro-impact test rig” research equipment, Moscow Power Engineering Institute. Test rig diagram is shown in Figure 5.

The hydro-impact rig is a rotary-type test facility, operating as follows: two test samples are attached to the ends of a rod rotating in a vacuum chamber and crossed by a vertical stream of liquid droplets emanating from a special droplet generator. For the experiments, the liquid supply system was upgraded to reduce the droplet size and bring the test conditions to the real pattern of droplet impact in the last stages of low-pressure cylinders of high-power steam turbines. The spinneret responsible for supplying droplets to the vacuum chamber was replaced by a special axial nozzle, which made it possible to achieve atomization of liquid in the form of a mist and to achieve uniform distribution of moisture in the plane of impact with the sample. Tests of experimental samples on the erosion test rig were conducted at specified impact velocities of samples (S_imp, m/s) with liquid droplets of known diameter (d_k, µm). Figure 6 shows the diagram of droplet impact with the surface of the tested samples implemented in the erosion test rig.

Several successive erosion tests at impact speed 250 m/s and average droplet diameter 150 μm were carried out to test the effect of droplet impact on change of stresses in a near-surface layer of a material of 20Kh13 steel samples with and without surface modification.

The surface profile was measured and the resulting curvature of the sample, which characterises the change in stresses, was evaluated after a total droplet impact time of 10, 195 and 600 min.

After the tests were completed, the surface and near-surface condition of the samples were evaluated and examined using electron microscope, and cross-sectional metallographic specimens were made.

3. Results and Discussion

As a result of several successive erosion tests during 10, 195 and 600 min at impact speed of 250 m/s and average droplet diameter of 150 µm, fractograms of surface and near-surface layer of 20Kh13 steel samples without surface modification have been obtained.

Representative view of destruction of surface and near-surface layer of 20Kh13 steel under impact for 600 min (d_k = 150 µm, and S_imp = 250 m/s) is shown in Figure 7.

After 10 and 195 min of droplet impact, no visible surface damage was observed on the surface of 20Kh13 steel samples. After 600 min of testing, an initial mass removal from the sample and formation of rather large erosive caverns on the surface (see Figure 7a) with sizes from 20 to 590 µm (average size—150 µm) were fixed. This indicated the end of incubation period of the wear process of 20Kh13 steel without surface modification.

The thickness of the near-surface layer in which the material microstructure changes (see Figure 7b) is 5 to 30 µm (20 µm on average), and the thickness of the layer in which the material fails (cavern depth) during erosion testing ranges from 15 to 110 µm.

To determine the values of average reduced normal curvature of 20Kh13 steel samples without surface modification caused by droplet impact, surface profiles were measured after 10, 195 and 600 min of erosion tests. The representative view of smoothed surface profiles of 20Kh13 steel samples after erosion tests of different duration after subtraction of preliminary profiles (pre-test profiles) is shown in Figure 8.

Average values of reduced normal curvature δ/l, shown in

Table 1. Results of determining the average reduced normal curvature of 20Kh13 steel samples without modification and stresses in the surface layer caused by droplet impact of various durations at S_imp = 250 m/s, d_k = 150 µm.

Droplet Impact Time t, min	(δ/l)·103	Stress Value σ, GPa
10	−0.109 ± 0.032	−0.20 ± 0.06
195	1.88 ± 0.39	3.40 ± 0.70
600	2.50 ± 0.50	4.60 ± 0.90

, were determined using the surface profiles.

The evaluation of change of internal stresses caused by droplet impact in 20Kh13 steel samples without surface modification was carried out by Formula (1) using determined values of average reduced normal curvature δ/l (see Figure 9a). While calculating stress using Formula (1), the Young modulus of sample material (20Kh13 steel) was assumed as E = 218 GPa, Poisson’s coefficient of sample material as ν = 0.27, sample thickness was t_s = 1 mm, thickness of layer modified due to droplet impact was t_f = 20 µm. Results of the stress evaluation are shown on Figure 9b.

Obtained results for 20Kh13 steel without modification show that droplet impact on the initial stage of the incubation period leads to the small values of stress of 0.20 ± 0.06 GPa, which sharply increase with increasing exposure time and then gradually reach the limit stress values of 4.6 ± 0.9 GPa, leading to the end of the incubation period after test time equal to 600 min.

As a result of similar successive erosion tests during 10, 195 and 600 min at impact speed of 250 m/s and average droplet diameter of 150 µm, fractograms of surface and near-surface layer of 20X13 steel samples with modifications (nitriding type I, type II, type III and Cr–CrC-based coating) modification have been obtained.

Representative views of destruction of surface and near-surface layer of 20Kh13 steel with different types of modification under impact for 600 min (d_k = 150 µm and S_imp = 250 m/s) are shown in Figure 10.

Results of visual analysis and microscopic examination of damages of surface and near-surface layer of samples with different types of modification during 600 min of tests are given in

Table 2. Results of visual analysis and microscopic examination of damages of surface and near-surface layer of samples with different types of modification during 600 min of tests.

Type of Modification	Depth of Modified Layer/Coating Thickness	Surface Defects (Caverns, Chips)	Thickness of Near-Surface Layer (Cavern Depth)
Nitriding type I	20–28 μm	2–90 μm	10–55 μm
Nitriding type II	55–58 μm	5–150 μm	5–50 μm
Nitriding type III	98–107 μm	7–200 μm	10–95 μm
Cr–CrC based coating	16–20 μm	3–300 μm	10–20 μm

.

