How to weld INCOLOY 800 alloy - Knowledge

1. How to Weld INCOLOY 800

INCOLOY 800 (a nickel-iron-chromium alloy) is primarily welded for high-temperature and corrosion-resistant applications (e.g., petrochemical reactors, heat exchangers). Successful welding requires strict control of intergranular corrosion (IGC) risk, hot cracking prevention, and maintenance of mechanical properties. Below is a detailed, step-by-step guide:

Step 1: Pre-Weld Preparation

Proper preparation is critical to avoid contamination and defects:

Material Cleaning: Remove all contaminants (oil, grease, paint, oxide scale, dirt) from the weld area (at least 25 mm/1 inch on both sides of the joint) using:

Solvents (e.g., acetone, isopropyl alcohol) for organic residues.

Stainless steel wire brushes (never carbon steel, to prevent iron contamination) or abrasive pads (aluminum oxide or silicon carbide) for oxide scales.

For heavy scale, use pickling solutions (e.g., 10-20% nitric acid + 1-2% hydrofluoric acid) followed by thorough rinsing with deionized water and drying.

Joint Design: Prioritize designs that minimize heat input and ensure full penetration:

Recommended joint types: Butt joints (single-V or double-V, with a root gap of 2-3 mm), lap joints (with fillet welds), or socket welds.

Avoid narrow, deep joints (e.g., single-U with small root openings) that trap heat and increase cracking risk.

Preheating: Generally, no preheating is required for INCOLOY 800, as it has good ductility at room temperature. However, for thick sections (>25 mm/1 inch) or when welding in cold environments (<5°C/41°F), preheat to 50-150°C (122-302°F) to reduce thermal stress.

Step 2: Welding Process Selection

The Gas Tungsten Arc Welding (GTAW/TIG) process is the most preferred for INCOLOY 800, as it offers precise heat control, minimal spatter, and high-quality welds. Other acceptable processes include:

Gas Metal Arc Welding (GMAW/MIG): For high-deposition rates (e.g., thick sections), but requires careful control of wire feed speed and shielding gas.

Shielded Metal Arc Welding (SMAW): Only for non-critical, thick-section applications (limited by electrode availability and potential slag inclusions).

Avoid processes with high heat input (e.g., Submerged Arc Welding/SAW), as they increase grain growth and IGC susceptibility.

Step 3: Filler Metal Selection

Filler metals must match or exceed the base metal's corrosion resistance and high-temperature strength. The most common choices are:

ERNiFeCr-2 (GTAW): Matches INCOLOY 800's composition (nickel-iron-chromium) and provides excellent resistance to IGC and high-temperature oxidation.

EniFeCr-2 (SMAW): Corresponding covered electrode for SMAW applications.

ERNiCr-3 (GTAW): A nickel-chromium filler (e.g., INCONEL 625) used when enhanced corrosion resistance (e.g., to sulfur-containing environments) is required, though it has a higher nickel content than the base metal.

Ensure filler metals are stored in a dry environment (relative humidity <50%) to prevent moisture absorption, which can cause hydrogen-induced cracking.

Step 4: Welding Parameters (GTAW as Example)

Parameters vary by material thickness, but the key is to minimize heat input (to avoid grain coarsening) while ensuring full penetration:

Material Thickness	Tungsten Electrode (AWS)	Current (DCEN¹)	Arc Voltage	Travel Speed	Shielding Gas (Argon)
1-3 mm (0.04-0.12 in)	1.6 mm (1/16 in), EWTh-2	40-80 A	8-12 V	50-100 mm/min	10-15 L/min
3-6 mm (0.12-0.24 in)	2.4 mm (3/32 in), EWTh-2	80-120 A	10-14 V	40-80 mm/min	12-18 L/min
6-12 mm (0.24-0.47 in)	3.2 mm (1/8 in), EWTh-2	120-180 A	12-16 V	30-60 mm/min	15-20 L/min

¹DCEN = Direct Current Electrode Negative (standard for GTAW of nickel alloys, as it concentrates heat on the workpiece).

Backing Gas: For butt joints, use argon (10-15 L/min) as backing gas to protect the root from oxidation (critical for corrosion resistance).

Heat Input Calculation: Aim for heat input <2 kJ/mm (50 kJ/in) to prevent excessive grain growth.

