Open AccessArticle Effect of Heat Input on Microstructure and High-Cycle Fatigue Properties of the CGHAZs in Wind Power Steel 1 State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China 2 CITIC Metal Co., Ltd., Beijing 100004, China * Authors to whom correspondence should be addressed. Metals 2026, 16(6), 635; https://doi.org/10.3390/met16060635 (registering DOI) Submission received: 10 April 2026 / Revised: 3 June 2026 / Accepted: 4 June 2026 / Published: 9 June 2026 Abstract Wind turbine towers rely on welded joints for structural continuity, and the coarse-grained heat-affected zone (CGHAZ) at these joints is the principal site of fatigue damage under service loading. This study characterises the influence of welding heat input on the microstructural constitution, high-cycle fatigue response, and fracture mechanisms of Gleeble-simulated CGHAZs in a Nb-microalloyed wind power steel. Thermal cycles representative of submerged arc welding at 15, 25, 35, and 45 kJ/cm were applied, and the resulting microstructures were examined by optical microscopy, SEM, EBSD, and TEM. Raising the heat input produced systematic microstructural coarsening: the densities of low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) fell by approximately 40% and 26%, respectively, while the mean equivalent diameter (MED) and prior austenite grain (PAG) size grew by roughly 64% and 67%. Life partitioning showed that crack nucleation accounted for more than 84% of total fatigue cycles in every condition, identifying it as the life-governing damage stage. Over the 15-to-45 kJ/cm range, the CGHAZ fatigue strength at 2 × 10 6 cycles deteriorated from 246.9 MPa to 208.5 MPa (a 15.6% reduction), while the mean fatigue striation spacing widened from 0.142 μm to 0.183 μm (an increase of 28.9%). These results demonstrate that judicious heat-input selection is a practical and effective means of preserving CGHAZ fatigue integrity in wind tower steel fabrication, and they address a previously unresolved gap concerning high-cycle fatigue fracture mechanisms in this critical microstructural zone. Driven by the global energy transition and the “Dual Carbon” goals, the wind power industry is accelerating its evolution towards large megawatt capacities and deep-sea applications. The trend imposes stringent requirements on the lightweight design and service life of tower structures [ 1, 2], making the structural integrity of the tower a decisive factor for the reliability and lifecycle of the wind turbine [ 3, 4]. Wind turbine towers are primarily assembled in sections via welding. At the welded joints, fatigue cracks tend to initiate preferentially at the weld toe position due to stress concentration effects induced by geometric profiles [ 5]. Microscopically, this corresponds to the coarse-grained heat-affected zone (CGHAZ). As a core process parameter, welding heat input not only dictates welding efficiency but also profoundly influences the phase transformation path, microstructural morphology, and final mechanical properties of the CGHAZ by regulating the welding thermal cycle [ 4, 6]. Given that the CGHAZ is the vulnerable region of the welded joint [ 7], and statistics indicate that approximately 80% of failures in welded structures originate from fatigue damage in this zone [ 8], it has become an urgent need and a research priority to investigate the mechanism of welding heat input on the fatigue performance of the CGHAZ. Achieving precise control over the microstructure and service performance of this critical zone is essential for enhancing the operational safety of wind turbine towers. The extant research has principally concentrated on the relationship between microstructural evolution and mechanical properties (e.g., impact toughness and hardness) in the CGHAZ of low-carbon steels under varying heat inputs. An increase in heat input (25–300 kJ/cm) for Q420C steel led to significant grain coarsening in the CGHAZs and the massive precipitation of proeutectoid/massive ferrite, resulting in a drastic reduction in impact toughness [ 6]. Similarly, Yuan et al. [ 9] revealed that higher heat input caused grain coarsening and an increase in bainite content, which decreased both joint hardness and impact toughness in the CGHAZs of Q355D low-alloy steel. Zeng et al. [ 10] observed that an increase in heat input resulted in the coarsening of the microstructure and an increase