Effect of Preheating Temperature on Shield Tunneling Cutter Head Weld Overlay Microstructure and Properties

Literature Overview and Engineering Context

Shield tunneling machines (TBM) cutter heads are subjected to extreme abrasion and impact during underground excavation, necessitating hardfacing weld overlays to extend component service life. The weld overlay materials used are typically high-carbon, high-chromium alloys designed to achieve hardness levels exceeding 55 HRC. However, the base material of the cutter head is usually a low-carbon or low-alloy steel (such as Q345 or 42CrMo), which presents significant metallurgical challenges during welding due to the large difference in carbon equivalent and thermal conductivity between the base metal and overlay material.

This study systematically investigates the effect of preheating temperature on the microstructure, hardness, and mechanical properties of weld overlay layers deposited on TBM cutter heads. The research employs a range of preheating temperatures from 100°C to 400°C and evaluates the resulting weld metal microstructure, crack susceptibility, and service performance through a combination of metallographic analysis, hardness testing, and mechanical property evaluation.

Preheating Temperature and Microstructural Evolution

Preheating temperature is one of the most influential parameters in determining the cooling rate, solidification mode, and phase transformation behavior of the weld overlay. The study examines four preheating temperature levels: 100°C, 200°C, 300°C, and 400°C, and documents the corresponding microstructural changes in both the overlay layer and the heat-affected zone (HAZ).

The following table summarizes the key microstructural observations at different preheating temperatures:

Preheat Temp (°C)	Overlay Microstructure	HAZ Microstructure	Cooling Rate (K/s)	Crack Susceptibility
100	Fine martensite + retained austenite	Fine martensite, minimal grain growth	8–12	Low
200	Coarse martensite + retained austenite	Coarse martensite, moderate grain growth	5–8	Moderate
300	Coarse martensite + carbide network	Coarse grain, possible tempering	3–5	Moderate
400	Coarse martensite + excessive retained austenite	Significant grain growth, softening	1–3	High

The microstructural evolution follows a clear trend: increasing preheating temperature reduces the cooling rate, which promotes coarser martensite formation, increases retained austenite content, and accelerates grain growth in the HAZ. At 100°C preheat, the rapid cooling produces a fine, hard martensitic structure with minimal retained austenite, which is optimal for wear resistance. However, this condition also generates high residual stresses and increases the risk of cold cracking in the HAZ.

Phase Transformation Analysis

The phase transformation behavior in the weld overlay is governed by the cooling rate and the carbon equivalent of the base metal. At low preheating temperatures (100°C), the cooling rate exceeds the critical cooling rate for martensitic transformation, resulting in nearly complete martensite formation. The retained austenite content is typically 5–10%, which provides some toughening without significantly reducing hardness.

As preheating temperature increases to 200–300°C, the cooling rate decreases, and the retained austenite content rises to 15–25%. The retained austenite acts as a toughening phase but also reduces the effective hardness of the overlay. At 400°C preheat, the retained austenite content can exceed 30%, leading to a significant softening of the overlay layer and a corresponding reduction in wear resistance.

The HAZ microstructure is equally important. At 100°C preheat, the HAZ exhibits a fine martensitic structure with minimal grain growth, maintaining good toughness. However, the high cooling rate also increases the risk of cold cracking due to the high carbon equivalent of the base metal. At 400°C preheat, the HAZ grain size increases significantly (from ASTM No. 8 to No. 5 or coarser), which reduces the toughness and increases the susceptibility to intergranular cracking.

Mechanical Properties and Hardness Distribution

The mechanical properties of the weld overlay layer and HAZ were evaluated as a function of preheating temperature, with particular attention to hardness distribution, impact energy, and micro-hardness profiles.

The following table presents the key mechanical property data:

Preheat Temp (°C)	Overlay Hardness (HV)	HAZ Hardness (HV)	Impact Energy (J)	Surface Hardness (HRC)
100	780–820	350–380	25–35	62–65
200	720–760	320–350	30–40	58–62
300	680–720	280–320	35–45	55–58
400	620–660	250–280	40–50	50–55

The data reveals an inverse relationship between preheating temperature and overlay hardness, which is consistent with the microstructural observations. At 100°C preheat, the overlay achieves the highest hardness (780–820 HV, corresponding to 62–65 HRC), which is optimal for abrasion resistance. However, the impact energy is relatively low (25–35 J), indicating reduced toughness.

