CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Treatment on Microstructure and Mechanical Properties of Q345B Weld Overlay Composite Plate
Introduction
Q345B is a widely used low-carbon low-alloy structural steel in China, characterized by good weldability, formability, and mechanical properties. Weld overlay composite plates based on Q345B are extensively used in pressure vessels, pipelines, and structural components where a combination of strength and corrosion resistance is required. The heat treatment of these composite plates is a critical process step that significantly influences the microstructure, mechanical properties, and long-term service performance of both the overlay layer and the base metal. This study note examines the effects of various heat treatment cycles on Q345B-based weld overlay composite plates.
Microstructural Evolution During Heat Treatment
The microstructure of Q345B steel consists primarily of ferrite and pearlite, with a small amount of acicular ferrite depending on the rolling and cooling conditions. The weld overlay layer, typically composed of stainless steel such as 304, 316, or 321, has an austenitic or ferritic-austenitic microstructure. The heat treatment of the composite plate induces microstructural changes in three distinct regions: the base metal heat-affected zone (HAZ), the overlay layer, and the interface transition zone.
Base Metal Heat-Affected Zone
The HAZ of Q345B steel is sensitive to heat treatment conditions. During normalizing heat treatment at 850-900 °C, the ferrite-pearlite microstructure transforms to austenite and then re-forms upon cooling. The cooling rate from the normalizing temperature determines the final microstructure:
- Slow cooling (furnace cooling): Produces coarse pearlite and proeutectoid ferrite, resulting in lower strength but higher toughness.
- Moderate cooling (air cooling): Produces finer pearlite with some acicular ferrite, providing a good balance of strength and toughness.
- Fast cooling (water quenching): Produces martensite and bainite, resulting in high strength but reduced toughness and increased residual stresses.
For Q345B, which has a carbon equivalent (CE) of approximately 0.35-0.45, the risk of quench cracking is relatively low but not negligible, especially in thick sections. The normalizing treatment is the most commonly recommended heat treatment for Q345B weld overlay composite plates.
Overlay Layer Microstructure
The overlay layer microstructure is also affected by heat treatment. For austenitic stainless steel overlays (304, 316), heat treatment above 450 °C can cause sensitization, where chromium carbides precipitate at grain boundaries, reducing corrosion resistance. The sensitization risk increases with increasing carbon content and heat treatment temperature.
For 321 or 347 stainless steel overlays, which are stabilized with titanium or niobium, the sensitization risk is significantly reduced because the stabilizing elements preferentially form carbides instead of chromium carbides. However, prolonged exposure at temperatures above 600 °C can lead to sigma phase formation, which reduces toughness and corrosion resistance.
| Heat Treatment | Temperature (°C) | Cooling Method | Base Metal Effect | Overlay Effect |
|---|---|---|---|---|
| Normalizing | 850-900 | Air cooling | Refines ferrite-pearlite | Minimal change for austenitic SS |
| Stress relief | 550-650 | Furnace cooling | Reduces residual stresses | Possible sensitization for 304/316 |
| Solution treatment | 1050-1100 | Water quenching | Not applicable to composite | Dissolves carbides, restores corrosion resistance |
| Annealing | 650-750 | Furnace cooling | Softens base metal | Relieves overlay stresses |
Mechanical Property Analysis
The mechanical properties of Q345B weld overlay composite plates are strongly influenced by heat treatment. The key mechanical properties to consider include tensile strength, yield strength, elongation, impact toughness, and hardness.
Tensile and Yield Strength
Normalizing treatment generally increases the yield strength of Q345B from the typical 345 MPa to 400-500 MPa, depending on the cooling rate and microstructure refinement. The tensile strength typically increases from 470-630 MPa to 550-700 MPa after normalizing. However, excessive strength increases can reduce ductility and toughness, which may be detrimental for pressure vessel applications.
The overlay layer tensile properties are less affected by heat treatment for austenitic stainless steels, as they retain their austenitic structure over a wide temperature range. However, the overlay hardness can decrease slightly after stress relief treatment due to the softening effect of grain growth and carbide dissolution.
Impact Toughness
Impact toughness is particularly important for pressure vessel applications, especially in low-temperature service. The Charpy V-notch impact energy of Q345B steel is typically 34 J at -20 °C in the as-rolled condition. Normalizing treatment can improve the impact toughness by refining the pearlite colony size and promoting acicular ferrite formation.
