Hybrid Thermo-Vibrational Welding with Active Cooling for Preheat-Free Joining of Martensitic 15Kh5M Steel: Microstructural Refinement and Heat-Affected Zone Control

Abstract

Martensitic chromium-molybdenum steels such as 15Kh5M are widely used in high-temperature oil and gas equipment, but their weldability is limited by high hardenability and susceptibility to cold cracking, which usually necessitate energy-intensive preheating. This study evaluates an alternative route based on the combination of root-pass mechanical vibration (50 Hz, ~1 mm amplitude) and post-pass water-air jet cooling during mechanized GMAW. Three welding variants were compared: conventional preheated welding, vibration-assisted welding without preheating, and hybrid thermo-vibrational welding with active cooling. Among the tested conditions, the hybrid route produced the narrowest heat-affected zone, reducing its width from about 7 mm to about 3 mm, which is consistent with a compressed thermal cycle. Microhardness in the heat-affected zone decreased from 380 to 440 HV in the preheated condition to 330–370 HV in the hybrid condition. Optical microscopy further indicated a finer and more homogeneous transformed microstructure in the hybrid case. Results indicate that simultaneous vibro-treatment and controlled cooling effectively mitigate harmful metallurgical effects typically induced by rapid cooling, enabling preheat-free fabrication of thick-walled components. The proposed hybrid approach may offer energy savings, shorter production cycles, and improved automation compatibility in field welding applications.

1. Introduction

Martensitic chromium steels such as 15Kh5M are widely used in the fabrication of high-temperature equipment for oil refining, petrochemical processing, and furnace tubing due to their favorable combination of thermal stability, creep resistance, and relatively low coefficient of thermal expansion [1,2,3,4]. These materials, however, present significant challenges during welding, primarily because of their high hardenability and tendency toward martensitic phase transformations under rapid cooling conditions typical of welding processes. The formation of brittle, untempered martensite in the heat-affected zone (HAZ) leads to elevated hardness, residual stresses, and an increased risk of cold cracking—especially in the presence of diffusible hydrogen [5,6,7,8].

To mitigate these issues, conventional welding procedures typically require preheating to temperatures between 300 °C and 400 °C, followed by immediate post-weld heat treatment (PWHT), usually in the form of high-temperature tempering [9,10,11,12]. While effective in reducing thermal gradients and allowing hydrogen diffusion, this approach is energy-intensive, slows down production rates, and complicates field welding operations, particularly for large-diameter pipelines or thick-walled components [10,11,12,13]. Moreover, even with preheating, coarse martensitic structures can still develop along the fusion line, where hardness values often exceed 400 HV, making these zones prone to delayed fracture [6,7,8,14,15].

Alternative strategies have been explored to improve weldability without relying on extensive thermal interventions. Among them, mechanical vibration applied during welding has shown promise in modifying solidification behavior, refining grain structure, and relieving internal stresses through dynamic thermo-mechanical effects [16,17,18,19]. Vibro-treatment at frequencies around 50 Hz has been reported to reduce residual stresses of the second and third kind in welded joints, thereby improving crack resistance prior to final heat treatment [18,19,20]. However, most studies focus only on vibration as a standalone technique, and its integration into mechanized gas metal arc welding (GMAW) systems remains limited, especially for thick-section Cr-Mo steels.

Another emerging approach involves active control of the thermal cycle through forced cooling immediately after or during welding. By accelerating cooling below the martensite start temperature, it becomes possible to suppress excessive grain growth and limit the extent of the HAZ [21,22,23,24]. When properly controlled, such cooling regimes can lead to finer, more uniform microstructures—sometimes shifting transformation products toward lower-bainite or mixed bainitic-martensitic morphologies—which exhibit better toughness and lower hardness than fully martensitic zones. Yet, uncontrolled or overly intense cooling may increase quenching severity and actually promote cracking, so precise regulation is essential.

The idea of combining vibrational action with modified thermal cycles is not entirely new, but published work rarely addresses the synergistic effect of simultaneous vibro-treatment and localized forced cooling during GMAW of medium-alloy martensitic steels like 15Kh5M. Most existing technologies remain either purely thermal (preheating + PWHT) or mechanically assisted without real-time thermal management. There is also little data on how such hybrid treatments affect the width and hardness distribution across the HAZ, particularly when applied selectively, such as only to the root pass, where constraint and stress concentration are highest.

