Microstructure and Properties of BHW35 Steel Hot-Wire TIG Weld Overlay Joint

Literature Overview

This 2013 publication by Wu Guangfeng from Chongqing University of Technology investigates the microstructure and mechanical properties of a hot-wire TIG (HWT) weld overlay joint deposited on BHW35 steel. The research was supported by the Chongqing Higher Education Outstanding Achievement Transformation Major Project (KJZH11215) and was published in the journal Welding. BHW35 is a high-strength low-alloy (HSLA) steel with a nominal yield strength of approximately 350 MPa, widely used in structural applications, pressure vessels, and pipelines. The study examines the HWT process as a method for depositing wear-resistant or corrosion-resistant overlay layers on BHW35 steel substrates, addressing the unique metallurgical challenges associated with this combination.

Core Technical Points

Hot-wire TIG (HWT) welding is a variant of gas tungsten arc welding in which the filler wire is preheated by a high-frequency current or a separate heating element before entering the arc. This preheating increases the wire melting rate by 30–50% compared to conventional GTAW, resulting in higher deposition rates, lower heat input per unit of deposited metal, and reduced dilution of the base material. The HWT process is particularly advantageous for weld overlay applications where low dilution and high productivity are required.

The BHW35 steel substrate presents specific challenges for weld overlay. Its relatively high carbon equivalent (CE ≈ 0.45–0.55%) makes it susceptible to cold cracking in the heat-affected zone. The preheating requirement for BHW35 welding (typically 100–200 °C) must be maintained during the overlay process. The overlay material selection must ensure adequate bond strength with BHW35 while providing the desired functional properties (wear resistance, corrosion resistance, or thermal fatigue resistance).

Microstructural Analysis

The microstructure of the HWT weld overlay joint on BHW35 steel is characterized by three distinct zones: the heat-affected zone (HAZ) in the base metal, the weld metal, and the dilution zone at the interface. In the HAZ of BHW35, the microstructure typically consists of fine-grained martensite and bainite, with the grain size depending on the peak temperature and cooling rate. The cooling rate in the HAZ for HWT is generally lower than for conventional GTAW due to the higher deposition rate and reduced heat input per unit length.

The weld metal microstructure depends on the filler material composition. For a typical low-alloy steel filler, the weld metal consists of a mixed ferrite-pearlite microstructure with some retained austenite. For a higher-alloy filler (such as a Cr-Mo or Ni-Cr alloy), the microstructure may include martensite, austenite, and carbide precipitates. The dilution zone at the substrate-overlay interface is a critical region where the composition transitions from the base metal to the overlay composition. This zone typically exhibits a gradient in microstructure and properties, with the highest hardness often occurring at the interface due to the combined effect of base metal dilution and overlay alloying.

The hot-wire effect on the microstructure is primarily manifested in the reduced cooling rate and increased dilution control. The preheated wire melts more rapidly, resulting in a larger molten pool and slower solidification rate. This promotes the formation of finer, more equiaxed grains in the weld metal compared to conventional GTAW. The reduced dilution rate (10–25% versus 25–40% for conventional GTAW) ensures that the overlay composition is closer to the intended filler composition, which is critical for achieving the target mechanical and functional properties.

Engineering Practice and Process Qualification

The HWT process for weld overlay on BHW35 steel requires careful qualification per applicable standards. For pressure vessel applications, the qualification procedure must comply with NB/T 47014 or ASME IX, depending on the governing code. The qualification includes deposition of test coupons, metallographic examination, hardness profiling, tensile testing of the overlay, bond strength testing, and impact testing of the HAZ.

In practice, the HWT process is most commonly used for overlay applications where the overlay thickness is 2–8 mm and the substrate is a flat or mildly curved surface. The process is less suitable for heavily curved surfaces or complex geometries where the torch and wire positioning become difficult. For thick overlays (>8 mm), multi-pass welding with interpass grinding is required, and the HWT process may be combined with other processes such as SAW for the intermediate passes and GTAW or HWT for the cap passes.

Quality control for HWT overlay joints includes visual inspection for surface defects, magnetic particle inspection for surface cracks, ultrasonic testing for subsurface defects, and metallographic examination for microstructure and dilution verification. The overlay must also pass a hydrostatic pressure test or pneumatic tightness test if it is part of a pressure-retaining component.

This study provides valuable insights into the microstructural evolution and mechanical performance of HWT weld overlay joints on BHW35 steel. The hot-wire TIG process offers a compelling combination of low dilution, high deposition rate, and good process control for weld overlay applications. Engineers considering HWT for production welding should invest in process qualification and operator training to ensure consistent quality. The findings underscore the importance of substrate preheating, interpass temperature control, and post-weld heat treatment in preventing cracking and achieving the target mechanical properties of the overlay joint.