The excavation of deep tunnels through hard rock formations in mountainous regions presents significant engineering challenges. Tunnel Boring Machines (TBMs) have become the premier solution for such projects due to their efficiency, safety, and ability to produce a smooth tunnel profile. The core of the TBM’s rock-breaking capability lies in its rotating cutterhead, equipped with an array of disc cutters. These cutters function by generating extremely high point loads, causing compressive failure and chipping of the rock. The mechanism of rock fragmentation under a disc cutter can be conceptually related to the induced stress field. The contact pressure and the resulting subsurface stress distribution are critical for chip formation. While a simplified model for the force required for indentation can be expressed as:
$$ F = k \cdot \sigma_c \cdot d \cdot t $$
where \( F \) is the thrust force per cutter, \( \sigma_c \) is the uniaxial compressive strength of the rock, \( d \) is the cutter diameter, \( t \) is the penetration depth, and \( k \) is an empirical constant related to the cutter geometry and spacing.
Following fragmentation, the resulting muck (rock debris) accumulates at the tunnel invert. Efficient removal of this muck is paramount for maintaining continuous advance rates and, crucially, for preventing secondary crushing of the debris which can lead to accelerated and severe damage to the expensive disc cutters and other cutterhead components. This is the primary function of the muck scraper (or bucket lip). As the cutterhead rotates, these strategically placed scraper plates, typically mounted on the periphery of the buckets, scoop the muck from the tunnel floor and guide it into the bucket cavity for subsequent transfer to the conveyor system. The operational environment for these scrapers is exceptionally harsh, involving direct and continuous abrasion from hard rock fragments, significant impact loads, and occasional contact with the tunnel walls.
Traditionally, scraper plates have been manufactured from materials like ZG30CrMnSiRe cast steel. However, in demanding hard rock conditions, their wear life is often insufficient, leading to frequent stoppages for replacement. These interruptions not only incur direct material and labor costs but also severely impact the overall project schedule. Therefore, the development of a superior, more durable material for these critical wear parts is of high economic and operational importance. This research focuses on the development, heat treatment optimization, and production application of a novel low-alloy wear-resistant steel casting specifically engineered for TBM scraper plates, with the goal of significantly extending service life and reducing total cost of ownership.
| Element | Target Composition Range (wt.%) | Primary Function in Steel Casting |
|---|---|---|
| C | 0.35 – 0.50 | Primary hardening element; forms carbides and determines hardenability. |
| Si | 0.60 – 1.20 | Deoxidizer; solid solution strengthener; improves fluidity in casting. |
| Mn | 0.80 – 1.50 | Enhances hardenability; mitigates sulphur embrittlement by forming MnS. |
| Cr | 1.00 – 1.80 | Improves hardenability and wear resistance; forms stable carbides. |
| Mo | 0.20 – 0.50 | Powerful hardenability agent; reduces temper embrittlement; refines microstructure. |
| V / Nb | 0.05 – 0.15 | Forms fine, hard carbides (VC, NbC) for secondary hardening and grain refinement. |
| P, S | < 0.030 | Impurities kept to a minimum to ensure good toughness and casting integrity. |
The chemical composition was carefully designed to achieve an optimal balance of high hardness for abrasion resistance and sufficient toughness to withstand impact. The medium carbon content provides a strong martensitic matrix upon quenching. Manganese and chromium are key for ensuring adequate hardenability in section thicknesses relevant to scraper plates (approximately 40mm). Molybdenum is added to suppress temper embrittlement in the low-temperature tempering range required for high hardness, while micro-alloying with vanadium or niobium promotes grain refinement and secondary hardening.
The experimental steel casting was melted in a 100 kg medium-frequency induction furnace under a controlled atmosphere to minimize oxidation and gas pickup. After achieving the target temperature and chemistry, the melt was poured into pre-made sand molds to produce standard Y-block test castings, which are representative of the thermal mass and cooling rates of a typical scraper plate section. The casting process is the first critical step in determining the final properties, as it sets the initial as-cast microstructure, grain size, and soundness (freedom from shrinkage cavities or inclusions).