This study aims to evaluate the feasibility of a novel thermo-vibrational welding process combined with controlled post-pass cooling for producing high-integrity welds in 15Kh5M steel without preheating. The main innovation lies in the integration of two complementary techniques—mechanical vibration (50 Hz, ~1 mm amplitude) applied during root pass deposition, and targeted water-air jet cooling—into a single automated GMAW setup. This approach seeks to compress the effective thermal cycle, refine the microstructure, reduce HAZ width, and lower microhardness levels below the critical threshold associated with cold cracking susceptibility. Importantly, this specific combination of parameters and application strategy has not been previously documented in open literature, especially in the context of industrial-scale tube welding for oil and gas applications.

The work focuses on comparing three welding conditions: conventional preheated welding, vibration-assisted welding without preheating, and hybrid thermo-vibrational welding with active cooling. Microstructural analysis, hardness mapping, and macroscopic assessment were used to determine the effectiveness of each method in avoiding harmful martensitic bands and minimizing residual stress concentrations. If successful, this technique could offer a practical pathway toward preheat-free fabrication of martensitic steel components—reducing energy consumption, increasing productivity, and simplifying automation in harsh-service environments.

2. Materials and Methods

The study was focused on evaluating the influence of thermo-vibrational treatment combined with controlled cooling during mechanized gas metal arc welding (GMAW) of martensitic chromium-molybdenum steel 15Kh5M, which is commonly used in high-temperature oil and gas equipment. The main objective was to assess whether preheating could be eliminated without increasing the risk of cold cracking by applying vibration during root pass deposition and using active cooling to modify the thermal cycle.

2.1. Material Preparation

Experiments were carried out on tubular specimens with dimensions Ø152 × 8 mm made of 15Kh5M steel (analogous to 10Kh2M1, 12Kh9M). The tubes were made of 15Kh5M steel supplied by OOO “Pervaya Stal Urala” (Yekaterinburg, Russia), corresponding to GOST 20072–74 [25]. The nominal chemical composition of the base metal according to the material specification/supplier certificate is given in

Table 1. Chemical composition of 15Kh5M steel (in wt.%).

Element	C	Si	Mn	Cr	Mo	Ni	S	P
Content	≤0.15	≤0.50	≤0.60	4.5–6.0	0.45–0.60	≤0.30	≤0.025	≤0.030

Note: Balance is Fe; trace elements (W, V, Ti, Cu) are present in amounts below 0.05 wt.% and do not significantly affect the weldability or mechanical behavior.

(wt.%). For welding, a solid wire electrode DT-SG CrMo5 (diameter 1.2 mm, AWS A5.28/ER50-B2, equivalent to Sv-10Kh5M) made in Germany was used. The nominal composition of the filler metal is also given in Table 1. The shielding gas mixture consisted of 80% Ar + 20% CO₂.

2.2. Welding Procedures and Parameters

All welding operations were performed using semi-automatic GMAW in the flat position (1G), summarized in

Table 2. Welding parameters and conditions for experimental variants.

Bevel Prep and Layer Sequence	Tube D × s, mm	Welding Condition (Sample No.)	Layer	Current, A	Voltage, V
	Ø152 × 8	Preheat 300–350 °C	1	205–210	23–24
			2–3	215–220	25–26
	Ø152 × 8	No preheat, with vibro-treatment	1	200–205	24–25
			2–3	220–240	25–26
	Ø152 × 8	No preheat, vibro + cooling	1	205–210	20–22
			2–3	220–230	24–26

. The experimental setup was assembled at Ufa State Petroleum Technological University (Ufa, Russia) using a standard industrial welding power source. The parameters listed in Table 2 were selected on the basis of preliminary technological trials aimed at ensuring stable arc operation, acceptable bead formation, sufficient root penetration, and the absence of visible welding defects in each processing route. For comparability, the welding parameters were kept within the typical operating range for mechanized GMAW of Cr-Mo steels, while the root-pass current and voltage in the hybrid cooling condition were adjusted to limit heat input under preheat-free conditions.

Welding speed was maintained manually within a narrow range (~10–12 cm/min), monitored visually and recorded per run. Interpass temperature was controlled using infrared thermometry, especially in the hybrid cooling case, to ensure it remained below 150 °C before depositing subsequent layers. Figure 1 shows the welding process.

