December 20, 2018

AISI316Ti vs. AISI316L / 1.4571 vs. 1.4404

As you might know, these material grades are often interchangeable, and in much cases offered as an alternative when the original item is unavailable. But what is the story behind these grades?

316Ti is essentially 316 with the addition of some titanium(Ti) to reduce the risk of losing corrosion resistance in the heat-affected zone. An alternative to prevent this, is to reduce the carbon(C) level, which results in the grade 316L. So we may conclude that these grades offer the same results with different approaches. 316ti with an addition and 316L with a reduction.

One could say it makes more sense to remove an element, instead of simply adding one. However, in the early days of stainless steel making, there was no technology around to reduce the C content to the desired level. So this resulted in the development of the Ti-stabilized types. Nowadays it’s possible to abstract C from the molten steel through processes like AOD (Argon Oxygen Decarbusation) which makes 316L possible to replace 316Ti. But the 316Ti type has been traditionally specified by German engineers, and costs of changing the specifications are keeping this grade alive.

The type of corrosion which both types prevent is the intercrystalline corrosion (ICC), also known as intergranular attack. This corrosion occurs when the material is brought in a heat zone of about 450°-800°C, which could be created by welding, for example. The Chromium (Cr) reacts with the C and forms chromium-carbides of the type Cr23C6 at the grain boundaries. The Cr content in this crystal structure is very high compared to the C. This results in the formation of zones without Cr adjacent to the grain boundaries and of course, a weakening of the Cr-protection on these spots where the Cr content is below 12%.

The Ti in 316Ti fights against this mechanism by forming titanium carbo-nitrides instead of chromium carbides, thus maintaining the original structure of Cr. The Ti content must be 5 times the actual C content to obtain this result. In 316L, the low C content (less than 0,03%) prevents the Cr to form an alliance with the carbon, simply because there is not enough carbon to react with.

The stabilizing element Ti has multiple other (dis)advantages. 316Ti material has an improved mechanical strength because of the higher C content. E.g. the proof strength (Rp0.2) at room temperature 210MPa for 316Ti and 190MPa for 316L. This difference rises with the temperature. This implies they are not automatically a substitute for each other, when strength matters.

316Ti is sensible to a form of intergranular corrosion, named Knife Line Attack (KLA). This corrosion type occurs between the weld and the heat affected zone (HAZ), through the precipitation of chromium carbide in this area.

This happens after slow cooling or subsequent heating of the material.  This corrosion-attack appears as an extremely narrow line. To solve this problem, a heat treatment above 1035°C is recommended, followed by rapid cooling.

With the acquisition of Arcus Edelstahl in Germany, the Arcus Group has direct access to a great stock including Titanium stabilized types like 316Ti.

Please consult our experienced sales team for any enquiries or questions you might have.

Author: Daniel Hage

+31 (0) 78 648 3648
daniel.hage@arcus.nl

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