Construction Works Alloyed Steel HG785 Induced Draught Fan

This article's table of contents introduction:

1. What is HG785?
2. Why use HG785 for an ID Fan?
3. Critical Issues & Risks (Must Check)
4. Recommended Alternatives (If HG785 is not optimal)
5. Summary for "Construction Works"

It appears you are asking about the specifications or considerations for using HG785 alloyed steel in the construction of an Induced Draught (ID) Fan — likely for an industrial application such as a power plant, cement kiln, or steel mill.

Here is a breakdown of the key engineering considerations regarding the material (HG785) and the application (ID Fan).

What is HG785?

• Definition: HG785 is a high-strength, low-alloy (HSLA) structural steel, commonly used in Chinese standards (often equivalent to Q690 or S690QL under EN standards).
• Yield Strength: ~690 MPa (Minimum yield point).
• Tensile Strength: 785 – 940 MPa.
• Properties: High strength-to-weight ratio, good weldability (with proper preheat), and good toughness.
• Common Use: Boom structures, cranes, heavy machinery components where weight reduction is critical.

Why use HG785 for an ID Fan?

• Weight Reduction: ID fans are large (often several meters in diameter). Using a high-strength steel like HG785 allows for thinner plates in the impeller blades, backplate, and housing. This reduces rotational inertia, motor load, and structural support costs.
• Fatigue Performance: If the fan operates at variable speeds or has significant rotational mass, HG785 offers superior fatigue resistance compared to mild steel (e.g., Q235B).
• Combined Stress: The blade roots experience high centrifugal stress. HG785 helps manage this without excessive thickness.

Critical Issues & Risks (Must Check)

A. Weldability & Cold Cracking

• Problem: HG785 is sensitive to hydrogen-induced cracking (HIC) and cold cracking.
• Solution:
  • Preheat: Minimum 150°C – 200°C (depending on thickness). Thicker flanges or hubs may require higher preheat.
  • Interpass Temperature: Must be controlled (usually < 250°C).
  • Electrodes: Use low-hydrogen consumables (e.g., E81T1-Ni1 for FCAW or matching solid wire with flux).
  • Post-Weld Heat Treatment (PWHT): Often required for thick sections to relieve residual stress.

B. Corrosion & Erosion (The ID Fan Killer)

• The Problem: ID fans handle hot, dirty, abrasive flue gas. HG785 has no inherent corrosion resistance. It will rust and erode faster than stainless steel (e.g., 316L) or coated steel.
• The Risk: If you use bare HG785 in the impeller (blades), the combination of:
  • High tip speed (erosion by fly ash)
  • Condensation of sulfuric acid (if gas temp drops below dew point) will rapidly destroy the blades.
• Solution: HG785 is typically used as the structural core, and then:
  • Cladding: Weld overlay with Inconel or 309L SS on the leading edges.
  • Coatings: Tungsten carbide thermal spray or ceramic epoxy (high temperature rated).
  • Avoid: Using bare HG785 in the gas path unless the gas is clean and dry.

C. High Temperature Performance

• Typical ID Fan Inlet Temperature: 120°C – 180°C (up to 400°C in some boiler applications).
• HG785 Temperature Limit: The strength degrades significantly above 300°C. At 400°C, the allowable stress drops by roughly 40-50%.
• If gas temperature > 200°C:
  • You must derate the fan speed/stress.
  • Consider Chromium-Molybdenum steel (e.g., 15CrMo) or heat-resistant stainless (e.g., 310S) for the impeller, while using HG785 only for the cold housing.

Recommended Alternatives (If HG785 is not optimal)

Summary for "Construction Works"

If you are writing a specification or construction plan:

• Scope of Use: HG785 is acceptable for the hub, bearing shaft, and structural casing (outside the gas duct).
• For the Impeller (Rotating Blades): Strongly recommend against bare HG785 unless the gas is clean and cold.
• Welding Procedure: Must be qualified by WPQR (Welding Procedure Qualification Record) with preheat and PWHT defined.
• Destructive Testing: Must include Charpy V-notch impact tests at -20°C or -40°C (to confirm toughness).
• Coating: All exposed surfaces to flue gas must be coated with a high-temperature epoxy or ceramic paint.

Verdict: HG785 is a valid material for ID fan construction, but it is a niche application used primarily to save weight in large fans. It requires rigorous welding control and corrosion/erosion protection. For typical dirty flue gas, Hardox or Stainless Steel cladding is more common.