How to Select the Welding Flux
Created on 10.20
I. Fundamental Concepts of Flux
Flux is an auxiliary chemical substance used during welding. It removes impurities such as oxide films and oil residues from the surfaces of base metal and filler metal, while forming a protective layer to isolate the joint from air and prevent re-oxidation. It also lowers the melting point and surface tension of the filler metal to enhance its fluidity, ultimately contributing to improved weld quality. By manufacturing process, fluxes are primarily categorised into three types: fused, sintered, and bonded. By application, they are classified as submerged arc welding flux, surfacing welding flux, and electroslag welding flux. Alternatively, they may be categorised by welded material: - Fluxes for low-carbon steel - Fluxes for low-alloy steel - Fluxes for stainless steel - Fluxes for nickel and nickel alloys - Fluxes for titanium and titanium alloys These find extensive application across electronics, machinery, and other sectors.
II. Functions of Flux
1. Cleaning and Degreasing: Removes oxides, grease, and other contaminants from the surfaces of base metals and solder, clearing obstacles for metal bonding.
2. Isolation and Protection: Forms a protective layer of gas or slag to shield the high-temperature welding zone from air, preventing metal re-oxidation.
3. Auxiliary forming: Protecting, stabilising, and assisting weld bead formation.
III. Types of Flux (Classified by Production Process and Form)
Varieties | Manufacturing Process | Core Features |
Fusion welding flux | The raw material is melted at high temperatures, then cooled and crushed. | Composition is stable, welds exhibit robust mechanical properties |
Sintered flux | Powder mixing, followed by pressing and high-temperature sintering | Composition is uniform and compatible with a wide range of metals. |
Bonded flux | The powdered material is bonded with a binder and then dried. | Simple to manufacture, flexible to use |
IV. Common Issues with Flux Usage
Frequently Asked Questions | Core investigation focus | Corresponding solutions |
Welds exhibiting porosity | 1. Moisture absorption in flux, releasing water during welding 2. Excessively fine flux particle size, resulting in poor permeability 3. Inadequate cleaning of the welding area, leaving residual oil contamination | 1. Place the flux in the drying oven and dry at the specified temperature (typically 150–250°C). 2. Replace with flux of suitable particle size (e.g., 10–60 mesh for submerged arc welding). 3. Prior to welding, wipe the base metal surface with alcohol or acetone to remove grease and contaminants. |
Cracks have formed in the weld. | 1. Excessive hydrogen content in the flux (non-low-hydrogen type) 2. Mismatch between flux and base metal composition 3. Excessively rapid cooling after welding | 1. Switch to low-hydrogen or ultra-low-hydrogen flux 2. Select flux appropriate for the material type (e.g., low-silicon flux for stainless steel) 3. Apply slow cooling treatment to welds post-welding (e.g., covering with insulation wool) |
Solder does not flow/poor spreading | 1. Insufficient flux activity prevents removal of the oxide layer 2. Inadequate flux quantity results in insufficient coverage 3. Welding temperature has not reached the flux activation temperature | 1. Replace with a more active flux (such as a specialised active flux for brazing copper) 2. Appropriately increase the flux quantity to ensure complete coverage of the weld area 3. Raise the welding temperature to the active temperature range specified on the flux |
V. Correct Method for Using Flux
1. Pre-welding Preparation
① Drying the flux: Upon opening the packaging, if the flux is damp (e.g., clumping, damp to the touch), it must be dried immediately.
When drying flux, first spread it evenly on a clean iron plate and place it in an electric furnace or flame furnace for drying. The height of the flux pile within the drying furnace should not exceed 50mm.
Fused flux: Dry at 150–200°C for 1–2 hours.
Sintered flux/bonded flux: Dry at 200–250°C for 2–3 hours, following the product instructions.
② Cleaning the welding area: Remove scale from the base metal surface using sandpaper, then wipe away oil and dust with alcohol/acetone to ensure the metal surface is clean and free from contaminants that could cause porosity.
2. Application Quantity and Method
For submerged arc welding, fusion welding flux/sintered flux must cover the welding wire to a depth of 10–15mm, ensuring complete air exclusion in the weld zone.
3. During Welding: Matching Process Parameters
① Temperature matching: Ensure the soldering temperature reaches the flux's “active temperature range” (as per product specifications). Insufficient temperature will cause flux failure and prevent solder flow.
② Avoid interference: During welding operations, when using granular flux, ensure the flux reservoir remains adequately filled to prevent the welding wire from being exposed to air.
4. Post-Welding Treatment
① Promptly remove slag: After welding is complete, allow the weld to cool before chipping off surface slag with a slag hammer to prevent corrosion caused by prolonged slag adhesion.
② Handling of residual flux: Seal unused flux for storage. If dampened, it must be re-dried before reuse. Do not mix directly with fresh flux (to prevent contamination).
VI. Common Submerged Arc Welding Wire and Matching Flux Selection Comparison Table
Welding wire material types | Recommended flux types | Core features | Applicable scenarios |
Low-carbon steel / Low-alloy steel (e.g. H08A, H08MnA) | Acidic flux (such as HJ431) | High welding efficiency, easy slag removal, low cost | Ordinary structural components, thin-plate welding, scenarios with moderate toughness requirements |
Low-alloy steel / High-strength steel (e.g. H10Mn2, H08MnMoA) | Alkaline flux (such as HJ250, HJ350) | High weld toughness, excellent crack resistance, and superior strength matching | Structures with stringent weld performance requirements, such as construction machinery, pressure vessels, and bridges |
Stainless steel (such as ER308L, ER316L) | Stainless steel-specific neutral flux (e.g. HJ260, SJ601) | Composition is stable, corrosion resistance is excellent, and weld bead formation is good. | Scenarios requiring corrosion resistance, such as stainless steel equipment, chemical pipelines, and food processing machinery. |
Heat-resistant steel / Alloy structural steel (e.g. H08CrMoA, H13CrMoA) | High-alkalinity neutral flux (such as HJ401, HJ402) | Good high-temperature stability, uniform weld microstructure | Heat-resistant conditions for boilers, pipelines, high-temperature equipment, etc. |
VII. Weldynasty Flux
Weldynasty brand flux comprises two variants : sintered flux SJ101 and Iron Man sintering flux SJ101 (suitable for pressure vessels, ships, pipelines, etc.). It presents as grey spherical granules with particle sizes of (10-60 mesh) and (10-40 mesh). Compatible submerged arc welding wires include: H08MnA, H10Mn2, H10MnSi, H08MnMoA, H08Mn2MoA, H08Mn2SiA, and others.
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