The impact toughness of the overlay layer is generally high for austenitic stainless steels due to their face-centered cubic crystal structure, which provides excellent ductility even at cryogenic temperatures. However, the interface region may exhibit reduced toughness due to the formation of a martensitic transition zone, which can be embrittled by excessive carbon diffusion from the base metal.
Hardness Distribution
The hardness distribution across the composite plate is a critical quality indicator. The base metal hardness of Q345B is typically 120-180 HV, while the overlay layer hardness depends on the specific alloy composition. For 304 stainless steel, the hardness is typically 150-200 HV in the annealed condition and 200-250 HV after cold working.
The hardness gradient across the interface should be gradual to avoid stress concentrations. A sharp hardness transition can lead to cracking under cyclic loading. Heat treatment can help to homogenize the hardness distribution by promoting diffusion and reducing residual stresses.
Heat Treatment Cycle Optimization
The optimization of heat treatment cycles for Q345B weld overlay composite plates requires balancing multiple objectives:
- Base metal strength and toughness: Normalizing improves both strength and toughness but must be controlled to avoid excessive strength that reduces ductility.
- Overlay corrosion resistance: Heat treatment temperatures must be kept below the sensitization range for unstabilized austenitic stainless steels (304, 316), or the overlay must be solution treated separately.
- Residual stress reduction: Stress relief is essential for reducing welding residual stresses that can cause distortion, cracking, and reduced fatigue life.
- Interface bond quality: The heat treatment must not degrade the metallurgical bond at the interface.
A recommended heat treatment sequence for Q345B/304 composite plates is:
- Solution treatment of the overlay: 1050-1100 °C for 30 minutes, followed by water quenching. This step is performed before welding to prepare the overlay material.
- Normalizing of the composite plate: 860-890 °C for 30 minutes per 25 mm thickness, followed by air cooling. This refines the base metal microstructure and reduces residual stresses.
- Stress relief: 550-600 °C for 1 hour per 25 mm thickness, followed by furnace cooling. This step reduces residual stresses in the overlay and interface region.
For composite plates with stabilized stainless steel overlays (321, 347), the sensitization concern is reduced, and a simpler two-step heat treatment (normalizing followed by stress relief) is acceptable.
Defect Analysis and Countermeasures
Heat treatment can introduce or reveal defects in Q345B weld overlay composite plates. The following defects should be monitored:
- Sensitization of the overlay: Chromium carbide precipitation at grain boundaries reduces corrosion resistance. Countermeasure: use stabilized overlays or limit heat treatment temperature.
- Sigma phase formation: Long exposure at 600-900 °C can form brittle sigma phase in the overlay. Countermeasure: limit holding time at elevated temperatures.
- Cracking in the base metal HAZ: Quench cracking can occur if the cooling rate is too fast. Countermeasure: use controlled cooling rates and adequate preheating.
- Distortion: Uneven thermal expansion during heat treatment can cause warping. Countermeasure: use gradual heating and cooling rates, and support the plate during heat treatment.
- Decarburization: Exposure to high-temperature air can deplete carbon from the surface. Countermeasure: use protective atmosphere or coating during heat treatment.
Study Insights and Practical Recommendations
The study of heat treatment effects on Q345B weld overlay composite plates reveals that the heat treatment cycle must be tailored to the specific overlay material and service conditions. A one-size-fits-all approach is inappropriate and can lead to compromised performance or premature failure.
The key insight is that the heat treatment must be considered as an integral part of the fabrication process, not as a standalone step. The welding procedure, material selection, and heat treatment must be designed together to achieve the desired microstructure and mechanical properties. This integrated approach requires close collaboration between the welding engineer, materials engineer, and design engineer.
For pressure vessel applications governed by GB/T 150 or ASME VIII, the heat treatment must comply with the code requirements for the base material, and the overlay material must be verified for corrosion resistance after heat treatment. The engineer should conduct intergranular corrosion testing on heat-treated coupons to confirm that the overlay retains adequate corrosion resistance.
In conclusion, the heat treatment of Q345B weld overlay composite plates is a critical process step that significantly influences the microstructure, mechanical properties, and service performance. A systematic approach to heat treatment design, incorporating metallurgical principles, code requirements, and practical manufacturing constraints, is essential for producing high-quality composite plates for demanding pressure vessel applications.