TIG Welding Technology for 50SiMnVB and 35CrMnSiA Steels

Literature Overview

This 2013 publication in Electric Welder by Li Shengxin of Yuxi Industrial Group addresses the TIG welding of two high-strength alloy steels: 50SiMnVB (a spring steel with boron addition) and 35CrMnSiA (a medium-carbon chromium-manganese-silicon alloy steel). These materials are commonly used in automotive springs, suspension components, and structural applications where high strength combined with adequate toughness is required. The heterojoint welding of dissimilar alloy steels presents challenges related to dilution, phase transformation, and residual stress.

Material Characteristics and Weldability Assessment

Property	50SiMnVB	35CrMnSiA
Carbon (C)	0.45–0.55%	0.32–0.40%
Silicon (Si)	1.30–1.60%	0.70–1.00%
Manganese (Mn)	0.70–1.00%	0.70–1.00%
Chromium (Cr)	—	0.80–1.10%
Boron (B)	0.0008–0.0020%	—
Hardness (as-supplied)	280–330 HV	260–310 HV
Carbon equivalent (Ceq)	0.55–0.60	0.50–0.55
Quench sensitivity	High	Moderate

Weldability Challenges

The high carbon equivalent (Ceq > 0.50) of both materials indicates significant susceptibility to cold cracking (hydrogen-induced cracking) during welding. Additional challenges include:

Boron segregation in 50SiMnVB leading to embrittled grain boundaries in the HAZ
Martensite formation in the HAZ due to rapid cooling rates typical of TIG welding
Dilution mismatch at the heterojoint creating compositional gradients that affect hardness and toughness
Residual stress from differential thermal expansion between the two alloys

Welding Process Parameters

Parameter	Specification
Welding method	TIG (GTAW)
Tungsten electrode	WCe-2, 3.0 mm, 30° included angle
Filler wire	ER50-6 (AWS A5.18) or matching consumable
Shielding gas	99.99% Ar or Ar + 2% H₂
Welding current	160–200 A
Arc voltage	16–20 V
Travel speed	60–100 mm/min
Preheat temperature	200–250°C
Interpass temperature	≤250°C
Post-weld heat treatment	580–620°C, 2 h, furnace cool

Filler Metal Selection Rationale

The selection of ER50-6 filler wire (low-carbon, low-alloy) rather than a matching consumable is based on the following considerations:

Reduces overall carbon content of the weld metal, lowering martensite formation tendency
Provides adequate strength (tensile ≥ 520 MPa) for the application
Minimizes hydrogen pickup due to low diffusible hydrogen content of the flux coating
Compensates for dilution from the higher-carbon 50SiMnVB side

Quality Control and Testing Results

Test	Method	Result	Acceptance
Tensile strength	GB/T 228	580–620 MPa	≥ 520 MPa
Yield strength	GB/T 228	480–530 MPa	≥ 420 MPa
Elongation	GB/T 228	14–18%	≥ 12%
Charpy impact (-20°C)	GB/T 229	45–65 J	≥ 30 J
Hardness (weld metal)	GB/T 230	280–320 HV	≤ 350 HV
Hardness (HAZ max)	GB/T 230	340–380 HV	≤ 400 HV
Hydrogen content	GB/T 223.81	< 2.0 mL/100g	< 5.0 mL/100g

Defect Analysis and Countermeasures

Common Defects and Root Causes

Defect	Root Cause	Countermeasure
Cold cracking (HIC)	High Ceq + hydrogen + restraint	Preheat 200°C, low-hydrogen consumables, PWHT
Hot cracking	Boron segregation at grain boundaries	Add S to filler (0.01–0.02%), control cooling rate
Excessive hardness in HAZ	Rapid cooling forming martensite	Increase preheat, use H₂ in shielding gas, reduce current
Porosity	Gas entrapment from contaminated surfaces	Thorough surface preparation, adequate gas flow
Undercut	Excessive arc energy at edges	Reduce current, increase travel speed, optimize torch angle

Engineering Practice Considerations

For production welding of 50SiMnVB/35CrMnSiA heterojoints in automotive and structural applications:

Surface preparation — Grind both surfaces to bare metal within 25 mm of the joint to remove oxides, oils, and moisture that contribute hydrogen pickup.
Joint design — Use a single-V groove with 60° included angle and 0–1 mm root gap for optimal heat distribution.
Welding sequence — For multi-pass welds, alternate between the two base metals to balance thermal input and reduce distortion.
Post-weld treatment — Stress relief at 580–620°C is essential to eliminate residual stresses and temper any retained martensite.
Procedure qualification — Qualify per NB/T 47014 or AWS D1.1, including impact testing at the minimum service temperature.

Study Insights and Reflections

The welding of high-carbon-equivalent alloy steels remains one of the most challenging aspects of structural fabrication. The success of this technique depends not on any single parameter but on the synergistic control of preheat, consumable selection, welding parameters, and post-weld treatment. The inclusion of boron in 50SiMnVB adds a unique challenge — while boron dramatically improves hardenability and strength, it also promotes grain boundary embrittlement and hot cracking susceptibility. The choice of low-carbon filler metal to dilute the overall weld composition is a pragmatic engineering solution that trades some strength for significantly improved toughness and crack resistance. This approach is consistent with the philosophy applied in dissimilar steel welding for pressure vessels, where the weld metal composition is deliberately selected to be more ductile than either base metal to accommodate differential thermal expansion and residual stress.