ASTM B163 Inconel 625 Nickel Alloy Tube

ASTM B163 Inconel 625 Nickel Alloy Tube is the nickel alloy that engineering teams reach for when the service is too aggressive for stainless steel, too demanding for 316L, and too hot for most other nickel grades. The alloy, UNS N06625, combines two capabilities that rarely travel together: outstanding corrosion resistance in seawater and chloride environments, and useful strength at temperatures up to about 650-700°C.

 

That dual character is why 625 tube shows up in seawater cooled heat exchangers, offshore hydraulic control lines, chemical reactor coils, and boiler feedwater heater bundles - applications where the tube must survive both the process chemistry and the operating temperature. 

Quick facts at a glance

Item	Value
UNS designation	N06625 (W.Nr 2.4856)
Standards	ASTM B444 (seamless pipe & tube), ASTM B163 (heat-exchanger tube), ASME SB-444 / SB-163
Delivery condition	Solution annealed (Gr 1) or cold-worked (Gr 2) per B444
Density	8.44 g/cm³
Melting range	1290 - 1350°C (approx.)
Certificate	EN 10204 3.1 / 3.2, with PMI on request

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Chemical composition per ASTM B444

The heat-analysis limits below are taken from ASTM B444 for UNS N06625. Nickel is the balance element at 58.0% minimum, and the combination of molybdenum with niobium is what gives the alloy its exceptional strength and pitting resistance.

 

Element	Composition limit, %
Ni	58.0 min (balance)
Cr	20.0 - 23.0
Mo	8.0 - 10.0
Nb + Ta	3.15 - 4.15
Fe	5.0 max
Co	1.0 max
C	0.10 max
Mn	0.50 max
Si	0.50 max
Al	0.40 max
Ti	0.40 max
P	0.015 max
S	0.015 max

 

Three elements carry the alloy's personality. Molybdenum at 8.0-10.0% gives 625 its outstanding resistance to pitting and crevice corrosion in chloride media, making the tube effectively immune to chloride stress-corrosion cracking in normal service. Niobium (columbium) at 3.15-4.15% is the element that competitors struggle to match: it provides a major part of the alloy's strength without requiring a precipitation heat treatment, and it stabilizes the weld metal against cracking.

 

Chromium at 20.0-23.0% provides oxidation resistance up to about 980°C and balances the alloy's resistance across oxidizing and reducing media. The iron content is held to 5.0% maximum to keep the matrix fully nickel-rich and the pitting resistance high.

 

Mechanical properties of 625 tube

ASTM B444 divides the product into two conditions. Grade 1 is solution annealed and is the standard delivery for heat-exchanger and process tube; Grade 2 is cold-worked for higher strength. The minimum requirements for Grade 1 annealed tube are:

 

Property	Grade 1 (annealed) minimum	Grade 2 (cold-worked) minimum
Tensile strength	827 MPa (120 ksi)	965 MPa (140 ksi)
Yield strength, 0.2% offset	414 MPa (60 ksi)	621 MPa (90 ksi)
Elongation in 2 in (50.8 mm)	30%	20%

 

Typical annealed production values are higher: around 930 MPa tensile and 517 MPa yield with 40% elongation. The yield strength of annealed 625 is roughly double that of 316L stainless, which is one of the reasons thin-wall 625 tube is used where a stainless tube of the same wall would not carry the design pressure. At elevated temperature the alloy holds useful strength to about 650°C in continuous service, and it has been used up to 815°C in heat-treating applications where oxidation resistance rather than load carrying is the governing requirement.

 

Tube sizes, dimensions, and tolerances

Inconel 625 tube is supplied in the following envelope, with project-specific sizes available on request:

 

Parameter	Range
Outside diameter	6.0 mm to 114.3 mm (larger as seamless pipe per B444)
Wall thickness	0.5 mm to 12.7 mm
Length	Fixed 6 m, random lengths, or cut-to-length for bundles
End finish	Plain end, beveled, or expanded for tubesheet rolling
Surface	Bright annealed or pickled, OD/ID per specification

 

Because tube is sized by exact OD x wall rather than by schedule, the tolerances are expressed as exact values. ASTM B444 specifies an outside-diameter tolerance of about +/-0.10 mm on small-diameter tube, tightening with size, and a wall tolerance of +/-10% on the nominal wall. For heat-exchanger tube supplied to ASTM B163, the tolerance system is the same precision class, because the tube must roll into the tubesheet and seal under test pressure.

