Failure mode
Weld Cracking & Sensitisation in 5xxx Plate
A welded 5xxx structure is fine at delivery and then develops intergranular corrosion or cracking along the heat-affected zone after months in service — most often in marine or elevated-temperature duty.
What it actually is
A welded 5xxx structure is fine at delivery and then develops intergranular corrosion or cracking along the heat-affected zone after months in service — most often in marine or elevated-temperature duty.
Mechanism
Why it happens
Four causes account for most of it. They are not equally common, and they are not equally within the mill's control.
Sensitisation
Alloys above roughly 3% magnesium precipitate the beta phase along grain boundaries when held in the 50–200°C band for long periods. A continuous network at the boundary is what makes it intergranular.
Wrong temper specified
H116 and H321 are produced and tested for marine service. H32 and H34 are general-purpose work-hardened tempers that were never qualified for that duty, though they are often substituted on price.
Filler wire mismatch
5xxx plate needs a 5xxx filler such as 5356 or 5183. A 4xxx silicon filler drops the magnesium content of the weld and changes its corrosion behaviour.
HAZ softening
In work-hardened tempers the heat-affected zone loses strength. The joint is designed for the annealed condition whether or not the drawing accounts for it.
Verification
How it is measured
Susceptibility is measured by ASTM G67, the nitric acid mass loss test (NAMLT). A result at or below about 15 mg/cm² is the usual acceptance line for marine-grade 5xxx; above roughly 25 mg/cm² the material is considered sensitised. ASTM B928 covers the marine-grade alloy and temper requirements that sit behind it.
Specification
How to specify around it
Most of these cost nothing to write into an order and are difficult to enforce afterwards. This is the part worth copying into your next RFQ.
1
Name H116 or H321
For hull and marine structure, write the temper into the order. Do not accept "5083" without it.
2
Require the ASTM G67 result
Ask for the NAMLT value with the certificate. It is the only number that actually speaks to sensitisation.
3
State the service temperature
Continuous duty above about 65°C changes the alloy decision. Tell us the design temperature and we will say if 5xxx is the wrong family.
4
Match the filler metal
5356 or 5183 for 5xxx plate. Say it in the welding procedure, not just in the purchase order.
On marine and elevated-temperature enquiries we ask the service temperature before quoting, and where the duty justifies it we ask the mill for the ASTM G67 result alongside the 3.1 certificate. If the design temperature makes 5xxx the wrong family, we say so rather than quote it.
We do not claim this never happens. Metal is rolled, shipped across an ocean and unloaded by someone else. What we do is ask the question before the order runs, put the answer in writing, and be the single party that answers for it afterwards.