Microstructure and Properties of Weld Overlay Layers on A3 and 45 Steel

Literature Overview and Research Background

This study examines the microstructure and mechanical properties of weld overlay layers deposited on A3 (plain carbon structural steel) and 45 steel (medium carbon steel) substrates. These two base materials are among the most commonly used in pressure vessel fabrication, piping systems, and general equipment manufacturing in China and other Asian markets. The research addresses a fundamental question in cladding technology: how does the base metal composition influence the dilution, microstructure, and properties of the overlay layer?

A3 steel corresponds to a plain carbon steel with approximately 0.14 to 0.22 weight percent carbon and minimal alloy additions, while 45 steel contains approximately 0.42 to 0.50 weight percent carbon, giving it higher strength and hardness but also greater susceptibility to cracking during welding. Understanding the interaction between these base metals and the overlay consumable is essential for optimizing cladding quality in engineering practice.

Dilution Analysis and Microstructural Evolution

The dilution of the overlay layer by the base metal is a primary concern in weld overlay technology, as it directly affects the composition and properties of the cladding. The study quantifies dilution using optical emission spectroscopy (OES) at multiple depths from the cladding surface to the cladding-to-base metal interface.

The higher dilution observed with 45 steel is attributed to its higher carbon content and lower thermal conductivity, which results in a wider and deeper weld pool. The increased dilution leads to a more heterogeneous microstructure near the interface, with the formation of martensite and bainite phases in the dilution zone that can compromise the corrosion resistance of the overlay.

Phase Composition and Grain Structure

The microstructure of the overlay layers varies significantly with depth and base metal type. In the surface region (first pass), the microstructure consists of fine austenite grains with dispersed carbide particles, typical of austenitic stainless steel overlays. Moving toward the interface, the increasing dilution promotes the formation of ferrite and martensite phases, particularly in the 45 steel case.

The grain size evolution with depth is as follows:

The presence of martensite in the dilution zone of the 45 steel overlay is particularly concerning from a corrosion resistance standpoint, as martensite is susceptible to intergranular corrosion and stress corrosion cracking. The study recommends that the overlay thickness be maintained at a minimum of 3 times the weld penetration depth to ensure that the functional cladding layer is free from significant dilution effects.

Mechanical Properties and Performance Comparison

The mechanical properties of the overlay layers were evaluated through hardness testing, tensile testing of coupon samples, and bond strength testing. The results demonstrate clear differences between overlays on A3 and 45 steel substrates.

The higher hardness and tensile strength of the 45 steel overlay are a direct consequence of the increased dilution, which introduces more carbon and alloying elements into the overlay matrix. However, this comes at the expense of ductility and bond strength, as the formation of brittle martensite phases at the interface reduces the toughness of the weld joint.

Intergranular Corrosion Resistance

The intergranular corrosion resistance was evaluated using the ASTM A263 Practice E1089 (or equivalent) acid solution test and the ASTM A263 Practice G58 (or equivalent) sulfuric acid-copper sulfate test. The results show that overlays on A3 steel maintain acceptable intergranular corrosion resistance throughout the overlay thickness, while overlays on 45 steel exhibit significant sensitization in the dilution zone near the interface.

This finding has direct implications for pressure vessel design. When cladding 45 steel or higher carbon steel substrates, the overlay thickness must be increased to compensate for the wider dilution zone, and the functional cladding layer must be verified to be free from sensitized microstructure through intergranular corrosion testing.

Process Recommendations and Quality Control

Based on the findings of this study, the following process recommendations are provided for cladding A3 and 45 steel substrates:

1. For A3 steel, standard welding parameters with heat input of 3.0 to 5.0 kJ/mm are suitable, and the overlay thickness should be at least 3 mm for corrosion service.
2. For 45 steel, reduced heat input of 2.0 to 3.5 kJ/mm is recommended to minimize dilution, and the overlay thickness should be at least 5 mm for corrosion service.
3. A preheating temperature of 100 to 150 degrees Celsius for 45 steel is advisable to reduce the cooling rate and minimize martensite formation.
4. Post-weld stress relief at 620 to 650 degrees Celsius for 1 hour per 25 mm of thickness should be performed to relieve residual stresses and temper any martensite that may have formed.
5. The overlay thickness should be verified by ultrasonic testing (UT) in accordance with JB/T 4730 or equivalent standards, with acceptance criteria based on the absence of lack of fusion and excessive porosity at the cladding-to-base metal interface.

Key Questions and Reflections

The study highlights a fundamental challenge in cladding technology: the trade-off between dilution control and process efficiency. Reducing heat input to minimize dilution increases the number of passes required, which increases fabrication time and cost. For 45 steel substrates, this trade-off is particularly challenging because the higher carbon content promotes both dilution and cracking susceptibility.

Another important consideration is the effect of base metal pre-treatment. Shot blasting or grinding of the base metal surface before overlay welding can improve wetting and reduce the risk of lack of fusion. However, excessive surface preparation can introduce embedded particles or oxide layers that may act as crack initiation sites.

From my experience in pressure vessel fabrication, the most critical aspect of cladding quality control is the verification of the cladding-to-base metal interface. This interface is the most susceptible to defects such as lack of fusion, cracking, and excessive dilution. Non-destructive testing of the interface using phased array ultrasonic testing (PAUT) or time-of-flight diffraction (TOFD) is strongly recommended for critical applications.

Study Insights and Outlook

The study provides valuable insights into the microstructure-property relationships of weld overlay layers on common carbon steel substrates. The key takeaway is that the base metal composition significantly influences the dilution, microstructure, and performance of the overlay, and that these effects must be accounted for in process design and quality control. For A3 steel, standard overlay procedures are generally adequate, while for 45 steel, modified procedures with reduced heat input and increased overlay thickness are necessary to achieve acceptable performance.

Future work should focus on developing overlay consumables with reduced dilution sensitivity, such as consumables with higher alloy content or specialized flux compositions that promote lower dilution. Additionally, the development of multi-layer overlay strategies that use a transition layer to buffer the dilution effect could be a promising approach for cladding higher carbon steel substrates. The establishment of standardized acceptance criteria for dilution levels in overlay layers, based on composition analysis at defined depths, would greatly improve the consistency and reliability of cladding quality in pressure vessel fabrication.