The heart of this investigation lies in the systematic optimization of the heat treatment process. The as-cast condition is unsuitable for service due to its heterogeneous structure and low toughness. A quenching and tempering (Q&T) treatment is essential to develop the desired combination of properties. The key variables are the austenitizing (quenching) temperature and the tempering temperature. We designed a matrix to study the effect of austenitizing temperature:
| Specimen ID | Austenitizing Treatment | Tempering Treatment |
|---|---|---|
| A | 850°C for 2 hours, oil quench | 250°C for 2 hours, air cool |
| B | 880°C for 2 hours, oil quench | |
| C | 910°C for 2 hours, oil quench | |
| D | 880°C for 2 hours, oil quench | 200°C for 2 hours |
| E | 880°C for 2 hours, oil quench | 300°C for 2 hours |
Specimens for metallographic examination, hardness testing, and Charpy V-notch impact testing were machined from the heat-treated Y-blocks. Hardness was measured using both Rockwell C (HRC) and Brinell (HBW) scales, with reported values being the average of multiple indentations. Impact toughness was assessed at room temperature. Microstructural analysis was performed using optical microscopy (OM) and scanning electron microscopy (SEM) on polished and etched samples.
The microstructure of a properly heat-treated wear-resistant steel casting is paramount. Specimen A (850°C quench) exhibited a mixed microstructure of fine martensite with some unresolved ferrite/pearlite regions, indicating incomplete austenitization. This is described by the Austenite transformation equation, where the fraction of austenite \( f_\gamma \) is a function of temperature \( T \) and time \( t \), often following Avrami kinetics: \( f_\gamma = 1 – \exp(-k t^n) \). At 850°C, the driving force for the diffusion-controlled transformation was insufficient for complete homogenization. Specimen B (880°C quench) showed a fully transformed, uniform microstructure of fine lath martensite with a minimal amount of retained austenite. Specimen C (910°C quench) displayed a coarser lath martensitic structure with a noticeably higher proportion of retained austenite, a consequence of the higher austenitizing temperature reducing the martensite start (\(M_s\)) temperature, as approximated by: $$ M_s (°C) \approx 539 – 423C – 30.4Mn – 17.7Ni – 12.1Cr – 7.5Mo $$ where element symbols represent weight percent.
The mechanical property data from the heat treatment study is summarized below. The hardness can be related to the microstructure, where for a martensitic steel, hardness \( H \) is primarily a function of carbon content \( C \) and tempered condition: \( H \approx f(C, T_{temper}) \).
| Specimen ID | Impact Toughness (J/cm²) | Hardness (HRC) | Estimated Yield Strength (MPa)* |
|---|---|---|---|
| A (850°C+250°C) | 16.3 | 50.3 | ~1550 |
| B (880°C+250°C) | 17.2 | 52.5 | ~1650 |
| C (910°C+250°C) | 15.4 | 51.6 | ~1600 |
| D (880°C+200°C) | 14.8 | 54.1 | ~1700 |
| E (880°C+300°C) | 18.5 | 48.7 | ~1500 |
*Approximation based on HRC conversion relationships.
The results clearly indicate that Specimen B, subjected to the 880°C austenitizing and 250°C tempering treatment, achieved the optimal balance of hardness and impact toughness for this steel casting composition. The superior hardness (52.5 HRC) stems from the fully martensitic matrix with a high dislocation density and fine dispersion of transition carbides (ε-carbide) formed during low-temperature tempering. The toughness, while inherently limited in high-hardness martensite, is maximized at this condition due to the fine prior austenite grain size and the absence of pro-eutectoid phases that act as stress concentrators. Lower austenitizing temperatures (Specimen A) leave soft phases that reduce overall hardness and wear resistance. Higher temperatures (Specimen C) cause grain coarsening, which detrimentally affects both toughness and, to a lesser extent, hardness due to increased retained austenite. Tempering at 200°C (Specimen D) yields slightly higher hardness but at a significant cost to toughness, as the martensite is more brittle. Tempering at 300°C (Specimen E) sacrifices hardness for a gain in toughness as the martensite begins to recover and coarser cementite precipitates.