At 400°C preheat, the overlay hardness drops to 620–660 HV (50–55 HRC), representing a 20–25% reduction in wear resistance. While the impact energy increases to 40–50 J, this improvement in toughness comes at the expense of the primary function of the overlay, which is abrasion resistance.

Hardness Profiling Across the Overlay

The hardness distribution across the overlay thickness is not uniform and varies with preheating temperature. The study identifies three distinct regions:

Surface region (0–0.5 mm): Highest hardness due to rapid solidification and fine carbide precipitation
Mid-thickness region (0.5–1.5 mm): Moderate hardness with coarser carbide distribution
Near-interface region (1.5–2.5 mm): Lowest hardness due to base metal dilution and slower cooling

The hardness gradient is steeper at lower preheating temperatures, with a drop of 50–80 HV from surface to interface at 100°C preheat, compared to only 30–50 HV at 400°C preheat. This gradient affects the wear performance of the overlay, as the surface hardness is the primary determinant of abrasion resistance.

Crack Susceptibility and Preheating Temperature

Crack susceptibility is a critical consideration in TBM cutter head weld overlay, as cracks can lead to premature failure and catastrophic loss of the cutting edge. The study evaluates both cold cracking (hydrogen-induced cracking, HIC) and hot cracking susceptibility at different preheating temperatures.

Cold cracking is primarily governed by the carbon equivalent (CE) of the base metal, the hydrogen content, and the cooling rate. The preheating temperature directly influences the cooling rate and the diffusion of hydrogen from the weld zone. At 100°C preheat, the high cooling rate increases the susceptibility to cold cracking, particularly in the HAZ. The study recommends that when using 100°C preheat, the hydrogen content in the welding consumable should be limited to below 3 mL/100g, and the preheating should be supplemented with post-weld hydrogen bake-out at 200–250°C for 1 hour per 25 mm of weld thickness.

Hot cracking is less dependent on preheating temperature but is influenced by the retained austenite content and the thermal expansion mismatch between the overlay and base metal. At 400°C preheat, the increased retained austenite content can actually reduce hot cracking susceptibility by accommodating thermal strain, but the high temperature also increases the risk of HAZ grain growth and intergranular cracking.

The optimal preheating temperature is determined by balancing these competing factors. The study recommends a preheating temperature of 200–250°C for most TBM cutter head weld overlay applications, as this provides an acceptable balance between hardness (wear resistance) and crack resistance.

Engineering Practice Recommendations

Based on the study findings, the following process recommendations are provided for TBM cutter head weld overlay fabrication:

Preheating temperature: 200–250°C for general applications; 150–200°C for high-wear applications where hardness is prioritized
Interpass temperature: Maintained between 150–250°C to prevent excessive cooling rate between passes
Hydrogen control: Limit consumable hydrogen to below 3 mL/100g; use low-hydrogen flux or solid wire
Post-weld treatment: Hydrogen bake-out at 200–250°C for 1 hour per 25 mm of overlay thickness
Inspection: 100% magnetic particle testing (MT) of the overlay surface; spot ultrasonic testing (UT) for internal defects

The study emphasizes that the preheating temperature should be determined on a case-by-case basis, considering the base metal composition, overlay material, welding process, and service conditions. A systematic approach involving welding procedure qualification (WPQ) per NB/T 47014 or equivalent standards, followed by production monitoring and periodic requalification, is recommended to ensure consistent quality.

The research underscores that preheating temperature is not merely a parameter to be set arbitrarily but a critical variable that must be optimized through a combination of metallurgical understanding, process qualification, and empirical validation. The optimal preheating temperature is the one that achieves the required hardness while maintaining adequate crack resistance and mechanical integrity, and this balance must be established through rigorous testing and documentation for each specific application.