2.3. Vibro-Treatment Setup

Mechanical vibration was applied only during the root pass using a pneumatic vibratory device model VSh-10 (OOO “Stroykomplekt”, Yaroslavl, Russia) mounted directly onto the torch holder. The system generated harmonic oscillations with a frequency of 50 Hz and an amplitude of up to 1 mm, aligned transversely to the weld axis. Vibration parameters were continuously monitored using an Emerson CSI 1900 (Emerson Electric Co., St. Louis, MO, USA) vibration analyzer to confirm stability throughout the pass.

This method aimed to induce dynamic strain in the molten pool and early solidification zone, promoting dislocation movement and stress relaxation in the forming microstructure. Earlier studies suggest such treatment can reduce residual stresses of the second and third kind [26], though its integration into automated setups remains rare.

2.4. Forced Cooling Technique

In the third variant, immediate post-pass cooling was implemented using a custom-designed nozzle that delivered a fine water-air mist directly behind the arc at a distance of ~20–30 mm. Key parameters included:

• Nozzle slit length: 8–10 mm
• Air pressure: 0.25–0.30 MPa
• Water flow rate: 6–10 L/min
• Cooling zone width: 10–12 mm

The spray covered both the weld bead and the adjacent HAZ symmetrically. Cooling duration lasted approximately 30–40 s per pass until the surface temperature dropped below 100 °C, as measured by contact thermocouples placed near the fusion line.

The goal was not full quenching, but rather controlled acceleration of cooling through the critical martensite formation range (Ms ≈ 350–400 °C), thereby shortening the time available for coarse martensite development and reducing overall HAZ width.

2.5. Post-Weld Evaluation

After welding, all samples were allowed to cool naturally to room temperature before any inspection. Visual and macroscopic examination revealed no surface defects such as cracks, porosity, or undercut in any of the joints.

Cross-sectional metallographic specimens were cut from the mid-length of each tube weld. Samples were mounted, ground, polished, and etched with 3% nitric acid (nital) to reveal macro- and microstructures. Optical microscopy was performed using an Axio Scope.A1 (Carl Zeiss Microscopy, Jena, Germany) to analyze the weld metal, fusion line, heat-affected zone, and base metal.

Microhardness measurements were carried out using a PMT-3M (LOIP, Saint Petersburg, Russia) microhardness tester under a 100 g load on prepared cross-sections along the joint centerline. Indentations were spaced at 0.5 mm intervals and placed on the polished cross-sectional plane away from free edges to minimize edge effects.

While post-weld heat treatment (PWHT) was not performed in this phase of research—since the focus was on immediate post-solidification integrity—the results are considered relevant for assessing initial crack susceptibility prior to tempering.

2.6. Residual Stress Measurement

After microstructural and hardness evaluation, residual stress analysis was carried out on the surface of the weld joints using an X-ray diffractometer Uran-100 (AMTERTEK, Moscow, Russia) equipped with a Cr-Kα radiation source (λ = 2.2909 Å). Measurements were performed via the sin²ψ method at multiple points across the weld centerline, heat-affected zone, and base metal. The {211} lattice planes of α-iron were used for stress calculation, with tilt angles ranging from 0° to 45° in 5° increments. A step-scanning mode with 0.02° intervals and 5 s counting time per point ensured sufficient resolution. Surface preparation included gentle polishing without plastic deformation to preserve stress state integrity.

Three measurement zones were selected per sample: weld metal (WM), fusion line (FL), and base metal (BM). Reported values represent average stress levels with standard deviation based on three independent measurements.

3. Results and Discussion

The results obtained under the three welding conditions are presented below in terms of macrostructure, microstructure, hardness distribution, and residual stress.

3.1. Macrostructural Observations

Visual inspection and macroetching of cross-sections revealed no surface or subsurface defects such as cracks, lack of fusion, or porosity across all three welding conditions. However, significant differences were observed in the width and morphology of the heat-affected zone (HAZ), particularly near the fusion line. Figure 2 shows cross-sections of welded joints.

As shown in Figure 2, the HAZ appears widest in the sample welded with preheating (Figure 2a), where it extends up to about 7.0 ± 0.30 mm into the base metal. In contrast, the HAZ is visibly narrower in the vibration-only sample (Figure 2b, 6 ± 0.30 mm), and most notably reduced in the hybrid variant with vibro-treatment and forced cooling (Figure 2c), where its width decreases to about 3 ± 0.20 mm. Similar trends in HAZ geometry have been reported for other high-strength ferritic steels under uncontrolled thermal cycles, such as thermite-welded rail joints, where excessive heat input leads to pronounced grain coarsening and extended brittle zones [27]. This reduction—by a factor of 2.3 to 2.5—confirms that active thermal management compresses the effective thermal cycle, limiting the region subjected to critical heating and rapid cooling responsible for martensitic transformation.