in the size of the martensite/austenite (M/A) constituents, leading to a decline in impact toughness. Additionally, certain studies have identified that the impact of heat input on properties does not invariably result in a monotonous decrease [ 11, 12]. Wu et al. [ 13] studied V-Ti-N-Nb weathering steels, revealing that increasing heat input (10–70 kJ/cm) promotes M/A constituent formation and coarsening in the CGHAZ. This microstructural evolution significantly degrades toughness, establishing a critical link between thermal cycles and mechanical performance degradation. Xu et al. [ 14] noted that the HAZ microstructure remained relatively stable with increasing heat input for Q420FRE steel; significant material softening occurred, resulting in reduced impact toughness. Conversely, Fu et al. [ 15] confirmed from the perspective of cooling rates that for Q420qENH steel, increased cooling rates (i.e., lower heat input) promoted grain refinement and consequently increased hardness. The extant studies have elucidated the effects of heat input on microstructure and general mechanical properties. However, a critical gap in research persists concerning the effects of heat input on the microstructure and fatigue properties of CGHAZ, along with the underlying mechanisms. In this study, the CGHAZs of a wind power steel were simulated under typical heat inputs using a Gleeble 3800-GTC thermomechanical simulation system (Dynamic Systems Inc., Poestenkill, NY, USA). The main objectives of this study are to investigate the influence of heat input on the microstructure and fatigue performance of the simulated CGHAZs and to elucidate the underlying mechanisms by which high-cycle fatigue life is influenced by heat input. The research addressed a significant research gap concerning the high-cycle fatigue performance and fracture mechanisms of CGHAZs, providing a solid theoretical and experimental foundation for the development of wind power steel. 3.1. Microstructures Representative OM and SEM images of the four simulated CGHAZ conditions are shown in , and the quantitative phase fractions are summarised in . In all conditions the microstructure was bainite-dominated, consisting of lath bainitic ferrite (LBF), granular bainitic ferrite (GBF), martensite/austenite (M/A) constituents, and degenerated pearlite (DP). At 15 kJ/cm the microstructure was characterised by an LBF matrix with GBF, M/A, and DP as minority phases. As heat input was raised, GBF, M/A, and DP fractions grew monotonically at the expense of LBF, such that at 45 kJ/cm the microstructure was entirely granular, comprising GBF, M/A, and DP without measurable LBF. presents TEM micrographs of the simulated CGHAZ at different heat inputs. The microstructural features of the CGHAZ observed via TEM are fully consistent with the results obtained from OM and SEM observations in . The intragranular dislocation substructure became progressively sparser with increasing heat input, evidenced by fewer and more loosely distributed dislocation lines. SAED analysis confirmed that the bainitic ferrite matrix was body-centred cubic (BCC) (g). At 45 kJ/cm, higher-magnification images (e,f) revealed the DP constituent—composed of theta-carbide grains surrounded by degenerated pearlite ferrite (DPF), verified by the theta-phase SAED pattern in h—and M/A islands, whose SAED pattern (i) exhibited reflections from both BCC martensite and face-centred cubic (FCC) retained austenite. The red and blue lines in the image quality (IQ) plots (b,e,h,k) represent low-angle grain boundaries (LAGBs) with MTA ranges from 2 to 15° and high-angle grain boundaries (HAGBs) with MTAs greater than 15°, respectively. It is revealed that the HAGB fraction (MTA > 15°) contracted from 42.3% to 30.8% as heat input rose from 15 to 45 kJ/cm, while the LAGB fraction (MTA 2–15°) expanded correspondingly. Grain boundary densities, calculated following reference [ 19], showed HAGB density declining from 0.321 to 0.194 μm −1 and LAGB density falling from 0.437 to 0.325 μm −1—reductions of approximately 40% and 26%, respectively ( Depicted in are characteristic prior austenite grains (PAGs) from the CGHAZ, together with grain-size distributions measured across a range of heat-input conditions. The results revealed pronounced grain coarsening: the mean PAG diameter increased from 76.9 um at 15 kJ/cm to 128.3 um at 45 kJ/cm, an enlargement of approximately 67%. The Gaussian-fitted size distributions also broadened with rising heat input. displays TEM micrographs of precipitated particles in the simulated CGHAZ and corresponding energy-dispersive spectroscopy (EDS) point analysis. EDS point analysis of carbon extraction replicas identified the precipitates at all heat inputs as (Ti, Nb, V) (C, N) composite carbonitrides. Increasing heat input from 15 to 45 kJ/cm caused the mean particle diameter to grow from 52.2 nm to 74.8 nm and the average volume fraction to increase from 0.23% to 0.56%, reflecting particle coarsening driven by extended high-temperature dwell. 