On the surface of 20Kh13 steel samples with nitriding no visible surface damage was visually observed up to 600 min of droplet impact. After 600 min of testing on the surface of nitrided 20Kh13 steel samples (see Figure 10a,c,e) only a large number of individual small defects in form of caverns or cracks measuring from 2 to 7 μm and a small number of defects ranging in size from 100 to 200 μm were observed. Examination of cross-sectional metallographic specimens of nitrided 20Kh13 steel samples (see Figure 10b,d,f) showed that type II and type III nitrided samples fail within the thickness of the nitrided layer (cracks and chips in some areas of the layer up to 50 and 100 μm, respectively), while type I nitrided samples fail in some areas to the base material with cavity depths up to 55 μm.

During erosion tests of Cr–CrC-coated 20Kh13 steel samples, no visible surface damage was visually observed up to 600 min. After 600 min of test, only isolated defects in the form of delamination of top layers of Cr–CrC-coated 20Kh13 sample (see Figure 10g) up to 300 µm in size were observed, although the presence of 20 µm deep chips in cross-sectional metallographic specimens (see Figure 10h) reaching the base material was also noted.

To determine mean reduced normal curvature values of 20Kh13 steel samples with surface modification which are affected both by modification itself and by droplet impact, surface profiles were measured before and after modification as well as after 10, 195 and 600 min of erosion tests. Obtained values of average reduced normal curvature δ/l of nitrided and coated 20Kh13 steel samples before modification, after modification and subsequent erosion tests of 10, 195 and 600 min are given in

Table 3. Results of determining the average reduced normal curvature of 20Kh13 steel samples after surface modification and combined effect of surface modification and droplet impact for different durations at S_imp = 250 m/s, d_k = 150 μm.

Type of 20Kh13 Steel Surface Modification	tf, μm	(δ/l)·103
		Modification	Modification + Droplet Impact with Duration:
			10 min	195 min	600 min
nitriding (type I)	24	0.95 ± 0.24	0.81 ± 0.20	2.00 ± 0.40	2.30 ± 0.50
nitriding (type II)	56	2.90 ± 0.50	2.60 ± 0.04	3.10 ± 0.60	3.20 ± 0.60
nitriding (type III)	104	6.10 ± 1.00	4.80 ± 0.80	7.00 ± 0.90	7.20 ± 0.90
Cr–CrC based coating	18	0.80 ± 0.13	0.62 ± 0.15	1.46 ± 0.30	1.84 ± 0.30

.

Obtained values of average reduced normal curvature caused by combined action of nitriding and droplet impact as well as combined action of Cr–CrC coating and droplet impact are shown in Figure 11a. Stresses caused by the combined action of nitriding/coating and droplet impact are evaluated by Formula (1) using the curvature data. The thickness of the corresponding nitrided layer or coating was used as the t_f value. Results of stress evaluation are given in

Table 4. Results of determining stresses in the surface layer of 20Kh13 steel caused by surface modification and combined action of surface modification and droplet impact of different duration (at S_imp = 250 m/s, d_k = 150 μm).

Type of 20Kh13 Steel Surface Modification	tf, μm	Stress Value σ, GPa
		Modification	Modification + Droplet Impact with Duration:
			10 min	195 min	600 min
nitriding (type I)	24	1.37 ± 0.36	1.17 ± 0.30	2.90 ± 0.60	3.40 ± 0.70
nitriding (type II)	56	1.94 ± 0.33	1.72 ± 0.30	2.10 ± 0.40	2.10 ± 0.40
nitriding (type III)	104	2.12 ± 0.38	1.72 ± 0.32	2.44 ± 0.34	2.51 ± 0.33
Cr–CrC based coating	18	1.62 ± 0.30	1.24 ± 0.32	2.90 ± 0.60	3.70 ± 0.60

and Figure 11b.

At the initial stage of erosion tests, stresses in the near-surface layer slightly decrease: 1.1–1.2 times after 10 min of droplet impact for 20Kh13 steel with nitriding and 1.3 times for 20Kh13 steel with coating. At the stage from 10 to 195 min of testing, stress increases at the highest rate. The lowest stress of 2.1 GPa in the near-surface layer was observed in 20Kh13 samples with nitriding type II, and the highest stress of 3.4 GPa was observed in 20Kh13 samples without modification or coating. The stress increase from 195 to 600 min for nitrided samples is insignificant. For 20Kh13 samples without surface modification and 20Kh13 with coating in the stage from 195 min to 600 min the increase of stresses is significant but at a lower rate than in the stage from 10 to 195 min. The obtained high stress values in the near-surface layer at the droplet impact duration of 195 and 600 min correspond to the beginning of surface destruction accompanied by the development of caverns and cracks in the material with surface modification. The greater nature of the internal stresses in the uncoated material leads to the end of its incubation period. The observed pattern of stress changes in the surface layer of 20Kh13 steel samples under the combined action of surface modification and droplet impact of different durations can be used as a comparative criterion of erosion resistance, determined by internal stress arising from the impact of droplets against the hardened surface.

4. Conclusions

To examine 20Kh13 steel without surface modification, 20Kh13 steel with different types of nitriding and 20Kh13 steel with Cr–CrC-based coating, erosion tests at impact velocity of 250 m/s and average droplet diameter of 150 μm (nozzle spraying) during drop impact time of 10, 195 and 600 min were carried out. The values of average reduced normal curvature caused by both droplet impact and the combined effect of surface modification and droplet impact of various durations were determined. The internal stresses caused by the combined effect of surface modification and droplet impact were evaluated.

The results suggest that the initial droplet impact on uncoated, coated and nitrided samples leads to a sharp rise in internal stress followed by its slow growth during a period of surface damage accumulation, with no apparent effect on the stress state of the modified layer.