Effect of Post-Weld Heat Treatment on Overlay Microstructure and Mechanical Properties of 45 Steel Substrate Molds

Overview and Technical Context

Post-weld heat treatment (PWHT) is a critical process step in the cladding and repair of tool and die components, particularly those fabricated from medium-carbon steel substrates such as 45 steel (equivalent to AISI 1045 or S45C). The overlay layer applied to 45 steel molds serves to restore worn surfaces, improve wear resistance, and extend component service life. However, the welding process inevitably introduces residual stresses, microstructural transformations in both the cladding layer and the heat-affected zone (HAZ), and potential defects that can compromise the long-term performance of the repaired component. Understanding the effects of PWHT on these parameters is essential for ensuring that the repaired mold meets the required mechanical properties and service life expectations.

The 45 steel substrate presents specific challenges for cladding operations because of its medium carbon content (0.42-0.50% C), which makes it susceptible to hardening and cracking during welding. The carbon equivalent of 45 steel (CE = 0.42 + 0.40 + 0.12 + 0.03 = approximately 0.57%) places it in the high-hardenable range, requiring careful control of preheat and post-weld cooling rates to prevent hydrogen-induced cracking (HIC) and martensitic transformation in the HAZ.

Core Technical Content

Microstructural Evolution Without Post-Weld Treatment

The as-welded microstructure of a typical overlay layer on 45 steel consists of a complex arrangement of phases that depends on the cladding material composition, welding process parameters, and cooling rate. For a common high-chromium cast iron or hardfacing alloy overlay, the as-welded microstructure typically includes:

• Tempered martensite matrix with dispersed carbides (in the overlay layer)
• Retained austenite (5-15% depending on composition and cooling rate)
• Fine eutectic carbide networks at grain boundaries
• A dilution zone at the overlay-base metal interface where the composition transitions between the cladding alloy and the 45 steel substrate

The HAZ in the 45 steel substrate undergoes a thermal cycle that can produce a variety of microstructures depending on the peak temperature and cooling rate. Regions experiencing peak temperatures above the Ac3 temperature undergo complete austenitization and subsequent transformation to martensite or bainite upon cooling, resulting in hardness values of 40-55 HRC. Regions experiencing peak temperatures between Ac1 and Ac3 undergo partial austenitization, producing a mixed microstructure of untransformed ferrite-pearlite and newly formed martensite or bainite.

The residual stress state in the as-welded condition is compressive in the overlay layer near the surface and tensile in the HAZ and base metal beneath the cladding. The magnitude of these residual stresses can reach 300-500 MPa, which is sufficient to cause distortion, cracking, or premature fatigue failure in service. The residual stress distribution is influenced by the thermal expansion mismatch between the overlay layer and the substrate, the拘束 (constraint) of the surrounding base metal, and the cooling rate during solidification.

Effects of Solution Treatment

Solution treatment involves heating the entire component to a temperature above the Ac3 temperature of the 45 steel substrate (typically 830-880°C) and holding for a sufficient duration to homogenize the microstructure, followed by controlled cooling. For 45 steel, the Ac3 temperature is approximately 800°C, so solution treatment temperatures of 830-880°C ensure complete austenitization of the substrate HAZ.

The primary effects of solution treatment on the overlay microstructure include:

• Dissolution of secondary carbides in the overlay layer, producing a homogeneous austenitic or martensitic structure depending on the alloy composition
• Grain refinement in the HAZ through complete austenitization and controlled recrystallization
• Relief of residual stresses through stress relief mechanisms activated at elevated temperatures
• Potential sensitization of stainless steel overlay layers if the treatment temperature is within the sensitization range (450-850°C for 304/316 stainless steels)

The hardness of the overlay layer after solution treatment typically decreases by 5-15 HRC compared to the as-welded condition due to carbide dissolution and grain coarsening. However, this hardness reduction is often accompanied by improved toughness and fatigue resistance, which are more critical for mold applications subject to impact loading and cyclic stress.

Effects of Quenching and Tempering

The quenching and tempering cycle applied after solution treatment is the most effective method for achieving the desired balance of hardness, toughness, and wear resistance in the overlay layer and HAZ. The quenching medium, quenching rate, and tempering temperature are critical parameters that must be optimized for the specific overlay alloy and application requirements.

The tempering temperature selection is governed by the required balance between hardness and toughness. Lower tempering temperatures (450-500°C) preserve higher hardness values but result in reduced toughness and increased susceptibility to temper brittleness. Higher tempering temperatures (550-650°C) produce lower hardness but significantly improved toughness and reduced residual stress. For mold applications subject to impact loading, tempering temperatures of 550-600°C are generally recommended to achieve an optimal balance of properties.

Effects of Normalizing

Normalizing treatment involves heating the component to 780-850°C (above Ac3 for 45 steel) and air cooling. This treatment produces a refined pearlite-ferrite microstructure in the HAZ and promotes carbide coarsening and spheroidization in the overlay layer. Normalizing is less aggressive than quenching and tempering but provides effective stress relief and microstructural homogenization for applications where extreme hardness is not required.

