INFLUENCE OF WAVE STRAIN HARDENING ON CORROSION RESISTANCE OF WELDED JOINTS OF STRUCTURAL STEELS OF AGRICULTURAL MACHINERY

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The influence of wave strain hardening on the corrosion resistance of welded joints of structural steels of agricultural machinery и

RATIONALE

Corrosive destruction of structural steels is one of the main reasons for the reduction of service life and premature failure of critical components of agricultural machinery, such as frames, axles, suspension elements, working bodies of tillage implements (plows, harrows) and welded joints of combine bodies [1, 2]. Operation in aggressive environments (soil electrolytes, fertilizers, high humidity) leads to significant economic losses, estimated at 3-5% of the GDP of countries with a developed agro-industrial complex (AIC) [2]. Particularly vulnerable are areas of welded joints, where a heterogeneous microstructure is formed (large grain, residual stresses, carbide precipitation), reducing corrosion resistance by 30-50% compared to the base metal [3, 4]. In addition to the main traditional methods of corrosion protection (painting, galvanizing), the corrosion resistance is also affected by the manufacturing (hardening) technology of parts, in particular, the methods of surface plastic deformation (SPD). For example, shot blasting or knurling, used to harden the surface layer, demonstrate an ambiguous effect on corrosion behavior: moderate deformation can improve resistance by creating a nanocrystalline structure and compressive stresses, while excessive deformation provokes the formation of defects that activate corrosion [5, 6]. Existing studies often do not take into account the specifics of welded seams and rarely optimize the processing parameters for specific steel grades used in the agro-industrial complex (09G2S, 30KhGSA, 40Kh, 45, 10HSND) [7]. An analysis of literature data [5, 6, 8-10] revealed the lack of a systematic approach to the use of wave strain hardening (WSH) to improve the corrosion resistance of welded joints of agricultural structural steels. WSH is characterized by the transfer of deformation energy through a waveguide and the effect of prolonged pulses on the rolled material (Fig. 3), and has unique advantages: it forms a deep (up to 15 mm) hardened layer with high (-1000 MPa) compressive stresses and creates an ultrafine-grained surface structure [8, 9]. However, a comprehensive study of the relationship between WSH modes, microstructural changes (grain size, rotation angle, phase transformation) and corrosion resistance for a wide range of steels and their welds has not yet been conducted. Optimum ranges of the degree of deformation that guarantee improved corrosion characteristics without the risk of their deterioration when the critical level of strain hardening is exceeded have not been determined. This limits the implementation of promising VDU technology in the production and repair of agricultural machinery. Developing scientifically based recommendations on VDU parameters (coverage coefficient K, degree of deformation) for specific steel grades and their welded joints will significantly (by 20-40% according to preliminary experiments) increase corrosion resistance and, as a consequence, the durability of critical units of agricultural machinery operating under conditions of intensive corrosion and mechanical wear. This corresponds to the strategic objectives of import substitution and increasing the competitiveness of domestic agricultural machinery.

The aim of this study is to establish quantitative patterns of the influence of wave strain hardening parameters (overlap coefficient K=0.3 and K=0.6) on the corrosion resistance, microstructure and microhardness of widely used structural steels (09G2S, 30KhGSA, 40Kh, 45, 10HSND) and their welded joints, as well as to develop, on this basis, practical recommendations for optimizing WSH modes to improve the operational reliability of parts and units of agricultural machinery.

 

METHODS

Objects of the study. The work examined five grades of structural steels widely used in the production of critical units of agricultural machinery: low-alloy steel 09G2S (for combine and trailer frames), chromium-silicon steels 30KhGSA and 40Kh (gear transmissions, axles), carbon steel 45 (fasteners), and low-carbon steel 10KhSND (welded structures of plows). The chemical composition of the materials corresponded to the manufacturers' certificates: 09G2S (0.09-0.15% C, 1.3-1.7% Mn); 30KhGSA (0.28-0.34% C, 0.8-1.1% Cr); 40Kh (0.36-0.44% C, 0.8-1.1% Cr); 45 (0.42-0.50% C); 10ХСНД (0.10% C, 0.8% Cr, 0.5% Ni) [11].

Sample preparation. Rectangular plates measuring 75×80 mm were cut from 10 mm thick sheet metal. To study the welded joints, the samples were butt-welded in pairs using semiautomatic argon welding (modes: current 180-220 A, voltage 22-28 V, wire feed speed 15 cm/min). The quality of the welds was controlled visually and by ultrasonic flaw detection (deviations did not exceed 5% of GOST 3242-79). The surface of all samples was ground to a roughness of Ra=0.8 μm, followed by degreasing in acetone [12].

Wave strain hardening technique. Hardening was carried out on an experimental setup using a mechanical pulse generator (RU patent No. 2098259) with a specific impact energy of 4-8 J/mm and a pulse frequency of 9-14 Hz, using a rod roller with a diameter of 10 mm. The overlap coefficient (K) is a complex technological parameter of the VDU that determines the multiplicity of dynamic loading of the deformation zone and, consequently, the uniformity and degree of hardening of the surface layer. K characterizes the degree of overlap of plastic imprints from successive impacts and is calculated using formula (1):

,                                                                  (1)

where δ is the imprint size, mm; s is the feed rate of the workpiece relative to the tool, mm/min; f is the impact frequency, Hz. [8].

