Alloy 600, designated UNS N06600, is a nickel-chromium-iron alloy that combines a stable austenitic matrix with about 15 percent chromium. It is supplied as round bar, seamless pipe, plate, wire, strip, flange and forged fittings produced by hot rolling, forging, precision rolling, extrusion, continuous casting and cold drawing. Weldability is good, but the alloy has a shallow penetration characteristic and a sensitivity to chromium carbide precipitation, so welding procedures must be written with care. The sections below describe a sound gas tungsten arc welding route for pipe, plate and bar assemblies used in chemical, nuclear and heat-treatment equipment.
Material Overview and Weldability
Because the alloy is austenitic and does not undergo an allotropic transformation, weld metal and heat-affected zone keep a coarse cast or equiaxed grain structure. Thermal conductivity is low compared with carbon steel, so heat concentrates at the joint and the molten pool stays fluid longer. Penetration is lower than an operator expects from carbon steel, so raising current does not deliver a proportionally deeper bead and instead risks alloy burn-out and a loss of joint properties.
| Element, percent | Typical limit |
|---|---|
| Nickel | 72.0 min |
| Chromium | 14.0 to 17.0 |
| Iron | 6.0 to 10.0 |
| Carbon | 0.15 max |
| Manganese | 1.0 max |
| Silicon | 0.50 max |
| Sulfur | 0.015 max |
| Copper | 0.50 max |
The low carbon and controlled sulfur levels are the reason the alloy can be welded without hot cracking under a properly balanced procedure.
Welding Method and Process Parameters
Gas tungsten arc welding with direct current electrode negative is the preferred process because it gives concentrated heat input and reliable gas protection. The parameters below are a practical starting point for manual and orbital welding of pipe and plate in the 2 to 10 mm wall thickness range, and should be qualified by procedure testing for the actual component.
| Variable | Recommended setting |
|---|---|
| Process and polarity | GTAW, direct current electrode negative |
| Tungsten electrode | 2.4 mm diameter, tip flat about 0.4 mm, included angle 30 to 45 degrees |
| Filler metal diameter | 2.4 mm for root passes, 2.4 to 3.2 mm for fill and cap |
| Welding current | 90 to 130 A |
| Arc voltage | 10 to 14 V |
| Travel speed | 60 to 120 mm per minute |
| Shielding gas | Argon of 99.99 percent purity minimum, 10 to 15 L per minute |
| Back purge | Argon, 8 to 12 L per minute until the root has cooled below 150 C |
| Interpass temperature | 150 C maximum |
| Heat input | Kept low; high line energy promotes alloy burn-out and sensitisation |
Filler Metal and Shielding Gas Selection
The correct filler is a nickel-chromium-iron wire of matching composition with roughly 20 percent chromium and a niobium addition for strength, classified as ERNiCr-3 under the nickel alloy filler specification and supplied in 1.6, 2.4 and 3.2 mm diameters. Wipe the wire with acetone immediately before use to remove drawing lubricant and handling soil. The tungsten tip geometry matters: a small flat of about 0.4 mm with an included angle of 30 to 45 degrees stabilises the arc and gives a clean penetration profile. Argon purity must exceed 99.99 percent with a low moisture content, because water vapour and sulfur-bearing contamination are the usual causes of porosity and of reduced corrosion resistance in the finished joint.
Joint Preparation and Cleaning Before Welding
Degrease the inside and outside surfaces of the pipe or plate before any mechanical preparation.
Clean at least 50 mm on both sides of the groove with acetone to remove oil, rust, marking ink and cutting fluid, then allow the surfaces to dry completely.
Machine or grind the groove with a comparatively wide included angle and a small root face so that full penetration can be achieved at the low current levels the alloy requires.
Use dedicated stainless steel or nickel alloy brushes; carbon steel brushes embed iron particles that later rust and contaminate the weld.
Keep sulfur-bearing markers, lead pencils, lubricants and cutting compounds away from the joint, and purge the bore thoroughly before striking the arc.
Sensitisation and Intergranular Corrosion Control
Alloy 600 contains chromium as its main alloying element and therefore behaves like a chromium-bearing stainless steel with respect to intergranular corrosion. The sensitivity depends on the thermal history: after welding thermal cycles or improper heating, chromium carbides precipitate in the weld metal, the heat-affected zone or the parent metal and leave the grain boundaries chromium depleted. This condition, known as sensitisation, allows accelerated attack along the grain boundaries in certain electrolyte solutions. For UNS N06600 the sensitisation range is 550 to 850 C, which is not very high for a nickel alloy and is therefore often described as medium-temperature sensitisation.
Components for chemical equipment need particular attention to this range. Two decisions matter: the selected heat-treatment temperature and the time that weld metal and heat-affected zone spend between 550 and 850 C. Where the service environment is an electrolyte that can cause intergranular corrosion, a stress relief treatment at about 650 C must not be applied. The acceptable alternatives are a high-temperature softening anneal or a solution treatment, with rapid cooling through the sensitisation range. Baking an expansion joint with an oxy-acetylene flame before forming, a practice sometimes proposed to soften the alloy, is not recommended: the flame cannot reach solution treatment temperature, the heated zone falls inside the sensitisation range, and the heat does nothing to help forming. Unless a final solution treatment follows, uncontrolled pre-heating is harmful to nickel-based alloys.
Frequently Asked Questions
Q: Which welding process is best for alloy 600?
Gas tungsten arc welding with direct current electrode negative. It gives concentrated heat, good shielding and the cleanest root on thin-wall pipe and tube, and it is easily qualified for both manual and orbital work.
Q: Which filler metal should be used?
A matching nickel-chromium-iron wire classified as ERNiCr-3, in 2.4 mm diameter for root passes. For dissimilar joints to carbon or stainless steel the same filler family is normally used because it tolerates dilution well.
Q: Is preheating required before welding?
No. Preheat is not needed and uncontrolled heating is harmful because the alloy can sensitise between 550 and 850 C. The joint should be dry and at ambient temperature, with interpass temperature kept at or below 150 C.
Q: Is post-weld heat treatment necessary?
Not as a routine step. A 650 C stress relief must be avoided where intergranular corrosion is a risk. If heat treatment is required, a high-temperature softening anneal or solution treatment followed by rapid cooling is the correct route.
Q: Why does the weld pool penetrate less than expected?
Nickel-based alloys have lower penetration and lower thermal conductivity than carbon steel, so the arc energy stays near the surface. A wider groove angle, a small root face and a properly pointed tungsten electrode solve the problem; increasing current only overheats the alloy.
Q: Can the alloy be welded to stainless or carbon steel?
Yes. With a matching nickel-chromium-iron filler and a qualified procedure, dissimilar joints are made routinely in heat exchangers, vessels and transition spools.