3.2. Mechanical Properties shows the high-cycle fatigue performance of simulated CGHAZ under different heat inputs. The corresponding S-N curves (Woehler curves) for each CGHAZ specimen are shown in a, b, c, and d, respectively. It can be observed that the fatigue data obtained from the CGHAZ specimens exhibited a relatively concentrated distribution. The S-N curves of the failed CGHAZ specimens were fitted using the following equations [ 22, 23]: l o g σ a = l o g σ f ′ − b ∗ l o g ( N f ) (1) where σa denotes the stress amplitude, σ f ′ represents the fatigue strength coefficient, Nf indicates the number of cycles to failure, and b is the fatigue strength index. e shows the fitting results of Nf versus stress amplitude, with the fitting equation for the CGHAZ specimen as follows: 15 kJ/cm: log σa = 2.69 − 0.05 ∗ log( Nf) (2) 25 kJ/cm: log σa = 2.62 − 0.04 ∗ log( Nf) (3) 35 kJ/cm: log σa = 2.65 − 0.05 ∗ log( Nf) (4) 45 kJ/cm: log σa = 2.73 − 0.06 ∗ log( Nf) (5) Therefore, consistent with the trends observed for tensile strength and hardness, the fatigue strength of the simulated CGHAZ specimens gradually decreased from 246.9 MPa to 208.5 MPa as the heat input increased from 15 kJ/cm to 45 kJ/cm. 3.3. Fatigue Fracture Characteristics Fatigue life breaks down into two contributions—the life spent initiating a crack and the life spent propagating it. Examination of the stable crack growth regions revealed a systematic increase in mean fatigue striation spacing with heat input: from 0.142 μm at 15 kJ/cm to 0.183 um at 45 kJ/cm, representing a widening of 28.9%. Because each striation corresponds to crack advance during a single load cycle [ 24, 25], this trend directly reflects accelerated crack growth kinetics at higher heat inputs, consistent with the reduced HAGB density available to deflect or temporarily arrest the advancing crack front. Quantitative life partitioning revealed that crack propagation consumed between 17,324 and 27,541 cycles—equivalent to only 0.64–15.52% of the total failure life—across the four conditions. Accordingly, crack initiation life constituted more than 84% of total fatigue life in every case, establishing nucleation as the dominant fatigue damage mechanism. This dominance of the initiation stage is consistent with findings reported in the literature for comparable welded steel microstructures [ 26, 27 4.1. Microstructure Evolutionary presents the dilatometric curve of the CGHAZ measured on the Gleeble 3800-GTC system during the thermal simulation, with the green and blue dotted lines denoting the tangents to the corresponding curve segments. With increasing heat input, the onset temperature (Ar 3) of the γ→α phase transformation in the simulated CGHAZ rose from 596 °C to 681 °C, while the finish temperature (Ar 1) increased from 407 °C to 520 °C. The Ar3 and Ar1 temperatures were determined from continuous cooling dilatometry curves using the tangent method. The cooling rate significantly decreased with increasing heat input, providing ample time for carbon atom and alloy element diffusion. This promoted the nucleation and growth of α phase and pearlite, thereby elevating the Ar3 and Ar1 temperatures [ 28]. Consequently, the ferrite structure coarsened significantly with increasing heat input. Simultaneously, the elevated temperatures led to carbon enrichment, resulting in increased M/A and DP content (). Additionally, prolonged diffusion at elevated temperatures caused coarsening of precipitation particles, with particle size increasing as heat input rises (). As heat input was increased, the cooling rate of the CGHAZ was observed to decrease and the high-temperature holding time was prolonged, leading to coarsening of prior austenite grains (PAGs) () and a consequent reduction in total grain boundary