At VDU with K=0, the tool impact imprints are adjacent to each other without overlapping; at 0 < K < 1, they partially overlap; at K=1, the tool impacts are applied to one point without its displacement. In this study, hardening with K=0.3 and K=0.6 was used, corresponding to the modes of partial overlap of imprints. The choice of these values ​​is due to their potential for the formation of an optimal heterogeneous structure (combining high hardness and toughness), proven in previous studies [8, 9] as effective for improving performance characteristics (resistance to contact chipping, cyclic strength under alternating loads). For each combination of "steel grade/sample type/K", 5 identical samples were processed.

Corrosion tests. Corrosion resistance was assessed by weight loss after exposure to salt fog (5% NaCl solution, t=25±2°C, duration 240 h) in accordance with GOST 9.908-85. Samples were weighed before and after testing on a CE224-C analytical balance. Weight losses were recalculated in g/mm³ taking into account the surface area.

Property analysis methods. The microstructure was examined using a Zeiss Axio Observer.D1m optical microscope at magnifications of 100-1000× using Thixomet Pro software for image analysis. The following were determined: grain size using the chord method (GOST 5639-82), hardened layer thickness, crystal rotation angles, and phase composition. For welded joints, changes in the orientation of columnar crystals in the heat-affected zone were additionally analyzed.

Statistical processing. Statistical data processing was performed in the STATISTICA program. Differences in corrosion losses, microhardness and grain size between sample groups (non-hardened, hardened with K=0.3, hardened with K=0.6) were assessed using one-way analysis of variance (ANOVA). Data are presented as arithmetic mean and standard deviation (M±SD). Differences were considered statistically significant at p<0.05.

RESULTS

Experimental studies have revealed a complex dependence of the corrosion resistance of structural steels on the modes of wave strain hardening. For low-alloy steel 09G2S in the base metal, treatment with an overlap coefficient of K = 0.6 provided a statistically significant reduction in corrosion losses by 2.7% (from 7.50 ± 0.12 to 7.30 ± 0.11 ×10-5 g/mm3; p < 0.05), which was accompanied by a decrease in the grain size from 6.9 × 6.9 μm to 6.1 × 5.3 μm and their rotation by 50°. In welded joints of the same steel, the maximum effect was achieved at K=0.3: losses decreased by 28% (to 4.80 ± 0.09 ×10-5 g/mm3), but increasing the coefficient to K=0.6 led to an abnormal increase in losses to 5.90 ± 0.14 ×10-5 g/mm3. Microstructural analysis showed that this effect correlates with the fragmentation of columnar crystals in the weld – their length decreased from 200 to 110 µm.

The highest sensitivity to VDU ​​in the base metal was recorded for chromium-silicon steel 30KhGSA. Treatment with K=0.6 reduced corrosion losses by 33% (to 4.50 ± 0.08 ×10-5 g/mm3; p<0.01) with a decrease in grain size from 8.9×9.3 μm to 7.6×7.3 μm. In contrast to the base metal, 30KhGSA welds demonstrated degradation of properties: already at K=0.3, losses increased by 11% (to 5.10 ± 0.13 ×10-5 g/mm3), and at K=0.6 they reached 5.30 ± 0.16 ×10-5 g/mm3. Micrographs revealed the formation of coarse bainitic packets and an increase in the angle of inclination of crystals relative to the weld axis to 49°. Photos of the microstructure of the hardened VDU with K=0.3 weld seam and without hardening are shown in Figures 1 and 2.

For 40X steel, a multidirectional reaction was revealed. In the base metal, VDU with K=0.3 increased losses by 46% (up to 7.00 ± 0.22 ×10-5 g/mm3), while in welded joints the same mode ensured a decrease of 21% (up to 6.20 ± 0.17 ×10-5 g/mm3). The optimal result for welded joints was achieved at K=0.6: losses decreased to 4.50 ± 0.12 ×10-5 g/mm3 (–42%; p<0.001), and the depth of the hardened layer was 6.3 mm. The weld microstructure after treatment was characterized by the predominance of acicular bainite and grain rotation by 45–50°.

The most negative dynamics was observed in carbon steel 45. In the base metal, VDU with K=0.6 increased corrosion losses by 26% (up to 3.40 ± 0.10 × 10-5 g/mm3) despite the increase in the degree of hardening to 9.5%. In welded joints, degradation intensified: at K=0.6, losses reached 7.00 ± 0.23 × 10-5 g/mm3 (+35%; p<0.05). Microstructural analysis revealed a unique anomaly – an increase in grain length from 6.4 to 6.6 μm (+3%) with a decrease in width to 5.5 μm (–17%), which was accompanied by the formation of microdefects in the ferrite-pearlite structure.