The normalizing treatment is particularly suitable for overlay layers composed of high-chromium cast irons or nickel-based alloys where the as-welded microstructure contains excessive retained austenite or where the HAZ hardness exceeds the acceptable limit. Normalizing reduces the HAZ hardness from 45-55 HRC (as-welded) to 25-35 HRC (normalized), significantly reducing the risk of cracking during subsequent machining or service.

Process Parameter Optimization

Heating Rate and Soak Time

The heating rate during PWHT must be controlled to prevent thermal shock and distortion of the mold component. For thick-section molds (wall thickness > 50 mm), heating rates of 50-80°C/h are recommended, while thinner sections can tolerate rates of 100-150°C/h. The soak time at temperature should be sufficient to achieve thermal equilibrium throughout the component cross-section, typically calculated as 1 hour per 25 mm of maximum section thickness, with a minimum soak time of 2 hours.

The selection of heating rate is critical for preventing thermal stresses that can cause distortion or cracking during the PWHT cycle. Rapid heating of thick-section molds can produce differential thermal expansion between the surface and the core, generating tensile stresses at the surface that may exceed the material's yield strength at the elevated temperature. This can result in creep deformation, surface cracking, or distortion of critical mold geometry.

Cooling Rate Control

The cooling rate during PWHT is equally important and must be selected based on the desired final microstructure and the risk of cracking or distortion. Air cooling from solution treatment temperatures produces a martensitic transformation in the HAZ, resulting in high hardness but also high residual stress. Oil quenching produces a similar but slightly slower cooling rate, which can reduce the risk of quench cracking while still achieving acceptable hardness. Furnace cooling (slow cooling) produces a pearlitic or bainitic microstructure with lower hardness but significantly reduced residual stress and improved dimensional stability.

For mold repair applications where dimensional accuracy is critical, furnace cooling is often preferred despite the lower resulting hardness because the reduced distortion preserves the mold geometry and eliminates the need for post-PWHT re-machining. For applications where maximum hardness is the priority and dimensional tolerance is less critical, oil quenching followed by tempering is the preferred approach.

Defect Analysis and Countermeasures

The PWHT process can introduce or exacerbate certain defects if not properly controlled. Understanding these defect mechanisms and implementing appropriate countermeasures is essential for producing high-quality repaired molds.

The overlay spalling defect is particularly relevant to 45 steel mold repair because the thermal expansion coefficient mismatch between the overlay layer and the substrate can generate significant interfacial stresses during the PWHT cycle. If the overlay layer has a lower thermal expansion coefficient than the substrate (as is typical for hardfacing alloys), cooling from the PWHT temperature produces compressive stress in the overlay layer and tensile stress at the interface, which can lead to delamination or spalling if the bond strength is insufficient.

Preventing overlay spalling requires careful control of the PWHT temperature to avoid exceeding the bond strength limit, the use of adequate preheating before welding to reduce the initial residual stress state, and the selection of overlay alloys with thermal expansion coefficients that are compatible with the 45 steel substrate. In cases where spalling is observed, the affected area must be ground away and re-cladded with a compatible alloy before reapplying PWHT.

Engineering Practice Considerations

In the context of mold repair operations, the PWHT procedure must be integrated into a comprehensive repair plan that includes surface preparation, cladding process selection, welding parameter optimization, and post-weld inspection. The repair plan should be developed in accordance with the applicable standards and the specific requirements of the mold application.

For injection molding molds, the PWHT procedure must ensure that the overlay layer hardness is compatible with the mold steel substrate and that the residual stress state does not compromise the dimensional stability of the mold during repeated heating and cooling cycles in service. The typical service life of a properly repaired mold is 80-90% of the original mold life, which represents a significant economic benefit compared to complete mold replacement.

For forging and pressing molds, the PWHT procedure must ensure that the overlay layer maintains sufficient toughness to resist impact-induced cracking during the forming operation. The Charpy V-notch impact energy of the overlay layer at the service temperature should be at least 25-30 J to ensure adequate resistance to brittle fracture under impact loading.

Key Technical Reflections

The study of PWHT effects on 45 steel mold overlay layers reveals that the heat treatment process is not merely a stress relief operation but a fundamental microstructural engineering tool that can be used to optimize the balance of hardness, toughness, and wear resistance in the repaired component. The key insight is that the PWHT parameters must be selected based on a systematic evaluation of the specific application requirements, the overlay alloy composition, and the substrate condition, rather than applying a generic heat treatment procedure to all repair scenarios.

The integration of PWHT into the mold repair workflow requires careful coordination between the welding, heat treatment, and inspection departments to ensure that the entire repair process is optimized for the desired final properties. This coordination is facilitated by the use of standardized procedure qualification testing in accordance with NB/T 47014 or ASME IX, which provides a documented basis for the PWHT parameters and acceptance criteria.

The long-term reliability of repaired molds depends on the quality of the PWHT operation, and investment in proper heat treatment equipment, instrumentation, and personnel training is essential for achieving consistent repair quality. The economic case for proper PWHT is compelling when the cost of a failed repair (including downtime, replacement parts, and lost production) is compared to the relatively modest cost of a properly executed heat treatment cycle.