Step 5: Post-Weld Heat Treatment (PWHT)

PWHT is required to mitigate IGC risk and restore ductility, as welding can cause precipitation of chromium carbides at grain boundaries (depleting chromium near boundaries, making the alloy susceptible to corrosion). The standard PWHT for INCOLOY 800 is:

Solution Annealing: Heat to 980-1050°C (1796-1922°F), hold for 15-30 minutes (per 25 mm thickness), then water quench (rapid cooling) to prevent carbide re-precipitation.

Stress Relieving: For applications with high thermal cycling, a stress relief at 700-760°C (1292-1400°F) (hold 1-2 hours, air cool) may be used instead, but it does not fully eliminate IGC risk.

Note: Skip PWHT only for thin sections (<3 mm) in non-corrosive, low-temperature environments.

Step 6: Post-Weld Inspection & Cleaning

Visual Inspection: Check for surface defects (cracks, porosity, undercut, incomplete penetration).

Non-Destructive Testing (NDT): Use X-ray or ultrasonic testing (UT) for internal defects; use liquid penetrant testing (PT) for surface cracks (especially critical for corrosion-prone applications).

Post-Weld Cleaning: Remove weld spatter or discoloration with a stainless steel brush, then pickle (10% nitric acid) and rinse to restore corrosion resistance.

2. What is the chemical composition of INCOLOY 800?

INCOLOY 800 is a nickel-iron-chromium alloy defined by ASTM B409 (sheet/plate) and ASTM B408 (bar). Its composition is tightly controlled to balance corrosion resistance, high-temperature strength, and fabricability. Below are the typical and maximum allowable ranges (by weight percentage, wt%) per ASTM standards:

Element	Symbol	Typical Composition (wt%)	ASTM Maximum Allowance (wt%)	Key Function
Nickel	Ni	30.0-35.0	30.0-35.0	Enhances corrosion resistance (especially to acids) and high-temperature stability.
Iron	Fe	46.0-51.0	Balance (≈46-51)	Base metal, provides structural strength and cost-effectiveness.
Chromium	Cr	19.0-23.0	19.0-23.0	Critical for oxidation resistance (forms a protective Cr₂O₃ oxide layer) and resistance to IGC.
Carbon	C	0.05-0.10	0.10	Controls strength; limited to <0.10% to reduce carbide precipitation (IGC risk).
Manganese	Mn	0.50-1.00	1.50	Improves weldability and prevents hot cracking.
Silicon	Si	0.30-0.60	1.00	Enhances oxide layer stability; limited to avoid brittleness.
Copper	Cu	≤0.75	0.75	Trace element; improves resistance to sulfuric acid.
Aluminum	Al	0.15-0.40	0.40	Aids in forming a protective oxide layer at high temperatures.
Titanium	Ti	0.15-0.40	0.60	"Stabilizes" carbon (forms TiC instead of Cr₂₃C₆), reducing IGC risk.
Phosphorus	P	≤0.030	0.030	Impurity; limited to avoid brittleness and corrosion.
Sulfur	S	≤0.015	0.015	Impurity; limited to prevent hot cracking and reduce corrosion.

3. What is the hardness of INCOLOY 800?

The hardness of INCOLOY 800 depends on its heat treatment condition (e.g., annealed, welded, stress-relieved), as heat treatment alters its microstructure (grain size, carbide precipitation). Below are the typical hardness values for common conditions, measured via standard scales (Brinell, Rockwell, Vickers):

Heat Treatment Condition	Brinell Hardness (HB)	Rockwell Hardness (HRB)	Vickers Hardness (HV)	Key Notes
Solution Annealed¹	140-180	70-85	150-190	Most common condition (after manufacturing or PWHT). Soft, ductile, and optimized for corrosion resistance.
As-Welded	180-220	85-95	190-230	Harder than annealed due to localized grain coarsening and residual stress from welding.
Stress-Relieved²	160-200	75-90	170-210	Hardness decreases slightly compared to as-welded, as residual stresses are reduced (but not fully eliminated).
Cold-Worked³	200-250	90-100	210-260	Rare for INCOLOY 800 (it is primarily used in annealed form), but cold working (e.g., rolling) increases hardness and strength at the cost of ductility.