Steel 10HSND demonstrated high stability. In welded joints, the K=0.3 mode reduced losses by 38% (to 4.30 ± 0.08 × 10-5 g/mm3; p<0.01), and in the base metal, treatment with K=0.6 resulted in a 22% reduction (to 6.30 ± 0.15 × 10-5 g/mm3). It is important to note that the microstructure retained a fine-grained ferrite-carbide matrix without phase transformations.

Summarizing the microstructural changes, VDU caused a grain rotation of 37–50° in all steels. A decrease in grain size was recorded for 09G2S (–12% in length), 30KhGSA (–15%), 40Kh (–7%) and 10HSND (–8%), while an atypical increase in grain length by 3% was observed in steel 45. In welds, the key effect was a decrease in the length of columnar crystals by 45–50% (09G2S, 40Kh) and a reorientation of bainite (30KhGSA). Full quantitative data are presented in Table 1.

Practical implications for agricultural machinery follow from the established patterns:

1. For the working parts of plows and harrows (steel 10HSND), the optimal mode is K=0.3 (deformation degree ≤45%), reducing losses by 38% without the risk of degradation of the ferrite carbide matrix.
2. It is advisable to process axles and shafts made of steel 30KhGSA with K=0.6, but their welded joints should not be subjected to VDU ​​due to an increase in losses by 16%.
3. Carbon steels (45) are unsuitable for VDU in critical units. For fasteners, hardening is only permissible up to 30%.

CONCLUSION

It has been established that wave strain hardening with overlap coefficients K=0.3–0.6 increases the corrosion resistance of alloy steels (30KhGSA, 40Kh, 10KhSND) and their welded joints, reducing mass losses by 21–42% due to the formation of a deep hardened layer, reducing grain size by 7–23% and creating compressive stresses, however, for carbon steel 45, the use of VDU is limited due to an increase in corrosion losses by 26–56%.

ADDITIONAL INFORMATION

Funding source. The research was carried out with the support of a grant from the Russian Science Foundation № 24-29-00666, https://rscf.ru/project/24-29-00666/.

Competing interests. The authors declare that they have no competing interests.

 

Author contributions: S.V. Barinov: literature review, development of experimental methods, literature review, study of microstructure and microhardness, data processing, writing the text of the manuscript; N.A. Grigorieva: conducting corrosion tests, processing and analysis of experimental data, writing the text of the manuscript; D.M. Shestopalov: conducting corrosion tests, processing and analyzing experimental data, writing the text of the manuscript. All the authors approved the version of the manuscript to be published and agreed to be accountable for all aspects of the work, ensuring that issues related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

 

TABLE 1. SUMMARY OF EXPERIMENTAL DATA

Brand	Sample	Parameter	Material without hardening	K=0,3	K=0,6
09G2S	Base metal	Losses, g/mm3 (×105)	7,50 ± 0,12	7,40 ± 0,15	7,30 ± 0,11
	Welded seam	Losses, g/mm3 (×105)	6,70 ± 0,18	4,80 ± 0,09	5,90 ± 0,14
30KhGSA	Base metal	Losses, g/mm3 (×105)	6,70 ± 0,21	4,70 ± 0,07	4,50 ± 0,08
	Welded seam	Losses, g/mm3 (×105)	4,60 ± 0,11	5,10 ± 0,13	5,30 ± 0,16
40X	Base metal	Losses, g/mm3 (×105)	4,80 ± 0,15	7,00 ± 0,22	6,50 ± 0,19
	Welded seam	Losses, g/mm3 (×105)	7,80 ± 0,25	6,20 ± 0,17	4,50 ± 0,12
45	Base metal	Losses, g/mm3 (×105)	2,70 ± 0,08	4,20 ± 0,14	3,40 ± 0,10
	Welded seam	Losses, g/mm3 (×105)	5,20 ± 0,16	5,30 ± 0,18	7,00 ± 0,23
10HSND	Base metal	Losses, g/mm3 (×105)	8,10 ± 0,24	7,10 ± 0,20	6,30 ± 0,15
	Welded seam	Losses, g/mm3 (×105)	6,90 ± 0,19	4,30 ± 0,08	5,20 ± 0,13

 

DRAWINGS

Fig. 1 – Microstructure of the weld in a sample made of 30KhGSA steel: a) non-hardened sample; b) VDU with K=0.3 (×200)

  

Fig. 2 – Macrostructure of a weld in a sample of 30KhGSA steel with WSH with K=0.3 (×10)

About the authors

Sergey Barinov

Federal State Budgetary Educational Institution of Higher Education "Vladimir State University named after Alexander Grigorievich and Nikolai Grigorievich Stoletov"

Author for correspondence.
Email: box64@rambler.ru
ORCID iD: 0000-0002-1341-446X
Scopus Author ID: https://www.scopus.com/authid/detail.uri?authorId=57147171600

Доцент, доцент кафедры "Технология машиностроения"

Russian Federation

Natalia Alexandrovna Grigorieva

Email: natali-kukanova@mail.ru
ORCID iD: 0009-0000-2096-5449
Russian Federation

Danila Mikhailovich Shestopalov

Email: shestopalov.danila@yandex.ru
ORCID iD: 0009-0000-7596-2261
Russian Federation

References

Supplementary files