Wear-Resistant Overlay Welding for Coal Chemical Equipment Experimental Study and Application

Overview of the Technical Context

Coal chemical equipment operates under extreme conditions of high temperature, abrasive slurry, and corrosive chemical media, which makes the selection and fabrication of wear-resistant overlay layers a critical engineering challenge. The literature under review focuses on the experimental development and field application of wear-resistant overlay welding specifically tailored for coal chemical processing units, covering equipment such as slurry pumps, cyclone separators, coal-water slurry pipelines, and reactor internals. The study systematically addresses the material selection, welding process parameters, microstructure evolution, and performance evaluation of multi-layer overlay systems designed to resist both mechanical abrasion and chemical corrosion simultaneously.

Material Selection and Overlay System Design

The core of the wear-resistant overlay system relies on a graded layer architecture consisting of a transition layer, an intermediate bonding layer, and a functional hardfacing layer. The base material is typically Q345R or 16MnR carbon steel, which provides structural strength for pressure-bearing components. The transition layer employs a low-carbon stainless steel or nickel-based filler metal to prevent chromium carbide precipitation at the fusion boundary and to minimize intergranular corrosion susceptibility. The functional hardfacing layer utilizes chromium carbide-based or tungsten carbide-based alloys to achieve surface hardness exceeding 60 HRC while maintaining adequate toughness to resist spalling under impact loading.

The selection of filler metal composition is governed by the specific service conditions. In coal-water slurry applications where the slurry particle size exceeds 0.5 mm, tungsten carbide particles with a size distribution of 10-50 micrometers are preferred because their hardness of approximately 1500 HV provides superior resistance to three-body abrasion. In contrast, for finer slurry particles below 0.1 mm where two-body abrasion dominates, chromium carbide-based alloys with a hardness of 1200-1400 HV offer better fatigue resistance and crack tolerance.

Welding Process Parameters and Microstructure Control

The overlay welding process employed in the study primarily uses submerged arc welding (SAW) for the transition and intermediate layers, followed by gas metal arc welding (GMAW) or plasma transferred arc (PTA) for the functional hardfacing layer. The selection of SAW for the bonding layers is driven by its high deposition rate, which reduces the number of passes and minimizes the cumulative heat input per layer, thereby controlling the dilution rate of base material elements into the overlay.

Key process parameters for the SAW bonding layers include a welding current of 450-550 A, arc voltage of 28-34 V, and travel speed of 200-280 mm/min, with a flux coverage thickness of no less than 3 mm. For the GMAW hardfacing layer, the current is reduced to 180-250 A with an arc voltage of 22-28 V and a travel speed of 300-450 mm/min. The heat input per pass is carefully controlled to remain below 1.5 kJ/mm to prevent excessive grain growth and carbide coarsening in the hardfacing layer.

The microstructural analysis reveals that the transition layer exhibits a ferrite-austenite dual-phase structure with a Cr concentration gradient ranging from approximately 8 wt% at the fusion line to 22 wt% at the layer interface. This gradient effectively prevents the formation of continuous intergranular chromium carbide networks, which is a critical requirement for resisting intergranular corrosion in chloride-containing environments. The hardfacing layer microstructure consists of a matrix of martensite and retained austenite with uniformly dispersed Cr7C3 carbides, which provide the primary wear resistance through a combination of load-bearing capacity and crack-arresting mechanisms.

Performance Evaluation and Field Application Results

The overlay system was evaluated through a series of standardized tests including hardness profiling, impact energy measurement, corrosion resistance testing, and wear rate determination. The hardness profile across the overlay thickness shows a gradual increase from 220 HB at the fusion boundary to 62 HRC at the surface, indicating a well-controlled dilution gradient. The impact energy at room temperature for the transition layer exceeds 30 J, which satisfies the minimum toughness requirement specified in GB/T 150 for pressure vessel applications.

In field applications at a coal chemical plant processing coal-water slurry with a concentration of 60% and a flow velocity of 3.5 m/s, the overlay-welded slurry pump impellers demonstrated a service life of 18 months, representing a 4.5-fold improvement over the baseline of 4 months achieved with uncoated cast iron impellers. The wear rate measured by ASTM G98 sliding wear tests against alumina abrasives was 0.012 mm³/N·m for the hardfacing layer, compared to 0.085 mm³/N·m for the base material. The corrosion rate in a simulated coal chemical environment (5% NaCl + 2% H2S at 80°C) was measured at 0.02 mm/year, well below the acceptable threshold of 0.1 mm/year specified in engineering practice.

Key Technical Insights and Engineering Implications

The study underscores several critical engineering insights that have direct implications for industrial practice. First, the dilution control strategy is paramount; maintaining a dilution rate below 25% in the transition layer is essential for preventing intergranular corrosion, and this can be achieved through careful selection of filler metal composition with higher Cr and Ni content than the base material. Second, the layer thickness design must balance wear resistance with structural integrity; a total overlay thickness of 6-10 mm is recommended for slurry service, with the hardfacing layer comprising 2-3 mm of the total thickness. Third, post-weld heat treatment is mandatory for thick overlay systems to relieve residual stresses and temper any untempered martensite, typically performed at 600-650°C for 2 hours per 25 mm of thickness. Fourth, the quality of the base material surface preparation directly influences the bond strength; grit blasting to Sa 2.5 grade with a surface roughness of 60-100 micrometers is required to ensure adequate mechanical interlocking and wetting.

The application of this technology in coal chemical equipment has demonstrated significant economic benefits, with the overlay system reducing unplanned maintenance frequency by 70% and extending equipment overhaul intervals from 6 months to 24 months. The study also highlights the importance of non-destructive testing in quality assurance, with magnetic particle testing (MT) applied to all overlay welds and ultrasonic testing (UT) performed on the fusion boundary to detect any lack of fusion or porosity defects. The implementation of a systematic quality control plan based on PDCA methodology has been shown to reduce weld defect rates from 12% to below 3% over successive production batches.

In conclusion, the experimental study provides a comprehensive framework for the design, fabrication, and quality control of wear-resistant overlay systems in coal chemical equipment, demonstrating that through careful material selection, process parameter optimization, and rigorous quality assurance, significant improvements in equipment service life and operational reliability can be achieved under the demanding conditions of coal chemical processing.