 

When the drawing specifies "625 tube", the enquiry should state the exact OD, wall, and the standard - B444 for general tube, B163 for heat-exchanger bundles - because the two standards differ in the examination and certification requirements.

 

Corrosion performance

Inconel 625 tube is a dual-purpose alloy, and the corrosion half of its personality is defined by the high molybdenum and niobium content. In seawater and marine environments, 625 is effectively immune to chloride stress-corrosion cracking and offers outstanding resistance to pitting and crevice corrosion - the failure modes that eliminate standard stainless grades in warm chloride service.

 

In acid media, the alloy resists dilute hydrochloric, sulfuric, phosphoric, and nitric acids across a broad concentration range, and it is one of the few alloys that handles oxidizing and reducing conditions alternately without a significant corrosion penalty.

 

Environment	625 tube behavior
Seawater and marine atmospheres	Excellent; immune to chloride SCC
Chloride pitting / crevice service	Excellent; PREN ~ 51
Dilute sulfuric / hydrochloric acid	Good to very good, concentration dependent
Phosphoric acid (wet process)	Very good
Nitric acid	Good, especially with chromium-rich surface
Caustic solutions	Good up to moderate concentrations

 

For the tube buyer the practical consequence is simple: 625 tube is the standard specification for seawater heat exchangers on offshore platforms, for hydraulic and chemical injection lines in subsea controls, and for any chloride-bearing process where the tube must survive for the life of the plant without a corrosion allowance. It is more expensive than 6% molybdenum super austenitic stainless, and where the temperature and pressure allow, engineers sometimes substitute the cheaper grade - but where the design calls for a nickel alloy, 625 is the benchmark tube material.

 

Typical applications

Inconel 625 seamless tube is specified across the following services:

 

• Seawater-cooled heat exchangers and condensers in coastal and offshore plants
• Offshore hydraulic, pneumatic, and chemical injection control lines (small-bore tube)
• Chemical reactor coils, dip pipes, and spargers in chloride and acid service
• Boiler feedwater heater bundles and high-temperature steam tubes
• Pulp and paper bleach plant heat-exchanger tube
• Flue gas desulfurization (FGD) reheater tubes where 6Mo stainless is marginal

 

In each of these services the tube is bought for its resistance to a specific failure mode - pitting, SCC, or high-temperature oxidation - and the engineer selects 625 when the cheaper candidates have been eliminated by the duty. The alloy is also the default field-weldable nickel alloy, which makes it the practical choice for repairs and retrofits where the weld is made in place and cannot be post-weld heat treated.

 

Welding and fabrication

Inconel 625 is welded with matching filler metal ERNiCrMo-3 (AWS A5.14), which is the same chemistry family as the base alloy. No preheat is required and no post-weld heat treatment is normally required, because the alloy derives its strength from the solid-solution and niobium-stabilized matrix rather than from a heat-treatment response.

 

This is the key fabrication advantage of 625 over the precipitation-hardenable grades: a weld made in the field to 625 tubing retains its corrosion resistance and ductility in the as-welded condition.

 

For tubesheet joints, the tube is typically rolled and then seal-welded with GTAW, with the weld made under inert gas shielding on both sides where access allows. The alloy work-hardens faster than stainless, so tube bending should use mandrel bending for tight radii, and cutting should use sharp tooling with controlled feed rates.

 

What to specify on the order

To avoid the specification mix-ups that delay 625 tube deliveries, state the following on the enquiry:

 

• Standard and grade: "ASTM B444 Gr 1" or "ASTM B163", UNS N06625, not just "625 tube"
• Exact dimensions: OD x wall in millimetres, and whether the wall is minimum-wall or nominal-wall basis
• Length and end condition: fixed 6 m, random, or cut lengths; plain end or beveled
• Condition: annealed (Gr 1) or cold-worked (Gr 2)
• Examination: hydrostatic or eddy-current per the standard, and the certificate format (EN 10204 3.1/3.2)

 

Inconel 625 tube is a premium nickel-alloy product whose price reflects the raw material and the tight dimensional control of tube manufacture. The buying decision should rest on the corrosion and temperature duty, not on price alone - and the enquiry should name the standard, the grade, and the exact dimensions so that the delivered tube is the tube the design requires.

 

 

 

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