Dissimilar High-Strength Steel TIG Welding of Unequal Thickness B340LA/B1500HS Plates

Literature Overview

This paper by Li Binpo, Liu Peng, Zhao Baozhong, and Xu Wentao (Shandong Electric Power Engineering Consulting Institute and Shandong Jianzhu University, 2014) investigates the weld formation, microstructure, and hardness of TIG-welded joints between unequal-thickness dissimilar high-strength steel plates: B340LA (a low-alloy steel) and B1500HS (a high-strength steel). The research was funded by the Shandong Provincial Outstanding Young and Middle-aged Scientists Research Award Fund. Published in Electrician (Vol. 44, No. 8, pp. 40–43), the study addresses a practical challenge in nuclear power plant construction, where dissimilar steel joints of unequal thickness are common in structural and piping components.

Core Technical Content

Material Characteristics

Property	B340LA	B1500HS
Yield Strength (MPa)	≥340	≥1500
Tensile Strength (MPa)	490–630	≥1500
Carbon Equivalent (Ceq)	~0.40	~0.65
Microstructure	Ferrite-pearlite	Quenched and tempered martensite
Application	General structural	High-pressure piping, reactor internals

The large difference in strength and carbon equivalent between these two materials creates significant challenges for welding, including differential thermal contraction, hydrogen-induced cracking susceptibility, and microstructural incompatibility at the fusion boundary.

Weld Formation and Microstructure

The study demonstrates that TIG welding can produce sound weld joints with good external appearance and no obvious defects. The weld metal microstructure is primarily composed of pearlite and ferrite. However, at higher welding speeds, a significant amount of lath martensite appears in the weld metal, which dramatically increases the hardness.

Hardness vs. Welding Speed

Welding Speed	Weld Metal Hardness (HV)	Dominant Microstructure
Low speed	~550	Ferrite + pearlite
High speed	~1200	Lath martensite + ferrite + pearlite

This dramatic increase in hardness with welding speed is attributed to the reduced heat input at higher speeds, which results in higher cooling rates and the formation of hard, brittle martensitic phases in the weld metal.

Dissimilar Steel Welding Challenges

Welding dissimilar high-strength steels of unequal thickness presents several unique challenges:

Thermal mismatch: The unequal thickness causes asymmetric heat flow, leading to weld misalignment and angular distortion.
Differential hardness: The high-strength steel side has inherently higher hardness, creating stress concentrations at the fusion boundary.
Hydrogen cracking: Both steels have elevated carbon equivalents, increasing susceptibility to cold cracking, particularly in the heat-affected zone of the high-strength steel.
Microstructural incompatibility: The transition from martensitic base metal to ferrite-pearlite weld metal creates a steep hardness gradient.
Residual stress: The combination of differential thermal expansion and thickness mismatch generates complex residual stress fields.

Process Optimization for Dissimilar Steel TIG Welding

Preheat: Minimum 150–200°C for both materials to reduce cooling rate and prevent hydrogen cracking.
Filler metal selection: A filler metal with carbon equivalent intermediate between the two base metals (e.g., ER80S-D2 or ER90S-D2) should be used to balance strength and toughness.
Welding direction: Weld from the high-strength steel side first to minimize the effect of differential contraction.
Travel speed control: Moderate travel speed is essential—too slow causes excessive heat input and grain growth, while too fast produces brittle martensite.
Interpass temperature: Maintain 150–250°C to prevent cold cracking without causing excessive softening.
Post-weld heat treatment: Stress relief at 550–650°C to reduce residual stresses and temper any martensitic phases.

Microstructural Analysis and Quality Assessment

The presence of lath martensite at high welding speeds is a critical finding. Martensite in weld metal of high-strength steels is generally undesirable because it has low toughness and high susceptibility to hydrogen cracking. The hardness of HV1200 is well above the threshold for acceptable weld metal hardness in most pressure vessel codes (typically limited to HV350 for carbon and low-alloy steels).

Defect Analysis

Defect Type	Cause	Countermeasure
Cold cracking (HIC)	High Ceq, fast cooling, hydrogen	Preheat, low-hydrogen filler, PWHT
Hardness exceedance	Martensite formation at high speed	Reduce travel speed, increase heat input
Angular distortion	Thickness mismatch, asymmetric heat flow	Backing plate, clamping fixtures
Fusion defect at thick side	Insufficient penetration on thick side	Adjust torch angle, increase current
Undercut at thin side	Excessive heat input on thin side	Reduce current, increase travel speed on thin side

Engineering Practice Implications

In nuclear power plant construction, dissimilar steel joints of unequal thickness are unavoidable due to the need to connect different components with varying pressure ratings and structural requirements. The findings of this study have direct implications for welding procedure qualification (WPQ) in nuclear service:

ASME Section III compliance: Welding procedures for dissimilar steel joints must include specific qualification tests for each material combination, including tensile tests with test coupons taken from the weld and HAZ of each base metal.
NQA-1 quality requirements: Welders must demonstrate proficiency in dissimilar steel welding, and all consumables must be qualified per NQA-1.
In-service inspection: Dissimilar steel joints in nuclear service require enhanced inspection programs due to the potential for differential corrosion and stress corrosion cracking.

Critical Reflections

The study's primary contribution is demonstrating that TIG welding can produce sound joints between these dissimilar high-strength steels, which is encouraging for engineering applications. However, the hardness of HV1200 at high welding speeds is a serious concern that the paper does not adequately address. This hardness level is incompatible with most code requirements and would likely result in weld rejection under ASME or EN codes. The paper should have provided more detailed discussion of the implications of this finding and recommended process windows that limit hardness to acceptable levels.

Additionally, the paper does not report on toughness (Charpy V-notch impact energy), which is a critical property for dissimilar steel joints in nuclear service. The absence of creep or fatigue data is also a limitation, given that these joints may be subjected to cyclic loading during reactor operation.

Study Insights and Implications

This research highlights the fundamental challenge of welding dissimilar high-strength steels: the process parameters that produce sound weld formation may not necessarily produce acceptable mechanical properties. Engineers must carefully balance penetration requirements with hardness and toughness requirements, often necessitating multi-pass welding with different parameters for each pass. The finding that welding speed directly controls the transition from ductile ferrite-pearlite to brittle martensitic microstructure provides a practical tool for process optimization, but it must be used within the constraints of code-specified hardness limits. For nuclear power plant applications, where safety margins are paramount, the recommended approach would be to adopt conservative welding parameters that ensure hardness remains below HV350, even at the cost of increased welding time and higher heat input. Future work should focus on developing filler metals specifically designed for B340LA/B1500HS dissimilar joints, with compositions that promote ferrite-pearlite microstructure even at high cooling rates, thereby decoupling penetration from hardness.