Technical knowledge
Solution annealing and sensitisation: what the heat treatment actually achieves
What solution annealing does, how sensitisation removes chromium from grain boundaries, which grades are at risk, and how to specify and verify a correct thermal treatment.
The mechanism in plain terms
Carbon dissolved in austenitic stainless steel has low solubility at intermediate temperatures. Given time in the 500 to 800 C band, it combines with chromium and precipitates at grain boundaries. Because the carbide takes chromium out of the surrounding metal, the grain boundary region loses the chromium content needed for passivity and corrodes preferentially.
Which grades are at risk
| Grade type | Risk | Control |
|---|---|---|
| 304, 316 normal carbon | High | Use the L grade or stabilised grade |
| 304L, 316L | Low for welding | Still at risk after long high temperature exposure |
| 321, 347 stabilised | Low | Titanium or niobium binds the carbon |
| Duplex grades | Related risk from intermetallics | Control heat input and cooling |
| Nickel alloys such as 625 and 825 | Low | Niobium or titanium stabilisation |
Specifying the treatment
- Name the temperature range appropriate to the grade, typically above 1040 C for austenitic stainless.
- Require rapid quenching, normally water, to freeze the structure.
- State the hold time based on section thickness.
- Require the chart showing actual temperatures and cooling method.
- Add intergranular corrosion testing where the service or the specification justifies it.
What it looks like in the field
Sensitisation damage typically appears as cracking or attack adjacent to welds, or as general attack in a heat affected band. It is frequently misdiagnosed as poor material. Confirming the thermal history and, where necessary, examining the microstructure is the fastest route to the correct conclusion.
Questions about this topic
Can sensitised material be restored?
Yes, by a full solution anneal within the correct temperature range followed by rapid quenching, provided the component geometry tolerates it. Verify afterwards rather than assuming the treatment worked.
Is 316L immune to sensitisation?
It is resistant under normal welding conditions because the carbon content is very low. Long service exposure at elevated temperature can still cause precipitation, so the exposure history matters.
Do nickel alloys sensitise?
Stabilised grades such as Inconel 625 and Incoloy 825 are designed to resist it. Unstabilised nickel alloys and some high carbon variants can be affected, so check the grade and the thermal history.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email demi@nickel-alloy.com or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
Share your duty conditions and we will shortlist candidate alloys within one working day.