area. This grain coarsening effect directly reduced the density of HAGBs associated with grain boundaries. With increasing heat input, although the GB content increased and the LB content decreased, the reduction in total grain boundary area caused by PAG coarsening remained dominant. Moreover, the decreased LB content diminished the contribution of intragranular lath substructures to LAGBs. Consequently, both LAGB and HAGB densities exhibited a decreasing trend within the CGHAZ. Furthermore, due to the prevalence of intragranular substructures, the LAGB density consistently remained higher than that of HAGBs ( presents the thermodynamic calculation results of precipitated phases obtained using Thermo-Calc TM 4.0 software with the TCFE13 database. The precipitation temperatures of Ti (N,C), V (C,N), and NbC phases were determined to be approximately 1439 °C, 765 °C, and 1110 °C, respectively. During the welding heating process, the NbC and V (C,N) phases completely dissolved into the austenite matrix, whereas the Ti (N,C) phase, owing to its high thermal stability, underwent only partial dissolution. Upon subsequent cooling, the precipitates formed sequentially in the order of Ti (N,C), NbC, and V (C,N). Notably, the partially retained Ti (N,C) particles served as preferential nucleation sites for subsequent precipitation, promoting the epitaxial growth of NbC and V (C,N) phases on their surfaces. This resulted in the formation of complex (Ti,Nb,V) (C,N) precipitates with a core–shell structure [ 29, 30]. With increasing heat input, both the average diameter and volume fraction of the (Ti,Nb,V) (C,N) particles in the CGHAZ increased accordingly. The presence of precipitated particles in steel exerted resistance to the migration of γ grain boundaries [ 31, 32]. According to the Zener model, these precipitated particles exerted a pinning force on γ grains boundaries [ 33], thereby restricting their migration capability (Equation (6)). P z = 3 f v γ γ γ 2 r (6) where P z represents the pinning force exerted by precipitated particles on the γ grain boundaries, f v denotes the volume fraction of the particles, γ γ γ stands for the interfacial energy between two adjacent γ grains, and r signifies the radius of the particles. As the heat input increased, the residence time at high temperatures was prolonged. Consequently, the precipitated particles coarsened, and their average size increased. Meanwhile, some precipitates dissolved back into the matrix, which led to a decrease in the particle number density. These changes collectively resulted in a weakening of the pinning force exerted by the precipitated particles on the γ grain boundaries. The diminished pinning effect reduced the resistance to grain boundary migration, which in turn led to an increase in the average size of the PAGs and a corresponding increase in the MED within the CGHAZ ( and 4.2. Fatigue Damage Mechanism To elucidate the fatigue fracture mechanism in the CGHAZs under varying heat inputs, it was essential to analyze the initiation and propagation of fatigue cracks. The results presented above indicated that the crack initiation behavior played a dominant role in the fatigue fracture process. The primary factor contributing to the initiation of fatigue cracks was the presence of local strain concentrations within the alloy materials [ 34], which were the result of irreversible plastic deformation [ 35]. It has been demonstrated that grain boundaries impede dislocation motion in polycrystalline materials [ 35, 36]. During cyclic deformation, HAGBs hindered dislocation movement, leading to the accumulation of dislocation pile-ups and the formation of stress concentrations at these boundaries, which induced the initiation of fatigue cracks [ 37, 38]. However, dislocations possessed the capacity to migrate through LAGBs into adjacent grains [ 39], making LAGB cracking a formidable challenge. The increased heat input reduced the density of both LAGBs and HAGBs in the CGHAZs. Consequently, this effect diminished the resistance to fatigue crack initiation. The difference in grain size leads to changes in the number of dislocations piled up per unit length ahead of grain boundaries [ 40]. According to Taylor theory and the Orowan formula, the relationship between shear stress ( τ) and dislocation density ( ρ) during plastic deformation can be expressed as follows [ 41]: τ = τ 0 + K G b ρ (7) where