New Stellite Alloy Weld Overlay Technology for Blast Furnace No. 5

Literature Overview

This study note examines a technical paper describing the application of advanced Stellite alloy weld overlay technology on Blast Furnace No. 5 at an integrated steel plant. Blast furnaces operate under extreme thermal cycling, mechanical abrasion, and chemical erosion conditions that historically caused rapid degradation of critical refractory-adjacent metal components. The paper addresses the specific challenge of extending the service life of furnace bell, throat, and tuyere zone components by employing a multi-layer Stellite-based overlay scheme that replaces conventional single-pass hardfacing approaches.

The core premise is that conventional Stellite 6 hardfacing, while offering excellent abrasion resistance, suffers from poor bonding with thick carbon steel substrates in large-diameter furnace components due to thermal distortion and cracking. The new technology introduces a graded transition layer combined with modified flux composition to achieve reliable metallurgical bonding and crack-free overlay deposits.

Core Technical Approach

Substrate Preparation and Preheat Strategy

Blast furnace components typically consist of Q235 or 16MnR carbon steel with wall thicknesses ranging from 25 mm to 60 mm. The high carbon equivalent of the base material combined with thick section geometry creates severe hydrogen cracking susceptibility. The paper specifies a preheat temperature of 200–250 °C applied using induction heating or gas torch, maintained throughout the welding sequence and followed by post-weld heat treatment at 550–600 °C for stress relief.

Multi-Layer Overlay Scheme

The innovation lies in a three-layer overlay system:

1. Bonding layer (Layer 1): A nickel-iron alloy (similar to Stellite 6 but with reduced cobalt content, approximately 35% Co, 20% Cr, 1% C) applied via submerged arc welding (SAW) using a specially formulated flux. This layer serves as a metallurgical bridge between the ferritic-pearlitic base metal and the subsequent cobalt-chromium overlay. The reduced cobalt content minimizes dilution-induced brittleness while maintaining sufficient hardenability.
2. Transition layer (Layer 2): Stellite 6 alloy applied in two to three passes using either electroslag welding (ESW) or multi-wire submerged arc welding. The ESW process is preferred for thick sections because of its high deposition rate (up to 15 kg/h) and deep, uniform penetration. Wire diameter of 4.0 mm or 5.0 mm is used, with travel speed of 80–120 mm/min and current of 400–600 A.
3. Surface layer (Layer 3): A thin (2–3 mm) layer of modified Stellite 6 with added niobium (0.5–1.0% Nb) and tungsten (3–5% W) to enhance high-temperature strength and reduce thermal fatigue cracking. This layer is applied using gas metal arc welding (GMAW) with a self-shielded flux-cored wire for field flexibility.

Flux Modification

The paper highlights a critical innovation in flux chemistry. Conventional saw fluxes (such as HJ431) produce deposits with excessive sulfur and phosphorus content, which promote intergranular cracking in cobalt-chromium alloys. The modified flux contains:

• Reduced MnO content (from 45% to 30%) to minimize manganese dilution in the overlay
• Added Al₂O₃ (8–12%) to improve slag fluidity and reduce spatter
• 2–3% BaCO₃ as a desulfurizer
• 1–2% CaF₂ to stabilize arc and reduce porosity

This flux modification reduces deposit sulfur content from typical 0.035% to below 0.015%, significantly improving crack resistance.

Defect Analysis and Countermeasures

Common Defects Observed

Metallurgical Analysis

Metallographic examination of the optimized overlay revealed:

• The bonding layer shows a uniform martensitic structure with 550–600 HV hardness, providing good ductility for bonding
• The transition layer exhibits a typical Stellite 6 microstructure: matrix of austenite with M₇C₃ carbide precipitates, hardness 350–450 HV
• The surface layer shows a refined carbide distribution with NbC and WC precipitates, achieving 500–580 HV hardness
• Dilution from base metal into the first pass is approximately 15–20%, decreasing to less than 5% in subsequent passes
• No intergranular cracking or microcracking was observed in the optimized scheme

Engineering Practice Integration

Field Application Results

The technology was applied to the tuyere zone of Blast Furnace No. 5, covering approximately 12 m² of overlay surface. Key results include:

• Service life improvement: From 8 months (conventional hardfacing) to over 24 months, representing a threefold increase
• Overlay thickness: 8–12 mm total, with wear allowance of 3–5 mm
• Repair cost reduction: Despite higher initial material cost, the extended service life reduced total repair cost by approximately 40%
• Downtime reduction: Overlay can be performed in situ with reduced furnace outage time

Quality Control Protocol

The following inspection regime was implemented:

1. Visual inspection (VT): After each pass, checking for undercuts, excessive reinforcement, and surface defects
2. Magnetic particle testing (MT): 100% inspection of all overlay surfaces after completion, per NB/T 47013.4
3. Ultrasonic testing (UT): Bond strength verification at critical locations, per NB/T 47013.2, using contact UT with phased array technique
4. Hardness testing: Minimum 10 points per m², verifying 350–580 HV range depending on layer
5. Impact testing: Charpy V-notch impact at -20 °C on coupon specimens, minimum 27 J

Study Insights and Reflections

The most significant insight from this literature is the recognition that weld overlay technology for blast furnace components cannot be treated as a simple hardfacing exercise. The combination of thick sections, high thermal input, and extreme service conditions demands a systems-level approach that integrates substrate preparation, process parameter optimization, flux chemistry modification, and rigorous quality control.

The concept of a graded multi-layer overlay is particularly instructive. By introducing a nickel-iron bonding layer, the metallurgical compatibility between dissimilar materials is improved without sacrificing the wear resistance of the surface layer. This approach mirrors the philosophy used in explosion-cladded bimetallic plates, where a transition zone is deliberately engineered to accommodate the mismatch in thermal expansion and mechanical properties.

Another key lesson is the importance of flux chemistry in cobalt-based overlay welding. The sulfur content in the deposit, while seemingly minor, has a disproportionate effect on crack resistance in high-alloy alloys. The modified flux approach demonstrates that consumable optimization can be as impactful as process parameter adjustment.

The field application results validate the technical approach, but the study also raises questions about long-term durability under cyclic thermal conditions. The blast furnace operates with frequent hot-blast cycles, and the thermal fatigue performance of the overlay after 24 months of service warrants continued monitoring. Future work should incorporate thermal cycling testing and in-service metallurgical examination to validate the predicted service life.

This literature serves as an excellent case study for engineers working on large-scale industrial hardfacing applications. It demonstrates that innovation in weld overlay technology requires not only advanced materials but also careful process engineering and quality assurance integration. The multi-layer approach with modified flux chemistry provides a replicable methodology for similar applications in other heavy industry sectors.