This narrowing of the HAZ is technologically important because it reduces the volume of metal prone to high hardness and residual stresses, which are major contributors to hydrogen-assisted cracking. Moreover, the sharp boundary between weld metal and base metal becomes less pronounced in the cooled sample, suggesting smoother thermal gradients and potentially lower stress concentrations at the fusion line.

3.2. Microstructural Evolution

Microstructural analysis (Figure 3, Figure 4 and Figure 5) further supports these observations and reveals how different welding strategies influence phase transformations in the HAZ and weld metal.

In the preheated sample (Figure 3), both the weld metal and the heat-affected zone exhibit a comparatively coarse acicular transformed microstructure throughout the near-fusion region. The HAZ appears fully transformed over a wider band, consistent with a prolonged thermal cycle and extended exposure above the upper critical temperature. Such conditions are known to promote grain coarsening and the formation of hard, brittle transformation products in Cr–Mo and related martensitic steels, which in turn can degrade toughness and increase susceptibility to cracking under service loading [6,7,8,9,28]. These observations are in line with reports for high-strength pipeline and power-plant steels, where extended high-temperature exposure in the HAZ leads to coarser transformed morphologies and elevated hardness [19,20,21,22,23,24,29].

For the vibration-assisted sample without preheating (Figure 4), the optical micrographs show a certain degree of refinement in the transformed morphology within the HAZ compared with the preheated condition. Locally, somewhat finer lath-like features can be distinguished, suggesting that mechanical vibration during solidification may facilitate dislocation rearrangement and partial stress relaxation in the evolving microstructure. Nevertheless, the overall transformed region remains relatively wide and structurally coarse, and the microhardness values in the HAZ stay at a similar level to those of the preheated joint. This indicates that vibration alone does not sufficiently modify the thermal cycle or suppress the formation of hard transformation products when cooling proceeds without additional control. Similar conclusions about the limited microstructural effect of vibration in the absence of complementary thermal management have been drawn in studies on vibration-assisted welding of structural and pipeline steels [16,17,18,19,26].

The most pronounced microstructural change is observed in the hybrid thermo-vibrational condition with active cooling (Figure 5). In this case, the HAZ adjacent to the fusion line appears noticeably narrower and exhibits a finer, more homogeneous transformed morphology compared with the other two conditions. The near-fusion region contains smaller transformed packets and a less sharply defined boundary between weld metal and base metal in optical observation. This refinement is consistent with the measured reduction in HAZ width (from about 7 mm to about 3 mm) and the lower HAZ microhardness (330–370 HV) obtained in the same condition. Comparable trends—namely, a decrease in transformed-zone width and a reduction in hardness with increasing cooling rate or more carefully shaped thermal cycles—have been reported for martensitic-bainitic class steels and ferritic stainless steels, where controlled cooling can promote finer transformed microstructures and improved toughness levels [20,21,22,23,24].

It should be emphasized that the present microstructural assessment is based solely on optical microscopy. Although the observed refinement and homogenization in the hybrid condition are consistent with a less severe local hardening response, the exact phase balance and crystallographic features of the transformed zones cannot be resolved without phase-sensitive techniques such as X-ray diffraction or EBSD. In particular, any references to specific constituents (e.g., bainitic or martensitic subtypes) remain qualitative and are intentionally kept conservative in this work. A more detailed phase and orientation analysis therefore represents an important direction for future studies.

The microstructural differences between the three conditions should also be considered together with the cooling strategy. In the hybrid route, the water-air mist applied immediately after each pass shortens the effective time spent in the high-temperature range where extensive grain coarsening occurs, while the controlled intensity of cooling (via air pressure and water flow rate) avoids excessive quenching severity. Similar approaches of tailoring the cooling path have been shown to refine transformed morphologies and moderate hardness in Cr-containing steels and cast irons by adjusting cooling rates through the martensitic transformation region [20,21,22,23]. In the present work, the combination of vibration and active cooling thus acts primarily by compressing and shaping the thermal cycle, which manifests as a narrower transformed zone and a finer optical microstructure in the HAZ of the hybrid sample.