τ0 represents the initial shear stress required to initiate dislocation motion in the absence of other dislocations, K is an empirical constant, and G is the shear modulus, while b is the Burgers vector. During plastic deformation, assuming the shear stress acting on each grain follows Schmid’s law—which describes the relationship between grain crystallographic orientation and stress—the relationship among these variables can be expressed as τ = Ω σ (8) where Ω represents the Schmid factor, and σ denotes the applied normal flow stress on both ends of the specimen, which is comparable to the material’s yield strength. According to the derivation by Zhang et al. [ 40], the relationship among the number of dislocation pile-ups per unit length ( n), the parameter ρ, and Ω is given by the following equation: n = ρ = Ω σ − σ 0 K G b (9) where σ0 is the dislocation glide threshold stress, reflecting the resistance to deformation within the crystal. According to the Hall–Petch relationship, the following relation exists between the grain size ( d) and yield stress: σ − σ 0 = k d − 1 / 2 (10) where k is a constant that characterizes the degree of influence of grain boundaries on strength. In this study, d is the MED determined from EBSD data, as reported in [ 17, 18]. Based on Equations (9) and (10), the relationship between the number of dislocation pile-ups and grain size satisfies n = k Ω K G b d (11) As shown in Equation (11), the number of dislocation pile-ups at grain boundaries continuously increases with the grain size of polycrystalline materials. When the number of dislocation pile-ups reaches a critical value ( nc), localized cracking occurs at the grain boundaries [ 40], leading to fatigue cracks [ 42]. An increase in heat input results in an increased MED of the CGHAZ (), thereby increasing the number of dislocation pile-ups. This finding suggested that the propensity for fatigue cracking may increase. Therefore, higher heat input produced a combined effect in CGHAZ specimens, manifesting as a decrease in LAGB alongside an increase in MED. This effect served to weaken the inhibitory effect on fatigue crack initiation. In summary, the study indicated that an increase in heat input promoted both the initiation and propagation of fatigue cracks, thereby exerting a negative influence by reducing both the crack initiation lifetime and propagation lifetime. However, it was imperative to recognize that crack initiation lifetimes accounted for over 84% of the total failure lifetimes. Compared to the propagation lifetime, the increase in heat input exerted a more significant negative impact on the reduction of crack initiation lifetime. It was determined that fatigue crack initiation was the predominant factor leading to fatigue failure fractures. Finally, as the heat input increases, the fatigue strength of the simulated CGHAZ decreased continuously from 246.9 MPa to 208.5 MPa. The present study investigated the effects of heat input on the microstructure and high-cycle fatigue properties of the simulated CGHAZs in wind power steel using the Gleeble 3800-GTC system. The characteristics and damage mechanisms of fatigue fractures were thoroughly elucidated. The key conclusions are summarized as follows. (1) Heat input governed the phase balance of the simulated CGHAZ: GBF, M/A, and DP fractions rose progressively while LBF declined, so that at 45 kJ/cm the microstructure was entirely granular. Concurrently, grain boundary densities of both LAGBs and HAGBs decreased, the MED enlarged, and prior austenite grains coarsened. These changes originate from the slower cooling rates and prolonged high-temperature exposure at elevated heat inputs, which weaken (Ti, Nb, V) (C, N) Zener pinning and permit uninhibited austenite grain boundary migration. (2) All static mechanical indices—yield strength, tensile strength, and Vickers hardness—decreased monotonically over the 15-to-45 kJ/cm range (from 555 to 481 MPa, 720 to 608 MPa, and 250.9 to 207.2 HV0.5, respectively), while uniform elongation increased modestly from 7.9% to 10.1%. The CGHAZ fatigue limit at 2 × 10 6 cycles fell correspondingly from 246.9 MPa to 208.5 MPa, a 15.6% reduction, confirming that elevated heat input degrades both static and cyclic performance. (3) Fatigue crack initiation was identified as the life-controlling damage mechanism, accounting for more than 84% of total failure life across