316 vs 316l vs 316Ti Comparison of Grades 316 and 316L to 316Ti

Grade 316Ti stainless steel tube has been traditionally specified by German engineers and users with the Werkstoff number 1.4571.The former steel grade in the UK was 320S31.

This grade is essentially a standard carbon 316 type with titanium stabilisation and is similar in principle to the titanium stabilisation of the 304 (1.4301) type to produce 321 (1.4541). The addition of titanium is made to reduce the risk of intergranular corrosion (IC) following heating in the temperature range 425-815C.

Intergranular corrosion When austenitic stainless steel are subject to prolonged heating in the temperature range 425-815C, the carbon in the steel diffuses to the grain boundaries and precipitates chromium carbide. This removes chromium from the solid solution and leaves a lower chromium content adjacent to the grain boundaries. Steel in this condition are termed 'sensitised'. The grain boundaries become prone to preferential atack on subsequent exposure to a corrosive environment. This type of corrosion is known as intergranular corrosion (IC), also known in the past as 'weld decay'.

The addition of titanium reduces the risk of Intergranular corrosion since titanium carbo-nitrides are formed in preference to chromium carbides which has the effect of maintaining the correct distribution of chromium throughout the structure of the stainless steel tubing.

The result is that areas adjacent to grain boundaries, where the arbo-nitrides form, is not depleted of chromium to a level at which localised corrosion can occur in the grain boundary area.



The alternative approach to reducing the risk of Intergranular corrosion attack is to reduce the carbon level to below 0.03%. In this way a grade of 316 is produced with, in practice, the same resistance to Intergranular corrosion as 316Ti 320S31/1.4571 . This is the basis of the 316L types (1.4404 and 316S13/1.4432).

Under most conditions it can be taken that the two grades are interchangeable, 316L (1.4404) being suitable for applications where 316Ti (1.4571) is specified. In aqueous corrosion media or environments at ambient temperatures, there is no practical advantage in specifying the 316Ti type in preference to the 316L. In some circumstances the 316L (1.4404 / 1.4432) grades may be better choices.

Mechanical Properties

The presence of titanium to 316Ti (1.4571) does, however, give some improvements to mechanical strength, sespecially, at elevated temperatures above about 600 C. and care must therefore be exercised in selecting 1.4404 as a substitute under these conditions. 316Ti (1.4571) may however have inferior impact properties at ambient temperature compared to the 1.4404 / 1.4432.

Machinability

The machinability of 316Ti (1.4571) can also be an issue as the titanium carbo-nitrides particles can result in higher tool wear and may not cold form or cold head as readily as the 1.4404 / 1.4432 types.

Polishing

The titanium carbo-nitrides in 316Ti (1.4571) can also result in problems where high standards of polished surface finish are required. The titanium carbo-nitrides particles can result in 'comet-tail' streaks on the polished surface as they are dragged out during polishing. This is similar to the 1.4541(321) grade, which was not recommended for a 'No8' bright mechanical polish in now obsolete BS1449 Pt2 (now replaced by BSEN 10088:2-1995 finish 1P/2P).

Corrosion Resistance

There is also some evidence that the 316Ti (1.4571) type may have inferior pitting and stress corrosion cracking resistance, compared to the 1.4404 / 1.4432 types, although the general corrosion resistance can be assumed to be generally similar. The titanium stabilised 316Ti (1.4571) grade may also be prone to 'knife line attack' in the heat-affected zones of welds, very close to the fusion zone where the carbo-nitrides have redissolved in the solid steel matrix.

 Weldability

 The weldability of the 316Ti (1.4571) and 1.4404 / 1.4432 can be assumed to be similar. Neither of the grades can be expected to weld 'easier' or 'better' than-the-other. Niobium stabilised fillers (welding consumables) should be used for welding the 316Ti (1.4571), especially where elevated temperature weld strength may be important. In other circumstances a '316L' filler should give a matching weld metal aqueous corrosion resistance to that of the 'parent' 316Ti (1.4571) '316Ti' material.

316Ti Stainless Steel Tubing

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