3.3. Microhardness Distribution

Hardness mapping provides quantitative confirmation of the microstructural trends described above. As illustrated in Figure 6, the highest hardness values are observed in the heat-affected zone of the conventionally preheated joint, reaching 380–440 HV. Although preheating reduces thermal shock, it does not prevent the formation of hard transformation products in the HAZ; instead, it can lead to uneven tempering if post-weld heat treatment is delayed. The elevated hardness in this region indicates persistent sensitivity to cracking, even under traditional procedures.

In the vibration-assisted condition without preheating, the hardness levels in the HAZ remain similarly elevated (380–430 HV), despite the slight structural refinement noted in Section 3.2. This suggests that while vibro-treatment helps to relax localized stresses, it has only a limited influence on the hardness response when the thermal cycle is not additionally controlled [30].

The hybrid thermo-vibrational route with active cooling yields the lowest hardness values in the HAZ (330–370 HV) among the tested conditions. This reduction is consistent with the narrower transformed zone and the finer, more homogeneous optical microstructure observed in the corresponding micrographs, and it indicates a less severe local hardening response.

Interestingly, the weld metal itself also shows slightly lower hardness (around 340–360 HV) in the hybrid condition compared with the other variants, which may be attributed to faster solidification and a finer dendritic/transformed morphology associated with the compressed thermal cycle.

3.4. Residual Stress Distribution

The integration of vibro-treatment and active cooling not only influenced microstructure and hardness but also significantly altered the residual stress state in the weld joint. In the conventionally preheated condition, tensile residual stresses dominate near the fusion line, reaching about +240 ± 20 MPa. Such tensile stresses arise from non-uniform thermal contraction during cooling and are known to promote crack initiation, particularly in hardened heat-affected zones of Cr–Mo and high-strength ferritic steels [31,32,33,34,35].

In the vibration-assisted condition without preheating, the tensile stresses at the fusion line are reduced to +175 ± 22 MPa, corresponding to a decrease of approximately 27% relative to the preheated joint. This trend is consistent with the general notion that mechanical vibration during solidification can facilitate dislocation rearrangement and partial plastic flow, thereby mitigating residual tensile-stress accumulation, even though the overall hardness response and transformed morphology remain similar to the preheated condition.

The most pronounced change is observed in the hybrid thermo-vibrational condition with active cooling, where the residual stresses at the fusion boundary shift from tensile to compressive, reaching –38 ± 15 MPa. This compressive state reflects the combined effect of a modified thermal cycle, accelerated cooling in the near-surface region, and microstructural refinement. For martensitic steels, the volumetric expansion accompanying the austenite-to-martensite transformation may additionally contribute to compressive stress generation in the near-weld zone, partially counteracting the typical tensile contraction stresses. Away from the fusion line, the HAZ regions in the hybrid condition tend to exhibit low-magnitude compressive or near-neutral stresses, which is consistent with the narrowed thermal cycle inferred from macrostructural measurements and the reduced HAZ width discussed in Section 3.1.

To place these findings in a broader context,

Table 3. Representative residual stress values reported for welded joints of alloyed steels after different treatments.

Treatment	Material/Steel Grade	Residual Stress, MPa (Fusion Line/HAZ)	Method	Source
Conventional GMAW with preheating (300–350 °C)	15Kh5M martensitic Cr-Mo steel	+240 ± 20 (tensile, fusion line)	XRD (sin2ψ), Uran-100	This work
Vibration-assisted GMAW, no preheating (50 Hz, 1 mm)	15Kh5M martensitic Cr-Mo steel	+175 ± 22 (tensile, fusion line; ≈27% reduction)	XRD (sin2ψ), Uran-100	This work
Hybrid thermo-vibrational GMAW with active water-air cooling	15Kh5M martensitic Cr-Mo steel	–38 ± 15 (compressive, fusion line)	XRD (sin2ψ), Uran-100	This work
Conventional arc welding (as-welded, no PWHT)	P91 (9Cr-1Mo) steel pipe girth weld	+320 to +450 (tensile peak at HAZ/BM boundary); compressive in weld metal (final pass)	Neutron diffraction	Ref. [36]
Arc welding, as-welded state (TIG/SMAW)	P92 (9Cr-2W) steel weldment	+380 to +600 (longitudinal tensile in fusion zone/HAZ); near-zero or compressive in weld metal	Neutron diffraction/hole-drilling	Ref. [37]
Arc welding, as-welded (GMAW/MIG)	X70 pipeline steel (high-strength ferritic)	+241 ± 17 (transverse tensile, HAZ); +260–300 (fusion line)	XRD (sin2ψ)	Ref. [31]
Subresonant vibratory weld conditioning (VSR, 39.4 Hz) applied post-weld	304 stainless steel plate weld	262 → 206 MPa (reduction ≈ 21%)	XRD	Ref. [38]
Local forced cooling of joint zone (water cooling behind arc)	Structural steel plate (arc welding)	28–30% reduction in tensile RS in weld zone	XRD/strain gauges	Ref. [34]