all conditions. Higher heat input reduced LAGB density and increased grain size, collectively diminishing the microstructure’s resistance to crack nucleation: a sparser grain boundary network provided fewer obstacles to dislocation accumulation, and larger grains promoted greater dislocation pile-up numbers, hastening attainment of the critical value required for boundary microcracking. (4) EBSD crack path mapping demonstrated that HAGBs effectively deflected and retarded advancing fatigue cracks by forcing changes in the crystallographic crack plane, whereas LAGBs provided negligible impedance. The loss of HAGB density with increasing heat input therefore directly weakened crack-propagation resistance, producing wider fatigue striations (0.142 to 0.183 μm) and shorter propagation lives. (5) For wind power steel applications, increasing welding heat input exerts a dual detrimental influence on CGHAZ fatigue performance: it simultaneously lowers resistance to crack initiation and accelerates crack propagation. Since initiation life dominates, the degradation of crack-nucleation resistance is the principal concern in practice. Controlling heat input within appropriate limits thus represents an effective fabrication strategy for safeguarding the high-cycle fatigue integrity of welded wind turbine tower structures. Author Contributions Conceptualization, G.Z. and Q.W.; Methodology, G.Z. and Y.K.; Formal analysis, G.Z. and L.Z.; Investigation, G.Z., L.Z. and Y.K.; Resources, J.H.; Data curation, G.Z., J.H. and L.Z.; Writing—original draft, G.Z.; Writing—review and editing, Q.W. and Z.L.; Visualization, G.Z.; Supervision, Q.W. and Z.L.; Project administration, G.Z.; Funding acquisition, Q.W. and Z.L. All authors have read and agreed to the published version of the manuscript. Funding This work was supported by the Major Scientific and Technological Innovation Project of CITIC Group (Grant Number 2022zxkya06100) and the CITIC Metal—CBMM Collaborative R&D Project for Niobium Technology Advancement (Grant Number M2149-2024). Data Availability Statement The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors. Conflicts of Interest Authors Guodong Zhang, Jiangli He and Zhongzhu Liu were employed by the company CITIC Metal Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abbreviations The following abbreviations are used in this manuscript: CGHAZ Coarse-grained heat-affected zone LAGBs Low-angle grain boundaries HAGBs High-angle grain boundaries MED Mean equivalent diameter PAGs Prior austenite grains TMCP Thermomechanical control process OM Optical microscope SEM Scanning electron microscope EBSD Electron backscatter diffraction TEM Transmission electron microscope EDS Energy-dispersive X-ray spectroscopy LBF Lath bainitic ferrite GBF Granular bainitic ferrite DPF Degenerated pearlite ferrite SAED Selected area electron diffraction BCC Body-centered cubic FCC Face-centered cubic IPFs Inverse pole figures MTAs Misorientation angles IQ Image quality KAM Kernel average misorientation References National Energy Administration; Ministry of Industry and Information Technology; SASAC; State Administration for Market Regulation. Guidelines on Promoting High-Quality Development of Energy Equipment. Installation 2025, 1–4. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. Share and Cite Zhang, G.; Zhu, L.; He, J.; Kong, Y.; Wang, Q.; Liu, Z. Effect of Heat Input on Microstructure and High-Cycle Fatigue Properties of the CGHAZs in Wind Power Steel. Metals 2026, 16, 635. https://doi.org/10.3390/met16060635 Zhang G, Zhu L, He J, Kong Y, Wang Q, Liu Z. Effect of Heat Input on Microstructure and High-Cycle Fatigue Properties of the CGHAZs in Wind Power Steel. Metals. 2026; 16(6):635. https://doi.org/10.3390/met16060635 Zhang, Guodong, Liyuan Zhu, Jiangli He, Yisen Kong, Qingfeng Wang, and Zhongzhu Liu. 2026. "Effect of Heat Input on Microstructure and High-Cycle Fatigue Properties of the CGHAZs in Wind Power Steel" Metals 16, no. 6: 635. https://doi.org/10.3390/met16060635 Zhang, G., Zhu, L., He, J., Kong, Y., Wang, Q., & Liu, Z. (2026). Effect of Heat Input on Microstructure and High-Cycle Fatigue Properties of the CGHAZs in Wind Power Steel. Metals, 16(6), 635. https://doi.org/10.3390/met16060635 Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here. Article Metrics Article metric data becomes available approximately 24 hours after publication online.