The dominant wear mechanism for the scraper is abrasive wear, with a significant contribution from impact. The wear volume \( V \) due to abrasion can be modeled by the Archard-type equation modified for abrasion: $$ V = K_{abr} \cdot \frac{L \cdot s}{H} $$ where \( K_{abr} \) is the abrasive wear coefficient, \( L \) is the load, \( s \) is the sliding distance, and \( H \) is the material hardness. This underscores why high hardness is critical. However, pure hardness is insufficient if the material lacks the toughness to prevent micro-cracking and spalling under impact. The developed steel casting achieves a synergistic effect: the hard martensitic matrix resists cutting and ploughing by abrasive particles, while its sufficient toughness prevents catastrophic fracture. The fine carbides act as reinforcements, impeding the motion of dislocations and providing additional resistance to wear.
Based on the laboratory optimization, the industrial production process for the TBM scraper plates was established. The component, with a major wall thickness of 40 mm, a length of 230 mm, and a finished weight of approximately 12 kg, is ideally suited for sand casting. A precise foundry practice was implemented using resin-coated sand molds to achieve good dimensional accuracy and surface finish. A two-cavity mold layout was designed for efficiency. The gating and risering system was carefully simulated and designed to ensure sound, defect-free castings; the key parameters can be controlled by the Bernoulli and continuity equations for fluid flow: $$ \frac{P}{\rho g} + \frac{v^2}{2g} + z = \text{constant} $$ and $$ Q = A_1 v_1 = A_2 v_2 $$ where proper gating ratios (\(A_{sprue}:A_{runner}:A_{gate}\)) are critical to minimize turbulence and aspiration.
Post-casting, the scrapers undergo a strict thermal cycle. First, they are subjected to a stress-relief anneal after shakeout to relieve internal casting stresses. Following rough machining, the critical Q&T treatment is applied: austenitization at 880°C ± 10°C in a controlled atmosphere furnace, followed by an oil quench and a low-temperature temper at 250°C ± 10°C. This standardized heat treatment ensures the replication of the optimal laboratory-developed properties in every production steel casting. Finally, the parts are finish-machined to precise dimensional tolerances and inspected.
The field performance of the new low-alloy wear-resistant steel casting scraper plates was monitored on a hard rock TBM drive. Compared to the previously used ZG30CrMnSiRe scrapers, the new material demonstrated a dramatic improvement. The service life before requiring replacement increased by approximately 60-70%. This translates directly into fewer operational stoppages, reduced inventory and logistics costs for spare parts, and a significant increase in machine availability (uptime). A simplified cost-benefit analysis over a 10km tunnel drive shows the substantial advantage:
| Parameter | Traditional Material | New Low-Alloy Steel Casting |
|---|---|---|
| Estimated Scraper Life (hours) | 150 | 250 |
| Number of Replacements | ~67 | ~40 |
| Total Replacement Downtime (hours)* | ~67 | ~40 |
| Cost of Scrapers | X | ~1.3X |
| Total Effective Cost (Replacement + Downtime) | High | Substantially Lower |
*Assuming ~1 hour per change-out.
In conclusion, the systematic development and optimization of a low-alloy wear-resistant steel casting for TBM scraper plates has proven highly successful. Through controlled chemistry design and a focus on heat treatment process variables, specifically identifying 880°C austenitization followed by 250°C tempering as the optimum, a material with an outstanding balance of hardness (≥52 HRC) and impact toughness (~17 J/cm²) was achieved. This property profile directly addresses the demanding abrasive-impact wear conditions in hard rock tunneling. The implementation of robust sand-casting and controlled heat treatment practices ensured the reliable translation of laboratory results into high-performance production components. Field validation confirmed a service life extension of over 60%, delivering compelling economic benefits through reduced downtime and lower total operating costs. This work underscores the critical role of tailored steel casting metallurgy and precision heat treatment in advancing the durability and efficiency of critical wear parts in heavy industrial machinery. Future work may explore further micro-alloying additions or advanced heat treatment cycles like austempering to push the performance envelope even further.