summarizes representative residual-stress levels reported for welded joints of alloyed steels under various treatment conditions, including conventional arc welding, post-weld vibratory conditioning, and local thermal/cooling treatments. Despite differences in material grade, joint geometry and processing details, the tensile and compressive stress ranges obtained in the present study fall within the general intervals reported for treated and untreated welded joints of comparable steels. The table is therefore intended only to indicate that the magnitude and sign of the residual stresses achieved by the thermo-vibrational route are consistent with typical values observed for alloyed-steel welds, rather than to establish a direct quantitative equivalence between different processes.

3.5. Synergistic Effects and Industrial Implications

The key innovation of this work lies not in using vibration or cooling individually, but in their synergistic integration during mechanized GMAW. This approach aligns with a broader trend in materials processing toward dynamic external energy inputs—such as high-density current pulses, which have been shown to enhance plastic flow and reduce yield stress in copper under stepwise loading through electroplastic effects [39]. While the physical mechanisms differ (mechanical vs. electromagnetic stimulation), both strategies aim to modify dislocation dynamics and internal stress state during deformation or solidification. In this context, vibratory weld conditioning can be viewed as a mechanical analogue to such field-assisted treatments, promoting microstructural refinement and stress relaxation through controlled dynamic strain. Neither technique alone eliminates the need for preheating effectively, but together they produce a “compressed” thermomechanical cycle that limits the extent of the heat-affected zone, refines the transformed microstructure, reduces peak hardness, suppresses coarse transformed regions, and lowers residual stress accumulation.

This combination has not been widely reported for industrial-scale tube welding of 15Kh5M, especially without subsequent immediate PWHT. From an operational standpoint, reducing or eliminating preheating offers substantial potential benefits: lower energy consumption, shorter cycle times, improved working conditions, and easier automation—particularly relevant for field welding in oil and gas infrastructure. Furthermore, the use of a simple pneumatic vibrator (VSh-10) and a custom water-air nozzle makes the setup adaptable to existing semi-automatic welding rigs without major retooling.

One limitation worth noting is that the current trials did not include full mechanical testing (e.g., bend tests, impact toughness, or hydrogen-induced cracking tests). Therefore, the present conclusions should be interpreted as metallurgical in nature. Nevertheless, the combination of microstructural refinement, reduced HAZ hardness, and a more favorable residual stress state indicates a promising direction for enhancing joint performance in service environments where susceptibility to cold cracking is a concern.

4. Conclusions

The combination of mechanical vibration and active cooling during mechanized GMAW of martensitic 15Kh5M steel produced the most favorable set of metallurgical indicators among the tested conditions for preheat-free welding. Compared with the conventional preheated procedure, the hybrid thermo-vibrational route resulted in a narrower heat-affected zone and a lower hardness level in the HAZ. Optical microscopy also revealed a finer and more homogeneous transformed microstructure in the hybrid condition.

Vibration alone, applied without preheating and without additional cooling control, had only a limited effect on hardness reduction, although some refinement of the transformed morphology was observed. By contrast, the combination of vibration with targeted water–air mist cooling immediately after each pass led to a marked decrease in HAZ width to about 3 mm and to more uniform microhardness values in the range of 330–370 HV, which are lower than those measured in the preheated and vibration-only conditions.

Importantly, no surface defects or discontinuities were detected in any of the welds, including those produced without preheating. Together with the refined optical microstructure, reduced HAZ hardness, and the transition from tensile to compressive residual stresses at the fusion boundary in the hybrid variant, these findings indicate a more favorable metallurgical state of the joint when the thermo-vibrational approach with active cooling is used.

At the same time, it should be emphasized that the present conclusions are based on metallographic observations, hardness mapping, and residual-stress measurements; phase-sensitive characterization (e.g., XRD/EBSD/EDS) and full mechanical testing (impact toughness, bend tests, hydrogen-assisted cracking resistance) were beyond the scope of this study. Within these limitations, the results support the feasibility of preheat-free welding of 15Kh5M steel under the investigated conditions and justify further work aimed at comprehensive mechanical and service-performance validation.